EXHIBIT 4.1
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                        AMSC ACQUISITION COMPANY, INC.

                                        

                             SERIES A AND SERIES B
                         12 1/4% SENIOR NOTES DUE 2008


                                   INDENTURE
                                        



                                March 31, 1998



                      State Street Bank and Trust Company

                                    Trustee




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                             CROSS-REFERENCE TABLE*


Trust Indenture Act Section                                   Indenture Section
                                                      
310 (a)(1)........................................................  7.10
(a)(2)............................................................  7.10
(a)(3)............................................................  N.A.
(a)(4)............................................................  N.A.
(a)(5)............................................................  7.10
(i)(b)............................................................  7.10
(ii)(c)...........................................................  N.A.
311(a)............................................................  7.11
(b)...............................................................  7.11
(iii)(c)..........................................................  N.A.
312 (a)...........................................................  2.05
(b)............................................................... 13.03
(iv)(c)........................................................... 13.03
313(a)............................................................  7.06
(b)(2)............................................................  7.07
(v)(c)............................................................  7.06;
                                                                   13.02
(vi)(d)...........................................................  7.06
314(a)............................................................  4.03;
                                                                   13.02
(c)(1)............................................................ 13.04
(c)(2)............................................................ 13.04
(c)(3)............................................................  N.A.
(vii)(e).......................................................... 13.05
(f)...............................................................  N.A.
315 (a)...........................................................  7.01
(b)...............................................................  7.05,
                                                                   13.02
(A)(c)............................................................  7.01
(d)...............................................................  7.01
(e)...............................................................  6.11
316 (a)(last sentence)............................................  2.09
(a)(1)(A).........................................................  6.05
(a)(1)(B).........................................................  6.04
(a)(2)............................................................  N.A.
(b)...............................................................  6.07
(B)(c)............................................................  2.12
317 (a)(1)........................................................  6.08
(a)(2)............................................................  6.09
(b)...............................................................  2.04
318 (a)........................................................... 13.01
(b)...............................................................  N.A.
(c)............................................................... 13.01

N.A. means not applicable.
*This Cross-Reference Table is not part of the Indenture.

 
                               TABLE OF CONTENTS

                                                                            Page

ARTICLE 1.      DEFINITIONS AND INCORPORATION BY REFERENCE.................. 1

 SECTION 1.01.  DEFINITIONS................................................. 1

 SECTION 1.02.  OTHER DEFINITIONS........................................... 15 

 SECTION 1.03.  INCORPORATION BY REFERENCE OF TRUST INDENTURE ACT........... 15

 SECTION 1.04.  RULES OF CONSTRUCTION....................................... 16

ARTICLE 2.      THE NOTES................................................... 16

 SECTION 2.01.  FORM AND DATING............................................. 16

 SECTION 2.02.  EXECUTION AND AUTHENTICATION................................ 18

 SECTION 2.03.  REGISTRAR AND PAYING AGENT.................................. 18

 SECTION 2.04.  PAYING AGENT TO HOLD MONEY IN TRUST......................... 18

 SECTION 2.05.  HOLDER LISTS................................................ 19

 SECTION 2.06.  TRANSFER AND EXCHANGE....................................... 19

 SECTION 2.07.  REPLACEMENT NOTES........................................... 31

 SECTION 2.08.  OUTSTANDING NOTES........................................... 31

 SECTION 2.09.  TREASURY NOTES.............................................. 32

 SECTION 2.10.  TEMPORARY NOTES............................................. 32

 SECTION 2.11.  CANCELLATION................................................ 32

 SECTION 2.12.  DEFAULTED INTEREST.......................................... 32

ARTICLE 3.      REDEMPTION AND PREPAYMENT................................... 33

 SECTION 3.01.  NOTICES TO TRUSTEE.......................................... 33

 SECTION 3.02.  SELECTION OF NOTES TO BE REDEEMED........................... 33

 SECTION 3.03.  NOTICE OF REDEMPTION........................................ 33

 SECTION 3.04.  EFFECT OF NOTICE OF REDEMPTION.............................. 34

 SECTION 3.05.  DEPOSIT OF REDEMPTION PRICE................................. 34

                                       i

 
 SECTION 3.06.  NOTES REDEEMED IN PART...................................... 34

 SECTION 3.07.  OPTIONAL REDEMPTION......................................... 35

 SECTION 3.08.  MANDATORY REDEMPTION........................................ 35

 SECTION 3.09.  OFFER TO PURCHASE BY APPLICATION OF EXCESS PROCEEDS......... 35

ARTICLE 4.      COVENANTS................................................... 37

 SECTION 4.01.  PAYMENT OF NOTES............................................ 37

 SECTION 4.02.  MAINTENANCE OF OFFICE OR AGENCY............................. 37

 SECTION 4.03.  REPORTS..................................................... 38

 SECTION 4.04.  COMPLIANCE CERTIFICATE...................................... 38

 SECTION 4.05.  TAXES....................................................... 39

 SECTION 4.06.  STAY, EXTENSION AND USURY LAWS.............................. 39

 SECTION 4.07.  RESTRICTED PAYMENTS......................................... 39

 SECTION 4.08.  DIVIDEND AND OTHER PAYMENT RESTRICTIONS AFFECTING 
                SUBSIDIARIES................................................ 41

 SECTION 4.09.  INCURRENCE OF INDEBTEDNESS AND ISSUANCE OF PREFERRED STOCK.. 42

 SECTION 4.10.  ASSET SALES................................................. 44

 SECTION 4.11.  TRANSACTIONS WITH AFFILIATES................................ 45

 SECTION 4.12.  LIENS....................................................... 45

 SECTION 4.13.  LINE OF BUSINESS............................................ 46

 SECTION 4.14.  CORPORATE EXISTENCE......................................... 46

 SECTION 4.15.  OFFER TO REPURCHASE UPON CHANGE OF CONTROL.................. 46

 SECTION 4.16.  MAINTENANCE OF INSURANCE.................................... 47

 SECTION 4.17.  LIMITATION ON SALE AND LEASEBACK TRANSACTIONS............... 49

 SECTION 4.18.  LIMITATION ON ISSUANCES AND SALES OF CAPITAL STOCK OF 
                WHOLLY OWNED SUBSIDIARIES................................... 49

 SECTION 4.19.  ADDITIONAL NOTE GUARANTEES.................................. 49

 SECTION 4.20.  PAYMENTS FOR CONSENTS....................................... 49

ARTICLE 5.      SUCCESSORS.................................................. 50

 SECTION 5.01.  MERGER, CONSOLIDATION, OR SALE OF ASSETS.................... 50

                                      ii

 
 SECTION 5.02.  SUCCESSOR CORPORATION SUBSTITUTED........................... 50

ARTICLE 6.      DEFAULTS AND REMEDIES....................................... 51

 SECTION 6.01.  EVENTS OF DEFAULT........................................... 51

 SECTION 6.02.  ACCELERATION................................................ 53

 SECTION 6.03.  OTHER REMEDIES.............................................. 53

 SECTION 6.04.  WAIVER OF PAST DEFAULTS..................................... 54

 SECTION 6.05.  CONTROL BY MAJORITY......................................... 54

 SECTION 6.06.  LIMITATION ON SUITS......................................... 54

 SECTION 6.07.  RIGHTS OF HOLDERS OF NOTES TO RECEIVE PAYMENT............... 54

 SECTION 6.08.  COLLECTION SUIT BY TRUSTEE.................................. 55

 SECTION 6.09.  TRUSTEE MAY FILE PROOFS OF CLAIM............................ 55

 SECTION 6.10.  PRIORITIES.................................................. 55

 SECTION 6.11.  UNDERTAKING FOR COSTS....................................... 56

ARTICLE 7.      TRUSTEE..................................................... 56

 SECTION 7.01.  DUTIES OF TRUSTEE........................................... 56

 SECTION 7.02.  RIGHTS OF TRUSTEE........................................... 57

 SECTION 7.03.  INDIVIDUAL RIGHTS OF TRUSTEE................................ 57

 SECTION 7.04.  TRUSTEE'S DISCLAIMER........................................ 58

 SECTION 7.05.  NOTICE OF DEFAULTS.......................................... 58

 SECTION 7.06.  REPORTS BY TRUSTEE TO HOLDERS OF THE NOTES.................. 58

 SECTION 7.07.  COMPENSATION AND INDEMNITY.................................. 58

 SECTION 7.08.  REPLACEMENT OF TRUSTEE...................................... 59

 SECTION 7.09.  SUCCESSOR TRUSTEE BY MERGER, ETC............................ 60

 SECTION 7.10.  ELIGIBILITY; DISQUALIFICATION............................... 60

 SECTION 7.11.  PREFERENTIAL COLLECTION OF CLAIMS AGAINST COMPANY........... 60

ARTICLE 8.      LEGAL DEFEASANCE AND COVENANT DEFEASANCE.................... 60

                                      iii

 
 SECTION 8.01.  OPTION TO EFFECT LEGAL DEFEASANCE OR COVENANT DEFEASANCE.... 60

 SECTION 8.02.  LEGAL DEFEASANCE AND DISCHARGE.............................. 60

 SECTION 8.03.  COVENANT DEFEASANCE......................................... 61

 SECTION 8.04.  CONDITIONS TO LEGAL OR COVENANT DEFEASANCE.................. 61

 SECTION 8.05.  DEPOSITED MONEY AND GOVERNMENT SECURITIES TO BE HELD IN 
                TRUST; OTHER MISCELLANEOUS PROVISIONS....................... 63

 SECTION 8.06.  REPAYMENT TO COMPANY........................................ 63

 SECTION 8.07.  REINSTATEMENT............................................... 63

ARTICLE 9.      AMENDMENT, SUPPLEMENT AND WAIVER............................ 64

 SECTION 9.01.  WITHOUT CONSENT OF HOLDERS OF NOTES......................... 64

 SECTION 9.02.  WITH CONSENT OF HOLDERS OF NOTES............................ 64

 SECTION 9.03.  COMPLIANCE WITH TRUST INDENTURE ACT......................... 66

 SECTION 9.04.  REVOCATION AND EFFECT OF CONSENTS........................... 66

 SECTION 9.05.  NOTATION ON OR EXCHANGE OF NOTES............................ 66

 SECTION 9.06.  TRUSTEE TO SIGN AMENDMENTS, ETC............................. 66

ARTICLE 10.     SUBORDINATION OF HOLDINGS GUARANTEE......................... 67

 SECTION 10.01. AGREEMENT TO SUBORDINATE.................................... 67
                
 SECTION 10.02. CERTAIN DEFINITIONS......................................... 67
                
 SECTION 10.03. LIQUIDATION; DISSOLUTION; BANKRUPTCY........................ 67
                
 SECTION 10.04. DEFAULT ON DESIGNATED SENIOR INDEBTEDNESS................... 68
                
 SECTION 10.05. ACCELERATION OF NOTES....................................... 68
                
 SECTION 10.06. WHEN DISTRIBUTION MUST BE PAID OVER......................... 69
                
 SECTION 10.07. NOTICE BY COMPANY........................................... 69
                
 SECTION 10.08. SUBROGATION................................................. 69
                
 SECTION 10.09. RELATIVE RIGHTS............................................. 69
                
 SECTION 10.10. SUBORDINATION MAY NOT BE IMPAIRED BY COMPANY................ 70
                
 SECTION 10.11. DISTRIBUTION OR NOTICE TO REPRESENTATIVE.................... 70
                
                                      iv

 
 SECTION 10.12. RIGHTS OF TRUSTEE AND PAYING AGENT.......................... 70
                
 SECTION 10.13. AUTHORIZATION TO EFFECT SUBORDINATION....................... 70
                
 SECTION 10.14. AMENDMENTS.................................................. 71

ARTICLE 11.     SUBSIDIARY GUARANTEES....................................... 71

 SECTION 11.01. SUBSIDIARY GUARANTEE........................................ 71
                
 SECTION 11.02. LIMITATION ON SUBSIDIARY GUARANTOR LIABILITY................ 72
                
 SECTION 11.03. EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTEE.............. 72
                
 SECTION 11.04. SUBSIDIARY GUARANTORS MAY CONSOLIDATE, ETC., ON CERTAIN 
                TERMS....................................................... 73
                
 SECTION 11.05. RELEASES FOLLOWING SALE OF ASSETS........................... 73

ARTICLE 12.     HOLDINGS GUARANTEE.......................................... 74

 SECTION 12.01. HOLDINGS GUARANTEE.......................................... 74

 SECTION 12.02. LIMITATION ON HOLDINGS LIABILITY............................ 75

 SECTION 12.03. EXECUTION AND DELIVERY OF HOLDINGS GUARANTEE................ 75

 SECTION 12.04. HOLDINGS MAY CONSOLIDATE, ETC., ON CERTAIN TERMS............ 75

 SECTION 12.05. SUBORDINATION............................................... 76

ARTICLE 13.     MISCELLANEOUS............................................... 76

 SECTION 13.01. TRUST INDENTURE ACT CONTROLS................................ 76

 SECTION 13.02. NOTICES..................................................... 76

 SECTION 13.03. COMMUNICATION BY HOLDERS OF NOTES WITH OTHER HOLDERS OF 
                NOTES....................................................... 77

 SECTION 13.04. CERTIFICATE AND OPINION AS TO CONDITIONS PRECEDENT.......... 77

 SECTION 13.05. STATEMENTS REQUIRED IN CERTIFICATE OR OPINION............... 78

 SECTION 13.06. RULES BY TRUSTEE AND AGENTS................................. 78

 SECTION 13.07. NO PERSONAL LIABILITY OF DIRECTORS, OFFICERS, EMPLOYEES AND
                STOCKHOLDERS................................................ 78

 SECTION 13.08. GOVERNING LAW............................................... 78

 SECTION 13.09. NO ADVERSE INTERPRETATION OF OTHER AGREEMENTS............... 79

 SECTION 13.10. SUCCESSORS.................................................. 79

                                       v

 
 SECTION 13.11. SEVERABILITY................................................ 79

 SECTION 13.12. COUNTERPART ORIGINALS....................................... 79

 SECTION 13.13. TABLE OF CONTENTS, HEADINGS, ETC............................ 79


EXHIBITS

Exhibit A1      FORM OF NOTE
Exhibit A2      FORM OF TEMPORARY REGULATION S NOTE
Exhibit B       FORM OF CERTIFICATE OF TRANSFER
Exhibit C       FORM OF CERTIFICATE OF EXCHANGE
Exhibit D       FORM OF CERTIFICATE OF ACQUIRING INSTITUTIONAL ACCREDITED 
                INVESTOR
Exhibit E       FORM OF NOTE GUARANTEE
Exhibit F       FORM OF SUPPLEMENTAL INDENTURE


SCHEDULES

Schedule I      Schedule of Guarantors

                                      vi

 
          INDENTURE dated as of March 31, 1998 among AMSC Acquisition Company,
Inc. a Delaware corporation (the "COMPANY"), American Mobile Satellite
Corporation, American Mobile Satellite Sales Corporation, AMSC Sales Corporation
Ltd., AMSC Subsidiary Corporation, ARDIS Company, Motorola ARDIS, Inc., Motorola
ARDIS Acquisition, Inc., ARDIS Holding Company, and Radio Data Network Holding
Corporation (collectively, the "GUARANTORS") and State Street Bank and Trust
Company, as trustee (the "TRUSTEE").

          The Company, the Guarantors and the Trustee agree as follows for the
benefit of each other and for the equal and ratable benefit of the Holders of
the 12 1/4% Series A Senior Notes due 2008 (the "SERIES A NOTES") and the 12
1/4% Series B Senior Notes due 2008 (the "SERIES B NOTES" and, together with the
Series A Notes, the "NOTES"):

                                   ARTICLE 1.

                   DEFINITIONS AND INCORPORATION BY REFERENCE

Section 1.01.  Definitions.

          "144A GLOBAL NOTE" means a global note in the form of Exhibit A1
hereto bearing the Global Note Legend and the Private Placement Legend and
deposited with or on behalf of, and registered in the name of, the Depositary or
its nominee that will be issued in a denomination equal to the outstanding
principal amount of the Notes sold in reliance on Rule 144A.

          "ACQUIRED INDEBTEDNESS" means, with respect to any specified Person,
(i) Indebtedness of any other Person existing at the time such other Person is
merged with or into or became a Subsidiary of such specified Person, including,
without limitation, Indebtedness incurred in connection with, or in
contemplation of, such other Person merging with or into or becoming a
Subsidiary of such specified Person, and (ii) Indebtedness secured by a Lien
encumbering any asset acquired by such specified Person.

          "ACTEL AGREEMENT" means the agreement between Holdings and African
Continental Telecommunications Ltd., dated as of December 4, 1997, pursuant to
which Holdings will lease its MSAT-2 satellite to African Continental
Telecommunications Ltd., as such agreement is in effect on the date hereof.

          "AFFILIATE" of any specified Person means any other Person directly or
indirectly controlling or controlled by or under direct or indirect common
control with such specified Person.  For purposes of this definition, "control"
(including, with correlative meanings, the terms "controlling," "controlled by"
and "under common control with"), as used with respect to any Person, shall mean
the possession, directly or indirectly, of the power to direct or cause the
direction of the management or policies of such Person, whether through the
ownership of voting securities, by agreement or otherwise; provided that
beneficial ownership of 10% or more of the Voting Stock of a Person shall be
deemed to be control.

          "AGENT" means any Registrar, Paying Agent or co-registrar.

          "APPLICABLE PROCEDURES" means, with respect to any transfer or
exchange of or for beneficial interests in any Global Note, the rules and
procedures of the Depositary, Euroclear and Cedel that apply to such transfer or
exchange.

 
          "ASSET SALE" means (i) the sale, lease, conveyance or other
disposition of any assets or rights (including, without limitation, by way of a
sale and leaseback) other than sales of inventory in the ordinary course of
business consistent with past practices (provided that the sale, lease,
conveyance or other disposition of all or substantially all of the assets of the
Company and its Subsidiaries taken as a whole will be governed by Section 4.15
hereof and/or Section 5.01 hereof and not by the provisions of Section 4.10
hereof), and (ii) the issue or sale by the Company or any of its Subsidiaries of
Equity Interests of any of the Company's Subsidiaries, in the case of either
clause (i) or (ii), whether in a single transaction or a series of related
transactions (a) that have a fair market value in excess of $1.0 million or (b)
for net proceeds in excess of $1.0 million.  Notwithstanding the foregoing, the
following items shall not be deemed to be Asset Sales: (i) a transfer of assets
by the Company to a Wholly Owned Subsidiary or by a Wholly Owned Subsidiary to
the Company or to another Wholly Owned Subsidiary, (ii) an issuance of Equity
Interests by a Wholly Owned Subsidiary to the Company or to another Wholly Owned
Subsidiary, and (iii) a Restricted Payment that is permitted by Section 4.07
hereof.

          "ATTRIBUTABLE INDEBTEDNESS" in respect of a sale and leaseback
transaction means, at the time of determination, the present value (discounted
at the rate of interest implicit in such transaction, determined in accordance
with GAAP) of the obligation of the lessee for net rental payments during the
remaining term of the lease included in such sale and leaseback transaction
(including any period for which such lease has been extended or may, at the
option of the lessor, be extended).

          "BANKRUPTCY LAW" means Title 11, U.S. Code or any similar federal or
state law for the relief of debtors.

          "BOARD OF DIRECTORS" means the Board of Directors of the Company, or
any authorized committee of the Board of Directors.

          "BORROWING BASE" means, as of any date, an amount equal to 85% of the
face amount of all accounts receivable owned by the Company and its Subsidiaries
as of such date that are not more than 90 days past due, all calculated on a
consolidated basis and in accordance with GAAP.  To the extent that information
is not available as to the amount of accounts receivable or trade payables as of
a specific date, the Company may utilize the most recent available information
for purposes of calculating the Borrowing Base.

          "BUSINESS DAY" means any day other than a Legal Holiday.

          "CAPITAL LEASE OBLIGATION" means, at the time any determination
thereof is to be made, the amount of the liability in respect of a capital lease
that would at such time be required to be capitalized on a balance sheet in
accordance with GAAP.

          "CAPITAL STOCK" means (i) in the case of a corporation, corporate
stock, (ii) in the case of an association or business entity, any and all
shares, interests, participations, rights or other equivalents (however
designated) of corporate stock, (iii) in the case of a partnership or limited
liability company, partnership or membership interests (whether general or
limited) and (iv) any other interest or participation that confers on a Person
the right to receive a share of the profits and losses of, or distributions of
assets of, the issuing Person.

          "CASH EQUIVALENTS" means (i) U.S. dollars, (ii) securities issued or
directly and fully guaranteed or insured by the U.S. government or any agency or
instrumentality thereof (provided that the full faith and credit of the United
States is pledged in support thereof) having maturities of not more than 

                                       2

 
six months from the date of acquisition, (iii) certificates of deposit and
eurodollar time deposits with maturities of six months or less from the date of
acquisition, bankers' acceptances with maturities not exceeding six months and
overnight bank deposits, in each case with any domestic commercial bank having
capital and surplus in excess of $500.0 million and a Thompson Bank Watch Rating
of "B" or better, (iv) repurchase obligations with a term of not more than seven
days for underlying securities of the types described in clauses (ii) and (iii)
above entered into with any financial institution meeting the qualifications
specified in clause (iii) above, (v) commercial paper having the highest rating
obtainable from Moody's Investors Service, Inc. or Standard & Poor's Corporation
and in each case maturing within six months after the date of acquisition and
(vi) money market funds the assets of which constitute Cash Equivalents of the
kinds described in clauses (i)-(v) of this definition.

          "CEDEL" means Cedel Bank, SA.

          "CHANGE OF CONTROL" means the occurrence of any of the following: (i)
the sale, lease, transfer, conveyance or other disposition (other than by way of
merger or consolidation), in one or a series of related transactions, of all or
substantially all of the assets of the Company and its Subsidiaries taken as a
whole to any "person" (as such term is used in Section 13(d)(3) of the Exchange
Act) other than a Principal or a Related Party of a Principal (as defined
below), (ii) the adoption of a plan relating to the liquidation or dissolution
of the Company, (iii) the consummation of any transaction (including, without
limitation, any merger or consolidation) the result of which is that any
"person" (as defined above), other than the Principals and their Related
Parties, becomes the "beneficial owner" (as such term is defined in Rule 13d-3
and Rule 13d-5 under the Exchange Act, except that a person shall be deemed to
have "beneficial ownership" of all securities that such person has the right to
acquire, whether such right is currently exercisable or is exercisable only upon
the occurrence of a subsequent condition), directly or indirectly, of more than
40% of the Voting Stock of Holdings (measured by voting power rather than number
of shares), (iv) the consummation of the first transaction (including, without
limitation, any merger or consolidation) the result of which is that any
"person" (as defined above) becomes the "beneficial owner" (as defined above),
directly or indirectly, of more of the Voting Stock of Holdings (measured by
voting power rather than number of shares) than is at the time "beneficially
owned" (as defined above) by the Principals and their Related Parties in the
aggregate, (v) the first day on which a majority of the members of the Board of
Directors of the Company or Holdings are not Continuing Directors or (vi) the
first day on which Holdings ceases to own 80% of the outstanding Voting Stock of
the Company.

          "COMMUNICATIONS ASSET" means a terrestrial or satellite antenna,
licensed site, base station, communications ground segment, network operations
center or other telecommunications facility (other than a satellite) that is
used or useful in a Permitted Business.

          "COMPANY" means AMSC Acquisition Company, Inc., and any and all
successors thereto.

          "CONSOLIDATED ANNUALIZED CASH FLOW" means, with respect to any Person
for any period, the product of (x) the Consolidated Cash Flow of such Person for
the most recently completed fiscal quarter for which internal financial
statements are available, times (y) four.

          "CONSOLIDATED CASH FLOW" means, with respect to any Person for any
period, the Consolidated Net Income of such Person for such period plus (i) an
amount equal to any extraordinary loss plus any net loss realized in connection
with an Asset Sale (to the extent such losses were deducted in computing such
Consolidated Net Income), plus (ii) provision for taxes based on income or
profits of 

                                       3

 
such Person and its Subsidiaries for such period, to the extent that such
provision for taxes was included in computing such Consolidated Net Income, plus
(iii) consolidated interest expense of such Person and its Subsidiaries for such
period, whether paid or accrued and whether or not capitalized (including,
without limitation, amortization of debt issuance costs and original issue
discount, non-cash interest payments, the interest component of any deferred
payment obligations, the interest component of all payments associated with
Capital Lease Obligations imputed interest with respect to Attributable
Indebtedness, commissions, discounts and other fees and charges incurred in
respect of letter of credit or bankers' acceptance financings, and net payments
(if any) pursuant to Hedging Obligations), to the extent that any such expense
was deducted in computing such Consolidated Net Income, plus (iv) depreciation,
amortization (including amortization of goodwill and other intangibles but
excluding amortization of prepaid cash expenses that were paid in a prior
period) and other non-cash expenses (excluding any such non-cash expense to the
extent that it represents an accrual of or reserve for cash expenses in any
future period or amortization of a prepaid cash expense that was paid in a prior
period) of such Person and its Subsidiaries for such period to the extent that
such depreciation, amortization and other non-cash expenses were deducted in
computing such Consolidated Net Income, minus (v) non-cash items increasing such
Consolidated Net Income for such period, in each case, on a consolidated basis
and determined in accordance with GAAP. Notwithstanding the foregoing, the
provision for taxes on the income or profits of, and the depreciation and
amortization and other non-cash expenses of, a Subsidiary of the referent Person
shall be added to Consolidated Net Income to compute Consolidated Cash Flow only
to the extent that a corresponding amount would be permitted at the date of
determination to be dividended to the Company by such Subsidiary without prior
governmental approval (that has not been obtained), and without direct or
indirect restriction pursuant to the terms of its charter and all agreements,
instruments, judgments, decrees, orders, statutes, rules and governmental
regulations applicable to that Subsidiary or its stockholders.

          "CONSOLIDATED INDEBTEDNESS" means, with respect to any Person as of
any date of determination, the sum, without duplication, of (i) the total amount
of Indebtedness of such Person and its Subsidiaries, plus (ii) the total amount
of Indebtedness of any other Person, to the extent that such Indebtedness has
been Guaranteed by the referent Person or one or more of its Subsidiaries, plus
(iii) the aggregate liquidation value of all Disqualified Stock of such Person
and all preferred stock of Subsidiaries of such Person, in each case, determined
on a consolidated basis in accordance with GAAP.

          "CONSOLIDATED INTEREST EXPENSE" means, with respect to any Person for
any period, the sum of (i) the consolidated interest expense of such Person and
its Subsidiaries for such period, whether paid or accrued (including, without
limitation, amortization or original issue discount, non-cash interest payments,
the interest component of any deferred payment obligations, the interest
component of all payments associated with Capital Lease Obligations, imputed
interest with respect to Attributable Indebtedness, commissions, discounts and
other fees and charges incurred in respect of letter of credit or bankers'
acceptance financings, and net payments (if any) pursuant to Hedging
Obligations) and (ii) the consolidated interest expense of such Person and its
Subsidiaries that was capitalized during such period, and (iii) any interest
expense on Indebtedness of another Person that is guaranteed by such Person or
one of its Subsidiaries or secured by a Lien on assets of such Person or one of
its Subsidiaries (whether or not such Guarantee or Lien is called upon) and (iv)
the product of (a) all dividend payments on any series of preferred stock of
such Person or any of its Subsidiaries, times (b) a fraction, the numerator of
which is one and the denominator of which is one minus the then current combined
federal, state and local statutory tax rate of such Person, expressed as a
decimal, in each case, on a consolidated basis and in accordance with GAAP.

                                       4

 
          "CONSOLIDATED NET INCOME" means, with respect to any Person for any
period, the aggregate of the Net Income of such Person and its Subsidiaries for
such period, on a consolidated basis, determined in accordance with GAAP;
provided that (i) the Net Income (but not loss) of any Person that is not a
Subsidiary or that is accounted for by the equity method of accounting shall be
included only to the extent of the amount of dividends or distributions paid in
cash to the referent Person or a Wholly Owned Subsidiary thereof that is a
Subsidiary Guarantor, (ii) the Net Income of any Subsidiary shall be excluded to
the extent that the declaration or payment of dividends or similar distributions
by that Subsidiary of that Net Income is not at the date of determination
permitted without any prior governmental approval (that has not been obtained)
or, directly or indirectly, by operation of the terms of its charter or any
agreement, instrument, judgment, decree, order, statute, rule or governmental
regulation applicable to that Subsidiary or its stockholders, (iii) the Net
Income of any Person acquired in a pooling of interests transaction for any
period prior to the date of such acquisition shall be excluded and (iv) the
cumulative effect of a change in accounting principles shall be excluded.

          "CONSOLIDATED NET WORTH" means, with respect to any Person as of any
date, the sum of (i) the consolidated equity of the common stockholders of such
Person and its consolidated Subsidiaries as of such date plus (ii) the
respective amounts reported on such Person's balance sheet as of such date with
respect to any series of preferred stock (other than Disqualified Stock) that by
its terms is not entitled to the payment of dividends unless such dividends may
be declared and paid only out of net earnings in respect of the year of such
declaration and payment, but only to the extent of any cash received by such
Person upon issuance of such preferred stock, less (x) all write-ups (other than
write-ups resulting from foreign currency translations and write-ups of tangible
assets of a going concern business made within 12 months after the acquisition
of such business) subsequent to the date hereof in the book value of any asset
owned by such Person or a consolidated Subsidiary of such Person, (y) all
investments as of such date in unconsolidated Subsidiaries and in Persons that
are not Subsidiaries (except, in each case, Permitted Investments), and (z) all
unamortized debt discount and expense and unamortized deferred charges as of
such date, all of the foregoing determined in accordance with GAAP.

          "CONSOLIDATED TANGIBLE NET WORTH" of any Person at any time means the
Consolidated Net Worth of such Person at such time less goodwill and any other
intangible assets reflected on the consolidated balance sheet of such Person at
such time.

          "CONTINUING DIRECTORS" means, as of any date of determination, any
member of the Board of Directors of the Company who (i) was a member of such
Board of Directors on the date hereof, (ii) was nominated for election or
elected to such Board of Directors with the approval of a majority of the
Continuing Directors who were members of such Board at the time of such
nomination or election or (iii) was a designee of any Principal at a time when
such Principal owned more than 10% of the Voting Stock of Holdings.

          "CORPORATE TRUST OFFICE OF THE TRUSTEE" shall be at the address of the
Trustee specified in Section 13.02 hereof or such other address as to which the
Trustee may give notice to the Company.

          "CREDIT FACILITIES" means, with respect to the Company, one or more
debt facilities (including, without limitation, the Revolving Credit Facility)
or commercial paper facilities with banks or other institutional lenders
providing for revolving credit loans, term loans, receivables financing
(including through the sale of receivables to such lenders or to special purpose
entities formed to borrow from such lenders against such receivables) or letters
of credit, in each case, as amended, restated, modified, renewed, refunded,
replaced or refinanced in whole or in part from time to time. Indebtedness under
Credit Facilities outstanding on the date on which Notes are first issued and
authenticated 

                                       5

 
hereunder shall be deemed to have been incurred on such date in reliance on the
exception provided by clause (i) of the definition of Permitted Indebtedness.

          "CUMULATIVE CONSOLIDATED CASH FLOW" means the aggregate Consolidated
Cash Flow of a Person from a particular point in time.

          "CUSTODIAN" means the Trustee, as custodian with respect to the Notes
in global form, or any successor entity thereto.

          "DEBT TO CASH FLOW RATIO" means, with respect to any Person as of any
date of determination (the "CALCULATION DATE") the ratio of (a) the Consolidated
Indebtedness of the Company as of such date to (b) the Consolidated Annualized
Cash Flow of the Company for the most recent full fiscal Reference Period ending
immediately prior to such date for which internal financial statements are
available, determined on a pro forma basis after giving effect to all
acquisitions or dispositions of assets made by the Company and its Subsidiaries
from the beginning of such Reference Period through and including such date of
determination (including any related financing transactions) as if such
acquisitions and dispositions had occurred at the beginning of such quarter. In
addition, for purposes of making the computation referred to above, (i)
acquisitions that have been made by the Company or any of its Subsidiaries,
including through mergers or consolidations and including any related financing
transactions, during the reference period or subsequent to such reference period
and on or prior to the Calculation Date shall be deemed to have occurred on the
first day of the reference period and Consolidated Cash Flow for such reference
period shall be calculated without giving effect to clause (iii) of the proviso
set forth in the definition of Consolidated Net Income, and (ii) the
Consolidated Cash Flow attributable to discontinued operations, as determined in
accordance with GAAP, and operations or businesses disposed of prior to the
Calculation Date, shall be excluded.

          "DEBT REGISTRATION RIGHTS AGREEMENT" means the Debt Registration
Rights Agreement, dated as of March 31, 1998, by and among the Company and the
other parties named on the signature pages thereof, as such agreement may be
amended, modified or supplemented from time to time.

          "DEFAULT" means any event that is or with the passage of time or the
giving of notice or both would be an Event of Default.

          "DEFINITIVE NOTE" means a certificated Note registered in the name of
the Holder thereof and issued in accordance with Section 2.06 hereof, in the
form of Exhibit A1 hereto except that such Note shall not bear the Global Note
Legend and shall not have the "Schedule of Exchanges of Interests in the Global
Note" attached thereto.

          "DEPOSITARY" means, with respect to the Notes issuable or issued in
whole or in part in global form, the Person specified in Section 2.03 hereof as
the Depositary with respect to the Notes, and any and all successors thereto
appointed as depositary hereunder and having become such pursuant to the
applicable provision of this Indenture.

          "DISQUALIFIED STOCK" means any Capital Stock that, by its terms (or by
the terms of any security into which it is convertible, or for which it is
exchangeable, at the option of the holder thereof), or upon the happening of any
event, matures or is mandatorily redeemable, pursuant to a sinking fund
obligation or otherwise, or redeemable at the option of the Holder thereof, in
whole or in part, on or prior to the date that is 91 days after the date on
which the Notes mature; provided, however, that any Capital Stock that would
constitute Disqualified Stock solely because the holders thereof have the right
to 

                                       6

 
require the Company to repurchase such Capital Stock upon the occurrence of a
Change of Control or an Asset Sale shall not constitute Disqualified Stock if
the terms of such Capital Stock provide that the Company may not repurchase or
redeem any such Capital Stock pursuant to such provisions unless such repurchase
or redemption complies with Section 4.07 hereof.

          "EQUITY INTERESTS" means Capital Stock and all warrants, options or
other rights to acquire Capital Stock (but excluding any debt security that is
convertible into, or exchangeable for, Capital Stock).

          "EQUITY OFFERING" means (i) an offering of common stock of the Company
or a capital contribution to the Company's common equity of the net cash
proceeds of a public offering of Holdings or (ii) one or more Strategic Equity
Investments (other than in connection with a Change of Control).

          "EUROCLEAR" means Morgan Guaranty Trust Company of New York, Brussels
office, as operator of the Euroclear system.

          "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended.

          "EXCHANGE NOTES" means the Notes issued in the Exchange Offer pursuant
to Section 2.06(f) hereof.

          "EXCHANGE OFFER" has the meaning set forth in the Debt Registration
Rights Agreement.

          "EXCHANGE OFFER REGISTRATION STATEMENT" has the meaning set forth in
the Debt Registration Rights Agreement.

          "EXISTING INDEBTEDNESS" means Indebtedness of the Company and its
Subsidiaries (other than Indebtedness under the Revolving Credit Facility) in
existence on the date hereof, until such amounts are repaid.

          "GAAP" means generally accepted accounting principles set forth in the
opinions and pronouncements of the Accounting Principles Board of the American
Institute of Certified Public Accountants and statements and pronouncements of
the Financial Accounting Standards Board or in such other statements by such
other entity as have been approved by a significant segment of the accounting
profession, which are in effect from time to time.

          "GLOBAL NOTES" means, individually and collectively, each of the
Restricted Global Notes and the Unrestricted Global Notes, in the form of
Exhibits A1 and A2 hereto issued in accordance with Section 2.01, 2.06(b)(iv),
2.06(d)(i), 2.06(d)(ii), 2.06(d)(iii) or 2.06(f) hereof.

          "GLOBAL NOTE LEGEND" means the legend set forth in Section
2.06(g)(ii), which is required to be placed on all Global Notes issued under
this Indenture.

          "GOVERNMENT SECURITIES" means direct obligations of, or obligations
guaranteed by, the United States of America, and the payment for which the
United States pledges its full faith and credit.

          "GUARANTEE" means a guarantee (other than by endorsement of negotiable
instruments for collection in the ordinary course of business), direct or
indirect, in any manner (including, without limitation, by way of a pledge of
assets or through letters of credit or reimbursement agreements in respect
thereof), of all or any part of any Indebtedness.

                                       7

 
          "GUARANTORS" means (i) Holdings and (ii) the Subsidiary Guarantors.

          "HEDGING OBLIGATIONS" means, with respect to any Person, the
obligations of such Person under (i) interest rate swap agreements, interest
rate cap agreements and interest rate collar agreements and (ii) other
agreements or arrangements designed to protect such Person against fluctuations
in interest rates.

          "HOLDER" means a Person in whose name a Note is registered.

          "HOLDINGS" means American Mobile Satellite Corporation and all
successors thereto.

          "IAI GLOBAL NOTE" means the Global Note in the form of Exhibit A1
hereto bearing the Global Note Legend and the Private Placement Legend and
deposited with or on behalf of and registered in the name of the Depositary or
its nominee that will be issued in a denomination equal to the outstanding
principal amount of the Notes sold to Institutional Accredited Investors.

          "INDEBTEDNESS" means, with respect to any Person, any indebtedness of
such Person, whether or not contingent, in respect of borrowed money or
evidenced by bonds, notes, debentures or similar instruments or letters of
credit (or reimbursement agreements in respect thereof) or banker's acceptances
or representing Capital Lease Obligations or the balance deferred and unpaid of
the purchase price of any property or representing any Hedging Obligations,
except any such balance that constitutes an accrued expense or trade payable, if
and to the extent any of the foregoing (other than letters of credit and Hedging
Obligations) would appear as a liability upon a balance sheet of such Person
prepared in accordance with GAAP, as well as all Indebtedness of others secured
by a Lien on any asset of such Person (whether or not such Indebtedness is
assumed by such Person) and, to the extent not otherwise included, the Guarantee
by such Person of any indebtedness of any other Person.  The amount of any
Indebtedness outstanding as of any date shall be (i) the accreted value thereof,
in the case of any Indebtedness issued with original issue discount, and (ii)
the principal amount thereof, together with any interest thereon that is more
than 30 days past due, in the case of any other Indebtedness.

          "INDENTURE" means this Indenture, as amended or supplemented from time
to time.

          "INDIRECT PARTICIPANT" means a Person who holds a beneficial interest
in a Global Note through a Participant.

          "INSTITUTIONAL ACCREDITED INVESTOR" means an institution that is an
"accredited investor" as defined in Rule 501(a)(1), (2), (3) or (7) under the
Securities Act, who are not also QIBs.

          "INVESTMENTS" means, with respect to any Person, all investments by
such Person in other Persons (including Affiliates) in the forms of direct or
indirect loans (including guarantees of Indebtedness or other obligations),
advances or capital contributions (excluding commission, travel and similar
advances to officers and employees made in the ordinary course of business),
purchases or other acquisitions for consideration of Indebtedness, Equity
Interests or other securities, together with all items that are or would be
classified as investments on a balance sheet prepared in accordance with GAAP.
If the Company or any Subsidiary of the Company sells or otherwise disposes of
any Equity Interests of any direct or indirect Subsidiary of the Company such
that, after giving effect to any such sale or disposition, such Person is no
longer a Subsidiary of the Company, the Company shall be deemed to have made an
Investment on the date of any such sale or disposition equal to the fair market
value of the 

                                       8

 
Equity Interests of such Subsidiary not sold or disposed of in an amount
determined as provided in the final paragraph of Section 4.07 hereof.

          "LEGAL HOLIDAY" means a Saturday, a Sunday or a day on which banking
institutions in the City of New York, the city in which the corporate trust
office of the Trustee is located, or at a place of payment are authorized by
law, regulation or executive order to remain closed.  If a payment date is a
Legal Holiday at a place of payment, payment may be made at that place on the
next succeeding day that is not a Legal Holiday, and no interest shall accrue on
such payment for the intervening period.

          "LETTER OF TRANSMITTAL" means the letter of transmittal to be prepared
by the Company and sent to all Holders of the Notes for use by such Holders in
connection with the Exchange Offer.

          "LIEN" means, with respect to any asset, any mortgage, lien, pledge,
charge, security interest or encumbrance of any kind in respect of such asset,
whether or not filed, recorded or otherwise perfected under applicable law
(including any conditional sale or other title retention agreement, any lease in
the nature thereof, any option or other agreement to sell or give a security
interest in and any filing of or agreement to give any financing statement under
the Uniform Commercial Code (or equivalent statutes) of any jurisdiction).

          "LIQUIDATED DAMAGES" means all liquidated damages then owing pursuant
to Section 5 of the Debt Registration Rights Agreement.

          "NET INCOME" means, with respect to any Person, the net income (loss)
of such Person, determined in accordance with GAAP and before any reduction in
respect of preferred stock dividends, excluding, however, (i) any gain (but not
loss), together with any related provision for taxes on such gain (but not
loss), realized in connection with (a) any Asset Sale (including, without
limitation, dispositions pursuant to sale and leaseback transactions) or (b) the
disposition of any securities by such Person or any of its Subsidiaries or the
extinguishment of any Indebtedness of such Person or any of its Subsidiaries and
(ii) any extraordinary or nonrecurring gain (but not loss), together with any
related provision for taxes on such extraordinary or nonrecurring gain (but not
loss).

          "NET PROCEEDS" means the aggregate cash and Cash Equivalent proceeds
received by the Company or any of its Subsidiaries in respect of any Asset Sale
(including, without limitation, any cash received upon the sale or other
disposition of any non-cash consideration received in any Asset Sale), net of
the direct costs relating to such Asset Sale (including, without limitation,
legal, accounting and investment banking fees, and sales commissions) and any
relocation expenses incurred as a result thereof, taxes paid or payable as a
result thereof (after taking into account any available tax credits or
deductions and any tax sharing arrangements), amounts required to be applied to
the repayment of Indebtedness secured by a Lien on the asset or assets that were
the subject of such Asset Sale and any reserve for adjustment in respect of the
sale price of such asset or assets established in accordance with GAAP.

          "NON-U.S. PERSON" means a Person who is not a U.S. Person.

          "NOTE GUARANTEE" means the Guarantee by each Guarantor of the
Company's payment obligations under this Indenture and the Notes, executed
pursuant to the provisions of this Indenture.

          "NOTES" has the meaning assigned to it in the preamble to this
Indenture.

                                       9

 
          "OBLIGATIONS" means any principal, interest, penalties, fees,
indemnifications, reimbursements, damages and other liabilities payable under
the documentation governing any Indebtedness.

          "OFFERING" means the offering of the Notes by the Company.

          "OFFICER" means, with respect to any Person, the Chairman of the
Board, the Chief Executive Officer, the President, the Chief Operating Officer,
the Chief Financial Officer, the Treasurer, any Assistant Treasurer, the
Controller, the Secretary or any Vice-President of such Person.

          "OFFICERS' CERTIFICATE" means a certificate signed on behalf of the
Company by two Officers of the Company, one of whom must be the principal
executive officer, the principal financial officer, the treasurer or the
principal accounting officer of the Company, that meets the requirements of
Sections 13.04 and 13.05 hereof.

          "OPINION OF COUNSEL" means an opinion from legal counsel who is
reasonably acceptable to the Trustee, that meets the requirements of Section
13.05 hereof.  The counsel may be an employee of or counsel to the Company, any
Subsidiary of the Company or the Trustee.

          "PARTICIPANT" means, with respect to the Depositary, Euroclear or
Cedel, a Person who has an account with the Depositary, Euroclear or Cedel,
respectively (and, with respect to The Depository Trust Company, shall include
Euroclear and Cedel).

          "PARTICIPATING BROKER-DEALER" has the meaning set forth in the Debt
Registration Rights Agreement.

          "PERMITTED BUSINESS" means the business of the Company on the date
hereof and businesses reasonably related or incidental thereto.

          "PERMITTED INVESTMENTS" means (a) any Investment in the Company or in
a Wholly Owned Subsidiary of the Company that is a Subsidiary Guarantor (b) any
Investment in Cash Equivalents; (c) any Investment by the Company or any
Subsidiary of the Company in a Person, if as a result of such Investment (i)
such Person becomes a Wholly Owned Subsidiary of the Company and a Subsidiary
Guarantor or (ii) such Person is merged, consolidated or amalgamated with or
into, or transfers or conveys substantially all of its assets to, or is
liquidated into, the Company or a Wholly Owned Subsidiary of the Company that is
a Subsidiary Guarantor; (d) any Investment made as a result of the receipt of
non-cash consideration from an Asset Sale that was made pursuant to and in
compliance with Section 4.10 hereof; and (e) any acquisition of assets solely in
exchange for the issuance of Equity Interests (other than Disqualified Stock) of
the Company.

          "PERMITTED LIENS" means (i) Liens in favor of the Company; (ii) Liens
on property of a Person existing at the time such Person is merged into or
consolidated with the Company or any Subsidiary of the Company; provided that
such Liens were in existence prior to the contemplation of such merger or
consolidation and do not extend to any assets other than those of the Person
merged into or consolidated with the Company; (iii) Liens on property existing
at the time of acquisition thereof by the Company or any Subsidiary of the
Company, provided that such Liens were in existence prior to the contemplation
of such acquisition; (iv) Liens to secure the performance of statutory
obligations, surety or appeal bonds, performance bonds or other obligations of a
like nature incurred in the ordinary course of business; (v) Liens to secure
Indebtedness (including Capital Lease Obligations) permitted by clause (iv) 

                                       10

 
or clause (x) of Section 4.09 hereof covering only the assets acquired with such
Indebtedness; (vi) Liens existing on the date hereof; (vii) Liens for taxes,
assessments or governmental charges or claims that are not yet delinquent or
that are being contested in good faith by appropriate proceedings promptly
instituted and diligently concluded, provided that any reserve or other
appropriate provision as shall be required in conformity with GAAP shall have
been made therefor; and (viii) Liens incurred in the ordinary course of business
of the Company or any Subsidiary of the Company with respect to obligations that
do not exceed $5.0 million at any one time outstanding and that (a) are not
incurred in connection with the borrowing of money or the obtaining of advances
or credit (other than trade credit in the ordinary course of business) and (b)
do not in the aggregate materially detract from the value of the property or
materially impair the use thereof in the operation of business by the Company or
such Subsidiary.

          "PERMITTED REFINANCING INDEBTEDNESS" means any Indebtedness of the
Company or any of its Subsidiaries issued in exchange for, or the net proceeds
of which are used to extend, refinance, renew, replace, defease or refund other
Indebtedness of the Company or any of its Subsidiaries (other than intercompany
Indebtedness); provided that: (i) the principal amount (or accreted value, if
applicable) of such Permitted Refinancing Indebtedness does not exceed the
principal amount of (or accreted value, if applicable), plus accrued interest
on, the Indebtedness so extended, refinanced, renewed, replaced, defeased or
refunded (plus the amount of reasonable expenses incurred in connection
therewith); (ii) such Permitted Refinancing Indebtedness has a final maturity
date later than the final maturity date of, and has a Weighted Average Life to
Maturity equal to or greater than the Weighted Average Life to Maturity of, the
Indebtedness being extended, refinanced, renewed, replaced, defeased or
refunded; (iii) if the Indebtedness being extended, refinanced, renewed,
replaced, defeased or refunded is subordinated in right of payment to the Notes,
such Permitted Refinancing Indebtedness has a final maturity date later than the
final maturity date of, and is subordinated in right of payment to, the Notes on
terms at least as favorable to the Holders of Notes as those contained in the
documentation governing the Indebtedness being extended, refinanced, renewed,
replaced, defeased or refunded; and (iv) such Indebtedness is incurred either by
the Company or by the Subsidiary who is the obligor on the Indebtedness being
extended, refinanced, renewed, replaced, defeased or refunded.

          "PERSON" means any individual, corporation, limited liability company,
joint stock company, trust, unincorporated organization, government or any
agency or political subdivision thereof or any other entity.

          "PLEDGE AGREEMENT" means the Pledge and Security Agreement between the
Company and the Trustee, dated the date hereof and as amended from time to time.

          "PRINCIPALS" means Hughes Communications Satellite Services, Inc.,
Singapore Telecommunications Ltd., AT&T Wireless Services, Inc. and Motorola,
Inc.

          "PRIVATE PLACEMENT LEGEND" means the legend set forth in Section
2.06(g)(i) to be placed on all Notes issued under this Indenture except where
otherwise permitted by the provisions of this Indenture.

          "QIB" means a "qualified institutional buyer" as defined in Rule 144A.

          "REFERENCE PERIOD" means the most recently ended full fiscal quarter.

          "REGULATION S" means Regulation S promulgated under the Securities
Act.

                                       11

 
          "REGULATION S GLOBAL NOTE" means a Regulation S Temporary Global Note
or Regulation S Permanent Global Note, as appropriate.

          "REGULATION S PERMANENT GLOBAL NOTE" means a permanent global Note in
the form of Exhibit A1 hereto bearing the Global Note Legend and the Private
Placement Legend and deposited with or on behalf of and registered in the name
of the Depositary or its nominee, issued in a denomination equal to the
outstanding principal amount of the Regulation S Temporary Global Note upon
expiration of the Restricted Period.

          "REGULATION S TEMPORARY GLOBAL NOTE" means a temporary global Note in
the form of Exhibit A2 hereto bearing the Private Placement Legend and deposited
with or on behalf of and registered in the name of the Depositary or its
nominee, issued in a denomination equal to the outstanding principal amount of
the Notes initially sold in reliance on Rule 903 of Regulation S.

          "RELATED PARTY" with respect to any Principal means (A) any
controlling stockholder, 80% (or more) owned Subsidiary, or spouse or immediate
family member (in the case of an individual) of such Principal or (B) any trust,
corporation, partnership or other entity, the beneficiaries, stockholders,
partners, owners or Persons beneficially holding an 80% or more controlling
interest of which consist of such Principal and/or such other Persons referred
to in the immediately preceding clause (A).

          "REPLACEMENT SATELLITE" means any satellite constructed to replace
MSAT-1 or MSAT-2 in the event of a failure of such MSAT-1 or MSAT-2.

          "RESPONSIBLE OFFICER," when used with respect to the Trustee, means
any officer within the Corporate Trust Administration of the Trustee (or any
successor group of the Trustee) or any other officer of the Trustee customarily
performing functions similar to those performed by any of the above designated
officers and also means, with respect to a particular corporate trust matter,
any other officer to whom such matter is referred because of his knowledge of
and familiarity with the particular subject.

          "RESTRICTED DEFINITIVE NOTE" means a Definitive Note bearing the
Private Placement Legend.

          "RESTRICTED GLOBAL NOTE" means a Global Note bearing the Private
Placement Legend.

          "RESTRICTED INVESTMENT" means any Investment other than a Permitted
Investment.

          "RESTRICTED PERIOD" means the 40-day restricted period as defined in
Regulation S.

          "REVOLVING CREDIT FACILITY" means that certain Revolving Credit
Facility, dated as of March 31, 1998, by and among the Company and Morgan
Guaranty Trust Company of New York, Toronto Dominion Bank, Bank of America
National Trust and Savings Association and certain other lenders (collectively,
the "BANKS"), providing for up to $100.0 million of revolving credit borrowings,
including any related notes, guarantees, collateral documents, instruments and
agreements executed in connection therewith, and in each case as amended,
modified, renewed, refunded, replaced or refinanced from time to time.

          "RULE 144" means Rule 144 promulgated under the Securities Act.

          "RULE 144A" means Rule 144A promulgated under the Securities Act.

                                       12

 
          "RULE 903" means Rule 903 promulgated under the Securities Act.

          "RULE 904" means Rule 904 promulgated under the Securities Act.

          "SATELLITE LEASE ARRANGEMENTS" means the sale or lease of the
Company's MSAT-2 satellite pursuant to the ACTEL Agreement or a replacement
agreement (an "MSAT-2 LEASE AGREEMENT") provided that any such replacement
agreement shall be on commercially reasonable terms, provided that (A) the
consideration received by the Company in respect at such sale or lease (x)
consists solely of cash and (y) constitutes fair market value (as determined by
the Board of Directors of the Company set forth in resolution thereof delivered
to the Company, which determination shall be based upon an opinion or appraisal
issued by an appraisal or investment banking firm of national standing); (B) the
Company shall have acquired (through purchase or lease) capacity on MSAT-1 or a
reasonable substitute thereof either (x) pursuant to the TMI Lease Agreement or
(y) any other agreement with a term not less than the maximum term of the MSAT-2
Lease Agreement then in effect and otherwise on commercially reasonable terms if
(in the case of this clause (y)) in the opinion of a nationally recognized
independent expert (a) the capacity acquired pursuant to such replacement
agreement is sufficient to permit the Company to conduct an operations as
conducted and as contemplated to be conducted through the term of the MSAT-2
Lease Agreement then in effect and (y) the total consideration paid by the
Company for such replacement satellite capacity is no greater than the fair
market value thereof.

          "SEC" means the Securities and Exchange Commission.

          "SECURITIES ACT" means the Securities Act of 1933, as amended.

          "SEPARATION DATE" means that earlier of (i) 180 days from the date of
issuance, (ii) such date as the Initial Purchasers may, in their discretion,
deem appropriate, (iii) in the event a Change of Control occurs, the date that
Company mails notice thereof to holders of the Notes, (iv) the commencement of
the Exchange Offer and (v) the effectiveness of the shelf registration statement
relating to the Notes.

          "SHAREHOLDER GUARANTORS" means Hughes Communications Satellite
Services, Inc., Singapore Telecommunications Ltd. and Baron Capital Partners,
L.P.

          "SHELF REGISTRATION STATEMENT" means the Shelf Registration Statement
as defined in the Debt Registration Rights Agreement.

          "SIGNIFICANT SUBSIDIARY" means any Subsidiary that would be a
"significant subsidiary" as defined in Article 1, Rule 1-02 of Regulation S-X,
promulgated pursuant to the Securities Act, as such Regulation is in effect on
the date hereof.

          "STATED MATURITY" means, with respect to any installment of interest
or principal on any series of Indebtedness, the date on which such payment of
interest or principal was scheduled to be paid in the original documentation
governing such Indebtedness, and shall not include any contingent obligations to
repay, redeem or repurchase any such interest or principal prior to the date
originally scheduled for the payment thereof.

          "STRATEGIC EQUITY INVESTMENT" means an investment in Capital Stock
(other than Disqualified Stock) of the Company or Holdings in an aggregate
amount of not less than $50.0 million.

                                       13

 
          "STRATEGIC INVESTOR" means a Person engaged in one or more Permitted
Businesses that has, or is a Wholly-Owned Subsidiary of a Person that has, an
equity market capitalization in excess of $1.0 billion.

          "SUBSIDIARY" means, with respect to any Person, (i) any corporation,
association or other business entity of which more than 50% of the total voting
power of shares of Capital Stock entitled (without regard to the occurrence of
any contingency) to vote in the election of directors, managers or trustees
thereof is at the time owned or controlled, directly or indirectly, by such
Person or one or more of the other Subsidiaries of that Person (or a combination
thereof) and (ii) any partnership (a) the sole general partner or the managing
general partner of which is such Person or a Subsidiary of such Person or (b)
the only general partners of which are such Person or of one or more
Subsidiaries of such Person (or any combination thereof).

          "SUBSIDIARY GUARANTORS" means (i) American Mobile Satellite Sales
Corporation, AMSC Sales Corporation Ltd., AMSC Subsidiary Corporation and ARDIS
Company and (ii) any other Subsidiary of the Company that executes a Subsidiary
Guarantee in accordance with Section 4.19 hereof, and their respective
successors and assigns.

          "TERM LOAN FACILITY" means that certain Credit Agreement, dated as of
March 31, 1998, by and among Holdings and the Banks, providing for up to $100.0
million of borrowings, including any related notes, guarantees, collateral
documents, instruments and agreements executed in connection therewith, and in
each case as amended, modified, renewed, refunded, replaced or refinanced from
time to time.

          "TIA" means the Trust Indenture Act of 1939 (15 U.S.C. (S)(S) 77aaa-
77bbbb) as in effect on the date on which this Indenture is qualified under the
TIA.

          "TRUSTEE" means the party named as such above until a successor
replaces it in accordance with the applicable provisions of this Indenture and
thereafter means the successor serving hereunder.

          "UNIT AGREEMENT" means the Unit Agreement among the Company, Holdings
and the Trustee, dated the date hereof and as amended from time to time.

          "UNRESTRICTED GLOBAL NOTE" means a permanent global Note in the form
of Exhibit A1 attached hereto that bears the Global Note Legend and that has the
"Schedule of Exchanges of Interests in the Global Note" attached thereto, and
that is deposited with or on behalf of and registered in the name of the
Depositary, representing a series of Notes that do not bear the Private
Placement Legend.

          "UNRESTRICTED DEFINITIVE NOTE" means one or more Definitive Notes that
do not bear and are not required to bear the Private Placement Legend.

          "U.S. PERSON" means a U.S. person as defined in Rule 902(o) under the
Securities Act.

          "VENDOR FINANCING INDEBTEDNESS" means Indebtedness of the Company or a
Subsidiary Guarantor the proceeds of which are utilized solely to acquire
ground-based Communications Assets used or usable in a Permitted Business of the
Company or such Subsidiary Guarantor.

          "VOTING STOCK" of any Person as of any date means the Capital Stock of
such Person that is at the time entitled to vote in the election of the Board of
Directors of such Person.

                                       14

 
          "WARRANT REGISTRATION RIGHTS AGREEMENT " the registration rights
agreement, dated as of March 31, 1998, by and among Holdings and the other
parties named on the signature pages thereof, as such agreement may be amended,
modified or supplemented from time to time.

          "WEIGHTED AVERAGE LIFE TO MATURITY" means, when applied to any
Indebtedness at any date, the number of years obtained by dividing (i) the sum
of the products obtained by multiplying (a) the amount of each then remaining
installment, sinking fund, serial maturity or other required payments of
principal, including payment at final maturity, in respect thereof, by (b) the
number of years (calculated to the nearest one-twelfth) that will elapse between
such date and the making of such payment, by (ii) the then outstanding principal
amount of such Indebtedness.

          "WHOLLY OWNED SUBSIDIARY" of any Person means a Subsidiary of such
Person all of the outstanding Capital Stock or other ownership interests of
which (other than directors' qualifying shares) shall at the time be owned by
such Person or by one or more Wholly Owned Subsidiaries of such Person and one
or more Wholly Owned Subsidiaries of such Person.


SECTION 1.02.   OTHER DEFINITIONS.

                                                      Defined in  
       Term                                            Section   
 
       "AFFILIATE TRANSACTION"...........................4.11
       "ASSET SALE"......................................4.10
       "ASSET SALE OFFER"................................3.09
        "BANKRUPTCY LAW".................................4.01
       "CHANGE OF CONTROL OFFER".........................4.15
       "CHANGE OF CONTROL PAYMENT".......................4.15
       "CHANGE OF CONTROL PAYMENT DATE"..................4.15
       "COVENANT DEFEASANCE".............................8.03
       "EVENT OF DEFAULT"................................6.01
       "EXCESS PROCEEDS".................................4.10
       "INCUR"...........................................4.09
       "LEGAL DEFEASANCE"................................8.02
       "NOTE AUTHENTICATION ORDER".......................2.02
       "OFFER AMOUNT"....................................3.09
       "OFFER PERIOD"....................................3.09
       "PAYING AGENT"....................................2.03
       "PERMITTED INDEBTEDNESS"..........................4.09
       "PURCHASE DATE"...................................3.09
       "REGISTRAR".......................................2.03
       "RESTRICTED PAYMENTS".............................4.07

SECTION 1.03.  INCORPORATION BY REFERENCE OF TRUST INDENTURE ACT

          Whenever this Indenture refers to a provision of the TIA, the
provision is incorporated by reference in and made a part of this Indenture.

          The following TIA terms used in this Indenture have the following
meanings:

                                       15

 
          "INDENTURE SECURITIES" means the Notes;

          "INDENTURE SECURITY HOLDER" means a Holder of a Note;

          "INDENTURE TO BE QUALIFIED" means this Indenture;

          "INDENTURE TRUSTEE" or "INSTITUTIONAL TRUSTEE" means the Trustee; and

          "OBLIGOR" on the Notes and the Note Guarantees means the Company and
the Guarantors, respectively, and any successor obligor upon the Notes and the
Note Guarantees, respectively.

          All other terms used in this Indenture that are defined by the TIA,
defined by TIA reference to another statute or defined by SEC rule under the TIA
have the meanings so assigned to them.

SECTION 1.04.  RULES OF CONSTRUCTION.

          Unless the context otherwise requires:

            (1) a term has the meaning assigned to it;

            (2) an accounting term not otherwise defined has the meaning
     assigned to it in accordance with GAAP;

            (3)  "or" is not exclusive;

            (4) words in the singular include the plural, and in the plural
     include the singular;

            (5) provisions apply to successive events and transactions; and

            (6) references to sections of or rules under the Securities Act
     shall be deemed to include substitute, replacement of successor sections or
     rules adopted by the SEC from time to time.

                                   ARTICLE 2.
                                   THE NOTES

SECTION 2.01.  FORM AND DATING.

          (a)  General.

          The Notes and the Trustee's certificate of authentication shall be
substantially in the form of Exhibits A1 and A2 hereto.  The Notes may have
notations, legends or endorsements required by law, stock exchange rule or
usage.  Each Note shall be dated the date of its authentication.  The Notes
shall be in denominations of $1,000 and integral multiples thereof.

          The terms and provisions contained in the Notes shall constitute, and
are hereby expressly made, a part of this Indenture and the Company, the
Guarantors and the Trustee, by their execution and delivery of this Indenture,
expressly agree to such terms and provisions and to be bound 

                                       16

 
thereby. However, to the extent any provision of any Note conflicts with the
express provisions of this Indenture, the provisions of this Indenture shall
govern and be controlling.

          (b)  Global Notes.

          Notes issued in global form shall be substantially in the form of
Exhibits A1 or A2 attached hereto (including the Global Note Legend thereon and
the "Schedule of Exchanges of Interests in the Global Note" attached thereto).
Notes issued in definitive form shall be substantially in the form of Exhibit A1
attached hereto (but without the Global Note Legend thereon and without the
"Schedule of Exchanges of Interests in the Global Note" attached thereto).  Each
Global Note shall represent such of the outstanding Notes as shall be specified
therein and each shall provide that it shall represent the aggregate principal
amount of outstanding Notes from time to time endorsed thereon and that the
aggregate principal amount of outstanding Notes represented thereby may from
time to time be reduced or increased, as appropriate, to reflect exchanges and
redemptions.  Any endorsement of a Global Note to reflect the amount of any
increase or decrease in the aggregate principal amount of outstanding Notes
represented thereby shall be made by the Trustee or the Note Custodian, at the
direction of the Trustee, in accordance with instructions given by the Holder
thereof as required by Section 2.06 hereof.

          (c)  Temporary Global Notes.

          Notes offered and sold in reliance on Regulation S shall be issued
initially in the form of the Regulation S Temporary Global Note, which shall be
deposited on behalf of the purchasers of the Notes represented thereby with the
Trustee, at its New York office, as custodian for the Depositary, and registered
in the name of the Depositary or the nominee of the Depositary for the accounts
of designated agents holding on behalf of Euroclear or Cedel Bank, duly executed
by the Company and authenticated by the Trustee as hereinafter provided.  The
Restricted Period shall be terminated upon the receipt by the Trustee of (i) a
written certificate from the Depositary, together with copies of certificates
from Euroclear and Cedel Bank certifying that they have received certification
of non-U.S. beneficial ownership of 100% of the aggregate principal amount of
the Regulation S Temporary Global Note (except to the extent of any beneficial
owners thereof who acquired an interest therein during the Restricted Period
pursuant to another exemption from registration under the Securities Act and who
will take delivery of a beneficial ownership interest in a 144A Global Note or
an IAI Global Note bearing a Private Placement Legend, all as contemplated by
Section 2.06(g)(i) hereof), and (ii) an Officers' Certificate from the Company.
Following the termination of the Restricted Period, beneficial interests in the
Regulation S Temporary Global Note shall be exchanged for beneficial interests
in Regulation S Permanent Global Notes pursuant to the Applicable Procedures.
Simultaneously with the authentication of Regulation S Permanent Global Notes,
the Trustee shall cancel the Regulation S Temporary Global Note.  The aggregate
principal amount of the Regulation S Temporary Global Note and the Regulation S
Permanent Global Notes may from time to time be increased or decreased by
adjustments made on the records of the Trustee and the Depositary or its
nominee, as the case may be, in connection with transfers of interest as
hereinafter provided.

          (d) Euroclear and Cedel Procedures Applicable.

          The provisions of the "Operating Procedures of the Euroclear System"
and "Terms and Conditions Governing Use of Euroclear" and the "General Terms and
Conditions of Cedel Bank" and "Customer Handbook" of Cedel Bank shall be
applicable to transfers of beneficial interests in the Regulation S Temporary
Global Note and the Regulation S Permanent Global Notes that are held by
Participants through Euroclear or Cedel Bank.

                                       17

 
SECTION 2.02.  EXECUTION AND AUTHENTICATION.

          Two Officers shall sign the Notes for the Company by manual or
facsimile signature.

          If an Officer whose signature is on a Note no longer holds that office
at the time a Note is authenticated, the Note shall nevertheless be valid.

          A Note shall not be valid until authenticated by the manual signature
of the Trustee.  The signature shall be conclusive evidence that the Note has
been authenticated under this Indenture.

          The Trustee shall, upon a written order of the Company signed by two
Officers (an "NOTE AUTHENTICATION ORDER"), authenticate Notes for original issue
up to the aggregate principal amount stated in paragraph 4 of the Notes.  The
aggregate principal amount of Notes outstanding at any time may not exceed such
amount except as provided in Section 2.07 hereof.

          The Trustee may appoint an authenticating agent acceptable to the
Company to authenticate Notes.  An authenticating agent may authenticate Notes
whenever the Trustee may do so.  Each reference in this Indenture to
authentication by the Trustee includes authentication by such agent.  An
authenticating agent has the same rights as an Agent to deal with Holders or an
Affiliate of the Company.

SECTION 2.03.  REGISTRAR AND PAYING AGENT.

          The Company shall maintain an office or agency where Notes may be
presented for registration of transfer or for exchange ("REGISTRAR") and an
office or agency where Notes may be presented for payment ("PAYING AGENT").  The
Registrar shall keep a register of the Notes and of their transfer and exchange.
The Company may appoint one or more co-registrars and one or more additional
paying agents.  The term "Registrar" includes any co-registrar and the term
"Paying Agent" includes any additional paying agent.  The Company may change any
Paying Agent or Registrar without notice to any Holder.  The Company shall
notify the Trustee in writing of the name and address of any Agent not a party
to this Indenture.  If the Company fails to appoint or maintain another entity
as Registrar or Paying Agent, the Trustee shall act as such.  The Company or any
of its Subsidiaries may act as Paying Agent or Registrar.

          The Company initially appoints The Depository Trust Company ("DTC") to
act as Depositary with respect to the Global Notes.

          The Company initially appoints the Trustee to act as the Registrar and
Paying Agent and to act as Note Custodian with respect to the Global Notes.

SECTION 2.04.  PAYING AGENT TO HOLD MONEY IN TRUST.

          The Company shall require each Paying Agent other than the Trustee to
agree in writing that the Paying Agent will hold in trust for the benefit of
Holders or the Trustee all money held by the Paying Agent for the payment of
principal, premium or Liquidated Damages, if any, or interest on the Notes, and
will notify the Trustee of any default by the Company in making any such
payment.  While any such default continues, the Trustee may require a Paying
Agent to pay all money held by it to the Trustee.  The Company at any time may
require a Paying Agent to pay all money held by it to the Trustee.  Upon payment
over to the Trustee, the Paying Agent (if other than the Company or a
Subsidiary) shall have no further liability for the money.  If the Company or a
Subsidiary acts as Paying 

                                       18

 
Agent, it shall segregate and hold in a separate trust fund for the benefit of
the Holders all money held by it as Paying Agent. Upon any bankruptcy or
reorganization proceedings relating to the Company, the Trustee shall serve as
Paying Agent for the Notes.

SECTION 2.05.  HOLDER LISTS.

          The Trustee shall preserve in as current a form as is reasonably
practicable the most recent list available to it of the names and addresses of
all Holders and shall otherwise comply with TIA (S) 312(a).  If the Trustee is
not the Registrar, the Company shall furnish to the Trustee at least seven
Business Days before each interest payment date and at such other times as the
Trustee may request in writing, a list in such form and as of such date as the
Trustee may reasonably require of the names and addresses of the Holders of
Notes and the Company shall otherwise comply with TIA (S) 312(a).

SECTION 2.06.  TRANSFER AND EXCHANGE.

          (a) Transfer and Exchange of Global Notes.

          A Global Note may not be transferred as a whole except by the
Depositary to a nominee of the Depositary, by a nominee of the Depositary to the
Depositary or to another nominee of the Depositary, or by the Depositary or any
such nominee to a successor Depositary or a nominee of such successor
Depositary.  All Global Notes will be exchanged by the Company for Definitive
Notes if (i) the Company delivers to the Trustee notice from the Depositary that
it is unwilling or unable to continue to act as Depositary or that it is no
longer a clearing agency registered under the Exchange Act and, in either case,
a successor Depositary is not appointed by the Company within 120 days after the
date of such notice from the Depositary or (ii) the Company in its sole
discretion determines that the Global Notes (in whole but not in part) should be
exchanged for Definitive Notes and delivers a written notice to such effect to
the Trustee; provided that in no event shall the Regulation S Temporary Global
Note be exchanged by the Company for Definitive Notes prior to (x) the
expiration of the Restricted Period and (y) the receipt by the Registrar of any
certificates required pursuant to Rule 903(c)(3)(ii)(B) under the Securities
Act.  Upon the occurrence of either of the preceding events in (i) or (ii)
above, Definitive Notes shall be issued in such names as the Depositary shall
instruct the Trustee.  Global Notes also may be exchanged or replaced, in whole
or in part, as provided in Sections 2.07 and 2.10 hereof.  Every Note
authenticated and delivered in exchange for, or in lieu of, a Global Note or any
portion thereof, pursuant to this Section 2.06 or Section 2.07 or 2.10 hereof,
shall be authenticated and delivered in the form of, and shall be, a Global
Note.  A Global Note may not be exchanged for another Note other than as
provided in this Section 2.06(a), however, beneficial interests in a Global Note
may be transferred and exchanged as provided in Section 2.06(b),(c) or (f)
hereof.

          (b) Transfer and Exchange of Beneficial Interests in the Global Notes.

          The transfer and exchange of beneficial interests in the Global Notes
shall be effected through the Depositary, in accordance with the provisions of
this Indenture and the Applicable Procedures.  Beneficial interests in the
Restricted Global Notes shall be subject to restrictions on transfer comparable
to those set forth herein to the extent required by the Securities Act.
Transfers of beneficial interests in the Global Notes also shall require
compliance with either subparagraph (i) or (ii) below, as applicable, as well as
one or more of the other following subparagraphs, as applicable:

               (i) Transfer of Beneficial Interests in the Same Global Note.
     Beneficial interests in any Restricted Global Note may be transferred to
     Persons who take delivery thereof 

                                       19

 
     in the form of a beneficial interest in the same Restricted Global Note in
     accordance with the transfer restrictions set forth in the Private
     Placement Legend; provided, however, that prior to the expiration of the
     Restricted Period, transfers of beneficial interests in the Temporary
     Regulation S Global Note may not be made to a U.S. Person or for the
     account or benefit of a U.S. Person (other than an Initial Purchaser).
     Beneficial interests in any Unrestricted Global Note may be transferred to
     Persons who take delivery thereof in the form of a beneficial interest in
     an Unrestricted Global Note. No written orders or instructions shall be
     required to be delivered to the Registrar to effect the transfers described
     in this Section 2.06(b)(i).

               (ii) All Other Transfers and Exchanges of Beneficial Interests in
     Global Notes.  In connection with all transfers and exchanges of beneficial
     interests that are not subject to Section 2.06(b)(i) above, the transferor
     of such beneficial interest must deliver to the Registrar either (A) (1) a
     written order from a Participant or an Indirect Participant given to the
     Depositary in accordance with the Applicable Procedures directing the
     Depositary to credit or cause to be credited a beneficial interest in
     another Global Note in an amount equal to the beneficial interest to be
     transferred or exchanged and (2) instructions given in accordance with the
     Applicable Procedures containing information regarding the Participant
     account to be credited with such increase or (B) (1) a written order from a
     Participant or an Indirect Participant given to the Depositary in
     accordance with the Applicable Procedures directing the Depositary to cause
     to be issued a Definitive Note in an amount equal to the beneficial
     interest to be transferred or exchanged and (2) instructions given by the
     Depositary to the Registrar containing information regarding the Person in
     whose name such Definitive Note shall be registered to effect the transfer
     or exchange referred to in (1) above; provided that in no event shall
     Definitive Notes be issued upon the transfer or exchange of beneficial
     interests in the Regulation S Temporary Global Note prior to (x) the
     expiration of the Restricted Period and (y) the receipt by the Registrar of
     any certificates required pursuant to Rule 903 under the Securities Act.
     Upon consummation of an Exchange Offer by the Company in accordance with
     Section 2.06(f) hereof, the requirements of this Section 2.06(b)(ii) shall
     be deemed to have been satisfied upon receipt by the Registrar of the
     instructions contained in the Letter of Transmittal delivered by the Holder
     of such beneficial interests in the Restricted Global Notes.  Upon
     satisfaction of all of the requirements for transfer or exchange of
     beneficial interests in Global Notes contained in this Indenture and the
     Notes or otherwise applicable under the Securities Act, the Trustee shall
     adjust the principal amount of the relevant Global Note(s) pursuant to
     Section 2.06(h) hereof.

               (iii)  Transfer of Beneficial Interests to Another Restricted
     Global Note.  A beneficial interest in any Restricted Global Note may be
     transferred to a Person who takes delivery thereof in the form of a
     beneficial interest in another Restricted Global Note if the transfer
     complies with the requirements of Section 2.06(b)(ii) above and the
     Registrar receives the following:

                    (A) if the transferee will take delivery in the form of a
          beneficial interest in the 144A Global Note, then the transferor must
          deliver a certificate in the form of Exhibit B hereto, including the
          certifications in item (1) thereof;

                    (B) if the transferee will take delivery in the form of a
          beneficial interest in the Regulation S Temporary Global Note or the
          Regulation S Global Note, then the transferor must deliver a
          certificate in the form of Exhibit B hereto, including the
          certifications in item (2) thereof; and

                                       20

 
                    (C) if the transferee will take delivery in the form of a
          beneficial interest in the IAI Global Note, then the transferor must
          deliver a certificate in the form of Exhibit B hereto, including the
          certifications and certificates and Opinion of Counsel required by
          item (3) thereof, if applicable.

               (iv) Transfer and Exchange of Beneficial Interests in a
     Restricted Global Note for Beneficial Interests in the Unrestricted Global
     Note.  A beneficial interest in any Restricted Global Note may be exchanged
     by any holder thereof for a beneficial interest in an Unrestricted Global
     Note or transferred to a Person who takes delivery thereof in the form of a
     beneficial interest in an Unrestricted Global Note if the exchange or
     transfer complies with the requirements of Section 2.06(b)(ii) above and:

                    (A) such exchange or transfer is effected pursuant to the
          Exchange Offer in accordance with the Debt Registration Rights
          Agreement and the holder of the beneficial interest to be transferred,
          in the case of an exchange, or the transferee, in the case of a
          transfer, certifies in the applicable Letter of Transmittal or via the
          Depositary's book-entry system that it is not (1) a broker-dealer, (2)
          a Person participating in the distribution of the Exchange Notes or
          (3) a Person who is an affiliate (as defined in Rule 144) of the
          Company;

                    (B) such transfer is effected pursuant to the Shelf
          Registration Statement in accordance with the Debt Registration Rights
          Agreement;

                    (C) such transfer is effected by a Participating Broker-
          Dealer pursuant to the Exchange Offer Registration Statement in
          accordance with the Debt Registration Rights Agreement; or

                    (D) the Registrar receives the following:

                         (1) if the holder of such beneficial interest in a
               Restricted Global Note proposes to exchange such beneficial
               interest for a beneficial interest in an Unrestricted Global
               Note, a certificate from such holder in the form of Exhibit C
               hereto, including the certifications in item (1)(a) thereof; or

                         (2) if the holder of such beneficial interest in a
               Restricted Global Note proposes to transfer such beneficial
               interest to a Person who shall take delivery thereof in the form
               of a beneficial interest in an Unrestricted Global Note, a
               certificate from such holder in the form of Exhibit B hereto,
               including the certifications in item (4) thereof;

               and, in each such case set forth in this subparagraph (D), if the
               Registrar so requests or if the Applicable Procedures so require,
               an Opinion of Counsel in form reasonably acceptable to the
               Registrar to the effect that such exchange or transfer is in
               compliance with the Securities Act and that the restrictions on
               transfer contained herein and in the Private Placement Legend are
               no longer required in order to maintain compliance with the
               Securities Act.

          If any such transfer is effected pursuant to subparagraph (B) or (D)
above at a time when an Unrestricted Global Note has not yet been issued, the
Company shall issue and, upon receipt of a Note 

                                       21

 
Authentication Order in accordance with Section 2.02 hereof, the Trustee shall
authenticate one or more Unrestricted Global Notes in an aggregate principal
amount equal to the aggregate principal amount of beneficial interests
transferred pursuant to subparagraph (B) or (D) above.

          Beneficial interests in an Unrestricted Global Note cannot be
exchanged for, or transferred to Persons who take delivery thereof in the form
of, a beneficial interest in a Restricted Global Note.

          (c) Transfer or Exchange of Beneficial Interests for Definitive Notes.

               (i) Beneficial Interests in Restricted Global Notes to Restricted
     Definitive Notes.  If any holder of a beneficial interest in a Restricted
     Global Note proposes to exchange such beneficial interest for a Restricted
     Definitive Note or to transfer such beneficial interest to a Person who
     takes delivery thereof in the form of a Restricted Definitive Note, then,
     upon receipt by the Registrar of the following documentation:

                    (A) if the holder of such beneficial interest in a
          Restricted Global Note proposes to exchange such beneficial interest
          for a Restricted Definitive Note, a certificate from such holder in
          the form of Exhibit C hereto, including the certifications in item
          (2)(a) thereof;

                    (B) if such beneficial interest is being transferred to a
          QIB in accordance with Rule 144A under the Securities Act, a
          certificate to the effect set forth in Exhibit B hereto, including the
          certifications in item (1) thereof;

                    (C) if such beneficial interest is being transferred to a
          Non-U.S. Person in an offshore transaction in accordance with Rule 903
          or Rule 904 under the Securities Act, a certificate to the effect set
          forth in Exhibit B hereto, including the certifications in item (2)
          thereof;

                    (D) if such beneficial interest is being transferred
          pursuant to an exemption from the registration requirements of the
          Securities Act in accordance with Rule 144 under the Securities Act, a
          certificate to the effect set forth in Exhibit B hereto, including the
          certifications in item (3)(a) thereof;

                    (E) if such beneficial interest is being transferred to an
          Institutional Accredited Investor in reliance on an exemption from the
          registration requirements of the Securities Act other than those
          listed in subparagraphs (B) through (D) above, a certificate to the
          effect set forth in Exhibit B hereto, including the certifications,
          certificates and Opinion of Counsel required by item (3) thereof, if
          applicable;

                    (F) if such beneficial interest is being transferred to the
          Company or any of its Subsidiaries, a certificate to the effect set
          forth in Exhibit B hereto, including the certifications in item (3)(b)
          thereof; or

                    (G) if such beneficial interest is being transferred
          pursuant to an effective registration statement under the Securities
          Act, a certificate to the effect set forth in Exhibit B hereto,
          including the certifications in item (3)(c) thereof,

                                       22

 
     the Trustee shall cause the aggregate principal amount of the applicable
     Global Note to be reduced accordingly pursuant to Section 2.06(h) hereof,
     and the Company shall execute and, upon receipt of a Note Authentication
     Order, the Trustee shall authenticate and deliver to the Person designated
     in the instructions a Definitive Note in the appropriate principal amount.
     Any Definitive Note issued in exchange for a beneficial interest in a
     Restricted Global Note pursuant to this Section 2.06(c) shall be registered
     in such name or names and in such authorized denomination or denominations
     as the holder of such beneficial interest shall instruct the Registrar
     through instructions from the Depositary and the Participant or Indirect
     Participant.  The Trustee shall deliver such Definitive Notes to the
     Persons in whose names such Notes are so registered.  Any Definitive Note
     issued in exchange for a beneficial interest in a Restricted Global Note
     pursuant to this Section 2.06(c)(i) shall bear the Private Placement Legend
     and shall be subject to all restrictions on transfer contained therein.

               Notwithstanding Sections 2.06(c)(i)(A) and (C) hereof, a
     beneficial interest in the Regulation S Temporary Global Note may not be
     exchanged for a Definitive Note or transferred to a Person who takes
     delivery thereof in the form of a Definitive Note prior to (x) the
     expiration of the Restricted Period and (y) the receipt by the Registrar of
     any certificates required pursuant to Rule 903(c)(3)(ii)(B) under the
     Securities Act, except in the case of a transfer pursuant to an exemption
     from the registration requirements of the Securities Act other than Rule
     903 or Rule 904.

               (v) Beneficial Interests in Restricted Global Notes to
     Unrestricted Definitive Notes.  A holder of a beneficial interest in a
     Restricted Global Note may exchange such beneficial interest for an
     Unrestricted Definitive Note or may transfer such beneficial interest to a
     Person who takes delivery thereof in the form of an Unrestricted Definitive
     Note only if:

                    (A) such exchange or transfer is effected pursuant to the
          Exchange Offer in accordance with the Debt Registration Rights
          Agreement and the holder of such beneficial interest, in the case of
          an exchange, or the transferee, in the case of a transfer, certifies
          in the applicable Letter of Transmittal that it is not (1) a broker-
          dealer, (2) a Person participating in the distribution of the Exchange
          Notes or (3) a Person who is an affiliate (as defined in Rule 144) of
          the Company;

                    (B) such transfer is effected pursuant to the Shelf
          Registration Statement in accordance with the Debt Registration Rights
          Agreement;

                    (C) such transfer is effected by a Participating Broker-
          Dealer pursuant to the Exchange Offer Registration Statement in
          accordance with the Debt Registration Rights Agreement; or

                    (D) the Registrar receives the following:

                         (1) if the holder of such beneficial interest in a
               Restricted Global Note proposes to exchange such beneficial
               interest for a Definitive Note that does not bear the Private
               Placement Legend, a certificate from such holder in the form of
               Exhibit C hereto, including the certifications in item (1)(b)
               thereof; or

                                       23

 
                         (2) if the holder of such beneficial interest in a
               Restricted Global Note proposes to transfer such beneficial
               interest to a Person who shall take delivery thereof in the form
               of a Definitive Note that does not bear the Private Placement
               Legend, a certificate from such holder in the form of Exhibit B
               hereto, including the certifications in item (4) thereof;

               and, in each such case set forth in this subparagraph (D), if the
               Registrar so requests or if the Applicable Procedures so require,
               an Opinion of Counsel in form reasonably acceptable to the
               Registrar to the effect that such exchange or transfer is in
               compliance with the Securities Act and that the restrictions on
               transfer contained herein and in the Private Placement Legend are
               no longer required in order to maintain compliance with the
               Securities Act.

               (vi) Beneficial Interests in Unrestricted Global Notes to
     Unrestricted Definitive Notes.  If any holder of a beneficial interest in
     an Unrestricted Global Note proposes to exchange such beneficial interest
     for a Definitive Note or to transfer such beneficial interest to a Person
     who takes delivery thereof in the form of a Definitive Note, then, upon
     satisfaction of the conditions set forth in Section 2.06(b)(ii) hereof, the
     Trustee shall cause the aggregate principal amount of the applicable Global
     Note to be reduced accordingly pursuant to Section 2.06(h) hereof, and the
     Company shall execute and, upon receipt of a Note Authentication Order, the
     Trustee shall authenticate and deliver to the Person designated in the
     instructions a Definitive Note in the appropriate principal amount.  Any
     Definitive Note issued in exchange for a beneficial interest pursuant to
     this Section 2.06(c)(iii) shall be registered in such name or names and in
     such authorized denomination or denominations as the holder of such
     beneficial interest shall instruct the Registrar through instructions from
     the Depositary and the Participant or Indirect Participant.  The Trustee
     shall deliver such Definitive Notes to the Persons in whose names such
     Notes are so registered.  Any Definitive Note issued in exchange for a
     beneficial interest pursuant to this Section 2.06(c)(iii) shall not bear
     the Private Placement Legend.

          (d) Transfer and Exchange of Definitive Notes for Beneficial
Interests.

               (i) Restricted Definitive Notes to Beneficial Interests in
     Restricted Global Notes.  If any Holder of a Restricted Definitive Note
     proposes to exchange such Note for a beneficial interest in a Restricted
     Global Note or to transfer such Restricted Definitive Notes to a Person who
     takes delivery thereof in the form of a beneficial interest in a Restricted
     Global Note, then, upon receipt by the Registrar of the following
     documentation:

                    (A) if the Holder of such Restricted Definitive Note
          proposes to exchange such Note for a beneficial interest in a
          Restricted Global Note, a certificate from such Holder in the form of
          Exhibit C hereto, including the certifications in item (2)(b) thereof;

                    (B) if such Restricted Definitive Note is being transferred
          to a QIB in accordance with Rule 144A under the Securities Act, a
          certificate to the effect set forth in Exhibit B hereto, including the
          certifications in item (1) thereof;

                    (C) if such Restricted Definitive Note is being transferred
          to a Non-U.S. Person in an offshore transaction in accordance with
          Rule 903 or Rule 904 under 

                                       24

 
          the Securities Act, a certificate to the effect set forth in Exhibit B
          hereto, including the certifications in item (2) thereof;

                    (D) if such Restricted Definitive Note is being transferred
          pursuant to an exemption from the registration requirements of the
          Securities Act in accordance with Rule 144 under the Securities Act, a
          certificate to the effect set forth in Exhibit B hereto, including the
          certifications in item (3)(a) thereof;

                    (E) if such Restricted Definitive Note is being transferred
          to an Institutional Accredited Investor in reliance on an exemption
          from the registration requirements of the Securities Act other than
          those listed in subparagraphs (B) through (D) above, a certificate to
          the effect set forth in Exhibit B hereto, including the
          certifications, certificates and Opinion of Counsel required by item
          (3) thereof, if applicable;

                    (F) if such Restricted Definitive Note is being transferred
          to the Company or any of its Subsidiaries, a certificate to the effect
          set forth in Exhibit B hereto, including the certifications in item
          (3)(b) thereof; or

                    (G) if such Restricted Definitive Note is being transferred
          pursuant to an effective registration statement under the Securities
          Act, a certificate to the effect set forth in Exhibit B hereto,
          including the certifications in item (3)(c) thereof,

     the Trustee shall cancel the Restricted Definitive Note, increase or cause
     to be increased the aggregate principal amount of, in the case of clause
     (A) above, the appropriate Restricted Global Note, in the case of clause
     (B) above, the 144A Global Note, in the case of clause (c) above, the
     Regulation S Global Note, and in all other cases, the IAI Global Note.

               (vii)  Restricted Definitive Notes to Beneficial Interests in
     Unrestricted Global Notes.  A Holder of a Restricted Definitive Note may
     exchange such Note for a beneficial interest in an Unrestricted Global Note
     or transfer such Restricted Definitive Note to a Person who takes delivery
     thereof in the form of a beneficial interest in an Unrestricted Global Note
     only if:

                    (A) such exchange or transfer is effected pursuant to the
          Exchange Offer in accordance with the Debt Registration Rights
          Agreement and the Holder, in the case of an exchange, or the
          transferee, in the case of a transfer, certifies in the applicable
          Letter of Transmittal that it is not (1) a broker-dealer, (2) a Person
          participating in the distribution of the Exchange Notes or (3) a
          Person who is an affiliate (as defined in Rule 144) of the Company;

                    (B) such transfer is effected pursuant to the Shelf
          Registration Statement in accordance with the Debt Registration Rights
          Agreement;

                    (C) such transfer is effected by a Participating Broker-
          Dealer pursuant to the Exchange Offer Registration Statement in
          accordance with the Debt Registration Rights Agreement; or

                    (D) the Registrar receives the following:

                                       25

 
                         (1) if the Holder of such Definitive Notes proposes to
               exchange such Notes for a beneficial interest in the Unrestricted
               Global Note, a certificate from such Holder in the form of
               Exhibit C hereto, including the certifications in item (1)(c)
               thereof; or

                         (2) if the Holder of such Definitive Notes proposes to
               transfer such Notes to a Person who shall take delivery thereof
               in the form of a beneficial interest in the Unrestricted Global
               Note, a certificate from such Holder in the form of Exhibit B
               hereto, including the certifications in item (4) thereof;

               and, in each such case set forth in this subparagraph (D), if the
               Registrar so requests or if the Applicable Procedures so require,
               an Opinion of Counsel in form reasonably acceptable to the
               Registrar to the effect that such exchange or transfer is in
               compliance with the Securities Act and that the restrictions on
               transfer contained herein and in the Private Placement Legend are
               no longer required in order to maintain compliance with the
               Securities Act.

     Upon satisfaction of the conditions of any of the subparagraphs in this
     Section 2.06(d)(ii), the Trustee shall cancel the Definitive Notes and
     increase or cause to be increased the aggregate principal amount of the
     Unrestricted Global Note.

               (iii)  Unrestricted Definitive Notes to Beneficial Interests in
     Unrestricted Global Notes.  A Holder of an Unrestricted Definitive Note may
     exchange such Note for a beneficial interest in an Unrestricted Global Note
     or transfer such Definitive Notes to a Person who takes delivery thereof in
     the form of a beneficial interest in an Unrestricted Global Note at any
     time.  Upon receipt of a request for such an exchange or transfer, the
     Trustee shall cancel the applicable Unrestricted Definitive Note and
     increase or cause to be increased the aggregate principal amount of one of
     the Unrestricted Global Notes.

               If any such exchange or transfer from a Definitive Note to a
     beneficial interest is effected pursuant to subparagraphs (ii)(B), (ii)(D)
     or (iii) above at a time when an Unrestricted Global Note has not yet been
     issued, the Company shall issue and, upon receipt of a Note Authentication
     Order in accordance with Section 2.02 hereof, the Trustee shall
     authenticate one or more Unrestricted Global Notes in an aggregate
     principal amount equal to the principal amount of Definitive Notes so
     transferred.

          (e) Transfer and Exchange of Definitive Notes for Definitive Notes.

          Upon request by a Holder of Definitive Notes and such Holder's
compliance with the provisions of this Section 2.06(e), the Registrar shall
register the transfer or exchange of Definitive Notes.  Prior to such
registration of transfer or exchange, the requesting Holder shall present or
surrender to the Registrar the Definitive Notes duly endorsed or accompanied by
a written instruction of transfer in form satisfactory to the Registrar duly
executed by such Holder or by his attorney, duly authorized in writing.  In
addition, the requesting Holder shall provide any additional certifications,
documents and information, as applicable, required pursuant to the following
provisions of this Section 2.06(e).

               (i) Restricted Definitive Notes to Restricted Definitive Notes.
     Any Restricted Definitive Note may be transferred to and registered in the
     name of Persons who take 

                                       26

 
     delivery thereof in the form of a Restricted Definitive Note if the
     Registrar receives the following:

                    (A) if the transfer will be made pursuant to Rule 144A under
          the Securities Act, then the transferor must deliver a certificate in
          the form of Exhibit B hereto, including the certifications in item (1)
          thereof;

                    (B) if the transfer will be made pursuant to Rule 903 or
          Rule 904, then the transferor must deliver a certificate in the form
          of Exhibit B hereto, including the certifications in item (2) thereof;
          and

                    (C) if the transfer will be made pursuant to any other
          exemption from the registration requirements of the Securities Act,
          then the transferor must deliver a certificate in the form of Exhibit
          B hereto, including the certifications, certificates and Opinion of
          Counsel required by item (3) thereof, if applicable.

               (ii) Restricted Definitive Notes to Unrestricted Definitive
     Notes.  Any Restricted Definitive Note may be exchanged by the Holder
     thereof for an Unrestricted Definitive Note or transferred to a Person or
     Persons who take delivery thereof in the form of an Unrestricted Definitive
     Note if:

                    (A) such exchange or transfer is effected pursuant to the
          Exchange Offer in accordance with the Debt Registration Rights
          Agreement and the Holder, in the case of an exchange, or the
          transferee, in the case of a transfer, certifies in the applicable
          Letter of Transmittal that it is not (1) a broker-dealer, (2) a Person
          participating in the distribution of the Exchange Notes or (3) a
          Person who is an affiliate (as defined in Rule 144) of the Company;

                    (B) any such transfer is effected pursuant to the Shelf
          Registration Statement in accordance with the Debt Registration Rights
          Agreement;

                    (C) any such transfer is effected by a Participating Broker-
          Dealer pursuant to the Exchange Offer Registration Statement in
          accordance with the Debt Registration Rights Agreement; or

                    (D) the Registrar receives the following:

                         (1) if the Holder of such Restricted Definitive Notes
               proposes to exchange such Notes for an Unrestricted Definitive
               Note, a certificate from such Holder in the form of Exhibit C
               hereto, including the certifications in item (1)(d) thereof; or

                         (2) if the Holder of such Restricted Definitive Notes
               proposes to transfer such Notes to a Person who shall take
               delivery thereof in the form of an Unrestricted Definitive Note,
               a certificate from such Holder in the form of Exhibit B hereto,
               including the certifications in item (4) thereof;

               and, in each such case set forth in this subparagraph (D), if the
               Registrar so requests, an Opinion of Counsel in form reasonably
               acceptable to the Company to the effect that such exchange or
               transfer is in compliance with the Securities 

                                       27

 
               Act and that the restrictions on transfer contained herein and in
               the Private Placement Legend are no longer required in order to
               maintain compliance with the Securities Act.

               (iii)  Unrestricted Definitive Notes to Unrestricted Definitive
     Notes.  A Holder of Unrestricted Definitive Notes may transfer such Notes
     to a Person who takes delivery thereof in the form of an Unrestricted
     Definitive Note.  Upon receipt of a request to register such a transfer,
     the Registrar shall register the Unrestricted Definitive Notes pursuant to
     the instructions from the Holder thereof.

          (f)  Exchange Offer.

          Upon the occurrence of the Exchange Offer in accordance with the Debt
Registration Rights Agreement, the Company shall issue and, upon receipt of a
Note Authentication Order in accordance with Section 2.02, the Trustee shall
authenticate (i) one or more Unrestricted Global Notes in an aggregate principal
amount equal to the principal amount of the beneficial interests in the
Restricted Global Notes tendered for acceptance by Persons that certify in the
applicable Letters of Transmittal that (x) they are not broker-dealers, (y) they
are not participating in a distribution of the Exchange Notes and (z) they are
not affiliates (as defined in Rule 144) of the Company, and accepted for
exchange in the Exchange Offer and (ii) Definitive Notes in an aggregate
principal amount equal to the principal amount of the Restricted Definitive
Notes accepted for exchange in the Exchange Offer.  Concurrently with the
issuance of such Notes, the Trustee shall cause the aggregate principal amount
of the applicable Restricted Global Notes to be reduced accordingly, and the
Company shall execute and, upon receipt of a Note Authentication Order, the
Trustee shall authenticate and deliver to the Persons designated by the Holders
of Definitive Notes so accepted Definitive Notes in the appropriate principal
amount.

          (g)  Legends

          The following legends shall appear on the face of all Global Notes and
Definitive Notes issued under this Indenture unless specifically stated
otherwise in the applicable provisions of this Indenture.

               (i)  Private Placement Legend.

                    (A) Except as permitted by subparagraph (B) below, each
          Global Note and each Definitive Note (and all Notes issued in exchange
          therefor or substitution thereof) shall bear the legend in
          substantially the following form:

               "THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT
     OF 1933, AS AMENDED (THE "SECURITIES ACT") OR ANY STATE SECURITIES LAWS.
     NEITHER THIS SECURITY NOR ANY INTEREST OR PARTICIPATION HEREIN MAY BE
     OFFERED, SOLD, ASSIGNED, TRANSFERRED, PLEDGED, ENCUMBERED OR OTHERWISE
     DISPOSED OF IN THE ABSENCE OF SUCH REGISTRATION OR UNLESS SUCH TRANSACTION
     IS EXEMPT FROM, OR NOT SUBJECT TO, REGISTRATION.

               THE HOLDER OF THIS SECURITY BY ITS ACCEPTANCE HEREOF AGREES TO
     (A) OFFER, SELL, PLEDGE OR OTHERWISE TRANSFER THIS SECURITY ONLY (1) TO
     HOLDINGS, (2) PURSUANT TO A REGISTRATION STATEMENT WHICH 

                                       28

 
     HAS BEEN DECLARED EFFECTIVE UNDER THE SECURITIES ACT, (3) TO A PERSON IT
     REASONABLY BELIEVES IS A "QUALIFIED INSTITUTIONAL BUYER" AS DEFINED IN RULE
     144A IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE 144A, (4) PURSUANT
     TO OFFERS AND SALES TO NON-U.S. PERSONS THAT OCCUR OUTSIDE THE UNITED
     STATES IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE 904 OF REGULATION
     S UNDER THE SECURITIES ACT, (5) TO AN INSTITUTIONAL "ACCREDITED INVESTOR"
     (AS DEFINED IN RULE 501(A)(1), (2), (3) OR (7) OF REGULATION D UNDER THE
     SECURITIES ACT (AN "IAI") THAT, PRIOR TO SUCH TRANSFER, FURNISHES TO THE
     TRUSTEE, AND, IN THE CASE OF ANY TRANSFER TO ANY IAI OF SECURITIES WHICH
     ARE AN AGGREGATE PRINCIPAL AMOUNT OF $100,000 OR LESS, AN OPINION OF
     COUNSEL IF HOLDINGS SO REQUESTS OR (6) PURSUANT TO ANY OTHER AVAILABLE
     EXEMPTION FROM THE REGISTRATION REQUIREMENTS UNDER THE SECURITIES ACT (AND
     BASED ON AN OPINION OF COUNSEL IF HOLDINGS SO REQUESTS), SUBJECT IN EACH OF
     THE FOREGOING CASES TO APPLICABLE SECURITIES LAWS OF ANY STATE OF THE
     UNITED STATES OR ANY OTHER APPLICABLE JURISDICTION AND (B) THAT IT WILL,
     AND EACH SUBSEQUENT HOLDER IS REQUIRED TO, NOTIFY ANY PURCHASER FROM IT OF
     THE SECURITY EVIDENCED HEREBY OF THE RESALE RESTRICTIONS SET FORTH IN (A)
     ABOVE."

                    (B) Notwithstanding the foregoing, any Global Note or
          Definitive Note issued pursuant to subparagraphs (b)(iv), (c)(ii),
          (c)(iii), (d)(ii), (d)(iii), (e)(ii), (e)(iii) or (f) to this Section
          2.06 (and all Notes issued in exchange therefor or substitution
          thereof) shall not bear the Private Placement Legend.

               (ii)  Global Note Legend.  Each Global Note shall bear a legend
     in substantially the following form:

               "THIS GLOBAL NOTE IS HELD BY THE DEPOSITARY (AS DEFINED IN THE
     INDENTURE GOVERNING THIS NOTE) OR ITS NOMINEE IN CUSTODY FOR THE BENEFIT OF
     THE BENEFICIAL OWNERS HEREOF, AND IS NOT TRANSFERABLE TO ANY PERSON UNDER
     ANY CIRCUMSTANCES EXCEPT THAT (I) THE TRUSTEE MAY MAKE SUCH NOTATIONS
     HEREON AS MAY BE REQUIRED PURSUANT TO SECTION 2.07 OF THE INDENTURE, (II)
     THIS GLOBAL NOTE MAY BE EXCHANGED IN WHOLE BUT NOT IN PART PURSUANT TO
     SECTION 2.06(a) OF THE INDENTURE, (III) THIS GLOBAL NOTE MAY BE DELIVERED
     TO THE TRUSTEE FOR CANCELLATION PURSUANT TO SECTION 2.11 OF THE INDENTURE
     AND (IV) THIS GLOBAL NOTE MAY BE TRANSFERRED TO A SUCCESSOR DEPOSITARY WITH
     THE PRIOR WRITTEN CONSENT OF THE COMPANY."

               (iii)  Regulation S Temporary Global Note Legend.  The Regulation
     S Temporary Global Note shall bear a legend in substantially the following
     form:

               "THE RIGHTS ATTACHING TO THIS REGULATION S TEMPORARY GLOBAL NOTE,
     AND THE CONDITIONS AND PROCEDURES GOVERNING ITS EXCHANGE FOR CERTIFICATED
     NOTES, ARE AS SPECIFIED IN THE INDENTURE (AS DEFINED HEREIN).  NEITHER THE
     HOLDER NOR THE BENEFICIAL OWNERS OF THIS REGULATION S TEMPORARY GLOBAL NOTE
     SHALL BE ENTITLED TO RECEIVE PAYMENT OF INTEREST HEREON."

                                       29

 
          (h) Cancellation and/or Adjustment of Global Notes.

          At such time as all beneficial interests in a particular Global Note
have been exchanged for Definitive Notes or a particular Global Note has been
redeemed, repurchased or canceled in whole and not in part, each such Global
Note shall be returned to or retained and canceled by the Trustee in accordance
with Section 2.11 hereof.  At any time prior to such cancellation, if any
beneficial interest in a Global Note is exchanged for or transferred to a Person
who will take delivery thereof in the form of a beneficial interest in another
Global Note or for Definitive Notes, the principal amount of Notes represented
by such Global Note shall be reduced accordingly and an endorsement shall be
made on such Global Note by the Trustee or by the Depositary at the direction of
the Trustee to reflect such reduction; and if the beneficial interest is being
exchanged for or transferred to a Person who will take delivery thereof in the
form of a beneficial interest in another Global Note, such other Global Note
shall be increased accordingly and an endorsement shall be made on such Global
Note by the Trustee or by the Depositary at the direction of the Trustee to
reflect such increase.

          (i) General Provisions Relating to Transfers and Exchanges.

               (i) To permit registrations of transfers and exchanges, the
     Company shall execute and the Trustee shall authenticate Global Notes and
     Definitive Notes upon the Company's order or at the Registrar's request.

               (ii) No service charge shall be made to a holder of a beneficial
     interest in a Global Note or to a Holder of a Definitive Note for any
     registration of transfer or exchange, but the Company may require payment
     of a sum sufficient to cover any transfer tax or similar governmental
     charge payable in connection therewith (other than any such transfer taxes
     or similar governmental charge payable upon exchange or transfer pursuant
     to Sections 2.10, 3.06, 3.09, 4.10, 4.15 and 9.05 hereof).

               (iii)  The Registrar shall not be required to register the
     transfer of or exchange any Note selected for redemption in whole or in
     part, except the unredeemed portion of any Note being redeemed in part.

               (iv) All Global Notes and Definitive Notes issued upon any
     registration of transfer or exchange of Global Notes or Definitive Notes
     shall be the valid obligations of the Company, evidencing the same debt,
     and entitled to the same benefits under this Indenture, as the Global Notes
     or Definitive Notes surrendered upon such registration of transfer or
     exchange.

               (v) The Company shall not be required (A) to issue, to register
     the transfer of or to exchange any Notes during a period beginning at the
     opening of business 15 days before the day of any selection of Notes for
     redemption under Section 3.02 hereof and ending at the close of business on
     the day of selection, (B) to register the transfer of or to exchange any
     Note so selected for redemption in whole or in part, except the unredeemed
     portion of any Note being redeemed in part or (c) to register the transfer
     of or to exchange a Note between a record date and the next succeeding
     Interest Payment Date.

               (vi) Prior to due presentment for the registration of a transfer
     of any Note, the Trustee, any Agent and the Company may deem and treat the
     Person in whose name any Note is registered as the absolute owner of such
     Note for the purpose of receiving payment of 

                                       30

 
     principal of and interest on such Notes and for all other purposes, and
     none of the Trustee, any Agent or the Company shall be affected by notice
     to the contrary.

               (vii)  The Trustee shall authenticate Global Notes and Definitive
     Notes in accordance with the provisions of Section 2.02 hereof.

               (viii)  All certifications, certificates and Opinions of Counsel
     required to be submitted to the Registrar pursuant to this Section 2.06 to
     effect a registration of transfer or exchange may be submitted by
     facsimile.

SECTION 2.07.  REPLACEMENT NOTES

          If any mutilated Note is surrendered to the Trustee or the Company and
the Trustee receives evidence to its satisfaction of the destruction, loss or
theft of any Note, the Company shall issue and the Trustee, upon receipt of a
Note Authentication Order, shall authenticate a replacement Note if the
Trustee's requirements are met.  If required by the Trustee or the Company, an
indemnity bond must be supplied by the Holder that is sufficient in the judgment
of the Trustee and the Company to protect the Company, the Trustee, any Agent
and any authenticating agent from any loss that any of them may suffer if a Note
is replaced.  The Company may charge for its expenses in replacing a Note.

          Every replacement Note is an additional obligation of the Company and
shall be entitled to all of the benefits of this Indenture equally and
proportionately with all other Notes duly issued hereunder.

SECTION 2.08.  OUTSTANDING NOTES.

          The Notes outstanding at any time are all the Notes authenticated by
the Trustee except for those canceled by it, those delivered to it for
cancellation, those reductions in the interest in a Global Note effected by the
Trustee in accordance with the provisions hereof, and those described in this
Section as not outstanding.  Except as set forth in Section 2.09 hereof, a Note
does not cease to be outstanding because the Company or an Affiliate of the
Company holds the Note.

          If a Note is replaced pursuant to Section 2.07 hereof, it ceases to be
outstanding unless the Trustee receives proof satisfactory to it that the
replaced Note is held by a bona fide purchaser.

          If the principal amount of any Note is considered paid under Section
4.01 hereof, it ceases to be outstanding and interest on it ceases to accrue.

          If the Paying Agent (other than the Company, a Subsidiary or an
Affiliate of any thereof) holds, on a redemption date or maturity date, money
sufficient to pay Notes payable on that date, then on and after that date such
Notes shall be deemed to be no longer outstanding and shall cease to accrue
interest.

                                       31

 
SECTION 2.09.  TREASURY NOTES.

          In determining whether the Holders of the required principal amount of
Notes have concurred in any direction, waiver or consent, Notes owned by the
Company, or by any Person directly or indirectly controlling or controlled by or
under direct or indirect common control with the Company, shall be considered as
though not outstanding, except that for the purposes of determining whether the
Trustee shall be protected in relying on any such direction, waiver or consent,
only Notes that the Trustee knows are so owned shall be so disregarded.

SECTION 2.10.  TEMPORARY NOTES

          Until certificates representing Notes are ready for delivery, the
Company may prepare and the Trustee, upon receipt of a Note Authentication
Order, shall authenticate temporary Notes.  Temporary Notes shall be
substantially in the form of certificated Notes but may have variations that the
Company considers appropriate for temporary Notes and as shall be reasonably
acceptable to the Trustee. Without unreasonable delay, the Company shall prepare
and, upon receipt of a Note Authentication Order, the Trustee shall authenticate
definitive Notes in exchange for temporary Notes.

          Holders of temporary Notes shall be entitled to all of the benefits of
this Indenture.

SECTION 2.11.  CANCELLATION.

          The Company at any time may deliver Notes to the Trustee for
cancellation.  The Registrar and Paying Agent shall forward to the Trustee any
Notes surrendered to them for registration of transfer, exchange or payment.
The Trustee and no one else shall cancel all Notes surrendered for registration
of transfer, exchange, payment, replacement or cancellation and shall destroy
canceled Notes (subject to the record retention requirement of the Exchange
Act).  Certification of the destruction of all canceled Notes shall be delivered
to the Company.  The Company may not issue new Notes to replace Notes that it
has paid or that have been delivered to the Trustee for cancellation.

SECTION 2.12.  DEFAULTED INTEREST.

          If the Company defaults in a payment of interest on the Notes, it
shall pay the defaulted interest in any lawful manner plus, to the extent
lawful, interest payable on the defaulted interest, to the Persons who are
Holders on a subsequent special record date, in each case at the rate provided
in the Notes and in Section 4.01 hereof.  The Company shall notify the Trustee
in writing of the amount of defaulted interest proposed to be paid on each Note
and the date of the proposed payment.  The Company shall fix or cause to be
fixed each such special record date and payment date, provided that no such
special record date shall be less than 10 days prior to the related payment date
for such defaulted interest.  At least 15 days before the special record date,
the Company (or, upon the written request of the Company, the Trustee in the
name and at the expense of the Company) shall mail or cause to be mailed to
Holders a notice that states the special record date, the related payment date
and the amount of such interest to be paid.

                                       32

 
                                   ARTICLE 3.
                           REDEMPTION AND PREPAYMENT

SECTION 3.01.  NOTICES TO TRUSTEE.

          If the Company elects to redeem Notes pursuant to the optional
redemption provisions of Section 3.07 hereof and paragraph 5 of the Notes, it
shall furnish to the Trustee, at least 30 days but not more than 60 days before
a redemption date, an Officers' Certificate setting forth (i) the clause of this
Indenture pursuant to which the redemption shall occur, (ii) the redemption
date, (iii) the principal amount of Notes to be redeemed, (iv) the redemption
price and (v) the CUSIP numbers of the Notes to be redeemed.

SECTION 3.02.  SELECTION OF NOTES TO BE REDEEMED

          If less than all of the Notes are to be redeemed or purchased in an
offer to purchase at any time, the Trustee shall select the Notes to be redeemed
or purchased among the Holders of the Notes in compliance with the requirements
of the principal national securities exchange, if any, on which the Notes are
listed or, if the Notes are not so listed, on a pro rata basis, by lot or in
accordance with any other method the Trustee considers fair and appropriate.  In
the event of partial redemption by lot, the particular Notes to be redeemed
shall be selected, unless otherwise provided herein, not less than 30 nor more
than 60 days prior to the redemption date by the Trustee from the outstanding
Notes not previously called for redemption.

          The Trustee shall promptly notify the Company in writing of the Notes
selected for redemption and, in the case of any Note selected for partial
redemption, the principal amount thereof to be redeemed.  Notes and portions of
Notes selected shall be in amounts of $1,000 or whole multiples of $1,000;
except that if all of the Notes of a Holder are to be redeemed, the entire
outstanding amount of Notes held by such Holder, even if not a multiple of
$1,000, shall be redeemed.  Except as provided in the preceding sentence,
provisions of this Indenture that apply to Notes called for redemption also
apply to portions of Notes called for redemption.

SECTION 3.03.  NOTICE OF REDEMPTION

          Subject to the provisions of Section 3.09 hereof, at least 30 days but
not more than 60 days before a redemption date, the Company shall mail or cause
to be mailed, by first class mail, a notice of redemption to each Holder whose
Notes are to be redeemed at its registered address.

          The notice shall identify the Notes to be redeemed, including the
CUSIP numbers, and shall state:

               (a) the redemption date;

               (b) the redemption price;

               (c) if any Note is being redeemed in part, the portion of the
     principal amount of such Note to be redeemed and that, after the redemption
     date upon surrender of such Note, a new Note or Notes in principal amount
     equal to the unredeemed portion shall be issued upon cancellation of the
     original Note;

               (d) the name and address of the Paying Agent;

                                       33

 
               (e) that Notes called for redemption must be surrendered to the
     Paying Agent to collect the redemption price;

               (f) that, unless the Company defaults in making such redemption
     payment, interest on Notes called for redemption ceases to accrue on and
     after the redemption date;

               (g) the paragraph of the Notes and/or Section of this Indenture
     pursuant to which the Notes called for redemption are being redeemed; and

               (h) that no representation is made as to the correctness or
     accuracy of the CUSIP number, if any, listed in such notice or printed on
     the Notes.

          At the Company's request, the Trustee shall give the notice of
redemption in the Company's name and at its expense; provided, however, that the
Company shall have delivered to the Trustee, at least 45 days prior to the
redemption date, an Officers' Certificate requesting that the Trustee give such
notice and setting forth the information to be stated in such notice as provided
in the preceding paragraph.

SECTION 3.04.  EFFECT OF NOTICE OF REDEMPTION

          Once notice of redemption is mailed in accordance with Section 3.03
hereof, Notes called for redemption become irrevocably due and payable on the
redemption date at the redemption price.  A notice of redemption may not be
conditional.

SECTION 3.05.  DEPOSIT OF REDEMPTION PRICE

          One Business Day prior to the redemption date, the Company shall
deposit with the Trustee or with the Paying Agent money sufficient to pay the
redemption price of and accrued interest on all Notes to be redeemed on that
date.  The Trustee or the Paying Agent shall promptly return to the Company any
money deposited with the Trustee or the Paying Agent by the Company in excess of
the amounts necessary to pay the redemption price of, and accrued interest on,
all Notes to be redeemed.

          If the Company complies with the provisions of the preceding
paragraph, on and after the redemption date, interest shall cease to accrue on
the Notes or the portions of Notes called for redemption.  If a Note is redeemed
on or after an interest record date but on or prior to the related interest
payment date, then any accrued and unpaid interest shall be paid to the Person
in whose name such Note was registered at the close of business on such record
date.  If any Note called for redemption shall not be so paid upon surrender for
redemption because of the failure of the Company to comply with the preceding
paragraph, interest shall be paid on the unpaid principal, from the redemption
date until such principal is paid, and to the extent lawful on any interest not
paid on such unpaid principal, in each case at the rate provided in the Notes
and in Section 4.01 hereof.

SECTION 3.06.  NOTES REDEEMED IN PART.

          Upon surrender of a Note that is redeemed in part, the Company shall
issue and, upon the Company's written request, the Trustee shall authenticate
for the Holder at the expense of the Company a new Note equal in principal
amount to the unredeemed portion of the Note surrendered.

                                       34

 
SECTION 3.07.  OPTIONAL REDEMPTION.

          (a) Except as set forth in clause (b) of this Section 3.07, the Notes
shall not be subject to redemption at the Company's option prior to April 1,
2003.  The Notes shall be subject to redemption at any time at the option of the
Company, in whole or in part, upon not less than 30 nor more than 60 days'
notice, at the redemption prices (expressed as percentages of principal amount)
set forth below plus accrued and unpaid interest and Liquidated Damages thereon,
if any, to the applicable redemption date, if redeemed during the twelve-month
period beginning on April 1 of the years indicated below:

          YEAR                                     PERCENTAGE
          ----                                     ----------

          2003..................................... 106.125%
          2004..................................... 104.083%
          2005..................................... 102.042%
          2006 and thereafter...................... 100.000%

          (b) Notwithstanding the foregoing, during the first 36 months after
the date hereof, the Company may redeem up to 35% of the aggregate principal
amount of Notes originally issued hereunder at a redemption price of 112.25% of
the principal amount thereof, plus accrued and unpaid interest and Liquidated
Damages thereon, if any, to the redemption date, with the net cash proceeds to
the Company of an Equity Offering; provided that (i) at least $217.8 million in
aggregate principal amount of the Notes remain outstanding immediately after the
occurrence of each such redemption (excluding Notes held by the Company and its
Subsidiaries) and (ii) each such redemption shall occur within 45 days after the
date of the closing of any such Equity Offering.

          (c) Any redemption pursuant to this Section 3.07 and paragraph 5 of
the Notes shall be made pursuant to the provisions of Section 3.01 through 3.06
hereof.

SECTION 3.08.  MANDATORY REDEMPTION.

          The Company shall not be required to make mandatory redemption
payments with respect to the Notes, except as set forth in Section 4.15.

SECTION 3.09.  OFFER TO PURCHASE BY APPLICATION OF EXCESS PROCEEDS.

          In the event that, pursuant to Section 4.10 hereof, the Company shall
be required to commence an offer to all Holders to purchase Notes (an "ASSET
SALE OFFER"), it shall follow the procedures specified below.

          The Asset Sale Offer shall remain open for a period of 20 Business
Days following its commencement and no longer, except to the extent that a
longer period is required by applicable law (the "OFFER PERIOD").  No later than
five Business Days after the termination of the Offer Period (the "PURCHASE
DATE"), the Company shall purchase the principal amount of Notes required to be
purchased pursuant to Section 4.10 hereof (the "OFFER AMOUNT") or, if less than
the Offer Amount has been tendered, all Notes tendered in response to the Asset
Sale Offer.  Payment for any Notes so purchased shall be made in the same manner
as interest payments are made.

          If the Purchase Date is on or after an interest record date and on or
before the related interest payment date, any accrued and unpaid interest shall
be paid to the Person in whose name a Note 

                                       35

 
is registered at the close of business on such record date, and no additional
interest shall be payable to Holders who tender Notes pursuant to the Asset Sale
Offer.

          Upon the commencement of an Asset Sale Offer, the Company shall send,
by first class mail, a notice to the Trustee and each of the Holders, with a
copy to the Trustee.  The notice shall contain all instructions and materials
necessary to enable such Holders to tender Notes pursuant to the Asset Sale
Offer.  The Asset Sale Offer shall be made to all Holders.  The notice, which
shall govern the terms of the Asset Sale Offer, shall state:

               (a) that the Asset Sale Offer is being made pursuant to this
     Section 3.09 and Section 4.10 hereof and the length of time the Asset Sale
     Offer shall remain open;

               (b) the Offer Amount, the purchase price and the Purchase Date;

               (c) that any Note not tendered or accepted for payment shall
     continue to accrete or accrue interest;

               (d) that, unless the Company defaults in making such payment, any
     Note accepted for payment pursuant to the Asset Sale Offer shall cease to
     accrete or accrue interest after the Purchase Date;

               (e) that Holders electing to have a Note purchased pursuant to an
     Asset Sale Offer may only elect to have all of such Note purchased and may
     not elect to have only a portion of such Note purchased;

               (f) that Holders electing to have a Note purchased pursuant to
     any Asset Sale Offer shall be required to surrender the Note, with the form
     entitled "Option of Holder to Elect Purchase" on the reverse of the Note
     completed, or transfer by book-entry transfer, to the Company, a
     depositary, if appointed by the Company, or a Paying Agent at the address
     specified in the notice at least three days before the Purchase Date;

               (g) that Holders shall be entitled to withdraw their election if
     the Company, the depositary or the Paying Agent, as the case may be,
     receives, not later than the expiration of the Offer Period, a telegram,
     telex, facsimile transmission or letter setting forth the name of the
     Holder, the principal amount of the Note the Holder delivered for purchase
     and a statement that such Holder is withdrawing his election to have such
     Note purchased;

               (h) that, if the aggregate principal amount of Notes surrendered
     by Holders exceeds the Offer Amount, the Company shall select the Notes to
     be purchased on a pro rata basis (with such adjustments as may be deemed
     appropriate by the Company so that only Notes in denominations of $1,000,
     or integral multiples thereof, shall be purchased); and

               (i) that Holders whose Notes were purchased only in part shall be
     issued new Notes equal in principal amount to the unpurchased portion of
     the Notes surrendered (or transferred by book-entry transfer).

          On or before the Purchase Date, the Company shall, to the extent
lawful, accept for payment, on a pro rata basis to the extent necessary, the
Offer Amount of Notes or portions thereof tendered pursuant to the Asset Sale
Offer, or if less than the Offer Amount has been tendered, all Notes tendered,
and shall deliver to the Trustee an Officers' Certificate stating that such
Notes or portions 

                                       36

 
thereof were accepted for payment by the Company in accordance with the terms of
this Section 3.09. The Company, the Depositary or the Paying Agent, as the case
may be, shall promptly (but in any case not later than five days after the
Purchase Date) mail or deliver to each tendering Holder an amount equal to the
purchase price of the Notes tendered by such Holder and accepted by the Company
for purchase, and the Company shall promptly issue a new Note, and the Trustee,
upon written request from the Company shall authenticate and mail or deliver
such new Note to such Holder, in a principal amount equal to any unpurchased
portion of the Note surrendered. Any Note not so accepted shall be promptly
mailed or delivered by the Company to the Holder thereof. The Company shall
publicly announce the results of the Asset Sale Offer on the Purchase Date.

          Other than as specifically provided in this Section 3.09, any purchase
pursuant to this Section 3.09 shall be made pursuant to the provisions of
Sections 3.01 through 3.06 hereof.

                                   ARTICLE 4.
                                   COVENANTS

SECTION 4.01.  PAYMENT OF NOTES.

          The Company or a Guarantor shall pay or cause to be paid the principal
of, premium, if any, and interest and Liquidated Damages, if any, on the Notes
on the dates and in the manner provided in the Notes.  Principal, premium, if
any, and interest and Liquidated Damages, if any, shall be considered paid on
the date due if the Paying Agent, if other than the Company or a Subsidiary
thereof, holds as of 10:00 a.m. Eastern Time on the due date money deposited by
the Company in immediately available funds and designated for and sufficient to
pay all principal, premium, if any, and interest and Liquidated Damages, if any,
then due.  The Company shall pay all Liquidated Damages, if any, in the same
manner on the dates and in the amounts set forth in the Debt Registration Rights
Agreement.

          The Company or a Guarantor shall pay interest (including post-petition
interest in any proceeding under any Bankruptcy Law) on overdue principal at the
rate equal to 1% per annum in excess of the then applicable interest rate on the
Notes to the extent lawful; it shall pay interest (including post-petition
interest in any proceeding under any Bankruptcy Law) on overdue installments of
interest and Liquidated Damages (without regard to any applicable grace period)
at the same rate to the extent lawful.

SECTION 4.02.  MAINTENANCE OF OFFICE OR AGENCY.

          The Company shall maintain in the Borough of Manhattan, the City of
New York, an office or agency (which may be an office of the Trustee or an
affiliate of the Trustee, Registrar or co-registrar) where Notes may be
surrendered for registration of transfer or for exchange and where notices and
demands to or upon the Company in respect of the Notes and this Indenture may be
served.  The Company shall give prompt written notice to the Trustee of the
location, and any change in the location, of such office or agency.  If at any
time the Company shall fail to maintain any such required office or agency or
shall fail to furnish the Trustee with the address thereof, such presentations,
surrenders, notices and demands may be made or served at the Corporate Trust
Office of the Trustee.

          The Company may also from time to time designate one or more other
offices or agencies where the Notes may be presented or surrendered for any or
all such purposes and may from time to time rescind such designations; provided,
however, that no such designation or rescission shall in any manner relieve the
Company of its obligation to maintain an office or agency in the Borough of
Manhattan, the City of New York for such purposes.  The Company shall give

                                       37

 
prompt written notice to the Trustee of any such designation or rescission and
of any change in the location of any such other office or agency.

          The Company hereby designates the Corporate Trust Office of the
Trustee as one such office or agency of the Company in accordance with Section
2.03.

SECTION 4.03.  REPORTS.

          (a) Whether or not required by the rules and regulations of the
Commission, so long as any Notes are outstanding, the Company shall furnish to
the Trustee and the Holders of Notes (i) all quarterly and annual financial
information that would be required to be contained in a filing with the
Commission on Forms 10-Q and 10-K if the Company were required to file such
forms, including a "Management's Discussion and Analysis of Financial Condition
and Results of Operations" and, with respect to the annual information only, a
report thereon by the Company's certified independent accountants and (ii) all
current reports that would be required to be filed with the Commission on Form
8-K if the Company were required to file such reports, in each case within the
time periods specified in the Commission's rules and regulations.  For so long
as Holdings is a guarantor of the Notes, the Company may satisfy its obligations
in this covenant with respect to financial information relating to the Company
by furnishing financial information relating to Holdings; provided that the same
is accompanied by consolidated financial information of the Company on a stand-
alone basis that explains in reasonable detail the differences between the
information relating to Holdings, on the one hand, and the information relating
to the Company on a stand-alone basis, on the other hand.  In addition,
following the consummation of the exchange offer contemplated by the Debt
Registration Rights Agreement, whether or not required by the rules and
regulations of the Commission, the Company shall file a copy of all such
information and reports with the Commission for public availability within the
time periods specified in the Commission's rules and regulations (unless the
Commission will not accept such a filing) and make such information reasonably
available to securities analysts and prospective investors upon request.  The
Company shall at all times comply with TIA (S) 314(a).

          (b) For so long as any Notes remain outstanding, Holdings, the Company
and the Guarantors shall furnish to the Holders and to securities analysts and
prospective investors, upon their request, the information required to be
delivered pursuant to Rule 144A(d)(4) under the Securities Act.

SECTION 4.04.  COMPLIANCE CERTIFICATE.

          (a) The Company and each Guarantor (to the extent that such Guarantor
is so required under the TIA) shall deliver to the Trustee, within 90 days after
the end of each fiscal year, an Officers' Certificate stating that a review of
the activities of the Company and its Subsidiaries during the preceding fiscal
year has been made under the supervision of the signing Officers with a view to
determining whether the Company has kept, observed, performed and fulfilled its
obligations under this Indenture, and further stating, as to each such Officer
signing such certificate, that to the best of his or her knowledge the Company
has kept, observed, performed and fulfilled each and every covenant contained in
this Indenture and is not in default in the performance or observance of any of
the terms, provisions and conditions of this Indenture (or, if a Default or
Event of Default shall have occurred, describing all such Defaults or Events of
Default of which he or she may have knowledge and what action the Company is
taking or proposes to take with respect thereto) and that to the best of his or
her knowledge no event has occurred and remains in existence by reason of which
payments on account of the principal of or interest, if any, on the Notes is
prohibited or if such event has occurred, a description of the event and what
action the Company is taking or proposes to take with respect thereto.

                                       38

 
          (b) So long as not contrary to the then current recommendations of the
American Institute of Certified Public Accountants, the year-end financial
statements delivered pursuant to Section 4.03(a) above shall be accompanied by a
written statement of the Company's independent public accountants (who shall be
a firm of established national reputation) that in making the examination
necessary for certification of such financial statements, nothing has come to
their attention that would lead them to believe that the Company has violated
any provisions of Article 4 or Article 5 hereof or, if any such violation has
occurred, specifying the nature and period of existence thereof, it being
understood that such accountants shall not be liable directly or indirectly to
any Person for any failure to obtain knowledge of any such violation.

          (c) The Company shall, so long as any of the Notes are outstanding,
deliver to the Trustee, forthwith upon any Officer becoming aware of any Default
or Event of Default, an Officers' Certificate specifying such Default or Event
of Default and what action the Company is taking or proposes to take with
respect thereto.

SECTION 4.05.  TAXES.

          The Company shall pay, and shall cause each of its Subsidiaries to
pay, prior to delinquency, all material taxes, assessments, and governmental
levies except such as are contested in good faith and by appropriate proceedings
or where the failure to effect such payment is not adverse in any material
respect to the Holders of the Notes.

SECTION 4.06.  STAY, EXTENSION AND USURY LAWS.

          The Company and each of the Guarantors covenants (to the extent that
it may lawfully do so) that it shall not at any time insist upon, plead, or in
any manner whatsoever claim or take the benefit or advantage of, any stay,
extension or usury law wherever enacted, now or at any time hereafter in force,
that may affect the covenants or the performance of this Indenture; and the
Company and each of the Guarantors (to the extent that it may lawfully do so)
hereby expressly waives all benefit or advantage of any such law, and covenants
that it shall not, by resort to any such law, hinder, delay or impede the
execution of any power herein granted to the Trustee, but shall suffer and
permit the execution of every such power as though no such law has been enacted.

SECTION 4.07. RESTRICTED PAYMENTS.

          The Company shall not, and shall not permit any of its Subsidiaries
to, directly or indirectly: (i) declare or pay any dividend or make any other
payment or distribution on account of the Company's or any of its Subsidiaries'
Equity Interests (including, without limitation, any payment in connection with
any merger or consolidation involving the Company or any of its Subsidiaries) or
to the direct or indirect holders of the Company's or any of its Subsidiaries'
Equity Interests in their capacity as such (other than dividends or
distributions payable in Equity Interests (other than Disqualified Stock) of the
Company or to the Company or a Subsidiary of the Company); (ii) purchase, redeem
or otherwise acquire or retire for value (including, without limitation, in
connection with any merger or consolidation involving the Company) any Equity
Interests of the Company or any direct or indirect parent of the Company or
other Affiliate of the Company (other than any such Equity Interests owned by
the Company or any Wholly Owned Subsidiary of the Company); (iii) make any
payment on or with respect to, or purchase, redeem, defease or otherwise acquire
or retire for value any Indebtedness that is subordinated to the Notes, except a
payment of interest or principal at Stated Maturity; or (iv) make any Restricted
Investment (all such payments and other actions set forth in clauses (i) through
(iv) above 

                                       39

 
being collectively referred to as "RESTRICTED PAYMENTS"), unless, at the time of
and after giving effect to such Restricted Payment:

               (a) no Default or Event of Default shall have occurred and be
     continuing or would occur as a consequence thereof; and

               (b) the Company would, at the time of such Restricted Payment and
     after giving pro forma effect thereto as if such Restricted Payment had
     been made at the beginning of the Reference Period, have been permitted to
     incur at least $1.00 of additional Indebtedness pursuant to the Debt to
     Cash Flow Ratio test set forth in the first paragraph of Section 4.09
     hereof; and

               (c) such Restricted Payment, together with the aggregate amount
     of all other Restricted Payments made by the Company and its Subsidiaries
     after the date hereof (excluding Restricted Payments permitted by clauses
     (ii), (iii) and (iv) of the next succeeding paragraph), is less than the
     sum, without duplication, of (i) 50% of the Consolidated Net Income of the
     Company for the period (taken as one accounting period) from the beginning
     of the first fiscal quarter commencing after the date hereof to the end of
     the Company's most recently ended fiscal quarter for which internal
     financial statements are available at the time of such Restricted Payment
     (or, if such Consolidated Net Income for such period is a deficit, less
     100% of such deficit), plus (ii) 100% of the aggregate net cash proceeds
     received by the Company since the date hereof as a contribution to its
     common equity capital or from the issue or sale of Equity Interests of the
     Company (other than Disqualified Stock) or from the issue or sale of
     Disqualified Stock or debt securities of the Company that have been
     converted into such Equity Interests (other than Equity Interests (or
     Disqualified Stock or convertible debt securities) sold to a Subsidiary of
     the Company), plus (iii) to the extent that any Restricted Investment that
     was made after the date hereof is sold for cash or otherwise liquidated or
     repaid for cash, the lesser of (A) the cash return of capital with respect
     to such Restricted Investment (less the cost of disposition, if any) and
     (B) the initial amount of such Restricted Investment.

          The foregoing provisions will not prohibit (i) the payment of any
dividend within 60 days after the date of declaration thereof, if at said date
of declaration such payment would have complied with the provisions of this
Indenture; (ii) the redemption, repurchase, retirement, defeasance or other
acquisition of any subordinated Indebtedness or Equity Interests of the Company
in exchange for, or out of the net cash proceeds of the substantially concurrent
sale (other than to a Subsidiary of the Company) of, other Equity Interests of
the Company (other than any Disqualified Stock); provided that the amount of any
such net cash proceeds that are utilized for any such redemption, repurchase,
retirement, defeasance or other acquisition shall be excluded from clause (c)
(ii) of the preceding paragraph; (iii) the defeasance, redemption, repurchase or
other acquisition of subordinated Indebtedness with the net cash proceeds from
an incurrence of Permitted Refinancing Indebtedness; (iv) the payment of any
dividend by a Subsidiary of the Company to the holders of its common Equity
Interests on a pro rata basis; (v) from and after April 1, 2001 the payment of
dividends to Holdings the proceeds of which are used to satisfy interest expense
obligations of Holdings under the Term Loan Facility, as in effect on the date
hereof; provided that (x) no Default or Event of Default shall have occurred and
be continuing immediately before or immediately after such transaction and (y)
the aggregate amount of such payments does not exceed the sum of (1) $6.5
million and (2) the difference between (A) the Company's Cumulative Consolidated
Cash Flow from and after April 1, 2001 and (B) 1.75 times Consolidated Interest
Expense accrued on a cumulative basis from and after April 1, 2001; (vi) the
payment of dividends to Holdings the proceeds of which are used to satisfy
ordinary course administrative expenses 

                                       40

 
of Holdings, but in no event to exceed, (together with all payments made
pursuant to management agreements), $2.0 million in any given fiscal year; (vii)
the payment of any dividend required pursuant to the Tax Sharing Agreement
between the Company and Holdings, as such is in effect on the date hereof and
(viii) other Investments in any Person (other than Holdings or an Affiliate of
Holdings that is not also a Subsidiary of the Company) having an aggregate fair
market value (measured on the date each such Investment was made and without
giving effect to subsequent changes in value), when taken together with all
other Investments made pursuant to this clause (viii) that are at the time
outstanding, not to exceed the greater of (x) $10.0 million and (y) 2.5% of the
Consolidated Tangible Net Worth of the Company at such time.

          The amount of all Restricted Payments (other than cash) shall be the
fair market value on the date of the Restricted Payment of the asset(s) or
securities proposed to be transferred or issued by the Company or such
Subsidiary, as the case may be, pursuant to the Restricted Payment. The fair
market value of any non-cash Restricted Payment shall be determined by the Board
of Directors whose resolution with respect thereto shall be delivered to the
Trustee, such determination to be based upon an opinion or appraisal issued by
an accounting, appraisal or investment banking firm of national standing if such
fair market value exceeds $5.0 million. Not later than the date of making any
Restricted Payment, the Company shall deliver to the Trustee an Officers'
Certificate stating that such Restricted Payment is permitted and setting forth
the basis upon which the calculations required by the covenant "Restricted
Payments" were computed, together with a copy of any fairness opinion or
appraisal required by this Indenture.

SECTION 4.08.  DIVIDEND AND OTHER PAYMENT RESTRICTIONS AFFECTING SUBSIDIARIES.

          The Company shall not, and shall not permit any of its Subsidiaries
to, directly or indirectly, create or otherwise cause or suffer to exist or
become effective any encumbrance or restriction on the ability of any Subsidiary
to (i)(a) pay dividends or make any other distributions to the Company or any of
its Subsidiaries (1) on its Capital Stock or (2) with respect to any other
interest or participation in, or measured by, its profits, or (b) pay any
indebtedness owed to the Company or any of its Subsidiaries, (ii) make loans or
advances to the Company or any of its Subsidiaries or (iii) transfer any of its
properties or assets to the Company or any of its Subsidiaries. However, the
foregoing restrictions will not apply to encumbrances or restrictions existing
under or by reason of (a) Existing Indebtedness as in effect on the date hereof,
(b) the Revolving Credit Facility as in effect on the date hereof, and any
amendments, modifications, restatements, renewals, increases, supplements,
refundings, replacements or refinancings thereof, provided that such amendments,
modifications, restatements, renewals, increases, supplements, refundings,
replacement or refinancings are no more restrictive, taken as a whole, with
respect to such dividend and other payment restrictions than those contained in
the Revolving Credit Facility as in effect on the date hereof, (c) this
Indenture and the Notes, (d) applicable law, (e) any instrument governing
Indebtedness or Capital Stock of a Person acquired by the Company or any of its
Subsidiaries as in effect at the time of such acquisition (except to the extent
such Indebtedness was incurred in connection with or in contemplation of such
acquisition), which encumbrance or restriction is not applicable to any Person,
or the properties or assets of any Person, other than the Person, or the
property or assets of the Person, so acquired, provided that, in the case of
Indebtedness, such Indebtedness was permitted by the terms of this Indenture to
be incurred, (f) customary non-assignment provisions in leases entered into in
the ordinary course of business and consistent with past practices, (g) purchase
money obligations for property acquired in the ordinary course of business that
impose restrictions of the nature described in clause (iii) above on the
property so acquired, (h) any agreement for the sale of a Subsidiary that
restricts distributions by that Subsidiary pending its sale, (i) Permitted
Refinancing Indebtedness, provided that the restrictions contained in the
agreements governing such Permitted Refinancing Indebtedness are no 

                                       41

 
more restrictive, taken as a whole, than those contained in the agreements
governing the Indebtedness being refinanced, (j) secured Indebtedness otherwise
permitted to be incurred pursuant to the provisions of Section 4.12 hereof that
limit the right of the debtor to dispose of the assets securing such
Indebtedness, (k) provisions with respect to the disposition or distribution of
assets or property in joint venture agreements and other similar agreements
entered into in the ordinary course of business and (l) restrictions on cash or
other deposits or net worth imposed by customers under contracts entered into in
the ordinary course of business.

SECTION 4.09.  INCURRENCE OF INDEBTEDNESS AND ISSUANCE OF PREFERRED STOCK.

          The Company shall not, and shall not permit any of its Subsidiaries
to, directly or indirectly, create, incur, issue, assume, guarantee or otherwise
become directly or indirectly liable, contingently or otherwise, with respect to
(collectively, "INCUR") any Indebtedness (including Acquired Indebtedness) and
the Company will not issue any Disqualified Stock and will not permit any of its
Subsidiaries to issue any shares of preferred stock; provided, however, that the
Company may incur Indebtedness (including Acquired Indebtedness) or issue shares
of Disqualified Stock if the Company's Debt to Cash Flow Ratio at the time of
incurrence of such Indebtedness or the issuance of such Disqualified Stock,
after giving pro forma effect to such incurrence or issuance as of such date and
to the use of proceeds therefrom as if the same had occurred at the beginning of
the most recently ended Reference Period of the Company for which internal
financial statements are available, would have been no greater than 4.0 to 1.0.

          The Company shall not incur any Indebtedness that is contractually
subordinated in right of payment to any other Indebtedness of the Company unless
such Indebtedness is also contractually subordinated in right of payment to the
Notes on substantially identical terms; provided, however, that no Indebtedness
of the Company shall be deemed to be contractually subordinated in right of
payment to any other Indebtedness of the Company solely by virtue of being
unsecured.

          The provisions of the first paragraph of this section will not apply
to the incurrence of any of the following items of Indebtedness (collectively,
"PERMITTED INDEBTEDNESS"):

        (i) the incurrence by the Company of Indebtedness (including letters of
     credit, with letters of credit being deemed to have a principal amount
     equal to the maximum potential liability of the Company and its
     Subsidiaries thereunder) under Credit Facilities; provided that the
     aggregate principal amount of all Indebtedness outstanding under all Credit
     Facilities after giving effect to such incurrence does not exceed an amount
     equal to the greater of (a) $100.0 million less the aggregate amount of all
     Net Proceeds of Asset Sales applied to repay revolving credit Indebtedness
     under a Credit Facility pursuant to Section 4.10 hereof other than Net
     Proceeds received pursuant to the Satellite Lease Arrangements and (b) the
     Borrowing Base at the time of such incurrence;

        (ii) the incurrence by the Company and its Subsidiaries of the Existing
     Indebtedness;

        (iii) the incurrence by the Company of Indebtedness represented by the
     Notes and the Exchange Notes and the guarantee by the Subsidiary Guarantors
     thereof;

        (iv) the incurrence by the Company or any of its Subsidiaries of
     Indebtedness represented by Capital Lease Obligations, mortgage financings
     or purchase money 

                                       42

 
     obligations, in each case incurred for the purpose of financing all or any
     part of the purchase price or cost of construction or improvement of
     property, plant or equipment used in the business of the Company or such
     Subsidiary, in an aggregate principal amount not to exceed (subject to
     clause (x) below) $17.5 million at any time outstanding;

        (v) the incurrence by the Company or any of its Subsidiaries of
     Indebtedness in connection with the acquisition of assets or a new
     Subsidiary; provided that such Indebtedness was incurred by the prior owner
     of such assets or such Subsidiary prior to such acquisition by the Company
     or one of its Subsidiaries and was not incurred in connection with, or in
     contemplation of, such acquisition by the Company or one of it
     Subsidiaries; and provided further that the principal amount (or accreted
     value, as applicable) of such Indebtedness, together with any other
     outstanding Indebtedness incurred pursuant to this clause (v), does not
     exceed (subject to clause (x) below) $17.5 million;

        (vi) the incurrence by the Company or any of its Subsidiaries of
     Permitted Refinancing Indebtedness in exchange for, or the net proceeds of
     which are used to refund, refinance or replace Indebtedness (other than
     intercompany Indebtedness) that is either Existing Indebtedness or was
     permitted by this Indenture to be incurred under the first paragraph hereof
     or clauses (iii), (iv), (v) or (vi) of this paragraph;

        (vii) the incurrence by the Company or any of its Subsidiaries of
     intercompany Indebtedness between or among the Company and any of its
     Wholly Owned Subsidiaries; provided, however, that (i) if the Company is
     the obligor on such Indebtedness, such Indebtedness is expressly
     subordinated to the prior payment in full in cash of all Obligations with
     respect to the Notes and (ii)(A) any subsequent issuance or transfer of
     Equity Interests that results in any such Indebtedness being held by a
     Person other than the Company or a Subsidiary thereof and (B) any sale or
     other transfer of any such Indebtedness to a Person that is not either the
     Company or a Wholly Owned Subsidiary thereof shall be deemed, in each case,
     to constitute an incurrence of such Indebtedness by the Company or such
     Subsidiary, as the case may be, that was not permitted by this clause
     (vii);

        (viii) the incurrence by the Company of Hedging Obligations that are
     incurred for the purpose of fixing or hedging interest rate risk with
     respect to any floating rate Indebtedness that is permitted by the terms of
     this Indenture to be outstanding;

        (ix) the guarantee by the Company or any of the Subsidiary Guarantors of
     Indebtedness of the Company or a Subsidiary of the Company that was
     permitted to be incurred by another provision of this covenant;

        (x)  the incurrence by the Company or any Subsidiary Guarantor of Vendor
     Financing Indebtedness in an aggregate principal amount (or accreted value,
     as applicable) not to exceed $15.0 million outstanding at anytime; and

        (xi) the incurrence by the Company of additional Indebtedness in an
     aggregate principal amount (or accreted value, as applicable) at any time
     outstanding not to exceed $17.5 million; provided that the total amount of
     Indebtedness incurred by the Company pursuant to clauses (iv), (v) and (xi)
     hereof does not, in the aggregate, exceed $35.0 million.

                                       43

 
          For purposes of determining compliance with this covenant, (a) in the
event that an item of Indebtedness meets the criteria of more than one of the
categories of Permitted Indebtedness described in clauses (i) through (xi) above
or is entitled to be incurred pursuant to the first paragraph of this covenant,
the Company shall, in its sole discretion, classify such item of Indebtedness
and will only be required to include the amount and type of such Indebtedness in
one of the above clauses, and (b) an item of Indebtedness may be divided and
classified in more than one of the types of Indebtedness described herein.
Accrual of interest, accretion or amortization of original issue discount, the
payment of interest on any Indebtedness in the form of additional Indebtedness
with the same terms, and the payment of dividends on Disqualified Stock in the
form of additional shares of the same class of Disqualified Stock will not be
deemed to be an incurrence of Indebtedness or an issuance of Disqualified Stock
for purposes of this covenant.

SECTION 4.10.  ASSET SALES

          The Company shall not, and shall not permit any of its Subsidiaries
to, consummate an Asset Sale unless (i) the Company (or the Subsidiary, as the
case may be) receives consideration at the time of such Asset Sale at least
equal to the fair market value (evidenced by a resolution of the Board of
Directors set forth in an Officers' Certificate delivered to the Trustee) of the
assets or Equity Interests issued or sold or otherwise disposed of and (ii) at
least 75% of the consideration therefor received by the Company or such
Subsidiary is in the form of cash or Cash Equivalents; provided that the amount
of (x) any liabilities (as shown on the Company's or such Subsidiary's most
recent balance sheet), of the Company or any Subsidiary (other than contingent
liabilities and liabilities that are by their terms subordinated to the Notes or
any guarantee thereof) that are assumed by the transferee of any such assets
pursuant to a customary novation agreement that releases the Company or such
Subsidiary from further liability and (y) any securities, notes or other
obligations received by the Company or any such Subsidiary from such transferee
that are contemporaneously (subject to ordinary settlement periods) converted by
the Company or such Subsidiary into cash (to the extent of the cash received),
shall be deemed to be cash for purposes of this provision.

          Within 180 days after the receipt of any Net Proceeds from an Asset
Sale, the Company may apply such Net Proceeds, at its option, (a) to repay
Indebtedness under a Credit Facility, (b) to the acquisition of all of the
assets of, or all of the Voting Stock of, another Permitted Business, the making
of a capital expenditure or the acquisition of other long-term assets that are
used or useful in a Permitted Business (and the payment of related expenses) or
(c) in the case of Net Proceeds received by the Company pursuant to the
Satellite Lease Arrangements, for working capital purposes. Pending the final
application of any such Net Proceeds, the Company may temporarily reduce
revolving credit borrowings or otherwise invest such Net Proceeds in any manner
that is not prohibited by this Indenture. Any Net Proceeds from Asset Sales that
are not applied or invested as provided in the first sentence of this paragraph
will be deemed to constitute "EXCESS PROCEEDS." When the aggregate amount of
Excess Proceeds exceeds $10.0 million, the Company will be required to make an
Asset Sale Offer to purchase the maximum principal amount of Notes that may be
purchased out of the Excess Proceeds, at an offer price in cash in an amount
equal to 100% of the principal amount thereof plus accrued and unpaid interest
and Liquidated Damages thereon, if any, to the date of purchase, in accordance
with the procedures set forth in this Indenture. To the extent that any Excess
Proceeds remain after consummation of an Asset Sale Offer, the Company may use
such Excess Proceeds for any purpose not otherwise prohibited by this Indenture.
If the aggregate principal amount of Notes tendered into such Asset Sale Offer
surrendered by Holders thereof exceeds the amount of Excess Proceeds, the
Trustee shall select the Notes to be purchased on a pro rata basis.  Upon
completion of such offer to purchase, the amount of Excess Proceeds shall be
reset at zero.

                                       44

 
          Notwithstanding the foregoing, the Company and its Subsidiaries will
be permitted to consummate Asset Sales of Communications Assets without
complying with the first paragraph of this covenant if (x) at least 75% of the
consideration received by the Company and its Subsidiaries in respect of such
Asset Sale constitutes either Communications Assets, cash or Cash Equivalents,
and (y) any Net Proceeds received by the Company or any of its Subsidiaries in
connection with such Asset Sale are applied in accordance with the foregoing
paragraph and (z) the Company and its Subsidiaries receives consideration at the
time of such Asset Sale at least equal to the fair market value (evidenced by a
resolution of the Board of Directors set forth in a Officer's Certificate
delivered to the trustee, such determination to be based upon an opinion or
appraisal issued by an accounting, appraisal or investment banking firm of
national standing if such fair market value exceeds $1.0 million) of the
Communications Assets sold or otherwise disposed of.

SECTION 4.11.  TRANSACTIONS WITH AFFILIATES.

          The Company shall not, and shall not permit any of its Subsidiaries
to, make any payment to, or sell, lease, transfer or otherwise dispose of any of
its properties or assets to, or purchase any property or assets from, or enter
into or make or amend any transaction, contract, agreement, understanding, loan,
advance or guarantee with, or for the benefit of, any Affiliate (each of the
foregoing, an "AFFILIATE TRANSACTION"), unless (i) such Affiliate Transaction is
on terms that are no less favorable to the Company or the relevant Subsidiary
than those that would have been obtained in a comparable transaction by the
Company or such Subsidiary with an unrelated Person and (ii) the Company
delivers to the Trustee (a) with respect to any Affiliate Transaction or series
of related Affiliate Transactions involving aggregate consideration in excess of
$1.0 million, a resolution of the Board of Directors set forth in an Officers'
Certificate certifying that such Affiliate Transaction complies with clause (i)
above and that such Affiliate Transaction has been approved by a majority of the
disinterested members of the Board of Directors and (b) with respect to any
Affiliate Transaction or series of related Affiliate Transactions involving
aggregate consideration in excess of $10.0 million, an opinion as to the
fairness to the Holders of such Affiliate Transaction from a financial point of
view issued by an accounting, appraisal or investment banking firm of national
standing. Notwithstanding the foregoing, the following items shall not be deemed
to be Affiliate Transactions: (i) reasonable fees and compensation paid to and
indemnity provisions provided on behalf of officers, directors or employees of
the Company or any of its Subsidiaries in the ordinary course of business, (ii)
transactions between or among the Company and its Subsidiaries, (iii)
transactions pursuant to agreements in effect on the Issue Date and amendments,
modifications and replacements thereof, provided that such amendments,
modifications and replacements are on terms that are fair and reasonable to the
Company and have been unanimously approved by the disinterested members of the
Board of Directors, (iv) transactions between the Company and Holdings pursuant
to the Tax Sharing Agreement, as such is in effect on the date hereof (or as
amended thereafter so long as no payment by the Company and its Subsidiaries
under any such amended Tax Sharing Agreement shall exceed the amount of the
payment that would have been permitted at such time under the Tax Sharing
Agreement as in effect on the date hereof), and (v) Restricted Payments (other
than Restricted Investments) that are permitted Section 4.07 hereof.

SECTION 4.12.  LIENS.

          The Company shall not, and shall not permit any of its Subsidiaries
to, directly or indirectly, create, incur, assume or suffer to exist any Liens
on any asset now owned or hereafter acquired, or any income or profits therefrom
or assign or convey any right to receive income therefrom, except Permitted
Liens.

                                       45

 
SECTION 4.13.  BUSINESS ACTIVITIES.

          The Company shall not, and shall not permit any Subsidiary to, engage
in any business other than Permitted Businesses, except to such extent as would
not be material to the Company and its Subsidiaries taken as a whole.

SECTION 4.14.  CORPORATE EXISTENCE.

          Subject to Article 5 hereof, the Company shall do or cause to be done
all things necessary to preserve and keep in full force and effect (i) its
corporate existence, and the corporate, partnership or other existence of each
of its Subsidiaries, in accordance with the respective organizational documents
(as the same may be amended from time to time) of the Company or any such
Subsidiary and (ii) the rights (charter and statutory), licenses and franchises
of the Company and its Subsidiaries; provided, however, that the Company shall
not be required to preserve any such right, license or franchise, or the
corporate, partnership or other existence of any of its Subsidiaries, if the
Board of Directors shall determine that the preservation thereof is no longer
desirable in the conduct of the business of the Company and its Subsidiaries,
taken as a whole, and that the loss thereof is not adverse in any material
respect to the Holders of the Notes.

SECTION 4.15.  OFFER TO REPURCHASE UPON CHANGE OF CONTROL.

          (a) Upon the occurrence of a Change of Control, the Company shall make
an offer (the "CHANGE OF CONTROL OFFER") to each Holder of Notes to repurchase
all or any part (equal to $1,000 or an integral multiple thereof) of such
Holder's Notes pursuant to the offer described below at an offer price in cash
equal to 101% of the aggregate principal amount thereof plus accrued and unpaid
interest and Liquidated Damages thereon, if any, to the date of purchase (the
"CHANGE OF CONTROL PAYMENT"). Within ten days following any Change of Control,
the Company shall mail a notice to each Holder describing the transaction or
transactions that constitute the Change of Control and offering to repurchase
Notes on the date specified in such notice, which date shall be no earlier than
30 days and no later than 60 days from the date such notice is mailed (the
"CHANGE OF CONTROL PAYMENT DATE"), pursuant to the procedures required by this
Indenture and described in such notice.  The Company will comply with the
requirements of Rule 14e-1 under the Exchange Act and any other securities laws
and regulations thereunder to the extent such laws and regulations are
applicable in connection with the repurchase of the Notes as a result of a
Change of Control.

          (b) On the Change of Control Payment Date, the Company shall, to the
extent lawful, (1) accept for payment all Notes or portions thereof properly
tendered pursuant to the Change of Control Offer, (2) deposit with the Paying
Agent an amount equal to the Change of Control Payment in respect of all Notes
or portions thereof so tendered and (3) deliver or cause to be delivered to the
Trustee the Notes so accepted together with an Officers' Certificate stating the
aggregate principal amount of Notes or portions thereof being purchased by the
Company.  The Paying Agent shall promptly mail to each Holder of Notes so
tendered the Change of Control Payment for such Notes, and the Trustee shall
promptly authenticate and mail (or cause to be transferred by book entry) to
each Holder a new Note equal in principal amount to any unpurchased portion of
the Notes surrendered, if any; provided that each such new Note will be in a
principal amount of $1,000 or an integral multiple thereof.  The Company shall
publicly announce the results of the Change of Control Offer on or as soon as
practicable after the Change of Control Payment Date.  Unless the Company
defaults in the payment for any Notes properly tendered pursuant to the Change
of Control Offer, any Notes accepted for payment pursuant to the Change of
Control Offer shall cease to accrue interest after the Change of Control Payment
Date.

                                       46

 
          (c) The Company shall not be required to make a Change of Control
Offer upon a Change of Control if a third party makes the Change of Control
Offer in the manner, at the times and otherwise in compliance with the
requirements set forth herein applicable to a Change of Control Offer made by
the Company and purchases all Notes validly tendered and not withdrawn under
such Change of Control Offer.

SECTION 4.16.  MAINTENANCE OF INSURANCE.

          The Company shall obtain or maintain (as applicable) in full force and
effect:

               (i) in-orbit operations insurance with respect to each of MSAT-1
     or MSAT-2, at all times representing the value and prior insurance
     settlements of such satellite (taking into account the foregone useful life
     of such satellite) and the pro rata cost of a launch vehicle, payable in
     the event that such satellite ceases to be used for commercial revenue
     producing service (provided that such insurance may contain customary
     provisions for deductible payments and minimum thresholds for satellite
     failure); provided, however, that at the time the Company is required to
     procure or renew in-orbit operations insurance with respect to MSAT-1 or
     MSAT-2, the Company may reduce the amount to be insured by (x) the amount
     of cash, Cash Equivalents and short-term investments (amounts allocated or
     expected to be allocated for capital expenditures or working capital
     requirements), currently available to the Company to construct a
     Replacement Satellite as determined in good faith by the Board of Directors
     of the Company (evidenced by a resolution approved by at least a majority
     of the Board of Directors of the Company and set forth in an Officers'
     Certificate delivered to the Trustee), and (y) the value of any long lead-
     time spare parts that the Company has procured to date for any satellite
     that is comparable to the technological capability of the MSAT-1 or MSAT-2
     being insured, as such value is determined in good faith by the Board of
     Directors of the Company (evidenced by a resolution approved by at least a
     majority of the Board of Directors of the Company and set forth in an
     Officers' Certificate delivered to the Trustee), provided further that any
     insurance maintained by the Company will reflect the rights of the lessee
     or purchaser under the terms of any Satellite Lease Arrangements.

               (ii) launch and in-orbit checkout insurance with respect to each
     Replacement Satellite, which insurance shall be procured promptly prior to
     the launch of each such satellite and shall be in effect on the launch date
     and remain in effect through the launch and the initial check-out period of
     such Replacement Satellite, in an amount sufficient to provide for the
     construction, launch and insurance of a Replacement Satellite to be payable
     in the event of a launch or satellite failure during the initial check-out
     period; provided, however, that at the time the Company is required to
     procure launch and in-orbit check-out insurance with respect to a
     Replacement Satellite, the Company may reduce the amount to be insured if
     another Replacement Satellite is fully operational, is being used in
     commercial service and is insured in accordance with clause (i) above, by
     (x) the amount of cash, cash equivalents and short-term investments
     currently available to the Company to construct a Replacement Satellite as
     determined in good faith by the Board of Directors of the Company
     (evidenced by a resolution approved by at least a majority of the Board of
     Directors of the Company and set forth in an Officers' Certificate
     delivered to the Trustee), and (y) the value of any long lead-time spare
     parts that the Company has procured to date for any satellite that is
     comparable to the technological capability of the Replacement Satellite
     being insured, as such value is determined in good faith by the Board of
     Directors of the Company (evidenced by a resolution approved by at least a

                                       47

 
     majority of the Board of Directors of the Company and set forth in an
     Officers' Certificate delivered to the Trustee).

          The obligation of the Company to maintain insurance pursuant to this
covenant may be satisfied by any combination of:

               (i) insurance commitments obtained from any recognized insurance
     provider,

               (ii) insurance commitments obtained from any entity other than an
     entity referred to in clause (i) if the Board of Directors of the Company
     determines in good faith (evidenced by a majority resolution of the Board
     of Directors of the Company and set forth in an Officer's Certificate
     delivered to the Trustee) that such entity is creditworthy and otherwise
     capable of bearing the financial risk of providing such insurance and
     making payments in respect of any claims on a timely basis; and

               (iii)  unrestricted cash segregated and maintained by the Company
     in a segregated account established with an Eligible Institution (the
     "INSURANCE ACCOUNT") solely for disbursement in accordance with the terms
     of this covenant ("CASH INSURANCE"), and to be held in trust for the sole
     and express benefit of the holders of the Notes.

               Within 30 days following any date on which the Company is
     required to obtain insurance pursuant to this Indenture, the Company will
     deliver to the Trustee an insurance certificate certifying the amount of
     insurance then carried, and in full force and effect, and an Officer's
     Certificate stating that such insurance, together with any other insurance
     or Cash Insurance maintained by the Company, complies with this Indenture.
     In addition, the Company will cause to be delivered to the Trustee no less
     than once each year an insurance certificate setting forth the amount of
     insurance then carried, which insurance certificate shall entitle the
     Trustee to:

               (i) notice of any claim under any such insurance policy; and

               (ii) at least 30 days' notice from the provider of such insurance
     prior to the cancellation of any such insurance and an Officers'
     Certificate that complies with the first sentence of this paragraph.

          In the event that the Company maintains any Cash Insurance in
satisfaction of any part of their obligation to maintain insurance pursuant to
this covenant, the Company shall deliver, in lieu of any insurance certificate
otherwise required by this covenant, an Officers' Certificate to the Trustee
certifying the amount of such Cash Insurance.

          In the event that the Company receives any proceeds of any insurance
that it is required to maintain pursuant to this covenant, the Company shall
promptly deposit such proceeds into an escrow account established with an
Eligible Institution for such purpose. If the Company maintains any Cash
Insurance in satisfaction of any part of its obligation to maintain insurance
pursuant to this covenant, the Company shall transfer the cash maintained in the
Insurance Account to such escrow account upon the occurrence of the event (e.g.
a launch failure) that would have entitled the Company to the payment of
insurance had the Company purchased insurance from a recognized insurance
provider.  The Company may use monies on deposit in such escrow account for the
design, development, construction, 

                                       48

 
procurement, launch and insurance of any Replacement Satellite if: (i) the
Company delivers to the Trustee a certificate of the Company's President
certifying that such Replacement Satellite is comparable to the technological
capability of the satellite being replaced, (ii) within 30 days following the
receipt of such insurance proceeds, the Company delivers to the Trustee an
Officers' Certificate certifying that (A) the Company will use its reasonable
best efforts to ensure that such Replacement Satellites are launched within 24
months following delivery from the escrow account of such insurance proceeds;
and (B) the Company will have sufficient funds to service the Company's
projected debt service requirements until the scheduled launch of such
Replacement Satellite and to develop, construct, launch and insure such
Replacement Satellite.

SECTION 4.17.  LIMITATION ON SALE AND LEASEBACK TRANSACTIONS.

          The Company shall not, and shall not permit any of its Subsidiaries
to, enter into any sale and leaseback transaction; provided that the Company may
enter into a sale and leaseback transaction if (i) the Company could have (a)
incurred Indebtedness in an amount equal to the Attributable Indebtedness
relating to such sale and leaseback transaction pursuant to the Debt to Cash
Flow Ratio test set forth in Section 4.09 hereof and (b) incurred a Lien to
secure such Indebtedness pursuant Section 4.12 hereof, (ii) the gross cash
proceeds of such sale and leaseback transaction are at least equal to the fair
market value (as determined in good faith by the Board of Directors and set
forth in an Officers' Certificate delivered to the Trustee) of the property that
is the subject of such sale and leaseback transaction and (iii) the transfer of
assets in such sale and leaseback transaction is permitted by, and the Company
applies the proceeds of such transaction in compliance with Section 4.10 hereof.

SECTION 4.18.   LIMITATION ON ISSUANCES AND SALES OF CAPITAL STOCK OF WHOLLY
                OWNED SUBSIDIARIES.

          The Company (i) shall not, and shall not permit any Wholly Owned
Subsidiary of the Company to, transfer, convey, sell, lease or otherwise dispose
of any Equity Interests in any Wholly Owned Subsidiary of the Company to any
Person (other than the Company or a Wholly Owned Subsidiary of the Company),
unless (a) such transfer, conveyance, sale, lease or other disposition is of all
the Equity Interests in such Wholly Owned Subsidiary and (b) the cash Net
Proceeds from such transfer, conveyance, sale, lease or other disposition are
applied in accordance Section 4.10 hereof and (ii) shall not permit any Wholly
Owned Subsidiary of the Company to issue any of its Equity Interests (other
than, if necessary, shares of its Capital Stock constituting directors'
qualifying shares) to any Person other than to the Company or a Wholly Owned
Subsidiary of the Company.

SECTION 4.19.  ADDITIONAL SUBSIDIARY GUARANTEES

          If the Company or any of its Subsidiaries shall acquire or create
another Subsidiary after the date hereof, then such newly acquired or created
Subsidiary shall become a Subsidiary Guarantor and execute a Supplemental
Indenture in the form attached hereto as Exhibit F and deliver an Opinion of
Counsel and such other documents necessary and in accordance with the terms of
Article 11 and otherwise hereof.

SECTION 4.20.  PAYMENTS FOR CONSENTS

          The Company nor any of its Subsidiaries shall, directly or indirectly,
pay or cause to be paid any consideration, whether by way of interest, fee or
otherwise, to any Holder of any Notes for or as an inducement to any consent,
waiver or amendment of any of the terms or provisions of this Indenture 

                                       49

 
or the Notes unless such consideration is offered to be paid or is paid to all
Holders of the Notes that consent, waive or agree to amend in the time frame set
forth in the solicitation documents relating to such consent, waiver or
agreement.

                                   ARTICLE 5.
                                   SUCCESSORS

SECTION 5.01.  MERGER, CONSOLIDATION, OR SALE OF ASSETS.

          The Company shall not consolidate or merge with or into (whether or
not the Company is the surviving corporation), or sell, assign, transfer, lease,
convey or otherwise dispose of all or substantially all of its properties or
assets in one or more related transactions, to another corporation, Person or
entity unless (i) the Company is the surviving corporation or the entity or the
Person formed by or surviving any such consolidation or merger (if other than
the Company) or to which such sale, assignment, transfer, lease, conveyance or
other disposition shall have been made is a corporation organized or existing
under the laws of the United States, any state thereof or the District of
Columbia; (ii) the entity or Person formed by or surviving any such
consolidation or merger (if other than the Company) or the entity or Person to
which such sale, assignment, transfer, lease, conveyance or other disposition
shall have been made assumes all the obligations of the Company under the Unit
Agreement, the Pledge Agreement, the Debt Registration Rights Agreement, the
Warrant Registration Rights Agreement and the Notes and this Indenture pursuant
to a supplemental indenture in a form reasonably satisfactory to the Trustee;
(iii) immediately after such transaction no Default or Event of Default exists;
and (iv) except in the case of a merger of the Company with or into a Wholly
Owned Subsidiary of the Company, the Company or the entity or Person formed by
or surviving any such consolidation or merger (if other than the Company), or to
which such sale, assignment, transfer, lease, conveyance or other disposition
shall have been made (A) will have a Consolidated Net Worth immediately after
the transaction equal to or greater than the Consolidated Net Worth of the
Company immediately preceding the transaction and (B) will, at the time of such
transaction and after giving pro forma effect thereto as if such transaction had
occurred at the beginning of the Reference Period, be permitted to incur at
least $1.00 of additional Indebtedness pursuant to the Debt to Cash Flow Ratio
test set forth Section 4.09 hereof.

SECTION 5.02.  SUCCESSOR CORPORATION SUBSTITUTED.

          Upon any consolidation or merger, or any sale, assignment, transfer,
lease, conveyance or other disposition of all or substantially all of the assets
of the Company in accordance with Section 5.01 hereof, the successor corporation
formed by such consolidation or into or with which the Company is merged or to
which such sale, assignment, transfer, lease, conveyance or other disposition is
made shall succeed to, and be substituted for (so that from and after the date
of such consolidation, merger, sale, lease, conveyance or other disposition, the
provisions of this Indenture referring to the "Company" shall refer instead to
the successor corporation and not to the Company), and may exercise every right
and power of the Company under this Indenture with the same effect as if such
successor Person had been named as the Company herein; provided, however, that
the predecessor Company shall not be relieved from the obligation to pay the
principal of and interest on the Notes except in the case of a sale of all of
the Company's assets that meets the requirements of Section 5.01 hereof.

                                       50

 
                                   ARTICLE 6.
                             DEFAULTS AND REMEDIES

SECTION 6.01.  EVENTS OF DEFAULT.

          An "EVENT OF DEFAULT" occurs if:

          (a) the Company defaults in the payment when due of interest on, or
Liquidated Damages with respect to, the Notes and such default continues for a
period of 30 days (whether or not prohibited, with respect to the Holdings
Guarantee, by the subordination provisions of Article 10);

          (b) the Company defaults in the payment when due of principal of or
premium, if any, on the Notes when the same becomes due and payable at maturity,
upon redemption (including in connection with an offer to purchase) or otherwise
(whether or not prohibited, with respect to the Holdings Guarantee, by the
subordination provisions of Article 10);

          (c) the Company fails to comply with any of the provisions of Section
4.07, 4.09, 4.10 or 4.15 hereof;

          (d) the Company fails to observe or perform any other covenant,
representation, warranty or other agreement in this Indenture or the Notes for
60 days after notice to the Company by the Trustee or the Holders of at least
25% in aggregate principal amount of the Notes then outstanding voting as a
single class;

          (e) a default occurs under any mortgage, indenture or instrument under
which there may be issued or by which there may be secured or evidenced any
Indebtedness for money borrowed by the Company or any of its Subsidiaries (or
the payment of which is guaranteed by the Company or any of its Subsidiaries),
whether such Indebtedness or guarantee now exists, or is created after the date
of this Indenture, which default (a) is caused by a failure to pay principal of
or premium, if any, or interest or such Indebtedness prior to the expiration of
the grace period provided in such Indebtedness on the date of such default (a
"Payment Default") or (b) results in the acceleration of such Indebtedness prior
to its express maturity and, in each case, the principal amount of such
Indebtedness, together with the principal amount of any other such Indebtedness
the maturity of which has been so accelerated, aggregates $5.0 million or more;

          (f) a default occurs under any mortgage, indenture or instrument under
which there may be issued or by which there may be secured or evidenced any
Indebtedness for money borrowed by Holdings (or the payment of which is
guaranteed by Holdings) whether such Indebtedness or guarantee now exists, or is
created after the date of this Indenture, which default (a) is a Payment Default
or (b) results in the acceleration of such Indebtedness prior to its express
maturity and, in each case, the principal amount of such Indebtedness, together
with the principal amount of any other such Indebtedness the maturity of which
has been so accelerated, aggregates $5.0 million or more;

          (g) a final judgment or final judgments for the payment of money are
entered by a court or courts of competent jurisdiction against the Company or
any of its Subsidiaries or any group of Subsidiaries that, taken as a whole,
would constitute a Subsidiary and such judgment or judgments remain undischarged
for a period (during which execution shall not be effectively stayed) of 60
days, provided that the aggregate of all such undischarged judgments exceeds
$5.0 million;

                                       51

 
          (h) a final judgment or final judgments for the payment of money are
entered by a court or courts of competent jurisdiction against Holdings and such
judgment or judgments remain undischarged for a period (during which execution
shall not be effectively stayed) of 60 days, provided that the aggregate of all
such undischarged judgments exceeds $5.0 million;

          (i) material breach by Holdings, the Company or any Subsidiary of any
representation or warranty set forth in the Unit Agreement or the Pledge
Agreement, or material default by Holdings, the Company or any Subsidiary in the
performance of any covenant set forth in the Unit Agreement or the Pledge
Agreement, or repudiation by Holdings, the Company or any Subsidiary of its
obligations under the Unit Agreement or the Pledge Agreement or the
unenforceability of the Unit Agreement or the Pledge Agreement against Holdings,
the Company or any Subsidiary for any reason;

          (j) the termination or revocation of any of the permits, licenses,
approvals, orders, certificates, franchises or authorizations of governmental or
regulatory authorities, including those relating to the Federal Communications
Act of 1934, as amended, owned or held by Holdings, the Company or any of the
Subsidiary Guarantors that are material to the Company and its Subsidiaries,
taken as a whole, (collectively, "LICENSES") or any other material impairment
occurs of the rights under any such Licenses of the holder of such License;

          (k) the Company or any Guarantor:

               (i) commences a voluntary case,

               (ii) consents to the entry of an order for relief against it in
     an involuntary case,

               (iii) consents to the appointment of a custodian of it or for all
     or substantially all of its property,

               (iv) makes a general assignment for the benefit of its creditors,
     or

               (v) generally is not paying its debts as they become due; or

          (l) a court of competent jurisdiction enters an order or decree under
any Bankruptcy Law that:

               (i) is for relief against the Company or any of the Guarantors in
     an involuntary case;

               (ii) appoints a custodian of the Company or any of the Guarantors
     for all or substantially all of the property of the Company or any of the
     Guarantors; or

               (iii) orders the liquidation of the Company or any of the
     Guarantors;

               (iv) and the order or decree remains unstayed and in effect for
     60 consecutive days; or

          (m) except as permitted by this Indenture, any Note Guarantee is held
in any judicial proceeding to be unenforceable or invalid or shall cease for any
reason to be in full force and effect or 

                                       52

 
any Guarantor, or any Person acting on behalf of any Guarantor, shall deny or
disaffirm its obligations under such Guarantor's Note Guarantee.

SECTION 6.02.  ACCELERATION.

          If any Event of Default (other than an Event of Default specified in
clause (k) or (l) of Section 6.01 hereof with respect to the Company Guarantor
or any Guarantor occurs and is continuing, the Trustee or the Holders of at
least 25% in principal amount of the then outstanding Notes may declare all the
Notes to be due and payable immediately.  Notwithstanding the foregoing, if an
Event of Default specified in clause (k) or (l) of Section 6.01 hereof occurs
with respect to the Company or any Guarantor, all outstanding Notes shall be due
and payable immediately without further action or notice.  The Holders of a
majority in aggregate principal amount of the then outstanding Notes by written
notice to the Trustee may on behalf of all of the Holders rescind an
acceleration and its consequences if the rescission would not conflict with any
judgment or decree and if all existing Events of Default (except nonpayment of
principal, interest or premium that has become due solely because of the
acceleration) have been cured or waived.

          If an Event of Default occurs by reason of any willful action (or
inaction) taken (or not taken) by or on behalf of the Company with the intention
of avoiding payment of the premium that the Company would have had to pay if the
Company then had elected to redeem the Notes pursuant to Section 3.07 hereof,
then, upon acceleration of the Notes, an equivalent premium shall also become
and be immediately due and payable, to the extent permitted by law, anything in
this Indenture or in the Notes to the contrary notwithstanding. If an Event of
Default occurs prior to April 1, 2003 by reason of any willful action (or
inaction) taken (or not taken) by or on behalf of the Company with the intention
of avoiding the prohibition on redemption of the Notes prior to such date, then,
upon acceleration of the Notes, a premium shall become and be immediately due
and payable in an amount, for each of the years beginning on April 1 of the
years set forth below, as set forth below (expressed as a percentage of the
aggregate principal amount to the date of payment that would otherwise be due
but for the provisions of this sentence):

          YEAR                                  PERCENTAGE
          ----                                  ----------

          1998...................................116.335%
          1999...................................114.293%
          2000...................................112.251%
          2001...................................110.209%
          2002...................................108.167%

SECTION 6.03.  OTHER REMEDIES.

          If an Event of Default occurs and is continuing, the Trustee may
pursue any available remedy to collect the payment of principal, premium, if
any, and interest and Liquidated Damages, if any, on the Notes or to enforce the
performance of any provision of the Notes or this Indenture.

          The Trustee may maintain a proceeding even if it does not possess any
of the Notes or does not produce any of them in the proceeding.  A delay or
omission by the Trustee or any Holder of a Note in exercising any right or
remedy accruing upon an Event of Default shall not impair the right or remedy or
constitute a waiver of or acquiescence in the Event of Default.  All remedies
are cumulative to the extent permitted by law.

                                       53

 
SECTION 6.04.  WAIVER OF PAST DEFAULTS.

          Holders of not less than a majority in aggregate principal amount of
the then outstanding Notes by written notice to the Trustee may on behalf of the
Holders of all of the Notes waive an existing Default or Event of Default and
its consequences hereunder, except a continuing Default or Event of Default in
the payment of the principal of, premium and Liquidated Damages, if any, or
interest on, the Notes (including in connection with an offer to purchase)
(provided, however, that the Holders of a majority in aggregate principal amount
of the then outstanding Notes may rescind an acceleration and its consequences,
including any related payment default that resulted from such acceleration).
Upon any such waiver, such Default shall cease to exist, and any Event of
Default arising therefrom shall be deemed to have been cured for every purpose
of this Indenture; but no such waiver shall extend to any subsequent or other
Default or impair any right consequent thereon.

SECTION 6.05.  CONTROL BY MAJORITY.

          Holders of a majority in principal amount of the then outstanding
Notes may direct the time, method and place of conducting any proceeding for
exercising any remedy available to the Trustee or exercising any trust or power
conferred on it.  However, the Trustee may refuse to follow any direction that
conflicts with law or this Indenture that the Trustee determines may be unduly
prejudicial to the rights of other Holders of Notes or that may involve the
Trustee in personal liability.

SECTION 6.06.  LIMITATION ON SUITS.

          A Holder of a Note may pursue a remedy with respect to this Indenture
or the Notes only if:

               (a) the Holder of a Note gives to the Trustee written notice of a
     continuing Event of Default;

               (b) the Holders of at least 25% in principal amount of the then
     outstanding Notes make a written request to the Trustee to pursue the
     remedy;

               (c) such Holder of a Note or Holders of Notes offer and, if
     requested, provide to the Trustee indemnity satisfactory to the Trustee
     against any loss, liability or expense;

               (d) the Trustee does not comply with the request within 60 days
     after receipt of the request and the offer and, if requested, the provision
     of indemnity; and

               (e) during such 60-day period the Holders of a majority in
     principal amount of the then outstanding Notes do not give the Trustee a
     direction inconsistent with the request.

          A Holder of a Note may not use this Indenture to prejudice the rights
of another Holder of a Note or to obtain a preference or priority over another
Holder of a Note.

SECTION 6.07.  RIGHTS OF HOLDERS OF NOTES TO RECEIVE PAYMENT.

          Notwithstanding any other provision of this Indenture, the right of
any Holder of a Note to receive payment of principal, premium and Liquidated
Damages, if any, and interest on the Note, on or after the respective due dates
expressed in the Note (including in connection with an offer to 

                                       54

 
purchase), or to bring suit for the enforcement of any such payment on or after
such respective dates, shall not be impaired or affected without the consent of
such Holder.

SECTION 6.08.  COLLECTION SUIT BY TRUSTEE.

          If an Event of Default specified in Section 6.01(a) or (b) occurs and
is continuing, the Trustee is authorized to recover judgment in its own name and
as trustee of an express trust against the Company for the whole amount of
principal of, premium and Liquidated Damages, if any, and interest remaining
unpaid on the Notes and interest on overdue principal and, to the extent lawful,
interest and such further amount as shall be sufficient to cover the costs and
expenses of collection, including the reasonable compensation, expenses,
disbursements and advances of the Trustee, its agents and counsel.

SECTION 6.09.  TRUSTEE MAY FILE PROOFS OF CLAIM.

          The Trustee is authorized to file such proofs of claim and other
papers or documents as may be necessary or advisable in order to have the claims
of the Trustee (including any claim for the reasonable compensation, expenses,
disbursements and advances of the Trustee, its agents and counsel) and the
Holders of the Notes allowed in any judicial proceedings relative to the Company
(or any other obligor upon the Notes), its creditors or its property and shall
be entitled and empowered to collect, receive and distribute any money or other
property payable or deliverable on any such claims and any custodian in any such
judicial proceeding is hereby authorized by each Holder to make such payments to
the Trustee, and in the event that the Trustee shall consent to the making of
such payments directly to the Holders, to pay to the Trustee any amount due to
it for the reasonable compensation, expenses, disbursements and advances of the
Trustee, its agents and counsel, and any other amounts due the Trustee under
Section 7.07 hereof.  To the extent that the payment of any such compensation,
expenses, disbursements and advances of the Trustee, its agents and counsel, and
any other amounts due the Trustee under Section 7.07 hereof out of the estate in
any such proceeding, shall be denied for any reason, payment of the same shall
be secured by a Lien on, and shall be paid out of, any and all distributions,
dividends, money, securities and other properties that the Holders may be
entitled to receive in such proceeding whether in liquidation or under any plan
of reorganization or arrangement or otherwise.  Nothing herein contained shall
be deemed to authorize the Trustee to authorize or consent to or accept or adopt
on behalf of any Holder any plan of reorganization, arrangement, adjustment or
composition affecting the Notes or the rights of any Holder, or to authorize the
Trustee to vote in respect of the claim of any Holder in any such proceeding.

SECTION 6.10.  PRIORITIES.

          If the Trustee collects any money pursuant to this Article, it shall
pay out the money in the following order:

          First:  to the Trustee, its agents and attorneys for amounts due under
Section 7.07 hereof, including payment of all compensation, expense and
liabilities incurred, and all advances made, by the Trustee and the costs and
expenses of collection;

          Second:  to Holders of Notes for amounts due and unpaid on the Notes
for principal, premium and Liquidated Damages, if any, and interest, ratably,
without preference or priority of any kind, according to the amounts due and
payable on the Notes for principal, premium and Liquidated Damages, if any and
interest, respectively; and

                                       55

 
          Third:  to the Company or to such party as a court of competent
jurisdiction shall direct.

          The Trustee may fix a record date and payment date for any payment to
Holders of Notes pursuant to this Section 6.10.

SECTION 6.11.  UNDERTAKING FOR COSTS.

          In any suit for the enforcement of any right or remedy under this
Indenture or in any suit against the Trustee for any action taken or omitted by
it as a Trustee, a court in its discretion may require the filing by any party
litigant in the suit of an undertaking to pay the costs of the suit, and the
court in its discretion may assess reasonable costs, including reasonable
attorneys' fees, against any party litigant in the suit, having due regard to
the merits and good faith of the claims or defenses made by the party litigant.
This Section does not apply to a suit by the Trustee, a suit by a Holder of a
Note pursuant to Section 6.07 hereof, or a suit by Holders of more than 10% in
principal amount of the then outstanding Notes.

                                  ARTICLE 7.
                                   TRUSTEE

SECTION 7.01.  DUTIES OF TRUSTEE.

          (a) If an Event of Default has occurred and is continuing, the Trustee
shall exercise such of the rights and powers vested in it by this Indenture, and
use the same degree of care and skill in its exercise, as a prudent man would
exercise or use under the circumstances in the conduct of his own affairs.

          (b) Except during the continuance of an Event of Default:

               (i) the duties of the Trustee shall be determined solely by the
express provisions of this Indenture and the Trustee need perform only those
duties that are specifically set forth in this Indenture and no others, and no
implied covenants or obligations shall be read into this Indenture against the
Trustee; and

               (ii) in the absence of bad faith on its part, the Trustee may
conclusively rely, as to the truth of the statements and the correctness of the
opinions expressed therein, upon certificates or opinions furnished to the
Trustee and conforming to the requirements of this Indenture.  However, the
Trustee shall examine the certificates and opinions to determine whether or not
they conform to the requirements of this Indenture.

          (c) The Trustee may not be relieved from liabilities for its own
negligent action, its own negligent failure to act, or its own willful
misconduct, except that:

               (i) this paragraph does not limit the effect of paragraph (b) of
     this Section;

               (ii) the Trustee shall not be liable for any error of judgment
made in good faith by a Responsible Officer, unless it is proved that the
Trustee was negligent in ascertaining the pertinent facts; and

               (iii) the Trustee shall not be liable with respect to any action
it takes or omits to take in good faith in accordance with a direction received
by it pursuant to Section 6.05 hereof.

                                       56

 
          (d) Whether or not therein expressly so provided, every provision of
this Indenture that in any way relates to the Trustee is subject to paragraphs
(a), (b), (c), (e) and (f) of this Section and Section 7.02.

          (e) No provision of this Indenture shall require the Trustee to expend
or risk its own funds or incur any liability.  The Trustee shall be under no
obligation to exercise any of its rights and powers under this Indenture at the
request of any Holders, unless such Holder shall have offered to the Trustee
security and indemnity satisfactory to it against any loss, liability or
expense.

          (f) The Trustee shall not be liable for interest on any money received
by it except as the Trustee may agree in writing with the Company.  Money held
in trust by the Trustee need not be segregated from other funds except to the
extent required by law.

SECTION 7.02.  RIGHTS OF TRUSTEE.

          (a) The Trustee may conclusively rely upon any document believed by it
to be genuine and to have been signed or presented by the proper Person.  The
Trustee need not investigate any fact or matter stated in the document.

          (b) Before the Trustee acts or refrains from acting, it may require an
Officers' Certificate or an Opinion of Counsel or both.  The Trustee shall not
be liable for any action it takes or omits to take in good faith in reliance on
such Officers' Certificate or Opinion of Counsel.  The Trustee may consult with
counsel and the written advice of such counsel or any Opinion of Counsel shall
be full and complete authorization and protection from liability in respect of
any action taken, suffered or omitted by it hereunder in good faith and in
reliance thereon.

          (c) The Trustee may act through its attorneys and agents and shall not
be responsible for the misconduct or negligence of any agent appointed with due
care.

          (d) The Trustee shall not be liable for any action it takes or omits
to take in good faith that it believes to be authorized or within the rights or
powers conferred upon it by this Indenture.

          (e) Unless otherwise specifically provided in this Indenture, any
demand, request, direction or notice from the Company or any Guarantor shall be
sufficient if signed by an Officer of the Company or Guarantor issuing such
demand, request or notice.

          (f) The Trustee shall be under no obligation to exercise any of the
rights or powers vested in it by this Indenture at the request or direction of
any of the Holders unless such Holders shall have offered to the Trustee
reasonable security or indemnity against the costs, expenses and liabilities
that might be incurred by it in compliance with such request or direction.

SECTION 7.03.  INDIVIDUAL RIGHTS OF TRUSTEE.

          The Trustee in its individual or any other capacity may become the
owner or pledgee of Notes and may otherwise deal with the Company or any
Affiliate of the Company with the same rights it would have if it were not
Trustee.  However, in the event that the Trustee acquires any conflicting
interest it must eliminate such conflict within 90 days, apply to the SEC for
permission to continue as trustee or resign.  Any Agent may do the same with
like rights and duties.  The Trustee is also subject to Sections 7.10 and 7.11
hereof.

                                       57

 
SECTION 7.04.  TRUSTEE'S DISCLAIMER.

          The Trustee shall not be responsible for and makes no representation
as to the validity or adequacy of this Indenture or the Notes, it shall not be
accountable for the Company's use of the proceeds from the Notes or any money
paid to the Company or upon the Company's direction under any provision of this
Indenture, it shall not be responsible for the use or application of any money
received by any Paying Agent other than the Trustee, and it shall not be
responsible for any statement or recital herein or any statement in the Notes or
any other document in connection with the sale of the Notes or pursuant to this
Indenture other than its certificate of authentication.

SECTION 7.05.  NOTICE OF DEFAULTS.

          If a Default or Event of Default occurs and is continuing and if it is
known to the Trustee, the Trustee shall mail to Holders of Notes a notice of the
Default or Event of Default within 90 days after it occurs.  Except in the case
of a Default or Event of Default in payment of principal of, premium, if any, or
interest on any Note, the Trustee may withhold the notice if and so long as a
committee of its Responsible Officers in good faith determines that withholding
the notice is in the interests of the Holders of the Notes.

SECTION 7.06.  REPORTS BY TRUSTEE TO HOLDERS OF THE NOTES.

          Within 60 days after each May 15 beginning with the May 15 following
the date of this Indenture, and for so long as Notes remain outstanding, the
Trustee shall mail to the Holders of the Notes a brief report dated as of such
reporting date that complies with TIA (S) 313(a) (but if no event described in
TIA (S) 313(a) has occurred within the twelve months preceding the reporting
date, no report need be transmitted).  The Trustee also shall comply with TIA
(S) 313(b)(2).  The Trustee shall also transmit by mail all reports as required
by TIA (S) 313(c).

          A copy of each report at the time of its mailing to the Holders of
Notes shall be mailed to the Company and filed with the SEC and each stock
exchange on which the Notes are listed in accordance with TIA (S) 313(d).  The
Company shall promptly notify the Trustee when the Notes are listed on any stock
exchange.

SECTION 7.07.  COMPENSATION AND INDEMNITY.

          The Company and the Guarantors shall pay to the Trustee from time to
time reasonable compensation for its acceptance of this Indenture and services
hereunder.  The Trustee's compensation shall not be limited by any law on
compensation of a trustee of an express trust.  The Company and the Guarantors
shall reimburse the Trustee promptly upon request for all reasonable
disbursements, advances and expenses incurred or made by it in addition to the
compensation for its services.  Such expenses shall include the reasonable
compensation, disbursements and expenses of the Trustee's agents and counsel.

          The Company and the Guarantors shall indemnify the Trustee against any
and all losses, liabilities or expenses incurred by it arising out of or in
connection with the acceptance or administration of its duties under this
Indenture, including the costs and expenses of enforcing this Indenture against
the Company and the Guarantors (including this Section 7.07) and defending
itself against any claim (whether asserted by the Company and the Guarantors or
any Holder or any other person) or liability in connection with the exercise or
performance of any of its powers or duties hereunder, except to the extent any
such loss, liability or expense may be attributable to its negligence or bad
faith.  The Trustee 

                                       58

 
shall notify the Company promptly of any claim for which it may seek indemnity.
Failure by the Trustee to so notify the Company shall not relieve the Company
and the Guarantors of its obligations hereunder. The Company shall defend the
claim and the Trustee shall cooperate in the defense. The Trustee may have
separate counsel and the Company shall pay the reasonable fees and expenses of
such counsel. The Company need not pay for any settlement made without its
consent, which consent shall not be unreasonably withheld.

          The obligations of the Company and the Guarantors under this Section
7.07 shall survive the satisfaction and discharge of this Indenture.

          To secure the Company's and the Guarantors' payment obligations in
this Section, the Trustee shall have a Lien prior to the Notes on all money or
property held or collected by the Trustee, except that held in trust to pay
principal and interest on particular Notes.  Such Lien shall survive the
satisfaction and discharge of this Indenture.

          When the Trustee incurs expenses or renders services after an Event of
Default specified in Section 6.01(k) or (l) hereof occurs, the expenses and the
compensation for the services (including the fees and expenses of its agents and
counsel) are intended to constitute expenses of administration under any
Bankruptcy Law.

          The Trustee shall comply with the provisions of TIA (S) 313(b)(2) to
the extent applicable.

SECTION 7.08.  REPLACEMENT OF TRUSTEE.

          A resignation or removal of the Trustee and appointment of a successor
Trustee shall become effective only upon the successor Trustee's acceptance of
appointment as provided in this Section.

          The Trustee may resign in writing at any time and be discharged from
the trust hereby created by so notifying the Company.  The Holders of Notes of a
majority in principal amount of the then outstanding Notes may remove the
Trustee by so notifying the Trustee and the Company in writing.  The Company may
remove the Trustee if:

               (a) the Trustee fails to comply with Section 7.10 hereof;

               (b) the Trustee is adjudged a bankrupt or an insolvent or an
     order for relief is entered with respect to the Trustee under any
     Bankruptcy Law;

               (c) a custodian or public officer takes charge of the Trustee or
     its property; or

               (d) the Trustee becomes incapable of acting.

          If the Trustee resigns or is removed or if a vacancy exists in the
office of Trustee for any reason, the Company shall promptly appoint a successor
Trustee.  Within one year after the successor Trustee takes office, the Holders
of a majority in principal amount of the then outstanding Notes may appoint a
successor Trustee to replace the successor Trustee appointed by the Company.

          If a successor Trustee does not take office within 60 days after the
retiring Trustee resigns or is removed, the retiring Trustee, the Company, or
the Holders of Notes of at least 10% in 

                                       59

 
principal amount of the then outstanding Notes may petition any court of
competent jurisdiction for the appointment of a successor Trustee.

          If the Trustee, after written request by any Holder of a Note who has
been a Holder of a Note for at least six months, fails to comply with Section
7.10, such Holder of a Note may petition any court of competent jurisdiction for
the removal of the Trustee and the appointment of a successor Trustee.

          A successor Trustee shall deliver a written acceptance of its
appointment to the retiring Trustee and to the Company.  Thereupon, the
resignation or removal of the retiring Trustee shall become effective, and the
successor Trustee shall have all the rights, powers and duties of the Trustee
under this Indenture.  The successor Trustee shall mail a notice of its
succession to Holders of the Notes.  The retiring Trustee shall promptly
transfer all property held by it as Trustee to the successor Trustee, provided
all sums owing to the Trustee hereunder have been paid and subject to the Lien
provided for in Section 7.07 hereof.  Notwithstanding replacement of the Trustee
pursuant to this Section 7.08, the Company's obligations under Section 7.07
hereof shall continue for the benefit of the retiring Trustee.

SECTION 7.09.  SUCCESSOR TRUSTEE BY MERGER, ETC.

          If the Trustee consolidates, merges or converts into, or transfers all
or substantially all of its corporate trust business to, another corporation,
the successor corporation without any further act shall be the successor
Trustee.

SECTION 7.10.  ELIGIBILITY; DISQUALIFICATION.

          There shall at all times be a Trustee hereunder that is a corporation
organized and doing business under the laws of the United States of America or
of any state thereof that is authorized under such laws to exercise corporate
trustee power, that is subject to supervision or examination by federal or state
authorities and that has a combined capital and surplus of at least $100 million
as set forth in its most recent published annual report of condition.

          This Indenture shall always have a Trustee who satisfies the
requirements of TIA (S) 310(a)(1), (2) and (5).  The Trustee is subject to TIA
(S) 310(b).

SECTION 7.11.  PREFERENTIAL COLLECTION OF CLAIMS AGAINST COMPANY.

          The Trustee is subject to TIA (S) 311(a), excluding any creditor
relationship listed in TIA (S) 311(b).  A Trustee who has resigned or been
removed shall be subject to TIA (S) 311(a) to the extent indicated therein.

                                   ARTICLE 8.
                    LEGAL DEFEASANCE AND COVENANT DEFEASANCE

SECTION 8.01.  OPTION TO EFFECT LEGAL DEFEASANCE OR COVENANT DEFEASANCE.

          The Company may, at the option of its Board of Directors evidenced by
a resolution set forth in an Officers' Certificate, at any time, elect to have
either Section 8.02 or 8.03 hereof be applied to all outstanding Notes upon
compliance with the conditions set forth below in this Article 8.

SECTION 8.02. LEGAL DEFEASANCE AND DISCHARGE.

                                       60

 
          Upon the Company's exercise under Section 8.01 hereof of the option
applicable to this Section 8.02, the Company shall, subject to the satisfaction
of the conditions set forth in Section 8.04 hereof, be deemed to have been
discharged from its obligations with respect to all outstanding Notes and to
have each Guarantor's obligation discharged with respect to its Note Guarantee
on the date the conditions set forth below are satisfied (hereinafter, "LEGAL
DEFEASANCE").  For this purpose, Legal Defeasance means that the Company shall
be deemed to have paid and discharged the entire Indebtedness represented by the
outstanding Notes, which shall thereafter be deemed to be "outstanding" only for
the purposes of Section 8.05 hereof and the other Sections of this Indenture
referred to in (a) and (b) below, and to have satisfied all its other
obligations under such Notes and this Indenture (and the Trustee, on demand of
and at the expense of the Company, shall execute proper instruments
acknowledging the same), except for the following provisions which shall survive
until otherwise terminated or discharged hereunder:  (a) the rights of Holders
of outstanding Notes to receive solely from the trust fund described in Section
8.04 hereof, and as more fully set forth in such Section, payments in respect of
the principal of, premium, if any, and interest and Liquidated Damages, if any,
on such Notes when such payments are due, (b) the Company's obligations with
respect to such Notes under Article 2 and Section 4.02 hereof, (c) the rights,
powers, trusts, duties and immunities of the Trustee hereunder and the Company's
obligations in connection therewith and (d) this Article 8.  Subject to
compliance with this Article 8, the Company may exercise its option under this
Section 8.02 notwithstanding the prior exercise of its option under Section 8.03
hereof.

SECTION 8.03.  COVENANT DEFEASANCE.

          Upon the Company's exercise under Section 8.01 hereof of the option
applicable to this Section 8.03, the Company and each Guarantor shall, subject
to the satisfaction of the conditions set forth in Section 8.04 hereof, be
released from their obligations under the covenants contained in Sections 4.07,
4.08, 4.09, 4.10, 4.11, 4.12, 4.13, 4.15, 4.16, 4.17, 4.18, 4.19 and 4.20 hereof
with respect to the outstanding Notes on and after the date the conditions set
forth in Section 8.04 are satisfied (hereinafter, "COVENANT DEFEASANCE"), and
the Notes shall thereafter be deemed not "outstanding" for the purposes of any
direction, waiver, consent or declaration or act of Holders (and the
consequences of any thereof) in connection with such covenants, but shall
continue to be deemed "outstanding" for all other purposes hereunder (it being
understood that such Notes shall not be deemed outstanding for accounting
purposes).  For this purpose, Covenant Defeasance means that, with respect to
the outstanding Notes, the Company and each Guarantor may omit to comply with
and shall have no liability in respect of any term, condition or limitation set
forth in any such covenant, whether directly or indirectly, by reason of any
reference elsewhere herein to any such covenant or by reason of any reference in
any such covenant to any other provision herein or in any other document and
such omission to comply shall not constitute a Default or an Event of Default
under Section 6.01 hereof, but, except as specified above, the remainder of this
Indenture and such Notes shall be unaffected thereby.  In addition, upon the
Company's exercise under Section 8.01 hereof of the option applicable to this
Section 8.03 hereof, subject to the satisfaction of the conditions set forth in
Section 8.04 hereof, Sections 6.01(d) through 6.01(m) hereof shall not
constitute Events of Default.

SECTION 8.04.  CONDITIONS TO LEGAL OR COVENANT DEFEASANCE.

          The following shall be the conditions to the application of either
Section 8.02 or 8.03 hereof to the outstanding Notes:

          In order to exercise either Legal Defeasance or Covenant Defeasance:

                                       61

 
          (a) the Company must irrevocably deposit with the Trustee, in trust,
for the benefit of the Holders, cash in United States dollars, non-callable
Government Securities, or a combination thereof, in such amounts as will be
sufficient, in the opinion of a nationally recognized firm of independent public
accountants, to pay the principal of, premium and Liquidated Damages, if any,
and interest on the outstanding Notes on the stated date for payment thereof or
on the applicable redemption date, as the case may be, and the Company must
specify whether the Notes are being defeased to maturity or to a particular
redemption date;

          (b) in the case of an election under Section 8.02 hereof, the Company
shall have delivered to the Trustee an Opinion of Counsel in the United States
reasonably acceptable to the Trustee confirming that (A) the Company has
received from, or there has been published by, the Internal Revenue Service a
ruling or (B) since the date of this Indenture, there has been a change in the
applicable federal income tax law, in either case to the effect that, and based
thereon such Opinion of Counsel shall confirm that, the Holders of the
outstanding Notes will not recognize income, gain or loss for federal income tax
purposes as a result of such Legal Defeasance and will be subject to federal
income tax on the same amounts, in the same manner and at the same times as
would have been the case if such Legal Defeasance had not occurred;

          (c) in the case of an election under Section 8.03 hereof, the Company
shall have delivered to the Trustee an Opinion of Counsel in the United States
reasonably acceptable to the Trustee confirming that the Holders of the
outstanding Notes will not recognize income, gain or loss for federal income tax
purposes as a result of such Covenant Defeasance and will be subject to federal
income tax on the same amounts, in the same manner and at the same times as
would have been the case if such Covenant Defeasance had not occurred;

          (d) no Default or Event of Default shall have occurred and be
continuing on the date of such deposit (other than a Default or Event of Default
resulting from the incurrence of Indebtedness all or a portion of the proceeds
of which will be used to defease the Notes pursuant to this Article 8
concurrently with such incurrence) or insofar as Sections 6.01(k) or 6.01(l)
hereof is concerned, at any time in the period ending on the 91st day after the
date of deposit;

          (e) such Legal Defeasance or Covenant Defeasance shall not result in a
breach or violation of, or constitute a default under, any material agreement or
instrument (other than this Indenture) to which the Company or any of its
Subsidiaries is a party or by which the Company or any of its Subsidiaries is
bound;

          (f) the Company shall have delivered to the Trustee an Opinion of
Counsel (which may be subject to customary exceptions) to the effect that on the
91st day following the deposit, the trust funds will not be subject to the
effect of any applicable bankruptcy, insolvency, reorganization or similar laws
affecting creditors' rights generally;

          (g) the Company shall have delivered to the Trustee an Officers'
Certificate stating that the deposit was not made by the Company with the intent
of preferring the Holders over any other creditors of the Company or with the
intent of defeating, hindering, delaying or defrauding any other creditors of
the Company; and

          (h) the Company shall have delivered to the Trustee an Officers'
Certificate and an Opinion of Counsel, each stating that all conditions
precedent provided for or relating to the Legal Defeasance or the Covenant
Defeasance have been complied with.

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SECTION 8.05.  DEPOSITED MONEY AND GOVERNMENT SECURITIES TO BE HELD IN TRUST;
               OTHER MISCELLANEOUS PROVISIONS.

          Subject to Section 8.06 hereof, all money and non-callable Government
Securities (including the proceeds thereof) deposited with the Trustee (or other
qualifying trustee, collectively for purposes of this Section 8.05, the
"TRUSTEE") pursuant to Section 8.04 hereof in respect of the outstanding Notes
shall be held in trust and applied by the Trustee, in accordance with the
provisions of such Notes and this Indenture, to the payment, either directly or
through any Paying Agent (including the Company acting as Paying Agent) as the
Trustee may determine, to the Holders of such Notes of all sums due and to
become due thereon in respect of principal, premium, if any, and interest, but
such money need not be segregated from other funds except to the extent required
by law.

          The Company and the Guarantors shall pay and indemnify the Trustee
against any tax, fee or other charge imposed on or assessed against the cash or
non-callable Government Securities deposited pursuant to Section 8.04 hereof or
the principal and interest received in respect thereof other than any such tax,
fee or other charge which by law is for the account of the Holders of the
outstanding Notes.

          Anything in this Article 8 to the contrary notwithstanding, the
Trustee shall deliver or pay to the Company from time to time upon the request
of the Company any money or non-callable Government Securities held by it as
provided in Section 8.04 hereof which, in the opinion of a nationally recognized
firm of independent public accountants expressed in a written certification
thereof delivered to the Trustee (which may be the opinion delivered under
Section 8.04(a) hereof), are in excess of the amount thereof that would then be
required to be deposited to effect an equivalent Legal Defeasance or Covenant
Defeasance.

SECTION 8.06.  REPAYMENT TO COMPANY.

          Any money deposited with the Trustee or any Paying Agent, or then held
by the Company, in trust for the payment of the principal of, premium, if any,
or interest on any Note and remaining unclaimed for two years after such
principal, and premium, if any, or interest has become due and payable shall be
paid to the Company on its request or (if then held by the Company) shall be
discharged from such trust; and the Holder of such Note shall thereafter, as a
secured creditor, look only to the Company for payment thereof, and all
liability of the Trustee or such Paying Agent with respect to such trust money,
and all liability of the Company as trustee thereof, shall thereupon cease;
provided, however, that the Trustee or such Paying Agent, before being required
to make any such repayment, may at the expense of the Company cause to be
published once, in the New York Times and The Wall Street Journal (national
edition), notice that such money remains unclaimed and that, after a date
specified therein, which shall not be less than 30 days from the date of such
notification or publication, any unclaimed balance of such money then remaining
will be repaid to the Company.

SECTION 8.07.  REINSTATEMENT.

          If the Trustee or Paying Agent is unable to apply any United States
dollars or non-callable Government Securities in accordance with Section 8.02 or
8.03 hereof, as the case may be, by reason of any order or judgment of any court
or governmental authority enjoining, restraining or otherwise prohibiting such
application, then the Company's obligations under this Indenture and the Notes
shall be revived and reinstated as though no deposit had occurred pursuant to
Section 8.02 or 8.03 hereof until such time as the Trustee or Paying Agent is
permitted to apply all such money in accordance 

                                       63

 
with Section 8.02 or 8.03 hereof, as the case may be; provided, however, that,
if the Company makes any payment of principal of, premium, if any, or interest
on any Note following the reinstatement of its obligations, the Company shall be
subrogated to the rights of the Holders of such Notes to receive such payment
from the money held by the Trustee or Paying Agent.

                                   ARTICLE 9.
                        AMENDMENT, SUPPLEMENT AND WAIVER

SECTION 9.01.  WITHOUT CONSENT OF HOLDERS OF NOTES.

          Notwithstanding Section 9.02 of this Indenture, the Company, the
Guarantors and the Trustee may amend or supplement this Indenture, the Note
Guarantees or the Notes without the consent of any Holder of a Note:

               (a) to cure any ambiguity, defect or inconsistency;

               (b) to provide for uncertificated Notes in addition to or in
     place of certificated Notes or to alter the provisions of Article 2 hereof
     (including the related definitions) in a manner that does not materially
     adversely affect any Holder;

               (c) to provide for the assumption of the Company's or a
     Guarantor's obligations to the Holders of the Notes by a successor to the
     Company or a Guarantor pursuant to Article 5, Article 11 or Article 12
     hereof;

               (d) to make any change that would provide any additional rights
     or benefits to the Holders of the Notes or that does not adversely affect
     the legal rights hereunder of any Holder of the Note;

               (e) to comply with requirements of the SEC in order to effect or
     maintain the qualification of this Indenture under the TIA; or

               (f) to allow any Guarantor to execute a supplemental indenture
     and/or a Note Guarantee with respect to the Notes.

          Upon the request of the Company accompanied by a resolution of its
Board of Directors authorizing the execution of any such amended or supplemental
Indenture, and upon receipt by the Trustee of the documents described in Section
7.02 hereof, the Trustee shall join with the Company and the Guarantors in the
execution of any amended or supplemental Indenture authorized or permitted by
the terms of this Indenture and to make any further appropriate agreements and
stipulations that may be therein contained, but the Trustee shall not be
obligated to enter into such amended or supplemental Indenture that affects its
own rights, duties or immunities under this Indenture or otherwise.

SECTION 9.02.  WITH CONSENT OF HOLDERS OF NOTES.

          Except as provided below in this Section 9.02, the Company and the
Trustee may amend or supplement this Indenture (including Section 3.09, 4.10 and
4.15 hereof) , the Note Guarantees and the Notes may be amended or supplemented
with the consent of the Holders of at least a majority in principal amount of
the Notes then outstanding voting as a single class (including consents obtained
in connection with a tender offer or exchange offer for, or purchase of, the
Notes), and, subject to Sections 6.04 and 6.07 hereof, any existing Default or
Event of Default (other than a Default or Event of Default 

                                       64

 
in the payment of the principal of, premium, if any, or interest on the Notes,
except a payment default resulting from an acceleration that has been rescinded)
or compliance with any provision of this Indenture, the Note Guarantees or the
Notes may be waived with the consent of the Holders of a majority in principal
amount of the then outstanding Notes voting as a single class (including
consents obtained in connection with a tender offer or exchange offer for, or
purchase of, the Notes). Section 2.08 hereof shall determine which Notes are
considered to be "outstanding" for purposes of this Section 9.02.

          Upon the request of the Company accompanied by a resolution of its
Board of Directors authorizing the execution of any such amended or supplemental
Indenture, and upon the filing with the Trustee of evidence satisfactory to the
Trustee of the consent of the Holders of Notes as aforesaid, and upon receipt by
the Trustee of the documents described in Section 7.02 hereof, the Trustee shall
join with the Company in the execution of such amended or supplemental Indenture
unless such amended or supplemental Indenture directly affects the Trustee's own
rights, duties or immunities under this Indenture or otherwise, in which case
the Trustee may in its discretion, but shall not be obligated to, enter into
such amended or supplemental Indenture.

          It shall not be necessary for the consent of the Holders of Notes
under this Section 9.02 to approve the particular form of any proposed amendment
or waiver, but it shall be sufficient if such consent approves the substance
thereof.

          After an amendment, supplement or waiver under this Section becomes
effective, the Company shall mail to the Holders of Notes affected thereby a
notice briefly describing the amendment, supplement or waiver.  Any failure of
the Company to mail such notice, or any defect therein, shall not, however, in
any way impair or affect the validity of any such amended or supplemental
Indenture or waiver.  Subject to Sections 6.04 and 6.07 hereof, the Holders of a
majority in aggregate principal amount of the Notes then outstanding voting as a
single class may waive compliance in a particular instance by the Company with
any provision of this Indenture or the Notes.  However, without the consent of
each Holder affected, an amendment or waiver under this Section 9.02 may not
(with respect to any Notes held by a non-consenting Holder):

               (a) reduce the principal amount of Notes whose Holders must
     consent to an amendment, supplement or waiver;

               (b) reduce the principal of or change the fixed maturity of any
     Note or alter or waive any of the provisions with respect to the redemption
     of the Notes except as provided above with respect to Sections 3.09, 4.10
     and 4.15 hereof;

               (c) reduce the rate of or change the time for payment of
     interest, including default interest, on any Note;

               (d) waive a Default or Event of Default in the payment of
     principal of or premium, if any, or interest or Liquidated Damages, if any,
     on the Notes (except a rescission of acceleration of the Notes by the
     Holders of at least a majority in aggregate principal amount of the then
     outstanding Notes and a waiver of the payment default that resulted from
     such acceleration;

               (e) make any Note payable in money other than that stated in the
     Notes;

                                       65

 
               (f) make any change in the provisions of this Indenture relating
     to waivers of past Defaults or the rights of Holders of Notes to receive
     payments of principal of or premium, interest or Liquidated Damages, if
     any, on the Notes;

               (g) waive a redemption payment with respect to any Note (other
     than a payment required by either of Section 4.10 or 4.15 hereof);

               (h) release any Guarantor from any of its obligations under its
     Note Guarantee or this Indenture, except in accordance with the terms of
     this Indenture; or

               (i) make any change in the foregoing amendment and waiver
     provisions.

SECTION 9.03.  COMPLIANCE WITH TRUST INDENTURE ACT.

          Every amendment or supplement to this Indenture or the Notes shall be
set forth in a amended or supplemental Indenture that complies with the TIA as
then in effect.

SECTION 9.04.  REVOCATION AND EFFECT OF CONSENTS.

          Until an amendment, supplement or waiver becomes effective, a consent
to it by a Holder of a Note is a continuing consent by the Holder of a Note and
every subsequent Holder of a Note or portion of a Note that evidences the same
debt as the consenting Holder's Note, even if notation of the consent is not
made on any Note.  However, any such Holder of a Note or subsequent Holder of a
Note may revoke the consent as to its Note if the Trustee receives written
notice of revocation before the date the waiver, supplement or amendment becomes
effective.  An amendment, supplement or waiver becomes effective in accordance
with its terms and thereafter binds every Holder.

SECTION 9.05.  NOTATION ON OR EXCHANGE OF NOTES.

          The Trustee may place an appropriate notation about an amendment,
supplement or waiver on any Note thereafter authenticated.  The Company in
exchange for all Notes may issue and the Trustee shall, upon receipt of a Note
Authentication Order, authenticate new Notes that reflect the amendment,
supplement or waiver.

          Failure to make the appropriate notation or issue a new Note shall not
affect the validity and effect of such amendment, supplement or waiver.

SECTION 9.06.  TRUSTEE TO SIGN AMENDMENTS, ETC.

          The Trustee shall sign any amended or supplemental Indenture
authorized pursuant to this Article 9 if the amendment or supplement does not
adversely affect the rights, duties, liabilities or immunities of the Trustee.
The Company may not sign an amendment or supplemental Indenture until the Board
of Directors approves it.  In executing any amended or supplemental indenture,
the Trustee shall be entitled to receive and (subject to Section 7.01 hereof)
shall be fully protected in relying upon, in addition to the documents required
by Section 13.04 hereof, an Officer's Certificate and an Opinion of Counsel
stating that the execution of such amended or supplemental indenture is
authorized or permitted by this Indenture.

                                       66

 
                                  ARTICLE 10.
                      SUBORDINATION OF HOLDINGS GUARANTEE

SECTION 10.01. AGREEMENT TO SUBORDINATE.

          Holdings agrees, and each Holder by accepting a Note agrees, that the
Obligations of Holdings with respect to its Holdings Guarantee is subordinated
in right of payment, to the extent and in the manner provided in this Article
10, to the prior payment in full of all Senior Indebtedness of Holdings (whether
outstanding on the date hereof or hereafter created, incurred, assumed or
guaranteed), and that the subordination is for the benefit of the holders of
Senior Indebtedness of Holdings.

SECTION 10.02. CERTAIN DEFINITIONS.

          "Designated Senior Indebtedness" means (i) any Indebtedness
outstanding under the Term Loan Facility or (ii) any other Senior Indebtedness
of Holdings the principal amount of which is $25.0 million or more and that has
been designated by Holdings as "Designated Senior Indebtedness."

          "Permitted Junior Securities" means Equity Interests in Holdings or
debt securities that are subordinated to all Senior Indebtedness of Holdings
(and any debt securities issued in exchange for Senior Indebtedness of Holdings)
to substantially the same extent as, or to a greater extent than, the Holdings
Guarantee is subordinated to Senior Indebtedness of Holdings pursuant to this
Indenture.

          "Representative" means the indenture trustee or other trustee, agent
or representative for any Senior Indebtedness.

          "Senior Indebtedness" means Obligations with respect to the Term Loan
Facility and any other Indebtedness of Holdings now or hereafter incurred except
such Indebtedness specifically designated by Holdings as subordinated
Indebtedness at the time of its incurrence.  Notwithstanding anything to the
contrary in the foregoing, Senior Indebtedness will not include (w) any
liability for federal, state, local or other taxes owed or owing by Holdings,
(x) any Indebtedness of Holdings to any of its Subsidiaries or other Affiliates,
(y) any trade payables or (z) any Indebtedness that is incurred in violation of
this Indenture.

          A distribution may consist of cash, securities or other property, by
set-off or otherwise.

SECTION 10.03. LIQUIDATION; DISSOLUTION; BANKRUPTCY.

          Upon any distribution to creditors of Holdings in a liquidation or
dissolution of Holdings or in a bankruptcy, reorganization, insolvency,
receivership or similar proceeding relating to Holdings or its property, in an
assignment for the benefit of creditors or any marshaling of Holding's assets
and liabilities:

          (1) holders of Senior Indebtedness of Holdings shall be entitled to
receive payment in full in cash of all Obligations due in respect of such Senior
Indebtedness of Holdings (including interest after the commencement of any such
proceeding at the rate specified in the instrument governing the applicable
Senior Indebtedness of Holdings, whether or not an allowable claim in such
proceeding) before Holders of the Notes shall be entitled to receive any payment
with respect to the Holdings Guarantee (except that Holders may receive (i)
Permitted Junior Securities and (ii) payments and other distributions made from
any defeasance trust created pursuant to Section 8.01 hereof); and

                                       67

 
          (2) until all Obligations with respect to Senior Indebtedness of
Holdings (as provided in subsection (1) above) are paid in full in cash, any
distribution to which Holders would be entitled but for this Article 10 shall be
made to holders of Senior Indebtedness of Holdings (except that Holders of Notes
may receive (i) Permitted Junior Securities and (ii) payments and other
distributions made from any defeasance trust created pursuant to Section 8.01
hereof), as their interests may appear.

SECTION 10.04. DEFAULT ON DESIGNATED SENIOR INDEBTEDNESS.

          Holdings may not make any payment or distribution to the Trustee or
any Holder in respect of Obligations with respect to the Holdings Guarantee and
may not acquire from the Trustee or any Holder any Notes for cash or property
(other than (i) Permitted Junior Securities and (ii) payments and other
distributions made from any defeasance trust created pursuant to Section 8.01
hereof) until all principal and other Obligations with respect to the Senior
Indebtedness of Holdings have been paid in full if:

     (i) a default in the payment of any principal or other Obligations with
  respect to Designated Senior Indebtedness of Holdings occurs and is continuing
  beyond any applicable grace period in the agreement, indenture or other
  document governing such Designated Senior Indebtedness of Holdings; or

     (ii) a default, other than a payment default, on Designated Senior
  Indebtedness of Holdings occurs and is continuing that then permits holders of
  such Designated Senior Indebtedness to accelerate its maturity and the Trustee
  receives a notice of the default (a "PAYMENT BLOCKAGE NOTICE") from a Person
  who may give it pursuant to Section 10.12 hereof.  If the Trustee receives any
  such Payment Blockage Notice, no subsequent Payment Blockage Notice shall be
  effective for purposes of this Section unless and until (i) at least 360 days
  shall have elapsed since the effectiveness of the immediately prior Payment
  Blockage Notice and (ii) all scheduled payments of principal, premium, if any,
  and interest on the Notes that have come due have been paid in full in cash.
  No nonpayment default that existed or was continuing on the date of delivery
  of any Payment Blockage Notice to the Trustee shall be, or be made, the basis
  for a subsequent Payment Blockage Notice unless such default shall have been
  waived for a period of not less than 180 days.

          Holdings may and shall resume payments on and distributions in respect
of the Holdings Guarantee and may acquire them upon the earlier of:

          (1) the date upon which the default is cured or waived, or

          (2) in the case of a default referred to in Section 10.04(ii) hereof,
179 days pass after notice is received if the maturity of such Designated Senior
Indebtedness has not been accelerated,

if this Article 10 otherwise permits the payment, distribution or acquisition at
the time of such payment or acquisition.

SECTION 10.05. ACCELERATION OF NOTES.

          If payment of the Notes is accelerated because of an Event of Default,
Holdings shall promptly notify holders of Senior Indebtedness of Holdings of the
acceleration.

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SECTION 10.06. WHEN DISTRIBUTION MUST BE PAID OVER.

          In the event that the Trustee or any Holder receives any payment of
any Obligations with respect to the Holdings Guarantee at a time when the
Trustee or such Holder, as applicable, has actual knowledge that such payment is
prohibited by Article 10 hereof, such payment shall be held by the Trustee or
such Holder, in trust for the benefit of, and shall be paid forthwith over and
delivered, upon written request, to, the holders of Senior Indebtedness of
Holdings as their interests may appear or their Representative under the
indenture or other agreement (if any) pursuant to which Senior Indebtedness of
Holdings may have been issued, as their respective interests may appear, for
application to the payment of all Obligations with respect to Senior
Indebtedness of Holdings remaining unpaid to the extent necessary to pay such
Obligations in full in accordance with their terms, after giving effect to any
concurrent payment or distribution to or for the holders of Senior Indebtedness
of Holdings.

          With respect to the holders of Senior Indebtedness of Holdings, the
Trustee undertakes to perform only such obligations on the part of the Trustee
as are specifically set forth in this Article 10, and no implied covenants or
obligations with respect to the holders of Senior Indebtedness of Holdings shall
be read into this Indenture against the Trustee.  The Trustee shall not be
deemed to owe any fiduciary duty to the holders of Senior Indebtedness of
Holdings, and shall not be liable to any such holders if the Trustee shall pay
over or distribute to or on behalf of Holders or Holdings or any other Person
money or assets to which any holders of Senior Indebtedness of Holdings shall be
entitled by virtue of this Article 10, except if such payment is made as a
result of the willful misconduct or gross negligence of the Trustee.

SECTION 10.07. NOTICE BY COMPANY.

          Holdings shall promptly notify the Trustee and the Paying Agent of any
facts known to Holdings that would cause a payment of any Obligations with
respect to Holdings Guarantee to violate this Article 10, but failure to give
such notice shall not affect the subordination of the Holdings Guarantee to the
Senior Indebtedness of Holdings as provided in this Article 10.

SECTION 10.08. SUBROGATION.

          After all Senior Indebtedness of Holdings is paid in full and until
the Notes are paid in full, Holders of Notes shall be subrogated (equally and
ratably with all other Indebtedness pari passu with the Notes) to the rights of
holders of Senior Indebtedness of Holdings to receive distributions applicable
to Senior Indebtedness of Holdings to the extent that distributions otherwise
payable to the Holders of Notes have been applied to the payment of Senior
Indebtedness of Holdings.  A distribution made under this Article 10 to holders
of Senior Indebtedness of Holdings that otherwise would have been made to
Holders of Notes is not, as between Holdings and Holders, a payment by Holdings
on the Notes.

SECTION 10.09. RELATIVE RIGHTS.

          This Article 10 defines the relative rights of Holders of Notes and
holders of Senior Indebtedness of Holdings.  Nothing in this Indenture shall:

          (1) impair, as between the Company and Holders of Notes, the
obligation of the Company, which is absolute and unconditional, to pay principal
of and interest on the Notes in accordance with their terms;

                                       69

 
          (2) affect the relative rights of Holders of Notes and creditors of
the Company other than their rights in relation to holders of Senior
Indebtedness; or

          (3) prevent the Trustee or any Holder of Notes from exercising its
available remedies upon a Default or Event of Default, subject to the rights of
holders and owners of Senior Indebtedness to receive distributions and payments
otherwise payable to Holders of Notes.

          If the Company fails because of this Article 10 to pay principal of or
interest on a Note on the due date, the failure is still a Default or Event of
Default.

SECTION 10.10.  SUBORDINATION MAY NOT BE IMPAIRED BY COMPANY.

          No right of any holder of Senior Indebtedness of Holdings to enforce
the subordination of the Indebtedness evidenced by the Holdings Guarantee shall
be impaired by any act or failure to act by Holdings or any Holder or by the
failure of Holdings or any Holder to comply with this Indenture.

SECTION 10.11.  DISTRIBUTION OR NOTICE TO REPRESENTATIVE.

          Whenever a distribution is to be made or a notice given to holders of
Senior Indebtedness of Holdings, the distribution may be made and the notice
given to their Representative.

          Upon any payment or distribution of assets of Holdings referred to in
this Article 10, the Trustee and the Holders of Notes shall be entitled to rely
upon any order or decree made by any court of competent jurisdiction or upon any
certificate of such Representative or of the liquidating trustee or agent or
other Person making any distribution to the Trustee or to the Holders of Notes
for the purpose of ascertaining the Persons entitled to participate in such
distribution, the holders of the Senior Indebtedness of Holdings and other
Indebtedness of Holdings, the amount thereof or payable thereon, the amount or
amounts paid or distributed thereon and all other facts pertinent thereto or to
this Article 10.

SECTION 10.12.  RIGHTS OF TRUSTEE AND PAYING AGENT.

          Notwithstanding the provisions of this Article 10 or any other
provision of this Indenture, the Trustee shall not be charged with knowledge of
the existence of any facts that would prohibit the making of any payment or
distribution by the Trustee, and the Trustee and the Paying Agent may continue
to make payments on the Notes, unless the Trustee shall have received at its
Corporate Trust Office at least five Business Days prior to the date of such
payment written notice of facts that would cause the payment of any Obligations
with respect to the Holdings Guarantee to violate this Article 10.  Only
Holdings or a Representative may give the notice.  Nothing in this Article 10
shall impair the claims of, or payments to, the Trustee under or pursuant to
Section 7.07 hereof.

          The Trustee in its individual or any other capacity may hold Senior
Indebtedness of Holdings with the same rights it would have if it were not
Trustee.  Any Agent may do the same with like rights.

SECTION 10.13.  AUTHORIZATION TO EFFECT SUBORDINATION.

          Each Holder of Notes, by the Holder's acceptance thereof, authorizes
and directs the Trustee on such Holder's behalf to take such action as may be
necessary or appropriate to effectuate the subordination as provided in this
Article 10, and appoints the Trustee to act as such Holder's attorney-in-fact
for any and all such purposes.  If the Trustee does not file a proper proof of
claim or 

                                       70

 
proof of debt in the form required in any proceeding referred to in Section 6.09
hereof at least 30 days before the expiration of the time to file such claim,
the credit agents are hereby authorized to file an appropriate claim for and on
behalf of the Holders of the Notes.

SECTION 10.14.  AMENDMENTS.

          The provisions of this Article 10 shall not be amended or modified
without the written consent of the holders of all Senior Indebtedness of
Holdings.

                                  ARTICLE 11.
                             SUBSIDIARY GUARANTEES

SECTION 11.01.  SUBSIDIARY GUARANTEE.

          Subject to this Article 11, each of the Subsidiary Guarantors hereby,
jointly and severally (with Holdings with respect to the Holdings Guarantee),
unconditionally guarantees to each Holder of a Note authenticated and delivered
by the Trustee and to the Trustee and its successors and assigns, irrespective
of the validity and enforceability of this Indenture, the Notes or the
obligations of the Company hereunder or thereunder, that:  (a) the principal of
and interest on the Notes will be promptly paid in full when due, whether at
maturity, by acceleration, redemption or otherwise, and interest on the overdue
principal of and interest on the Notes, if any, if lawful, and all other
obligations of the Company to the Holders or the Trustee hereunder or thereunder
will be promptly paid in full or performed, all in accordance with the terms
hereof and thereof; and (b) in case of any extension of time of payment or
renewal of any Notes or any of such other obligations, that same will be
promptly paid in full when due or performed in accordance with the terms of the
extension or renewal, whether at stated maturity, by acceleration or otherwise.
Failing payment when due of any amount so guaranteed or any performance so
guaranteed for whatever reason, the Guarantors shall be jointly and severally
obligated to pay the same immediately.  Each Subsidiary Guarantor agrees that
this is a guarantee of payment and not a guarantee of collection.

          The Subsidiary Guarantors hereby agree that their obligations
hereunder shall be unconditional, irrespective of the validity, regularity or
enforceability of the Notes or this Indenture, the absence of any action to
enforce the same, any waiver or consent by any Holder of the Notes with respect
to any provisions hereof or thereof, the recovery of any judgment against the
Company, any action to enforce the same or any other circumstance which might
otherwise constitute a legal or equitable discharge or defense of a guarantor.
Each Subsidiary Guarantor hereby waives diligence, presentment, demand of
payment, filing of claims with a court in the event of insolvency or bankruptcy
of the Company, any right to require a proceeding first against the Company,
protest, notice and all demands whatsoever and covenant that this Subsidiary
Guarantee shall not be discharged except by complete performance of the
obligations contained in the Notes and this Indenture.

          If any Holder or the Trustee is required by any court or otherwise to
return to the Company, the  Subsidiary Guarantors or any custodian, trustee,
liquidator or other similar official acting in relation to either the Company or
the Subsidiary Guarantors, any amount paid by either to the Trustee or such
Holder, this Note Guarantee, to the extent theretofore discharged, shall be
reinstated in full force and effect.

          Each Subsidiary Guarantor agrees that it shall not be entitled to any
right of subrogation in relation to the Holders in respect of any obligations
guaranteed hereby until payment in full of all obligations guaranteed hereby.
Each Subsidiary Guarantor further agrees that, as between the Subsidiary

                                       71

 
Guarantors, on the one hand, and the Holders and the Trustee, on the other hand,
(x) the maturity of the obligations guaranteed hereby may be accelerated as
provided in Article 6 hereof for the purposes of this Subsidiary Guarantee,
notwithstanding any stay, injunction or other prohibition preventing such
acceleration in respect of the obligations guaranteed hereby, and (y) in the
event of any declaration of acceleration of such obligations as provided in
Article 6 hereof, such obligations (whether or not due and payable) shall
forthwith become due and payable by the Subsidiary Guarantors for the purpose of
this Subsidiary Guarantee.  The Guarantors shall have the right to seek
contribution from any non-paying Guarantor so long as the exercise of such right
does not impair the rights of the Holders under the Subsidiary Guarantees.

SECTION 11.02.  LIMITATION ON SUBSIDIARY GUARANTOR LIABILITY.

          Each Subsidiary Guarantor, and by its acceptance of Notes, each
Holder, hereby confirms that it is the intention of all such parties that the
Subsidiary Guarantee of such Guarantor not constitute a fraudulent transfer or
conveyance for purposes of Bankruptcy Law, the Uniform Fraudulent Conveyance
Act, the Uniform Fraudulent Transfer Act or any similar federal or state law to
the extent applicable to any Subsidiary Guarantee.  To effectuate the foregoing
intention, the Trustee, the Holders and the Subsidiary Guarantors hereby
irrevocably agree that the obligations of such Subsidiary Guarantor under its
Subsidiary Guarantee and this Article 11 shall be limited to the maximum amount
as will, after giving effect to such maximum amount and all other contingent and
fixed liabilities of such Subsidiary Guarantor that are relevant under such
laws, and after giving effect to any collections from, rights to receive
contribution from or payments made by or on behalf of any other Guarantor in
respect of the obligations of such other Guarantor under this Article 11 or
Article 12 hereto, result in the obligations of such Guarantor under its Note
Guarantee not constituting a fraudulent transfer or conveyance.

SECTION 11.03.  EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTEE.

          To evidence its Subsidiary Guarantee set forth in Section 11.01, each
Subsidiary Guarantor hereby agrees that a notation of such Subsidiary Guarantee
substantially in the form included in Exhibit E shall be endorsed by an Officer
of such Subsidiary Guarantor on each Note authenticated and delivered by the
Trustee and that this Indenture shall be executed on behalf of such Guarantor by
its President or one of its Vice Presidents.

          Each Subsidiary Guarantor hereby agrees that its Subsidiary Guarantee
set forth in Section 11.01 shall remain in full force and effect notwithstanding
any failure to endorse on each Note a notation of such Subsidiary Guarantee.

          If an Officer whose signature is on this Indenture or on the
Subsidiary Guarantee no longer holds that office at the time the Trustee
authenticates the Note on which a Subsidiary Guarantee is endorsed, the
Subsidiary Guarantee shall be valid nevertheless.

          The delivery of any Note by the Trustee, after the authentication
thereof hereunder, shall constitute due delivery of the Subsidiary Guarantee set
forth in this Indenture on behalf of the Subsidiary Guarantors.

          In the event that the Company creates or acquires any new Subsidiaries
subsequent to the date of this Indenture, if required by Section 4.19 hereof,
the Company shall cause such Subsidiaries to execute supplemental indentures to
this Indenture and Note Guarantees in accordance with Section 4.19 hereof and
this Article 11, to the extent applicable.

                                       72

 
SECTION 11.04.  SUBSIDIARY GUARANTORS MAY CONSOLIDATE, ETC., ON CERTAIN TERMS.

          No Subsidiary Guarantor may consolidate with or merge with or into
(whether or not such Subsidiary Guarantor is the surviving Person) another
Person whether or not affiliated with such Subsidiary Guarantor unless:

          (a) subject to Section 11.05 hereof, the Person formed by or surviving
any such consolidation or merger (if other than a Subsidiary Guarantor or the
Company) unconditionally assumes all the obligations of such Subsidiary
Guarantor, pursuant to a supplemental indenture in form and substance reasonably
satisfactory to the Trustee, under the Unit Agreement, the Notes, this
Indenture, the Pledge Agreement, the Debt Registration Rights Agreement and the
Subsidiary Guarantee on the terms set forth herein or therein;

          (b) immediately after giving effect to such transaction, no Default or
Event of Default exists; and

          (c) except in the case of any such merger or consolidation with the
Company or another Subsidiary Guarantor, the Company would be permitted by
virtue of the Company's Debt to Cash Flow Ratio, immediately after giving effect
to such transaction, to incur at least $1.00 of additional Indebtedness pursuant
to the Debt to Cash Flow Ratio test set forth in the first paragraph of Section
4.09 hereof.

          In case of any such consolidation, merger, sale or conveyance and upon
the assumption by the successor Person, by supplemental indenture, executed and
delivered to the Trustee and satisfactory in form to the Trustee, of the
Subsidiary Guarantee endorsed upon the Notes and the due and punctual
performance of all of the covenants and conditions of this Indenture to be
performed by the Subsidiary Guarantor, such successor Person shall succeed to
and be substituted for the Subsidiary Guarantor with the same effect as if it
had been named herein as a Subsidiary Guarantor.  Such successor Person
thereupon may cause to be signed any or all of the Subsidiary Guarantees to be
endorsed upon all of the Notes issuable hereunder which theretofore shall not
have been signed by the Company and delivered to the Trustee.  All the
Subsidiary Guarantees so issued shall in all respects have the same legal rank
and benefit under this Indenture as the Subsidiary Guarantees theretofore and
thereafter issued in accordance with the terms of this Indenture as though all
of such Subsidiary Guarantees had been issued at the date of the execution
hereof.

          Except as set forth in Articles 4 and 5 hereof, and notwithstanding
clauses (a) and (b) above, nothing contained in this Indenture or in any of the
Notes shall prevent any consolidation or merger of a Subsidiary Guarantor with
or into the Company or another Subsidiary Guarantor, or shall prevent any sale
or conveyance of the property of a Subsidiary Guarantor as an entirety or
substantially as an entirety to the Company or another Subsidiary Guarantor.

SECTION 11.05.  RELEASES FOLLOWING SALE OF ASSETS.

          In the event of a sale or other disposition of all of the assets of
any Subsidiary Guarantor, by way of merger, consolidation or otherwise, or a
sale or other disposition of all of the capital stock of any Subsidiary
Guarantor, then such Subsidiary Guarantor (in the event of a sale or other
disposition, by way of merger, consolidation or otherwise, of all of the capital
stock of such Subsidiary Guarantor) or the corporation acquiring the property
(in the event of a sale or other disposition of all or substantially all of the
assets of such Subsidiary Guarantor) will be released and relieved of any
obligations under its 

                                       73

 
Subsidiary Guarantee; provided that the Net Proceeds of such sale or other
disposition are applied in accordance with the applicable provisions of this
Indenture, including without limitation Section 4.10 hereof. Upon delivery by
the Company to the Trustee of an Officers' Certificate and an Opinion of Counsel
to the effect that such sale or other disposition was made by the Company in
accordance with the applicable provisions of this Indenture, including without
limitation Section 4.10 hereof, the Trustee shall execute any documents
reasonably required in order to evidence the release of any Subsidiary Guarantor
from its obligations under its Subsidiary Guarantee.

          Any Subsidiary Guarantor not released from its obligations under its
Subsidiary Guarantee shall remain liable for the full amount of principal of and
interest on the Notes and for the other obligations of any Subsidiary Guarantor
under this Indenture as provided in this Article 11.

                                  ARTICLE 12.
                               HOLDINGS GUARANTEE

SECTION 12.01.  HOLDINGS GUARANTEE.

          Subject to this Article 12, Holdings hereby, jointly and severally
(with the Subsidiary Guarantors, with respect to their Subsidiary Guarantees),
unconditionally guarantees to each Holder of a Note authenticated and delivered
by the Trustee and to the Trustee and its successors and assigns, irrespective
of the validity and enforceability of this Indenture, the Notes or the
obligations of the Company hereunder or thereunder, that:  (a) the principal of
and interest on the Notes will be promptly paid in full when due, whether at
maturity, by acceleration, redemption or otherwise, and interest on the overdue
principal of and interest on the Notes, if any, if lawful, and all other
obligations of the Company to the Holders or the Trustee hereunder or thereunder
will be promptly paid in full or performed, all in accordance with the terms
hereof and thereof; and (b) in case of any extension of time of payment or
renewal of any Notes or any of such other obligations, that same will be
promptly paid in full when due or performed in accordance with the terms of the
extension or renewal, whether at stated maturity, by acceleration or otherwise.
Failing payment when due of any amount so guaranteed or any performance so
guaranteed for whatever reason, the Guarantors shall be jointly and severally
obligated to pay the same immediately.  Holdings agrees that this is a guarantee
of payment and not a guarantee of collection.

          Holdings hereby agrees that its obligation hereunder shall be
unconditional, irrespective of the validity, regularity or enforceability of the
Notes or this Indenture, the absence of any action to enforce the same, any
waiver or consent by any Holder of the Notes with respect to any provisions
hereof or thereof, the recovery of any judgment against the Company, any action
to enforce the same or any other circumstance which might otherwise constitute a
legal or equitable discharge or defense of a guarantor.  Holdings hereby waives
diligence, presentment, demand of payment, filing of claims with a court in the
event of insolvency or bankruptcy of the Company, any right to require a
proceeding first against the Company, protest, notice and all demands whatsoever
and covenant that this Holdings Guarantee shall not be discharged except by
complete performance of the obligations contained in the Notes and this
Indenture.

          If any Holder or the Trustee is required by any court or otherwise to
return to the Company, Holdings or any custodian, trustee, liquidator or other
similar official acting in relation to either the Company or Holdings, any
amount paid by either to the Trustee or such Holder, this Holdings Guarantee, to
the extent theretofore discharged, shall be reinstated in full force and effect.

          Holdings agrees that it shall not be entitled to any right of
subrogation in relation to the Holders in respect of any obligations 

                                       74

 
guaranteed hereby until payment in full of all obligations guaranteed hereby.
Holdings further agrees that, as between Holdings, on the one hand, and the
Holders and the Trustee, on the other hand, (x) the maturity of the obligations
guaranteed hereby may be accelerated as provided in Article 6 hereof for the
purposes of this Note Guarantee, notwithstanding any stay, injunction or other
prohibition preventing such acceleration in respect of the obligations
guaranteed hereby, and (y) in the event of any declaration of acceleration of
such obligations as provided in Article 6 hereof, such obligations (whether or
not due and payable) shall forthwith become due and payable by Holdings for the
purpose of this Holdings Guarantee. Holdings shall have the right to seek
contribution from any non-paying Guarantor so long as the exercise of such right
does not impair the rights of the Holders under the Holdings Guarantee.

SECTION 12.02.  LIMITATION ON HOLDINGS LIABILITY.

          Holdings, and by its acceptance of Notes, each Holder, hereby confirms
that it is the intention of all such parties that the Holdings Guarantee of such
Holdings not constitute a fraudulent transfer or conveyance for purposes of
Bankruptcy Law, the Uniform Fraudulent Conveyance Act, the Uniform Fraudulent
Transfer Act or any similar federal or state law to the extent applicable to the
Holdings Guarantee.  To effectuate the foregoing intention, the Trustee, the
Holders and Holdings hereby irrevocably agree that the obligations of Holdings
under its Holdings Guarantee and this Article 12 shall be limited to the maximum
amount as will, after giving effect to such maximum amount and all other
contingent and fixed liabilities of Holdings that are relevant under such laws,
and after giving effect to any collections from, rights to receive contribution
from or payments made by or on behalf of any other Guarantor in respect of the
obligations of such other Guarantor under Article 11 hereof, result in the
obligations of such Guarantor under its Note Guarantee not constituting a
fraudulent transfer or conveyance.

SECTION 12.03.  EXECUTION AND DELIVERY OF HOLDINGS GUARANTEE.

          To evidence its Holdings Guarantee set forth in Section 12.01,
Holdings hereby agrees that a notation of such Holdings Guarantee substantially
in the form included in Exhibit E shall be endorsed by an Officer of Holdings on
each Note authenticated and delivered by the Trustee and that this Indenture
shall be executed on behalf of Holdings by its President or one of its Vice
Presidents.

          Holdings hereby agrees that its Holdings Guarantee set forth in
Section 12.01 shall remain in full force and effect notwithstanding any failure
to endorse on each Note a notation of such Holdings Guarantee.

          If an Officer whose signature is on this Indenture or on the Holdings
Guarantee no longer holds that office at the time the Trustee authenticates the
Holdings on which a Holdings Guarantee is endorsed, the Holdings Guarantee shall
be valid nevertheless.

          The delivery of any Note by the Trustee, after the authentication
thereof hereunder, shall constitute due delivery of the Holdings Guarantee set
forth in this Indenture on behalf of Holdings.

SECTION 12.04.  HOLDINGS MAY CONSOLIDATE, ETC., ON CERTAIN TERMS.

          Holdings may not consolidate with or merge with or into (whether or
not such Guarantor is the surviving Person) another Person whether or not
affiliated with such Guarantor unless:

          (a) the Person formed by or surviving any such consolidation or merger
(if other than a Guarantor or the Company) unconditionally assumes all the
obligations of such Guarantor, 

                                       75

 
pursuant to a supplemental indenture in form and substance reasonably
satisfactory to the Trustee, under the Notes, the Unit Agreement, this
Indenture, the Pledge Agreement, the Debt Registration Rights Agreement, the
Warrant Registration Rights Agreement, and the Holdings Guarantee on the terms
set forth herein or therein; and

          (b) immediately after giving effect to such transaction, no Default or
Event of Default exists.

          In case of any such consolidation, merger, sale or conveyance and upon
the assumption by the successor Person, by supplemental indenture, executed and
delivered to the Trustee and satisfactory in form to the Trustee, of the
Holdings Guarantee endorsed upon the Notes and the due and punctual performance
of all of the covenants and conditions of this Indenture to be performed by
Holdings, such successor Person shall succeed to and be substituted for Holdings
with the same effect as if it had been named herein as a Guarantor.  Such
successor Person thereupon may cause to be signed Holdings Guarantee to be
endorsed upon all of the Notes issuable hereunder which theretofore shall not
have been signed by the Company and delivered to the Trustee.  All the Holdings
Guarantee so issued shall in all respects have the same legal rank and benefit
under this Indenture as the Holdings Guarantee theretofore and thereafter issued
in accordance with the terms of this Indenture as though all of such Holdings
Guarantee had been issued at the date of the execution hereof.

          Except as set forth in Articles 4 and 5 hereof, and notwithstanding
clauses (a) and (b) above, nothing contained in this Indenture or in any of the
Notes shall prevent any consolidation or merger of Holdings with or into the
Company or another Guarantor, or shall prevent any sale or conveyance of the
property of a Guarantor as an entirety or substantially as an entirety to the
Company or another Guarantor.

SECTION 12.05.  SUBORDINATION.

          The Holdings Guarantee is subordinated as set forth in Article 10.

                                  ARTICLE 13.
                                 MISCELLANEOUS

SECTION 13.01.  TRUST INDENTURE ACT CONTROLS.

          If any provision of this Indenture limits, qualifies or conflicts with
the duties imposed by TIA (S) 318(c), the imposed duties shall control.

SECTION 13.02.  NOTICES.

          Any notice or communication by the Company, any Guarantor or the
Trustee to the others is duly given if in writing and delivered in Person or
mailed by first class mail (registered or certified, return receipt requested),
telex, telecopier or overnight air courier guaranteeing next day delivery, to
the others' address

          If to the Company and/or any Guarantor:

          AMSC Acquisition Company, Inc.
          10802 Parkridge Blvd.
          Reston, VA  20191-5416

                                       76

 
          Telecopier No.:  703-758-6134
          Attention:  Randy S. Segal, Esq.

          With a copy to:

          Arnold & Porter
          555 12th Street, N.W.
          Washington, D.C.  20004-1202
          Telecopier No.:  202-942-5999
          Attention:  Richard E. Baltz, Esq.

          If to the Trustee:

          State Street Bank and Trust Company
          Goodwin Square
          225 Asylum Street
          Hartford, Connecticut 06103
          Telecopier No.: (860) 244-1897
          Attention: Steven Cimalore

          The Company, any Guarantor or the Trustee, by notice to the others may
designate additional or different addresses for subsequent notices or
communications.

          All notices and communications (other than those sent to Holders)
shall be deemed to have been duly given: at the time delivered by hand, if
personally delivered; five Business Days after being deposited in the mail,
postage prepaid, if mailed; when answered back, if telexed; when receipt
acknowledged, if telecopied; and the next Business Day after timely delivery to
the courier, if sent by overnight air courier guaranteeing next day delivery.

          Any notice or communication to a Holder shall be mailed by first class
mail, certified or registered, return receipt requested, or by overnight air
courier guaranteeing next day delivery to its address shown on the register kept
by the Registrar.  Any notice or communication shall also be so mailed to any
Person described in TIA (S) 313(c), to the extent required by the TIA.  Failure
to mail a notice or communication to a Holder or any defect in it shall not
affect its sufficiency with respect to other Holders.

          If a notice or communication is mailed in the manner provided above
within the time prescribed, it is duly given, whether or not the addressee
receives it.

          If the Company mails a notice or communication to Holders, it shall
mail a copy to the Trustee and each Agent at the same time.

SECTION 13.03.  COMMUNICATION BY HOLDERS OF NOTES WITH OTHER HOLDERS OF NOTES.

          Holders may communicate pursuant to TIA (S) 312(b) with other Holders
with respect to their rights under this Indenture or the Notes.  The Company,
the Trustee, the Registrar and anyone else shall have the protection of TIA (S)
312(c).

SECTION 13.04.  CERTIFICATE AND OPINION AS TO CONDITIONS PRECEDENT.

                                       77

 
          Upon any request or application by the Company to the Trustee to take
any action under this Indenture, the Company shall furnish to the Trustee:

          (a) an Officers' Certificate in form and substance reasonably
satisfactory to the Trustee (which shall include the statements set forth in
Section 13.05 hereof) stating that, in the opinion of the signers, all
conditions precedent and covenants, if any, provided for in this Indenture
relating to the proposed action have been satisfied; and

          (b) an Opinion of Counsel in form and substance reasonably
satisfactory to the Trustee (which shall include the statements set forth in
Section 13.05 hereof) stating that, in the opinion of such counsel, all such
conditions precedent and covenants have been satisfied.

SECTION 13.05.  STATEMENTS REQUIRED IN CERTIFICATE OR OPINION.

          Each certificate or opinion with respect to compliance with a
condition or covenant provided for in this Indenture (other than a certificate
provided pursuant to TIA (S) 314(a)(4)) shall comply with the provisions of TIA
(S) 314(e) and shall include:

          (a) a statement that the Person making such certificate or opinion has
read such covenant or condition;

          (b) a brief statement as to the nature and scope of the examination or
investigation upon which the statements or opinions contained in such
certificate or opinion are based;

          (c) a statement that, in the opinion of such Person, he or she has
made such examination or investigation as is necessary to enable him to express
an informed opinion as to whether or not such covenant or condition has been
satisfied; and

          (d) a statement as to whether or not, in the opinion of such Person,
such condition or covenant has been satisfied.

SECTION 13.06.  RULES BY TRUSTEE AND AGENTS.

          The Trustee may make reasonable rules for action by or at a meeting of
Holders. The Registrar or Paying Agent may make reasonable rules and set
reasonable requirements for its functions.

SECTION 13.07.  NO PERSONAL LIABILITY OF DIRECTORS, OFFICERS, EMPLOYEES AND
                STOCKHOLDERS.

          No past, present or future director, officer, employee, incorporator
or stockholder of the Company or any Guarantor, as such, shall have any
liability for any obligations of the Company or such Guarantor under the Notes,
the Note Guarantees, this Indenture or for any claim based on, in respect of, or
by reason of, such obligations or their creation.  Each Holder by accepting a
Note waives and releases all such liability.  The waiver and release are part of
the consideration for issuance of the Notes.

SECTION 13.08.  GOVERNING LAW.

          THE INTERNAL LAW OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO
CONSTRUE THIS INDENTURE, THE NOTES AND THE NOTE GUARANTEES WITHOUT GIVING EFFECT
TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE 

                                       78

 
EXTENT THAT THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE
REQUIRED THEREBY.

SECTION 13.09.  NO ADVERSE INTERPRETATION OF OTHER AGREEMENTS.

          This Indenture may not be used to interpret any other indenture, loan
or debt agreement of the Company or its Subsidiaries or of any other Person.
Any such indenture, loan or debt agreement may not be used to interpret this
Indenture.

SECTION 13.10.  SUCCESSORS.

          All agreements of the Company in this Indenture and the Notes shall
bind its successors.  All agreements of the Trustee in this Indenture shall bind
its successors.

SECTION 13.11.  SEVERABILITY.

          In case any provision in this Indenture or in the Notes shall be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not in any way be affected or impaired thereby.

SECTION 13.12.  COUNTERPART ORIGINALS.

          The parties may sign any number of copies of this Indenture.  Each
signed copy shall be an original, but all of them together represent the same
agreement.

SECTION 13.13.  TABLE OF CONTENTS, HEADINGS, ETC.

          The Table of Contents, Cross-Reference Table and Headings of the
Articles and Sections of this Indenture have been inserted for convenience of
reference only, are not to be considered a part of this Indenture and shall in
no way modify or restrict any of the terms or provisions hereof.

                         [Signatures on following page]

                                       79

 
                                  SIGNATURES
Dated as of March 31, 1998


                              AMSC ACQUISITION COMPANY, INC.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMERICAN MOBILE SATELLITE CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMERICAN MOBILE SATELLITE SALES CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMSC SALES CORPORATION LTD.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMSC SUBSIDIARY CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:


Indenture signature page(s)

 
                              ARDIS COMPANY


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS, INC.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS ACQUISITION, INC.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              ARDIS HOLDING COMPANY


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              RADIO DATA NETWORK HOLDING CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:


Indenture signature page(s)

 
                              STATE STREET BANK AND TRUST COMPANY


                              By:
                                 ---------------------------------
                              Name:
                              Title:


Indenture signature page(s)

 
                                  EXHIBIT A1

                                (Face of Note)

[INSERT THE GLOBAL NOTE LEGEND, IF APPLICABLE PURSUANT TO THE PROVISIONS OF THE
INDENTURE]

[INSERT THE PRIVATE PLACEMENT LEGEND, IF APPLICABLE PURSUANT TO THE PROVISIONS
OF THE INDENTURE]

                                                            CUSIP/CINS 00178DAA5

              12 1/4% [Series A] [Series B] Senior Notes due 2008

No.                       $

                         AMSC ACQUISITION COMPANY, INC.

promises to pay to ______________ or registered assigns, the principal sum of
_______________ Dollars on April 1, 2008.

Interest Payment Dates: April 1 and October 1.

Record Dates:  March 15 and September 15.

                                      A1-1

 
                                    Dated:  _____ __, ____.

                                    AMSC Acquisition Company, Inc.

                                    By:
                                       -------------------------------- 
                                    Name:
                                    Title:

                                    By:
                                       -------------------------------- 
                                    Name:
                                    Title:

This is one of the [Global]
Notes referred to in the
within-mentioned Indenture:

State Street Bank and Trust Company,
as Trustee
By:
   -------------------------------- 

                                      A1-2

 
                                (Back of Note)

              12 1/4% [Series A] [Series B] Senior Notes due 2008

          Capitalized terms used herein shall have the meanings assigned to them
in the Indenture referred to below unless otherwise indicated.



          1.  INTEREST. AMSC Acquisition Company, Inc., a Delaware corporation
(the "COMPANY"), promises to pay interest on the principal amount of this Note
at 12 1/4% per annum from March 31, 1998 until maturity and shall pay the
Liquidated Damages payable pursuant to Section 5 of the Debt Registration Rights
Agreement referred to below. The Company will pay interest and Liquidated
Damages semi-annually on April 1 and October 1 of each year, or if any such day
is not a Business Day, on the next succeeding Business Day (each an "INTEREST
PAYMENT DATE"). Interest on the Notes will accrue from the most recent date to
which interest has been paid or, if no interest has been paid, from the date of
issuance; provided that if there is no existing Default in the payment of
interest, and if this Note is authenticated between a record date referred to on
the face hereof and the next succeeding Interest Payment Date, interest shall
accrue from such next succeeding Interest Payment Date; provided, further, that
the first Interest Payment Date shall be October 1, 1998. The Company shall pay
interest (including post-petition interest in any proceeding under any
Bankruptcy Law) on overdue principal and premium, if any, from time to time on
demand at a rate that is 1% per annum in excess of the rate then in effect; it
shall pay interest (including post-petition interest in any proceeding under any
Bankruptcy Law) on overdue installments of interest and Liquidated Damages
(without regard to any applicable grace periods) from time to time on demand at
the same rate to the extent lawful. Interest will be computed on the basis of a
360-day year of twelve 30-day months.

          2.  METHOD OF PAYMENT. The Company will pay interest on the Notes
(except defaulted interest) and Liquidated Damages to the Persons who are
registered Holders of Notes at the close of business on the March 15 or
September 15 next preceding the Interest Payment Date, even if such Notes are
canceled after such record date and on or before such Interest Payment Date,
except as provided in Section 2.12 of the Indenture with respect to defaulted
interest. The Notes will be payable as to principal, premium and Liquidated
Damages, if any, and interest at the office or agency of the Company maintained
for such purpose within or without the City and State of New York, or, at the
option of the Company, payment of interest and Liquidated Damages may be made by
check mailed to the Holders at their addresses set forth in the register of
Holders, and provided that payment by wire transfer of immediately available
funds will be required with respect to principal of and interest, premium and
Liquidated Damages on, all Global Notes and all other Notes the Holders of which
shall have provided wire transfer instructions to the Company or the Paying
Agent. Such payment shall be in such coin or currency of the United States of
America as at the time of payment is legal tender for payment of public and
private debts.

          3.  PAYING AGENT AND REGISTRAR. Initially, State Street Bank and Trust
Company, the Trustee under the Indenture, will act as Paying Agent and
Registrar. The Company may change any Paying Agent or Registrar without notice
to any Holder. The Company or any of its Subsidiaries may act in any such
capacity.

          4.  INDENTURE. The Company issued the Notes under an Indenture dated
as of March 31, 1998 ("INDENTURE") among the Company, the Guarantors and the
Trustee. The terms of the Notes include those stated in the Indenture and those
made part of the Indenture by reference to the Trust Indenture Act of 1939, as
amended (15 U.S. Code (S)(S) 77aaa-77bbbb). The Notes are subject to all such

                                      A1-3

 
terms, and Holders are referred to the Indenture and such Act for a statement of
such terms. To the extent any provision of this Note conflicts with the express
provisions of the Indenture, the provisions of the indenture shall govern and be
controlling. The Notes are obligations of the Company limited to $335 million in
aggregate principal amount.

          5.  OPTIONAL REDEMPTION.

          (a) Except as set forth in subparagraph (b) of this Paragraph 5, the
Notes shall not be subject to redemption at the Company's option prior to 
April 1, 2003. The Notes shall be subject to redemption at any time at the
option of the Company, in whole or in part, upon not less than 30 nor more than
60 days' notice, at the redemption prices (expressed as percentages of principal
amount) set forth below plus accrued and unpaid interest and Liquidated Damages
thereon, if any, to the applicable redemption date, if redeemed during the
twelve-month period beginning on April 1 of the years indicated below:

         YEAR                                                PERCENTAGE
         ----                                                ----------
                                                             
         2003................................................ 106.125%
         2004................................................ 104.083%
         2005................................................ 102.042%
         2006 and thereafter................................. 100.000%

          (b) Notwithstanding the provisions of subparagraph (a) of this
Paragraph 5, during the first 36 months after the date of the Indenture, the
Company may redeem up to 35% of the aggregate principal amount of Notes
originally issued hereunder at a redemption price of 112.25% of the principal
amount thereof, plus accrued and unpaid interest and Liquidated Damages thereon,
if any, to the redemption date, with the net cash proceeds to the Company of an
Equity Offering; provided that (i) at least $217.8 million in aggregate
principal amount of the Notes remain outstanding immediately after the
occurrence of each such redemption (excluding Notes held by the Company and its
Subsidiaries) and (ii) each such redemption shall occur within 45 days after the
date of the closing of any such Equity Offering.

          6.  MANDATORY REDEMPTION.

          Except as set forth in paragraph 7 below, the Company shall not be
required to make mandatory redemption payments with respect to the Notes.

          7.  REPURCHASE AT OPTION OF HOLDER.

          (a) If there is a Change of Control, the Company shall be required to
make an offer (a "CHANGE OF CONTROL OFFER") to repurchase all or any part (equal
to $1,000 or an integral multiple thereof) of each Holder's Notes at a purchase
price equal to 101% of the aggregate principal amount thereof plus accrued and
unpaid interest and Liquidated Damages thereon, if any, to the date of purchase
(the "CHANGE OF CONTROL PAYMENT").  Within 10 days following any Change of
Control, the Company shall mail a notice to each Holder setting forth the
procedures governing the Change of Control Offer as required by the Indenture.

          (b) If the Company or a Subsidiary consummates any Asset Sales, within
five days of each date on which the aggregate amount of Excess Proceeds exceeds
$10.0 million, the Company shall commence an offer to all Holders of Notes (as
"ASSET SALE OFFER") pursuant to Section 3.09 of the Indenture to purchase the
maximum principal amount of Notes that may be purchased out of the Excess

                                      A1-4

 
Proceeds at an offer price in cash in an amount equal to 100% of the principal
amount thereof plus accrued and unpaid interest and Liquidated Damages thereon,
if any, to the date fixed for the closing of such offer, in accordance with the
procedures set forth in the Indenture. To the extent that the aggregate amount
of Notes tendered pursuant to an Asset Sale Offer is less than the Excess
Proceeds, the Company (or such Subsidiary) may use such deficiency for general
corporate purposes. If the aggregate principal amount of Notes surrendered by
Holders thereof exceeds the amount of Excess Proceeds, the Trustee shall select
the Notes to be purchased on a pro rata basis. Holders of Notes that are the
subject of an offer to purchase will receive an Asset Sale Offer from the
Company prior to any related purchase date and may elect to have such Notes
purchased by completing the form entitled "Option of Holder to Elect Purchase"
on the reverse of the Notes.

          8.  NOTICE OF REDEMPTION.  Notice of redemption will be mailed at
least 30 days but not more than 60 days before the redemption date to each
Holder whose Notes are to be redeemed at its registered address.  Notes in
denominations larger than $1,000 may be redeemed in part but only in whole
multiples of $1,000, unless all of the Notes held by a Holder are to be
redeemed.  On and after the redemption date interest ceases to accrue on Notes
or portions thereof called for redemption.

          9.  DENOMINATIONS, TRANSFER, EXCHANGE.  The Notes are in registered
form without coupons in denominations of $1,000 and integral multiples of
$1,000.  The transfer of Notes may be registered and Notes may be exchanged as
provided in the Indenture.  The Registrar and the Trustee may require a Holder,
among other things, to furnish appropriate endorsements and transfer documents
and the Company may require a Holder to pay any taxes and fees required by law
or permitted by the Indenture.  The Company need not exchange or register the
transfer of any Note or portion of a Note selected for redemption, except for
the unredeemed portion of any Note being redeemed in part.  Also, the Company
need not exchange or register the transfer of any Notes for a period of 15 days
before a selection of Notes to be redeemed or during the period between a record
date and the corresponding Interest Payment Date.

          10.  PERSONS DEEMED OWNERS.  The registered Holder of a Note may be
treated as its owner for all purposes.

          11.  AMENDMENT, SUPPLEMENT AND WAIVER.  Subject to certain exceptions,
the Indenture, the Note Guarantees or the Notes may be amended or supplemented
with the consent of the Holders of at least a majority in principal amount of
the then outstanding Notes voting as a single class, and any existing default or
compliance with any provision of the Indenture, the Note Guarantees or the Notes
may be waived with the consent of the Holders of a majority in principal amount
of the then outstanding Notes voting as a single class.  Without the consent of
any Holder of a Note, the Indenture, the Note Guarantees or the Notes may be
amended or supplemented to cure any ambiguity, defect or inconsistency, to
provide for uncertificated Notes in addition to or in place of certificated
Notes, to provide for the assumption of the Company's or Guarantor's obligations
to Holders of the Notes in case of a merger or consolidation, to make any change
that would provide any additional rights or benefits to the Holders of the Notes
or that does not adversely affect the legal rights under the Indenture of any
such Holder, to comply with the requirements of the SEC in order to effect or
maintain the qualification of the Indenture under the Trust Indenture Act, or to
allow any Guarantor to execute a supplemental indenture to the Indenture and/or
a Note Guarantee with respect to the Notes.

          12.  DEFAULTS AND REMEDIES.  Events of Default include: (i) default in
the payment when due of interest on, or Liquidated Damages with respect to, the
Notes and such default continues for a period of 30 days (whether or not
prohibited, with respect to the Holdings Guarantee, by the subordination

                                      A1-5

 
provisions of Article 10 of the Indenture); (ii) default in the payment when due
of principal of or premium, if any, on the Notes when the same becomes due and
payable at maturity, upon redemption (including in connection with an offer to
purchase) or otherwise (whether or not prohibited, with respect to the Holdings
Guarantee, by the subordination provisions of Article 10 of the Indenture),
(iii) failure by the Company to comply with any of the provisions of Section
4.07, 4.09, 4.10 or 4.15 of the Indenture; (iv) failure by the Company for 60
days after notice to the Company by the Trustee or the Holders of at least 25%
in principal amount of the Notes then outstanding voting as a single class to
comply with certain other agreements in the Indenture or the Notes; (v) default
under certain other agreements relating to Indebtedness of the Company or any of
its Subsidiaries which default results in the acceleration of such Indebtedness
prior to its express maturity; (vi) default under certain other agreements
relating to Indebtedness of Holdings which default results in the acceleration
of such Indebtedness prior to its express maturity; (viii) certain final
judgments against the Company or its Subsidiaries for the payment of money that
remain undischarged for a period of 60 days; (ix) certain final judgments
against Holdings for the payment of money that remain undischarged for a period
of 60 days; (x) material breach by Holdings, the Company or any Subsidiary of
any representation or warranty set forth in the Unit Agreement or the Pledge
Agreement, or material default by Holdings, the Company or any Subsidiary in the
performance of any covenant set forth in the Unit Agreement or the Pledge
Agreement, or repudiation by Holdings, the Company or any Subsidiary of its
obligations under the Unit Agreement or the Pledge Agreement or the
unenforceability of the Unit Agreement or the Pledge Agreement against Holdings,
the Company or any Subsidiary for any reason; (xi) the termination or revocation
of any of the permits, licenses, approvals, orders, certificates, franchises or
authorizations of governmental or regulatory authorities, including those
relating to the Federal Communications Act of 1934, as amended, owned or held by
Holdings, the Company or any of the Subsidiary Guarantors that are material to
the Company and its Subsidiaries, taken as a whole, (collectively, "LICENSES")
or any other material impairment occurs of the rights under any such Licenses of
the holder of such License; (xii) certain events of bankruptcy or insolvency
with respect to the Company or any Guarantor; and (xiii) except as permitted by
the Indenture, any Note Guarantee shall be held in any judicial proceeding to be
unenforceable or invalid or shall cease for any reason to be in full force and
effect or any Guarantor or any Person acting on its behalf shall deny or
disaffirm its obligations under such Guarantor's Note Guarantee.  If any Event
of Default occurs and is continuing, the Trustee or the Holders of at least 25%
in principal amount of the then outstanding Notes may declare all the Notes to
be due and payable.  Notwithstanding the foregoing, in the case of an Event of
Default arising from certain events of bankruptcy or insolvency, all outstanding
Notes will become due and payable without further action or notice.  Holders may
not enforce the Indenture or the Notes except as provided in the Indenture.
Subject to certain limitations, Holders of a majority in principal amount of the
then outstanding Notes may direct the Trustee in its exercise of any trust or
power.  The Trustee may withhold from Holders of the Notes notice of any
continuing Default or Event of Default (except a Default or Event of Default
relating to the payment of principal or interest) if it determines that
withholding notice is in their interest.  The Holders of a majority in aggregate
principal amount of the Notes then outstanding by notice to the Trustee may on
behalf of the Holders of all of the Notes waive any existing Default or Event of
Default and its consequences under the Indenture except a continuing Default or
Event of Default in the payment of interest on, or the principal of, the Notes.
The Company is required to deliver to the Trustee annually a statement regarding
compliance with the Indenture, and the Company is required upon becoming aware
of any Default or Event of Default, to deliver to the Trustee a statement
specifying such Default or Event of Default.

          13.  TRUSTEE DEALINGS WITH COMPANY.  The Trustee, in its individual or
any other capacity, may make loans to, accept deposits from, and perform
services for the Company or its Affiliates, and may otherwise deal with the
Company or its Affiliates, as if it were not the Trustee.

                                      A1-6

 
          14.  NO RECOURSE AGAINST OTHERS.  A director, officer, employee,
incorporator or stockholder, of the Company, as such, shall not have any
liability for any obligations of the Company under the Notes or the Indenture or
for any claim based on, in respect of, or by reason of, such obligations or
their creation.  Each Holder by accepting a Note waives and releases all such
liability.  The waiver and release are part of the consideration for the
issuance of the Notes.

          15.  AUTHENTICATION.  This Note shall not be valid until authenticated
by the manual signature of the Trustee or an authenticating agent.

          16.  ABBREVIATIONS.  Customary abbreviations may be used in the name
of a Holder or an assignee, such as:  TEN COM (= tenants in common), TEN ENT (=
tenants by the entireties), JT TEN (= joint tenants with right of survivorship
and not as tenants in common), CUST (= Custodian), and U/G/M/A (= Uniform Gifts
to Minors Act).

          17.  ADDITIONAL RIGHTS OF HOLDERS OF RESTRICTED GLOBAL NOTES AND
RESTRICTED DEFINITIVE NOTES.  In addition to the rights provided to Holders of
Notes under the Indenture, Holders of Restricted Global Notes and Restricted
Definitive Notes shall have all the rights set forth in the Debt Registration
Rights Agreement dated as of March 31, 1998, between the Company and the parties
named on the signature pages thereof (the "DEBT REGISTRATION RIGHTS AGREEMENT").

          18.  CUSIP NUMBERS.  Pursuant to a recommendation promulgated by the
Committee on Uniform Security Identification Procedures, the Company has caused
CUSIP numbers to be printed on the Notes and the Trustee may use CUSIP numbers
in notices of redemption as a convenience to Holders.  No representation is made
as to the accuracy of such numbers either as printed on the Notes or as
contained in any notice of redemption and reliance may be placed only on the
other identification numbers placed thereon.

          The Company will furnish to any Holder upon written request and
without charge a copy of the Indenture and/or the Debt Registration Rights
Agreement.  Requests may be made to:

          AMSC Acquisition Company, Inc.
          10802 Parkridge Blvd.
          Reston, Virginia  20191-5416
          Attention:  Randy S. Segal, Esq.

                                      A1-7

 
                                ASSIGNMENT FORM

To assign this Note, fill in the form below: (I) or (we) assign and transfer
this Note to

- --------------------------------------------------------------------------------
                 (Insert assignee's soc. sec. or tax I.D. no.)

- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
(Print or type assignee's name, address and zip code)

and irrevocably appoint_________________________________________________________
to transfer this Note on the books of the Company.  The agent may substitute
another to act for him.



Date:_________________
                            Your Signature:
                                           -------------------------------------
                            (Sign exactly as your name appears on the face of
                            this Note)

                            Tax Identification No:______________________________


                            SIGNATURE GUARANTEE:

                            ---------------------------------

                            Signatures must be guaranteed by an "eligible
                            guarantor institution" meeting the requirements of
                            the Registrar, which requirements include membership
                            or participation in the Security Transfer Agent
                            Medallion Program ("STAMP") or such other "signature
                            guarantee program" as may be determined by the
                            Registrar in addition to, or in substitution for,
                            STAMP, all in accordance with the Securities
                            Exchange Act of 1934, as amended.

                                      A1-8

 
                       OPTION OF HOLDER TO ELECT PURCHASE

          If you want to elect to have this Note purchased by the Company
pursuant to Section 4.10 or 4.15 of the Indenture, check the box below:

          [_] Section 4.10     [_] Section 4.15

          If you want to elect to have only part of the Note purchased by the
Company pursuant to Section 4.10 or Section 4.15 of the Indenture, state the
amount you elect to have purchased: $________



Date:__________________
                            Your Signature:
                                           -------------------------------------
                            (Sign exactly as your name appears on the face of
                            this Note)

                            Tax Identification No:______________________________


                            SIGNATURE GUARANTEE:

                            ---------------------------------

                            Signatures must be guaranteed by an "eligible
                            guarantor institution" meeting the requirements of
                            the Registrar, which requirements include membership
                            or participation in the Security Transfer Agent
                            Medallion Program ("STAMP") or such other "signature
                            guarantee program" as may be determined by the
                            Registrar in addition to, or in substitution for,
                            STAMP, all in accordance with the Securities
                            Exchange Act of 1934, as amended.

                                      A1-9

 
             SCHEDULE OF EXCHANGES OF INTERESTS IN THE GLOBAL NOTE

          The following exchanges of a part of this Global Note for an interest
in another Global Note or for a Definitive Note, or exchanges of a part of
another Global Note or Definitive Note for an interest in this Global Note, have
been made:



                                                              Principal Amount
                   Amount of decrease   Amount of increase     of this Global       Signature of
                      in Principal         in Principal        Note following        authorized
                     Amount of this       Amount of this      such decrease (or      officer of
Date of Exchange       Global Note          Global Note           increase)        Trustee or Note
                                                                                     Custodian
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                                     A1-10

 
                                   EXHIBIT A2

                  (Face of Regulation S Temporary Global Note)

          THE RIGHTS ATTACHING TO THIS REGULATION S TEMPORARY GLOBAL NOTE, AND
THE CONDITIONS AND PROCEDURES GOVERNING ITS EXCHANGE FOR CERTIFICATED NOTES, ARE
AS SPECIFIED IN THE INDENTURE (AS DEFINED HEREIN).  NEITHER THE HOLDER NOR THE
BENEFICIAL OWNERS OF THIS REGULATION S TEMPORARY GLOBAL NOTE SHALL BE ENTITLED
TO RECEIVE PAYMENT OF INTEREST HEREON.

          UNLESS AND UNTIL IT IS EXCHANGED IN WHOLE OR IN PART FOR NOTES IN
DEFINITIVE FORM, THIS NOTE MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY THE
DEPOSITARY TO A NOMINEE OF THE DEPOSITARY OR BY A NOMINEE OF THE DEPOSITARY TO
THE DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY OR BY THE DEPOSITARY OR ANY
SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH SUCCESSOR
DEPOSITARY.  UNLESS THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED
REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (55 WATER STREET, NEW YORK, NEW
YORK) ("DTC"), TO THE COMPANY OR ITS AGENT FOR REGISTRATION OF TRANSFER,
EXCHANGE OR PAYMENT, AND ANY CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF
CEDE & CO. OR SUCH OTHER NAME AS MAY BE REQUESTED BY AN AUTHORIZED
REPRESENTATIVE OF DTC (AND ANY PAYMENT IS MADE TO CEDE & CO. OR SUCH OTHER
ENTITY AS MAY BE REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF DTC), ANY
TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON
IS WRONGFUL INASMUCH AS THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST
HEREIN.

          THE NOTE (OR ITS PREDECESSOR) EVIDENCED HEREBY WAS ORIGINALLY ISSUED
IN A TRANSACTION EXEMPT FROM REGISTRATION UNDER SECTION 5 OF THE UNITED STATES
SECURITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), AND THE NOTE
EVIDENCED HEREBY MAY NOT BE OFFERED, SOLD OR OTHERWISE TRANSFERRED IN THE
ABSENCE OF SUCH REGISTRATION OR AN APPLICABLE EXEMPTION THEREFROM.  EACH
PURCHASER OF THE NOTE EVIDENCED HEREBY IS HEREBY NOTIFIED THAT THE SELLER MAY BE
RELYING ON THE EXEMPTION PROVIDED BY RULE 144A UNDER THE SECURITIES ACT.  THE
HOLDER OF THE NOTE EVIDENCED HEREBY AGREES FOR THE BENEFIT OF THE COMPANY THAT
(A) SUCH NOTE MAY BE RESOLD, PLEDGED OR OTHERWISE TRANSFERRED, ONLY (1) (a) TO A
PERSON WHO THE SELLER REASONABLY BELIEVES IS A QUALIFIED INSTITUTIONAL BUYER (AS
DEFINED IN OF RULE 144A UNDER THE SECURITIES ACT) IN A TRANSACTION MEETING THE
REQUIREMENTS OF RULE 144A, (b) IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE
144 UNDER THE SECURITIES ACT, (c) OUTSIDE THE UNITED STATES TO A FOREIGN PERSON
IN A TRANSACTION MEETING THE REQUIREMENTS OF RULE 904 UNDER THE SECURITIES ACT
OR (d) IN ACCORDANCE WITH ANOTHER EXEMPTION FROM THE REGISTRATION REQUIREMENTS
OF THE SECURITIES ACT (AND BASED UPON AN OPINION OF COUNSEL IF THE COMPANY SO
REQUESTS), (2) TO THE COMPANY OR (3) PURSUANT TO AN EFFECTIVE REGISTRATION
STATEMENT AND, IN EACH CASE, IN ACCORDANCE WITH THE APPLICABLE SECURITIES LAWS
OF ANY STATE OF THE UNITED STATES OR ANY OTHER APPLICABLE JURISDICTION AND (B)
THE HOLDER WILL, AND EACH 

                                      A2-1

 
SUBSEQUENT HOLDER IS REQUIRED TO, NOTIFY ANY PURCHASER OF THE NOTE EVIDENCED
HEREBY OF THE RESALE RESTRICTIONS SET FORTH IN (1) ABOVE.

                                                            CUSIP/CINS U00178AA8

                     12 1/4% Series A Senior Notes due 2008

No._____                                                        $_______________

                         AMSC ACQUISITION COMPANY, INC.

promises to pay to ______________ or registered assigns, the principal sum of
_______________ Dollars on April 1, 2008.

Interest Payment Dates:  April 1 and October 1.

Record Dates: March 15 and September 15.

                                      A2-2

 
                                    Dated:  _____ __, ____.

                                    AMSC Acquisition Company, Inc.

                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:

                                    By:
                                       -----------------------------------------
                                    Name:
                                    Title:

This is one of the Global
Notes referred to in the
within-mentioned Indenture:

State Street Bank and Trust Company,
as Trustee

By:
   -----------------------------------------

                                      A2-3

 
                  (Back of Regulation S Temporary Global Note)

                     12 1/4% Series A Senior Notes due 2008

          Capitalized terms used herein shall have the meanings assigned to them
in the Indenture referred to below unless otherwise indicated.

          1.  INTEREST.  AMSC Acquisition Company, Inc., a Delaware corporation
(the "COMPANY"), promises to pay interest on the principal amount of this Note
at 12 1/4% per annum from March 31, 1998 until maturity and shall pay the
Liquidated Damages payable pursuant to Section 5 of the Debt Registration Rights
Agreement referred to below.  The Company will pay interest and Liquidated
Damages semi-annually on April 1 and October 1 of each year, or if any such day
is not a Business Day, on the next succeeding Business Day (each an "INTEREST
PAYMENT DATE").  Interest on the Notes will accrue from the most recent date to
which interest has been paid or, if no interest has been paid, from the date of
issuance; provided that if there is no existing Default in the payment of
interest, and if this Note is authenticated between a record date referred to on
the face hereof and the next succeeding Interest Payment Date, interest shall
accrue from such next succeeding Interest Payment Date; provided, further, that
the first Interest Payment Date shall be October 1, 1998.  The Company shall pay
interest (including post-petition interest in any proceeding under any
Bankruptcy Law) on overdue principal and premium, if any, from time to time on
demand at a rate that is 1% per annum in excess of the rate then in effect; it
shall pay interest (including post-petition interest in any proceeding under any
Bankruptcy Law) on overdue installments of interest and Liquidated Damages
(without regard to any applicable grace periods) from time to time on demand at
the same rate to the extent lawful.  Interest will be computed on the basis of a
360-day year of twelve 30-day months.

          Until this Regulation S Temporary Global Note is exchanged for one or
more Regulation S Permanent Global Notes, the Holder hereof shall not be
entitled to receive payments of interest hereon; until so exchanged in full,
this Regulation S Temporary Global Note shall in all other respects be entitled
to the same benefits as other Senior Subordinated Notes under the Indenture.

          2.  METHOD OF PAYMENT. The Company will pay interest on the Notes
(except defaulted interest) and Liquidated Damages to the Persons who are
registered Holders of Notes at the close of business on the March 15 or
September 15 next preceding the Interest Payment Date, even if such Notes are
canceled after such record date and on or before such Interest Payment Date,
except as provided in Section 2.12 of the Indenture with respect to defaulted
interest. The Notes will be payable as to principal, premium, interest and
Liquidated Damages at the office or agency of the Company maintained for such
purpose within or without the City and State of New York, or, at the option of
the Company, payment of interest and Liquidated Damages may be made by check
mailed to the Holders at their addresses set forth in the register of Holders,
and provided that payment by wire transfer of immediately available funds will
be required with respect to principal of and interest, premium and Liquidated
Damages on, all Global Notes and all other Notes the Holders of which shall have
provided wire transfer instructions to the Company or the Paying Agent. Such
payment shall be in such coin or currency of the United States of America as at
the time of payment is legal tender for payment of public and private debts.

          3.  PAYING AGENT AND REGISTRAR. Initially, State Street Bank & Trust
Company, the Trustee under the Indenture, will act as Paying Agent and
Registrar. The Company may change any Paying Agent or Registrar without notice
to any Holder. The Company or any of its Subsidiaries may act in any such
capacity.

                                      A2-4

 
          4.  INDENTURE. The Company issued the Notes under an Indenture dated
as of March 31, 1998 ("INDENTURE") among the Company, the Guarantors and the
Trustee. The terms of the Notes include those stated in the Indenture and those
made part of the Indenture by reference to the Trust Indenture Act of 1939, as
amended (15 U.S. Code (S)(S) 77aaa-77bbbb). The Notes are subject to all such
terms, and Holders are referred to the Indenture and such Act for a statement of
such terms. The Notes are general obligations of the Company limited to $335
million in aggregate principal amount.

          5.  OPTIONAL REDEMPTION.

          (a) Except as set forth in subparagraph (b) of this Paragraph 5, the
Notes shall not be subject to redemption at the Company's option prior to April
1, 2003.  The Notes shall be subject to redemption at any time at the option of
the Company, in whole or in part, upon not less than 30 nor more than 60 days'
notice, at the redemption prices (expressed as percentages of principal amount)
set forth below plus accrued and unpaid interest and Liquidated Damages thereon,
if any, to the applicable redemption date, if redeemed during the twelve-month
period beginning on April 1 of the years indicated below:

            YEAR                                                PERCENTAGE
            ----                                                ----------

            2003................................................  106.125%
            2004................................................  104.083%
            2005................................................  102.042%
            2006 and thereafter.................................  100.000%

          (b) Notwithstanding the provisions of subparagraph (a) of this
Paragraph 5, during the first 36 months after the date of the Indenture, the
Company may redeem up to 35% of the aggregate principal amount of Notes
originally issued hereunder at a redemption price of 112.25% of the principal
amount thereof, plus accrued and unpaid interest and Liquidated Damages thereon,
if any, to the redemption date, with the net cash proceeds to the Company of an
Equity Offering; provided that (i) at least $217.8 million in aggregate
principal amount of the Notes remain outstanding immediately after the
occurrence of each such redemption (excluding Notes held by the Company and its
Subsidiaries) and (ii) each such redemption shall occur within 45 days after the
date of the closing of any such Equity Offering.

          6.  MANDATORY REDEMPTION.

          Except as set forth in paragraph 7 below, the Company shall not be
required to make mandatory redemption payments with respect to the Notes.

          7.  REPURCHASE AT OPTION OF HOLDER.

          (a) If there is a Change of Control, the Company shall be required to
make an offer (a "CHANGE OF CONTROL OFFER") to repurchase all or any part (equal
to $1,000 or an integral multiple thereof) of each Holder's Notes at a purchase
price equal to 101% of the aggregate principal amount thereof plus accrued and
unpaid interest, if any, to the date of purchase (the "CHANGE OF CONTROL
PAYMENT"). Within 10 days following any Change of Control, the Company shall
mail a notice to each Holder setting forth the procedures governing the Change
of Control Offer as required by the Indenture.

          (b) If the Company or a Subsidiary consummates any Asset Sales, within
five days of each date on which the aggregate amount of Excess Proceeds exceeds
$10.0 million, the Company shall commence an offer to all Holders of Notes (as
"ASSET SALE OFFER") pursuant to Section 3.09 of the 

                                      A2-5

 
Indenture to purchase the maximum principal amount of Notes that may be
purchased out of the Excess Proceeds at an offer price in cash in an amount
equal to 100% of the principal amount thereof plus accrued and unpaid interest,
if any, to the date fixed for the closing of such offer, in accordance with the
procedures set forth in the Indenture. To the extent that the aggregate amount
of Notes tendered pursuant to an Asset Sale Offer is less than the Excess
Proceeds, the Company (or such Subsidiary) may use such deficiency for general
corporate purposes. If the aggregate principal amount of Notes surrendered by
Holders thereof exceeds the amount of Excess Proceeds, the Trustee shall select
the Notes to be purchased on a pro rata basis. Holders of Notes that are the
subject of an offer to purchase will receive an Asset Sale Offer from the
Company prior to any related purchase date and may elect to have such Notes
purchased by completing the form entitled "Option of Holder to Elect Purchase"
on the reverse of the Notes.

          8.  NOTICE OF REDEMPTION.  Notice of redemption will be mailed at
least 30 days but not more than 60 days before the redemption date to each
Holder whose Notes are to be redeemed at its registered address.  Notes in
denominations larger than $1,000 may be redeemed in part but only in whole
multiples of $1,000, unless all of the Notes held by a Holder are to be
redeemed.  On and after the redemption date interest ceases to accrue on Notes
or portions thereof called for redemption.

          9.  DENOMINATIONS, TRANSFER, EXCHANGE.  The Notes are in registered
form without coupons in denominations of $1,000 and integral multiples of
$1,000.  The transfer of Notes may be registered and Notes may be exchanged as
provided in the Indenture.  The Registrar and the Trustee may require a Holder,
among other things, to furnish appropriate endorsements and transfer documents
and the Company may require a Holder to pay any taxes and fees required by law
or permitted by the Indenture.  The Company need not exchange or register the
transfer of any Note or portion of a Note selected for redemption, except for
the unredeemed portion of any Note being redeemed in part.  Also, it need not
exchange or register the transfer of any Notes for a period of 15 days before a
selection of Notes to be redeemed or during the period between a record date and
the corresponding Interest Payment Date.

          This Regulation S Temporary Global Note is exchangeable in whole or in
part for one or more Global Notes only (i) on or after the termination of the
40-day restricted period (as defined in Regulation S) and (ii) upon presentation
of certificates (accompanied by an Opinion of Counsel, if applicable) required
by Article 2 of the Indenture.  Upon exchange of this Regulation S Temporary
Global Note for one or more Global Notes, the Trustee shall cancel this
Regulation S Temporary Global Note.

          10.  PERSONS DEEMED OWNERS.  The registered Holder of a Note may be
treated as its owner for all purposes.

          11.  AMENDMENT, SUPPLEMENT AND WAIVER.  Subject to certain exceptions,
the Indenture, the Note Guarantees or the Notes may be amended or supplemented
with the consent of the Holders of at least a majority in principal amount of
the then outstanding Notes voting as a single class, and any existing default or
compliance with any provision of the Indenture, the Note Guarantees or the Notes
may be waived with the consent of the Holders of a majority in principal amount
of the then outstanding Notes voting as a single class.  Without the consent of
any Holder of a Note, the Indenture, the Note Guarantees or the Notes may be
amended or supplemented to cure any ambiguity, defect or inconsistency, to
provide for uncertificated Notes in addition to or in place of certificated
Notes, to provide for the assumption of the Company's or Guarantor's obligations
to Holders of the Notes in case of a merger or consolidation, to make any change
that would provide any additional rights or benefits to the Holders of the Notes
or that does not adversely affect the legal rights under the Indenture of any
such 

                                      A2-6

 
Holder, to comply with the requirements of the SEC in order to effect or
maintain the qualification of the Indenture under the Trust Indenture Act, or to
allow any Guarantor to execute a supplemental indenture to the Indenture and/or
a Note Guarantee with respect to the Notes.

          12.  DEFAULTS AND REMEDIES.  Events of Default include: (i) default in
the payment when due of interest on, or Liquidated Damages with respect to, the
Notes and such default continues for a period of 30 days (whether or not
prohibited, with respect to the Holdings Guarantee, by the subordination
provisions of Article 10 of the Indenture); (ii) default in the payment when due
of principal of or premium, if any, on the Notes when the same becomes due and
payable at maturity, upon redemption (including in connection with an offer to
purchase) or otherwise (whether or not prohibited, with respect to the Holdings
Guarantee, by the subordination provisions of Article 10 of the Indenture),
(iii) failure by the Company to comply with any of the provisions of Section
4.07, 4.09, 4.10 or 4.15 of the Indenture; (iv) failure by the Company for 60
days after notice to the Company by the Trustee or the Holders of at least 25%
in principal amount of the Notes then outstanding voting as a single class to
comply with certain other agreements in the Indenture or the Notes; (v) default
under certain other agreements relating to Indebtedness of the Company or any of
its Subsidiaries which default results in the acceleration of such Indebtedness
prior to its express maturity; (vi) default under certain other agreements
relating to Indebtedness of Holdings which default results in the acceleration
of such Indebtedness prior to its express maturity; (viii) certain final
judgments against the Company or its Subsidiaries for the payment of money that
remain undischarged for a period of 60 days; (ix) certain final judgments
against Holdings for the payment of money that remain undischarged for a period
of 60 days; (x) material breach by Holdings, the Company or any Subsidiary of
any representation or warranty set forth in the Unit Agreement or the Pledge
Agreement, or material default by Holdings, the Company or any Subsidiary in the
performance of any covenant set forth in the Unit Agreement or the Pledge
Agreement, or repudiation by Holdings, the Company or any Subsidiary of its
obligations under the Unit Agreement or the Pledge Agreement or the
unenforceability of the Unit Agreement or the Pledge Agreement against Holdings,
the Company or any Subsidiary for any reason; (xi) the termination or revocation
of any of the permits, licenses, approvals, orders, certificates, franchises or
authorizations of governmental or regulatory authorities, including those
relating to the Federal Communications Act of 1934, as amended, owned or held by
Holdings, the Company or any of the Subsidiary Guarantors which are material to
the Company and its Subsidiaries, taken as a whole, (collectively, "LICENSES")
or any other material impairment occurs of the rights under any such Licenses of
the holder of any License; (xii) certain events of bankruptcy or insolvency with
respect to the Company or any Guarantor; and (xiii) except as permitted by the
Indenture, any Note Guarantee shall be held in any judicial proceeding to be
unenforceable or invalid or shall cease for any reason to be in full force and
effect or any Guarantor or any Person acting on its behalf shall deny or
disaffirm its obligations under such Guarantor's Note Guarantee.  If any Event
of Default occurs and is continuing, the Trustee or the Holders of at least 25%
in principal amount of the then outstanding Notes may declare all the Notes to
be due and payable.  Notwithstanding the foregoing, in the case of an Event of
Default arising from certain events of bankruptcy or insolvency, all outstanding
Notes will become due and payable without further action or notice.  Holders may
not enforce the Indenture or the Notes except as provided in the Indenture.
Subject to certain limitations, Holders of a majority in principal amount of the
then outstanding Notes may direct the Trustee in its exercise of any trust or
power.  The Trustee may withhold from Holders of the Notes notice of any
continuing Default or Event of Default (except a Default or Event of Default
relating to the payment of principal or interest) if it determines that
withholding notice is in their interest.  The Holders of a majority in aggregate
principal amount of the Notes then outstanding by notice to the Trustee may on
behalf of the Holders of all of the Notes waive any existing Default or Event of
Default and its consequences under the Indenture except a continuing Default or
Event of Default in the payment of interest on, or the principal of, the Notes.
The Company is required to deliver to the Trustee annually a 

                                      A2-7

 
statement regarding compliance with the Indenture, and the Company is required
upon becoming aware of any Default or Event of Default, to deliver to the
Trustee a statement specifying such Default or Event of Default.

          13.  TRUSTEE DEALINGS WITH COMPANY.  The Trustee, in its individual or
any other capacity, may make loans to, accept deposits from, and perform
services for the Company or its Affiliates, and may otherwise deal with the
Company or its Affiliates, as if it were not the Trustee.

          14.  NO RECOURSE AGAINST OTHERS.  A director, officer, employee,
incorporator or stockholder, of the Company or any of the Guarantors, as such,
shall not have any liability for any obligations of the Company or such
Guarantor under the Notes, the Note Guarantees or the Indenture or for any claim
based on, in respect of, or by reason of, such obligations or their creation.
Each Holder by accepting a Note waives and releases all such liability.  The
waiver and release are part of the consideration for the issuance of the Notes.

          15.  AUTHENTICATION.  This Note shall not be valid until authenticated
by the manual signature of the Trustee or an authenticating agent.

          16.  ABBREVIATIONS.  Customary abbreviations may be used in the name
of a Holder or an assignee, such as:  TEN COM (= tenants in common), TEN ENT (=
tenants by the entireties), JT TEN (= joint tenants with right of survivorship
and not as tenants in common), CUST (= Custodian), and U/G/M/A (= Uniform Gifts
to Minors Act).

          17.  ADDITIONAL RIGHTS OF HOLDERS OF RESTRICTED GLOBAL NOTES AND
RESTRICTED DEFINITIVE NOTES.  In addition to the rights provided to Holders of
Notes under the Indenture, Holders of Restricted Global Notes and Restricted
Definitive Notes shall have all the rights set forth in the Debt Registration
Rights Agreement dated as of March 31, 1998, between the Company and the parties
named on the signature pages thereof (the "DEBT REGISTRATION RIGHTS AGREEMENT").

          18.  CUSIP NUMBERS.  Pursuant to a recommendation promulgated by the
Committee on Uniform Security Identification Procedures, the Company has caused
CUSIP numbers to be printed on the Notes and the Trustee may use CUSIP numbers
in notices of redemption as a convenience to Holders.  No representation is made
as to the accuracy of such numbers either as printed on the Notes or as
contained in any notice of redemption and reliance may be placed only on the
other identification numbers placed thereon.

          The Company will furnish to any Holder upon written request and
without charge a copy of the Indenture and/or the Debt Registration Rights
Agreement.  Requests may be made to:

               AMSC Acquisition Company, Inc.
               10802 Parkridge Blvd.
               Reston, Virginia  20191-5416
               Attention:  Randy S. Segal, Esq.

                                      A2-8

 
                                ASSIGNMENT FORM

To assign this Note, fill in the form below: (I) or (we) assign and transfer
this Note to

- --------------------------------------------------------------------------------
                 (Insert assignee's soc. sec. or tax I.D. no.)

- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
             (Print or type assignee's name, address and zip code)

and irrevocably appoint_________________________________________________________
to transfer this Note on the books of the Company.  The agent may substitute
another to act for him.



Date:_____________
                            Your Signature:
                                           -------------------------------------
                            (Sign exactly as your name appears on the face of
                            this Note)

                            Tax Identification No:______________________________


                            SIGNATURE GUARANTEE:

                            _________________________________

                            Signatures must be guaranteed by an "eligible
                            guarantor institution" meeting the requirements of
                            the Registrar, which requirements include membership
                            or participation in the Security Transfer Agent
                            Medallion Program ("STAMP") or such other "signature
                            guarantee program" as may be determined by the
                            Registrar in addition to, or in substitution for,
                            STAMP, all in accordance with the Securities
                            Exchange Act of 1934, as amended.

                                      A2-9

 
                      OPTION OF HOLDER TO ELECT PURCHASE

          If you want to elect to have this Note purchased by the Company
pursuant to Section 4.10 or 4.15 of the Indenture, check the appropriate box
below:

     [_] Section 4.10     [_] Section 4.15

          If you want to elect to have only part of the Note purchased by the
Company pursuant to Section 4.10 or Section 4.15 of the Indenture, state the
amount you elect to have purchased:  $___________



Date:________________
                            Your Signature:
                                           -------------------------------------
                            (Sign exactly as your name appears on the face of
                            this Note)

                            Tax Identification No:______________________________


                            SIGNATURE GUARANTEE:

                            ---------------------------------

                            Signatures must be guaranteed by an "eligible
                            guarantor institution" meeting the requirements of
                            the Registrar, which requirements include membership
                            or participation in the Security Transfer Agent
                            Medallion Program ("STAMP") or such other "signature
                            guarantee program" as may be determined by the
                            Registrar in addition to, or in substitution for,
                            STAMP, all in accordance with the Securities
                            Exchange Act of 1934, as amended.

                                     A2-10

 
          SCHEDULE OF EXCHANGES OF REGULATION S TEMPORARY GLOBAL NOTE

          The following exchanges of a part of this Regulation S Temporary
Global Note for an interest in another Global Note, or of other Restricted
Global Notes for an interest in this Regulation S Temporary Global Note, have
been made:



                                                                               
                                                              Principal Amount      Signature of
                   Amount of decrease   Amount of increase     of this Global        authorized
                      in Principal         in Principal        Note following        officer of
                     Amount of this       Amount of this      such decrease (or    Trustee or Note
Date of Exchange       Global Note          Global Note           increase)           Custodian
- ---------------------------------------------------------------------------------------------------
                                                                      
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------
 
- ---------------------------------------------------------------------------------------------------


                                     A2-11

 
                                   EXHIBIT B

                        FORM OF CERTIFICATE OF TRANSFER

AMSC Acquisition Company, Inc.
10802 Parkridge Blvd.
Reston, Virginia  20191-5416
Telecopier No.:  703-758-6134
Attention:  Randy S. Segal, Esq.

State Street Bank and Trust Company
Goodwin Square
225 Asylum Street
Hartford, Connecticut  06103
Attention:  Steven Cimalore

          Re:  12 1/4% Senior Notes due 2008
               -----------------------------

                              (CUSIP __________)

          Reference is hereby made to the Indenture, dated as of March 31, 1998
(the "INDENTURE"), among AMSC Acquisition Company, Inc., as issuer (the
"COMPANY"), American Mobile Satellite Corporation, American Mobile Satellite
Sales Corporation, AMSC Sales Corporation Ltd., AMSC Subsidiary Corporation,
ARDIS Company, Motorola ARDIS, Inc., Motorola ARDIS Acquisition, Inc., ARDIS
Holding Company, and Radio Data Network Holding Corporation (collectively, the
"GUARANTORS") and State Street Bank and Trust Company, as trustee.  Capitalized
terms used but not defined herein shall have the meanings given to them in the
Indenture.

          ______________, (the "TRANSFEROR") owns and proposes to transfer the
Note[s] or interest in such Note[s] specified in Annex A hereto, in the
principal amount of $___________ in such Note[s] or interests (the "TRANSFER"),
to  __________ (the "TRANSFEREE"), as further specified in Annex A hereto.  In
connection with the Transfer, the Transferor hereby certifies that:

[CHECK ALL THAT APPLY]

1. [_] CHECK IF TRANSFEREE WILL TAKE DELIVERY OF A BENEFICIAL INTEREST IN THE
       ----------------------------------------------------------------------
144A GLOBAL NOTE OR A DEFINITIVE NOTE PURSUANT TO RULE 144A.  The Transfer is
- -----------------------------------------------------------                  
being effected pursuant to and in accordance with Rule 144A under the United
States Securities Act of 1933, as amended (the "SECURITIES ACT"), and,
accordingly, the Transferor hereby further certifies that the beneficial
interest or Definitive Note is being transferred to a Person that the Transferor
reasonably believed and believes is purchasing the beneficial interest or
Definitive Note for its own account, or for one or more accounts with respect to
which such Person exercises sole investment discretion, and such Person and each
such account is a "qualified institutional buyer" within the meaning of Rule
144A in a transaction meeting the requirements of Rule 144A and such Transfer is
in compliance with any applicable blue sky securities laws of any state of the
United States.  Upon consummation of the proposed Transfer in accordance with
the terms of the Indenture, the transferred beneficial interest or Definitive
Note will be subject to the restrictions on transfer enumerated in the Private
Placement Legend printed on the 144A Global Note and/or the Definitive Note and
in the Indenture and the Securities Act.

                                      B-1

 
2. [_] CHECK IF TRANSFEREE WILL TAKE DELIVERY OF A BENEFICIAL INTEREST IN THE
       ----------------------------------------------------------------------
TEMPORARY REGULATION S GLOBAL NOTE, THE REGULATION S GLOBAL NOTE OR A DEFINITIVE
- --------------------------------------------------------------------------------
NOTE PURSUANT TO REGULATION S.  The Transfer is being effected pursuant to and
- -----------------------------                                                 
in accordance with Rule 903 or Rule 904 under the Securities Act and,
accordingly, the Transferor hereby further certifies that (i) the Transfer is
not being made to a person in the United States and (x) at the time the buy
order was originated, the Transferee was outside the United States or such
Transferor and any Person acting on its behalf reasonably believed and believes
that the Transferee was outside the United States or (y) the transaction was
executed in, on or through the facilities of a designated offshore securities
market and neither such Transferor nor any Person acting on its behalf knows
that the transaction was prearranged with a buyer in the United States, (ii) no
directed selling efforts have been made in contravention of the requirements of
Rule 903(b) or Rule 904(b) of Regulation S under the Securities Act and, (iii)
the transaction is not part of a plan or scheme to evade the registration
requirements of the Securities Act and (iv) if the proposed transfer is being
made prior to the expiration of the Restricted Period, the transfer is not being
made to a U.S. Person or for the account or benefit of a U.S. Person (other than
an Initial Purchaser).  Upon consummation of the proposed transfer in accordance
with the terms of the Indenture, the transferred beneficial interest or
Definitive Note will be subject to the restrictions on Transfer enumerated in
the Private Placement Legend printed on the Regulation S Global Note, the
Temporary Regulation S Global Note and/or the Definitive Note and in the
Indenture and the Securities Act.

3. [_] CHECK AND COMPLETE IF TRANSFEREE WILL TAKE DELIVERY OF A BENEFICIAL
       -------------------------------------------------------------------
INTEREST IN THE IAI GLOBAL NOTE OR A DEFINITIVE NOTE PURSUANT TO ANY PROVISION
- ------------------------------------------------------------------------------
OF THE SECURITIES ACT OTHER THAN RULE 144A OR REGULATION S.  The Transfer is
- ----------------------------------------------------------                  
being effected in compliance with the transfer restrictions applicable to
beneficial interests in Restricted Global Notes and Restricted Definitive Notes
and pursuant to and in accordance with the Securities Act and any applicable
blue sky securities laws of any state of the United States, and accordingly the
Transferor hereby further certifies that (check one):

          (a) [_] such Transfer is being effected pursuant to and in accordance
with Rule 144 under the Securities Act;

                                      or

          (b) [_] such Transfer is being effected to the Company or a subsidiary
thereof;

                                      or

          (c) [_] such Transfer is being effected pursuant to an effective
registration statement under the Securities Act and in compliance with the
prospectus delivery requirements of the Securities Act;

                                       or

          (d) [_] such Transfer is being effected to an Institutional Accredited
Investor and pursuant to an exemption from the registration requirements of the
Securities Act other than Rule 144A, Rule 144 or Rule 904, and the Transferor
hereby further certifies that it has not engaged in any general solicitation
within the meaning of Regulation D under the Securities Act and the Transfer
complies with the transfer restrictions applicable to beneficial interests in a
Restricted Global Note or Restricted Definitive Notes and the requirements of
the exemption claimed, which certification is supported by (1) a certificate
executed by the Transferee in the form of Exhibit D to the Indenture and (2) if
such Transfer is 

                                      B-2

 
in respect of a principal amount of Notes at the time of transfer of less than
$250,000, an Opinion of Counsel provided by the Transferor or the Transferee (a
copy of which the Transferor has attached to this certification), to the effect
that such Transfer is in compliance with the Securities Act. Upon consummation
of the proposed transfer in accordance with the terms of the Indenture, the
transferred beneficial interest or Definitive Note will be subject to the
restrictions on transfer enumerated in the Private Placement Legend printed on
the IAI Global Note and/or the Definitive Notes and in the Indenture and the
Securities Act.

4. [_] Check if Transferee will take delivery of a beneficial interest in an
Unrestricted Global Note or of an Unrestricted Definitive Note.

          (a) [_] CHECK IF TRANSFER IS PURSUANT TO RULE 144. (i) The Transfer is
being effected pursuant to and in accordance with Rule 144 under the Securities
Act and in compliance with the transfer restrictions contained in the Indenture
and any applicable blue sky securities laws of any state of the United States
and (ii) the restrictions on transfer contained in the Indenture and the Private
Placement Legend are not required in order to maintain compliance with the
Securities Act. Upon consummation of the proposed Transfer in accordance with
the terms of the Indenture, the transferred beneficial interest or Definitive
Note will no longer be subject to the restrictions on transfer enumerated in the
Private Placement Legend printed on the Restricted Global Notes, on Restricted
Definitive Notes and in the Indenture.

          (b) [_] CHECK IF TRANSFER IS PURSUANT TO REGULATION S. (i) The 
Transfer is being effected pursuant to and in accordance with Rule 903 or Rule
904 under the Securities Act and in compliance with the transfer restrictions
contained in the Indenture and any applicable blue sky securities laws of any
state of the United States and (ii) the restrictions on transfer contained in
the Indenture and the Private Placement Legend are not required in order to
maintain compliance with the Securities Act. Upon consummation of the proposed
Transfer in accordance with the terms of the Indenture, the transferred
beneficial interest or Definitive Note will no longer be subject to the
restrictions on transfer enumerated in the Private Placement Legend printed on
the Restricted Global Notes, on Restricted Definitive Notes and in the
Indenture.

          (c) [_] CHECK IF TRANSFER IS PURSUANT TO OTHER EXEMPTION. (i) The
Transfer is being effected pursuant to and in compliance with an exemption from
the registration requirements of the Securities Act other than Rule 144, Rule
903 or Rule 904 and in compliance with the transfer restrictions contained in
the Indenture and any applicable blue sky securities laws of any State of the
United States and (ii) the restrictions on transfer contained in the Indenture
and the Private Placement Legend are not required in order to maintain
compliance with the Securities Act. Upon consummation of the proposed Transfer
in accordance with the terms of the Indenture, the transferred beneficial
interest or Definitive Note will not be subject to the restrictions on transfer
enumerated in the Private Placement Legend printed on the Restricted Global
Notes or Restricted Definitive Notes and in the Indenture.

                                      B-3

 
    This certificate and the statements contained herein are made for your
benefit and the benefit of the Company.


                                    --------------------------------- 
                                    [Insert Name of Transferor]


                                    By:
                                       ------------------------------ 
                                    Name:
                                    Title:

Dated:  ________ __, ____

                                      B-4

 
                      ANNEX A TO CERTIFICATE OF TRANSFER

1.  The Transferor owns and proposes to transfer the following:

                           [CHECK ONE OF (a) OR (b)]

     (a) [_] a beneficial interest in the:

         (i)   [_] 144A Global Note (CUSIP _________), or

         (ii)  [_] Regulation S Global Note (CUSIP _________), or

         (iii) [_] IAI Global Note (CUSIP ________); or

     (b) [_] a Restricted Definitive Note.

2.  After the Transfer the Transferee will hold:

                                  [CHECK ONE]

     (a) [_] a beneficial interest in the:
    
         (i)   [_] 144A Global Note (CUSIP ________), or

         (ii)  [_] Regulation S Global Note (CUSIP ________), or

         (iii) [_] IAI Global Note (CUSIP ________); or

         (iv)  [_] Unrestricted Global Note (CUSIP ________); or

     (b) [_] a Restricted Definitive Note; or

     (c) [_] an Unrestricted Definitive Note,

in accordance with the terms of the Indenture.

                                      B-5

 
                                   EXHIBIT C

                        FORM OF CERTIFICATE OF EXCHANGE

AMSC Acquisition Company, Inc.
10802 Parkridge Blvd.
Reston, Virginia  20191-5416
Telecopier No.:  703-758-6134
Attention:  Randy S. Segal, Esq.

State Street Bank and Trust Company
Goodwin Square
225 Asylum Street
Hartford, Connecticut 06103
Attention:  Steven Cimalore

          Re:  12 1/4% Senior Notes due 2008
               -----------------------------

                               (CUSIP __________)

          Reference is hereby made to the Indenture, dated as of March 31, 1998
(the "INDENTURE"), among AMSC Acquisition Company, Inc., as issuer (the
"COMPANY"), American Mobile Satellite Corporation, American Mobile Satellite
Sales Corporation, AMSC Sales Corporation Ltd., AMSC Subsidiary Corporation,
ARDIS Company, Motorola ARDIS, Inc., Motorola ARDIS Acquisition, Inc., ARDIS
Holding Company, and Radio Data Network Holding Corporation (collectively, the
"GUARANTORS") and State Street Bank and Trust Company, as trustee.  Capitalized
terms used but not defined herein shall have the meanings given to them in the
Indenture.

          ____________, (the "OWNER") owns and proposes to exchange the Note[s]
or interest in such Note[s] specified herein, in the principal amount of
$____________ in such Note[s] or interests (the "EXCHANGE").  In connection with
the Exchange, the Owner hereby certifies that:

1.  EXCHANGE OF RESTRICTED DEFINITIVE NOTES OR BENEFICIAL INTERESTS IN A
RESTRICTED GLOBAL NOTE FOR UNRESTRICTED DEFINITIVE NOTES OR BENEFICIAL INTERESTS
IN AN UNRESTRICTED GLOBAL NOTE

          (a) [_] CHECK IF EXCHANGE IS FROM BENEFICIAL INTEREST IN A RESTRICTED
                  -------------------------------------------------------------
GLOBAL NOTE TO BENEFICIAL INTEREST IN AN UNRESTRICTED GLOBAL NOTE.  In
- -----------------------------------------------------------------     
connection with the Exchange of the Owner's beneficial interest in a Restricted
Global Note for a beneficial interest in an Unrestricted Global Note in an equal
principal amount, the Owner hereby certifies (i) the beneficial interest is
being acquired for the Owner's own account without transfer, (ii) such Exchange
has been effected in compliance with the transfer restrictions applicable to the
Global Notes and pursuant to and in accordance with the United States Securities
Act of 1933, as amended (the "SECURITIES ACT"), (iii) the restrictions on
transfer contained in the Indenture and the Private Placement Legend are not
required in order to maintain compliance with the Securities Act and (iv) the
beneficial interest in an Unrestricted Global Note is being acquired in
compliance with any applicable blue sky securities laws of any state of the
United States.

          (b) [_] CHECK IF EXCHANGE IS FROM BENEFICIAL INTEREST IN A RESTRICTED
                  -------------------------------------------------------------
GLOBAL NOTE TO UNRESTRICTED DEFINITIVE NOTE.  In connection with the Exchange of
- -------------------------------------------                                     
the Owner's beneficial interest in a Restricted Global Note for an Unrestricted
Definitive Note, the Owner hereby certifies (i) 

                                      C-1

 
the Definitive Note is being acquired for the Owner's own account without
transfer, (ii) such Exchange has been effected in compliance with the transfer
restrictions applicable to the Restricted Global Notes and pursuant to and in
accordance with the Securities Act, (iii) the restrictions on transfer contained
in the Indenture and the Private Placement Legend are not required in order to
maintain compliance with the Securities Act and (iv) the Definitive Note is
being acquired in compliance with any applicable blue sky securities laws of any
state of the United States.

          (c) [_] CHECK IF EXCHANGE IS FROM RESTRICTED DEFINITIVE NOTE TO 
                  -------------------------------------------------------
BENEFICIAL INTEREST IN AN UNRESTRICTED GLOBAL NOTE.  In connection with the 
- --------------------------------------------------
Owner's Exchange of a Restricted Definitive Note for a beneficial interest in an
Unrestricted Global Note, the Owner hereby certifies (i) the beneficial interest
is being acquired for the Owner's own account without transfer, (ii) such
Exchange has been effected in compliance with the transfer restrictions
applicable to Restricted Definitive Notes and pursuant to and in accordance with
the Securities Act, (iii) the restrictions on transfer contained in the
Indenture and the Private Placement Legend are not required in order to maintain
compliance with the Securities Act and (iv) the beneficial interest is being
acquired in compliance with any applicable blue sky securities laws of any state
of the United States.

          (d) [_] CHECK IF EXCHANGE IS FROM RESTRICTED DEFINITIVE NOTE TO
                  -------------------------------------------------------
UNRESTRICTED DEFINITIVE NOTE.  In connection with the Owner's Exchange of a
- ----------------------------                                               
Restricted Definitive Note for an Unrestricted Definitive Note, the Owner hereby
certifies (i) the Unrestricted Definitive Note is being acquired for the Owner's
own account without transfer, (ii) such Exchange has been effected in compliance
with the transfer restrictions applicable to Restricted Definitive Notes and
pursuant to and in accordance with the Securities Act, (iii) the restrictions on
transfer contained in the Indenture and the Private Placement Legend are not
required in order to maintain compliance with the Securities Act and (iv) the
Unrestricted Definitive Note is being acquired in compliance with any applicable
blue sky securities laws of any state of the United States.

2.  EXCHANGE OF RESTRICTED DEFINITIVE NOTES OR BENEFICIAL INTERESTS IN
RESTRICTED GLOBAL NOTES FOR RESTRICTED DEFINITIVE NOTES OR BENEFICIAL INTERESTS
IN RESTRICTED GLOBAL NOTES

          (a) [_] CHECK IF EXCHANGE IS FROM BENEFICIAL INTEREST IN A RESTRICTED
                  -------------------------------------------------------------
GLOBAL NOTE TO RESTRICTED DEFINITIVE NOTE.  In connection with the Exchange of
- -----------------------------------------                                     
the Owner's beneficial interest in a Restricted Global Note for a Restricted
Definitive Note with an equal principal amount, the Owner hereby certifies that
the Restricted Definitive Note is being acquired for the Owner's own account
without transfer.  Upon consummation of the proposed Exchange in accordance with
the terms of the Indenture, the Restricted Definitive Note issued will continue
to be subject to the restrictions on transfer enumerated in the Private
Placement Legend printed on the Restricted Definitive Note and in the Indenture
and the Securities Act.

          (b) [_] CHECK IF EXCHANGE IS FROM RESTRICTED DEFINITIVE NOTE TO 
                  -------------------------------------------------------
BENEFICIAL INTEREST IN A RESTRICTED GLOBAL NOTE. In connection with the Exchange
- -----------------------------------------------
of the Owner's Restricted Definitive Note for a beneficial interest in the
[CHECK ONE] 144A Global Note, Regulation S Global Note, IAI Global Note with an
equal principal amount, the Owner hereby certifies (i) the beneficial interest
is being acquired for the Owner's own account without transfer and (ii) such
Exchange has been effected in compliance with the transfer restrictions
applicable to the Restricted Global Notes and pursuant to and in accordance with
the Securities Act, and in compliance with any applicable blue sky securities
laws of any state of the United States. Upon consummation of the proposed
Exchange in accordance with the terms of the Indenture, the beneficial interest
issued will be subject to the 

                                      C-2

 
restrictions on transfer enumerated in the Private Placement Legend printed on
the relevant Restricted Global Note and in the Indenture and the Securities Act.

                                      C-3

 
          This certificate and the statements contained herein are made for your
benefit and the benefit of the Company.

                                    __________________________________
                                         [Insert Name of Owner]


                                    By:  
                                       -------------------------------
                                    Name:
                                    Title:

Dated: ________________, ____

                                      C-4

 
                                   EXHIBIT D

                           FORM OF CERTIFICATE FROM
                  ACQUIRING INSTITUTIONAL ACCREDITED INVESTOR
                                        
AMSC Acquisition Company, Inc.
10802 Parkridge Blvd.
Reston, Virginia  20191-5416
Telecopier No.:  703-758-6134
Attention:  Randy S. Segal, Esq.

State Street Bank and Trust Company
Goodwin Square
225 Asylum Street
Hartford, Connecticut  06103
Attention:  Steven Cimalore

          Re:  12 1/4 % Senior Notes due 2008
               ------------------------------

                              (CUSIP __________)

          Reference is hereby made to the Indenture, dated as of March 31, 1998
(the "INDENTURE"), among AMSC Acquisition Company, Inc., as issuer (the
"COMPANY"), American Mobile Satellite Corporation, American Mobile Satellite
Sales Corporation, AMSC Sales Corporation Ltd., AMSC Subsidiary Corporation,
ARDIS Company, Motorola ARDIS, Inc., Motorola ARDIS Acquisition, Inc., ARDIS
Holding Company, and Radio Data Network Holding Corporation (collectively, the
"GUARANTORS") and State Street Bank and Trust Company, as trustee.  Capitalized
terms used but not defined herein shall have the meanings given to them in the
Indenture.

          In connection with our proposed purchase of $____________ aggregate
principal amount of:

          (a) [_] a beneficial interest in a Global Note, or

          (b) [_] a Definitive Note,

          we confirm that:

          1.  We understand that any subsequent transfer of the Notes or any
interest therein is subject to certain restrictions and conditions set forth in
the Indenture and the undersigned agrees to be bound by, and not to resell,
pledge or otherwise transfer the Notes or any interest therein except in
compliance with, such restrictions and conditions and the United States
Securities Act of 1933, as amended (the "SECURITIES ACT").

          2.  We understand that the offer and sale of the Notes have not been
registered under the Securities Act, and that the Notes and any interest therein
may not be offered or sold except as permitted in the following sentence.  We
agree, on our own behalf and on behalf of any accounts for which we are acting
as hereinafter stated, that if we should sell the Notes or any interest therein,

                                      D-11

 
we will do so only (A) to the Company or any subsidiary thereof, (B) in
accordance with Rule 144A under the Securities Act to a "qualified institutional
buyer" (as defined therein), (c) to an institutional "accredited investor" (as
defined below) that, prior to such transfer, furnishes (or has furnished on its
behalf by a U.S. broker-dealer) to you and to the Company a signed letter
substantially in the form of this letter and, if such transfer is in respect of
a principal amount of Notes, at the time of transfer of less than $250,000, an
Opinion of Counsel in form reasonably acceptable to the Company to the effect
that such transfer is in compliance with the Securities Act, (D) outside the
United States in accordance with Rule 904 of Regulation S under the Securities
Act, (E) pursuant to the provisions of Rule 144(k) under the Securities Act or
(F) pursuant to an effective registration statement under the Securities Act,
and we further agree to provide to any person purchasing the Definitive Note or
beneficial interest in a Global Note from us in a transaction meeting the
requirements of clauses (A) through (E) of this paragraph a notice advising such
purchaser that resales thereof are restricted as stated herein.

          3.  We understand that, on any proposed resale of the Notes or
beneficial interest therein, we will be required to furnish to you and the
Company such certifications, legal opinions and other information as you and the
Company may reasonably require to confirm that the proposed sale complies with
the foregoing restrictions.  We further understand that the Notes purchased by
us will bear a legend to the foregoing effect.  We further understand that any
subsequent transfer by us of the Notes or beneficial interest therein acquired
by us must be effected through one of the Initial Purchasers.

          4.  We are an institutional "accredited investor" (as defined in Rule
501(a)(1), (2), (3) or (7) of Regulation D under the Securities Act) and have
such knowledge and experience in financial and business matters as to be capable
of evaluating the merits and risks of our investment in the Notes, and we and
any accounts for which we are acting are each able to bear the economic risk of
our or its investment.

          5.  We are acquiring the Notes or beneficial interest therein
purchased by us for our own account or for one or more accounts (each of which
is an institutional "accredited investor") as to each of which we exercise sole
investment discretion.

          You and the Company are entitled to rely upon this letter and are
irrevocably authorized to produce this letter or a copy hereof to any interested
party in any administrative or legal proceedings or official inquiry with
respect to the matters covered hereby.


                              __________________________________________
                                 [Insert Name of Accredited Investor]



                              By:  
                                 ---------------------------------
                              Name:
                              Title:


Dated: __________________, ____

                                      D-12

 
                                   EXHIBIT E

                         FORM OF NOTATION OF GUARANTEE

                                        

          For value received, each Guarantor (which term includes any successor
Person under the Indenture) has, jointly and severally, unconditionally
guaranteed, to the extent set forth in the Indenture and subject to the
provisions in the Indenture dated as of March 31, 1998 (the "INDENTURE") among
AMSC Acquisition Company, Inc., the Guarantors listed on Schedule I thereto and
State Street Bank and Trust Company, as trustee (the "TRUSTEE"), (a) the due and
punctual payment of the principal of, premium, if any, and interest on the Notes
(as defined in the Indenture), whether at maturity, by acceleration, redemption
or otherwise, the due and punctual payment of interest on overdue principal and
premium, and, to the extent permitted by law, interest, and the due and punctual
performance of all other obligations of the Company to the Holders or the
Trustee all in accordance with the terms of the Indenture and (b) in case of any
extension of time of payment or renewal of any Notes or any of such other
obligations, that the same will be promptly paid in full when due or performed
in accordance with the terms of the extension or renewal, whether at stated
maturity, by acceleration or otherwise.  The obligations of the Guarantors to
the Holders of Notes and to the Trustee pursuant to the Note Guarantee and the
Indenture are expressly set forth in Articles 11 and 12 of the Indenture and
reference is hereby made to the Indenture for the precise terms of the Note
Guarantee.  Each Holder of a Note, by accepting the same, (a) agrees to and
shall be bound by such provisions, (b) authorizes and directs the Trustee, on
behalf of such Holder, to take such action as may be necessary or appropriate to
effectuate the subordination as provided in the Indenture and (c) appoints the
Trustee attorney-in-fact of such Holder for such purpose; provided, however,
that the Indebtedness evidenced by this Note Guarantee shall cease to be so
subordinated and subject in right of payment upon any defeasance of this Note in
accordance with the provisions of the Indenture.


                              AMERICAN MOBILE SATELLITE CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMERICAN MOBILE SATELLITE SALES CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:

                                      E-1

 
                              AMSC SALES CORPORATION LTD.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              AMSC SUBSIDIARY CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              ARDIS COMPANY


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS, INC.


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS ACQUISITION, INC.


                              By:
                                 ---------------------------------
                              Name:
                              Title:

                                      E-2

 
                              ARDIS HOLDING COMPANY


                              By:
                                 ---------------------------------
                              Name:
                              Title:



                              RADIO DATA NETWORK HOLDING CORPORATION


                              By:
                                 ---------------------------------
                              Name:
                              Title:

                                      E-3

 
                                   EXHIBIT F

                         FORM OF SUPPLEMENTAL INDENTURE
                    TO BE DELIVERED BY SUBSEQUENT GUARANTORS

                                        

          Supplemental Indenture (this "SUPPLEMENTAL INDENTURE"), dated as of
________ __, ____ among  __________________ (the "GUARANTEEING SUBSIDIARY"), a
direct or indirect subsidiary of AMSC Acquisition Company, Inc. (or its
permitted successor), a Delaware corporation (the "COMPANY"), the Company, the
other Guarantors (as defined in the Indenture referred to herein) and State
Street Bank and Trust Company, as trustee under the indenture referred to below
(the "TRUSTEE").

                              W I T N E S S E T H

          WHEREAS, the Company has heretofore executed and delivered to the
Trustee an indenture (the "INDENTURE"), dated as of March 31, 1998 providing for
the issuance of an aggregate principal amount of up to $335 million of 12 1/4%
Senior Notes due 2008 (the "NOTES");

          WHEREAS, the Indenture provides that under certain circumstances the
Guaranteeing Subsidiary shall execute and deliver to the Trustee a supplemental
indenture pursuant to which the Guaranteeing Subsidiary shall unconditionally
guarantee all of the Company's Obligations under the Notes and the Indenture on
the terms and conditions set forth herein (the "NOTE GUARANTEE"); and

          WHEREAS, pursuant to Section 9.01 of the Indenture, the Trustee is
authorized to execute and deliver this Supplemental Indenture.

          NOW THEREFORE, in consideration of the foregoing and for other good
and valuable consideration, the receipt of which is hereby acknowledged, the
Guaranteeing Subsidiary and the Trustee mutually covenant and agree for the
equal and ratable benefit of the Holders of the Notes as follows:

          1.  CAPITALIZED TERMS.  Capitalized terms used herein without
definition shall have the meanings assigned to them in the Indenture.

          2.  AGREEMENT TO GUARANTEE.  The Guaranteeing Subsidiary hereby agrees
as follows:

          (a)  Along with all Guarantors named in the Indenture, to jointly and
               severally Guarantee to each Holder of a Note authenticated and
               delivered by the Trustee and to the Trustee and its successors
               and assigns, irrespective of the validity and enforceability of
               the Indenture, the Notes or the obligations of the Company
               hereunder or thereunder, that:

               (i)  the principal of and interest on the Notes will be promptly
                    paid in full when due, whether at maturity, by acceleration,
                    redemption or otherwise, and interest on the overdue
                    principal of and interest on the Notes, if any, if lawful,
                    and all other obligations of the Company to the Holders or
                    the Trustee hereunder or thereunder will be promptly paid in
                    full or performed, all in accordance with the terms hereof
                    and thereof; and

                                      F-1

 
               (ii) in case of any extension of time of payment or renewal of
                    any Notes or any of such other obligations, that same will
                    be promptly paid in full when due or performed in accordance
                    with the terms of the extension or renewal, whether at
                    stated maturity, by acceleration or otherwise.  Failing
                    payment when due of any amount so guaranteed or any
                    performance so guaranteed for whatever reason, the
                    Guarantors shall be jointly and severally obligated to pay
                    the same immediately.

          (b)  The obligations hereunder shall be unconditional, irrespective of
               the validity, regularity or enforceability of the Notes or the
               Indenture, the absence of any action to enforce the same, any
               waiver or consent by any Holder of the Notes with respect to
               any provisions hereof or thereof, the recovery of any judgment
               against the Company, any action to enforce the same or any other
               circumstance which might otherwise constitute a legal or
               equitable discharge or defense of a guarantor.

          (c)  The following is hereby waived:  diligence  presentment, demand
               of payment, filing of claims with a court in the event of
               insolvency or bankruptcy of the Company, any right to require a
               proceeding first against the Company, protest, notice and all
               demands whatsoever.

          (d)  This Note Guarantee shall not be discharged except by complete
               performance of the obligations contained in the Notes and the
               Indenture.

          (e)  If any Holder or the Trustee is required by any court or
               otherwise to return to the Company, the Guarantors, or any
               custodian, Trustee, liquidator or other similar official acting
               in relation to either the Company or the Guarantors, any amount
               paid by either to the Trustee or such Holder, this Note
               Guarantee, to the extent theretofore discharged, shall be
               reinstated in full force and effect.

          (f)  The Guaranteeing Subsidiary shall not be entitled to any right of
               subrogation in relation to the Holders in respect of any
               obligations guaranteed hereby until payment in full of all
               obligations guaranteed hereby.

          (g)  As between the Guarantors, on the one hand, and the Holders and
               the Trustee, on the other hand, (x) the maturity of the
               obligations guaranteed hereby may be accelerated as provided in
               Article 6 of the Indenture for the purposes of this Note
               Guarantee, notwithstanding any stay, injunction or other
               prohibition preventing such acceleration in respect of the
               obligations guaranteed hereby, and (y) in the event of any
               declaration of acceleration of such obligations as provided in
               Article 6 of the Indenture, such obligations (whether or not due
               and payable) shall forthwith become due and payable by the
               Guarantors for the purpose of this Note Guarantee.

          (h)  The Guarantors shall have the right to seek contribution from any
               non-paying Guarantor so long as the exercise of such right does
               not impair the rights of the Holders under the Guarantee.

                                      F-2

 
          (i)  Pursuant to Section 11.02 of the Indenture, after giving effect
               to any maximum amount and any other contingent and fixed
               liabilities that are relevant under any applicable Bankruptcy or
               fraudulent conveyance laws, and after giving effect to any
               collections from, rights to receive contribution from or payments
               made by or on behalf of any other Guarantor in respect of the
               obligations of such other Guarantor under Article 11 of the
               Indenture shall result in the obligations of such Guarantor under
               its Note Guarantee not constituting a fraudulent transfer or
               conveyance.

          3.  EXECUTION AND DELIVERY.  Each Guaranteeing Subsidiary agrees that
the Note Guarantees shall remain in full force and effect notwithstanding any
failure to endorse on each Note a notation of such Note Guarantee.

          4.  GUARANTEEING SUBSIDIARY MAY CONSOLIDATE, ETC. ON CERTAIN TERMS.

     (a)  The Guaranteeing Subsidiary may not consolidate with or merge with or
          into (whether or not such Guarantor is the surviving Person) another
          corporation, Person or entity whether or not affiliated with such
          Guarantor unless:

          (i)  subject to Section 11.05 of the Indenture, the Person formed by
               or surviving any such consolidation or merger (if other than a
               Guarantor or the Company) unconditionally assumes all the
               obligations of such Guarantor, pursuant to a supplemental
               indenture in form and substance reasonably satisfactory to the
               Trustee, under the Notes, the Indenture and the Note Guarantee on
               the terms set forth herein or therein; and

          (ii) immediately after giving effect to such transaction, no Default
               or Event of Default exists.

     (b)  In case of any such consolidation, merger, sale or conveyance and upon
          the assumption by the successor corporation, by supplemental
          indenture, executed and delivered to the Trustee and satisfactory in
          form to the Trustee, of the Note Guarantee endorsed upon the Notes and
          the due and punctual performance of all of the covenants and
          conditions of the Indenture to be performed by the Guarantor, such
          successor corporation shall succeed to and be substituted for the
          Guarantor with the same effect as if it had been named herein as a
          Guarantor.  Such successor corporation thereupon may cause to be
          signed any or all of the Note Guarantees to be endorsed upon all of
          the Notes issuable hereunder which theretofore shall not have been
          signed by the Company and delivered to the Trustee.  All the Note
          Guarantees so issued shall in all respects have the same legal rank
          and benefit under the Indenture as the Note Guarantees theretofore and
          thereafter issued in accordance with the terms of the Indenture as
          though all of such Note Guarantees had been issued at the date of the
          execution hereof.

          (c) Except as set forth in Articles 4 and 5 of the Indenture, and
notwithstanding clauses (a) and (b) above, nothing contained in the Indenture or
in any of the Notes shall prevent any consolidation or merger of a Guarantor
with or into the Company or another Guarantor, or shall prevent any sale or
conveyance of the property of a Guarantor as an entirety or substantially as an
entirety to the Company or another Guarantor.

          5.  RELEASES.

                                      F-3

 
     (a)  In the event of a sale or other disposition of all of the assets of
          any Guarantor, by way of merger, consolidation or otherwise, or a sale
          or other disposition of all to the capital stock of any Guarantor,
          then such Guarantor (in the event of a sale or other disposition, by
          way of merger, consolidation or otherwise, of all of the capital stock
          of such Guarantor) or the corporation acquiring the property (in the
          event of a sale or other disposition of all or substantially all of
          the assets of such Guarantor) will be released and relieved of any
          obligations under its Note Guarantee; provided that the Net Proceeds
          of such sale or other disposition are applied in accordance with the
          applicable provisions of the Indenture, including without limitation
          Section 4.10 of the Indenture. Upon delivery by the Company to the
          Trustee of an Officers' Certificate and an Opinion of Counsel to the
          effect that such sale or other disposition was made by the Company in
          accordance with the provisions of the Indenture, including without
          limitation Section 4.10 of the Indenture, the Trustee shall execute
          any documents reasonably required in order to evidence the release of
          any Guarantor from its obligations under its Note Guarantee.

     (b)  Any Guarantor not released from its obligations under its Note
          Guarantee shall remain liable for the full amount of principal of and
          interest on the Notes and for the other obligations of any Guarantor
          under the Indenture as provided in Article 11 of the Indenture.

          6.  NO RECOURSE AGAINST OTHERS.  No past, present or future director,
officer, employee, incorporator, stockholder or agent of the Guaranteeing
Subsidiary, as such, shall have any liability for any obligations of the Company
or any Guaranteeing Subsidiary under the Notes, any Note Guarantees, the
Indenture or this Supplemental Indenture or for any claim based on, in respect
of, or by reason of, such obligations or their creation.  Each Holder of the
Notes by accepting a Note waives and releases all such liability.  The waiver
and release are part of the consideration for issuance of the Notes.  Such
waiver may not be effective to waive liabilities under the federal securities
laws and it is the view of the Commission that such a waiver is against public
policy.

          7.  NEW YORK LAW TO GOVERN.  THE INTERNAL LAW OF THE STATE OF NEW YORK
SHALL GOVERN AND BE USED TO CONSTRUE THIS SUPPLEMENTAL INDENTURE BUT WITHOUT
GIVING EFFECT TO APPLICABLE PRINCIPLES OF CONFLICTS OF LAW TO THE EXTENT THAT
THE APPLICATION OF THE LAWS OF ANOTHER JURISDICTION WOULD BE REQUIRED THEREBY.

          8.  COUNTERPARTS. The parties may sign any number of copies of this
Supplemental Indenture.  Each signed copy shall be an original, but all of them
together represent the same agreement.

          9.  EFFECT OF HEADINGS.  The Section headings herein are for
convenience only and shall not affect the construction hereof.

          10. THE TRUSTEE.  The Trustee shall not be responsible in any manner
whatsoever for or in respect of the validity or sufficiency of this Supplemental
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Guaranteeing Subsidiary and the Company.

                                      F-4

 
          IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed and attested, all as of the date first above
written.

Dated:  ________ __, ____

                              [GUARANTEEING SUBSIDIARY]


                              By:   
                                 ----------------------------
                              Name:
                              Title:


                              AMSC ACQUISITION COMPANY, INC.


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              AMERICAN MOBILE SATELLITE CORPORATION


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              AMERICAN MOBILE SATELLITE SALES CORPORATION


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              AMSC SALES CORPORATION LTD.


                              By:
                                 ----------------------------
                              Name:
                              Title:

                                      F-5

 
                              AMSC SUBSIDIARY CORPORATION


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              ARDIS COMPANY


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS, INC.


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              MOTOROLA ARDIS ACQUISITION, INC.


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              ARDIS HOLDING COMPANY


                              By:
                                 ----------------------------
                              Name:
                              Title:

                                      F-6

 
                              RADIO DATA NETWORK HOLDING CORPORATION


                              By:
                                 ----------------------------
                              Name:
                              Title:



                              STATE STREET BANK AND TRUST COMPANY


                              By:
                                 ----------------------------
                              Name:
                              Title:

                                      F-7

 
                                  SCHEDULE I

                             SCHEDULE OF GUARANTORS

          The following schedule lists each Guarantor under the Indenture as of
the Issue Date:

1.  American Mobile Satellite Corporation

2.  American Mobile Satellite Sales Corporation

3.  AMSC Sales Corporation Ltd.

4.  AMSC Subsidiary Corporation

5.  ARDIS Company

6.  Motorola ARDIS, Inc.

7.  Motorola ARDIS Acquisition, Inc.

8.  ARDIS Holding Company

9.  Radio Data Network Holding Corporation