Exhibit 10.64
                                                                   -------------


                              BRIDGE LOAN AGREEMENT


                  BRIDGE LOAN  AGREEMENT,  dated as of December 30, 1997,  (this
"Agreement")  made  by  and  among  AMSC  Subsidiary  Corporation,   a  Delaware
corporation  dually  incorporated as a Virginia Public Service  corporation (the
"Borrower"),  American Mobile Satellite Corporation, a Delaware corporation (the
"Parent  Guarantor")  and Hughes  Communications  Satellite  Services,  Inc.,  a
California corporation (the "Lender").

                              W I T N E S S E T H :

                  WHEREAS,  the  Borrower  has  requested  that the Lender  make
secured bridge term loans to the Borrower in an aggregate principal amount of up
to $10,000,000 which the Borrower will use for general corporate purposes; and

                  WHEREAS,  the Lender has agreed to make  secured  bridge  term
loans to the  Borrower,  but only upon the terms and  subject to the  conditions
contained herein.

                  NOW,  THEREFORE,  in  consideration of the premises and of the
mutual covenants herein contained and for other good and valuable consideration,
the  receipt  of which is  hereby  acknowledged,  the  parties  hereto  agree as
follows:

                  1.  Defined  Terms.  The  following  terms  when  used in this
Agreement  shall  have the  following  meanings  (such  meanings  being  equally
applicable to both the singular and plural forms of the terms defined):

                  "Agreement" shall mean this Bridge Loan Agreement, as the same
         may be amended,  modified or otherwise  supplemented  from time to time
         and  shall  refer to this  Agreement  as in  effect  on the  date  such
         reference becomes operative.

                  "AMRC" shall mean American Mobile Radio Corporation, a
         Delaware corporation.

                  "AMSC Pledge Agreement" shall mean the Pledge Agreement, dated
         as of the  date  hereof,  executed  by the  Parent  Guarantor,  as such
         agreement may be amended,  supplemented or otherwise modified from time
         to time,  pursuant to which the Parent  Guarantor  shall  pledge to the
         Lender the Pledged Collateral referred to therein, including the common
         stock of AMRC Holdings,  Inc. owned by the Parent Guarantor,  to secure
         the Secured Obligations of the Borrower.

                  "Business  Day" shall mean any day that is not a  Saturday,  a
         Sunday or a day on which banks are  required or  permitted to be closed
         in the States of New York or California.

                  "Closing Date" shall mean the date hereof.





                  "Corporation"  shall  mean each of the  Borrower,  the  Parent
         Guarantor and AMRC Holdings, Inc.

                  "Event of Default" shall have the meaning specified in
         Section 6(a) hereof.

                  "Governmental Authority" shall mean any nation or gov ernment,
         any state or other political  subdivision thereof, any central bank (or
         similar  monetary  or  regulatory   authority)   thereof,   any  entity
         exercising   executive,    legislative,    judicial,    regulatory   or
         administrative  functions  of or  pertaining  to  government,  and  any
         corporation  or other  entity  owned or  controlled,  through  stock or
         capital ownership or otherwise, by any of the foregoing.

                  "hereby," "herein," "hereof," "hereunder" and words of similar
         import  refer  to  this  Agreement  as  a  whole  (including,   without
         limitation,  any  schedules  hereto)  and not  merely  to the  specific
         section, paragraph or clause in which the respective word appears.

                  "Lender" shall mean Hughes Communications Satellite
         Services, Inc. or its successors or assigns.

                  "Lien"  shall  mean any  mortgage  or deed of  trust,  pledge,
         hypothecation,  assignment,  deposit arrangement,  lien, charge, claim,
         security interest, easement or encumbrance, or prefer ence, priority or
         other  security  agreement or  preferential  arrangement of any kind or
         nature whatsoever  (including,  without limitation,  any lease or title
         retention agreement,  any financing lease having substantially the same
         economic  effect  as  any of the  foregoing,  and  the  filing  of,  or
         agreement  to give,  any  financing  statement  perfecting  a  security
         interest under the UCC or comparable law of any jurisdiction).

                  "Loan" shall have the meaning specified in Section 2(a)
         hereof.

                  "Loan Documents" shall mean this Agreement,  the Term Note and
         the AMSC Pledge Agreement.




                                        2








                  "Material Adverse Effect" shall mean a material adverse change
         in, or a  material  adverse  effect  upon,  any of (a) the  operations,
         business, properties,  condition (financial or otherwise) of either the
         Parent Guarantor or the Borrower and its Subsidiaries taken as a whole;
         (b) the ability or prospective ability of the Parent Guarantor,  or the
         Borrower  to  perform  under  any  Loan  Document;  (c)  the  legality,
         validity,  binding effect or enforceability  of any Loan Document;  (d)
         the  perfection  or priority of any Lien  granted the Lender  under the
         AMSC Pledge Agreement; or (e) the Pledged Collateral.

                  "Maturity Date" shall mean March 31, 1999.

                  "Maximum Commitment Amount" shall mean $10,000,000.

                  "Maximum  Term Loan Amount"  shall mean the maximum  aggregate
         principal amount of Term Loans outstanding as of such date as set forth
         in Schedule I hereto.

                  "Morgan Credit  Agreement" shall mean the $150,000,000  Credit
         Agreement, dated as of June 28, 1996 among AMSC Subsidiary Corporation,
         American Mobile Satellite Corporation, the banks listed therein, Morgan
         Guaranty Trust Company of New York, as Documentation Agent, and Toronto
         Dominion (Texas),  Inc., as Administrative  Agent, as amended from time
         to time and giving effect to any waivers granted thereunder.

                  "Net  Proceeds"  shall mean for any asset  disposition,  lease
         agreement,  financing or equity transaction the aggregate cash proceeds
         received by the Parent  Guarantor or any of its Subsidiaries in respect
         of such transaction  (including any non-cash  proceeds thereof that are
         thereafter  sold or disposed of for, or otherwise  give rise to, cash),
         net of  direct  out-of-pocket  costs  of the  Parent  Guarantor  or its
         Subsidiaries relating to such transaction.

                  "Person"  shall  mean  any  individual,  sole  proprietorship,
         partnership,   joint  venture,  trust,   unincorporated   organization,
         association,  corporation,  institution,  public  benefit  corporation,
         entity or government (whether federal,  state,  county, city, municipal
         or  otherwise,  including,  without  limitation,  any  instrumentality,
         division, agency, body or department thereof).



                                        3








                  "Pledged  Collateral" shall mean all property and interests in
         property  and proceeds  thereof now owned or hereafter  acquired by the
         Parent  Guarantor  in or upon  which a Lien is  granted  under the AMSC
         Pledge Agreement.

                  "Pledged  Shares"  shall mean the capital stock covered by the
         AMSC Pledge Agreement.

                  "Projections"  shall mean the financial  projections  covering
         the months December,  1997 through March, 1999,  inclusive,  heretofore
         delivered  to  the  Lender  by the  Borrower,  referred  to in  Section
         3(a)(vi).

                  "Requirement  of  Law"  means,  as  to  any  Person,  any  law
         (statutory or common),  treaty,  rule or regulation or determination of
         an arbitrator or of a Governmental  Authority,  in each case applicable
         to or binding  upon the Person or any of its  property  or to which the
         Person or any of its property is subject;  in any case,  non-compliance
         with which by either of the Parent  Guarantor  or the Borrower or their
         Subsidiaries  could  reasonably be expected to have a Material  Adverse
         Effect.

                  "Secured  Obligations"  shall mean all of the unpaid principal
         amount of, and  accrued  interest  on, the Term Note,  and all  amounts
         payable to Lender under this  Agreement  and the AMSC Pledge  Agreement
         together with all costs of the Lender,  including,  without limitation,
         reasonable attorneys' fees, incurred in connection with the enforcement
         of any of its rights and remedies hereunder and thereunder.

                  "Subsidiary"  shall mean, with respect to any Person,  (a) any
         corporation  of which an aggregate of more than 50% of the  outstanding
         stock having  ordinary voting power to elect a majority of the board of
         directors of such corporation  (irrespective  of whether,  at the time,
         stock of any other class or classes of such  corporation  shall have or
         might have voting power by reason of the happening of any  contingency)
         is at the time,  directly or indirectly,  owned legally or beneficially
         by such Person and/or one or more Subsidiaries of such Person,  and (b)
         any partnership in which such Person and/or one or more Subsidiaries of
         such Person  shall have an  interest  (whether in the form of voting or
         participation in profits or capital contribution) of more than 50%.



                                        4








                  "Term Note" shall have the meaning specified in Section
         2(a) hereof.

                  "UCC" shall mean the Uniform  Commercial Code as the same may,
         from time to time,  be in  effect  in the State of New York;  provided,
         however,  in the event that, by reason of mandatory  provisions of law,
         any or all of the  attachment,  perfection  or priority of the Lender's
         security interest in any Pledged  Collateral is governed by the Uniform
         Commercial Code as in effect in a jurisdiction  other than the State of
         New York, the term "UCC" shall mean the Uniform  Commercial  Code as in
         effect in such other jurisdiction for purposes of the provisions hereof
         relating to such attachment, perfection or priority and for purposes of
         definitions related to such provisions.

                  2.       Loan Provisions

                  (a)  Loans.  On  the  terms  and  subject  to  the  conditions
contained  in this  Agreement,  the Lender  agrees to make term loans (the "Term
Loans") to the Borrower  from time to time from the Closing Date until March 31,
1998; provided that the aggregate amount of the Term Loans shall not on any date
exceed  the  Maximum  Term Loan  Amount  for such date set forth in  Schedule  I
hereto.  The  Borrower  shall give  Lender at least  three  Business  Days prior
written notice of each Loan,  specifying the amount and date thereof.  Each Loan
will be at least $100,000 or a larger multiple thereof.

                  The Term Loans shall collectively be referred to as the "Loan"
and the Loan shall be evidenced by a promissory note,  substantially in the form
attached hereto as Exhibit A ("Term Note"),  to be executed and delivered by the
Borrower on the Closing Date.

                  (b) Payments.  (i) All payments and  prepayments  made on each
Term Loan  shall be made in lawful  money of the  United  States of  America  in
immediately  available  funds no later than 12 o'clock Noon,  New York, New York
time, at Bank of America, Concord, California, Account No. 12356-06628,  Account
Name: Hughes  Electronics  Corp., ABA No. 121000358,  or such other place as the
Lender  shall  designate  on the date  such  payment  is due in  writing  to the
Borrower.

                         (ii) If any payments or prepayment on a Term Loan shall
                    become  due on a day  which  is  not a  Business  Day,  such
                    payment or prepayment  shall be made on the next  succeeding
                    Business  Day and such  extension of time shall in such case
                    be included in computing  interest in  connection  with such
                    payment or prepayment.


                                        5








                         (iii) All payments made by the Borrower hereunder shall
                    be made without setoff, counterclaim or other defense.

                  (c) Repayment of Principal.  The principal amount of each Term
Loan shall be payable, together with accrued and unpaid interest thereon, on the
Maturity Date.

                  (d) Mandatory Prepayment. If on or after the Closing Date, the
Parent Guarantor or any of its  Subsidiaries  (other than AMRC or AMRC Holdings,
Inc.) shall receive Net Proceeds from any asset  disposition,  lease  agreement,
financing or equity  transaction,  Borrower shall immediately apply all such Net
Proceeds to prepay the Loan. Any amounts so prepaid may not be reborrowed.

                  (e) Optional  Prepayment.  The Borrower  may, at any time upon
notice to the Lender stating the proposed date and aggregate principal amount of
the prepayment,  prepay,  without penalty or premium, the outstanding  principal
amount of the Loan in whole or in part,  together  with accrued  interest to the
date of such prepayment on the principal amount prepaid; provided, however, that
each partial prepayment shall be in an aggregate  principal amount not less than
$100,000  or integral  multiples  of  $100,000  in excess  thereof.  Any amounts
prepaid may not be reborrowed. Upon the giving of such notice of prepayment, the
principal  amount of the Loan  specified  to be  prepaid  shall  become  due and
payable on the date specified for such prepayment.

                  (f)  Interest.   The  Borrower   shall  pay  interest  on  the
outstanding  principal  amount of each Term Loan for each day from and including
the date such Term Loan is made to but excluding the date such Term Loan is paid
in full to the Lender, monthly in arrears on the last Business Day of each month
("Interest  Payment  Date")  commencing on the first such date after the date of
the initial Term Loan, at a rate equal to 12% per annum, based on a 360-day year
of twelve 30-day  months for the actual  number of days  elapsed;  provided that
while an Event of  Default  is  continuing  each Term Loan and all  amounts  due
hereunder shall bear interest at a rate of 14% per annum.




                                        6









                  (g)  Proceeds.  The Borrower  shall apply the proceeds of each
Term Loan for general  corporate  purposes.  None of such proceeds shall be used
directly or  indirectly  for the purpose of buying or  carrying  "margin  stock"
within the meaning of Regulation G of the Federal Reserve Board.

                  3.       Conditions Precedent.

                  (a) Conditions  Precedent to Initial Term Loan. The obligation
of the  Lender to make its  initial  Term Loan to the  Borrower  is  subject  to
satisfaction of the conditions precedent that the Lender shall have received, on
the Closing  Date,  the  following,  in form and substance  satisfactory  to the
Lender:

                         (i) Counterparts of this Agreement duly executed by the
                    Parent Guarantor, the Borrower and the Lender.

                         (ii) Term Note of the  Borrower  dated the Closing Date
                    payable  to the order of the  Lender in a  principal  amount
                    equal to the Maximum Commitment Amount.

                         (iii)  A  copy  of  the  articles  or   certificate  of
                    incorporation  (or other  organizational  documents)  of the
                    Parent  Guarantor,  the Borrower,  AMRC  Holdings,  Inc. and
                    AMRC,  certified  as of a recent  date by the  Secretary  of
                    State of the state of incorporation of such entity, together
                    with  certificates  of such officials  attesting to the good
                    standing of such entity.

                         (iv) An executed copy of a favorable opinion of counsel
                    to the  Borrower as to matters as the Lender may  reasonably
                    request.

                         (v) The AMSC  Pledge  Agreement,  duly  executed by the
                    Parent Guarantor, together with:

                           (A)  certificates  representing  the Pledged  Shares,
                  together  with  undated  stock  powers for such  certificates,
                  executed in blank; and

                           (B) evidence that all other actions necessary,  or in
                  the  opinion of the Lender,  desirable  to perfect and protect
                  the Lien created by the aforementioned AMSC Pledge



                                        7







                  Agreement  have  been  taken   including  the  filing  of  UCC
                  financing statements.

                         (vi)   Projections   by  the   Borrower   of  (i)   the
                    consolidated  results of operations and changes in financial
                    position  of  Borrower  and its  Subsidiaries  on a  monthly
                    basis, from December 1997 through March 1999, inclusive, and
                    (ii) the consolidated  balance sheet of the Borrower and its
                    Subsidiaries  as of the last day of each month  during  such
                    period  based  upon all  information  known to the  Borrower
                    which is pertinent thereto; such Projections shall have been
                    made  by the  Borrower  on a  reasonable  basis  and in good
                    faith.

                         (vii) Balance sheet,  income statement and statement of
                    cash  flow  of the  Borrower  and  its  Subsidiaries,  which
                    financial  statements shall present fairly,  in all material
                    respects,  the  financial  position of the  Borrower and its
                    Subsidiaries as at September 30, 1997.

                         (viii)  A  certificate,  signed  by an  officer  of the
                    Borrower,  stating  that on the Closing  Date the  following
                    statements are true and correct with respect to the Borrower
                    and its Subsidiaries:

                           (A)  All  necessary   approvals   from   Governmental
                  Authorities  and all  necessary  approvals  from third parties
                  required  to be  obtained  in  connection  with the making and
                  performance  of this  Agreement and the AMSC Pledge  Agreement
                  and the  transactions  contemplated  hereby have been obtained
                  and remain in effect.

                           (B) There exists no judgment,  order,  injunction  or
                  other  restraint  prohibiting or imposing  materially  adverse
                  conditions  upon the Loan  and the  transactions  contemplated
                  hereby.

                           (C)   There   exists   no   claim,    action,   suit,
                  investigation or proceeding  (including,  without  limitation,
                  shareholder  or  derivative  litigation)  pending  or,  to the
                  knowledge  of  the  Borrower  or  any  of  its   Subsidiaries,
                  threatened   in  any  court  or  before  any   arbitrator   or
                  Governmental  Authority  which  relates  to,  the Loan and the
                  transactions contemplated hereby or which has



                                        8








                  a reasonable likelihood of having a material adverse effect on
                  the  Loan  and  the  transactions  contemplated  hereby  or  a
                  Material Adverse Effect.

                           (D) There  shall not occur as a result of the  making
                  and   performance  of  this  Agreement  and  the  AMSC  Pledge
                  Agreement and the transactions  contemplated  hereby a default
                  (or any event which with the giving of notice or lapse of time
                  or both would be a default) under  contractual  obligations of
                  or relating to any Corporation.

                         (ix) Such additional documents,  information (including
                    financial  information)  and  materials  as the  Lender  may
                    reasonably request.

                  (b) Conditions  Precedent to Each Term Loan. The obligation of
the  Lender to make any Term  Loan  (including  the  initial  Term  Loan) to the
Borrower shall be subject to the further conditions precedent that:

                         (i) The following  statements shall be true on the date
                    of such Term Loan,  before and after giving  effect  thereto
                    and to the  application  of the proceeds  therefrom (and the
                    acceptance by the Borrower of the proceeds of such Term Loan
                    shall  constitute  a  representation  and  warranty  by  the
                    Borrower that on the date of such Term Loan such  statements
                    are true):

                           (A) The  representations and warranties of the Parent
                  Guarantor,  the  Borrower  and its  Subsidiaries  contained in
                  Section 4 hereof are  correct on and as of such date as though
                  made on and as of such date;

                           (B) No Event of Default or event  which with lapse of
                  time or lack of notice  would have  become an Event of Default
                  has  occurred and is  continuing  or will result from the Term
                  Loan being made on such date; and

                           (C) The financial performance of the Borrower and its
                  Subsidiaries  shall  be not  less  than  as set  forth  in the
                  Projections  provided  by the  Borrower  pursuant  to  Section
                  3(a)(vi) hereof.




                                        9








                         (ii) The making of the such Term Loan on such date does
                    not  violate  any  Requirement  of Law and is not  enjoined,
                    temporarily, preliminarily or permanently.

                         (iii) The Lender shall have  received  such  additional
                    documents,  information  and  materials  as the  Lender  may
                    reasonably request.


                  4.   Representations  and  Warranties.   Each  of  the  Parent
Guarantor, the Borrower and its Subsidiaries hereby represents and warrants that
as of the Closing Date:

                  (a) Each  Corporation is duly  incorporated,  validly existing
and in good standing under the laws of its jurisdiction of incorporation and has
the requisite corporate power to make and perform the Loan Documents to which it
is a party  and each  Loan  Document  has been  duly  authorized,  executed  and
delivered by each Corporation  party thereto and constitutes a legal,  valid and
binding obligation of such Corporation enforceable in accordance with its terms.

                  (b) The  execution,  delivery  and  performance  of each  Loan
Document by each  Corporation  party thereto does not: (i) contravene its or any
of its Subsidiaries' respective certificate of incorporation or by-laws or other
comparable governing documents, (ii) violate any other applicable Requirement of
Law, or any order or decree of any Governmental  Authority or arbitrator,  (iii)
conflict  with or result in the breach of, or  constitute  a default  under,  or
result in or permit the termination or  acceleration  of, any of its contractual
obligations or any contractual obligations of its Subsidiaries or (iv) result in
the creation or  imposition of any Lien upon any of its property or the property
of any of its Subsidiaries,  other than those in favor of the Lender pursuant to
the AMSC Pledge Agreement; and

                  (c)  The   execution,   delivery   and   performance   by  the
Corporations of the Loan Documents and the transactions  contemplated hereby and
thereby does not and will not require the consent of, authorization by, approval
of, notice to, or filing or registration with, any Governmental Authority or any
other  Person,  other than those  which have been,  prior to the  Closing  Date,
delivered to the Lender.



                                       10








                  (d) The  Projections  delivered to the Lender were prepared by
the Borrower on a reasonable basis and in good faith.

                  (e) There  exists no claim,  action,  suit,  investigation  or
proceeding (including, without limitation, shareholder or derivative litigation)
pending  or,  to the  knowledge  of  the  Borrower  or any of its  Subsidiaries,
threatened in any court or before any arbitrator or Governmental Authority which
relates  to, the Loan and the  transactions  contemplated  hereby or which has a
reasonable  likelihood of having a material  adverse  effect on the Loan and the
transactions contemplated hereby or a Material Adverse Effect.

                  (f) The  representations  and warranties made in Sections 4.7,
4.8, 4.9, 4.11,  4.12,  4.13, 4.14, 4.16 and 4.17 of the Morgan Credit Agreement
by the Borrower and the Parent Guarantor are correct (with references therein to
"the  Agreement"  meaning  this  Agreement,  to "Loan  Documents"  meaning  Loan
Documents  as defined  herein and to the  "Banks" or the  "Agents"  meaning  the
Lender)  as though  made to the Lender  with  respect to each Term Loan and this
Agreement.

                  5.  Covenants.  Each of the Parent  Guarantor and the Borrower
covenants  and  agrees  with the  Lender  that  from and  after the date of this
Agreement and until the Secured  Obligations  are fully  satisfied  such parties
shall comply with its covenants made in Article 5 of the Morgan Credit Agreement
(as such  Agreement is in effect on the date hereof without giving effect to any
amendments or waiver thereof after the date hereof and with  references  therein
to "the Agreement"  meaning this  Agreement,  to "Loan  Documents"  meaning Loan
Documents  as defined  herein and to the  "Banks" or the  "Agents"  meaning  the
Lender) as though  such  covenants  were made by such  party to the Lender  with
respect to the Loan and this Agreement; provided that the failure to comply with
any of the  provisions  of Sections  5.28 through 5.32 or the proviso in Section
5.33 of the Morgan Credit  Agreement shall not constitute a breach  hereunder so
long as such  failure  to comply  is not an Event of  Default  under the  Morgan
Credit Agreement.









                                       11







                  6.  Events of Default; Rights and Remedies


                  (a) The occurrence of any one or more of the following  events
(regardless  of the  reason  thereof)  shall  constitute  an "Event of  Default"
hereunder:

                         (i) The  Borrower  shall  fail to make any  payment  of
                    principal  of, or  interest  on,  the Term Note when due and
                    payable or declared due and payable.

                         (ii) The  Borrower  shall fail or  neglect to  perform,
                    keep or observe any other provision of this Agreement or any
                    Loan  Document  and the same shall remain  unremedied  for a
                    period  ending  thirty  (30) days after the  Borrower  shall
                    receive written notice of any such failure from the Lender.

                         (iii) Any representation or warranty of any corporation
                    herein or in any Loan Document  shall be untrue or incorrect
                    in any material respect,  as of the date when made or deemed
                    made.

                         (iv) Any  "Event  of  Default"  shall  occur  under the
                    Morgan Credit Agreement.

                         (v) A case or  proceeding  shall  have  been  commenced
                    against any Corporation or AMRC in a court having  competent
                    jurisdiction  seeking a decree or order in  respect  of such
                    Corporation or AMRC, (A) under title 11 of the United States
                    Code, as now constituted or hereafter amended,  or any other
                    applicable  federal,  state or foreign  bankruptcy  or other
                    similar   law,  (B)   appointing   a  custodian,   receiver,
                    liquidator,  assignee,  trustee or sequestrator  (or similar
                    official) of such  Corporation or AMRC or of any substantial
                    part of its  properties,  or (C) ordering the  winding-up or
                    liquidation  of the affairs of such  Corporation or AMRC and
                    such case or proceeding shall remain undismissed or unstayed
                    for sixty (60)  consecutive days or such court shall enter a
                    decree or order  granting the relief  sought in such case or
                    proceeding.

                         (vi) Any  Corporation or AMRC shall (A) file a petition
                    seeking  relief under title 11 of the United States Code, as
                    now   constituted  or  hereafter   amended,   or  any  other
                    applicable  federal,  state or foreign  bankruptcy  or other
                    similar law, (B) consent to the  institution  of proceedings
                    thereunder  or to the filing of any such  petition or to the
                    appointment   of  or  taking   possession  by  a  custodian,
                    receiver, liquidator,  assignee, trustee or sequestrator (or
                    similar  official)  of any  Corporation  or  AMRC  or of any
                    substantial  part of its  properties,  (C) fail generally to
                    pay its  debts as such  debts  become  due,  or (D) take any
                    corporate action in furtherance of any such action.



                                       12








                  (b)      Rights and Remedies Upon Event of Default . 
                           ------------------------------------------

If any Event of Default  specified  in Section  6(a) shall have  occurred and be
continuing, the Lender may by notice to the Borrower terminate its commitment to
make Term Loans hereunder and/or declare the Secured Obligations to be forthwith
due and payable,  whereupon all such Secured Obligations shall become and be due
and payable, without presentment, demand, protest or further notice of any kind,
all of which are expressly waived by the Borrower; provided, however, that  upon
                                                   --------  -------
the  occurrence  of an Event of Default  specified  in  Section  6(a)(v) or (vi)
hereof,  such  commitment to make Term Loans shall  automatically  terminate and
such Secured  Obligations  shall  automatically  become due and payable  without
declaration, notice or demand by the Lender.

                  7.       Parent Guaranty.

                  (a)  The  Parent   Guaranty.   The  Parent   Guarantor  hereby
unconditionally  guarantees  the full and  punctual  payment  (whether at stated
maturity,  upon  acceleration  or otherwise) of the principal of and interest on
the Term Note issued by the Borrower  pursuant to this  Agreement,  and the full
and  punctual  payment of all other  Secured  Obligations.  Upon  failure by the
Borrower to pay punctually any such amount, the Parent Guarantor shall forthwith
on demand pay the amount not so paid at the place and in the manner specified in
this Agreement.

                  (b)  Guaranty  Unconditional.  The  obligations  of the Parent
Guarantor  hereunder shall be  unconditional  and absolute and, without limiting
the generality of the foregoing, shall not be released,  discharged or otherwise
affected by:

                         (i) any  extension,  renewal,  settlement,  compromise,
                    waiver  or  release  in  respect  of any  obligation  of the
                    Borrower under this Agreement or the Term Note, by operation
                    of law or otherwise;




                                       13








                         (ii) any  modification or amendment of or supplement to
                    this Agreement or the Term Note or any Loan Document;

                         (iii)  any  release,   impairment,   non-perfection  or
                    invalidity  of any  direct  or  indirect  security  for  any
                    obligation of the Borrower  under this Agreement or the Term
                    Note;

                         (iv) any change in the corporate  existence,  structure
                    or  ownership  of  any   Corporation,   or  any  insolvency,
                    bankruptcy,   reorganization  or  other  similar  proceeding
                    affecting  any  Corporation  or its assets or any  resulting
                    release  or  discharge  of any  obligation  of the  Borrower
                    contained in this Agreement or the Term Note;

                         (v) the existence of any claim, set-off or other rights
                    which the Parent  Guarantor may have at any time against the
                    Borrower,  the  Lender  or  any  other  Person,  whether  in
                    connection herewith or any unrelated transactions,  provided
                    that nothing  herein shall prevent the assertion of any such
                    claim by separate suit or compulsory counterclaim;

                         (vi) any invalidity or unenforceability  relating to or
                    against the Borrower for any reason of this Agreement or the
                    Term Note, or any provision of applicable  law or regulation
                    purporting  to prohibit  the payment by the  Borrower of the
                    principal  of or  interest  on the  Term  Note or any  other
                    amount payable by the Borrower under this Agreement; or

                         (vii) any other act or  omission to act or delay of any
                    kind by the Borrower,  the Lender or any other Person or any
                    other  circumstance  whatsoever  which  might,  but  for the
                    provisions  of  this   paragraph,   constitute  a  legal  or
                    equitable  discharge of the Parent  Guarantor's  obligations
                    hereunder.

                  (c)  Discharge  Only Upon Payments In Full;  Reinstatement  In
Certain Circumstances. The Parent Guarantor's obligations hereunder shall remain
in full force and effect  until the  principal  of and interest on the Term Note
and all  Secured  Obligations  shall have been paid in full.  If at any time any
payment  of the  principal  of or  interest  on the  Term  Note  or any  Secured
Obligations  is  rescinded or must be  otherwise  restored or returned  upon the
insolvency,  bankruptcy  or  reorganization  of the Borrower or  otherwise,  the
Parent Guarantor's  obligations  hereunder with respect to such payment shall be
reinstated at such time as though such payment had been due but not made at such
time.



                                       14







                  (d)  Waiver by the  Parent  Guarantor.  The  Parent  Guarantor
irrevocably  waives  acceptance  hereof,  presentment,  demand,  protest and any
notice not provided for herein,  as well as any requirement that at any time any
action be taken by any Person against the Borrower or any other Person.

                  (e)  Subrogation.  Until  such  time as all  principal  of and
interest on the Term Note issued by the Borrower  pursuant to this Agreement and
all  Secured  Obligations  have  indefeasibly  been  paid in  full,  the  Parent
Guarantor shall not assert any rights to which it may be entitled,  by operation
of law or otherwise,  upon making any payment  hereunder to be subrogated to the
rights of the payee against the Borrower with respect to such payment or against
any  direct or  indirect  security  therefor,  or  otherwise  to be  reimbursed,
indemnified  or  exonerated  by or for the  account of the  Borrower  in respect
thereof.

                  (f)  Stay of  Acceleration.  If  acceleration  of the time for
payment of any amount  payable by the Borrower  under this Agreement or the Term
Note is stayed upon insolvency,  bankruptcy or  reorganization  of the Borrower,
all such  amounts  otherwise  subject  to  acceleration  under the terms of this
Agreement  shall  nonetheless  be  payable  by the  Parent  Guarantor  hereunder
forthwith on demand by the Lender.

                  8.  Reinstatement.  This Agreement  shall remain in full force
and effect and  continue  to be  effective  should any  petition  be filed by or
against the  Borrower for  liquidation  or  reorganization,  should the Borrower
become  insolvent or make an assignment for the benefit of creditors or should a
receiver or trustee be appointed for all or any  significant  part of any of the
Borrower's assets,  and shall continue to be effective or be reinstated,  as the
case may be, if at any time payment and performance of the Secured  Obligations,
or any part thereof,  is,  pursuant to applicable  law,  rescinded or reduced in
amount,  or must otherwise be restored or returned by any obligee of the Secured
Obligations,  whether as a "voidable preference",  "fraudulent  conveyance",  or
otherwise,  all as though such payment or performance  had not been made. In the
event that any payment, or any part thereof, is rescinded,  reduced, restored or
returned, the Secured Obligations shall be reinstated and deemed reduced only by
such amount paid and not so rescinded, reduced, restored or returned.



                                       15







                  9. Notices.  Except as otherwise provided herein,  whenever it
is  provided  herein  that  any  notice,  demand,  request,  consent,  approval,
declaration or other  communication  shall or may be given to or served upon any
of the parties by any other  party,  or whenever  any of the parties  desires to
give or serve upon any other communication with respect to this Agreement,  each
such  notice,  demand,  request,   consent,   approval,   declaration  or  other
communication  shall be in writing and either  shall be delivered in person with
receipt  acknowledged  or sent by registered or certified  mail,  return receipt
requested,  postage prepaid, or by telecopy and confirmed by telecopy answerback
addressed as follows:

                  (a)  If to the Lender, at:

                       Hughes Communications Satellite Services, Inc.
                          c/o Hughes Electronics Corp.
                          7200 Hughes Terrace
                          Los Angeles, California 90045

                           Attention:  Craig R. Valiza,
                           Telecopy:   (310) 568-6051

                           with a copy to:

                        Attention: Amnon Carr, Assistant Treasurer
                           Telecopy: (310) 348-8791


                  (b)   If to the Borrower, at:

                        AMSC Subsidiary Corporation
                           10802 Parkridge Boulevard
                           Reston, Virginia 20191
                           Attention:  Treasurer
                                       General Counsel

                           Telecopy:   Treasurer:  (703) 716-6366
                                       General Counsel:  (703) 758-6134





                                       16








or at such  other  address  as may be  substituted  by  notice  given as  herein
provided.  The giving of any notice required  hereunder may be waived in writing
by the party  entitled to receive such notice.  Every notice,  demand,  request,
consent, approval,  declaration or other communication hereunder shall be deemed
to have been duly  given or  served on the date on which  personally  delivered,
with receipt  acknowledged,  telecopied and confirmed by telecopy  answerback or
three (3) Business  Days after the same shall have been  deposited in the United
States  mail.  Failure  or delay in  delivering  copies of any  notice,  demand,
request,  consent,  approval,  declaration or other communication to the persons
designated  above  to  receive  copies  shall  in no way  adversely  affect  the
effectiveness of such notice, demand, request, consent, approval, declaration or
other communication.

                  10.  Severability.  Any provision of this  Agreement  which is
prohibited or unenforceable in any jurisdiction  shall, as to such jurisdiction,
be ineffective to the extent of such  prohibition  or  unenforceability  without
invalidating  the  remaining  provisions  hereof,  and any such  prohibition  or
unenforceability   in  any   jurisdiction   shall  not   invalidate   or  render
unenforceable such provision in any other jurisdiction.

                  11.  No  Waiver.  The  Lender  shall  not by any  act,  delay,
omission  or  otherwise  be deemed to have  waived any of its rights or remedies
hereunder, and no waiver shall be valid unless in writing, signed by the Lender,
and then only to the extent  therein  set  forth.  A waiver by the Lender of any
right or remedy hereunder on any one occasion shall not be construed as a bar to
any right or remedy  which the  Lender  would  otherwise  have had on any future
occasion.  No failure to exercise nor any delay in exercising on the part of the
Lender,  any right,  power or  privilege  hereunder,  shall  operate as a waiver
thereof,  nor shall any  single  or  partial  exercise  of any  right,  power or
privilege  hereunder  preclude  any  other or  future  exercise  thereof  or the
exercise of any other right, power or privilege. None of the terms or provisions
of this  Agreement  may be waived,  altered,  modified  or amended  except by an
instrument  in writing,  duly executed by the Lender and by the Borrower and the
Parent Guarantor.

                  12.  Amendments.  This Agreement,  the Term Note and the other
Loan  Documents  constitute  the  complete  agreement  between the parties  with
respect to the subject matter hereof and may not be modified, altered or amended
except by an  agreement in writing  signed by the  Borrower  and the Lender.  No
amendment  or waiver of any  provision of this  Agreement  or the Term Note,  no
consent to any departure by the Borrower  therefrom,  nor release of any Pledged
Collateral,  shall in any event be effective unless the same shall be in writing
signed by the Lender.



                                       17








                  13.  Assignments; Successors and Assigns; Governing Law.

                  (a) The Borrower and the Parent Guarantor may not sell, assign
or transfer any of this  Agreement or any portion  thereof,  including,  without
limitation, its rights, title, interests,  remedies, powers and duties hereunder
or thereunder, without the prior written consent of the Lender.

                  (b) This Agreement and all obligations of the Borrower and the
Parent  Guarantor  hereunder shall be binding upon the successors and assigns of
the Borrower and the Parent Guarantor,  and shall,  together with the rights and
remedies of the Lender hereunder, inure to the benefit of the Lender, all future
holders of the Term Note and their respective  successors and assigns.  No sales
of participations,  other sales, assignments, transfers or other dispositions of
any agreement governing or instrument  evidencing the Secured Obligations or any
portion  thereof or interest  therein  shall in any manner  affect the  security
interest granted to the Lender under any Loan Document.

                  (c) This Agreement  shall be governed by, and be construed and
interpreted  in  accordance  with,  the laws of the State of New  York,  without
regard to the provisions thereof relating to conflict of laws.

                  14. Survival.  The  representations  and warranties of each of
the Parent  Guarantor  and the  Borrower  in this  Agreement  shall  survive the
execution,  delivery and acceptance hereof by the parties hereto and the closing
of the transactions described herein or re lated hereto.

                  15.  Remedies  Cumulative.  The  Lender's  rights and remedies
under this Agreement  shall be cumulative and  nonexclusive  of any other rights
and  remedies  which the Lender may have  under any other  agreement,  including
without limitation,  the Term Note or the AMSC Pledge Agreement, by operation of
law or otherwise.




                                       18







                  16. WAIVER OF JURY TRIAL. EACH OF THE PARENT GUARANTOR AND THE
BORROWER  WAIVES  ALL  RIGHT TO TRIAL BY JURY IN ANY  ACTION  OR  PROCEEDING  TO
ENFORCE OR DEFEND ANY RIGHTS OR REMEDIES HEREUNDER,  UNDER THIS AGREEMENT, UNDER
THE TERM  NOTE  UNDER  THE AMSC  PLEDGE  AGREEMENT  OR  RELATING  TO EACH OF THE
FOREGOING.

                  17.  Section  Titles.  The Section  titles  contained  in this
Agreement  are and shall be without  substantive  meaning or content of any kind
whatsoever and are not a part of the agreement between the parties hereto.

                  18.  Counterparts.  This Agreement may be executed in any
number of counterparts, which shall, collectively and separately,
constitute one agreement.





                                       19







                  IN WITNESS WHEREOF, each of the parties hereto has caused this
Agreement to be executed and  delivered  by its duly  authorized  officer on the
date first set forth above.

                                           AMERICAN MOBILE SATELLITE CORPORATION



                                           By:/s/Gary M. Parsons
                                              ------------------
                                           Name:  Gary Parsons
                                                  ------------
                                           Title: CEO, President
                                                  --------------


                                           AMSC SUBSIDIARY CORPORATION



                                            By:/s/Gary M. Parsons
                                               ------------------
                                            Name:  Gary Parsons
                                                   ------------
                                            Title: CEO, President
                                                   --------------


                                           HUGHES COMMUNICATIONS SATELLITE
                                           SERVICES, INC.



                                           By:/s/Amnon Carr
                                              -------------
                                           Name:   Amnon Carr
                                                   ----------
                                           Title:  Assistant Treasurer
                                                   -------------------

















                                                                      Schedule I

                                   TERM LOANS





Date                                                    Maximum Term Loan Amount

                                                           
Prior to and including                                        $ 0
  January 4, 1998

January 5, 1998 to and                                        $ 1,000,000
  including January 11, 1998

January 12, 1998 to and                                       $ 2,000,000
  including January 18, 1998

January 19, 1998 to and                                       $ 3,000,000
  including January 25, 1998

January 26, 1998 to and                                       $ 4,000,000
  including February 1, 1998

February 2, 1998 to and                                       $ 8,000,000
  including February 8, 1998

February 9, 1998 and thereafter                               $10,000,000














                                                                       EXHIBIT A

                           AMSC SUBSIDIARY CORPORATION
                                    Term Note


$10,000,000                                                   New York, New York
                                                               December 30, 1997


                  FOR VALUE RECEIVED,  the undersigned,  a Delaware  corporation
dually  incorporated as a Virginia Public Service  corporation (the "Borrower"),
hereby PROMISES TO PAY to the order of HUGHES COMMUNICATIONS SATELLITE SERVICES,
INC., a Delaware  company (the "Lender"),  its successors or assigns,  in lawful
money of the United States of America and in immediately  available  funds,  the
amount of TEN MILLION DOLLARS  ($10,000,000)  (or, if less, the unpaid principal
amount of Loans made by Lender to the Borrower under the Loan Agreement referred
to  below)  on the  Maturity  Date and to pay  interest  from and after the date
hereof  on the  unpaid  principal  amount  hereof  at the rates and on the dates
provided in the Loan Agreement referred to below.

                  This Note is  issued  pursuant  to that  certain  Bridge  Loan
Agreement,  dated as of December 30, 1997, by and among the  Borrower,  American
Mobile  Satellite  Corporation  and Lender,  (as such  agreement may be amended,
modified or otherwise supplemented from time to time, the "Loan Agreement"), and
is  entitled to the benefit and  security  as  provided  for  therein,  to which
reference  is hereby  made for a  statement  of all of the terms and  conditions
under which the loans evidenced hereby are made. All capitalized  terms,  unless
otherwise  defined herein,  shall have the meanings ascribed to them in the Loan
Agreement.

                  If any  payment on this Note  becomes due and payable on a day
other than a Business  Day, the maturity  thereof  shall be extended to the next
succeeding  Business Day and,  with respect to payments of  principal,  interest
thereon shall be payable at the then applicable rate during such extension.

                  Upon and after the occurrence of an Event of Default,  without
demand,  notice or legal  process of any kind,  this Note may be  declared,  and
immediately shall become, or may automatically  (without any notice) become, due
and payable.





                  Demand,  presentment,  protest  and notice of  nonpayment  and
protest are hereby waived by the Borrower.

                  This Note has been  delivered  and  accepted at New York,  New
York and shall be interpreted, governed by and construed in accordance with, the
laws of the State of New York.

                                            AMSC SUBSIDIARY CORPORATION



                                            By:
                                            Name:
                                            Title: