AMERICAN MOBILE SATELLITE CORP
SC 13D/A, 1998-04-13
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. 5)*

                      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 31, 1998
- --------------------------------------------------------------------------------
             (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
                                                   
           NUMBER OF                  4,919,046 shares
            SHARES
      BENEFICIALLY OWNED    8         SHARED VOTING POWER
      BY EACH REPORTING
            PERSON                    0 shares
             WITH
                            9         SOLE DISPOSITIVE POWER

                                      4,919,046 shares   

                            10        SHARED DISPOSITIVE POWER

                                      0 shares

11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          4,919,046 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)

          15.9%

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
                                                   
           NUMBER OF                  0 shares
            SHARES
      BENEFICIALLY OWNED    8         SHARED VOTING POWER
      BY EACH REPORTING
            PERSON                    4,919,046 shares
             WITH
                            9         SOLE DISPOSITIVE POWER

                                      0 shares

                            10        SHARED DISPOSITIVE POWER

                                      4,919,046 shares

11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          4,919,046 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)

          15.9%

14        TYPE OF REPORTING PERSON

          HC

<PAGE>

                                                                               4

                             AMENDED SCHEDULE 13D 1/


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

         The first paragraph of Item 2 is amended and restated 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 80% owned by Temasek, with the remaining 20% owned by members
of the public. Temasek is wholly owned by the Government of Singapore. The
filing of this statement should not be construed as an admission that Temasek
is, for purposes of Section 13(d) of the Securities Exchange Act of 1934 (the
"Act"), the beneficial owner of any of the securities described herein.

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 

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

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

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>

                                                                               5

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 $27,666,667 principal amount
of subordinated convertible notes previously issued by the Issuer to Singapore
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 8,451.71 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 8,451.71 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's subsidiary, Singapore Telecom received
a warrant from the Issuer entitling it, through June 28, 2001, 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 ability of the Issuer's
subsidiary 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 borrowing ability of the Issuer's subsidiary. 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 was exercisable in full as of March
27, 1997.

<PAGE>

                                                                               6

         On March 31, 1998, as part of the consideration for Singapore Telecom's
guaranty of up to $25 million in principal amount of restructured long-term
financing extended to the Issuer and the Issuer's subsidiary by lending
institutions, (a) the Amended Warrant was further amended so as to change the
exercise price to $12.51 per share and the expiration date to March 31, 2005,
and (b) Singapore Telecom received another warrant from the Issuer entitling it,
through March 31, 2005, to purchase 125,000 shares of Common Stock at an
exercise price of $12.51 ("Warrant No. 2"). Both the Amended Warrant and Warrant
No. 2 are exercisable in full as of March 31, 1998.

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

Item 4.  Purpose of Transaction
- -------  ----------------------

         Item 4 is amended and restated in its entirety as follows:

         The shares of Common Stock held by Singapore Telecom were acquired for
investment purposes, and continue to be held for such purposes. Pursuant to the
cumulative voting rights that exist under the Issuer's Certificate of
Incorporation with respect to the election of the Issuer's board of directors,
and pursuant to the rights that exist under the Stockholders' Agreement
(described in Item 6 below) with respect to appointing directors to the
executive committee of the Issuer's board of directors, Singapore Telecom has
the right to be represented on the Issuer's board of directors and its executive
committee. Singapore Telecom presently has two representatives on the Issuer's
board of directors and one representative on the Issuer's executive committee.
Singapore Telecom representatives also participate in other committees of the
Issuer's board of directors.

         The ability of Singapore Telecom to acquire or dispose of shares of
Common Stock is limited to some degree by certain agreements, as described under
Item 6 below. Subject to such agreements, Singapore Telecom may, from time to
time, make purchases or dispositions of Common Stock of the Issuer either in the
open market or in private transactions, depending upon Singapore Telecom's
evaluation of the Issuer's business, prospects, and financial condition, the
market for the Common Stock of the Issuer, other opportunities available to
Singapore Telecom, general economic conditions, money and stock market
conditions, regulatory approvals or restrictions, and other factors.

         Except as described herein, the Reporting Persons have no present plan
or proposal that relates to or would result in:

(a) the acquisition by any person of additional securities of the Issuer, or the
disposition of securities of the Issuer;

(b) an extraordinary corporate transaction, such as a merger, reorganization, or
liquidation, involving the Issuer or any of its subsidiaries;

<PAGE>

                                                                               7

(c) a sale or transfer of a material amount of assets of the Issuer or any of 
its subsidiaries;

(d) any change in the present board of directors or management of the Issuer;

(e) any material change in the present capitalization or dividend policy of the
Issuer;

(f) any other material change in the Issuer's business or corporate structure;

(g) changes in the Issuer's charter or bylaws or other actions which may impede
the acquisition of control of the Issuer by any person;

(h) any act or course of conduct causing the Common Stock of the Issuer to cease
to be authorized to be quoted in an inter-dealer quotation system of a
registered national securities association;

(i) any act or course of conduct causing the Common Stock of the Issuer to
become eligible for termination of registration pursuant to Section 12(g)(4) of
the Act; or

(j) any action similar to any of those enumerated above.

         The Reporting Persons reserve the right to formulate such plans or
proposals, and to take such action, with respect to any or all of the foregoing
matters and any other matters as they may deem appropriate.

         To the best knowledge of the Reporting Persons, all shares identified
in Item 5 below as beneficially owned by persons listed in Schedule I or II were
acquired by such persons for investment purposes. Such persons may buy or sell 
shares of Common Stock in the future as they deem appropriate, but, to the best 
knowledge of the Reporting Persons, and except as otherwise indicated herein, 
such persons have no present plan or proposal that relates to or would result in
the actions or events specified in (a) through (j) above.

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

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

(a) Singapore Telecom owns beneficially 4,919,046 shares of Common Stock. Of
this amount, 4,106,546 shares of Common Stock are owned of record by Singapore
Telecom, and 812,500 shares of Common Stock could be obtained by Singapore
Telecom upon exercise in full of the Amended Warrant and Warrant No. 2. As noted
in Item 2 above, Singapore Telecom is approximately 80% owned by Temasek. The
filing of this Statement should not, however, be construed as an admission that
Temasek is, for purposes of Section 13(d) of the Act, the beneficial owner of
any of the Common Stock beneficially owned by Singapore Telecom.

<PAGE>

                                                                               8

         Based upon the information contained in the Issuer's Form 10-K dated
March 31, 1998 and other information received from the Issuer, the 4,919,046
shares of Common Stock beneficially owned by Singapore Telecom constitute
approximately 15.9% of the Common Stock outstanding as of March 31, 1998.3/

         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                                 2,000                          *

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, Inc. ("Investments") and the following affiliates of
Investments: Transit Communications, Inc. and Satellite Communications
Investments Corporation (collectively with Investments, the "AT&T Entities).4/

         The Reporting Persons expressly disclaim beneficial ownership of shares
of Common Stock beneficially owned 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 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.


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

3/       For the purpose of computing this percentage, the Amended Warrant and
         Warrant No. 2 were deemed to be exercised in full and the shares of
         Common Stock issuable upon such exercise were deemed to be outstanding.

4/       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>

                                                                               9

         Based upon the information set forth in the Issuer's Form 10-K dated
March 31, 1998 and other 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:

Name of Beneficial Owner                 Number of Shares          Percentage 5/
- --------------------------------------------------------------------------------

Hughes Communications
  Satellite Services, Inc. 6/               6,691,622                 22.1
                          
Hughes Electronics Corporation 7/           4,875,000                 13.9
                                            ---------                 ----

         Hughes Entities as a Group        11,566,622                 33.0

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

Space Technologies
  Investments, Inc. 8/                      1,855,539                  6.0

Transit Communications, Inc.                  681,818                  2.3

Satellite Communications
  Investments Corporation 8/                1,344,067                  4.4
                                            ---------                 ----

         AT&T Entities as a Group           3,881,424                 12.5


(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 

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

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

6/       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.

7/       Consists of 4,125,000 shares of Common Stock issuable 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's subsidiary, and 750,000
         shares of Common Stock issuable upon the exercise of another warrant
         that Hughes Electronics received as part of the consideration for a
         guaranty that it provided in connection with the restructuring of the
         long-term financing. Each warrant is exercisable through March 31, 2005
         at an exercise price of $12.51.

8/       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.

<PAGE>

                                                                              10

Issuer beneficially owned by it, subject to the effect of the agreements
referred to in Item 6. As noted in Item 2 above, Singapore Telecom is 
approximately 80% owned by Temasek. The filing of this statement should not, 
however, be construed as an admission that Temasek is, for purposes of Section 
13(d) of the Act, the beneficial owner of any of the Common Stock beneficially 
owned by Singapore Telecom.

         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 March 31, 1998, as part of the consideration
for Singapore Telecom's guaranty of up to $25 million in principal amount of the
restructured long-term financing extended to the Issuer and the Issuer's
subsidiary by lending institutions, (i) the Amended Warrant was further amended
so as to change the exercise price to $12.51 per share and the expiration date
to March 31, 2005, and (ii) Singapore Telecom received Warrant No. 2 from the
Issuer, entitling it, through March 31, 2005, to purchase 125,000 shares of
Common Stock at an exercise price of $12.51. Both the Amended Warrant and
Warrant No. 2 are exercisable in full as of March 31, 1998.

         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, except as set forth in the table below:

                Date of           Number of         Price per        Sale or 
Name            Transaction       Shares            Share            Purchase
- ----            -----------       ---------         ---------        -------- 
Raphael Leong
Sai Mooi          2/13/98           1,000             $8.25            Sale

         On March 31, 1998, as part of the consideration for Hughes Electronics'
guaranty of up to $150 million in principal amount of the restructured long-term
financing extended to the Issuer and the Issuer's subsidiary by lending
institutions, (i) the existing warrant for 4,125,000 shares of Common Stock held
by Hughes Electronics was amended so as to change the exercise price to $12.51
per share and the expiration date to March 31, 2005, and (ii) Hughes Electronics
received another warrant from the Issuer entitling it, through March 31, 2005,
to purchase 750,000 shares of Common Stock at an exercise price of $12.51. Both
of these warrants are exercisable in full as of March 31, 1998.

         The Reporting Persons are not aware of any 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:

                                * * * * * * * * *

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

         Pursuant to the terms of the Guaranty Issuance Agreement, the Issuer
issued to Singapore Telecom the warrant dated June 28, 1996. This Original
Warrant entitled Singapore Telecom to purchase from the Issuer 625,000 shares of
Common Stock at an exercise price of $24 per share. 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 Original Warrant was prevented in whole or in part for
either of the foregoing reasons, the Issuer was required to provide the holder
of the Original 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 Original Warrant also provided that the number of warrant shares
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 Original
Warrant could be exercised was limited to the extent that certain financial
performance tests restricted the ability of the Issuer's subsidiary to borrow
fully under the long-term loan agreements. (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. (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

<PAGE>

                                                                              12

687,500, (b) to change the exercise price to $13 per share, and (c) to delete
Section 15. On March 31, 1998, pursuant to the Second Guaranty Issuance
Agreement (described below), this Amended Warrant was further amended so as to
(a) change the exercise price to $12.51 per share and (b) change the expiration
date to March 31, 2005.

         On June 28, 1996, the Issuer and each of the Guarantors became parties
to the Registration Rights Agreement. This Agreement provided a holder of the
Original Warrant or the shares issuable upon the exercise thereof with certain
demand and piggyback registration rights. These registration rights remained in
effect with respect to the Amended Warrant. The same registration rights were
provided to the holders of the warrants issued to the other Guarantors (i.e.,
Hughes Electronics and Baron). On March 31, 1998, the Agreement was amended to
cover the new warrants received by the Guarantors and the shares of Common Stock
issuable upon the exercise thereof, and as otherwise described below.

                                * * * * * * * * *

The Second Guaranty Issuance Agreement
- --------------------------------------

         Each of the Guarantors, the Issuer, and AMSC Acquisition, Inc. (a
subsidiary of the Issuer) ("AMSC Acquisition") are parties to the Guaranty
Issuance Agreement dated March 31, 1998 (the "Second Guaranty Issuance
Agreement"). The Second Guaranty Issuance Agreement specifies the consideration
to be provided by the Issuer and AMSC Acquisition to the Guarantors for the
issuance by the Guarantors of new guaranties of the obligations of the Issuer
and AMSC Acquisition under the restructured long-term financing arrangements
that closed on March 31, 1998. The closing of the restructured long-term
financing occurred simultaneously with the closing of (a) the Issuer's purchase
of the ARDIS business from Motorola, Inc., and (b) a high-yield debt offering by
AMSC Acquisition (which included warrants for the purchase of Common Stock).

         Under the Second Guaranty Issuance Agreement, the Issuer agreed to (a)
amend existing warrants held by the Guarantors so as change the exercise price
from $13 per share to $12.51 per share and the expiration date from June 28,
2001 to March 31, 2005, (b) issue to each Guarantor a new warrant to purchase
its respective Pro Rata Share of 1,000,000 shares of Common Stock at an exercise
price of $12.51 per share, and (c) execute an amended registration rights
agreement covering the old warrants (as amended), the new warrants, and other
restricted securities held by the Guarantors. The Pro Rata Share of each
Guarantor is equal to the aggregate principal amount of it guaranties divided
by $200,000,000. (Section 1)

         In addition, in consideration for Guarantors' willingness to guarantee
the Issuer's obligations under the restructured long-term financing beyond five
years, and up to a maximum of eight years, the Second Guaranty Issuance
Agreement provides that the Issuer shall pay each Guarantor certain specified
fees for each one-year extension of the credit facility beyond the fifth year.
The Issuer also agreed to (a) reimburse each Guarantor for all reasonable
expenses associated with the

<PAGE>

                                                                              13

negotiation, preparation, administration, and enforcement of the Agreement, the
guaranties, and the related documents, and (b) to execute a security and pledge
agreement granting the Guarantors a lien and security interest in all of the
Issuer's assets to secure the Issuer's obligations under the Agreement,
including the Issuer's obligation to reimburse any Guarantor that is required to
make any payment under its guaranties. (Sections 1, 2, and 4)

         The Second Guaranty Issuance Agreement also contains a limited
intercreditor agreement among the Guarantors. If any Guarantor makes any payment
under its guaranties or acquires any notes or obligations under the long-term
loan agreements, thereafter all decisions to act or to refrain from acting with
respect to the enforcement of such notes or obligations or the reimbursement
obligations contained in Section 13 of the Agreement against the Issuer or AMSC
Acquisition (including enforcement with respect to any collateral security
therefor) must first be approved in writing by Guarantors having Pro Rata Shares
greater than 87%. In addition, if any Guarantor does not make a required payment
under one of its guaranties and such payment is made by any other Guarantor,
then the defaulting Guarantor is obligated 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 or AMSC Acquisition or with respect to any
collateral therefor shall first be paid to the paying Guarantor until such
payment obligation of the defaulting Guarantor has been fully satisfied.
(Section 13)

Warrant No. 2 and the Amended Registration Rights Agreement
- -----------------------------------------------------------

         Pursuant to the terms of the Second Guaranty Issuance Agreement, the
Issuer issued to Singapore Telecom the warrant dated March 31, 1998. This
Warrant No. 2 entitles Singapore Telecom to purchase from the Issuer 125,000
shares of Common Stock at an exercise price of $12.51 per share. Warrant No. 2
is exercisable in full as of March 31, 1998, has an expiration date of March 31,
2005, and otherwise contains rights and conditions similar to those contained in
the Amended Warrant as amended.

         On March 31, 1998, the Issuer and each of the Guarantors entered into
the Amended and Restated Registration Rights Agreement ("Amended Registration
Rights Agreement"). The Amended Registration Rights Agreement provides certain
demand and piggyback registration rights with respect to shares of Common Stock
issuable upon the exercise of the old warrants and the new warrants, as well as
to other restricted securities held by the Guarantors.

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 13, 1998

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

<PAGE>

                                                                              14

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 (previously filed)

Exhibit XII -- Amendment No. 1 to Warrant Certificates dated March 27, 1997
(previously filed)

Exhibit XIII -- Participation Rights Agreement dated December 31, 1997
(previously filed)

Exhibit XIV -- Guaranty Issuance Agreement dated March 31, 1998

Exhibit XV --  Amendment No. 2 to Warrant Certificates dated March 31, 1998

Exhibit XVI -- Warrant dated March 31, 1998

Exhibit XVII -- Amended and Restated Registration Rights Agreement dated March
31, 1998

<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 13, 1998


                                 TEMASEK HOLDINGS (PRIVATE) LTD.


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

Dated:  April 13, 1998

<PAGE>

                                                                              16

                                  EXHIBIT INDEX

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


Exhibit I       Joint Filing Agreement dated April 13, 1998

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 (previously filed)

Exhibit XII     Amendment No. 1 to Warrant Certificates dated March 27,
                1997 (previously filed)

Exhibit XIII    Participation Rights Agreement dated December 31, 1997
                (previously filed)

Exhibit XIV     Guaranty Issuance Agreement dated March 31, 1998

Exhibit XV      Amendment No. 2 to Warrant Certificates dated March 31,
                1998

Exhibit XVI     Warrant dated March 31, 1998

<PAGE>

                                                                              17

Exhibit XVII    Amended and Restated Registration Rights Agreement dated
                March 31, 1998



                                    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 13, 1998 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 13th day
of April 1998.

                                 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 XIV

                           GUARANTY ISSUANCE AGREEMENT
                           ---------------------------

         THIS GUARANTY ISSUANCE AGREEMENT (this "Agreement") dated as of March
31, 1998 is 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"), AMERICAN MOBILE SATELLITE CORPORATION, a Delaware corporation ("AMSC
Parent"), and AMSC ACQUISITION COMPANY, INC., a Delaware corporation and a new
wholly-owned subsidiary of AMSC Parent ("AMSC Acquisition").


                                    RECITALS:


         WHEREAS, each of Hughes, SingTel and Baron is, directly or indirectly,
a shareholder of AMSC Parent;

         WHEREAS, AMSC Subsidiary Corporation ("AMSC Sub"), a Delaware
corporation dually incorporated as a Virginia public service corporation and a
wholly-owned subsidiary of AMSC Parent, AMSC Parent and Hughes Communications
Satellite Services, Inc. ("Hughes Communications") have entered into that
certain Bridge Loan Agreement dated as of December 30. 1997 (the "Hughes Bridge
Loan"), pursuant to which AMSC Sub has issued a term note in the aggregate face
amount of $10,000,000 to Hughes Communications;

         WHEREAS, AMSC Parent, AMSC Sub, the banks parties thereto, Morgan
Guaranty Trust Company of New York ("Morgan"), as Documentation Agent, and
Toronto Dominion (Texas), Inc. ("Toronto Dominion"), as Administrative Agent,
have entered into that certain $150,000,000 Credit Agreement dated as of June
28, 1996, providing for up to $150,000,000 of term loans (the "Existing Term
Loan Agreement"), and that certain $75,000,000 Credit Agreement dated as of June
28, 1996, providing for up to $75,000,000 of revolving loans (the "Existing
Revolving Credit Agreement" and together with the Existing Term Loan Agreement,
the "Existing Credit Agreement");

<PAGE>

         WHEREAS, each of Hughes, SingTel and Baron has issued a guaranty of a
portion of the obligations of AMSC Sub under the Existing Credit Agreements
(collectively, the "Existing Guaranties");

         WHEREAS, AMSC Parent and AMSC Acquisition, the owner of all of the
outstanding shares of capital stock of AMSC Sub. now propose, inter alia, to
renegotiate the existing indebtedness under the Existing Credit Agreements by
(i) AMSC Acquisition entering into that certain $100,000,000 Revolving Credit
Agreement with Morgan, Toronto Dominion and the banks party thereto providing
for up to $100,000,000 of revolving loans (the "Revolving Credit Agreement"),
and (ii) AMSC Parent entering into that certain $100,000,000 Term Credit
Agreement with Morgan, Toronto Dominion and the banks party thereto providing
for up to $100,000,000 of term loans (the "Term Credit Agreement" and, together
with the Revolving Credit Agreement, the "Credit Agreements");

         WHEREAS, in order to obtain the financing under the Revolving Credit
Agreement, AMSC Acquisition and AMSC Parent have requested that each of Hughes,
SingTel and Baron issue a guaranty of a portion of the obligations of AMSC
Acquisition under the Revolving Credit Agreement in substantially the form
attached hereto as Exhibit A-l, and in order to obtain the financing under the
Term Credit Agreement, AMSC Parent has requested that each of Hughes, SingTel
and Baron issue a guaranty of a portion of the obligations of AMSC Parent under
the Term Credit Agreement in substantially the form attached hereto as Exhibit
A-2 (each, a "Guaranty" and collectively, the "Guaranties"), effective upon the
repayment in full of the Existing Credit Agreements and other conditions set
forth in Section 2 hereof; and

         WHEREAS, each of Hughes, SingTel and Baron is willing to issue several
Guaranties on the terms, and subject to the conditions, set forth herein (the
parties which issue Guaranties hereunder are hereafter referred to individually
as "Guarantor" and collectively, "Guarantors").

                                   AGREEMENT:

         NOW, THEREFORE, in consideration of the foregoing recitals, the parties
hereto hereby agree as follows:

         1. Consideration for the Issuance of the Guaranties. (a) In
consideration of the issuance of the respective Guaranties by the Guarantors,
and

                                       2

<PAGE>

concurrently with (and conditioned upon) the issuance of these Guaranties, AMSC
Parent shall (i) amend existing warrants held by the Guarantors (the "Existing
Warrants") such that (x) the exercise price is changed from $13 per share to
$12.51 per share and (y) the expiration date is changed from June 28, 2001 to
March 31, 2005 (as amended, the "Amended Warrants"), (ii) issue to each
Guarantor a new warrant (collectively the "New Warrants") to purchase its "Pro
Rata Share" (as defined below) of 1,000,000 shares of the Common Stock, par
value $.01 per share, of AMSC Parent at an exercise price of $12.51 per share,
on the terms described in the form of warrant attached hereto as Exhibit B, and
(iii) execute an amended registration rights agreement covering the Amended
Warrants, the New Warrants and other restricted securities held by the
Guarantors, in the form attached hereto as Exhibit C (the "Amended Registration
Rights Agreement"). For purposes of this Agreement, the "Pro Rata Share" of a
Guarantor shall be calculated by dividing such Guarantor's aggregate Guaranteed
Principal Amounts (as defined and specified in each of its Guaranties) by
$200,000,000.

                  (b) In consideration of Guarantors' willingness to guarantee
AMSC Parent's obligations under the Term Credit Agreement beyond five years, and
up to a maximum of eight years, AMSC Parent shall pay to each Guarantor the
following fees for each one-year extension of the credit facility pursuant to
Section 2.12 of the Term Credit Agreement, payable at the time of each such
extension:(i) one percent (1%) of the Guaranteed Principal Amount (as defined
and specified in its Guaranty of the obligations under the Term Credit
Agreement) upon the first one-year extension, if any, of the credit facility
under the Term Credit Agreement, (ii) two percent (2%) of the Guaranteed
Principal Amount (as defined and specified in its Guaranty of the obligations
under the Term Credit Agreement) upon the second one-year extension, if any, of
the credit facility under the Term Credit Agreement and (iii) three percent (3%)
of the Guaranteed Principal Amount (as defined and specified in its Guaranty of
the obligations under the Term Credit Agreement) upon the third one-year
extension, if any, of the credit facility under the Term Credit Agreement.

         2. Conditions to the Issuance of the Guaranties. The obligations of
each Guarantor to issue its Guaranties are severally subject to the following
conditions:

                           (1) Each Guarantor shall have received evidence
satisfactory to it that the obligations of AMSC Sub and AMSC Parent under the
Hughes Bridge Loan have been satisfied in full.

                           (2) Each Guarantor shall have received evidence
satisfactory to it that the obligations of AMSC Sub and AMSC Parent under the
Existing Credit Agreements shall have been satisfied in full, the Banks party to
the Existing Credit

                                       3

<PAGE>

Agreements shall have irrevocably released their respective rights thereunder
and each of the Existing Guaranties shall have been released;

                           (3) AMSC Parent shall have executed and delivered to
each Guarantor its Amended Warrant, its New Warrant and the Amended Registration
Rights Agreement, as set forth in Section 1 hereof;

                           (4) AMSC Parent shall have executed and delivered to
the Guarantors (a) a security and pledge agreement in favor of the Banks under
the Term Credit Agreement (as defined therein) granting the Banks a lien and
security interest in all of its assets to secure AMSC Parent's obligations under
the Term Credit Agreement (the "Term Loan Security and Pledge Agreement"), and
(b) a security and pledge agreement in favor of the Guarantors granting the
Guarantors a second lien and security interest in all of its assets to secure
AMSC Parent's obligation under Section 13 of this Agreement to reimburse a
Guarantor that makes any payment under its Guaranty of obligations under the
Revolving Credit Agreement (the "Reimbursement Security and Pledge Agreement"
and, together with the Term Loan Security and Pledge Agreement, the "Security
and Pledge Agreements"), and each Guarantor shall have received evidence
satisfactory to it that such liens and security interests have been duly
perfected;

                           (5) Each of AMSC Acquisition and AMSC Parent shall
have received all consents and approvals (including from shareholders) required
to enter into this Agreement, the Amended Warrants, the New Warrants, the
Amended Registration Rights Agreement, the Term Loan Security and Pledge
Agreement and the Reimbursement Security and Pledge Agreement (collectively, the
"GIA Documents") and to perform its obligations thereunder, and shall have
delivered copies of all such consents and approvals to each Guarantor;

                           (6) Each Guarantor shall have approved the form and
substance of each of the Credit Agreements and all documents and instruments
delivered in connection therewith, and shall have received evidence satisfactory
to it of the execution and delivery thereof;

                           (7) Each Guarantor shall have received copies,
certified by the Secretary of each of AMSC Acquisition and AMSC Parent, of
resolutions duly adopted by the Board of Directors of the applicable party
approving each of the Credit Agreements and GIA Documents and the transactions
contemplated thereby.

                           (8) Each Guarantor shall have received all documents
and instruments delivered in connection with the high yield debt offering by
AMSC

                                       4

<PAGE>

Acquisition and such offering shall have closed and resulted in minimum net
proceeds of $140,000,000 to AMSC Acquisition;

                           (9) Each Guarantor shall have received the written
opinions of counsel to AMSC Acquisition and AMSC Parent as to the due
authorization, execution and enforceability of each of the GIA Documents and the
lack of any conflict between each GIA Document and any other agreement to which
AMSC Acquisition or AMSC Parent may be a party, in form and substance
satisfactory to each Guarantor;

                           (10) Each Guarantor shall have received evidence
satisfactory to it that each of the other Guarantors shall have concurrently
issued its Guaranties;

                           (11) There shall have been no material adverse change
in the business, condition (financial or otherwise), operations, performance,
properties or prospects of AMSC Parent or AMSC Acquisition since September 30,
1997, except as set forth in the Preliminary Offering Memorandum dated March 9,
1998 with respect to the high yield debt offering by AMSC Acquisition; and

                           (12) Each Guarantor shall have received in full the
consideration under Section 1 hereof and the amounts outstanding under Section 4
hereof (to the extent invoices have been supplied to AMSC Parent).

         3. Covenants of AMSC Acquisition and AMSC Parent; Guaranty Issuance
Agreement Events of Default.

                  (a) (i) AMSC Parent shall maintain at all times during each
calendar quarter set forth below, such maintenance to be evidenced at the end of
each such calendar quarter on a consolidated basis, a ratio of (x) the
Indebtedness (as defined in the Credit Agreements) of AMSC Parent and its
Subsidiaries (as defined in the Credit Agreements) to (y) EBITDA (as defined
below) for AMSC Parent and its consolidated Subsidiaries determined on the basis
of the twelve-month period ending on the last day of such calendar quarter, in
each case not in excess of the ratio set forth below for such calendar quarter:

         For the calendar
         quarter ending on                          Maximum Ratio
         -----------------                          -------------
         September 30, 2000                         31.0 to 1.00
         December 31, 2000                          18.5 to 1.00

                                       5

<PAGE>

         March 31,2001                              13.0 to 1.00
         June 30, 2001                              10.5 to 1.00
         September 30, 2001                          8.5 to 1.00
         December 31, 2001                           7.0 to 1.00

         March 31, 2002                              6.0 to 1.00
         June 30, 2002                               5.5 to 1.00
         September 30, 2002                          5.0 to 1.00
         December 31, 2002                           4.5 to 1.00

For purposes of this Section 3(a)(i), "EBITDA" shall mean, for any period, for
AMSC Parent on a consolidated basis, determined in accordance with GAAP (as
defined in the Credit Agreements), the sum of (i) the net income (or net loss)
plus (ii) all amounts treated as expenses for depreciation and interest and the
amortization of intangibles of any kind to the extent included in the
determination of such net income (or loss), plus (iii) all taxes on or measured
by gross or net income to the extent included in the determination of such net
income (or loss). For purposes of the foregoing, notwithstanding any requirement
of GAAP to the contrary, "net income" shall exclude:

                  (A) any restoration to income of any contingency or
non-recurring charge reserves, except to the extent that provision for such
reserve was made out of income accrued during such period and except for normal
accruals and reversals in the ordinary course of business;

                  (B) any write-up or write-down of any asset, and all equity
and earnings or losses of unconsolidated investments in joint ventures,
Subsidiaries, and other business organizations;

                  (C) any net gain from the collection of the proceeds of life
insurance policies;

                  (D) any gain or loss arising from the acquisition of any
securities or Indebtedness and any net loss arising from the exercise or grant
of any warrant or option;

                  (E) any deferred credit representing the excess of equity in
any Person (as defined in the Credit Agreements) at the date of acquisition over
the cost of the investment in such Person;

                                       6

<PAGE>

                  (F) any proceeds received pursuant to the Satellite Lease
Agreement dated as of December 2, 1997 between AMSC Parent and African
Continental Telecommunications Ltd., less the expenses described in the letter
agreement dated December 2, 1997 from AMSC Parent to Guarantors;

                  (G) any aggregate net gain (or loss) during such period
arising from the sale, exchange, lease or other disposition of capital assets
(such term to include all fixed assets, whether tangible or intangible, all
inventory sold in conjunction with the disposition of fixed assets, and all
securities) other than (i) any sale, exchange or other disposition in the
ordinary course of business and (ii) any sale, exchange or disposition of
equipment utilized in the business of AMSC Parent;

                  (H) all extraordinary items; and

                  (I) any change in accruals for long-term (more than one year)
personnel-related costs, such as vacation time, pension liabilities and retires
insurance.

                           (ii) The aggregate amount of Service Revenue (as
defined below) of AMSC Parent and its Subsidiaries determined on the basis of
the twelvemonth period ending on any date set forth below shall not be less than
the amount set forth below for such twelve-month period:

         For the twelve-month
         period ending on                      Minimum Service Revenue
         --------------------                  -----------------------
         December 31, 1998                          $ 65,000,000

         March 31, 1999                               71,000,000
         June 30, 1999                                79,000,000
         September 30, 1999                           88,000,000
         December 31, 1999                            98,000,000

         March 31, 2000                              111,000,000
         June 30, 2000                               125,000,000
         September 30, 2000                          139,000,000
         December 31, 2000                           153,000,000

         March 31, 2001                              167,000,000
         June 30, 2001                               179,000,000
         September 30, 2001                          191,000,000
         December 31, 2001                           203,000,000

                                       7

<PAGE>

         March 31, 2002                              214,000,000
         June 30, 2002                               225,000,000
         September 30, 2002                          236,000,000
         December 31, 2002                           249,000,000


For purposes of this Section 3(a)(ii), "Service Revenue" shall mean, for any
period, for AMSC Parent on a consolidated basis, gross revenues from services
including, but limited not to, data service and mobile voice communications
service. For purposes of the foregoing, "Service Revenue" shall not include
revenues derived from sales of equipment or lease proceeds.

                  (b) The occurrence of any one or more of the following events
(regardless of the reason thereof) shall constitute a "Guaranty Issuance
Agreement Event of Default" hereunder:

                           (1) An "Event of Default" under any of the Credit
Agreements (as defined therein), except any Event of Default specified in
Section 6.01(k) or (m) of the Revolving Credit Agreement or Section 6.01(k) or
(n) of the Term Credit Agreement, shall occur;

                           (2) AMSC Acquisition or AMSC Parent shall fail or
neglect to perform, keep or observe any covenant or agreement contained in
Section l(b) Section 3(a) or Section 13 hereof; or

                           (3) Any representation or warranty made or deemed
made in Section 12 hereunder by AMSC Acquisition or AMSC Parent shall prove to
have been incorrect in any material respect when made or deemed made.

         Upon the occurrence of a Guaranty Issuance Agreement Event of Default,
Guarantors having Pro Rata Shares greater than 87% ("Requisite Guarantors") may,
at their option, deliver written notice of such occurrence to the Administrative
Agent or the Banks under each of the Credit Agreements.

         Any notice by Requisite Guarantors in accordance with this Section 3
shall constitute notice of the existence of a Guaranty Issuance Agreement Event
of Default under Section 6.01(u) of each Credit Agreement.

         4. Expenses. Each of AMSC Acquisition and AMSC Parent jointly and
severally agrees that it will, upon demand, pay to the each Guarantor the amount
of any and all reasonable expenses, including, without limitation, the
reasonable fees and

                                       8

<PAGE>

expenses of such Guarantor's counsel and of any experts and agents, which such
Guarantor may incur in connection with (i) the negotiation, preparation or
administration of each GIA Document and each Guaranty (including costs and
expenses of providing any consents or waivers), (ii) the exercise or enforcement
of any of the rights and remedies hereunder of such Guarantor, or (iii) the
failure by AMSC Acquisition or AMSC Parent to perform or observe any of the
provisions hereof.

         5. Amendments, Etc. No amendment or waiver of any provision of this
Agreement shall in any event be effective unless the same shall be in writing
and with respect to its enforcement against any party, signed by such party,
except that the provisions of Section 3(a) hereof may be amended or waived by
written agreement executed by Requisite Guarantors, AMSC Acquisition and AMSC
Parent. Any action by Requisite Guarantors to amend or waive any provisions of
Section 3(a) hereof in accordance with this Section 5 shall bind all Guarantors.

         6. No Waiver; Remedies. No failure on the part of any Guarantor to
exercise, and no delay in its exercise of, any right hereunder shall operate as
a waiver thereof; nor shall any single or partial exercise of any right
hereunder by any Guarantor preclude any other or further exercise thereof or the
exercise of any other right by such party or any other Guarantor except as
otherwise provided in Sections 3 or 5 hereof with respect to actions taken by
Requisite Guarantors. Any Guarantor may specifically waive any breach of this
Agreement by AMSC Acquisition or AMSC Parent; PROVIDED that (x) no such waiver
shall be effective or binding unless in writing, (y) except as otherwise
specifically provided in Section 5 hereof with respect to certain waivers by
Requisite Guarantors, no such waiver shall be effective as to any Guarantor that
has not provided a written waiver with respect to such breach, and (z) no such
waiver shall constitute a continuing waiver of similar or other breaches. Any
party may specifically waive any condition to its own obligations hereunder, and
such waiver shall not affect the obligations of any other party.

         7. Notices, Etc. (a) AMSC Parent and AMSC Acquisition shall promptly
provide to each Guarantor (i) a copy of any notice provided to or received from
the Banks or an agent thereof under either Credit Agreement, (ii) notice
concerning any reduction of commitments or prepayment of principal under either
Credit Agreement and (iii) notice concerning any Default, Event of Default or
anticipated Event of Default under either Credit Agreement.

                  (b) All notices, demands, requests, consents, approvals and
other instruments hereunder shall be in writing and shall be deemed to have been
properly given if given to the parties hereto at the addresses or facsimile
number set forth on

                                       9

<PAGE>

Exhibit D hereto, or such other address or facsimile number as may be notified
to the other parties hereto in a written notice. Notices, demands and requests
shall be effective if given by facsimile to the number specified in this Section
7 when confirmation of receipt is received; or if given by any other means, when
delivered.

         8. Separability of This Agreement. In case any term or provision of
this Agreement or any application thereof to any circumstance shall, in any
circumstances or jurisdiction and to any extent, be invalid, illegal or
unenforceable, such term or provision shall be ineffective as to such
jurisdiction to the extent of such invalidity, illegality or unenforceability,
without invalidating or rendering unenforceable any remaining terms and
provisions hereof or the application of such term or provision to circumstances
or jurisdictions other than those as to which it is held invalid, illegal or
unenforceable.

         9. Further Assurances. AMSC Acquisition and AMSC Parent hereby agree to
execute and deliver all such instruments and take all such action as Hughes,
SingTel or Baron may from time to time reasonably request in order to fully
effectuate the purposes of this Agreement.

         10. Headings. The headings contained in this Agreement are for
convenience of reference only and shall not modify, define or limit any of the
terms or provisions hereof.

         11. GOVERNING LAW AND DAMAGE LIMITATION. THIS AGREEMENT SHALL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK
APPLICABLE TO AGREEMENTS MADE AND TO BE PERFORMED ENTIRELY WITHIN SUCH STATE.
THE PARTIES HEREBY IRREVOCABLY WAIVE ANY RIGHT TO A JURY TRIAL WITH RESPECT TO
ANY ACTION OR PROCEEDING ARISING OUT OF OR RELATING TO THIS AGREEMENT. THE
PARTIES AGREE THAT NO PARTY SHALL BE LIABLE HEREUNDER FOR ANY SPECIAL, INDIRECT,
CONSEQUENTIAL OR INCIDENTAL DAMAGES, INCLUDING, WITHOUT LIMITATION, DAMAGES FOR
LOST PROFITS OR BUSINESS.

         12. Representations and Warranties of AMSC Acquisition and AMSC Parent.
Each of AMSC Acquisition and AMSC Parent represents and warrants to each
Guarantor that:

                  (a) Each of AMSC Acquisition and AMSC Parent is a corporation
duly organized, validly existing and in good standing under the laws of the

                                       10

<PAGE>

jurisdiction of its incorporation, and has all requisite corporate power and
authority to enter into and perform its obligations under this Agreement.

                  (b) This Agreement has been duly authorized by all necessary
corporate action on the part of, and has been duly executed and delivered by,
each of AMSC Acquisition and AMSC Parent, and none of the execution and delivery
hereof, the consummation of the transactions contemplated hereby (including the
issuance of the Amended Warrants and the New Warrants and the issuance of the
common stock of AMSC Parent upon exercise of the Amended Warrants or New
Warrants, the registration of such stock pursuant to the Amended Registration
Rights Agreement and the granting of liens and security interests by AMSC Parent
in all of its assets under the Security and Pledge Agreements) or compliance by
AMSC Acquisition and AMSC Parent with any of the terms and provisions hereof or
of any of the other Documents (i) requires any approval of stockholders
(including any consent under the rules of the National Association of Securities
Dealers, Inc.) or approval or consent of any trustee or holders of any
indebtedness or obligations of AMSC Acquisition or AMSC Parent other than such
approvals or consents as have been obtained, (ii) contravenes any law, judgment,
governmental rule or regulation or order applicable to or binding on AMSC
Acquisition or AMSC Parent or any of their respective properties, the
contravention of which would have a material adverse effect on the financial
condition of AMSC Acquisition and its subsidiaries taken as a whole or AMSC
Parent and its subsidiaries taken as a whole or on the ability of AMSC
Acquisition and AMSC Parent to perform any of its obligations under this
Agreement or any of the other Documents, (iii) contravenes or results in any
breach of or constitutes any default under, any indenture, mortgage, chattel
mortgage, deed of trust, conditional sales contract, bank loan or credit
agreement for borrowed money, contract or other agreement or instrument to which
AMSC Acquisition or AMSC Parent is a party or by which AMSC Acquisition or AMSC
Parent or any of their respective properties may be bound, or (iv) contravenes
its corporate charter or by-laws.

                  (c) Neither the execution, delivery and performance by AMSC
Acquisition and AMSC Parent of this Agreement nor the consummation of any of the
transactions contemplated hereby (including the issuance of the Amended Warrants
and the New Warrants and the issuance of the common stock of AMSC Parent upon
the exercise of any Amended Warrants or New Warrants and the granting of liens
and security interests by AMSC Parent in all of its assets under the Security
and Pledge Agreements) requires the consent, approval or authorization of, the
giving of notice to, or the registration, recording or filing of any document
with, or the taking of any other action in respect of, any governmental agency
or authority, other than any registration or other action required under the
Registration Rights Agreement or any recording, filing or other action under the
Security and Pledge Agreements.

                                       11

<PAGE>

                  (d) This Agreement constitutes, and the other Documents will,
upon execution thereof, constitute, the legal, valid and binding obligation of
each of AMSC Acquisition and AMSC Parent, enforceable against AMSC Acquisition
and AMSC Parent in accordance with their terms, except as such enforcement may
be subject to bankruptcy, insolvency, moratorium or other similar laws affecting
creditors' rights generally and to general principles of equity.

                  (e) AMSC Parent has delivered copies of the consolidated
balance sheet of AMSC Parent and its consolidated subsidiaries as of December
31, 1996, and related statements of consolidated income and cash flow and
stockholder's equity for the fiscal year then ended, accompanied by the report
of Arthur Andersen LLP, independent accountants. Such statements fairly present,
in accordance with generally accepted accounting principles, the financial
position of AMSC Parent and its consolidated subsidiaries as of such date and
the results of their operations and cash flows for such fiscal year.

                  (f) AMSC Parent has duly reserved shares of its Common Stock
for issuance upon exercise of the Amended Warrants and the New Warrants.

                  (g) An independent committee of the board of directors of AMSC
Parent (i) has concluded, based on such expert advice as it deems appropriate,
that the consideration provided to Guarantors pursuant to this Agreement is fair
to AMSC Acquisition and AMSC Parent from a financial point of view, and (ii) has
approved AMSC Parent's entering into this Agreement.

         13. Reimbursement Agreement. If Hughes, SingTel or Baron makes any
payment under its Guaranties, each of AMSC Acquisition and AMSC Parent agrees
that it shall jointly and severally reimburse such Guarantor for such payment,
and that such Guarantor will be fully subrogated to the extent of such payment
to the rights and remedies (including any collateral security or rights therein)
of the lenders under the Credit Agreements. If Hughes, SingTel or Baron acquires
any notes evidencing the loans under either Credit Agreement, or any obligations
in respect of loans made or to be made under either Credit Agreement, then such
Guarantor shall have the full benefit of all of the rights and remedies
(including any collateral security) of a lender under the applicable Credit
Agreement. Any such acquisition of notes does not impair or extinguish any
rights Guarantors have under this Agreement. Neither Hughes, SingTel nor Baron
shall have any duties to AMSC Acquisition or AMSC Parent with respect to the
exercise or non-exercise of any rights or remedies to which the lenders are
entitled under the notes or the Credit Agreements.

                                       12

<PAGE>

         14. Intercreditor Agreements. (a) If Hughes, SingTel or Baron makes any
payment under its Guaranties or acquires any notes or obligations under either
of the Credit Agreements, thereafter all decisions to act or refrain from acting
with respect to the enforcement of such notes or obligations (whether acquired
by subrogation or otherwise) or the reimbursement obligations contained in
Section 13 hereof against AMSC Acquisition or AMSC Parent (including enforcement
with respect to any collateral security therefor) must first be approved in
writing by Requisite Guarantors. Prior to taking any such action, each Guarantor
shall discuss with each other Guarantor the actions proposed to be taken.

                  (b) If any Guarantor does not make any required payment under
its Guaranties (a "defaulting Guarantor"), and such payment is made by any other
Guarantor (a "funding Guarantor") even though the funding Guarantor has no
obligation to make such payment, then the defaulting Guarantor shall reimburse
the funding Guarantor for such payments on demand, and any amounts which would
otherwise be payable to the defaulting Guarantor by AMSC Acquisition or AMSC
Parent or with respect to any collateral therefor shall first be paid to the
funding Guarantor until such payment obligation of the defaulting Guarantor has
been fully satisfied. If, notwithstanding the foregoing, the defaulting
Guarantor receives such payment, it shall hold the payment in trust for the
funding Guarantor. For purposes of this Agreement, any payment made by a funding
Guarantor shall be added to the Pro Rata Share of the funding Guarantor and
subtracted from the Pro Rata Share of the defaulting Guarantor until the
defaulting Guarantor has fully satisfied its payment obligation to the funding
Guarantor. The funding Guarantor shall be subrogated to the rights of the
lenders to enforce the Guaranty of the defaulting Guarantor to the extent of the
defaulting Guarantor's payment obligation to the funding Guarantor.

                                       13

<PAGE>

         IN WITNESS WHEREOF, each of the parties hereto has caused this
Agreement to be executed by its duly authorized officer.


AMSC ACQUISITION COMPANY, INC.           SINGAPORE TELECOMMUNICATIONS
                                         LTD.

By:    /s/ Gary M. Parsons               By:    /s/ Yap Chee Keong
- --------------------------               -------------------------
Name:                                    Name:  Yap Chee Keong
Title:                                   Title: Group Financial Controller


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

By:    /s/ Gary M. Parsons               By: Baron Capital Management, Inc.
- --------------------------               a general partner
Name:  
Title:
                                         By:    /s/ Morty Shaja
                                         ----------------------
                                         Name:  Morty Shaja
                                         Title: 

HUGHES ELECTRONICS
CORPORATION

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

                                       14

<PAGE>

                                  Exhibit A-1
                         [Form of Guaranty -- Omitted]

                                  Exhibit A-2
                         [Form of Guaranty -- Omitted]

                                   EXHIBIT B
                          [Form of Warrant -- Omitted]
                       [See Exhibit XVI to Schedule 13D]

                                   EXHIBIT C
    [Form of Amended and Restated Registration Rights Agreement -- Omitted]
                       [See Exhibit XVII to Schedule 13D]



                                      
<PAGE>

                                   EXHIBIT D

                                Notice Addresses



AMERICAN MOBILE SATELLITE CORPORATION
AMSC ACQUISITION COMPANY, INC.
10802 Parkridge Blvd.
Reston, Virginia 22091
Attention: Chief Financial Officer
Fax: (703) 758-6142


HUGHES ELECTRONICS CORPORATION
200 North Sepulveda Blvd.
18th Floor
MS-A150
El Segundo, California 90245-0956
Attention: Amnon Carr
Fax: (310) 640-1715


SINGAPORE TELECOMMUNICATIONS LTD.
31 Exeter Road
#03-00 Comcentre III
Singapore 239734 Republic of Singapore
Attention: Yap Chee Keong
Fax: 011-65-738-8144


BARON CAPITAL PARTNERS,  L.P.
767 Fifth Avenue
24th Floor
New York, New York 10153
Attention: Morty Schaja
Fax: (212) 759-7529

                                     




                                   EXHIBIT XV
                                 AMENDMENT NO. 2
              TO WARRANT CERTIFICATES FOR THE PURCHASE OF SHARES OF
              COMMON STOCK OF AMERICAN MOBILE SATELLITE CORPORATION


         AMENDMENT, dated as of March 31, 1998, 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 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, the Holders and AMSC Acquisition Company, Inc.
are entering into, on the date hereof, a new Guaranty Issuance Agreement related
to the restructuring of the Company's existing indebtedness and the issuance of
new guaranties by the Holders (the "1998 Guaranty Issuance Agreement").

         WHEREAS, as contemplated by the 1998 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.  AMENDMENTS.

                  (a) Section 1 of each of the Warrants is hereby amended by

                           (i) modifying the definition of "Exercise Price" to
                           read in its entirety as follows:

                           "Exercise Price" means initially $12.51 per Warrant
                           Share, as adjusted from time to time,

                           and

                           (ii) modifying the definition of "Expiration Date" to
                           read in its entirety as follows:

<PAGE>

                           "Expiration Date" means March 31, 2005, at 5:00 p.m.
                           New York City time.

                  (b) Section 10C. of each of the Warrants is hereby amended to 
                  read in its entirety as follows:

                           If as a result of any event or for any other reason,
                           any adjustment is made which increases the number of
                           shares of Common Stock issuable upon conversion,
                           exercise or exchange of, or in the conversion or
                           exercise price or exchange ratio applicable to, any
                           outstanding securities of the Company that are
                           convertible into, or exercisable or exchangeable for,
                           Common Stock of the Company, including, without
                           limitation, any action taken in connection with the
                           warrants dated as of March 31, 1998 issued in
                           connection with the offering by the Company and AMSC
                           Acquisition Company, Inc. ("AMSC Acquisition") of
                           units consisting of 12 1/4% Senior Notes due 2008 of
                           AMSC Acquisition and warrants to purchase Common
                           Stock, then a corresponding adjustment shall be made
                           hereunder to increase the Warrant Share Amount, but
                           only to the extent that no such adjustment has been
                           made pursuant to Sections 10.A of B hereof with
                           respect to such event or for such other reason.

                  (c) Section 16 of each of the Warrants is hereby amended by
                  replacing the words "Registration Rights Agreement dated as of
                  June 28, 1996" in the third line thereof with the words
                  "Amended and Restated Registration Rights Agreement dated as
                  of March 31, 1998."

         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 and the amendments effected by Amendment No. 1, to
replace the presently outstanding Warrant held by such Holder.

                                       2

<PAGE>

         IN WITNESS WHEREOF, each of the parties hereto has executed this
Amendment No. 2 by its duly authorized officer as of the day and year first set
forth above.


AMERICAN MOBILE SATELLITE                SINGAPORE TELECOMMUNICATIONS
CORPORATION                              LTD.

By:    /s/ Gary M. Parsons               By:    /s/ Yap Chee Keong
- --------------------------               -------------------------
Name:                                    Name:  Yap Chee Keong
Title:                                   Title: Group Financial Controller


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

By:    /s/ Amnon Carr                    By: Baron Capital Management, Inc.
- ---------------------                    a general partner
Name:  Amnon Carr
Title: Assistant Treasurer               By:    /s/ Ronald Baron
                                         -----------------------
                                         Name:  
                                         Title: 



                                   EXHIBIT XVI
   THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, OR ANY STATE SECURITIES LAWS AND MAY NOT BE TRANSFERRED OR SOLD EXCEPT
         IN COMPLIANCE THEREWITH OR PURSUANT TO AN EXEMPTION THEREFROM.


                      AMERICAN MOBILE SATELLITE CORPORATION

                      Warrant for the Purchase of Shares of
              Common Stock of American Mobile Satellite Corporation
              -----------------------------------------------------

No. 5                                                        Warrant to Purchase
                                                                  125,000 Shares
                                                                  --------------


         FOR VALUE RECEIVED, AMERICAN MOBILE SATELLITE CORPORATION, a Delaware
corporation (the "Company"), hereby certifies that Singapore Telecommunications
Ltd., its successor or permitted assigns (the "Holder"), is entitled, subject to
the provisions of this Warrant, to purchase from the Company, at the times
specified herein, One Hundred and Twenty Five Thousand (125,000) (the "Warrant
Share Amount") fully paid and non-assessable shares of Common Stock of the
Company, par value $.01 per share (the "Common Stock"), at a purchase price per
share equal to the Exercise Price (as hereinafter defined). The Warrant Share
Amount and the Exercise Price are subject to adjustment from time to time as
hereinafter set forth.

         1. DEFINITIONS. The following terms, as used herein, have the following
meanings:

         "Accepted Alien Ownership Percentage Limitation" means 24.99% or, in
the event of a modification of the Alien Ownership Restrictions subsequent to
the date hereof, such percentage limitation upon the Company's Alien ownership
as may be in effect from time to time as a result of such modification, less
0.01%.

         "Alien" means any alien or a representative thereof, or a foreign
government or a representative thereof, or a corporation or other entity
organized under the laws of any foreign government.

         "Alien Owned Percentage" means, with respect to any Person, the
percentage of total ownership in such Person owned of record, as well as the
percentage of total ownership in such Person voted, by Aliens; provided, that if
under the Alien Ownership Restrictions such Person would be deemed to have a
percentage of total ownership owned of record or voted by Aliens other than the
actual percentage so owned or voted, then such Person's Alien Ownership
Percentage shall be such deemed percentage.

         "Alien Ownership Restrictions" means Section 310(b) of the
Communications Act, as modified by any interpretation, ruling or order of the
Federal Communications Commission (or any successor agency) applicable to the
Company or any of its subsidiaries.

         "Board of Directors" means the Board of Directors of the Company.

<PAGE>

         "Board of Directors" means the Board of Directors of the Company.

         "Business Day" means any day except a Saturday, Sunday or other day on
which commercial banks in the City of New York are authorized by law to close.

         "Communications Act" means the Communications Act of 1934, as amended,
or any successor statute.

         "Current Market Price Per Common Share" has the meaning set forth in
Section 10.D.

         "Exercise Date" means the applicable date of exercise of this Warrant,
as indicated on the Warrant Exercise Notice delivered by the Holder.

         "Exercise Price" means initially $12.51per Warrant Share, as adjusted
from time to time.

         "Expiration Date" means, March 31, 2005 at 5:00 p.m. New York City
time.

         "Person" means an individual, corporation, partnership, limited
liability company, association, trust, or any other entity or organization,
including a government or political subdivision or an agency or instrumentality
thereof.

         "Securities Act" means the Securities, Act of 1933, as amended, and the
rules and regulations promulgated thereunder.

         "Warrant Exercise Notice" means the Warrant Exercise Notice forming a
part hereof.

         "Warrant Margin" means, on any date, the difference of (x) the greater
of (A) the average of the Closing Prices (as defined in Section 10.D) on each of
the 20 trading days immediately preceding such date and (B) the Closing Date on
the trading day two trading days prior to such date, minus (y) the Exercise
Price.

         "Warrant Share Amount" has the meaning set forth in the preamble
hereof.

         "Warrant Shares" means the shares of Common Stock deliverable upon
exercise of this Warrant, as adjusted from time to time.

         2. EXERCISE OF WARRANT.

                  A. The Holder is entitled to exercise this Warrant in whole or
in part at any time, or from time to time, to and including the Expiration Date
or, if such day is not a Business Day, then on the next succeeding day that
shall be a Business Day. To exercise this Warrant, the Holder shall execute and
deliver to the Company at its address set forth in Section 12 hereof a Warrant
Exercise Notice substantially in the form annexed hereto and shall deliver to
the Company (x) this Warrant Certificate, including the Warrant Exercise
Subscription Form forming a part hereof duly executed by the Holder, and (y)
subject to Section 2.B, payment of the

                                       2

<PAGE>

Exercise Price then in effect for such Warrant Shares. Upon such delivery and
payment, the Holder shall be deemed to be the holder of record of the Warrant
Shares subject to such exercise. notwithstanding that the stock transfer books
of the Company shall then be closed or that certificates representing such
Warrant Shares shall not then be actually delivered to the Holder.

                  B. The Exercise Price may be paid in cash or by certified or
official bank check or bank cashier's check payable to the order of the Company
or by wire transfer of immediately available funds to an account designated by
the Company or by cancellation of indebtedness owed to the Holder or by any
combination of such methods. In the alternative, the Holder may exercise its
right to receive Warrant Shares on a net basis, such that, without the exchange
of any funds, the Holder will receive that number of Warrant Shares (and such
other consideration) otherwise issuable (or payable) upon exercise of this
Warrant less that number of Warrant Shares having an aggregate Current Market
Price Per Common Share on the Exercise Date equal to the aggregate Exercise
Price that would otherwise have been paid by the Holder for the Warrant Shares.
The Company shall pay any and all documentary, stamp or similar issue or
transfer taxes payable in respect of the issue or delivery of this Warrant and
the issue and delivery of the Warrant Shares.

                  C. If the Holder exercises this Warrant in part, this Warrant
Certificate shall be surrendered by the Holder to the Company and a new Warrant
Certificate of the same tenor and for the unexercised number of Warrant Shares
shall be executed by the Company. The Company shall register the new Warrant
Certificate in the name of the Holder or in such name or names of its
transferee(s) pursuant to Section 8 hereof as may be directed in writing by the
Holder and deliver the new Warrant Certificate to the Person or Persons entitled
to receive the same.

                  D. Except as otherwise provided in Section 3, upon surrender
of this Warrant Certificate in conformity with the foregoing provisions, the
Company shall transfer to the Holder of this Warrant Certificate appropriate
evidence of ownership of the shares of Common Stock or other securities or
property (including any money) to which the Holder is entitled, registered or
otherwise placed in, or payable to the order of, the name or names of the Holder
or its transferee(s) as may be directed in writing by the Holder, and shall
deliver such evidence of ownership and any other securities or property
(including any money) to the Person or Persons entitled to receive the same,
together with an amount in cash in lieu of any fraction of a share as provided
in Section 7 below.

         3. OWNERSHIP LIMITATION. If at any time the exercise of any Warrants
pursuant to Section 2 would cause the Company's Alien Ownership Percentage to
exceed the Accepted Alien Ownership Percentage Limitation, then in lieu of
issuing shares of Common Stock pursuant to Section 2:

                  A. the Company shall issue to each Holder exercising Warrants
at such time (each an "Exercising Holder") whose Alien Ownership Percentage is
less than or equal to the Accepted Alien Ownership Percentage Limitation the
number of shares of Common Stock to which such Exercising Holder is entitled
pursuant to Section 2;

                                       3

<PAGE>

                  B. the Company shall issue to each Exercising Holder whose
Alien Ownership Percentage is greater than the Accepted Alien Ownership
Percentage Limitation (each, an "Affected Exercising Holder") a number of shares
of Common Stock equal to the quotient of (x) the product of (A) the number of
shares of Common Stock that, immediately after giving effect to any issuances of
Common Stock pursuant to the foregoing Section 3.A, could be issued to a Person
with a 100% Alien Ownership Percentage without causing the Company's Alien
Ownership Percentage to exceed the Accepted Alien Ownership Percentage
Limitation, multiplied by (B) the number of shares of Common Stock to which such
Affected Exercising Holder would be entitled pursuant to Section 2 but for the
application of this Section 3, divided by (y) the product of (A) the aggregate
number of shares of Common Stock to which all Affected Exercising Holders would
be entitled pursuant to Section 2 but for the application of this Section 3,
multiplied by (B) such Affected Exercising Holder's's Alien Ownership
Percentage; provided that in no event shall the number of shares of Common Stock
issuable to any Affected Exercising Holder pursuant to this Section 3B exceed
the number of shares of Common Stock to which such Affected Exercising Holder
would have been entitled pursuant to Section 2 but for the application of this
Section 3; and

                  C. the Company shall deliver by wire transfer of immediately
available funds to the account of each Affected Exercising Holder specified in
such Affected Exercising Holder's Warrant Exercise Notice, an amount equal to
the product of (x) the number of shares of Common Stock to which such Affected
Exercising Holder would have been entitled pursuant to Section 2 that are not
issuable to such Affected Exercising Holder pursuant to the foregoing Section
3.B, multiplied by (y) the Warrant Margin on the Exercise Date.

         4. NASD LIMIT. Notwithstanding the provisions of Sections 2 and 3, in
no event shall this Warrant be exercisable for an aggregate number of shares of
Common Stock equal to or greater than such number of shares as would require the
approval of the Company Stockholders pursuant to Rule 4460(i)(1)(D) of the
National Association of Securities Dealers, Inc. (the "NASD Limit") unless the
Company's stockholders have, prior to any exercise of this Warrant that would
require the issuance of Common Stock equal to or greater than the NASD Limit,
approved the exercise of Warrants for an aggregate number of shares of Common
Stock equal to or greater than the NASD Limit. If, upon any exercise of this
Warrant, shares of Common Stock that would otherwise be issuable upon such
exercise are not issuable due to the provisions of the foregoing sentence, then
in lieu of issuing shares of Common Stock pursuant to Sections 2 or 3:

              (i) the Company shall issue the maximum number of shares of Common
     Stock, if any, issuable up to the NASD Limit; PROVIDED, that if more than
     one holder of Warrants is exercising Warrants at such time, such issuance
     shall be prorated in proportion to the number of shares of Common Stock to
     which each holder of Warrants exercising Warrants at such time would be
     entitled but for the provisions of this Section 4; and

              (ii) the Company shall deliver by wire transfer of immediately
     available funds to the account of each Exercising Holder specified in such
     Exercising Holder's Warrant Exercise Notice, an amount equal to the product
     of (x) the number of shares of Common Stock to which such Exercising Holder
     would have been entitled

                                       4

<PAGE>

     pursuant to the foregoing Sections 2 and 3 that are not issuable to such
     Exercising Holder pursuant to the foregoing clause (i), and (y) the Warrant
     Margin on the Exercise Date.

         5. RESTRICTIVE LEGEND. Upon original issuance thereof, and until such
time as the same shall have been registered under the Securities Act or sold
pursuant to Rule 144 promulgated thereunder (or any similar rule or regulation),
each Warrant Certificate and any certificates evidencing Warrant Shares shall
bear a legend substantially in the form of the legend set forth on the first
page hereof, unless in the opinion of counsel reasonably satisfactory to the
Company, such legend is no longer required by the Securities Act.

         6. RESERVATION OF SHARES. The Company hereby agrees that at all times
it shall reserve for issuance and delivery upon exercise of this Warrant such
number of its authorized but unissued shares of Common Stock as will be
sufficient to permit the exercise in full of this Warrant. All such shares shall
be duly authorized and, when issued upon such exercise, shall be validly issued,
fully paid and non-assessable, free and clear of all liens, security interests,
charges and other encumbrances or restrictions on sale and free and clear of all
preemptive rights.

         7. FRACTIONAL SHARES. No fractional shares or scrip representing
fractional shares shall be issued upon the exercise of this Warrant and in lieu
of delivery of any such fractional share upon any exercise hereof, the Company
shall pay to the Holder an amount in cash equal to such fraction multiplied by
the Current Market Price Per Common Share on the Exercise Date.

         8. EXCHANGE, TRANSFER OR ASSIGNMENT OF WARRANT.

                  A. The Company shall from time to time register the exchange
or transfer of any outstanding Warrant Certificates in a Warrant register to be
maintained by the Company upon surrender thereof, accompanied by a written
instrument or instruments of transfer in form satisfactory to the Company, duly
executed by the registered Holder or Holders thereof or by the duly appointed
legal representative thereof or by a duly authorized attorney. Each taker and
holder of this Warrant Certificate by taking or holding the same, consents and
agrees that the registered holder hereof may be treated by the Company and all
other Persons dealing with this Warrant Certificate as the absolute owner hereof
for any purpose and as the Person entitled to exercise the rights represented
hereby.

                  B. Prior to any proposed transfer of the Warrants or the
Warrant Shares, unless such transfer is made pursuant to an effective
registration statement under the Securities Act, the Holder will deliver to the
Company, if so requested by the Company, an opinion of counsel reasonably
satisfactory in form and substance to the Company, to the effect that the
Warrants or Warrant Shares, as applicable, may be sold or otherwise transferred
without registration under the Securities Act. Subject to the preceding
sentence, the Holder of this Warrant shall be entitled, without obtaining the
consent of the Company, to assign and transfer this Warrant, at any time in
whole or from time to time in part, to any Person or Persons. Subject to the
foregoing, upon surrender of this Warrant to the Company, together with the
attached Warrant Assignment Form duly executed, the Company shall, without
charge, execute and deliver a new Warrant in the name of the assignee or
assignees named in such instrument of

                                       5

<PAGE>

assignment and, if the Holder's entire interest is not being assigned, in the
name of the Holder, and this Warrant shall promptly be cancelled.

         9. LOSS OR DESTRUCTION OF WARRANT CERTIFICATE. Upon receipt by the
Company of evidence satisfactory to it (in the exercise of is reasonable
discretion) of the loss, theft, destruction or mutilation of this Warrant
Certificate, and (if requested by the Company in the case of loss, theft or
destruction) of reasonably satisfactory indemnification, and upon surrender and
cancellation of this Warrant Certificate, if mutilated, the Company shall
execute and deliver a new Warrant Certificate of like tenor and date
representing the right to purchase an equivalent number of Warrant Shares.

         10. ANTI-DILUTION PROVISIONS.

                  A. In case the Company shall at any time after the date hereof
(i) declare a dividend or make a distribution on Common Stock payable in Common
Stock or other shares of the Company's capital stock, (ii) subdivide, split or
reclassify the outstanding Common Stock into a larger number of shares, (iii)
combine or reclassify the outstanding Common Stock into a smaller number of
shares, or (iv) issue any shares of its capital stock in a reclassification of
Common Stock (including any such reclassification in connection with a
consolidation or merger in which the Company is the continuing corporation),
then in each such case the Warrant Share Amount shall be adjusted to equal the
number of shares to which the holder of this Warrant would have relied upon the
occurrence of such event if this Warrant had been exercised immediately prior to
such time. Such adjustment shall be made successively whenever any event listed
above shall occur.

                  B. In case the Company shall fix a record date for the making
of a distribution to holders of Common Stock (including any such distribution
made in connection with a consolidation or merger in which the Company is the
continuing corporation) of evidences of indebtedness, assets or other property
(excluding cash dividends, other cash distributions from current or retained
earnings or dividends payable in Common Stock for which an adjustment has been
made pursuant to Section 10.A), the Warrant Share Amount to be in effect after
such record date shall be determined by multiplying the Warrant Share Amount in
effect immediately prior to such record date by a fraction, the numerator of
which shall be the Current Market Price Per Common Share, and the denominator of
which shall be such Current Market Price Per Common Share on such record date,
less the fair market value (determined by the Board of Directors of the Company;
provided that if the Holder shall object to any such determination, the Board of
Directors shall retain an independent appraiser reasonably satisfactory to the
Holder to determine such fair marker value) of the portion of the assets, other
property or evidence of indebtedness so to be distributed which is applicable to
one share of Common Stock. Such adjustments shall be made successively whenever
such a record date is fixed; and in the event that such distribution is not so
made, the Warrant Share Amount shall again be adjusted to be the Warrant Share
Amount which would then be in effect if such record date had not been fixed.

                  C. If as a result of any event or for any other reason, any
adjustment is made which increases the number of shares of Common Stock issuable
upon conversion, exercise or exchange of, or in the conversion or exercise price
or exchange ratio applicable to, any outstanding securities of the Company that
are convertible into, or exercisable or

                                       6

<PAGE>

exchangeable for, Common Stock of the Company, including, without limitation,
any action taken in connection with the warrants dated as of March 31, 1998
issued in connection with the offering by the Company and AMSC Acquisition
Company, Inc. ("AMSC Acquisition") of units consisting of 12 1/4% Senior Notes
due 2008 of AMSC Acquisition and warrants to purchase Common Stock, then a
corresponding adjustment shall be made hereunder to increase the Warrant Share
Amount, but only to the extent that no such adjustment has been made pursuant to
Sections 10.A or B hereof with respect to such event or for such other reason.

                  D. For the purpose of any computation under Section 10.B
hereof, on any determination date the "Current Market Price Per Common Share"
shall be deemed to be the average (weighted by daily trading volume) of the
Closing Prices (as defined below) per share of Common Stock for the 20
consecutive trading days immediately prior to such date. "Closing Price" means
(1) if shares of Common Stock then are listed and traded on the New York Stock
Exchange, Inc. ("NYSE"), the closing price on such day as reported on the NYSE
Composite Transactions Tape; (2) if shares of Common Stock then are not listed
and traded on the NYSE, the closing price on such day as reported by the
principal national securities exchange on which the shares are listed and
traded; (3) if shares of Common Stock then are not listed and traded on any such
securities exchange, the last reported sale price on such day on the National
Market of the National Association of Securities Dealers, Inc. Automated
Quotation System ("NASDAQ"); or (4) if shares of Common Stock then are not
traded on the NASDAQ National Market, the average of the highest reported bid
and lowest reported asked price on such day as reported by NASDAQ. If on any
determination date shares of Common Stock are not quoted by any such
organization, the Current Market Price Per Common Share shall be the fair market
value of such shares on such determination date as reasonably determined by the
Board of Directors. If the Holder shall object to any determination by the Board
of Directors of the Current Market Price Per Common Share, the Current Market
Price Per Common Share shall be the fair market value per share of Common Stock
as determined by an independent appraiser retained by the Company at its expense
and reasonably acceptable to the Holder. For purposes of any computation under
this Section 10, the number of shares of Common Stock outstanding at any given
time shall not include shares owned or held by or for the account of the Company

                  E. Upon each adjustment of the Warrant Share Amount pursuant
to this Section 10, the Exercise Price Applicable to each Warrant outstanding
prior to the making of the adjustment in the Warrant Share Amount shall
thereafter be adjusted to reflect an adjusted Exercise Price (calculated to the
nearest tenth of a cent) obtained from the following formula:

                              E(1) + E x    W
                                           ----           
                                           W(1)

Where:

                               E(1)  =  the adjusted Exercise Price
                                        per share following the
                                        adjustment of Warrant Share
                                        Amount.

                               E     =  the Exercise Price prior to adjustment.


                                       7

<PAGE>

                               W(1)  =  the adjusted Warrant Share Amount.

                               W     =  the Warrant Share Amount prior to 
                                        adjustment

                  F. No adjustment in the Warrant Share Amount or the Exercise
Price shall be required unless such adjustment would require an increase or
decrease of at least one percent of such amount; provided that any adjustments
which by reason of this Section 10.F are not required to be made shall be
carried forward and taken into account in any subsequent adjustment. All
calculations under this Section 10 shall be made to the nearest one tenth of a
cent of a share, as the case may be.

                  G. In the event that, at any time as a result of the
provisions of this Section 10, the holder of this Warrant upon subsequent
exercise shall become entitled to receive any shares of capital stock of the
Company other than Common Stock, the number of such other shares so receivable
upon exercise of this Warrant shall thereafter be subject to adjustment from
time to time in a manner and on terms as nearly equivalent as practicable to the
provisions contained herein.

                  H. Upon any adjustment pursuant to this Section 10, the
Company shall promptly thereafter (i) cause to be filed with the Company a
certificate of an officer of the Company serving forth the Warrant Share Amount
and Exercise Price after such adjustment and setting forth in reasonable detail
the method of calculation and the facts upon which such calculations are based,
and (ii) cause to be given to each registered Holder of this Warrant Certificate
at the address as set forth in Section 12 written notice of such adjustments.
Where appropriate, such notice may be given in advance and included as a part of
the notice required to be delivered pursuant to Section 13.B.

         11. REORGANIZATION, CONSOLIDATION, MERGER, OR SALE OF ASSETS. In case
of any reclassification, redesignation, reorganization or recapitalization by
the Company (other than as set forth in Section 10) or consolidation of the
Company with, or merger of the Company into, any other Person, any merger of
another Person into the Company (other than a merger which does nor result in
any reclassification, conversion, exchange or cancellation of outstanding shares
of Common Stock) or any sale or transfer of all or substantially all of the
assets of the Company or of the Person formed by such consolidation or resulting
from such merger or which acquires such assets, as the case may be, the Holder
shall have the right thereafter to exercise this Warrant for the kind and amount
of securities, cash and other property receivable upon such reclassification.
redesignation, reorganization, recapitalization, consolidation, merger, sale or
transfer by a holder of the number of shares of Common Stock for which this
Warrant may have been exercised in full immediately prior to such
reclassification, redesignation, reorganization, recapitalization,
consolidation, merger, sale or transfer, assuming (i) such holder of Common
Stock is not a Person with which the Company consolidated or into which the
Company merged or which merged into the Company or to which such sale or
transfer was made, as the case may be ("constituent Person"), or an Affiliate of
a constituent Person and (ii) in the case of a consolidation, merger, sale or
transfer which includes an election as to the consideration to be received by
the holders, such holder of Common Stock

                                       8

<PAGE>

failed to exercise its rights of election, as to the kind or amount of
securities, cash and other property receivable upon such consolidation, merger,
sale or transfer (provided that if the kind or amount of securities, cash and
other property receivable upon such consolidation, merger, sale or transfer is
not the same for each share of Common Stock held immediately prior to such
consolidation, merger, sale or transfer by other than a constituent Person or an
Affiliate thereof and in respect of which such rights of election shall not have
been exercised ("non-electing share"), then for the purpose of this Section 11
the kind and amount of securities, cash and other property receivable upon such
consolidation, merger, sale or transfer by each non-electing share shall be
deemed to be the kind and amount so receivable per share by a plurality of the
non-electing shares). Adjustments for events subsequent to the effective date of
such reclassification, redesignation, reorganization, recapitalization,
consolidation, merger and sale of assets shall be as nearly equivalent as may be
practicable to the adjustments provided for in this Warrant. In any such event,
effective provisions shall be made in the certificate or articles of
incorporation of the resulting or surviving corporation, in any contract of
sale, conveyance, lease or transfer, or otherwise so that the provisions set
forth herein for the protection of the rights of the Holder shall thereafter
continue to be applicable; and any such resulting or surviving corporation shall
expressly assume the obligation to deliver, upon exercise, such shares of stock,
other securities, cash and property. The provisions of this Section 11 shall
similarly apply to successive consolidations; mergers, sales, leases or
transfers.

         12. NOTICES. Any notice, demand or delivery authorized or required by
this Warrant Certificate shall be in writing and shall be given to the Holder or
the Company, as the case may be, at its address (or facsimile number) set forth
below, or such other address (or facsimile number) as shall have been furnished
to the party giving or making such notice, demand or delivery:

         If to the Company:   American Mobile Satellite Corporation
                              10802 Parkridge Blvd.
                              Reston, VA 22091
                              Facsimile: (703) 758-6134
                              Attention: Randy Segal, General Counsel

         If to the Holder:    Singapore Telecommunications Ltd.
                              31, Exeter Road, Comcenter, #22-00
                              Singapore, 239732
                              Republic of Singapore
                              Facsimile No. (65) 732-0673/(65) 734-8119
                              Attention: Mr. Ho Siaw Hong

         Each such notice, demand or delivery shall be effective (i) if given by
facsimile, when such facsimile is transmitted to the facsimile number specified
herein and the intended recipient confirms the receipt of such facsimile or (ii)
if given by any other means, when received at the address specified herein.

                                       9

<PAGE>

         13. NOTICES TO WARRANT HOLDERS.

                  A. The Company shall provide to each Holder, at its address
and in the manner set forth in Section 12, a notice of expiration of this
Warrant not less than 90 nor more than 120 days prior to the Expiration Date.

                  B. In the event:

                  (a) the Company shall authorize the issuance to holders of
         shares of Common Stock of rights, options or warrants to subscribe for
         or purchase shares of Common Stock or of any other subscription rights
         or warrants; or

                  (b) the Company shall authorize the distribution to holders of
         shares of Common Stock of assets, including cash, evidences of its
         indebtedness, or other securities; or

                  (c) of any reorganization, consolidation or merger to which
         the Company is a party and for which approval of any shareholders of
         the Company is required, or of the conveyance or transfer of the
         properties and assets of the Company substantially as an entirety, or
         of any reclassification or change of Common Stock issuable upon
         exercise of the Warrants, or a tender offer or exchange offer for
         shares of Common Stock; or

                  (d) of the voluntary or involuntary dissolution, liquidation
         or winding up of the Company; or

                  (e) the Company proposes to take any action that would require
         an adjustment to the Warrant Share Amount or the Exercise Price
         pursuant to Section 10 hereof;

then the Company shall cause to be given to each registered Holder of this
Warrant Certificate, at least 20 days prior to the applicable record date
hereinafter specified, or 20 days prior to the date of the event in the case of
events for which there is no record date a written notice stating (i) the date
as of which the holders of record of shares of Common Stock entitled to receive
any such rights, options, warrants or distribution are to be determined, or (ii)
the initial expiration date set forth in any tender offer or exchange offer for
shares of Common Stock or (iii) the date on which any such reorganization,
reclassification, consolidation, merger, conveyance, transfer, dissolution,
liquidation or winding up is expected to become effective or consummated, and
the date as of which it is expected that holders of record of shares of Common
Stock shall be entitled to exchange such shares for securities or other
property, if any, deliverable upon such reorganization, reclassification,
consolidation, merger, conveyance, transfer, dissolution, liquidation or winding
up. The failure to give the notice required by this Section 13.B or any defect
therein shall not affect the legality or validity of any distribution, right,
option, warrant, consolidation, merger, conveyance, transfer, dissolution,
liquidation or winding up, or the vote upon any action.

                                       10

<PAGE>

         14. RIGHTS OF THE HOLDER. Prior to the exercise of any Warrant, the
Holder shall not, by virtue hereof, be entitled to any rights of a stockholder
of the Company, including, without limitation, the right to vote, to receive
dividends or other distributions, to exercise any preemptive right or to receive
any notice of meetings of stockholders or any notice of any proceedings of the
Company except as may be specifically provided for herein. Nothing contained
herein shall impose any obligation on the Holder to purchase any securities or
impose any liabilities on such Holder as a stockholder of the Company, whether
such obligation or liabilities are asserted by the Company or by creditors of
the Company.

         15. REGISTRATION RIGHTS. The Holder of this Warrant is entitled to
certain registration rights with respect to the Warrant Shares issuable upon the
exercise thereof. Said registration rights are set forth in an Amended and
Restated Registration Rights Agreement dated as of March 31, 1998, by and among
the Company and certain holders of warrants of the Company named therein (the
"Registration Rights Agreement"). By acceptance of this Warrant Certificate, the
Holder hereof agrees that upon exercise of this Warrant, in whole or in part,
such Holder will be bound by the Registration Rights Agreement as a holder of
Registrable Securities thereunder. The Company agrees that upon transfer of this
Warrant, in whole or in part, pursuant to Section 8 hereof, the transferee shall
be entitled to become a party to the Registration Rights Agreement if not
already a party thereto. A copy of the Registration Rights Agreement may be
obtained by the Holder hereof upon written request to the Company.

         16. GOVERNING LAW AND WAIVER OF JURY TRIAL. THIS WARRANT CERTIFICATE
AND ALL RIGHTS ARISING HEREUNDER SHALL BE CONSTRUED AND DETERMINED IN ACCORDANCE
WITH THE INTERNAL LAWS OF THE STATE OF NEW YORK, AND THE PERFORMANCE THEREOF
SHALL BE GOVERNED AND ENFORCED IN ACCORDANCE WITH SUCH LAWS. THE PARTIES HERETO
IRREVOCABLY WAIVE ANY RIGHT TO A JURY TRIAL WITH RESPECT TO ANY ACTION OR
PROCEEDING ARISING OUT OF OR RELATING TO THIS AGREEMENT.

         17. AMENDMENTS; WAIVERS. Any provision of this Warrant Certificate may
be amended or waived if, and only if, such amendment or waiver is in writing and
signed, in the case of an amendment, by the Holder and the Company, or in the
case of a waiver thereof, by the party against whom the waiver is to be
effective. No failure or delay by either party in exercising any right, power or
privilege hereunder shall operate as a waiver nor shall any single or partial
exercise thereof preclude any other or further exercise thereof or the exercise
of any other right, power or privilege. The rights and remedies herein provided
shall be cumulative and not exclusive of any rights or remedies provided by law.

         18. COUNTERPARTS. This Warrant Certificate may be executed in any
number of counterparts, each of which shall be deemed to be an original and all
of which together shall constitute one and the same instrument.

                            (SIGNATURE PAGE FOLLOWS)

                                       11

<PAGE>

         IN WITNESS WHEREOF, the Company has duly caused this Warrant
Certificate to be signed by its duly authorized officer and to be dated as of
March __, 1998.

                                         AMERICAN MOBILE SATELLITE
                                         CORPORATION


                                         By: /s/ Gary M. Parsons
                                         -----------------------
                                         Name:  
                                         Title:


Acknowledged and Agreed:

SINGAPORE TELECOMMUNICATIONS LTD.


By: /s/ Yap Chee Keong
- ----------------------
Name:  Yap Chee Keong
Title: Group Financial Controller

<PAGE>

                             WARRANT EXERCISE NOTICE

                (To be delivered prior to exercise of the Warrant
             by execution of the Warrant Exercise Subscription Form)


To: American Mobile Satellite Corporation
    10802 Parkridge Blvd.
    Reston, VA 22091

         The undersigned hereby notifies you of its intention to exercise the
Warrant to purchase shares of Common Stock, par value $.01 per share, of
American Mobile Satellite Corporation. The undersigned intends to exercise the
Warrant to purchase ______ shares (the "Shares") [at $__ per Share (the
"Exercise Price")] [pursuant to the net exercise provisions of Section 2.B of
the Warrant]. [The undersigned intends to pay the aggregate Exercise Price for
the Shares in cash, certified or official bank or bank cashier's check or by
wire transfer of immediately available funds to an account to designated by the
Company or by cancellation of indebtedness owed to the Holder (or a combination
of such methods) as indicated below.]

         The undersigned hereby certifies that to the best of its knowledge its
Alien Ownership Percentage as of the date is _______________.


Date:_______________



                                      --------------------------------------
                                      (Signature of Owner)

                                      --------------------------------------
                                      (Street Address)

                                      --------------------------------------
                                      (City)         (State)      (Zip Code)

Payment: $____________cash

         $____________check

         $____________wire transfer

         $____________cancellation of indebtedness


[Wire Transfer Instructions, if required pursuant to Section 3 or 4 of the
Warrant:____________________________________________________________________]

                                       13

<PAGE>


                      WARRANT EXERCISE SUBSCRIPTION NOTICE

               (To be executed only upon exercise of the Warrant)


To: American Mobile Satellite Corporation
    10802 Parkridge Blvd.
    Reston, VA 22091

         The undersigned irrevocably exercises the Warrant for the purchase of
shares (the "Shares") of Common Stock, par value $.01 per share, of American
Mobile Satellite Corporation (the "Company") at $______ per Share (the "Exercise
Price") and herewith makes payment of $ (such payment being made in cash or by
certified or official bank or bank cashier's check payable to the order of the
Company or by wire transfer or by cancellation of indebtedness owed to the
Holder or any combination of such methods) (unless the undersigned Holder is
exercising the Warrant pursuant to the net exercise provisions of Section 2.B of
the Warrant), all on the terms and conditions specified in the within Warrant
Certificate, surrenders this Warrant Certificate and all right, title and
interest therein to the Company and directs that the Shares deliverable upon the
exercise of this Warrant be registered or placed in the name and at the address
specified below and delivered thereto. If said number of Shares is less than all
of the shares of Common Stock for which the Warrant is exercisable, the
undersigned requests that a new Warrant Certificate representing the remaining
balance of such shares be registered in the name of the undersigned or nominee
hereinafter set forth, and further that such certificate be delivered to the
undersigned at the address hereinafter set forth or to such other person or
entity as hereinafter set forth.

Date:____________________


                                      --------------------------------------
                                      (Signature of Owner)

                                      --------------------------------------
                                      (Street Address)

                                      --------------------------------------
                                      (City)         (State)      (Zip Code)

                                       14

<PAGE>

Securities and/or check to be issued to:

Please insert social security or identifying number:_________________________

Name:________________________________________________________________________

Street Address:______________________________________________________________

City, State and Zip Code:____________________________________________________

Any unexercised portion of the Warrant evidenced by the within Warrant
Certificate to be issued to:

Please insert social security or identifying number:_________________________

Name:________________________________________________________________________

Street Address:______________________________________________________________

City, State and Zip Code:____________________________________________________

                                       15

<PAGE>

                             WARRANT ASSIGNMENT FORM
                             -----------------------

                                                                Dated:__________

         FOR VALUE RECEIVED, ________________________________________________

hereby sells, assigns and transfers unto ____________________________________

(the "Assignee"), (please type or print in block letters)


- -----------------------------------------------------------------------------
                           (insert Assignee's address)


- -----------------------------------------------------------------------------
           (insert Assignee's social security and taxpayer ID number)

its right to purchase up to ____ shares of Common Stock represented by this
Warrant and does hereby irrevocably constitute and appoint Attorney, to transfer
the same on the books of the Company, with full power of substitution in the
premises.


                                           ----------------------------------
                                           Signature


Signature Guarantee

                                       16



                                  EXHIBIT XVII
                              AMENDED AND RESTATED
                          REGISTRATION RIGHTS AGREEMENT
                          -----------------------------

         AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT dated as of March
31, 1998 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.
(collectively, the "Guarantors") and each other Person who executes this
Agreement.

                              W I T N E S S E T H:

         WHEREAS, the Company and the Guarantors are parties to a certain
Registration Rights Agreement (the "1996 Registration Rights Agreement") dated
as of June 28, 1996 entered into in connection with the Guarantee Issuance
Agreement dated as of June 28, 1996, as previously amended, and the warrants to
purchase common stock, par value $0.01 per share, of the Company issued in
connection therewith; and

         WHEREAS, in order to induce the Guarantors to issue new Guaranties in
connection with the restructuring of the Company's existing indebtedness, the
Company has agreed, pursuant to a new Guaranty Issuance Agreement dated as of
March 31, 1998 (the "1998 Guaranty Issuance Agreement"), to amend and restate
the 1996 Registration Rights Agreement to (i) extend the expiration date for
demand registration rights with respect to the existing warrants, (ii) provide
registration rights for the warrants to be issued pursuant to the 1998 Guaranty
Issuance Agreement, and (iii) provide registration rights for other restricted
securities held by the Guarantors;

         NOW, THEREFORE, the parties hereto agree that the 1996 Registration
Rights Agreement is hereby amended and restated to read in full as follows:


                                    ARTICLE I

                                   DEFINITIONS

         SECTION 1.1. Definitions. The following terms, as used herein, have the
following meanings:

         "Affiliate", as applied to any specified Person, shall mean any other
Person directly or indirectly controlling or controlled by or under direct or
indirect common control with such specified Person. For the purposes of this
definition, "control", when used with respect to any Person, means the power to
direct the management and policies of such Person, directly or indirectly,
whether through the ownership of voting securities, by contract or otherwise;
and the terms "controlling" and "controlled" have meanings correlative to the
foregoing.

<PAGE>

         "Board of Directors" means the Board of Directors of the Company.

         "Bridge Shares" means the shares of Common Stock issued or issuable
upon exercise of the Bridge Warrants in accordance with the terms thereof and
any Common Stock issued as or issuable upon the conversion or exercise or any
warrant, option, fight, or other security which is issued as a dividend or other
distribution with respect to or in exchange for or in replacement of the shares
of Common Stock issued or issuable upon exercise of the Bridge Warrants.

         "Bridge Registration Rights Agreement" means the registration rights
agreement dated as of April 19, 1996 among the Company, Toronto Dominion
Investments, Inc., Morgan Guaranty Trust Company of New York and Hughes
Communications Satellite Services, Inc. with respect to the registration of the
Bridge Shares.

         "Bridge Warrants" means the warrants to purchase Common Stock
originally issued by the Company to Toronto Dominion Investments, Inc., Morgan
Guaranty Trust Company of New York and .Hughes Communications Satellite
Services, Inc. on January 19, 1996.

         "Commission" means the Securities and Exchange Commission, or any
successor agency.

         "Common Stock" means the common stock, par value $.01 per share, of the
Company.

         "Deferral Period" has the meaning set forth in Section 2.1.

         "Demand Registration" has the meaning set forth in Section 2.1.

         "Demand Registration Notice" has the meaning set forth in Section 2.1.

         "Demanding Group" has the meaning set forth in Section 2.1.

         "Exchange Act" means the Securities Exchange Act of 1934, as amended,
and the rules and regulations promulgated thereunder.

         "Holder" means the holder of any Registrable Securities.

         "Motorola Registration Rights Agreement" means the Registration Rights
Agreement dated as of March 31, 1998 between the Company and Motorola, Inc.

         "NASD" means the National Association of Securities Dealers, Inc.

         "Participation Rights Agreement" means the Participation Rights
Agreement dated as of December 31, 1997 among the Company, Motorola, Inc.
("Motorola"), Hughes, Singapore Telecom and AT&T Wireless Services, Inc.

                                       2

<PAGE>

         "Person" means an individual, corporation, partnership, limited
liability company, association, trust, or any other entity or organization,
including a government or political subdivision or an agency or instrumentality
thereof.

         "Piggy-Back Registration" has the meaning set forth in Section 2.2.

         "Registrable Securities" means the Warrant Shares and any other shares
of Common Stock beneficially owned by the Guarantors that constitute "restricted
securities" as such term is defined in Rule 144 under the Securities Act until
(i) a Registration Statement covering such Warrant Shares or other shares has
been declared effective by the Commission and they have been disposed of
pursuant to such effective Registration Statement, (ii) such Warrant Shares or
other shares are sold under circumstances in which all of the applicable
conditions of Rule 144 (or any similar provisions then in force) under the
Securities Act are met or under which they may be sold pursuant to Rule 144(k)
or (iii) the Company has delivered a new certificate or other evidence of
ownership for them not bearing the legend required pursuant to the Warrants or
the agreement pursuant to which they were issued and they may be freely resold
at one time without subsequent registration under the Securities Act.

         "Registration Statement" means any registration statement of the
Company relating to a Demand Registration pursuant to Section 2.1 or a
Piggy-Back Registration pursuant to Section 2.2, in each case, including the
prospectus included therein, all amendments and supplements thereto (including
post-effective amendments) and all exhibits and material incorporated by
reference therein.

         "Securities Act" means the Securities Act of 1933, as amended, and the
rules and regulations promulgated thereunder.

         "Selling Holder" means a Holder who is selling Registrable Securities
pursuant to a Registration Statement under the Securities Act.

         "Subordination Termination Date" has the meaning set forth in Section 4
of the Participation Rights Agreement.

         "Underwriter" means a securities dealer who purchases any Registrable
Securities as principal and not as part of such dealer's market-making
activities.

         "Unit Offering" means the offering of 335,000 Units, each consisting of
$1,000 principal amount of 12 1/4% Senior Notes of AMSC Acquisition Company,
Inc. and one warrant to purchase 3.75749 shares of Common Stock, pursuant to the
Offering Memorandum dated March 31, 1998.

         "Unit Warrants Registration Rights Agreement" means the Registration
Rights Agreement dated as of March 31, 1998 among the Company, Bear Stearns &
Co. Inc., J.P. Morgan Securities, Inc., TD Securities (USA) Inc. and Banc
America Robertson Stephens entered into in connection with the Unit Offering.

                                       3

<PAGE>

         "Warrants" means the warrants dated June 28, 1996, as amended through
the date hereof and as may be further amended from time to time, and the
warrants dated March 31, 1998, as the same may be amended from time to time, to
purchase Common Stock issued to the Guarantors.

         "Warrant Shares" means the shares of Common Stock issued or issuable
upon exercise of the Warrants, in each case in accordance with the terms
thereof, and any Common Stock or other securities issued or issuable upon the
exercise of any warrant, option, fight, or other security which is issued as a
dividend or other distribution with respect to or in exchange for or in
replacement of the shares of Common Stock issued or issuable upon exercise of
the Warrants.


                                   ARTICLE II

                               REGISTRATION RIGHTS


         SECTION 2.1. Demand Registration.

         (a) Right to Demand. At any time and from time to time prior to March
31, 2005, Holders of Registrable Securities representing at least 25% of the
aggregate number of Registrable Securities as a group (each, a "Demanding
Group") may make a written request of the Company for registration with the
Commission, under and in accordance with the provisions of the Securities Act,
of all or part of their Registrable Securities (a "Demand Registration"). Within
5 days after receipt of the request for a Demand Registration, the Company will
send written notice (the "Demand Registration Notice") of such registration
request and its intention to comply therewith to each Holder and, subject to
paragraph (c) below, the Company will include in such registration all
Registrable Securities of such Holders with respect to which the Company has
received written requests for inclusion therein within 20 days after the
Holder's receipt of the Demand Registration Notice and such Holders will be
deemed to be members of the Demanding Group. All requests made pursuant to this
paragraph (a) will specify the aggregate number of Registrable Securities
requested to be registered.

         Promptly after receipt of any request for registration under this
paragraph (a), but in no event later than 60 days after receipt of such request,
the Company shall file a Registration Statement with the Commission with respect
to the Registrable Securities included in such request and shall use its best
efforts to have such Registration Statement declared effective as promptly as
practicable; provided however, that the Company may postpone the filing of such
Registration Statement for a period of up to 90 days (the "Deferral Period") if
(x) the Board of Directors reasonably determines that (i) such a filing would
adversely affect any proposed financing, acquisition, divestiture or other
material transaction by the Company or (ii) such a filing would otherwise
represent an undue hardship for the Company, and (y) such determination is
reflected in a certificate signed by the Chief Executive Officer or President of
the Company. The Company shall not be entitled to request more than one such
deferral with respect to any Demand Registration within any 365-day period. If
the Company does elect to defer any such Demand Registration, the Holders
requesting such Demand

                                       4

<PAGE>

Registration may, at their election by written notice to the Company, (i)
confirm their request to proceed with such Demand Registration upon the
expiration of the Deferral Period or (ii) withdraw their request for such Demand
Registration in which case no such request for a Demand Registration shall be
deemed to have occurred for purposes of Section 2.1 (b) or for any other
purposes under this Agreement (and if such Deferral Period extends past March
31, 2005, the Holders shall nevertheless be entitled to make subsequent requests
for Demand Registration hereunder).

         (b) Number of Demand Registrations. The Demanding Group(s) shall
collectively be entitled to two Demand Registrations hereunder. A Demand
Registration shall not be counted as a Demand Registration hereunder (i) until
such Demand Registration has been declared effective by the Commission and
maintained continuously effective for a period of at least 120 days or such
shorter period as will terminate when all Registrable Securities included
therein have been sold in accordance with such Demand Registration and (ii)
unless the number of Registrable Securities in such Demand Registration by the
Demand Group is at least 80% of the number of shares originally requested to be
included by such group after giving effect to any reductions pursuant to
paragraph (c) below.

         (c) Priority on Demand Registrations. If in any Demand Registration the
managing Underwriter or Underwriters thereof advise the Company in writing that
in its or their reasonable opinion or, in the case of a Demand Registration not
being underwritten, the Company shall reasonably determine after consultation
with an investment banking firm of nationally recognized standing, that the
number of Registrable Securities proposed to be sold in such Demand Registration
exceeds the number that can be sold in such offering or will adversely affect
the success of such offering (including, without limitation, an impact on the
selling price or the number of Registrable Securities that any participant may
sell), the Company shall include in such registration only the number of
Registrable Securities, if any, which in the opinion of such Underwriter or
Underwriters, or the Company, as the case may be, can be sold without having an
adverse effect on the success of the offering and in accordance with the
following priority: (x) up to and including the Subordination Termination Date,
(i) first, pursuant to Section 2 of the Motorola Registration Rights Agreement,
securities requested to be included in such offering by Motorola, (ii) second,
subject to the priority rights of the holders of Bridge Shares pursuant to the
Bridge Registration Rights Agreement, Registrable Securities that are Warrant
Shares requested to be included in such offering by Holders in the Demanding
Group requesting such registration, allocated PRO RATA among such Demanding
Group (based upon the number of such Warrant Shares requested to be included in
such Demand Registration), (iii) third, other Registrable Securities requested
to be included in such offering by Holders in the Demanding Group requesting
such registration, allocated PRO RATA among such Demanding Group (based upon the
number of such other Registrable Securities requested to be included in such
Demand Registration), (iv) fourth, PRO RATA (based upon the number of
Registrable Securities or similar securities requested to be included in such
registration by such Holders and other Persons, if any) among the other Holders
of Registrable Securities and other Persons having similar rights who have
requested to include Registrable Securities or similar securities in such
registration pursuant to the piggy-back registration provisions

                                       5

<PAGE>

of Section 2.2 or other registration rights agreements other than the Motorola
Registration Rights Agreement and the Bridge Registration Rights Agreement, and
(v) fifth, securities proposed to be issued by the Company for its own account;
and (y) after the Subordination Termination Date, (i) first, Registrable
Securities requested to be included in such offering by Holders in the Demanding
Group requesting such registration and securities requested to be included in
such offering by Motorola pursuant to Section 2 of the Motorola Registration
Rights Agreement, allocated pro rata among Motorola and, subject to the priority
rights of the holders of Bridge Shares pursuant to the Bridge Registration
Rights Agreement, the members of such Demanding Group based upon the number of
securities requested to be included in such offering (provided that Registrable
Securities that are Warrant Shares shall have priority over other Registrable
Securities in the shares included at the request of the Demanding Group), (ii)
second, PRO RATA (based upon the number of Registrable Securities or similar
securities requested to be included in such registration by such Holders and
other Persons, if any) among the other Holders of Registrable Securities and
other persons having similar rights who have requested to include Registrable
Securities or similar securities in such registration pursuant to the piggy-back
registration provisions of Section 2.2 or other registration rights agreements
other than the Motorola Registration Rights Agreement and the Bridge
Registration Rights Agreement, and (iii) third, securities proposed to be issued
by the Company for its own account.

         (d) Selection of Underwriters. If any Demand Registration is to be in
the form of an underwritten offering, the managing Underwriter or Underwriters
that will administer the offering shall be selected by the holders of a majority
of the Registrable Securities to be included in such offering; provided that
such managing underwriter or underwriters must be of recognized national
standing and reasonably satisfactory to the Company. The Company shall (together
with all Holders of Registrable Securities proposing to distribute Registrable
Securities through such underwriting) enter into an underwriting agreement in
customary form with the Underwriter or Underwriters selected for such
underwriting in the manner set forth above.

         (e) Withdrawal. If any Holder of Registrable Securities disapproves of
the terms of any such underwriting, such Holder may elect to withdraw therefrom
by written notice to the Company and the managing Underwriter. If by the
withdrawal of such Registrable Securities a greater number of Registrable
Securities held by other Holders may be included in such registration (up to the
maximum of any limitation imposed by the Underwriters), then the Company shall
offer to all Holders who have requested inclusion of Registrable Securities in
the registration, the right to include additional Registrable Securities in the
priority and proportions specified in Section 2.1(c).

         SECTION 2.2. Piggy-Back Registration.

         (a) If the Company proposes to file a registration statement under the
Securities Act with respect to an offering by the Company for its own account or
for the account of any of its respective securityholders of any class of equity
security or security convertible into or exchangeable for any class of equity
security (other than a registration

                                       6

<PAGE>

statement on Form S-4 or S-8 (or any substitute form that may be adopted by the
Commission), or a registration filed in connection with an exchange offer or
offering of securities solely to the Company's existing securityholders or other
registrations solely in connection with employee stock options or other employee
benefit plans), then the Company shall give written notice of such proposed
filing to the Holders of Registrable Securities as soon as practicable (but in
no event less than 30 days before the anticipated filing date), and such notice
shall offer such Holders the opportunity to register such number of shares of
Registrable Securities as each such Holder may request ("Piggy-Back
Registration"). The Company shall use its best efforts to cause the managing
Underwriter or Underwriters of a proposed underwritten offering to permit the
Registrable Securities requested to be included in a Piggy-Back Registration to
be included on the same terms and conditions as any similar securities of the
Company included therein and to permit the sale or other disposition of such
Registrable Securities in accordance with the intended method of distribution
thereof.

         No registration effected under this Section 2.2, and no failure to
effect a registration under this Section 2.2, shall relieve the Company of its
obligations pursuant to Section 2.1, and no failure to effect a registration
under this Section 2.2 and complete the sale of shares in connection therewith
shall relieve the Company of any other obligation under this Agreement
(including, without limitation, the Company's obligations under Sections 3.2 and
4.1 ).

         (b) Notwithstanding anything contained herein, if the managing
Underwriter or Underwriters of an offering described in the foregoing paragraph
(a) deliver a written opinion to the Holders of the Registrable Securities
proposed to be included in such offering that (i) the size of the offering that
the Holders, the Company and any other Persons intend to make or (ii) the kind
of securities that the Holders, the Company and any other Persons intend to
include in such offering are such that the success of the offering would be
materially and adversely affected by inclusion of the Registrable Securities
requested to be included, then, subject to the priority fights of the Unit
Warrant Holders pursuant to and in accordance with the Unit Warrant Registration
Rights Agreement, Motorola pursuant to and in accordance with the terms of the
Participation Rights Agreement (through the Subordination Termination Date) and
the holders of Bridge Shares pursuant to and in accordance with the terms of the
Bridge Registration Rights Agreement, (A) if the size of the offering is the
basis of such Underwriter's opinion, such amount of securities to be offered for
the accounts of Holders and the amount of securities to be offered for the
account of the Company shall be reduced PRO RATA (based upon the number of
Registrable Securities or other securities proposed to be included in such
registration by the Holders and the Company) provided that the number of
Registrable Securities that are not Warrant Shares shall be reduced to zero
before the number of Warrant Shares included is reduced, and the amount of
securities to be offered for the account of any other Persons (other than the
Unit Warrant Holders, Motorola and the holders of Bridge Shares) shall be
reduced to zero; and (B) if the combination of securities to be offered is the
basis of such Underwriter's opinion, (x) the amount of securities to be offered
for the accounts of Holders and the amount of securities to be offered for the
account of the Company shall be reduced PRO RATA (based upon the number of
Registrable Securities or other securities proposed to be included in

                                       7

<PAGE>

such registration by the Holders and the Company) provided that the number of
Registrable Securities that are not Warrant Shares shall be reduced to zero
before the number of Warrant Shares included is reduced, and the amount of
securities to be offered for the account of such other Persons (other than the
Unit Warrant Holders, Motorola and the holders of Bridge Shares) shall be
reduced to zero to the extent necessary, in the judgment of managing
Underwriter, to substantially eliminate the adverse effect that inclusion of the
Registrable Securities requested to be included would have on such offering.
After the Subordination Termination Date, any reduction in the amount of
securities to be offered for the accounts of participating Holders and the
amount of securities to be offered for the account of Motorola shall be on a pro
rata basis.

         (c) The Holders of Registrable Securities included within such
PiggyBack Registration may withdraw all or any part of the Registrable
Securities from such Piggy-Back Registration at any time (before but not after
the effective date of such Registration Statement), by delivering written notice
of such withdrawal request to the Company.

         (d) If the Company shall determine for any reason (x) not to register
or (y) to delay a registration which includes Registrable Securities pursuant to
this Section 2.2, the Company may, at its election, give written notice of such
determination to the Holders of the Registrable Securities and, thereupon (i) in
the case of a determination not to register, shall be relieved of its obligation
to register any Registrable Securities in connection with such registration (but
not from its obligation to pay the Registration Expenses in connection
therewith), without prejudice, however, to the rights, if any, of any Holder or
Holders of Registrable Securities to request that such registration be effected
as a Demand Registration under Section 2.1, and (ii) in the case of a delay in
registering, shall be permitted to delay registering any Registrable Securities
for the same period as the delay in registering such other shares.


                                   ARTICLE III

                             REGISTRATION PROCEDURES

         SECTION 3.1. Filings; Information. Whenever Registrable Securities are
to be registered pursuant to Section 2.1 hereof, the Company will use its best
efforts to effect the registration and the sale of such Registrable Securities
in accordance with the intended method of disposition thereof as quickly as
practicable, and in connection with any such request:

         (a) The Company will as expeditiously as possible (and in any event
within the time period specified in Section 2.1 (a)) prepare and file with the
Commission a Registration Statement on any form for which the Company then
qualifies or which counsel for the Company and counsel for the Selling Holders
shall deem appropriate and which form shall be available for the sale of the
Registrable Securities to be registered thereunder in accordance with the
intended method of distribution thereof, and if the offering is an underwritten
offering, shall be reasonably satisfactory to the managing Underwriter or
Underwriters. The Company will use its best efforts to cause such filed

                                       8

<PAGE>

Registration Statement to become and remain continuously effective in accordance
with Section 2. l(b).

         (b) The Company will, prior to filing a Registration Statement or
prospectus or any amendment or supplement thereto, furnish to each Selling
Holder and each Underwriter, if any, of the Registrable Securities covered by
such Registration Statement copies of such Registration Statement as proposed to
be filed, and thereafter furnish to such Selling Holder and Underwriter, if any,
such number of copies of such Registration Statement, each amendment and
supplement thereto (in each case including all exhibits thereto and documents
incorporated by reference therein), the prospectus included in such Registration
Statement (including each preliminary prospectus) and such other documents as
such Selling Holder or Underwriter may reasonably request in order to facilitate
the disposition of the Registrable Securities owned by such Selling Holder.

         (c) After the filing of the Registration Statement, the Company will
promptly notify each Selling Holder of Registrable Securities covered by such
Registration Statement of any stop order issued or threatened by the Commission
and take all reasonable actions required to prevent the entry of such stop order
or to remove it if entered.

         (d) The Company will use its best efforts to (i) register or qualify
the Registrable Securities under such other securities or blue sky laws of such
jurisdictions in the United States as any Selling Holder or managing Underwriter
reasonably (in light of such Selling Holder's intended plan of distribution)
requests and (ii) cause such Registrable Securities to be registered with or
approved by such other governmental agencies or authorities as may be necessary
by virtue of the business and operations of the Company and do any and all other
acts and things that may be reasonably necessary or advisable to enable such
Selling Holder and the Underwriters, if any, to consummate the disposition of
the Registrable Securities owned by such Selling Holder; provided that the
Company will not be required to (A) qualify generally to do business in any
jurisdiction where it would not otherwise be required to qualify but for this
paragraph (d), (B) subject itself to taxation in any such jurisdiction or (C)
consent to general service of process in any such jurisdiction.

         (e) The Company will immediately notify each Selling Holder, at any
time when a prospectus relating thereto is required to be delivered under the
Securities Act, of the occurrence of an event requiring the preparation of a
supplement or amendment to such prospectus so that, as thereafter delivered to
the purchasers of such Registrable Securities, such prospectus will not contain
an untrue statement of a material fact or omit to state any material fact
required to be stated therein or necessary to make the statements therein not
misleading, in light of the circumstances in which they were made, and promptly
file with the Commission and make available to each Selling Holder any such
supplement or amendment.

         (f) The Company will enter into customary agreements (including an
underwriting agreement in customary form if the offering is an underwritten
offering) and take such other actions as are reasonably required in order to
expedite or facilitate the

                                       9

<PAGE>

disposition of such Registrable Securities, including, in the case of an
offering pursuant to Section 2.1, cooperating in the marketing efforts of the
Underwriters and the Selling Holders by, among other things, making available,
as reasonably requested by the Underwriters and the Selling Holders, senior
executive officers of the Company for attendance at, and active participation
with the Underwriters in, informational meetings with prospective purchasers of
the Registrable Securities being offered, including meeting with groups of such
purchasers or with individual purchasers, providing information and answering
questions about the Company at such meetings, and traveling to locations at
reasonable times and as reasonably selected by the Underwriters.

         (g) The Company will make available for inspection by any Selling
Holder, any Underwriter participating in any disposition pursuant to such
Registration Statement and any attorney, accountant or other professional
retained by any such Selling Holder or Underwriter (collectively, the
"Inspectors"), all financial and other records, pertinent corporate documents
and properties of the Company (collectively, the "Records") as shall be
reasonably necessary to enable them to exercise their due diligence
responsibility, and cause the Company's officers, directors and employees to
supply all information reasonably requested by any Inspectors in connection with
such Registration Statement. Records which the Company determines, in good
faith, to be confidential and which it notifies the Inspectors are confidential
shall not be disclosed by the Inspectors unless (i) the disclosure of such
Records is necessary to avoid or correct a misstatement or omission in such
Registration Statement or (ii) the release of such Records is ordered pursuant
to a subpoena or other order from a court of competent jurisdiction. Each
Selling Holder of such Registrable Securities agrees that information obtained
by it as a result of such inspections shall be deemed confidential and shall not
be used by it as the basis for any market transactions in the securities of the
Company or its Affiliates unless and until such is made generally available to
the public. Each Selling Holder of such Registrable Securities further agrees
that it will, upon learning that disclosure of such Records is sought in a court
of competent jurisdiction, give notice to the Company and allow the company, at
its expense, to undertake appropriate action to prevent disclosure of the
Records deemed confidential.

         (h) The Company will furnish to each Selling Holder and to each
Underwriter, if any, a signed counterpart, addressed to such Selling Holder or
Underwriters of(i) an opinion or opinions of counsel to the Company and (ii) a
comfort letter or comfort letters from the Company's independent public
accountants, each in customary form and covering such matters of the type
customarily covered by opinions or comfort letters, as the cased may be, as the
holders of a majority of the Registrable Securities included in such offering or
the managing Underwriter therefor reasonably requests.

         (i) If requested by the Selling Holders, the Company will provide a
CUSIP number for all Registrable Securities not later than the effective date of
the Registration Statement covering such Registrable Securities and provide the
Company's transfer agent(s) and registrar(s) for the Registrable Securities with
printed certificates for the Registrable Securities.

                                       10

<PAGE>

         (j) The Company will cooperate and assist in any filings required to be
made with the NASD and in the performance of any due diligence investigation by
any Underwriter (including any "qualified independent underwriter") that is
required to be retained in accordance with the rules and regulations of the
NASD, and use its best efforts to cause such Registration Statement to become
effective and approved by such governmental agencies or authorities as may be
necessary to enable the Selling Holders or Underwriters, if any, to consummate
the disposition of such Registrable Securities.

         (k) The Company will otherwise use its best efforts to comply with all
applicable rules and regulations of the Commission, and make available to its
security holders, as soon as reasonably practicable, an earnings statement
covering a period of 12 months, beginning within three months after the
commencement of any public offering of securities pursuant to the Registration
Statement, which earnings statement shall satisfy the provisions of Section 11
(a) of the Securities Act.

         (1) The Company will use its best efforts to cause all such Registrable
Securities to be listed on each securities exchange on which similar securities
issued by the Company are then listed.

         The Company may require each Selling Holder to promptly furnish in
writing to the Company such information regarding the distribution of the
Registrable Securities as the Company may from time to time reasonably request
and such other information as may be legally required in connection with such
registration.

         Each Selling Holder agrees that, upon receipt of any notice from the
Company of the happening of any event of the kind described in Section 3.1 (e)
hereof, such Selling Holder will forthwith discontinue disposition of
Registrable Securities pursuant to the Registration Statement covering such
Registrable Securities until such Selling Holder's receipt of the copies of the
supplemented or amended prospectus contemplated by Section 3.1 (e) hereof, and,
if so directed by the Company, such Selling Holder will deliver to the Company
(at the Company's expense) all copies, other than permanent file copies then in
such Selling Holder's possession, of the most recent prospectus covering such
Registrable Securities at the time of receipt of such notice. In the event the
Company shall give such notice, the Company shall extend the period during which
such Registration Statement shall be maintained effective (including the period
referred to in Section 3.1 (a) hereof) by the number of days during the period
from and including the date of the giving of notice pursuant to Section 3.1 (c)
hereof to the date when the Company shall make available to the Selling Holders
a prospectus supplemented or amended to conform with the requirements of Section
3.1 (e) hereof.

         SECTION 3.2. Expenses. The Company shall pay the following expenses
incurred in connection with any registration required hereunder (the
"Registration Expenses"), regardless of whether a Registration Statement becomes
effective: (i) all registration and filing fees, (ii) fees and expenses of
compliance with securities or blue sky laws (including reasonable fees and
disbursements of counsel in connection with blue sky qualifications of the
Registrable Securities), (iii) printing and engraving expenses, (iv) internal
expenses of the Company (including, without limitation, all salaries and

                                       11

<PAGE>

expenses of its officers and employees performing legal or accounting duties),
(v) all fees and expenses incurred in connection with the listing of the
Registrable Securities, (vi) reasonable fees and disbursements of counsel for
the Company and customary fees and expenses for independent certified public
accountants retained by the Company (including the expenses of any comfort
letters or costs associated with the delivery by independent certified public
accountants of a comfort letter or comfort letters requested pursuant to Section
3.1 (h) hereof), (vii) the reasonable fees and expenses of any special experts
retained by the Company in connection with such registration, (viii) reasonable
fees and expenses of one counsel (who shall be reasonably acceptable to the
Company) for the Holders, (ix) in connection with any underwritten offering or
proposed underwritten offering of Registrable Securities hereunder, the
reasonable fees and disbursements of the Underwriters and counsel for the
Underwriters (excluding any underwriting discounts or commissions with respect
to Registrable Securities not being sold for the account of the Company), and
reasonable expenses in connection with the marketing efforts of the Underwriters
and the Selling Holders, including expenses related to meetings with prospective
purchasers of the Registrable Securities and any travel costs related thereto
and (xi) fees and expenses associated with any NASD filing required to be made
in connection with the registration of the Registrable Securities, including, if
applicable, the reasonable fees and expenses of any "qualified independent
underwriter" (and its counsel) that is required to be retained in accordance win
the rules and regulations of the NASD.


                                   ARTICLE IV

                        INDEMNIFICATION AND CONTRIBUTION

         SECTION 4.1. Indemnification by the Company. The Company agrees to
indemnify and hold harmless each Selling Holder, its officers, directors and
agents, and each Person, if any, who controls such Selling Holder within the
meaning of Section 15 of the Securities Act or Section 20 of the Exchange Act
from and against any and all losses, claims, damages and liabilities caused by
any untrue statement or alleged untrue statement of a material fact contained in
any Registration Statement or prospectus relating to the Registrable Securities
(as amended or supplemented if the Company shall have furnished any amendments
or supplements thereto) or any preliminary prospectus, or caused by any omission
or alleged omission to state therein a material fact required to be stated
therein or necessary to make the statements therein not misleading (in the case
of a prospectus, in light of the circumstances under which they were made),
except insofar as such losses, claims, damages or liabilities are caused by any
such untrue statement or omission or alleged untrue statement or omission based
upon information furnished in writing to the Company by such Selling Holder or
on such Selling Holder's behalf expressly for use therein; provided, however,
that the foregoing indemnity agreement with respect to any preliminary
prospectus shall not inure to the benefit of any Selling Holder from whom the
Person asserting any such loss, claim, damage or liability purchased the
Registrable Securities if it is determined that it was the responsibility of
such Selling Holder to provide such Person with a current copy of the prospectus
and such current copy of the prospectus would have cured the defect giving rise
to such loss,

                                       12

<PAGE>

claim, damage or liability. In connection with any underwritten offering, the
Company also agrees to indemnify the Underwriters of the Registrable Securities,
their officers and directors and each Person who controls such Underwriters on
substantially the same basis as that of the indemnification of the Selling
Holders provided in this Section 4.1.

         SECTION 4.2. Indemnification by Selling Holders. Each Selling Holder
agrees, severally but not jointly, to indemnify and hold harmless the Company,
its officers, directors and agents and each Person, if any, who controls the
Company within the meaning of either Section 15 of the Securities Act or Section
20 of the Exchange Act to the same extent as the foregoing indemnity from the
Company to such Selling Holder, but only with reference to information related
to such Selling Holder furnished in writing to the Company by such Selling
Holder or on such Selling Holder's behalf expressly for use in any Registration
Statement or prospectus relating to the Registrable Securities, or any amendment
or supplement thereto, or any preliminary prospectus. In connection with any
underwritten offering, each Selling Holder also agrees to indemnify and hold
harmless the Underwriters of the Registrable Securities, their officers and
directors and each Person who controls such Underwriters on substantially the
same basis as that of the indemnification of the Company provided in this
Section 4.2. Notwithstanding anything in this Agreement to the contrary, in no
event shall any Selling Holder be obligated to provide indemnification hereunder
in connection with any offering in an amount that exceeds the proceeds of such
offering received by such Selling Holder.

         SECTION 4.3. Conduct of Indemnification Proceedings. In case any
proceeding (including any governmental investigation) shall be instituted
involving any Person in respect of which indemnity may be sought pursuant to
Section 4.1 or 4.2, such Person (an "Indemnified Party") shall promptly notify
the Person against whom such indemnity may be sought (an "Indemnifying Party")
in writing and the Indemnifying Party shall assume the defense thereof,
including the employment of counsel reasonably satisfactory to such Indemnified
Party, and shall assume the payment of all fees and expenses. In any such
proceeding, any Indemnified Party shall have the fight to retain its own
counsel, but the fees and expenses of such counsel shall be at the expense of
such Indemnified Party unless (i) the Indemnifying Party and the Indemnified
Party shall have mutually agreed to the retention of such counsel or (ii) the
named parties to any such proceeding (including any impleaded parties) include
both the Indemnified Party and the Indemnifying Party and representation of both
parties by the same counsel would be inappropriate due to actual or potential
differing interests between them. It is understood that the Indemnifying Party
shall not, in connection with any proceeding or related proceedings in the same
jurisdiction, be liable for the reasonable fees and expenses of more than one
separate firm of attorneys (in addition to any local counsel) at any time for
all such Indemnified Parties, and that all such fees and expenses shall be
reimbursed as they are incurred. In the case of any such separate firm for the
Indemnified Parties, such firm shall be designated in writing by the Indemnified
Parties. The Indemnifying Party shall not be liable for any settlement of any
proceeding effected without its written consent, but if settled with such
consent, or if there be a final judgment for the plaintiff, the Indemnifying
Party shall indemnify and hold harmless such Indemnified Parties from and
against any loss or liability (to the extent stated above) by reason of such
settlement or judgment. Notwithstanding the foregoing sentence, if at any time
an Indemnified

                                       13

<PAGE>

Party shall have requested an Indemnifying Party to reimburse the Indemnified
Party for fees and expenses of counsel as contemplated by the third sentence of
this paragraph, the Indemnifying Party agrees that it shall be liable for any
settlement of any proceeding effected without its written consent if (i) such
settlement is entered into more than 30 business days after receipt by such
Indemnifying Party of the aforesaid request and (ii) such Indemnifying Party
shall not have reimbursed the Indemnified Party in accordance with such request
prior to the date of such settlement. No Indemnifying Party shall, without the
prior written consent of the Indemnified Party, effect any settlement of any
pending or threatened proceeding in respect of which any Indemnified Party is or
could have been a party and indemnity could have been sought hereunder by such
indemnified party, unless such settlement includes an unconditional release of
such Indemnified Party from all liability arising out of such proceeding.

         SECTION 4.4. Contribution. If the indemnification provided for in this
Article 4 is unavailable to an Indemnified Party in respect of any losses,
claims, damages or liabilities referred to herein, then each Indemnifying Party,
in lieu of indemnifying such Indemnified Party, shall contribute to the amount
paid or payable by such Indemnified Party as a result of such losses, claims,
damages or liabilities. As between the Company on the one hand and each Selling
Holder on the other, the amount of contribution shall be in such proportion as
is appropriate to reflect the relative fault of the Company and of each Selling
Holder in connection with such statements or omissions, as well as any other
relevant equitable considerations. The relative fault of the Company on the one
hand and of each Selling Holder on the other shall be determined by reference
to, among other things, whether the untrue or alleged untrue statement of a
material fact or the omission or alleged omission to state a material fact
relates to information supplied by such party, and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission.

         The Company and the Selling Holders agree that it would not be just and
equitable if contribution pursuant to this Section 4.4 were determined by PRO
RATA allocation or by any other method of allocation which does not take account
of the equitable considerations referred to in the immediately preceding
paragraph. The amount paid or payable by an Indemnified Party as a result of the
losses, claims, damages or liabilities referred to in the immediately preceding
paragraph shall be deemed to include, subject to the limitations set forth
above, any legal or other expenses reasonably incurred by such Indemnified Party
in connection with investigating or defending any such action or claim.
Notwithstanding anything to the contrary in this Agreement, in no event shall
any Selling Holder be obligated to contribute in connection with any offering in
an amount that exceeds the proceeds of such offering received by such Selling
Holder, minus the amount of any damages which such Selling Holder has otherwise
been required to pay by reason of such untrue or alleged untrue statement or
omission or alleged omission. No Person guilty of fraudulent misrepresentation
(within the meaning of Section 11 (f) of the Securities Act) shall be entitled
to contribution from any Person who was not guilty of such fraudulent
misrepresentation. The Selling Holders' obligations to contribute pursuant to
this Section 4.4 are several and not joint.

                                       14

<PAGE>

                                    ARTICLE V

                                  MISCELLANEOUS

         SECTION 5.1. Participation in Underwritten Registrations. No Person may
participate in any underwritten registration hereunder unless such Person (a)
agrees to sell such Person's securities on the basis provided in any
underwriting arrangements approved by the Persons entitled hereunder to approve
such arrangements and (b) completes and executes all questionnaires, powers of
attorney, indemnities, underwriting agreements and other documents reasonably
required under the terms of such underwriting arrangements and these
Registration Rights.

         SECTION 5.2. Rule 144. The Company covenants that it will file any
reports required to be filed by it under the Securities Act and the Exchange Act
and that it will take such further action as any Holder may reasonably request,
all to the extent required from time to time to enable Holders to sell
Registrable Securities without registration under the Securities Act within the
limitation of the exemptions provided by (a) Rule 144 under the Securities Act,
as such Rule may be amended from time to time, or (b) any similar rule or
regulation hereafter adopted by the Commission. Upon the request of any Holder,
the Company will deliver to such Holder a written statement as to whether it has
complied with such requirements.

         SECTION 5.3. Holdback Agreements. (a) RESTRICTIONS ON PUBLIC SALE BY
HOLDER OF REGISTRABLE SECURITIES. In the case of an underwritten public
offering, to the extent not inconsistent with applicable law, each Holder whose
securities are included in a Registration Statement agrees, except as part of
such public offering, not to effect any public sale or distribution of the issue
being registered or a similar security of the Company, or any securities
convertible into or exchangeable or exercisable for such securities, including
sale pursuant to Rule 144 under the Securities Act, during the 14 days prior to,
and during the 90-day period beginning on, the commencement of a public
distribution of Registrable Securities, if and to the extent requested by the
managing Underwriter or Underwriters.

         (B) RESTRICTIONS ON PUBLIC SALE BY THE COMPANY AND OTHERS. The Company
agrees, on behalf of itself and its Affiliates, (i) not to effect any public
sale or distribution of any securities similar to those being registered in
accordance with Section 2.1 or Section 2.2 hereof, or any securities convertible
into or exchangeable or exercisable for such securities, (in each case other
than in connection with the Company's Employee Stock Purchase Plan, Employee
Stock Option Plan, Non-Employee Director Stock Ownership Plan, 401(k) Plan or
other similar employee stock option or incentive plan) during the 30 days prior
to, and during the 180-day period beginning on, the commencement of a public
distribution of Registrable Securities (or such other period of time as may be
required by the Underwriter effecting such public distribution); and (ii) that
any agreement entered into after the date of this Agreement pursuant to which
the Company issues or agrees to issue any privately placed securities shall
contain a provision under which holders of such securities agree not to effect
any public sale or distribution of any such securities during the periods
described in (i) above, in each case

                                       15

<PAGE>

including a sale pursuant to Rule 144 under the Securities Act; provided,
however, that the provisions of this paragraph (b) shall not prevent the
conversion or exchange of any securities pursuant to their terms into or for
other securities.

         SECTION 5.4. Specific Performance. Each Holder, in addition to being
entitled to exercise all rights provided herein or granted by law, including
recovery of liquidated or other damages, will be entitled to specific
performance of its rights under this Agreement. The Company agrees that monetary
damages would not be adequate compensation for any loss incurred by reason of a
breach by it of the provisions of this Agreement and hereby agrees to waive the
defense in any action for specific performance that a remedy at law would be
adequate.

         SECTION 5.5. Notices. Any notice, demand or delivery authorized or
required by this Agreement shall be in writing and shall be given to the Holder
or the Company, as the case may be, at its address (or facsimile number) set
forth below, or such other address (or facsimile number) as shall have been
furnished to the party giving or making such notice, demand or delivery:

     If to the Company:  American Mobile Satellite Corporation
                         10802 Parkridge Blvd.
                         Reston, VA 22091
                         Facsimile: (703) 758-6134
                         Attention: Randy Segal, General Counsel

     If to any Holder:   at the address and facsimile number set forth in
                         the 1998 Guaranty Issuance Agreement.

Each such notice, demand or delivery shall be effective (i) if given by
facsimile, when such facsimile is transmitted to the facsimile number specified
herein and the intended recipient confirms the receipt of such telecopy or (ii)
if given by any other means, when received at the address specified herein.

         SECTION 5.6. No Inconsistent Agreement.

         (a) Notwithstanding anything to the contrary contained herein, to the
extent that any of the provisions hereof conflict with any of the provisions of
(i) the Unit Warrants Registration Rights Agreement or (ii) the Participation
Rights Agreement, the provisions of the Unit Warrants Registration Rights
Agreement and the Participation Rights Agreement, in that order, each as in
effect on the date hereof, shall have priority over the provisions hereof.

         (b) The Company will not after the date of this Agreement enter into
any agreement with respect to its securities or any amendment to such an
agreement that is inconsistent with the rights granted to the Holders in this
Agreement, or otherwise conflicts with the provisions hereof, including any
amendment to the Unit Warrants Registration Rights Agreement or the
Participation Rights Agreement. Except as provided in Section 5.6 (a), the
Company hereby represents that the rights granted to the Holders hereunder do
not in any way conflict with and are not inconsistent with the rights

                                       16

<PAGE>

granted to the holders of the Company's securities under any agreement in effect
on the date hereof. In addition, the Company agrees that it will not amend its
Certificate of Incorporation, by-laws or other governing documents in any
respect that would materially and adversely affect the rights of the Holders
hereunder.

         SECTION 5.7. Further Assurances. Each Party shall cooperate and take
such action as may be reasonably requested by another party in order to carry
out the provisions and purposes of this Agreement and the transactions
contemplated hereby.

         SECTION 5.8. Headings. The headings in this Agreement are for
convenience of reference only and shall not limit or otherwise affect the
meaning hereof.

         SECTION 5.9. GOVERNING LAW AND WAIVER OF JURY TRIAL. THIS AGREEMENT
SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF
NEW YORK, WITHOUT REGARD TO THE CHOICE OF LAW PRINCIPLES THEREOF. THE PARTIES
HERETO IRREVOCABLY WAIVE ANY RIGHT TO A JURY TRIAL WITH RESPECT TO ANY ACTION OR
PROCEEDING ARISING OUT OF OR RELATING TO THIS AGREEMENT.

         SECTION 5.10. Severability. In the event that any one or more of the
provisions contained herein, or the application thereof in any circumstance, is
held invalid, illegal or unenforceable, the validity, legality and
enforceability of any such provision in every other respect and of the remaining
provisions contained herein shall not be affected or impaired thereby.

         SECTION 5.11. Amendments; Waivers. Any provision of this Agreement
may be amended or waived if, and only if, such amendment or waiver is in writing
and signed, in the case of an amendment, by all parties to this Agreement, or in
the case of a waiver, by the party against whom the waiver is to be effective.
No failure or delay by any party in exercising any right, power or privilege
hereunder shall operate as a waiver thereof nor shall any single or partial
exercise thereof preclude any other or further exercise thereof or the exercise
of any other right, power or privilege. The rights and remedies herein provided
shall be cumulative and not exclusive of any rights or remedies provided by law.

         SECTION 5.12. Counterparts. This Agreement may be executed in any
number of counterparts and by the parties hereto in separate counterparts, each
of which when so executed shall be deemed to be an original and all of which
taken together shall constitute one and the same agreement.


                            (SIGNATURE PAGE FOLLOWS)

                                       17

<PAGE>

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

                                       AMERICAN MOBILE SATELLITE
                                       CORPORATION

                                       By: /s/ Gary M. Parsons
                                       -----------------------
                                       Name:  
                                       Title:


                                       HUGHES ELECTRONICS CORPORATION

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


                                       SINGAPORE TELECOMMUNICATIONS LTD.

                                       By: /s/ Yap Chee Keong
                                       ----------------------
                                       Name:  Yap Chee Keong
                                       Title: Group Financial Controller


                                       BARON CAPITAL PARTNERS, L.P.

                                       By:  Baron Capital Management Inc., a 
                                            General Partner

                                       By: /s/ Ronald Baron 
                                       --------------------
                                       Name:  
                                       Title: 



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