AMERICAN MOBILE SATELLITE CORP
SC 13D/A, 1998-04-10
COMMUNICATIONS SERVICES, NEC
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================================================================================

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549


                                  SCHEDULE 13D

                    Under the Securities Exchange Act of 1934


                                (AMENDMENT NO. 6)


                      AMERICAN MOBILE SATELLITE CORPORATION
- --------------------------------------------------------------------------------
                                (Name of Issuer)


       COMMON STOCK, $.01 PAR VALUE                           02755R 10 3
- --------------------------------------------------------------------------------
      (Title of class of securities)                         (CUSIP number)


                               SCOTT B. TOLLEFSEN
                 HUGHES COMMUNICATIONS SATELLITE SERVICES, INC.
                                 1500 HUGHES WAY
                              LONG BEACH, CA 90810
                                 (310) 525-5150
- --------------------------------------------------------------------------------
          (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 [_].

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).

================================================================================

NYFS07...:\56\53356\0056\1324\SCH4028R.10A
<PAGE>

<TABLE>
<CAPTION>
- --------------------------------------------------------           --------------------------------------
CUSIP No.               02755R 10 3                                               13D-Page 2
- --------------------------------------------------------           --------------------------------------

- ---------------------------------------------------------------------------------------------------------
      1        NAME OF REPORTING PERSON:                 HUGHES COMMUNICATIONS SATELLITE SERVICES, INC.
<S>           <C>                                                                              <C>
               S.S. OR I.R.S. IDENTIFICATION NO.                    95-3881942
               OF ABOVE PERSON:
- ---------------------------------------------------------------------------------------------------------
      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 ITEM 2(d) [_] OR 2(e):
- ---------------------------------------------------------------------------------------------------------
      6        CITIZENSHIP OR PLACE OF ORGANIZATION:     CA

- ---------------------------------------------------------------------------------------------------------
      NUMBER OF           7    SOLE VOTING POWER:                   0
        SHARES
                       ----------------------------------------------------------------------------------
     BENEFICIALLY         8    SHARED VOTING POWER:                 6,691,622
       OWNED BY
                       ----------------------------------------------------------------------------------
         EACH             9    SOLE DISPOSITIVE POWER:              0
      REPORTING
                       ----------------------------------------------------------------------------------
     PERSON WITH         10    SHARED DISPOSITIVE POWER:            6,691,622

- ---------------------------------------------------------------------------------------------------------
     11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH          6,691,622
               REPORTING PERSON:

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

- ---------------------------------------------------------------------------------------------------------
     13        PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11):                         22.1%

- ---------------------------------------------------------------------------------------------------------
     14        TYPE OF REPORTING PERSON:                 CO

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


SEE INSTRUCTIONS BEFORE FILLING OUT!
<PAGE>
- --------------------------------------------------------           --------------------------------------
CUSIP No.               02755R 10 3                                               13D-Page 3
- --------------------------------------------------------           --------------------------------------

- ---------------------------------------------------------------------------------------------------------
      1        NAME OF REPORTING PERSON:                 HUGHES COMMUNICATIONS, INC.

               S.S. OR I.R.S. IDENTIFICATION NO.
               OF ABOVE PERSON:
- ---------------------------------------------------------------------------------------------------------
      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 ITEM 2(d) [_] OR 2(e):
- ---------------------------------------------------------------------------------------------------------
      6        CITIZENSHIP OR PLACE OF ORGANIZATION:     CA

- ---------------------------------------------------------------------------------------------------------
      NUMBER OF           7    SOLE VOTING POWER:                   0
        SHARES
                       ----------------------------------------------------------------------------------
     BENEFICIALLY         8    SHARED VOTING POWER:                 6,691,622
       OWNED BY
                       ----------------------------------------------------------------------------------
         EACH             9    SOLE DISPOSITIVE POWER:              0
      REPORTING
                       ----------------------------------------------------------------------------------
     PERSON WITH         10    SHARED DISPOSITIVE POWER:            6,691,622

- ---------------------------------------------------------------------------------------------------------
     11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH          6,691,622
               REPORTING PERSON:

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

- ---------------------------------------------------------------------------------------------------------
     13        PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11):                         22.1%

- ---------------------------------------------------------------------------------------------------------
     14        TYPE OF REPORTING PERSON:                 CO

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


SEE INSTRUCTIONS BEFORE FILLING OUT!
<PAGE>
- --------------------------------------------------------           --------------------------------------
CUSIP No.               02755R 10 3                                               13D-Page 4
- --------------------------------------------------------           --------------------------------------

- ---------------------------------------------------------------------------------------------------------
      1        NAME OF REPORTING PERSON:                 HUGHES ELECTRONICS CORPORATION
                                                         FORMERLY KNOWN AS
                                                         HUGHES NETWORK SYSTEMS, INC.

               S.S. OR I.R.S. IDENTIFICATION NO.
               OF ABOVE PERSON:
- ---------------------------------------------------------------------------------------------------------
      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 ITEM 2(d) [_] OR 2(e):
- ---------------------------------------------------------------------------------------------------------
      6        CITIZENSHIP OR PLACE OF ORGANIZATION:     DE

- ---------------------------------------------------------------------------------------------------------
      NUMBER OF           7    SOLE VOTING POWER:                   0
        SHARES
                       ----------------------------------------------------------------------------------
     BENEFICIALLY         8    SHARED VOTING POWER:                 11,566,622
       OWNED BY
                       ----------------------------------------------------------------------------------
         EACH             9    SOLE DISPOSITIVE POWER:              0
      REPORTING
                       ----------------------------------------------------------------------------------
     PERSON WITH         10    SHARED DISPOSITIVE POWER:            11,566,622

- ---------------------------------------------------------------------------------------------------------
     11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH          11,566,622
               REPORTING PERSON:

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

- ---------------------------------------------------------------------------------------------------------
     13        PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11):                         32.9%

- ---------------------------------------------------------------------------------------------------------
     14        TYPE OF REPORTING PERSON:                 CO

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


SEE INSTRUCTIONS BEFORE FILLING OUT!
<PAGE>
- --------------------------------------------------------           --------------------------------------
CUSIP No.               02755R 10 3                                               13D-Page 5
- --------------------------------------------------------           --------------------------------------

- ---------------------------------------------------------------------------------------------------------
      1        NAME OF REPORTING PERSON:                 GENERAL MOTORS CORPORATION

               S.S. OR I.R.S. IDENTIFICATION NO.
               OF ABOVE PERSON:
- ---------------------------------------------------------------------------------------------------------
      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 ITEM 2(d) [_] OR 2(e):
- ---------------------------------------------------------------------------------------------------------
      6        CITIZENSHIP OR PLACE OF ORGANIZATION:     DE

- ---------------------------------------------------------------------------------------------------------
      NUMBER OF           7    SOLE VOTING POWER:                   0
        SHARES
                       ----------------------------------------------------------------------------------
     BENEFICIALLY         8    SHARED VOTING POWER:                 11,566,622
       OWNED BY
                       ----------------------------------------------------------------------------------
         EACH             9    SOLE DISPOSITIVE POWER:              0
      REPORTING
                       ----------------------------------------------------------------------------------
     PERSON WITH         10    SHARED DISPOSITIVE POWER:            11,566,622

- ---------------------------------------------------------------------------------------------------------
     11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH          11,566,622
               REPORTING PERSON:

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

- ---------------------------------------------------------------------------------------------------------
     13        PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11):                         32.9%

- ---------------------------------------------------------------------------------------------------------
     14        TYPE OF REPORTING PERSON:                 CO

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

</TABLE>

SEE INSTRUCTIONS BEFORE FILLING OUT!
<PAGE>
            This constitutes Amendment No. 6 to the Schedule 13D (the
"Statement") filed with the Securities and Exchange Commission (the
"Commission") by Hughes Communications Satellite Services, Inc. ("HCSS"), Hughes
Communications, Inc. ("HCI"), Hughes Electronics Corporation, formerly known as
Hughes Network Systems, Inc. ("HE"), and General Motors Corporation
(collectively, the "Reporting Persons"), with respect to the Common Stock, $.01
par value (the "Common Stock"), of American Mobile Satellite Corporation, a
Delaware corporation (the "Company"). The percentage of Common Stock reported in
this Amendment as being beneficially owned by the Reporting Persons is based
upon the number of outstanding shares of Common Stock on March 27, 1998 as
identified in the Company's Annual Report on Form 10-K for the period ended
December 31, 1997 and other information received from the Company relating to
the issuance of additional shares of Common Stock to Motorola, Inc. in
connection with the acquisition of ARDIS Company, and after giving effect to the
number of shares of Common Stock beneficially owned by the Reporting Persons.



                                     6
<PAGE>
Item 4.     Purpose of the Transaction
            --------------------------

            The information in Item 6 is incorporated herein by reference.

            Except as described in Item 6 below, the Reporting
Persons currently have no plans or proposals which would result
in any of the actions described in clauses (a) through (j) of
Item 4 of Schedule 13D.

Item 6.     Contracts, Arrangements, Understandings or
            Relationships with Respect to Securities of the Issuer
            ------------------------------------------------------

            As previously described in Amendments Nos. 3, 4 and 5
to the Statement, HE, Singapore Telecommunications Ltd., Baron Capital Partners,
L.P. (collectively, the "Guarantors"), and the Company are parties to that
certain Guaranty Issuance Agreement dated June 28, 1996, as amended (the "Old
Guaranty Issuance Agreement"), pursuant to which, among other things, the
Company issued to the Guarantors warrants to purchase Common Stock (the
"Existing Warrants").

            As previously described in Amendment No. 5 to the Statement, on
December 31, 1997, the Company entered into a purchase agreement with Motorola,
Inc. for the acquisition (the "Acquisition") of ARDIS Company. In connection
with the consummation of the Acquisition on March 31, 1998, the Company and its
subsidiaries renegotiated and refinanced their existing $200.0 million bank
facility (that had been guaranteed by the Guarantors subject to the conditions
of the Old Guaranty Issuance



                                     7
<PAGE>
Agreement) to provide for two new facilities: (i) a $100.0 million unsecured
five-year reducing revolving credit facility under which AMSC Acquisition
Company, Inc., a wholly owned subsidiary of the Company, is the borrower (the
"Revolving Credit Facility") and (ii) a $100.0 million five-year term loan
facility with up to three additional one-year extensions, subject to lender
approval, under which the Company is the borrower (the "Term Loan Facility," and
collectively with the Revolving Credit Facility, the "New Bank Financing"). The
Term Loan Facility is secured by the assets of the Company and its subsidiaries.

            At the request of the Company, the Guarantors have entered into a
new Guaranty Issuance Agreement, dated as of March 31, 1998 (the "New Guaranty
Issuance Agreement"), pursuant to which the New Bank Financing will be severally
guaranteed by the Guarantors. A copy of the New Guaranty Issuance Agreement is
filed as an exhibit hereto and is incorporated herein by reference.

             In consideration for the agreement by the Guarantors to enter into
the New Guaranty Issuance Agreement, (i) the Existing Warrants were amended,
pursuant to the terms of that certain Amendment No. 2 to the Warrant
Certificates, dated as of March 31, 1998 ("Amendment No. 2 to the Warrants"),
among the Guarantors and the Company, a copy of which is filed as an exhibit
hereto and is incorporated herein by reference, by



                                     8
<PAGE>
adjusting the exercise price of the Existing Warrants to $12.51 per share and by
extending the expiration date thereof to March 31, 2005, (ii) the Company issued
to the Guarantors additional warrants to purchase an aggregate of 1,000,000
shares of Common Stock (the "New Warrants") with substantially the same terms as
the Existing Warrants, including the same exercise price and expiration date,
and (iii) the Registration Rights Agreement, dated June 28, 1996, previously
entered into with the Guarantors, was amended and restated pursuant to the terms
of an Amended and Restated Registration Rights Agreement dated as of March 31,
1998, a copy of which is filed as an exhibit hereto and is incorporated herein
by reference, to provide registration rights with respect to (a) all shares of
Common Stock beneficially owned by the Guarantors that constitute "restricted
securities" (as that term is defined in Rule 144 under the Securities Act of
1933, as amended) and (b) all shares of Common Stock issuable to the Guarantors
upon exercise of the Existing Warrants and the New Warrants. In addition, the
registration rights of the Guarantors' thereunder were extended to March 31,
2005.

            In addition, in consideration for the Guarantors' agreement to
guarantee the Company's obligations under the New Bank Financing beyond five
years, and up to a maximum of eight years, the New Guaranty Issuance Agreement
provides that the Company shall pay each Guarantor certain specified fees for
each



                                     9
<PAGE>
one-year extension of the New Bank Financing beyond the fifth year. The Company
also agreed to (a) reimburse each Guarantor for all reasonable expenses
associated with the negotiation, preparation, administration, and enforcement of
the New Guaranty Issuance 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 Company's assets to secure the
Company's obligations under the New Guaranty Issuance Agreement, including the
obligation to reimburse any Guarantor that is required to make any payment under
its guaranties.

            As a result of Amendment No. 2 to the Warrants and the
issuance of the New Warrants, HE has the current right to acquire
4,900,000 shares of Common Stock.

Item 7.      Material to be filed as Exhibits.
             ---------------------------------

      1. Guaranty Issuance Agreement, dated as of March 31, 1998, among the
Guarantors, the Company and AMSC Acquisition Company, Inc.

      2.     Warrant, dated as of March 31, 1998.

      3.     Amended and Restated Registration Rights Agreement, dated as of 
March 31, 1998, by and among the Company and the Guarantors.

      4.     Amendment No. 2 to the Warrant Certificate, dated as of March 31, 
1998, by and among the Company and the Guarantors.



                                     10
<PAGE>
                                    SIGNATURE

After reasonable inquiry and to the best of our knowledge and belief, we the
undersigned certify that the information set forth in this Statement is true,
complete and correct.

Dated: April 9, 1998

                                  HUGHES COMMUNICATIONS SATELLITE
                                  SERVICES, INC.

                                  By:  /s/ Scott B. Tollefsen
                                      --------------------------------------
                                  Name:  Scott B. Tollefsen
                                  Title: Senior Vice President, General
                                  Counsel and Secretary


                                  HUGHES COMMUNICATIONS, INC.

                                  By:  /s/ Scott B. Tollefsen
                                      --------------------------------------
                                  Name:  Scott B. Tollefsen
                                  Title: Senior Vice President, General
                                  Counsel and Secretary


                                  HUGHES ELECTRONICS CORPORATION

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


                                  GENERAL MOTORS CORPORATION

                                  By:  /s/ Martin I. Darvick
                                      --------------------------------------
                                  Name:  Martin I. Darvick
                                  Title: Assistant Secretary





                                     11
<PAGE>
                                  EXHIBIT INDEX


            1. Guaranty Issuance Agreement, dated as of March 31, 1998, among
the Guarantors, the Company and AMSC Acquisition Company, Inc.

            2.     Warrant, dated as of March 31, 1998.

            3.     Amended and Restated Registration Rights Agreement, dated as
of March 31, 1998, by and among the Company and the Guarantors.

            4.     Amendment No. 2 to the Warrant Certificate, dated as of 
March 31, 1998, by and among the Company and the Guarantors.





                                                                     EXHIBIT 1


                           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").


                                R E C I T A L S:


            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 Agreements");




NYFS07...:\56\53356\0056\1939\AGR2128U.21J
<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-1, 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").


                               A G R E E M E N T:


            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
twelve-month 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 1(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: Gary M. Parsons                     Name: Yap Chee Keong
Title: Chief Executive Officer,           Title: Group Financial Controller
         President and Treasurer


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: Gary M. Parsons                           
Title: Chief Executive Officer,
       President
                                          By: /s/ Morty Schaja
                                             -------------------------------
                                          Name: Morty Schaja
                                          Title: Chief Operating Officer

HUGHES ELECTRONICS
CORPORATION

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




                                  14


                                                                     EXHIBIT 2


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.                                                        Warrant to Purchase
                                                                750,000 Shares


            FOR VALUE RECEIVED, AMERICAN MOBILE SATELLITE CORPORATION, a
Delaware corporation (the "Company"), hereby certifies that Hughes Electronics
Corporation, 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.

            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.


NYFS07...:\56\53356\0056\0297\WRT4068L.190
<PAGE>
            "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.

            "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.


                                   2
<PAGE>
            "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.

            EXERCISE OF WARRANT.

                  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 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.

                  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


                                   3
<PAGE>
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.

                  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.

            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:

                  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;

                  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


                                   4
<PAGE>
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 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 3.B 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

                  A.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.

            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:

                  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


                                   5
<PAGE>
                  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 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.

            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.

            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.

            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.


                                   6
<PAGE>
            EXCHANGE, TRANSFER OR ASSIGNMENT OF WARRANT.

                  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.

                  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 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.

            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.


                                   7
<PAGE>
            ANTI-DILUTION PROVISIONS.

                  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 been entitled
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.

                   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.


                                   8
<PAGE>
                  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 or B hereof with respect to
such event or for such other reason.

                  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


                                   9
<PAGE>
                  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 the Warrant Share Amount.

                    E    = the Exercise Price prior to adjustment.

                    W(1) = the adjusted Warrant Share Amount.

                    W    = the Warrant Share Amount prior to adjustment.


                   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 l0 shall be made to the nearest one tenth of a
cent of a share, as the case may be.

                   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.

                   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


                                   10
<PAGE>
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.

            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 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,


                                   11
<PAGE>
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.

            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.

            NOTICES TO WARRANT HOLDERS.

                  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.


                                   12
<PAGE>
                  In the event:

                        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

                        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

                        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

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

                        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,


                                   13
<PAGE>
consolidation, merger, conveyance, transfer, dissolution, liquidation or winding
up, or the vote upon any action.

            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.

            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.

            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.

            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


                                   14
<PAGE>
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.

            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)








                                   15
<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 31, 1998.


                                    AMERICAN MOBILE SATELLITE CORPORATION
     
                                    By:  /s/ Gary M. Parsons
                                        ----------------------------------------
                                    Name:  Gary M. Parsons
                                    Title: President and Chief Executive Officer





HUGHES ELECTRONICS CORPORATION

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








                                   16
<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)




                                   17
<PAGE>
Payment:    $__________________ cash

            $ _________________ check

            $ _________________ wire transfer

            $ _________________ cancellation of indebtedness



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

- -----------------------------------------------------------------------------]










                                   18
<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








                                   19
<PAGE>
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)




                                   20
<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:_________________________________________________________








                                   21


                                                                     EXHIBIT 3

                              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:



NYFS07...:\56\53356\0056\0297\AGR4068K.510
<PAGE>
                                    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.

            "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, right, or other security which is issued as a dividend or other
distribution with respect to or in exchange for or in



                                  2
<PAGE>
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.



                                  3
<PAGE>
            "Demand Registration Notice" has the meaning set forth in Section
2.l.

            "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.

            "Person" means an individual, corporation, partnership, limited
liability company, association, trust, or any



                                  4
<PAGE>
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



                                  5
<PAGE>
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"


                                  6
<PAGE>
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.

            "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, right, 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.



                                  7
<PAGE>
                                   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.



                                  8
<PAGE>
            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 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


                                  9
<PAGE>
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



                                  10
<PAGE>
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



                                  11
<PAGE>
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 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



                                  12
<PAGE>
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 (ii) 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.



                                  13
<PAGE>
            (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.l(c).



                                  14
<PAGE>
            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 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


                                  15
<PAGE>
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.l, 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 rights of the Unit
Warrant Holders pursuant to and in accordance with the Unit Warrant Registration
Rights Agreement, Motorola pursuant to and in



                                  16
<PAGE>
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
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



                                  17
<PAGE>
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
Piggy-Back 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



                                  18
<PAGE>
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



                                       19
<PAGE>
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 Registration Statement to become and remain continuously
effective in accordance with Section 2.1(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



                                  20
<PAGE>
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



                                  21
<PAGE>
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 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



                                  22
<PAGE>
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



                                  23
<PAGE>
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



                                  24
<PAGE>
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.

            (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 securityholders, 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.

            (l) The Company will use its best efforts to cause all such
Registrable Securities to be listed on each securities



                                  25
<PAGE>
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



                                  26
<PAGE>
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
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



                                  27
<PAGE>
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.



                                  28
<PAGE>
                                   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



                                  29
<PAGE>
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, 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



                                  30
<PAGE>
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 right to retain its own
counsel, but the fees and



                                  31
<PAGE>
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
Party shall have requested an Indemnifying Party to reimburse the Indemnified
Party for fees and expenses



                                  32
<PAGE>
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


                                  33
<PAGE>
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



                                  34
<PAGE>
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.


                                  35
<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




                                  36
<PAGE>
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



                                  37
<PAGE>
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 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.



                                  38
<PAGE>
            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.


                                  39
<PAGE>
            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 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



                                  40
<PAGE>
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


                                  41
<PAGE>
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)




                                  42
<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: Gary M. Parsons
                              Title: President and Chief Executive Officer



                              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


<PAGE>
                              BARON CAPITAL PARTNERS, L.P.

                              By:  Baron Capital Management Inc., a General
                                    Partner

                              By:   Ronald Brown
                                  --------------------------------------------
                              Name: Ronald Brown
                              Title: President











                                  45


                                                                    EXHIBIT 4


                                 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.

            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,



NYFS07...:\56\53356\0056\0297\AMD4068L.130
<PAGE>
                  and



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

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

            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."



                                     2
<PAGE>
      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.









                                     3
<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
 CORPORATION                                TELECOMMUNICATIONS LTP.

By:  /s/ Gary M. Parsons                   By:  /s/ Yap Chee Keong
    ------------------------------             --------------------------------
Name: Gary M. Parsons                        Name: Yap Chee Keong
Title: President and Chief                   Title: Group Financial Controller
        Executive Officer
                              
                                           
HUGHES ELECTRONICS CORPORATION             BARON CAPITAL PARTNERS, L.P., 
                                            a Delaware limited partnership
By:  /s/ Amnon Carr
    ------------------------------
Name: Amnon Carr
Title: Assistant Treasurer                 By:  Baron Capital Management, Inc.,
                                             a General Partner


                                           By:  /s/ Ronald Brown
                                               -----------------------------
                                           Name: Ronald Brown
                                           Title: President




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