EXHIBIT 10.29a
                                 --------------

         AMENDMENT,  dated as of January  15,  1999 (this  "Amendment"),  by and
among  HUGHES  ELECTRONICS  CORPORATION,   a  Delaware  corporation  ("Hughes"),
SINGAPORE  TELECOMMUNICATIONS LTD., a Singapore corporation  ("SingTel"),  BARON
CAPITAL  PARTNERS,  L.P., a Delaware limited  partnership  ("Baron;  and Hughes,
SingTel and Baron,  collectively,  the "Guarantors"),  AMERICAN MOBILE SATELLITE
CORPORATION,  a  Delaware  corporation  ("AMSC  Parent"),  and AMSC  ACQUISITION
COMPANY,  INC., a Delaware  corporation  and a  wholly-owned  subsidiary of AMSC
Parent ("AMSC  Acquisition"),  to the Guaranty  Issuance  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 "Guaranty  Issuance  Agreement",
and the terms  defined  therein  being  used  herein as therein  defined  unless
otherwise  defined herein) by and among each of the Guarantors,  AMSC Parent and
AMSC Acquisition.

                              W I T N E S S E T H :

                  WHEREAS,  each  of  the  Guarantors,   AMSC  Parent  and  AMSC
Acquisition  wish to clarify  the  application  of  Section  13 of the  Guaranty
Issuance Agreement;

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

                  Section 1.  Amendment to the Guaranty Issuance Agreement.  The
Guaranty Issuance Agreement is, subject  to Section  2 hereof, hereby amended as
follows:

                   Section 13 thereof is amended by  inserting at the end of the
first sentence thereof the following:

          "; provided that if such payment results from a drawing under a "Baron
     Capital  Letter of  Credit"  (as such term is defined in each of the Credit
     Agreements) as  contemplated by the last sentence of Section 1(e) of either
     Guaranty by Baron during the 90 day period  preceding  the expiry  thereof,
     AMSC  Acquisition  and  AMSC  Parent  shall  have  no  such   reimbursement
     obligation  and Baron  shall  become a holder of notes and  assignee of the
     rights  and  obligations  of the  Lenders  in respect of Tranche C Loans as
     contemplated by such Section 1(e) of such Guaranty by Baron."

                  Section  2.   Effectiveness.   This  Amendment   shall  become
effective  as of the  date  first  set  forth  above  upon  the  execution  of a
counterpart hereof by each of the Guarantors, AMSC Parent and AMSC Acquisition.

                  Section 3.  Miscellaneous.

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


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                  (b)  Except as  specifically  amended  or waived  hereby,  the
Guaranty Issuance  Agreement shall remain in full force and effect and is hereby
ratified and confirmed.

                  (c)  The  execution,   delivery  and   effectiveness  of  this
Amendment shall not, except as expressly provided herein, operate as a waiver of
any right, power, or remedy which AMSC Parent, AMSC Acquisition or any Guarantor
hereto may have under the Guaranty Issuance 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) Each of AMSC  Parent  and  AMSC  Acquisition  jointly  and
severally agrees that it will, upon demand,  pay 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 may incur in connection with the  negotiation,  preparation
or administration of this Amendment.

                  (f) THIS  AMENDMENT  SHALL BE  GOVERNED  BY AND CON  STRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.





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                  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 above written.


AMSC ACQUISITION COMPANY, INC.             SINGAPORE TELECOMMUNICATIONS
                                           LTD.


By:    /s/Randy Segal                      By:    /s/Ho Siaw Hong
Name:  Randy Segal                         Name:  Ho Siaw Hong
Title: Vice President                      Title: Assistant Vice President
                                                  (Satellite Services)

AMERICAN MOBILE SATELLITE                  BARON CAPITAL PARTNERS, L.P.,
CORPORATION                                 a Delaware limited partnership

By:    /s/Randy Segal                      By:  Baron Capital Management, Inc.,
Name:  Randy Segal                              a general partner
Title: Vice President
                                                By:    /s/Morty Schaja
                                                Name:  Morty Schaja
                                                Title: Chief Operating Officer
HUGHES ELECTRONICS
CORPORATION

By:     /s/Mark A. McEachen
Name:   Mark A. McEachen
Title:  Corporate Vice President, Treasurer