AMERICAN MOBILE SATELLITE CORP
SC 13D/A, 1997-04-11
COMMUNICATIONS SERVICES, NEC
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                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D

                    UNDER THE SECURITIES EXCHANGE ACT OF 1934
                               (AMENDMENT NO. 3)*

                      American Mobile Satellite Corporation
- --------------------------------------------------------------------------------
                                (Name of Issuer)

                           Common Stock $.01 Par Value
- --------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                   02755R 10 3
              ----------------------------------------------------
                                 (CUSIP Number)
    Ms. Chan Su Shan, Company Secretary, Singapore Telecommunications Limited
       31 Exeter Road, Comcentre, Singapore 239732, Republic of Singapore
                             (011) (65) 838-2201 */
- --------------------------------------------------------------------------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                 March 27, 1997
     ----------------------------------------------------------------------
             (Date of Event which Requires Filing of this Statement)



If the filing person has previously filed a statement on Schedule 13G to report
the acquisition which is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-1(b)(3) or (4), check the following box [ ].

Note: Six copies of this statement, including all exhibits, should be filed with
the Commission. See Rule 13d-1(a) for other parties to whom copies are to be
sent.

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter
disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).

* With a copy to: Phillip L. Spector, Esq., Paul, Weiss, Rifkind, Wharton &
  Garrison, 1615 L Street, N.W., Suite 1300, Washington, DC 20036, (202) 
  223-7340.




<PAGE>




                                       SCHEDULE 13D


CUSIP NO.  02755R 10 3                                   PAGE    2  OF    PAGES
         ---------------------------                                           


1      NAME OF REPORTING PERSON
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

             Singapore Telecommunications Limited

2      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP                  (a) [ ]
                                                                         (b) [X]

3      SEC USE ONLY


4      SOURCE OF FUNDS

             WC

5      CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED 
       PURSUANT TO ITEMS 2(d) or 2(e)                                        [ ]


6      CITIZENSHIP OR PLACE OF ORGANIZATION

             Republic of Singapore

                      7      SOLE VOTING POWER
                                   4,794,046 shares
      NUMBER OF
       SHARES
 BENEFICIALLY OWNED
  BY EACH REPORTING
       PERSON
        WITH

       8              SHARED VOTING POWER

                            0 shares

       9              SOLE DISPOSITIVE POWER
                            4,794,046 shares

       10             SHARED DISPOSITIVE POWER

                            0 shares

11     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

             4,794,046 shares

12     CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES
                                                                             [X]


13     PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

             18.6%

14     TYPE OF REPORTING PERSON

             CO
- ------ --------------


<PAGE>


                                       SCHEDULE 13D




CUSIP NO. 02755R 10 3                                    PAGE    3  OF    PAGES
         ---------------------------                                           


1      NAME OF REPORTING PERSON
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

             Temasek Holdings (Private) Limited

2      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP                  (a) [ ]
                                                                         (b) [X]

3      SEC USE ONLY


4      SOURCE OF FUNDS

             AF

5      CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED 
       PURSUANT TO ITEMS 2(d) or 2(e)                                        [ ]


6      CITIZENSHIP OR PLACE OF ORGANIZATION

             Republic of Singapore

                      7      SOLE VOTING POWER
                                   0 shares
      NUMBER OF
       SHARES
 BENEFICIALLY OWNED
  BY EACH REPORTING
       PERSON
        WITH

       8              SHARED VOTING POWER

                            4,794,046 shares

       9              SOLE DISPOSITIVE POWER

                            0 shares

       10             SHARED DISPOSITIVE POWER

                            4,794,046 shares

11     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON



12     CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES  
                                                                             [X]


13     PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

             18.6%

14     TYPE OF REPORTING PERSON

             HC
- ------ --------------



 

<PAGE>

                                                                    4

                         AMENDED SCHEDULE 13D/1/
                         -----------------------


Item 2.  Identity and Background
- -------  -----------------------

            Item 2 is amended and restated in its entirety as follows:

            This statement is being filed by Singapore Telecommunications
Limited ("Singapore Telecom") and Temasek Holdings (Private) Limited
("Temasek"), both of which are Singapore corporations. (Singapore Telecom and
Temasek are collectively referred to herein as the "Reporting Persons".)
Singapore Telecom is approximately 82% owned by Temasek, with the remaining 18%
owned by members of the public. Temasek is wholly owned by the Government of
Singapore.

            Singapore Telecom is principally engaged in the business of
operating and providing telecommunications systems and services. The address of
its principal business and principal office is 31 Exeter Road, Comcentre,
Singapore 239732, Republic of Singapore.

            Temasek is an investment holding company of the Government of
Singapore, with holdings in a broad range of industry sectors. The address of
its principal business and principal office is 8 Shenton Way #38-03, Temasek
Tower, Singapore 068811, Republic of Singapore.

            Schedules I and II attached hereto list each executive officer and
director of Singapore Telecom and Temasek, respectively, and the business
address, present principal occupation or employment, and citizenship of each
such executive officer and director, as well as the name and address of any
corporation or other organization in which such employment is conducted.

            Neither of the reporting Persons nor, to the best knowledge of the
Reporting Persons, any of the persons named in Schedules I and II has, during
the last five years, been convicted in any criminal proceeding (excluding
traffic violations or similar misdemeanors).

            Neither of the Reporting Persons nor, to the best knowledge of the
Reporting Persons, any of the persons named in Schedules I and II has, during
the last five years, been a party to a civil proceeding of a judicial or
administrative body of competent jurisdiction as a result of which such person
was or is subject to a judgment, decree, or final order enjoining future
violations of, or prohibiting or mandating activities subject to, federal or
state securities laws or finding any violation with respect to such laws.

- --------
/1/ Amending the Amended and Restated Schedule 13D dated December 28, 1995.


 

<PAGE>




                                                                    5

Item 3.  Source and Amount of Funds or Other Consideration
- -------  -------------------------------------------------

            Item 3 is amended and restated in its entirety as follows:

            Prior to November 1992, all of Singapore Telecom's holdings of
Common Stock were held indirectly through Mtel Space Technologies, L.P. ("Mtel
L.P."), a Delaware partnership in which Singapore Telecom's legal predecessor
was a limited partner until November 1992./2/ The sole assets of Mtel L.P. were
shares of Common Stock. Singapore Telecom provided funds to Mtel L.P. both by
purchasing limited partnership units in Mtel L.P. ("Mtel LPUs") and by
purchasing convertible debentures issued by Mtel L.P. (the "Mtel Convertible
Debentures"). The Mtel Convertible Debentures were, subject to certain
conditions, convertible into Mtel LPUs which, upon conversion, were to be
redeemed immediately for shares of Common Stock held of record by Mtel L.P.

            At the time Mtel L.P. was restructured in November 1992, Singapore
Telecom had contributed $6,667,000 to Mtel L.P. through the purchase of Mtel
LPUs and had loaned Mtel L.P. $24,266,355 through the purchase of Mtel
Convertible Debentures. All such funds came from the working capital of
Singapore Telecom.

            As part of the Mtel L.P. restructuring, all Mtel LPUs held by
Singapore Telecom were redeemed for shares of Common Stock, and a portion of the
Mtel Convertible Debentures held by Singapore Telecom were converted. After the
restructuring, Singapore Telecom held directly 467,810 shares of Common Stock
(equivalent to 1,116,363 shares of Common Stock after the December 1993 stock
split by the Issuer). In addition, Singapore Telecom continued to hold
$14,660,015 principal amount of Mtel Convertible Debentures that, upon
conversion into Mtel LPUs, were to be redeemed immediately by Mtel L.P. for
318,841 shares (760,869 post-split shares) of Common Stock held of record by
Mtel L.P.

            On December 20, 1993, Singapore Telecom engaged in the transactions
that required the filing of an initial statement on Schedule 13D. On that date,
Singapore Telecom purchased from the issuer 911,854 shares of Common Stock for a
cash purchase price of $18 million. The funds used to make this purchase came
from the working capital of Singapore Telecom.

            On that same date, 1,317,460 shares were issued to Singapore Telecom
by the Issuer upon conversion by Singapore Telecom of $27,666,677 principal
amount of subordinated convertible notes previously issued by the Issuer to
Singapore
- --------
/2/   In April 1992, pursuant to the Telecommunication Authority of Singapore
      Act 1992, Singapore Telecom became the successor in interest to
      telecommunications businesses owned by the Telecommunication Authority of
      Singapore (which continues to exercise regulatory oversight over those
      businesses). Unless otherwise indicated by the context, "Singapore
      Telecom" will be used to refer both to Singapore Telecommunications
      Limited and to its legal predecessor.


 

<PAGE>




                                                                    6

Telecom. The Singapore Telecom funds loaned to the Issuer in connection with
such convertible notes ($20 million in August 1992 and $7,666,667 in October
1993) came from the working capital of Singapore Telecom.

            In December 1995, Singapore Telecom delivered to Mtel L.P. a notice
of conversion with respect to the remaining Mtel Convertible Debentures. Upon
conversion, Singapore Telecom received 8451.71 Mtel LPUs that, as noted above,
were to be redeemed immediately by Mtel L.P. in exchange for 760,869 shares of
Common Stock held of record by Mtel L.P. On December 27, 1995, Mtel L.P.
redeemed the 8451.71 Mtel LPUs and directed the Issuer to transfer the 760,869
shares of Common Stock to Singapore Telecom (effective as of that date).

            On July 1, 1996, upon the closing of a set of agreements providing
long-term bank financing for the Issuer, Singapore Telecom received a warrant
from the Issuer entitling it to purchase 625,000 shares of Common Stock at an
exercise price of $24 per share (the "Original Warrant"). The Original Warrant
was received as part of the consideration for Singapore Telecom's guaranty of up
to $25 million in principal amount of such long-term financing. The number of
shares of Common Stock for which the Original Warrant could be exercised was
limited to the extent that certain financial performance tests restricted the
Issuer's ability to borrow under the long-term financing arrangements. As of
July 1, 1996, the Original Warrant was exercisable for only 406,250 shares of
Common Stock.

            On March 27, 1997, Singapore Telecom and the other guarantors of the
long-term financing agreed to eliminate these financial performance tests that
restricted the Issuer's borrowing ability. As part of this agreement, Singapore
Telecom's warrant was amended (the "Amended Warrant") so as to entitle the
holder to purchase 687,500 shares of Common Stock at an exercise price of $13
per share. The Amended Warrant is exercisable in full as of March 27, 1997.

            To the best knowledge of the Reporting Persons, the funds used by
the persons listed in Schedules I and II to purchase the shares of Common Stock
specified in Item 5 below came from personal savings of such persons.

Item 5      Interest in Securities of the Issuer
- ------      ------------------------------------

            Item 5 is amended and restated in its entirety as follows:

(a) Singapore Telecom owns beneficially 4,794,046 shares of Common Stock. Of
this amount, 4,106,546 shares of Common Stock are owned of record by Singapore
Telecom, and 687,500 shares of Common Stock could be obtained by Singapore
Telecom upon exercise of the Amended Warrant. By reason of its ownership stake
in Singapore Telecom, Temasek may be deemed to be the beneficial owner of the
shares of Common Stock beneficially owned by Singapore Telecom.

            Based upon the information contained in the Issuer's Form 10-K dated
March 28, 1997, the 4,794,046 shares of Common Stock beneficially owned by


 

<PAGE>




                                                                    7

Singapore Telecom constitute approximately 18.6% of the Common Stock outstanding
as of February 28, 1997./3/

            To the best knowledge of the Reporting Persons, none of the persons
listed in Schedule I or II beneficially own or have the right to acquire shares
of Common Stock of the Issuer, except as set forth in the table below:

    Name of
Beneficial Owner           Number of Shares          Percentage
- ----------------           ----------------          ----------
Lim Toon                        4,000/4/                 *

Raphael Leong Sai               1,000                    *
Mooi

Chua Sock Koong                 5,000                    *


* Less than 0.1%

            The Reporting Persons may be deemed to comprise a group (within the
meaning of Section 13(d)(3) of the Act) with the following entities by virtue of
certain agreements described in Item 6 below: (1) Hughes Electronics Corporation
("Hughes Electronics") and Hughes Communications Satellite Services, Inc.
("Hughes" and, together with Hughes Electronics, the "Hughes Entities"), an
indirect wholly-owned subsidiary of Hughes Electronics; and (2) Space
Technologies Investments ("Investments") and the following affiliates of
Investments: Transit Communications, Inc., and Satellite Communications
Investments Corporation (collectively with Investments, the "AT&T Entities")./5/

            The Reporting Persons expressly disclaim beneficial ownership of the
shares of Common Stock held by the Hughes Entities and the AT&T Entities, and
the filing of this statement by the Reporting Persons shall not be construed as
an admission by the Reporting Persons that either of them is, for purposes of
Section
- --------
/3/   For the purpose of computing this percentage, the Amended Warrant was
      deemed to be exercised and the shares of Common Stock issuable upon such
      exercise were deemed to be outstanding.

/4/   Includes 2,000 shares of Common Stock issuable upon the exercise of
      options granted under the Issuer's Nonemployee Director Stock Option Plan.

/5/   Prior to the conversion of the remaining Mtel Convertible Debentures in
      December 1995, and the resulting transfer of the 760,869 shares of Common
      Stock from Mtel L.P. to Singapore Telecom, the following entities might
      also have been deemed to be part of such group: Mtel L.P., Mtel Space
      Technologies Corporation ("Mtel Corp.") (Mtel L.P.'s general partner), and
      Mtel Technologies, Inc. ("Mtel Corp. Affiliate") (Mtel L.P.'s limited
      partner) (collectively, the "Mtel Group").


 

<PAGE>




                                                                    8

13(d) of the Act, the beneficial owner of any of the shares of Common Stock held
by the Hughes Entities or the AT&T Entities.

            Based upon the information set forth in the Issuer's Proxy Statement
dated April 1, 1996 and the Issuer's Form 10-K dated March 28, 1997, and upon
additional information received from the Issuer, the Reporting Persons believe
that the Hughes Entities and the AT&T Entities beneficially own the number of
shares of Common Stock of the Issuer set forth in the table below, constituting
in each case that percentage of the outstanding Common Stock of the Issuer set
forth in the table:


<PAGE>

                                                                   9


Name of Beneficial Owner                        Number of Shares   Percentage/6/
- ------------------------                        ----------------   -------------

Hughes Communications
  Satellite Services, Inc./7/                          6,691,622          26.6

Hughes Electronics Corporation/8/                      4,125,000          14.1
                                                       ---------          ----

      Hughes Entities as a Group                      10,816,622          37.0

- --------------------------------------------------------------------------------

Space Technologies
  Investments, Inc./9/                                 1,855,539           7.2

Transit Communications, Inc.                             681,818           2.7

Satellite Communications
  Investments Corporation/9/                           1,344,067           5.3
                                                       ---------           ---


           AT&T Entities as a Group                    3,881,424          14.9


(b) Singapore Telecom has sole power to vote or to direct the vote, and sole
power to dispose or to direct the disposition of, the shares of Common Stock of
the Issuer beneficially owned by it, subject to the effect of the agreements
referred to in Item 6. By reason of its ownership stake in Singapore Telecom,
Temasek may be deemed to share the power to direct the vote, or to direct the
disposition of, the shares of Common Stock of the Issuer beneficially owned by
Singapore Telecom, subject to the effect of the agreements referred to in Item
6.

- --------

/6/   For the purpose of computing the percentage of the Common Stock of the
      Issuer beneficially owned by the entities listed here, warrants held by
      such entities were deemed to be exercised and the shares of Common Stock
      issuable upon such exercise were deemed to be outstanding.

/7/   Includes 25,000 shares of Common Stock issuable to Hughes upon the
      exercise of certain warrants previously issued by the Issuer. These
      Warrants are exercisable through January 19, 2001 at an exercise price of
      $.01 per share.

/8/   Consists of 4,125,000 shares of Common Stock issuable to Hughes
      Electronics upon the exercise of a warrant that Hughes Electronics
      received as part of the consideration for a guaranty that it provided in
      connection with long-term bank financing for the Issuer. This Warrant, as
      amended, is exercisable through June 28, 2001 at an exercise price of $13
      per share.

/9/   Includes 649,347 shares of Common Stock issuable to Investments and
      230,932 shares of Common Stock issuable to Satellite Communications
      Investments Corporation, respectively, upon the exercise of certain
      warrants previously issued by the Issuer. These warrants are exercisable
      through December 20, 1998 at an exercise price of $21 per share of Common
      Stock.


 

<PAGE>




                                                                    10

            To the best knowledge of the Reporting Persons, each of the persons
listed in Schedule I or II has sole power to vote and to direct the vote, and
sole power to dispose and direct the disposition of, the Common Stock of the
Issuer beneficially owned by such person.

(c) As noted under Item 3 above, on July 1, 1996, upon the closing of a set of
agreements providing long-term bank financing for the Issuer, Singapore Telecom
received the Original Warrant from the Issuer, which entitled it to purchase
625,000 shares of Common Stock at an initial exercise price of $24 per share.
The Warrant was received as part of the consideration for Singapore Telecom's
guaranty of up to $25 million in principal amount of such long-term financing.
On March 27, 1997, in connection with the agreement by Singapore Telecom and the
other guarantors to eliminate certain financial performance tests that
restricted the Issuer's ability to borrow under the long-term financing
arrangements, the Original Warrant was amended so as to entitle the holder to
purchase 687,500 shares of Common Stock at an exercise price of $13 per share.
The Amended Warrant is exercisable in full as of March 27, 1997.

            To the best knowledge of the Reporting Persons, none of the persons
listed in Schedule I or II has sold or purchased shares of Common Stock during
the past sixty days.

            On July 1, 1996, upon the closing of a set of agreements providing
long-term bank financing for the Issuer, Hughes Electronics received a warrant
from the Issuer that entitled it to purchase 3,750,000 shares of Common Stock at
an initial exercise price of $24 per share. This warrant was received as part of
the consideration for Hughes Electronics' guaranty of up to $150 million in
principal amount of such long-term financing. On March 27, 1997, in connection
with the agreement by Hughes Electronics and the other guarantors to eliminate
certain financial performance tests that restricted the Issuer's ability to
borrow under the long-term financing arrangements, this warrant was amended so
as to entitle the holder to purchase 4,125,000 shares of Common Stock at an
exercise price of $13 per share. The warrant, as amended, is exercisable in full
as of March 27, 1997.

            The Reporting persons are not aware of transactions in shares of
Common Stock that were effectuated by the AT&T Entities during the past 60 days.

(d) The Reporting Persons do not know of any other person having the right to
receive or the power to direct the receipt of dividends from, or the proceeds
from the sale of, the shares of Common Stock of the Issuer beneficially owned by
the Reporting Persons.

(e)   Not applicable.



 

<PAGE>




                                                                    11

Item 6      Contracts, Arrangements, Understandings or Relationships
- ------      with Respect to Securities of the Issuer                
            --------------------------------------------------------
            
            Item 6 is amended and restated only with respect to the subject
headings listed below:

                        *     *     *     *     *

Guaranty Issuance Agreement
- ---------------------------

            Singapore Telecom, Hughes Electronics, Baron Capital Partners, L.P.
(a stockholder of the Issuer) ("Baron," and collectively with Singapore Telecom
and Hughes Electronics, the "Guarantors"), the Issuer, and AMSC Subsidiary
Corporation (a subsidiary of the Issuer) ("Issuer Subsidiary") are parties to
the Guaranty Issuance Agreement dated June 28, 1996. The Guaranty Issuance
Agreement specifies the compensation to be provided by the Issuer and the Issuer
Subsidiary to Singapore Telecom, Hughes Electronics, and Baron for the issuance
by the Guarantors of guaranties of the obligations of the Issuer Subsidiary
under the long-term loan agreements that closed on July 1, 1996.

            Under the Guaranty Issuance Agreement, the Issuer Subsidiary agreed
to pay each Guarantor a fee equal to 1.5% of the principal amount of its
respective guaranty. In addition, the Issuer agreed to issue to each Guarantor a
warrant to purchase its respective Pro Rata Share of 5,000,000 shares of Common
Stock at an initial exercise price of $24 per share. The Pro Rata Share of each
Guarantor was equal to the principal amount of its guaranty divided by
$200,000,000. (Section 1)

            In addition, the Issuer and the Issuer Subsidiary agreed that the
aggregate outstanding principal amount of the loans under the long-term loan
agreements, plus any amounts paid by the Guarantors with respect to principal,
would not exceed the Borrowing Limit specified in the Performance Schedule. The
Borrowing Limit was set at $130,000,000 during the period July 1, 1996 through
November 15, 1996, and was to move up on a quarterly basis thereafter provided
that (a) the Issuer Subsidiary had met certain Performance Tests specified in
the Performance Schedule, or (b) Guarantors having a Pro Rata Share greater than
50% had waived compliance with the Performance Tests and consented to increased
borrowings by the Issuer Subsidiary. (Section 3)

            On March 27, 1997, the Guaranty Issuance Agreement was amended to
eliminate the Performance Tests set forth in the Performance Schedule. As
amended, the Performance Schedule sets forth a simple Borrowing Limit of
$180,000,000 for the second quarter of 1997, which increases to $190,000,000 for
the third quarter and $200,000,000 for the fourth quarter of 1997. The amendment
to the Guaranty Issuance Agreement also provides that, as consideration for the
elimination of the Performance Tests, the warrants issued to the Guarantors are
amended to reflect an increase in the aggregate number of warrant shares to
5,500,000 and a change in the exercise price to $13 per share.



 

<PAGE>




                                                                    12

            The Guaranty Issuance Agreement also contains a limited
intercreditor arrangement among the Guarantors. If any Guarantor makes any
payment under its guaranty or acquires any notes or obligations under the
long-term loan agreements, thereafter all decisions to act or refrain from
acting with respect to the enforcement of such notes or obligations against the
Issuer Subsidiary or the Issuer (including enforcement with respect to any
collateral security therefor) must be approved by Guarantors having Pro Rata
Shares equal to at least 80% of the outstanding obligations so paid or
purchased. In addition, if any Guarantor does not make a required payment under
its guaranty, and such payment is made by any other Guarantor, then the
defaulting Guarantor shall be liable to reimburse the paying Guarantor for such
payment on demand, and any amounts which would otherwise be payable to the
defaulting Guarantor by the Issuer Subsidiary or the Issuer or with respect to
any collateral shall first be paid to the paying Guarantor until such payment
has been fully reimbursed. (Section 13)

The Warrant and the Registration Rights Agreement
- -------------------------------------------------

            Pursuant to the terms of the Guaranty Issuance Agreement, the Issuer
issued to Singapore Telecom the Original Warrant dated June 28, 1996. The
Original Warrant entitled Singapore Telecom to purchase from the Issuer 625,000
shares of Common Stock (the "Warrant Share Amount") at a purchase price of $24
per share (the "Exercise Price"). The Original Warrant was exercisable as of
July 1, 1996, subject to certain restrictions, with an expiration date of June 
28, 2001. (Section 1 of the Warrant)

            The exercise of the Original Warrant was restricted where (a) such
exercise would cause the Issuer's Alien Ownership Percentage to exceed the
Accepted Alien Ownership Percentage Limitation (which is derived from alien
ownership restrictions under Section 310(b) of the Communications Act), or (b)
such exercise would require the Issuer to issue Common Stock without first
having the stockholder approval necessary under Rule 4460(i)(1)(D) of the
National Association of Securities Dealers, Inc. Under specified circumstances
where exercise of the Warrant was prevented in whole or in part for either of
the foregoing reasons, the Issuer was required to provide the holder of the
Warrant with a payment of funds in lieu of the shares of Common Stock that were
not issuable to such holder. (Sections 3 and 4 of the Warrant)

            The Warrant Share Amount and the Exercise Price were to be adjusted
under certain conditions, including stock splits and asset distributions to
holders of Common Stock. (Section 10 of the Warrant)

            In addition to the restrictions upon exercise of the Original
Warrant described above, the number of shares of Common Stock for which the
Warrant could be exercised was limited to the extent that certain financial
performance tests restricted the Issuer's ability to borrow fully under the
long-term loan agreements. Specifically, the Warrant provided that it could be
exercised at any given time only for the number of shares of Common Stock which
was equal to the applicable Warrant Share Amount as in effect from time to time,
minus the Warrant Share


 

<PAGE>




                                                                    13

Amount multiplied by a fraction, the numerator of which was the amount which,
due solely to the applicable Borrowing Limit (as defined in the Guaranty
Issuance Agreement) in effect at such time, was not available to the Issuer
Subsidiary under the long-term loan agreements, and the denominator of which was
$200,000,000. (Section 15 of the Warrant)

            The Original Warrant also provided that the holder was entitled to
certain registration rights under the Registration Rights Agreement dated June
28, 1996 with respect to the shares of Common Stock for which the warrant could
be exercised (the "Warrant Shares"). (Section 16 of the Warrant)

            On March 27, 1997, pursuant to the amendment to the Guaranty
Issuance Agreement described above, the Original Warrant was amended (a) to
increase the number of shares of Common Stock issuable upon exercise thereof to
687,500, (b) to adjust the Exercise Price to $13 per share, and (c) to delete
Section 15 of the Original Warrant. The Amended Warrant is fully exercisable as
of March 27, 1997.

            Singapore Telecom, Hughes Electronics, Baron, and the Issuer are
also parties to the Registration Rights Agreement dated June 28, 1996. The
Agreement provides a holder of the Original Warrant or Warrant Shares with
certain demand and piggyback registration rights. These registration rights
remain in effect with respect to the Amended Warrant and the shares of Common
Stock issuable upon the exercise thereof. The same registration rights are
provided to the holders of the warrants issued to the other Guarantors (i.e.,
Hughes Electronics and Baron).

Item 7.   Material to be Filed as Exhibits
- -------   --------------------------------
         
          Item 7 is amended and restated in its entirety as follows:
         
          Exhibit I -- Joint Filing Agreement dated April 11, 1997.
         
          Exhibit II -- Amended and Restated Stockholders' Agreement dated
          December 1, 1993 (previously filed)
         
          Exhibit III -- Right of First Offer Agreement dated November 30, 1993
          (previously filed)
         
          Exhibit IV -- Letter Agreement dated October 11, 1993 (previously 
          filed)
         
          Exhibit V -- Principal Stockholder Holdback and Waiver Agreement dated
          October 20, 1993  (previously filed)
         
          Exhibit VI -- Amended and Restated Limited Partnership Agreement of 
          Mtel Space Technologies, L.P. dated November 18, 1992  (previously
          filed)
         
          Exhibit VII -- Amendment No. 1 to Right of First Offer Agreement dated
          June 28, 1996  (previously filed)
   


 

<PAGE>




                                                                    14
  

          Exhibit VIII -- Guaranty Issuance Agreement dated June 28, 1996
          (previously filed)
          
          Exhibit IX -- Warrant dated June 28, 1996 (previously filed)
          
          Exhibit X -- Registration Rights Agreement dated June 28, 1996
          (previously filed)          

          Exhibit XI -- Amendment No. 1 to Guaranty Issuance Agreement dated
          March 27, 1997
          
          Exhibit XII -- Amendment No. 1 to Warrant Certificates dated March 27,
          1997
          
          
          
          
          
<PAGE>  
          
          
        
      
                                                                    15

Signatures
- ----------


            After reasonable inquiry and to the best of its knowledge and
belief, the undersigned certifies that the information set forth in this
statement is true, complete and correct.

                              SINGAPORE TELECOMMUNICATIONS LTD.



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

Dated: April 11, 1997


                              TEMASEK HOLDINGS (PRIVATE) LTD.



                              By: /s/ Ng Kin Meng
                                 --------------------------------------
                                 Name:    Ng Kin Meng
                                 Title:   Senior Vice President/
                                          Company Secretary

Dated: April 11, 1997


 

<PAGE>




                                                                    16

                              EXHIBIT INDEX


            The Exhibit Index is amended and restated in its entirety as
follows:



     Exhibit
       No.
     -------

Exhibit I         Joint Filing Agreement dated April 11, 1997.

Exhibit II        Amended and Restated Stockholders' Agreement
                  dated December 1, 1993 (previously filed)

Exhibit III       Right of First Offer Agreement
                  dated November 30, 1993 (previously filed)

Exhibit IV        Letter Agreement dated October 11, 1993 (previously filed)

Exhibit V         Principal Stockholder Holdback and Waiver
                  Agreement dated October 20, 1993 (previously filed)

Exhibit VI        Amended and Restated Limited Partnership
                  Agreement of Mtel Space Technologies, L.P.
                  dated November 18, 1992 (previously filed)

Exhibit VII       Amendment No. 1 to Right of First Offer
                  Agreement dated June 28, 1996 (previously filed)

Exhibit VIII      Guaranty Issuance Agreement dated June 28, 1996 (previously 
                  filed)

Exhibit IX        Warrant dated June 28, 1996 (previously filed)

Exhibit X         Registration Rights Agreement dated June 28,
                  1996 (previously filed)

Exhibit XI        Amendment No. 1 to Guaranty Issuance
                  Agreement dated March 27, 1997

Exhibit XII       Amendment No. 1 to Warrant Certificates dated
                  March 27, 1997



<PAGE>

<TABLE>
<CAPTION>

                                          SCHEDULES I AND II



                                               SCHEDULE I


                                  SINGAPORE TELECOMMUNICATIONS LIMITED
                                      DIRECTORS & EXECUTIVE OFFICERS



                                                                    Present Principal
Name                          Position      Business Address      Occupation/Employment      Citizenship
- ------------------------  ----------------  -----------------  ----------------------------  -----------
<S>                       <C>               <C>                <C>                           <C>

Mr. Koh Boon Hwee         Board Member      Singapore Telecom  Executive Chairman            Singapore
                          Chairman          31 Exeter Road     Wuthelam Holdings Pte Ltd.
                                            Comcentre          298 Tiong Bahru Road
                                            Singapore 239732   #08-00 Tiong Bahru Plaza
                                                               Singapore 168730


Mr. Wong Hung Khim        Board Member      Singapore Telecom  Chairman                      Singapore
                          Deputy Chairman   31 Exeter Road     Singapore Bus Service (1978)
                                            Comcentre             Ltd.
                                            Singapore 239732   205 Braddell Road
                                                               Singapore 579701


BG Lee Hsien Yang         Board Member      Singapore Telecom  President & CEO               Singapore
                          President & CEO   31 Exeter Road     Singapore Telecom
                                            Comcentre          31 Exeter Road
                                            Singapore 239732   Comcentre
                                                               Singapore 239732


Dr. Hong Hai              Board Member      Singapore Telecom  President & CEO               Singapore
                                            31 Exeter Road     Haw Par Brothers
                                            Comcentre          International Ltd.
                                            Singapore 239732   180 Clemenceau Avenue
                                                               #08-00 Haw Par Glass Tower
                                                               Singapore 239722

<PAGE>
                                                                                                      2
                                                                             


                                                               Present Principal
Name                      Position          Business Address   Occupation/Employment         Citizenship
- ------------------------  ----------------  -----------------  ----------------------------  -----------

Mr. Lim Chuan Poh         Board Member      Singapore Telecom  Deputy Secretary              Singapore
                                            31 Exeter Road     Ministry of Communications
                                            Comcentre          #39-00 PSA Building       
                                            Singapore 239732   460 Alexandra Road        
                                                               Singapore 119963          
                                                               


Mr. Lim Ho Kee            Board Member      Singapore Telecom  Executive Vice President      Singapore
                                            31 Exeter Road       & CEO
                                            Comcentre          (East Asia)
                                            Singapore 239732   Union Bank of Switzerland
                                                               80 Raffles Place
                                                               #36-00 UOB Plaza 1
                                                               Singapore 048624


Mr. Quek Poh Huat         Board Member      Singapore Telecom  President                     Singapore
                                            31 Exeter Road     Temasek Holdings Pte Ltd.
                                            Comcentre          8 Shenton Way #38-03 
                                            Singapore 239732   Temasek Tower    
                                                               Singapore 068811


Col. Lim Chuan Poh        Board Member      Singapore Telecom  Commander 9 Division          Singapore
                                            31 Exeter Road     Ministry of Defence 
                                            Comcentre          Selarang Camp
                                            Singapore 239732   1 Selarang Ring Road
                                                               Singapore 507087                                      

<PAGE>
                                                                                                  3
                                                                               

                                                               Present Principal
Name                      Position          Business Address   Occupation/Employment         Citizenship
- ------------------------  ----------------  -----------------  ----------------------------  -----------

Mr. Keith Tay Ah Kee      Board Member      Singapore Telecom  Asia Quest Associates         Singapore
                                            31 Exeter Road        Pte Ltd.
                                            Comcentre          161A Telok Ayer Street
                                            Singapore 239732   Singapore 068615
                                                               

Mrs. Yu-Foo Yee Shoon     Board Member      Singapore Telecom  Assistant Secretary           Singapore
                                            31 Exeter Road     General
                                            Comcentre          National Trades Union
                                            Singapore 239732   Congress
                                                               Trade Union House
                                                               Shenton Way
                                                               Singapore 068810


Mr. Lim Toon              Executive Vice    Singapore Telecom  Executive Vice President      Singapore
                          President         31 Exeter Road     (International Services)
                            (International  Comcentre          31 Exeter Road
                          Services)         Singapore 239732   Comcentre
                                                               Singapore 239732


Mr. Raphael Leong         Executive Vice    Singapore Telecom  Executive Vice President      Singapore
Sai Mooi                  President         31 Exeter Road      (Local Services)
                          (Local Services)  Comcentre          Singapore Telecom
                                            Singapore 239732   31 Exeter Road
                                                               Comcentre
                                                               Singapore 239732


<PAGE>
                                                                                                    4
                                                                               



                                                               Present Principal
Name                      Position          Business Address   Occupation/Employment         Citizenship
- ------------------------  ----------------  -----------------  ----------------------------  -----------

Ms. Chua Sock Koong       Senior Vice       Singapore Telecom  Senior Vice President         Singapore
                          President         31 Exeter Road     (Corporate Affairs
                          (Corporate        Comcentre            & Finance)
                          Affairs           Singapore 239732   Singapore Telecom
                          & Finance)                           31 Exeter Road
                                                               Comcentre
                                                               Singapore 239732

</TABLE>




<PAGE>

<TABLE>
<CAPTION>

                                                                                                      5


                               SCHEDULE II

                   TEMASEK HOLDINGS (PRIVATE) LIMITED
                     DIRECTORS & EXECUTIVE OFFICERS




                                                                              Present Principal
Name                         Position               Business Address          Occupation/Employment              Citizenship
- ----                         --------               ----------------          ---------------------              -----------
<S>                          <C>                    <C>                       <C>                                <C>
Mr. S. Dhanabalan            Chairman               Temasek Holdings          Senior Advisor                     Singapore
                                                    (Private) Limited         Nuri Holdings (S) Pte Ltd
                                                    8 Shenton Way #38-03      9 Oxley Rise
                                                    Temasek Tower             #02-01 The Oxley
                                                    Singapore 068811          Singapore 238697


Mr. Ngiam Tong Dow           Deputy Chairman        Temasek Holdings          Permanent Secretary                Singapore
                                                    (Private) Limited         (Budget & Revenue)
                                                    8 Shenton Way #38-03      Ministry of Finance
                                                    Temasek Tower             Budget Division
                                                    Singapore 068811          100 High Street #10-01
                                                                              The Treasury
                                                                              Singapore 179434




<PAGE>


                                                                                                      6



                                                                              Present Principal
Name                         Position               Business Address          Occupation/Employment              Citizenship
- ----                         --------               ----------------          ---------------------              -----------

Dr. Andrew Chew              Director               Temasek Holdings          Chairman                           Singapore
Guan Khuan                                          (Private) Limited         Central Provident Fund
                                                    8 Shenton Way #38-03      Board
                                                    Temasek Tower             CPF Board
                                                    Singapore 068811          CPF Building #41-00
                                                                              79 Robinson Road
                                                                              Singapore 068897

Mr. Fock Siew Wah            Director               Temasek Holdings          Chairman                           Singapore
                                                    (Private) Limited         Land Transport Authority
                                                    8 Shenton Way #38-03      460 Alexandra Road
                                                    Temasek Tower             PSA Building #28-00
                                                    Singapore 068811          Singapore 119963


Mr. Lim Siong Guan           Director               Temasek Holdings          Permanent Secretary                Singapore
                                                    (Private) Limited         (Public Service Division)
                                                    8 Shenton Way #38-03      Prime Minister's Office
                                                    Temasek Tower             100 High Street #07-01
                                                    Singapore 068811          The Treasury
                                                                              Singapore 179434



 

<PAGE>


                                                                                                      7



                                                                              Present Principal
Name                         Position               Business Address          Occupation/Employment              Citizenship
- ----                         --------               ----------------          ---------------------              -----------

Mr. Koh Boon Hwee            Director               Temasek Holdings          Executive Chairman                 Singapore
                                                    (Private) Limited         Wuthelam Holdings Pte Ltd
                                                    8 Shenton Way #38-03      298 Tiong Bahru Road
                                                    Temasek Tower             #08-01 Tiong Bahru Plaza
                                                    Singapore 068811          Singapore 168730



Mr. Kua Hong Pak             Director               Temasek Holdings          President & CEO                    Singapore
                                                    (Private) Limited         Times Publishing Group
                                                    8 Shenton Way #38-03      Times Publishing Limited
                                                    Temasek Tower             1 New Industrial Road
                                                    Singapore 068811          Singapore 536196

Mr. Quek Poh Huat            President              Temasek Holdings          Temasek Holdings (Private)         Singapore
                                                    (Private) Limited         Limited
                                                    8 Shenton Way #38-03      8 Shenton Way #38-03                   
                                                    Temasek Tower             Temasek Tower                          
                                                    Singapore 068811          Singapore 068811                       
                                                                              

Mr. Quek Chee Hoon           Executive Vice         Temasek Holdings          Temasek Holdings (Private)         Singapore
                             President              (Private) Limited         Limited
                                                    8 Shenton Way #38-03      8 Shenton Way #38-03
                                                    Temasek Tower             Temasek Tower
                                                    Singapore 068811          Singapore 068811
                                                    
Mrs. Ng Kin Meng             Company                Temasek Holdings          Temasek Holdings (Private)         Singapore
                             Secretary/             (Private) Limited         Limited
                             Senior Vice            8 Shenton Way #38-03      8 Shenton Way #38-03  
                             President              Temasek Tower             Temasek Tower         
                                                    Singapore 068811          Singapore 068811      
</TABLE>                                                                      






                                                                  

                                                               EXHIBIT I



                         JOINT FILING AGREEMENT



            In accordance with Rule 13d-1(f) under the Securities Exchange Act
of 1934, as amended, the persons named below agree to the joint filing on behalf
of each of them of a statement on Schedule 13D dated April 11, 1997 with respect
to the Common Stock, par value $.01 per share, of American Mobile Satellite
Corporation, a Delaware corporation. This Joint Filing Agreement shall be
included as an Exhibit to such joint filing. In evidence thereof each of the
undersigned, being duly authorized, hereby executes this Agreement this 11th day
of April 1997.

                              SINGAPORE TELECOMMUNICATIONS LTD.



                              By: /s/ Ho Siaw Hong
                                 --------------------------------------
                                 Name:    Ho Siaw Hong
                                 Title:   Assistant Vice President
                                          (Satellite Services)
    
                              TEMASEK HOLDINGS (PRIVATE) LTD.



                              By: /s/ Ng Kin Meng
                                 --------------------------------------
                                 Name:    Ng Kin Meng
                                 Title:   Senior Vice President/
                                          Company Secretary







                                                                    EXHIBIT XI



                               AMENDMENT NO. 1
                        TO GUARANTY ISSUANCE AGREEMENT


      AMENDMENT, dated as of March 27, 1997, to the Guaranty Issuance Agreement,
dated as of June 28, 1996, by and among Hughes Electronics Corporation,
Singapore Telecommunications Ltd., Baron Capital Partners, L.P., AMSC Subsidiary
Corporation and American Mobile Satellite Corporation (the "Agreement").

                             W I T N E S S E T H:

      WHEREAS, the parties hereto desire to effect certain changes to the
Agreement herein contained;

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

      SECTION 1. DEFINITIONS. Unless otherwise indicated, capitalized terms used
herein shall have the meanings set forth in the Agreement.

      SECTION 2. CONSIDERATION FOR THE ISSUANCE OF THE GUARANTIES. As
consideration for the execution of this Amendment No. 1 by the Guarantors, the
Warrants issued to each of the Guarantors in connection with the Guarantees are
hereby amended to reflect an increase in the aggregate number of Warrant Shares
to 5,500,000 and a change in the exercise price to $13.00. To implement the
foregoing, an amendment to each of such Warrants, in the form annexed hereto,
has been executed concurrently with the execution hereof.

      SECTION 3. CHANGE TO THE PERFORMANCE SCHEDULE. The Performance Schedule
annexed to the Agreement as Exhibit D is hereby amended in its entirety and
replaced with the Performance Schedule annexed hereto.

      SECTION 4. Section 3 of the Agreement is hereby amended in its entirety
and replaced with the following:

LIMITATIONS ON AMOUNT OF GUARANTIES. AMSC and AMSC Parent have delivered to
Guarantors AMSC's 1997 Budget, including its projected borrowing needs (the
"Plan"), which has formed the basis for the agreement of the Guarantors to enter
into this Amendment No. 1. As consideration for the execution of this Amendment
No. 1 by the Guarantors, AMSC agrees that the outstanding principal amount of
the loans which are guaranteed (such outstanding amount and any payments made by
Guarantors with respect to principal under




<PAGE>




the Credit Agreement, the "Guaranteed Amount") shall not, at any time, exceed
the then applicable borrowing limit (the "Borrowing Limit") specified on the
Performance Schedule attached hereto as Exhibit D (the "Performance Schedule").

      AMSC and AMSC Parent agree that the aggregate outstanding principal amount
of the loans under the Credit Agreements plus any amounts paid by the Guarantors
with respect to principal shall not exceed the Guaranteed Amount. The Guarantors
having Pro Rata Shares greater than 50% ("Requisite Guarantors") may, by written
notice delivered to AMSC, waive compliance with the then applicable Borrowing
Limit and consent to borrowings by AMSC which would increase the Guaranteed
Amount up to the "Borrowing Limit" specified by such waiver. A waiver granted
hereunder shall not obligate the Guarantors to grant a waiver for any subsequent
period or consent to any additional increase in the applicable Borrowing Limit.

      If any borrowing causes or would cause the Guaranteed Amount to exceed the
then applicable Borrowing Limit, then Requisite Guarantors may, by a written
notice delivered to AMSC (a "Guarantor's Notice"), decline to increase the
Guaranteed Amount to cover any increased borrowings. Under the terms of the
Guaranties, Guarantors will be required to purchase the outstanding notes upon
the occurrence of a "Guarantor Event" under the Credit Agreements, and the
commitments to extend further financing under the Credit Agreements will
terminate.

      Under the terms of the Credit Agreements, at the time of each borrowing,
AMSC will be required to certify that it is in compliance with the provisions of
this Agreement. AMSC or AMSC Parent can so certify if the outstanding amount of
the loans after such borrowing will be less than the then applicable Borrowing
Limit or if, and to the extent that, Requisite Guarantors shall have modified
such Borrowing Limit. At the request of AMSC and AMSC Parent, any Borrowing
Limit may be modified with the written consent of Requisite Guarantors. If
Requisite Guarantors propose to increase the applicable Borrowing Limit for any
period to an amount in excess of that set forth on the Performance Schedule,
such proposal shall be discussed with the other Guarantors prior to granting
such consent.

      Any action by Requisite Guarantors in accordance with this Section 3 shall
bind all Guarantors. Any notice delivered under this Section shall be delivered
to all Guarantors, but failure of all Guarantors to receive such notice shall
not affect the validity of such notice.

      Nothing in this Section shall limit the enforceability by the "Guaranteed
Parties" of any Guaranty in accordance with its terms.




 
                                      2

<PAGE>




      Within 45 days after the end of each fiscal quarter, AMSC Parent shall
deliver to each Guarantor the unaudited consolidated and consolidating balance
sheets of AMSC and AMSC Parent as of the end of such quarter and the related
consolidated and consolidating statements of income, stockholders' equity and
cash flows, and certified by the chief financial officer as fairly presenting,
in all material respects, in accordance with generally accepted accounting
principles (except for the absence of footnote disclosure), the financial
position and the results of operations of AMSC and AMSC Parent.

      SECTION 5.  MISCELLANEOUS.

            (a)   AMSC and AMSC Parent hereby represent to the Guarantors that,
                  as of the date hereof, and after giving effect to this
                  Amendment No. 1 and the transactions contemplated hereby, no
                  Default (as such term is defined in the Credit Agreements) has
                  occurred and is continuing.

            (b)   AMSC hereby reaffirms that the Registration Rights Agreement
                  is in full force and effect and that all of the shares of
                  common stock of AMSC Parent issuable upon exercise of the
                  Warrants, as such number of shares has been increased as
                  described in this Amendment No. 1, constitute Registrable
                  Securities (as such term is defined in the Registration Rights
                  Agreement).

            (c)   AMSC and AMSC Parent hereby represent to the Guarantors that
                  each representation and warranty set forth in Section 11 of
                  the Agreement is true and correct as of the date hereof,
                  except that (i) each reference therein to "this Agreement"
                  shall be deemed to be a reference to this Amendment No. 1,
                  (ii) all references to the Warrants, the Registration Rights
                  Agreement and the Common Stock shall give effect to the
                  transactions contemplated hereby, and (iii) the reference in
                  Section 11(e) of the Agreement to December 31, 1995 instead
                  shall be to December 31, 1996.

            (d)   Except as expressly amended hereby, the terms of the Agreement
                  remain unchanged and the Agreement, as amended hereby,
                  continues in full force and effect.

            (e)   Concurrently with the execution hereof, each Guarantor shall
                  receive the written opinion of counsel to AMSC and AMSC Parent
                  as to the due authorization, execution and enforceability of
                  this Amendment





 
                                      3

<PAGE>




                  No. 1 and Amendment No. 1 to the Warrant Certificates, in form
                  and substance satisfactory to each Guarantor; and

            (f)   AMSC Parent hereby advises each of the Guarantors that the
                  Board of Directors of AMSC Parent has received an opinion from
                  Donaldson, Lufkin & Jenrette Securities Corporation to the
                  effect that the transactions contemplated hereby, including
                  the increase in the number of shares covered by the Warrants
                  and the reduction of the exercise price of the Warrants, are
                  fair to AMSC and AMSC Parent from a financial point of view.

      SECTION 6. GOVERNING LAW. This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

      SECTION 7. COUNTERPARTS. This Amendment may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if the
signatures thereto and hereto were upon the same instrument.

      SECTION 8. EFFECTIVENESS. This Amendment shall become effective when AMSC
has received signature pages hereof signed by the Requisite Guarantors or
facsimile or other written confirmation that such parties have signed a
counterpart hereof.




 
                                      4

<PAGE>




      IN WITNESS WHEREOF, each of the parties hereto has caused this Amendment
No. 1 to the Agreement to be executed by its duly authorized officer.

AMSC SUBSIDIARY CORPORATION               SINGAPORE TELECOMMUNICATIONS
                                          LTD.

By: /s/ Richard J. Burnheimer             By: /s/ Ho Siaw Hong
    -------------------------------           -------------------------------
    Name: Richard J. Burnheimer               Name: Ho Siaw Hong
    Title: VP & Treasurer                     Title: Senior Director

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

                                          By: Baron Capital Management, Inc.,
By: /s/ Richard J. Burnheimer                 a General Partner
    -------------------------------          
    Name: Richard J. Burnheimer  
    Title: VP & Treasurer                 By: /s/ Morty Schaja
                                              -------------------------------
                                              Name: Morty Schaja
HUGHES ELECTRONICS CORPORATION                Title: V.P.

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





 
                                         5

<PAGE>







                                    EXHIBIT D

                                       TO

                          GUARANTEE ISSUANCE AGREEMENT
                     (As Amended by Amendment No. 1 thereto)

                              Performance Schedule
                                    ($000's)



                     01/01/97        04/01/97        07/01/97        10/01/97
                        to              to              to              to
                     03/31/97        06/30/97        09/30/97        12/31/97
                  --------------  --------------  --------------  --------------

Borrowing Limit      $170,000        $180,000        $190,000        $200,000








 
                                     6





                                                                 EXHIBIT XII




                              AMENDMENT NO. 1
           TO WARRANT CERTIFICATES FOR THE PURCHASE OF SHARES OF
           COMMON STOCK OF AMERICAN MOBILE SATELLITE CORPORATION


      AMENDMENT, dated as of March 27, 1997, 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, the Holders and AMSC are entering into on the date
hereof Amendment No. 1 ("Amendment No. 1") to the Guaranty Issuance Agreement;

      WHEREAS, as contemplated by Amendment No. 1, the parties hereto desire to
amend certain terms of the Warrants.

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

      SECTION 1.  AMENDMENTS.

            a. 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 $13.00 per Warrant Share, as adjusted from time to time.

            b.  The Warrant Share Amount reflected in the preamble to each of 
the Warrants shall be modified as follows:

               Holder                        Warrant Share Amount
               ------                        --------------------

Hughes Electronics Corporation                     4,125,000

Singapore Telecommunications Ltd.                    687,500

Baron Capital Partners, L.P.                         687,500







<PAGE>





            c.  Section 15 of the Warrants is hereby deleted in its entirety.

      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.

      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                 SINGAPORE TELECOMMUNICATIONS
  CORPORATION                               LTD.

By: /s/ Richard J. Burnheimer             By: /s/ Ho Siaw Hong
    -------------------------------           -------------------------------
    Name: Richard J. Burnheimer               Name: Ho Siaw Hong
    Title: VP & Treasurer                     Title: Senior Director

HUGHES ELECTRONICS CORPORATION            BARON CAPITAL PARTNERS, L.P., a
  CORPORATION                               Delaware limited partnership

By: /s/ Amnon Carr                        By: Baron Capital Management, Inc.,
    -------------------------------           a General Partner
    Name: Amnon Carr                                    
    Title: Assistant Treasurer
                                          By: /s/ Morty Schaja
                                              ------------------------------- 
                                              Name: Morty Schaja
                                              Title: V.P.



                                     



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