EXHIBIT 10.29b


                               AMENDMENT NO. 2 TO
                           GUARANTY ISSUANCE AGREEMENT

     AMENDMENT  , dated as of March 29,  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 collectively with
                                                  -----
Hughes and SingTel, 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,  AMSC Parent and AMSC  Acquisition wish to eliminate the financial
     -------
covenants  contained in the Guaranty  Issuance  Agreement and the Guarantors are
willing to accept the  elimination of such  covenants,  subject to the terms and
conditions set forth herein;

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

     Section 1. Consideration for Amendment.  As consideration for the execution
     ---------  ---------------------------
of this  Amendment,  the warrants issued to each of the Guarantors in connection
with the  Guaranties  will be  amended,  effective  April 1, 1999,  to reflect a
change in the exercise price of each of the warrants to $7.50 per share, subject
to  adjustment as provided  therein.  To implement  the  foregoing,  each of (i)
Amendment  No. 1 to the New Warrants,  in the form annexed  hereto as Exhibit A,
and (ii) Amendment No. 3 to the Amended Warrants,  in the form annexed hereto as
Exhibit B, have been executed by the parties hereto.

     Section 2.  Amendments  to Section 3 of the  Guaranty  Issuance  Agreement.
     ---------   --------------------------------------------------------------
Section 3 of the Guaranty Issuance Agreement is hereby amended as follows:

     (a) Amendment to Section 3(a).  Section 3(a) thereof is amended by deleting
         -------------------------
it in its entirety and  substituting in lieu thereof the phrase  "[Intentionally
omitted.]".



     (b) Amendment to Section 3(b).  Section 3(b) thereof is amended by deleting
         -------------------------
the phrase ", Section 3(a)" from Section 3(b)(2).

<PAGE>



     (c) Amendment to Section 5. Section 5 thereof is amended by deleting all of
         ----------------------
the text  therein  (including  the  second  sentence  thereof)  after the phrase
"signed  by  such   party"   and   substituting   in  lieu   thereof  a  period.


     (d)  Amendment  to Section 6.  Section 6 thereof is amended by (i) deleting
          -----------------------
the phrase  "Sections 3 or 5" in the first sentence  thereof and substituting in
lieu  thereof the phrase  "Section 3", and (ii)  deleting the phrase  "except as
otherwise  specifically  provided  in Section 5 hereof  with  respect to certain
waivers   by   Requisite   Guarantors,"   in  the   second   sentence   thereof.


     Section 3.  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 4. 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.

     (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  acknowledges  its joint and
several 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.
---------------------------------



<PAGE>




     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.
------------------------------


By: /s/Randy Segal
    -------------------------------
Name:  Randy Segal
Title: Senior Vice President

AMERICAN MOBILE SATELLITE CORPORATION
-------------------------------------


By: /s/Randy Segal
    --------------------------------
Name:  Randy Segal
Title: Senior Vice President

HUGHES ELECTRONICS CORPORATION
------------------------------


By: /s/Mark A. McEachen
    --------------------------------
Name:  Mark A. McEachen
Title: Corporate V.P. & Treasurer

SINGAPORE TELECOMMUNICATIONS LTD.
---------------------------------


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

BARON CAPITAL PARTNERS, L.P.,
----------------------------
   a Delaware limited partnership
By: Baron Capital Management, Inc.,
   a General Partner

By: /s/Ronald Baron
    --------------------------------
Name:  Ronald Baron
Title: Chief Executive Officer & Chairman



<PAGE>
                                                                       EXHIBIT A
                                 AMENDMENT NO. 3
              TO WARRANT CERTIFICATES FOR THE PURCHASE OF SHARES OF
              COMMON STOCK OF AMERICAN MOBILE SATELLITE CORPORATION


     AMENDMENT, dated as of April 1, 1999, to each of those Warrant Certificates
dated as of June 28, 1996 (the "Warrants" and capitalized  terms used herein and
not otherwise defined shall have the meanings ascribed thereto in the Warrants),
issued by American  Mobile  Satellite  Corporation  (the  "Company")  to each of
Hughes  Electronics  Corporation,  Singapore  Telecommunications  Ltd. and Baron
Capital Partners, L.P. (collectively, the "Holders").

                              W I T N E S S E T H:

     WHEREAS,  the  Company  previously  issued  to the  Holders  Warrants  that
represented  in the aggregate the right to purchase  5,000,000  shares of Common
Stock at an Exercise Price of $24.00 per share;

     WHEREAS, the Company and the Holders previously agreed to Amendment No.1 to
the Warrants dated as of March 27, 1997  ("Amendment  No. 1"), which amended the
Warrants  so that  they  represented  in the  aggregate  the  right to  purchase
5,500,000 shares of common stock at an Exercise Price of $13 per share;

     WHEREAS,  the Company and the Holders  previously agreed to Amendment No. 2
to the Warrants dated as of March 31, 1998  ("Amendment  No. 2"),  which,  inter
alia, amended the Exercise Price to be $12.51 per share;

     WHEREAS, the Company,  the Holders and AMSC Acquisition Company,  Inc. have
entered into Amendment No. 2 to Guaranty  Issuance  Agreement  dated as of March
29, 1999 related to the elimination of certain financial  covenants contained in
the  Guaranty  Issuance  Agreement   ("Amendment  No.  2  to  Guaranty  Issuance
Agreement"); and

     WHEREAS, as contemplated by Amendment No. 2 to Guaranty Issuance Agreement,
the parties hereto desire to amend certain terms of the Warrants.

     NOW, THEREFORE, the undersigned parties hereto agree as follows:

     SECTION 1. Amendment.
                ---------

          Section 1 of each of the Warrants is hereby  amended by modifying  the
     definition of "Exercise Price" to read in its entirety as follows:

               "Exercise  Price" means  initially  $7.50 per Warrant  Share,  as
          adjusted from time to time.





<PAGE>




          SECTION 2. Reaffirmance. Except as expressly amended hereby, the terms
                     ------------
     of the Warrants remain  unchanged and the Warrants,  as previously  amended
     and as amended hereby, are in full force and effect.

          SECTION 3. Issuance of  Replacement  Warrant.  Upon the request of any
                     ---------------------------------
     Holder,  the Company promptly shall issue a new Warrant,  incorporating the
     amendments  effected hereby and the amendments  effected by Amendment No. 1
     and Amendment No. 2, to replace the presently  outstanding  Warrant held by
     such Holder.

          IN WITNESS  WHEREOF,  each of the  parties  hereto has  executed  this
     Amendment No. 3 by its duly authorized officer as of the day and year first
     set forth above.




AMERICAN MOBILE SATELLITE CORPORATION          SINGAPORE TELECOMMUNICATIONS LTD.

By:                                            By:
Name:                                          Name:
Title:                                         Title:





BARON CAPITAL PARTNERS, L.P.,                  HUGHES ELECTRONICS CORPORATION
      a Delaware limited partnership
                                               By:
By: Baron Capital Management, Inc.,            Name:
      a General Partner                        Title:

By :
Name:
Title:





                                        2


<PAGE>
                                                                       EXHIBIT B
                                 AMENDMENT NO. 1
              TO WARRANT CERTIFICATES FOR THE PURCHASE OF SHARES OF
              COMMON STOCK OF AMERICAN MOBILE SATELLITE CORPORATION


     AMENDMENT, dated as of April 1, 1999, to each of those Warrant Certificates
dated as of March 31, 1998 (the "Warrants" and capitalized terms used herein and
not otherwise defined shall have the meanings ascribed thereto in the Warrants),
issued by American  Mobile  Satellite  Corporation  (the  "Company")  to each of
Hughes  Electronics  Corporation,  Singapore  Telecommunications  Ltd. and Baron
Capital Partners, L.P. (collectively, the "Holders").

                              W I T N E S S E T H:

     WHEREAS,  the  Company  previously  issued  to the  Holders  Warrants  that
represented  in the aggregate the right to purchase  1,000,000  shares of Common
Stock at an Exercise Price of $12.51 per share;

     WHEREAS, the Company,  the Holders and AMSC Acquisition Company,  Inc. have
entered into Amendment No. 2 to Guaranty  Issuance  Agreement  dated as of March
29, 1999 related to the elimination of certain financial  covenants contained in
the  Guaranty  Issuance  Agreement   ("Amendment  No.  2  to  Guaranty  Issuance
Agreement"); and

     WHEREAS, as contemplated by Amendment No. 2 to Guaranty Issuance Agreement,
the parties hereto desire to amend certain terms of the Warrants.

     NOW, THEREFORE, the undersigned parties hereto agree as follows:

     SECTION 1. Amendment.
                ---------

          Section 1 of each of the Warrants is hereby  amended by modifying  the
     definition of "Exercise Price" to read in its entirety as follows:

               "Exercise  Price" means  initially  $7.50 per Warrant  Share,  as
          adjusted from time to time.

     SECTION 2.  Reaffirmance.  Except as expressly amended hereby, the terms of
                 ------------
the Warrants remain unchanged and the Warrants,  as amended hereby,  are in full
force and effect.

     SECTION 3. Issuance of Replacement Warrant. Upon the request of any Holder,
                -------------------------------
the Company  promptly  shall issue a new Warrant,  incorporating  the amendments
effected  hereby to  replace  the  presently  outstanding  Warrant  held by such
Holder.





<PAGE>



     IN WITNESS WHEREOF,  each of the parties hereto has executed this Amendment
No. 1 by its duly  authorized  officer  as of the day and year  first  set forth
above.




AMERICAN MOBILE SATELLITE CORPORATION          SINGAPORE TELECOMMUNICATIONS LTD.

By:                                            By:
Name:                                          Name:
Title:                                         Title:





BARON CAPITAL PARTNERS, L.P.,                  HUGHES ELECTRONICS CORPORATION
  a Delaware limited partnership
                                               By:
By: Baron Capital Management, Inc.,            Name:
  a General Partner                            Title:
                                               

By :
Name:
Title:





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