Exhibit 10.64a
                                                       --------------

                                PLEDGE AGREEMENT


                  PLEDGE  AGREEMENT,  dated as of  December  30,  1997,  made by
American Mobile Satellite  Corporation,  a Delaware corporation (the "Pledgor"),
to Hughes Communications Satellite Services, Inc., a California corporation (the
"Secured Party").


                              W I T N E S S E T H:


                  WHEREAS,  each  of the  Pledgor,  as  Parent  Guarantor,  AMSC
Subsidiary Corporation, a Delaware corporation dually incorporated as a Virginia
Public Service  corporation (the "Borrower"),  and the Secured Party, as Lender,
have entered into a Bridge Loan  Agreement,  dated as of December 30, 1997 (said
Agreement,  as it may be amended or otherwise  modified from time to time, being
the "Bridge Loan Agreement" and capitalized terms not defined herein but defined
therein being used herein as therein defined);

                  WHEREAS,  the  Pledgor  is the legal and  beneficial  owner of
eighty  percent  (80%) of the issued and  outstanding  shares of common stock of
AMRC Holdings,  Inc., a Delaware  corporation  ("AMRC Holdings") as described in
Schedule I hereto (the "Pledged Shares"); and

                  WHEREAS,  it is a  condition  precedent  under the Bridge Loan
Agreement to the making of the Loan that the Pledgor  shall have made the pledge
contemplated by this Agreement.

                  NOW, THEREFORE,  in consideration of the premises and in order
to induce the Secured Party to make the Loan, the Pledgor hereby agrees with the
Secured Party as follows:

                  SECTION 1.  Pledge.  (a) The Pledgor hereby pledges
to the Secured Party, and grants to the Secured Party, a
security interest in, the following (the "Pledged Collateral"):

                         (i) all of the Pledged Shares;

                         (ii) all additional shares of stock or other securities
                    of AMRC  Holdings  from time to time acquired by the Pledgor
                    in any manner and all shares of stock or other securities of
                    AMRC Holdings held by or owned by any Person who,  after the
                    date  of  this  Agreement,  becomes,  as  a  result  of  any
                    occurrence,  a  Subsidiary  of the Pledgor  (any such shares
                    being "Additional Shares");

                         (iii) the certificates representing the shares referred
                    to in clauses (i) and (ii) above; and







                         (iv)  all  dividends,   cash,   instruments  and  other
                    property or proceeds, from time to time received, receivable
                    or  otherwise  distributed  in respect of or in exchange for
                    any or all of the foregoing.

                  (b) Upon payment in full of the Secured  Obligations under the
Bridge Loan Agreement, the pledge and security interest granted pursuant to this
Section 1 shall  terminate.  Upon such  termination,  the  Secured  Party  shall
execute  and  deliver  to the  Pledgor,  at the  Pledgor's  sole  expense,  such
documents as the Pledgor shall reasonably request to evidence the termination of
such pledges or security interests or the release of all Pledged Collateral,  as
the case may be.

                  SECTION 2. Security for  Obligations.  This Agreement  secures
and the Pledged  Collateral is security for the full and prompt payment when due
(whether  at  stated  maturity,  by  acceleration  or  otherwise)  of,  and  the
performance  of, the Secured  Obligations  of the Borrower under (and as defined
in) the Bridge Loan Agreement.

                  SECTION 3. Delivery of Pledged Collateral. All certificates or
instruments representing or evidencing the Pledged Collateral shall be delivered
to and held by or on behalf of the Secured Party pursuant hereto and shall be in
suitable form for transfer by delivery, or shall be accompanied by duly executed
instruments  of  transfer  or  assignment  in blank,  all in form and  substance
satisfactory  to the Secured Party.  The Secured Party shall have the right,  at
any time in its discretion and without notice to the Pledgor,  to transfer to or
to register in its name or in the name of any of its  nominees any or all of the
Pledged Collateral;  provided,  however, that notwithstanding anything contained
herein to the  contrary,  any such  transfer  or  registration  in the name of a
nominee shall be subject to the assumption by such  transferee or nominee of the
terms and conditions of the AMRC Holdings Shareholders'  Agreement, as described
in Section 7(a)(i)(E) below. In addition, the Secured Party shall have the right
at any time to exchange  certificates or instruments  representing or evidencing
any of the Pledged  Collateral  for  certificates  or  instruments of smaller or
larger denominations.

                  SECTION 4.  Representations and Warranties.  The Pledgor makes
the following representations:

                  (a) The  Pledged  Shares  (i) have  been duly  authorized  and
validly issued;  (ii) are fully paid and  non-assessable;  and (iii)  constitute
eighty percent (80%) of the issued and outstanding shares of AMRC Holdings.

                  (b) The  Pledgor  is the  legal  and  beneficial  owner of the
Pledged  Collateral free and clear of any Lien, except for the Lien and security
interest created by this Agreement.







                  (c)  The  pledge  of  the  Pledged  Shares  pursuant  to  this
Agreement  creates a valid and perfected first priority security interest in the
Pledged Collateral, securing the payment of all of the Secured Obligations.

                  (d) No consent,  authorization,  approval, or other action by,
and no notice to or filing with,  any  Governmental  Authority or third party is
required  either (i) for the pledge by the  Pledgor  of the  Pledged  Collateral
pursuant to this Agreement or for the due execution,  delivery or performance of
this Agreement by the Pledgor,  or (ii) for the exercise by the Secured Party of
the rights  provided for in this  Agreement or of the remedies in respect of the
Pledged Collateral pursuant to this Agreement,  except as may be required by the
rules and regulations of the Federal Communications  Commission (the "FCC") from
time to time in effect in connection with the disposition of, exercise of voting
rights with respect to, or transfer of control of, the Pledged Collateral, or by
laws affecting the offering and sale of securities generally, and except for any
written  consent of other  shareholders  required  under Section  2.01(a) of the
Shareholders'  Agreement,  dated as of May 16,  1997,  by and among the Pledgor,
WorldSpace, Inc. and the Parent Guarantor.

                  SECTION 5. Further  Assurances,  Etc.  (a) The Pledgor  agrees
that at any time and from time to time,  at the cost and expense of the Pledgor,
the Pledgor  will  promptly  execute and  deliver  all further  instruments  and
documents,  and take all further action, that may be necessary or desirable,  or
that the Secured Party may request, in order to perfect and protect the Lien and
security  interest  granted or purported  to be granted  hereby or to enable the
Secured  Party to exercise  and enforce its rights and remedies  hereunder  with
respect to any Pledged Collateral.

                  (b) The  Pledgor  agrees  to defend  the title to the  Pledged
Collateral  and the Lien  thereon and security  interest  therein of the Secured
Party  against the claim of any other Person and to maintain  and preserve  such
Lien and  security  interest  until  indefeasible  payment in full of all of the
Secured Obligations.

                  SECTION 6.  Voting Rights; Dividends; Etc.

                  (a) As long as no Event of Default  or event  which with lapse
of time or lack of notice  would  have  become an Event of  Default  shall  have
occurred and be continuing (or, in the case of subsection (a)(i) of this Section
6, as long as no notice  thereof  shall have been given by the Secured  Party to
the Pledgor):




                         (i) The Pledgor  shall be entitled to exercise  any and
                    all voting and other  consensual  rights  pertaining  to the
                    Pledged  Collateral  or any part thereof for any purpose not
                    inconsistent  with the terms of this  Agreement or any other
                    Loan Document; provided, however, that the Pledgor shall not
                    exercise or shall refrain from exercising any such right if,
                    in the Secured Party's judgment,  exercised reasonably, such
                    action would have a material  adverse effect on the value of
                    the Pledged  Collateral or any part  thereof;  and provided,
                    further,  that the Pledgor  shall give the Secured  Party at
                    least five Business  Days'  written  notice of the manner in
                    which it intends to exercise,  or its reasons for refraining
                    from exercising, any such right.

                         (ii) The  Pledgor  shall not be  entitled to receive or
                    retain (A) any cash dividends paid in respect of the Pledged
                    Collateral,  or (B) any other  dividends  paid in respect of
                    the Pledged Collateral,  including,  without limitation, any
                    of the following:

                                    (x) dividends paid or payable in respect of,
                           and   instruments   and  other   property   received,
                           receivable or otherwise distributed in respect of, or
                           in exchange for, any Pledged
                           Collateral,

                                    (y) dividends and other  distributions  paid
                           or  payable  in  cash  in  respect  of  any   Pledged
                           Collateral  in  connection  with a  partial  or total
                           liquidation or  dissolution  or in connection  with a
                           reduction    of   capital,    capital    surplus   or
                           paid-in-surplus, and

                                    (z)  cash   paid,   payable   or   otherwise
                           distributed in redemption of, or in exchange for, any
                           Pledged Collateral,

                  all of which, together with any cash dividends received by the
                  Pledgor in violation of clause (A) of this clause (ii),  shall
                  be forthwith delivered to the Secured Party to hold as Pledged
                  Collateral and shall, if received by the Pledgor,  be received
                  in trust for the benefit of the Secured  Party,  be segregated
                  from  the  other  property  or funds  of the  Pledgor,  and be
                  forthwith delivered to the Secured Party as Pledged Collateral
                  in  the  same  form  as  so  received   (with  any   necessary
                  indorsement).

                  (b) Upon the occurrence and during the continuance of an Event
of Default:

                         (i) Upon  notice by the Secured  Party to the  Pledgor,
                    all rights of the Pledgor to  exercise  the voting and other
                    consensual  rights  which it would  otherwise be entitled to
                    exercise  pursuant to Section 6(a)(i) above shall cease, and
                    all such rights shall thereupon become vested in the Secured
                    Party who shall  thereupon  have the sole right to  exercise
                    such voting and other consensual rights.







                         (ii) The  Pledgor  shall,  if  necessary  to permit the
                    Secured  Party to exercise the voting and other rights which
                    it may be entitled to exercise  pursuant to Section  6(b)(i)
                    above and to receive all dividends and  distributions  which
                    it may be entitled to receive under Section  6(b)(ii) above,
                    execute and deliver to the Secured Party,  from time to time
                    and upon written  notice of the Secured  Party,  appropriate
                    proxies,  dividend  payment orders and other  instruments as
                    the Secured  Party may  reasonably  request.  The  foregoing
                    shall  not in any way limit the  Secured  Party's  power and
                    authority granted pursuant to Section 8 hereof.

                  SECTION 7. Transfers and Other Liens;  Additional  Shares. (a)
The Pledgor agrees that it will not (i) sell or otherwise  dispose of any of the
Pledged  Collateral,  or grant any option or warrant with respect to, any of the
Pledged  Col  lateral,  other  than  those  options  which  may  be  granted  to
WorldSpace, Inc., a Maryland corporation ("WorldSpace"), pursuant to:

         (A)      the  Participation  Agreement,  dated as of March 14, 1997, by
                  and between WorldSpace,  American Mobile Radio Corporation,  a
                  Delaware corporation ("AMRC"), and Pledgor;

         (B)      the Memorandum of  Understanding,  dated as of April 15, 1997,
                  by and between WorldSpace, AMRC and Pledgor;

         (C)      the Bridge,  Additional  Amounts and  Working  Capital  Credit
                  Facility, dated as of May 16, 1997, among AMRC Holdings, AMRC,
                  Pledgor and WorldSpace;

         (D)      the Stock Subscription and Exchange Agreement, dated as of May
                  16, 1997, by and between WorldSpace,  AMRC Holdings,  AMRC and
                  Pledgor;

         (E)      the Shareholders' Agreement,  dated as of May 16, 1997, by and
                  between  AMRC  Holdings,  WorldSpace  and  Pledgor  (the "AMRC
                  Holdings Shareholders' Agreement");

         (F)      the Bridge  Option,  dated as of May __,  1997,  between  AMRC
                  Holdings and WorldSpace;

         (G)      the  Additional  Amounts  Option,  dated  as of May 16,  1997,
                  between AMRC and WorldSpace;

         (H)      the Working Capital Option,  dated as of May 16, 1997, between
                  AMRC Holdings and WorldSpace; and







         (I)      the Security Agreement, dated as of May 16, 1997, between AMRC
                  Holdings and WorldSpace

         (collectively, the "Existing WorldSpace Arrangements"),


or (ii)  create or permit to exist any Lien upon or with  respect  to any of the
Pledged  Collateral,  except  for the Lien  and the  security  interest  created
pursuant to this Agreement.

                  (b) The  Pledgor  agrees  that it will (i) not vote any of the
Pledged  Shares  in  favor  of the  issuance  of any  shares  of  stock or other
securities  of AMRC Holdings in addition to or in  substitution  for the Pledged
Shares,  (A) except  with the  written  consent  of the  Secured  Party,  to the
Pledgor,  and (B) except for any shares to be issued  pursuant  to the  Existing
WorldSpace Arrangements, (ii) pledge hereunder, immediately upon its acquisition
(directly or  indirectly)  thereof,  any and all  Additional  Shares,  and (iii)
promptly  (and in any event  within two  Business  Days)  deliver to the Secured
Party a Pledge  Amendment,  duly  executed  by the Pledgor or any other owner of
Additional  Shares,  in substantially  the form of Schedule II hereto (a "Pledge
Amendment"), in respect of the Additional Shares, together with all certificates
or  instruments  representing  or evidencing  the same.  The Pledgor  hereby (i)
authorizes  the Secured  Party to attach each  Pledge  Amendment  to this Pledge
Agreement, (ii) agrees that all Additional Shares listed on any Pledge Amendment
delivered  to the Secured  Party  shall for all  purposes  hereunder  constitute
Pledged  Shares,  and (iii) is deemed to have made,  with respect to any and all
Additional   Shares   pledged  by  the   Pledgor,   upon  such   delivery,   the
representations  and  warranties  contained  in Section 4 hereof with respect to
such Pledged Collateral.

                  SECTION 8. Secured Party Appointed Attorney-in-Fact and Proxy.
The Pledgor hereby  irrevocably  constitutes  and appoints the Secured Party and
any officer or agent thereof,  with full power of substitution,  as its true and
lawful  attorney-in-fact  and proxy with full irrevocable power and authority in
the place and stead of the  Pledgor and in the name of the Pledgor or in its own
name, from time to time in the Secured Party's discretion, exercised reasonably,
for the purpose of carrying out the terms of this Agreement, to take any and all
appropriate  action  and to  execute  and  deliver  any  and all  documents  and
instruments  which  the  Secured  Party  may  deem  necessary  or  advisable  to
accomplish the purposes of this Agreement,  including,  without  limitation,  to
receive,  indorse  and  collect  all  instruments  made  payable to the  Pledgor
representing  any  dividend or other  distribution  or payment in respect of the
Pledged Collateral or any part thereof,  to give full discharge for the same and
to vote or grant any  consent in respect of the  Pledged  Shares  authorized  by
Section 6(b) hereof.  The Pledgor hereby  ratifies,  to the extent  permitted by
law, all that any said attorney  shall lawfully do or cause to be done by virtue
hereof.  This power,  being coupled with an interest,  is irrevocable  until the
Secured Obligations are paid in full.







                  SECTION 9. Secured Party May Perform.  If the Pledgor fails to
perform any agreement contained herein, the Secured Party may itself perform, or
cause  performance  of, such  agreement,  and the expenses of the Secured  Party
incurred in connection  therewith  shall be payable by the Pledgor under Section
12 hereof and constitute Secured Obligations secured hereby.

                  SECTION 10. Reasonable Care. The Secured Party shall be deemed
to have exercised reasonable care in the custody and preservation of the Pledged
Collateral  in its  possession if the Pledged  Collateral is accorded  treatment
substantially  equal to that which the Secured Party accords its own property of
a similar  nature,  it being  understood  that the  Secured  Party  shall not be
responsible  for (i)  ascertaining  or  taking  action  with  respect  to calls,
conversions,  exchanges,  maturities,  tenders or other matters  relative to any
Pledged  Collateral,  whether or not the Secured  Party has or is deemed to have
knowledge of any such  matter,  or (ii) taking any  necessary  steps to preserve
rights against any Person with respect to any Pledged Collateral.

                  SECTION 11.  Remedies  upon Event of Default.  If any Event of
Default shall have occurred and be continuing, the Secured Party may, subject to
the rules and regulations of the FCC from time to time in effect, if applicable,
exercise any or all of the followings  rights,  remedies and recourses which may
now or hereafter exist in equity or at law:

                  (a) The Secured  Party may  exercise in respect of the Pledged
Collateral,  in addition to other  rights and  remedies  provided  for herein or
otherwise  available to it, all the rights and remedies of a secured party after
default under the Uniform  Commercial Code in effect in the State of New York at
that time,  and the Secured Party may also,  without  notice except as specified
below, sell the Pledged Collateral or any part thereof in one or more parcels at
public or private sale, at any exchange,  broker's board or at any office of the
Secured Party or elsewhere, for cash, on credit or for future delivery, and upon
such other terms as the Secured Party may deem commercially  reasonable;  except
that,  notwithstanding  anything contained herein to the contrary,  the right of
the Secured  Party to sell and assign or  otherwise  transfer any of the Pledged
Collateral  shall be subject,  at all times,  to the terms and conditions of the
AMRC Holdings  Shareholders'  Agreement.  The Pledgor agrees that, to the extent
notice of sale  shall be  required  by law,  at least  ten  days'  notice to the
Pledgor  of the time and place of any public  sale or the time  after  which any
private sale is to be made shall constitute reasonable notification. The Secured
Party shall not be obligated to make any sale of Pledged  Collateral  regardless
of notice of sale having been given. The Secured Party may adjourn any public or






private  sale from  time to time by  announcement  at the time and  place  fixed
therefor,  and such sale may,  without further  notice,  be made at the time and
place to which it was so adjourned. The Pledgor hereby waives any claims against
the  Secured  Party  arising  by  reason of the fact that the price at which any
Pledged  Collateral  may have been sold at such a private sale was less than the
price which might have been obtained at a public sale, even if the Secured Party
accepts the first offer  received and does not offer such Pledged  Collateral to
more than one offeree.

                  (b) The  Secured  Party may elect to obtain (at the  Pledgor's
expense) the advice of any independent  nationally-known investment banking firm
with respect to the method and manner of sale or other disposition of any of the
Pledged  Collateral,   the  best  price  reasonably  obtainable  therefor,   the
consideration of cash and/or credit terms, or any other details  concerning such
sale or disposition.

                  (c) If the Secured Party shall determine to exercise its right
to sell all or any of the Pledged  Collateral  pursuant to this  Section 11, the
Pledgor agrees that, upon request of the Secured Party, the Pledgor will, at its
own cost and expense:

                         (i) execute and  deliver,  and use its best  efforts to
                    cause the issuer of the Pledged Shares and its directors and
                    officers to execute and deliver,  all such  instruments  and
                    documents,  and do or cause to be done all such  other  acts
                    and things,  as may be  necessary  or, in the opinion of the
                    Secured  Party,  necessary  or  advisable  to register  such
                    Pledged Shares under the provisions of the Securities Act of
                    1933, as from time to time amended (the  "Securities  Act"),
                    and to cause the registration  statement relating thereto to
                    become  effective and to remain effective for such period as
                    prospectuses  are  required by law to be  furnished,  and to
                    make  all  amendments  and  supplements  thereto  and to the
                    related  prospectus  which,  in the  opinion of the  Secured
                    Party,  are necessary or advisable,  all in conformity  with
                    the  requirements  of the  Securities  Act and the rules and
                    regulations  of  the  Securities  and  Exchange   Commission
                    ("SEC") applicable thereto;

                         (ii)  use its  best  efforts  to  qualify  the  Pledged
                    Collateral under the state securities or "Blue Sky" laws and
                    to obtain all necessary  governmental approvals for the sale
                    of the  Pledged  Collateral,  as  requested  by the  Secured
                    Party;

                         (iii) make available to its security  holders,  as soon
                    as practicable,  an earning statement which will satisfy the
                    provisions of section 11(a) of the Securities Act; and






                         (iv) do or cause to be done  all  such  other  acts and
                    things as may be  necessary to make such sale of the Pledged
                    Collateral  or any part  thereof  valid and  binding  and in
                    compliance with applicable law.

The Pledgor further acknowledges the impossibility of ascertaining the amount of
damages which would be suffered by the Secured Party by reason of the failure by
the Pledgor to perform any of the  covenants  contained  in this Section 11 and,
consequently,  agrees  that,  if the  Pledgor  shall fail to perform any of such
covenants,  it shall pay, as liquidated damages and not as a penalty,  an amount
equal to the value of the Pledged Collateral on the date the Secured Party shall
demand compliance with this Section.

                  (d)  The   Pledgor   recognizes   that,   by   reason  of  the
aforementioned requirements and certain prohibitions contained in the Securities
Act and applicable  state securities laws, the Secured Party may, at its option,
elect not to require  the  Pledgor to  register  all or any part of the  Pledged
Collateral  and may therefore be  compelled,  with respect to any sale of all or
any part of the Pledged Collateral, to limit purchasers to those who will agree,
among other  things,  to acquire  such  securities  for their own  account,  for
investment,  and not with a view to the  distribution  or  resale  thereof.  The
Pledgor  acknowledges  and  agrees  that any such sale may  result in prices and
other  terms less  favorable  to the seller than if such sale were a public sale
without such restrictions and,  notwithstanding such circumstances,  agrees that
any such sale  shall be deemed  to have been made in a  commercially  reasonable
manner.  The Secured Party shall be under no obligation to delay the sale of any
of the Pledged Collateral for the period of time necessary to permit the Pledgor
to register such  securities for public sale under the Securities  Act, or under
applicable state securities laws, even if the Pledgor would agree to do so.

                  (e) If the Secured  Party  determines to exercise its right to
sell any or all of the Pledged  Collateral,  upon written  request,  the Pledgor
shall,  from time to time,  furnish to the Secured Party all such information as
the  Secured  Party may request in order to  determine  the number of shares and
other  instruments  included in the Pledged  Collateral which may be sold by the
Secured Party as exempt  transactions  under the Securities Act and rules of the
SEC thereunder, as the same are from time to time in effect.

                  (f) Any cash held by the Secured  Party as Pledged  Collateral
and all cash  proceeds  received by the Secured Party in respect of any sale of,
collection  from,  or other  realiza  tion  upon all or any part of the  Pledged
Collateral shall be applied by the Secured Party:






                  First, to the payment of the costs and expenses of such  sale,
including, without limitation,  reasonable expenses of the Secured Party and its
agents  including  the fees  and  expenses  of its  counsel,  and all  expenses,
liabilities  and advances  made or incurred by the Secured  Party in  connection
therewith or pursuant to Section 9 hereof;

                  Next, toward  the  payment in full of the Secured Obligations;
and

                  Finally,   after  payment  in  full  of  all  of  the  Secured
Obligations,  to the payment to the Pledgor, or its successors or assigns, or to
whomsoever may be lawfully  entitled to receive the same as a court of competent
jurisdiction may direct.

                  SECTION 12. Expenses.  The Pledgor will upon demand pay to the
Secured Party the amount of any and all reasonable expenses,  including, without
limitation,  the reasonable fees and expenses of the Secured Party's counsel and
of any experts and agents,  which the Secured Party may incur in connection with
(i) the  administration of this Agreement,  (ii) the custody or preservation of,
sale of,  collection  from,  or other  realiza  tion  upon,  any of the  Pledged
Collateral,  (iii) the exercise or enforcement of any of the rights and remedies
hereunder of the Secured Party, or (iv) the failure by the Pledgor to perform or
observe any of the provisions hereof.

                  SECTION 13.  Security  Interest  Absolute.   All rights of the
Secured  Party and security  interests  hereunder,  and all  obligations  of the
Pledgor hereunder, shall be absolute and unconditional irrespective of:

                         (i)  any  lack of  validity  or  enforceability  of any
                    provision  of the Bridge  Loan  Agreement  or any other Loan
                    Document  or any  other  agreement  or  instrument  relating
                    thereto;

                         (ii) any change in the time, manner or place of payment
                    of, or in any other term of, or any  increase  in the amount
                    of,  all or any of the  Secured  Obligations,  or any  other
                    amendment  or waiver of any term of, or any  consent  to any
                    departure from any requirement of, the Bridge Loan Agreement
                    or any other Loan Document;

                         (iii) any exchange,  release or  non-perfection  of any
                    Lien on any other collateral, or any release or amendment or
                    waiver  of any  term  of any  guaranty  of,  or  consent  to
                    departure  from any  requirement  of any guaranty of, all or
                    any of the Secured Obligations; or

                         (iv)  any  other  circumstance  which  might  otherwise
                    constitute  a defense  available  to, or a  discharge  of, a
                    borrower  or a pledgor  (other  than  payment in full of the
                    Secured Obligations).






                  SECTION 14.  Amendments,  Etc. No  amendment  or waiver of any
provision of this Agreement nor consent to any departure by the Pledgor herefrom
shall in any event be effective unless the same shall be in writing, approved by
the  Secured  Party and signed by the  Secured  Party,  and then such  waiver or
consent  shall be effective  only in the specific  instance and for the specific
purpose for which given.

                  SECTION  15.  Addresses  for  Notices.  All  notices and other
communications  provided for hereunder shall be in writing  (including  telecopy
communication)  and mailed,  telecopied  (with a subsequent  hard copy mailed or
delivered by hand,  Federal Express or any other nationally  recognized  courier
service)  or  delivered  by  hand,  Federal  Express  or  any  other  nationally
recognized courier service, if to the Pledgor or the Secured Party, addressed to
the Pledgor or the Secured  Party at its  address  specified  in the Bridge Loan
Agreement, or, as to each party, at such other address as shall be designated by
such party in a written notice to each other party complying as to delivery with
the terms of this Section. All such notices and other communications shall, when
mailed,  telecopied  or  delivered,  be effective  when  deposited in the mails,
telecopied with confirmation of receipt,  or delivered by hand,  Federal Express
or such other courier service to the addressee or its agent, respectively.

                  SECTION 16. Continuing Security Interest; Transfer of Notes or
Obligations.  This Pledge Agreement shall create a continuing  security interest
in the Pledged  Collateral  and shall (i) remain in full force and effect  until
indefeasible  payment in full of the Secured  Obligations,  (ii) be binding upon
the Pledgor,  its  successors  and assigns,  and (iii) inure,  together with the
rights and  remedies of the Secured  Party  hereunder,  to the benefit of and be
enforceable by the Secured Party and its respective successors,  transferees and
assigns,  subject  to and upon the  terms and  conditions  set forth in the AMRC
Shareholders'  Agreement.  Upon the payment in full of the Secured  Obligations,
the Pledgor shall be entitled to the return,  promptly as practicable,  upon its
request and at its expense,  of such of the Pledged Collateral as shall not have
been sold or otherwise applied pursuant to the terms hereof.

                  SECTION 17. Governing Law; Severability; Terms. This Agreement
shall be governed by, and be construed and  interpreted in accordance  with, the
law of the  State of New York  (but  without  giving  effect  to  principles  of
conflicts of laws). Wherever possible, each provision of this Agreement shall be
interpreted  in such manner as to be effective and valid under  applicable  law,
but if any provision of this  Agreement  shall be prohibited by or invalid under
applicable law, such provision  shall be ineffective  only to the extent of such
prohibition or invalidity and without  invalidating the remaining  provisions of
this Agreement. Unless otherwise defined herein or in the Bridge Loan Agreement,
terms  defined in Article 9 of the Uniform  Commercial  Code as in effect in the
State of New York are used herein as therein defined.





                  SECTION 18. Submission to Jurisdiction;  Jury Trial; Judgment.
(a) Any  legal  action or  proceeding  with  respect  to this  Agreement  or any
document related hereto may be brought in the courts of the State of New York or
the United  States of America for the  southern  district  of New York,  and, by
execution and delivery of this Agreement,  the Pledgor hereby accepts for itself
and in respect of its property, generally and unconditionally,  the jurisdiction
of the aforesaid courts.  The Pledgor hereby  irrevocably  waives any objection,
including,  without limitation, any objection to the laying of venue or based on
the grounds of forum non conveniens, which the Pledgor may now or hereafter have
to  the  bringing  of  any  such  action  or  proceeding   in  such   respective
jurisdictions  and consents to the granting of such legal or equitable relief as
is deemed appropriate by the court.

                  (b) To the  extent  that  the  Pledgor  has or  hereafter  may
acquire any immunity  from  jurisdiction  of any court or from any legal process
(whether through service or notice, attachment prior to judgment,  attachment in
aid of  execution,  execution  or  otherwise)  with  respect  to  itself  or its
property,  the Pledgor hereby irrevocably waives to the fullest extent permitted
by law such immunity in respect of its obligations  under this Agreement and the
other Loan Documents.  The Pledgor waives to the fullest extent permitted by law
any right it may have to trial by jury in  respect of any  litigation  based on,
arising out of,  under or in  connection  with this  Agreement or any other Loan
Document,  or any  course of  conduct,  course  of  dealing,  verbal or  written
statement or other action of any Loan Party or the Secured Party.


                  SECTION 19.  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 part of this Agreement.







                  IN WITNESS  WHEREOF,  the Pledgor has caused this Agreement to
be duly executed and delivered by its duly authorized  officer on the date first
above written.

                                                     AMERICAN MOBILE SATELLITE
                                                     CORPORATION


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


Accepted and Acknowledged:


HUGHES COMMUNICATIONS SATELLITE
SERVICES, INC.


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








                         SCHEDULE I TO PLEDGE AGREEMENT


         Attached to and forming a part of that certain Pledge Agreement,  dated
as of December 30, 1997,  by American  Mobile  Satellite  Corporation  to Hughes
Communications Satellite Services, Inc.


                                     Stock
                    Class           Certificate                       Number of
Stock Issuer       of Stock          No(s).           Par Value         Shares


AMRC Holdings,      Common            001               $0.10             100
Inc.











                         SCHEDULE II TO PLEDGE AGREEMENT

                                PLEDGE AMENDMENT



                           This Pledge  Amendment,  dated ,     ,  is  delivered
                                                            ----   
         pursuant to Section 7 of the Pledge  Agreement  referred to below.  The
         undersigned hereby agrees that this Pledge Amendment may be attached to
         the Pledge  Agreement,  dated as of  December  30,  1997,  between  the
         undersigned  and Hughes  Communications  Satellite  Services,  Inc., as
         Secured Party referred to therein and that the Additional Shares listed
         on this  Pledge  Amendment  shall  be and  become  part of the  Pledged
         Collateral  referred to in the Pledge  Agreement  and shall  secure all
         Secured Obligations of the undersigned. The terms defined in the Pledge
         Agreement  or Bridge  Loan  Agreement  are being used herein as therein
         defined.

                                                 [AMERICAN MOBILE SATELLITE
                                                 CORPORATION][              ]


                                                  By:
                                                     Name:
                                                     Title:



                                        Stock
                       Class         Certificate                     Number of
Stock Issuer          of Stock          No(s).      Par Value          Shares