Exhibit 10.66

                         Participation Rights Agreement

                  THIS PARTICIPATION RIGHTS AGREEMENT (this "Agreement") is made
as of December 31, 1997,  by and among  Motorola,  Inc., a Delaware  corporation
(the "Investor"),  American Mobile Satellite Corporation, a Delaware corporation
(the  "Company"),  and the  parties  listed on  Schedule A attached  hereto (the
"Stockholders").

                  American Mobile Satellite Corporation, a Delaware corporation,
the Investor and certain others are parties to a Stock Purchase  Agreement dated
as of  December  31,  1997 (the  "Purchase  Agreement").  In order to induce the
Investor to enter into the Purchase  Agreement,  the Stockholders have agreed to
the provisions set forth in this Agreement.  Unless  otherwise  provided in this
Agreement,  capitalized  terms used herein  shall have the meanings set forth in
Section 5 hereof.

                  The parties hereto agree as follows:

                  1.  Investor  Participation  Rights.  At any  time  after  the
Closing Date and prior to the date on which the Investor  beneficially owns less
than 5% of the Common Stock on a fully-diluted basis:

                  (a) At least 30 days prior to any transfer,  assignment or any
other disposition of Stockholder Shares (other than a transfer (i) to the public
pursuant  to Rule 144  under the  Securities  Act (or any  similar  rule then in
force) or (ii) in other  sales  through a broker or dealer in the  public  stock
market  over  an  exchange  or the  Nasdaq  Stock  Market  (a  "Transfer"),  the
transferring  Stockholder  (the  "Transferring  Stockholder")  shall  deliver  a
written  notice (the "Sale  Notice") to the  Investor,  specifying in reasonable
detail the identity of the prospective transferee(s),  the number of Stockholder
Shares to be transferred and the terms and conditions of the Transfer (including
the proposed price at which the Stockholder  Shares is to be  transferred).  The
Investor may elect to  participate  in the  contemplated  Transfer by delivering
written notice of such election to the Transferring  Stockholder  within 30 days
after delivery of the Sale Notice. If the Investor elects to participate in such
Transfer,  each of the  Transferring  Stockholder  and  the  Investor  shall  be
entitled to sell in the contemplated Transfer, at the same price and on the same
terms, a number of  Stockholder  Shares equal to the product of (A) the quotient
determined by dividing the number of Stockholder  Shares owned by such Person by
the aggregate number of Stockholder Shares owned by the Transferring Stockholder
and the  Investor  and (B) the  number of  Stockholder  Shares to be sold in the
contemplated Transfer.

          For  example  (by  way  of  illustration  only),  if the  Sale  Notice
          contemplated a sale of 100 shares of Common Stock by the  Transferring
          Stockholder,  and if the  Transferring  Stockholder  at such time owns
          shares which  constitute 30% of all Common Stock which are Stockholder
          Shares and if the Investor  elects to participate in such Transfer and
          the Investor owns shares of Common Stock which  constitutes 10% of all
          of the Common Stock which are  Stockholder  Shares,  the  Transferring
          Stockholder would be entitled to sell 75 shares of Common Stock (30% /
          40% x 100 shares) and the Investor would be entitled to sell 25 shares
          of Common Stock (10% / 40% x 100 shares).







                  (b) The Transferring  Stockholder will use its best efforts to
obtain the agreement of the prospective  transferee(s)  to the  participation of
the Investor in any contemplated Transfer, and the Transferring Stockholder will
not  Transfer any of its  Stockholder  Shares to the  prospective  transferee(s)
unless (i)  simultaneously  with such Transfer,  the  prospective  transferee(s)
purchases from the Investor at the same price and on the same terms,  the number
of  Stockholder  Shares  which  it is  entitled  to  sell  to  such  prospective
transferee  pursuant  to  Section  1 above  or  (ii)  simultaneously  with  such
Transfer,  the  Transferring  Stockholder  purchases  the number of  Stockholder
Shares  from the  Investor  at the same  price and on the same  terms  which the
Investor would have been entitled to sell pursuant to Section 1 above.

                  2.  Shareholder  Participation  Rights.  At any time after the
Closing Date and prior to the date on which the Investor  beneficially owns less
than 5% of the Common Stock on a fully-diluted basis:

                  (a) At least 30 days prior to any transfer,  assignment or any
other  disposition of Stockholder  Shares by the Investor (other than a transfer
(i) to the public  pursuant to Rule 144 under the Securities Act (or any similar
rule  then in force)  or (ii) in other  sales  through a broker or dealer in the
public stock market over an exchange or the Nasdaq Stock Market) (a "Transfer"),
the  Investor  shall  deliver  a  written  notice  (the  "Sale  Notice")  to the
Stockholders,  specifying in reasonable  detail the identity of the  prospective
transferee(s),  the number of Stockholder Shares to be transferred and the terms
and  conditions  of the  Transfer  (including  the  proposed  price at which the
Stockholder  Shares is to be transferred).  The Stockholders may elect, pro rata
based on the number of  Stockholder  Shares owned by them, to participate in the
contemplated  Transfer  by  delivering  written  notice of such  election to the
Investor  within 30 days after delivery of the Sale Notice.  If any  Stockholder
elects to  participate  in such  Transfer,  the Investor and each such  electing
Stockholder  (an  "Electing  Stockholder")  shall  be  entitled  to  sell in the
contemplated  Transfer,  at the same  price and on the same  terms,  a number of
Stockholder  Shares  equal to the  product  of (A) the  quotient  determined  by
dividing the number of Stockholder  Shares owned by such Person by the aggregate
number of Stockholder Shares owned by the Electing Stockholders and the Investor
and  (B) the  number  of  Stockholder  Shares  to be  sold  in the  contemplated
Transfer.

         For  example  (by  way  of  illustration  only),  if  the  Sale  Notice
         contemplated a sale of 100 shares of Common Stock by the Investor,  and
         if the  Investor at such time owns shares which  constitute  30% of all
         Common Stock which are Stockholder Shares and if Electing  Stockholders
         elect to participate in such Transfer and the Electing Stockholders own
         shares of Common Stock which constitutes 10% of all of the Common Stock
         which are Stockholder Shares, the Investor would be entitled to sell 75
         shares  of  Common  Stock  (30% / 40% x 100  shares)  and the  Electing
         Stockholders would be entitled to sell 25 shares of Common Stock (10% /
         40% x 100 shares).



                                      - 2 -





                  (b) The  Investor  will use its best  efforts  to  obtain  the
agreement of the prospective  transferee(s) to the participation of the Electing
Stockholders in any  contemplated  Transfer,  and the Investor will not Transfer
any of its  Stockholder  Shares  to the  prospective  transferee(s)  unless  (i)
simultaneously with such Transfer, the prospective  transferee(s) purchases from
the Electing Stockholders at the same price and on the same terms, the number of
Stockholder  Shares  which  they  are  entitled  to  sell  to  such  prospective
transferee  pursuant  to  Section  2 above  or  (ii)  simultaneously  with  such
Transfer,  the Investor will purchase the number of Stockholder  Shares from the
Electing Stockholders at the same price and on the same terms which the Electing
Stockholders would have been entitled to sell pursuant to Section 2 above.

                  3. Agreement to Vote for Transaction.  Each Stockholder agrees
that it shall vote all of its Stockholder Shares in favor of and take such other
action as may be  necessary  to  approve,  and hereby  consents  to the  Company
entering into, all of the transactions  contemplated by the Purchase  Agreement,
including the issuance of shares of Common Stock to the Investor.

                  4.  Registration  Rights  Agreement.  Pursuant to the Purchase
Agreement,  the Company  shall  provide the Investor  with certain  registration
rights  under a  registration  rights  agreement  substantially  in the  form of
Schedule  B  attached  hereto  (the  "Registration   Rights  Agreement").   Each
Stockholder  agrees and acknowledges  that pursuant to the  Registration  Rights
Agreement,  the Company shall provide the Investor with Demand Registrations and
Piggyback  Registrations (each as defined in the Registration Rights Agreement),
for  which  the  Investor  shall  have a  priority  of sale  of its  Registrable
Securities  (as defined in the  Registration  Rights  Agreement)  over all other
unregistered  securities  held by any other  stockholder  of the  Company.  Each
Stockholder  agrees to subordinate any registration  rights granted with respect
to the  unregistered  securities  of the  Company  owned  by it  (including  any
unregistered securities it may acquire in the future), to the Investor under the
Registration  Rights  Agreement,  and agrees  that it shall be bound by Sections
1(b) and 2(c)  therein  until the end of the 42nd month after the month in which
the Closing under the Purchase Agreement occurs (the "Subordination  Termination
Date").  After  the  Subordination   Termination  Date,  the  Investor  and  the
Stockholders  will be pari passu with  respect  to the  priority  of sale in any
piggyback  registration  rights  granted  to such  parties  as set  forth in the
Registration Rights Agreement.

                  5.       Definitions.

                  (a)      "Common Stock" means the Common Stock, par value $.01
                            ------------
per share, of the Company.

                  (b) "Stockholder  Shares" means (i) any shares of Common Stock
                       -------------------  
issued to the Stockholders and the Investor  (including shares issuable upon the
exercise of any AMSC Warrants) and (ii) any equity securities issued or issuable
directly or  indirectly  with respect to the Common Stock  referred to in clause
(i) above  (including  by way of stock  dividend or stock split or in connection
with a combination of shares,  recapitalization,  merger, consolidation or other
reorganization).  As to any particular shares  constituting  Stockholder Shares,
such  shares  will  cease to be  Stockholder  Shares  when  they  have  been (x)
effectively  registered  under the  Securities Act and disposed of in accordance
with the registration statement covering them, or (y) sold to the public through
a broker,  dealer or market maker pursuant to Rule 144 (or by similar  provision
then in force) under the Securities Act.


                                      - 3 -





                  (c) Unless otherwise stated, other capitalized terms contained
herein have the meanings set forth in the Purchase Agreement.

                  6.       Miscellaneous.

                  (a)  Entire  Agreement;  No  Inconsistent   Agreements.   This
Agreement  contains the entire agreement between the parties hereto with respect
to the transactions contemplated herein and supersede all previous negotiations,
commitments  and  writings.  The  Company  shall not  hereafter  enter  into any
agreement with respect to its securities which is inconsistent  with or violates
the rights granted to the Investor in this Agreement.

                  (b) Remedies.  Any Person having rights under any provision of
this Agreement shall be entitled to enforce such rights  specifically to recover
damages caused by reason of any breach of any provision of this Agreement and to
exercise  all  other  rights  granted  by law.  The  parties  hereto  agree  and
acknowledge  that money damages may not be an adequate  remedy for any breach of
the provisions of this  Agreement and that any party may in its sole  discretion
apply to any court of law or equity of competent  jurisdiction  (without posting
any bond or other security) for specific  performance  and for other  injunctive
relief in order to  enforce  or  prevent  violation  of the  provisions  of this
Agreement.

                  (c)  Amendments  and  Waivers.  Except as  otherwise  provided
herein,  the provisions of this Agreement may be amended or waived only upon the
prior written consent of the Investor and the Stockholders.

                  (d)  Successors  and Assigns.  All covenants and agreements in
this Agreement by or on behalf of any of the parties hereto shall bind and inure
to the benefit of the  respective  successors  and assigns of the parties hereto
whether so expressed or not. In addition,  whether or not any express assignment
has been made,  the  provisions of this  Agreement  which are for the benefit of
purchasers  or holders  of the  Investor's  Stockholder  Shares are also for the
benefit of, and enforceable by, any subsequent holder of such shares.

                  (e) Severability.  Whenever  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 is held to be invalid,
illegal or  unenforceable in any respect under any applicable law or rule in any
jurisdiction,  such invalidity,  illegality or unenforceability shall not affect
any other  provision or the  effectiveness  or validity of any  provision in any
other jurisdiction, and this Agreement shall be reformed, construed and enforced
in such jurisdiction as if such invalid,  illegal or unenforceable provision had
never been contained herein.

                  (f)    Counterparts.    This   Agreement   may   be   executed
simultaneously  in two or more  counterparts  (including  by means of telecopied
signature pages),  any one of which need not contain the signatures of more than
one party, but all such counterparts taken together shall constitute one and the
same Agreement.

                  (g) Descriptive  Headings.  The  descriptive  headings of this
Agreement are inserted for convenience only and do not constitute a part of this
Agreement.



                                      - 4 -





                  (h)  Governing  Law.  THIS  AGREEMENT  AND  THE  EXHIBITS  AND
SCHEDULES  HERETO SHALL BE GOVERNED BY, AND  CONSTRUED IN ACCORDANCE  WITH,  THE
LAWS OF THE STATE OF NEW YORK,  WITHOUT  GIVING  EFFECT TO ANY  CHOICE OF LAW OR
CONFLICT  OF LAW RULES OR  PROVISIONS  (WHETHER  OF THE STATE OF NEW YORK OR ANY
OTHER  JURISDICTION)  THAT  WOULD  CAUSE  THE  APPLICATION  OF THE  LAWS  OF ANY
JURISDICTION OTHER THAN THE STATE OF NEW YORK.

                  (i) Notices.  All notices,  demands or other communications to
be given or delivered  under or by reason of the  provisions  of this  Agreement
shall be in writing  and shall be deemed to have been  given when (i)  delivered
personally to the recipient,  (ii) sent to the recipient by reputable  overnight
courier service (charges prepaid),  (iii) sent by facsimile  transmission,  when
transmitted  and  receipt  is  confirmed  or (iv)  mailed  to the  recipient  by
certified or registered mail, return receipt requested and postage prepaid. Such
notices,  demands and other  communications shall be sent to the Stockholders at
their  respective  addresses  listed on  Schedule A  attached  hereto and to the
Investor at the address indicated below:

                  To the Investor:

                  Motorola, Inc.
                  1303 East Algonquin Road
                  Schaumburg, Illinois 60196
                  Attn: General Counsel
                  Facsimile: (847) 576-3628

                  With a copy (which will not constitute notice) to:

                  Kirkland & Ellis
                  200 East Randolph Drive
                  Chicago, Illinois 60601
                  Attn:  Mark B. Tresnowski, Esq.
                  Facsimile:  (312) 861-2200

                  To the Company:

                  American Mobile Satellite Corporation
                  10802 Parkridge Boulevard
                  Reston, Virginia 20191-5416
                  Attn:  General Counsel
                  Facsimile: (703) 758-6134



                                      - 5 -





                  With a copy (which will not constitute notice) to:

                  Arnold & Porter
                  555 12th Street, N.W.
                  Washington, D.C. 20004
                  Attn:  Samuel A. Flax, Esq.
                  Facsimile:  (202) 942-5999

or to such  other  address  or to the  attention  of such  other  person  as the
recipient party has specified by prior written notice to the sending party.


                                    * * * * *


                                      - 6 -





                  IN  WITNESS  WHEREOF,  the  parties  have duly  executed  this
Participation Rights Agreement as of the date first written above.


MOTOROLA, INC.,                       SATELLITE MOBILE TELEPHONE
a Delaware corporation                COMPANY, L.P.

By:/s/Julie A. Welch                  By:/s/Douglas I. Brandon

Name:  Julie A. Welch                 Name:  Douglas I. Brandon

Its:   Senior Operations Controller   Its:  VP - External Affairs & Law

HUGHES COMMUNICATIONS                 TRANSIT COMMUNICATIONS, INC.
 SATELLITE SERVICES, INC.

By:/s/Amnon Carr                      By:/s/Douglas I. Brandon

Name: Amnon Carr                      Name:  Douglas I. Brandon

Its: Assistant Treasurer,             Its:  VP - External Affairs & Law
     Hughes Electronics

SATELLITE COMMUNICATIONS              SINGAPORE
INVESTMENTS CORPORATION               TELECOMMUNICATIONS LTD.

By:/s/Douglas I. Brandon              By:/s/Yap Chee Keong

Name: Douglas I. Brandon              Name:  Yap Chee Keong

Its: VP - External Affairs & Law      Its:  Group Financial Controller

SPACE TECHNOLOGIES                    AMERICAN MOBILE SATELLITE
INVESTMENTS, INC.                     CORPORATION

By:/s/Douglas I. Brandon              By:/s/Gary M. Parsons

Name: Douglas I. Brandon              Name:  Gary M. Parsons

Its: VP - External Affairs & Law      Its:  Chief Executive Officer and 
                                             President


                                      - 7 -






                         [Continuation of Signature Page
                       to Participation Rights Agreement]

Solely with respect to its Warrants:

BARON CAPITAL PARTNERS, L.P.
  By:    Baron Capital Management, Inc.,
         a General Partnership

By:/s/Morty Schaja
      ------------
Name:  Morty Schaja
       ------------
Its:  S.V.P.
      ------

For purposes of Sections 3 of the Participation Rights Agreement only:

BARON CAPITAL MANAGEMENT,
INC.

By:/s/Morty Schaja
      ------------
Name:  Morty Schaja
       ------------
Its:  S.V.P.
      ------

BAMCO, INC.

By:/s/Morty Schaja
      ------------
Name:  Morty Schaja
       ------------
Its:  S.V.P.
      ------








                                   Schedule A


Hughes Communications Satellite Services, Inc.
1500 Hughes Way
Long Beach, California  90810

Space Technologies Investments, Inc.
1150 Connecticut Avenue, N.W.
4th Floor
Washington, D.C.  20036

Satellite Communications Investments Corporation
1150 Connecticut Avenue, N.W.
4th Floor
Washington, D.C.  20036

Satellite Mobile Telephone Company, L.P.
1150 Connecticut Avenue, N.W.
4th Floor
Washington, D.C.  20036

Transit Communications, Inc.
1150 Connecticut Avenue, N.W.
4th Floor
Washington, D.C.  20036

Singapore Telecommunications Ltd.
31C Exeter Road
#03-00 Comcentre III
Singapore 239734
Republic of Singapore

Baron Capital Management, Inc.
767 Fifth Avenue
24th Floor
New York, New York  10153

BAMCO, Inc.
767 Fifth Avenue
24th Floor
New York, New York  10153