EXHIBIT 10.33a



                               AMENDMENT NO. 1 TO
                              AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT



     AMENDMENT NO. 1, dated as of May 10, 1999 (this "Amendment"),  by and among
American Mobile Satellite  Corporation,  a Delaware corporation (the "Company"),
Hughes Electronics  Corporation  ("Hughes"),  Singapore  Telecommunications Ltd.
("Singapore   Telecom"),   and  Baron  Capital  Partners,   L.P.  ("Baron,"  and
collectively  with  Hughes and  Singapore  Telecom,  the  "Guarantors"),  to the
Amended and Restated  Registration  Rights  Agreement dated as of March 31, 1998
(said Agreement, as the same may be amended,  supplemented or otherwise modified
from time to time,  being the  "Registration  Rights  Agreement,"  and the terms
defined  therein being used herein as therein defined unless  otherwise  defined
herein), by and among the Company and the Guarantors.



                                   WITNESSETH:



     WHEREAS,  the Guarantors have certain piggyback  registration  rights under
the Registration Rights Agreement; and



     WHEREAS,  the Company filed a  registration  statement on Form S-3 with the
Securities and Exchange Commission (the "SEC") on January 29, 1999 in connection
with an  offering  of its common  stock  (the  "Offering"),  which  registration
statement was declared effective by the SEC on March 31, 1999; and



     WHEREAS,  Hughes  declined  to  exercise  its  piggyback  rights  under the
Registration Rights Agreement with respect to the Offering.



     NOW,  THEREFORE,  in  consideration  of the mutual covenants and agreements
contained herein, the parties hereto hereby agree as follows:

     Section 1.  Consideration  for  Amendment.  The  Company has agreed to this
Amendment in consideration  for Hughes's  election not to exercise its piggyback
rights under the Registration Rights Agreement with respect to the Offering.





     Section 2. Extension of Period for Demand  Registration  in Section 2.1(a).
Section 2.1(a) of the  Registration  Rights  Agreement is hereby amended so that
each and every reference therein to "March 31, 2005" is replaced with "March 31,
2007."

     Section 3. Additional Demand Registration for Hughes.  Subject to the terms
and  conditions  contained  herein,  Hughes  may, at any time prior to March 31,
2007,  make a written  request of the  Company  for a Demand  Registration  with
respect to its Registrable Securities (the "Hughes Demand Registration"),  which
request  shall be in addition to the two Demand  Registrations  provided  for in
Section 2.1 of the  Registration  Rights  Agreement (the "Guarantor Group Demand
Registrations").  The Hughes Demand  Registration may not be exercised by Hughes
(or its permitted assignee) until the earlier of (i) both of the Guarantor Group
Demand  Registrations having been exercised and completed in accordance with the
terms of Section  2.1, and (ii) such time as when  Guarantors  other than Hughes
(or such Guarantors' assignees) do not own any Registrable Securities.  In order
to exercise the Hughes Demand  Registration,  Hughes shall follow the procedures
set forth in Section 2.1, and  Hughes's  election to exercise the Hughes  Demand
Registration  shall be subject to all of the terms and  conditions  contained in
Section 2.1; provided,  that, in the case of a Hughes Demand  Registration,  the
first paragraph of Section 2.1(a) shall not be applicable, and all references in
Section 2.1 to the "Demanding Group" shall be deemed, where applicable, to refer
solely to "Hughes." If the Hughes Demand  Registration is exercised by Hughes in
accordance  with the terms  hereof,  the  Guarantors  other than  Hughes will be
entitled  to  exercise   piggyback   rights  pursuant  to  Section  2.2  of  the
Registration Rights Agreement,  with respect to any Registrable Securities owned
by such Guarantors at that time. In the case of a Hughes Demand  Registration in
which the Guarantors other than Hughes wish to exercise their piggyback  rights,
Registrable Securities owned by such other Guarantors and desired to be included
in such Hughes Demand  Registration  shall be accorded the priority set forth in
Section  2.1(c)(x)(iv),  in  the  case  of  registrations  occurring  up to  and
including the Subordination  Termination Date, or Section 2.1(c)(y)(ii),  in the
case of registrations  occurring after the  Subordination  Termination Date. For
the  avoidance  of  doubt,  in the case of a  Hughes  Demand  Registration,  the
Guarantors  other than Hughes  shall not be deemed to be part of the  "Demanding
Group" for the purpose of  determining  priority of  registration  in accordance
with Section 2.1(c), or for any other purpose.

     Section 4. Assignment of Rights Under  Registration  Rights Agreement.  The
Company and each of the Guarantors  hereby agrees that the  Registration  Rights
Agreement (as amended hereby) and the rights of any Guarantor  thereunder may be
transferred and assigned to any entity that acquires any Registrable  Securities
from time to time;  provided,  that no such assignment shall be effective unless
(i) the assigning  Guarantor (or its permitted assignee) notifies the Company in
writing of such assignment,  and (ii) the prospective assignee agrees in writing
to be bound by, and become a party to, the Registration Rights Agreement.

     Section 5. Miscellaneous.

     (a)Upon  the  effectiveness  of  this  Amendment,  each  reference  in  the
Registration  Rights Agreement to "this  Agreement,"  "hereunder,"  "herein," or
words of like import  shall mean and be a reference to the  Registration  Rights
Agreement as amended hereby.

     (b)Except as specifically amended hereby, the Registration Rights Agreement
shall remain in full force and effect and is hereby ratified and confirmed.

     (c)The  execution and delivery and  effectiveness  of this Amendment  shall
not,  except as  expressly  provided  herein,  operate as a waiver of any right,
power,  or  remedy  which  the  Company  or any  Guarantor  may have  under  the
Registration Rights Agreement.

     (d)This  Amendment  may be  executed in any number of  counterparts  and by
different  parties  hereto  in  separate  counterparts,  each of  which  when so
executed and delivered  shall be deemed to be an original and all of which taken
together shall constitute but one and the same instrument.

     (e)The Company acknowledges its obligation, under Section 4 of the Guaranty
Issuance  Agreement,  to pay, upon demand, to each Guarantor,  the amount of any






and all reasonable expenses,  including, without limitation, the reasonable fees
and expenses of such  Guarantor's  counsel and of any experts and agents,  which
such  Guarantor  has incurred or may incur in connection  with the  negotiation,
preparation or administration of this Amendment.

     (f)This Amendment shall be governed by and construed in accordance with the
laws of the state of New York.






     IN WITNESS  WHEREOF,  the parties  hereto have caused this  Amendment to be
duly  executed  and  delivered  by  their  respective  officers  thereunto  duly
authorized as of the date first written above.



AMERICAN MOBILE SATELLITE CORPORATION


By:  /s/ Randy S. Segal
     ------------------
     Name:   Randy S. Segal
     Title:  Senior Vice President and General Counsel




HUGHES ELECTRONICS CORPORATION


By:  /s/ Mark McEachen
     -----------------
     Name:   Mark McEachen
     Title:  Senior Vice President



SINGAPORE TELECOMMUNICATIONS LTD.


By:  /s/ Hoh Wing Chee 
     ----------------- 
     Name:   Hoh Wing Chee
     Title:  VP (International Network)











BARON CAPITAL PARTNERS, L.P.


By:  Baron Capital Management Inc.,
       A General Partner


By:  /s/ Linda S. Martinson
     ----------------------
     Name:   Linda S. Martinson
     Title:  Vice President and General Counsel