AMERICAN MOBILE SATELLITE CORP
SC 13D/A, 1999-04-13
COMMUNICATIONS SERVICES, NEC
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                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13D

                    UNDER THE SECURITIES EXCHANGE ACT OF 1934
                               (AMENDMENT NO. 6)*

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

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

                                   02755R 10 3
              ----------------------------------------------------
                                 (CUSIP Number)

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

                                  April 1, 1999
- --------------------------------------------------------------------------------
             (Date of Event which Requires Filing of this Statement)


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

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

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

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

*/  With a copy to:  Richard S. Elliott, Esq., Paul, Weiss, Rifkind, Wharton & 
    Garrison, 1615 L Street, N.W., Suite 1300, Washington, DC  20036, 
    (202) 223-7324.
<PAGE>

                                  SCHEDULE 13D


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


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

          Singapore Telecommunications Limited

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

3         SEC USE ONLY


4         SOURCE OF FUNDS

          WC

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

6         CITIZENSHIP OR PLACE OF ORGANIZATION

          Republic of Singapore

                            7         SOLE VOTING POWER
                                                   
           NUMBER OF                  4,836,746 shares
            SHARES
      BENEFICIALLY OWNED    8         SHARED VOTING POWER
      BY EACH REPORTING
            PERSON                    0 shares
             WITH
                            9         SOLE DISPOSITIVE POWER

                                      4,836,746 shares

                            10        SHARED DISPOSITIVE POWER

                                      0 shares

11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          4,836,746 shares

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

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

          14.6%

14        TYPE OF REPORTING PERSON

          CO

<PAGE>

                                  SCHEDULE 13D


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


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

          Temasek Holdings (Private) Limited

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

3         SEC USE ONLY


4         SOURCE OF FUNDS

          AF

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


6         CITIZENSHIP OR PLACE OF ORGANIZATION

          Republic of Singapore

                            7         SOLE VOTING POWER
                                                   
           NUMBER OF                  0 shares
            SHARES
      BENEFICIALLY OWNED    8         SHARED VOTING POWER
      BY EACH REPORTING
            PERSON                    4,836,746 shares
             WITH
                            9         SOLE DISPOSITIVE POWER

                                      0 shares

                            10        SHARED DISPOSITIVE POWER

                                      4,836,746 shares

11        AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          4,836,746 shares

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

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

          14.6%

14        TYPE OF REPORTING PERSON

          HC
<PAGE>

                                                                               4

                             AMENDED SCHEDULE 13D 1/


Item 3.  Source and Amount of Funds or Other Consideration

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

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

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

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

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

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

2/       In April 1992, pursuant to the Telecommunication Authority of Singapore
         Act 1992, Singapore Telecom became the successor in interest to
         telecommunications businesses owned by the Telecommunication Authority
         of Singapore (which continues to exercise regulatory oversight over
         those businesses). Unless otherwise indicated by the context,
         "Singapore Telecom" will be used to refer both to Singapore
         Telecommunications Limited and to its legal predecessor.
<PAGE>

                                                                               5

         On that same date, 1,317,460 shares were issued to Singapore Telecom by
the Issuer upon conversion by Singapore Telecom of $27,666,667 principal amount
of subordinated convertible notes previously issued by the Issuer to Singapore
Telecom. The Singapore Telecom funds loaned to the Issuer in connection with
such convertible notes ($20 million in August 1992 and $7,666,667 in October
1993) came from the working capital of Singapore Telecom.

         In December 1995, Singapore Telecom delivered to Mtel L.P. a notice of
conversion with respect to the remaining Mtel Convertible Debentures. Upon
conversion, Singapore Telecom received 8,451.71 Mtel LPUs that, as noted above,
were to be redeemed immediately by Mtel L.P. in exchange for 760,869 shares of
Common Stock held of record by Mtel L.P. On December 27, 1995, Mtel L.P.
redeemed the 8,451.71 Mtel LPUs and directed the Issuer to transfer the 760,869
shares of Common Stock to Singapore Telecom (effective as of that date).

         On July 1, 1996, upon the closing of a set of agreements providing
long-term bank financing for the Issuer's subsidiary, Singapore Telecom received
a warrant from the Issuer entitling it, through June 28, 2001, to purchase
625,000 shares of Common Stock at an exercise price of $24 per share (the
"Original Warrant"). The Original Warrant was received as part of the
consideration for Singapore Telecom's guaranty of up to $25 million in principal
amount of such long-term financing. The number of shares of Common Stock for
which the Original Warrant could be exercised was limited to the extent that
certain financial performance tests restricted the ability of the Issuer's
subsidiary to borrow under the long-term financing arrangements. As of July 1,
1996, the Original Warrant was exercisable for only 406,250 shares of Common
Stock.

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

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

         On March 29, 1999, Singapore Telecom and the other guarantors of the
restructured long-term financing agreed to eliminate certain financial covenants
applicable to the Issuer and the Issuer's subsidiary. As part of the
consideration for the elimination of the financial covenants, the Amended
Warrant and Warrant No. 2
<PAGE>

                                                                               6

were each amended on April 1, 1999 so as to change the exercise price to $7.50
per share.

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

Item 4.  Purpose of Transaction

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

         The shares of Common Stock held by Singapore Telecom were acquired for
investment purposes, and continue to be held for such purposes. Pursuant to the
cumulative voting rights that exist under the Issuer's Certificate of
Incorporation with respect to the election of the Issuer's board of directors,
and pursuant to the rights that exist under the Stockholders' Agreement
(described in Item 6 below) with respect to appointing directors to the
executive committee of the Issuer's board of directors, Singapore Telecom has
the right to be represented on the Issuer's board of directors and its executive
committee. Singapore Telecom presently has no representatives on the Issuer's
board of directors or the Issuer's executive committee.

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

         Subject to the foregoing, and in light of its current strategic focus
on business ventures in the Asia-Pacific Region, Singapore Telecom has engaged
in certain dispositions of Common Stock in the open market, and may engage in
further dispositions from time to time. The shares of Common Stock beneficially
owned by Singapore Telecom (including the shares of Common Stock that Singapore
Telecom could obtain by exercising the Amended Warrant and Warrant No. 2) are
covered by a registration statement filed by the Issuer, which became effective
on March 31, 1999. Singapore Telecom exercised its piggyback registration rights
under the Amended and Restated Registration Rights Agreement (described under
Item 6 below) with respect to this registration statement, which permits sales
of shares of Common Stock to the public on a delayed or continuous basis.

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

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

                                                                               7

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

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

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

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

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

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

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

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

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

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

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

Item 5.  Interest in Securities of the Issuer

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

(a) Singapore Telecom owns beneficially 4,836,746 shares of Common Stock. Of
this amount, 4,024,246 shares of Common Stock are owned of record by Singapore
Telecom, and 812,500 shares of Common Stock could be obtained by Singapore
Telecom upon exercise in full of the Amended Warrant and Warrant No. 2. As noted
in Item 2 above, Singapore Telecom is approximately 80% owned by Temasek. The
filing of this Statement should not, however, be construed as an admission that
<PAGE>

                                                                               8

Temasek is, for purposes of Section 13(d) of the Act, the beneficial owner of
any of the Common Stock beneficially owned by Singapore Telecom.

         Based upon the information contained in the Issuer's Form 10-K dated
March 30, 1999, the 4,836,746 shares of Common Stock beneficially owned by
Singapore Telecom constitute approximately 14.6% of the Common Stock outstanding
as of March 25, 1999. 3/

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

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

Lim Toon                        2,000                       *


*Less Than 0.1%

         The Reporting Persons may be deemed to comprise a group (within the
meaning of Section 13(d)(3) of the Act) with the following entities by virtue of
certain agreements described in Item 6 below: (1) Hughes Electronics Corporation
("Hughes Electronics") and Hughes Communications Satellite Services, Inc.
("Hughes" and, together with Hughes Electronics, the "Hughes Entities"), an
indirect wholly-owned subsidiary of Hughes Electronics; and (2) Space
Technologies Investments, Inc. ("Investments") and the following affiliates of
Investments: Transit Communications, Inc., and Satellite Communications
Investments Corporation (collectively with Investments, the "AT&T Entities). 4/

         The Reporting Persons expressly disclaim beneficial ownership of shares
of Common Stock beneficially owned by the Hughes Entities and the AT&T Entities,
and the filing of this statement by the Reporting Persons shall not be construed
as an admission by the Reporting Persons that either of them is, for purposes of
Section 13(d) of the Act, the beneficial owner of any of the shares of Common
Stock held by the Hughes Entities or the AT&T Entities.

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

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

                                                                               9

         Based upon the information set forth in the Issuer's Form 10-K dated
March 30, 1999, the Reporting Persons believe that the Hughes Entities and the
AT&T Entities beneficially own the number of shares of Common Stock set forth in
the table below, constituting in each case that percentage of the outstanding
Common Stock set forth in the table:

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

Hughes Communications
  Satellite Services, Inc. 5/              6,691,622               --

Hughes Electronics Corporation 6/          4,875,000               --
                                           ---------               --

    Hughes Entities as a Group            11,566,622             31.1 7/

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

Space Technologies Investments, Inc.       1,206,192               --

Transit Communications, Inc.                 681,818               --

Satellite Communications
  Investments Corporation                  1,113,135               --
                                           ---------               --

    AT&T Entities as a Group               3,001,145              9.3


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

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

6/       Consists of 4,125,000 shares of Common Stock issuable upon the exercise
         of a warrant that Hughes Electronics received as part of the
         consideration for a guaranty that it provided in connection with
         long-term bank financing for the Issuer's subsidiary, and 750,000
         shares of Common Stock issuable upon the exercise of another warrant
         that Hughes Electronics received as part of the consideration for a
         guaranty that it provided in connection with the restructuring of the
         long-term financing. Each warrant is exercisable through March 31, 2005
         at an exercise price of $7.50 per share.

7/       For the purpose of computing this percentage, warrants held by the
         Hughes Entities were deemed to be exercised in full and the shares of
         Common Stock issuable upon such exercise were deemed to be outstanding.
<PAGE>

                                                                              10

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

(c) As noted under Item 3 above, on April 1, 1999, as consideration for
Singapore Telecom's agreement (along with the agreement of the other guarantors
of the restructured long-term financing) to eliminate certain financial
covenants applicable to the Issuer and the Issuer's subsidiary, the Amended
Warrant and Warrant No. 2 were each amended so as to change the exercise price
from $12.51 per share to $7.50 per share.

         During the past 60 days, Singapore Telecom has engaged in certain open
market sales of Common Stock under SEC Rule 144. Those sales were as follows.

Date of Transaction         Number of Shares           Price Per Share
- -------------------         ----------------           ---------------

      2/03/99                    10,000                $5.00-$5.125
      2/05/99                    22,500                $4.50-$4.8125
      2/08/99                    19,000                $4.50
      2/10/99                    10,400                $4.625-$4.6875
      2/11/99                       100                $4.50
      3/01/99                    10,300                $4.25
      3/04/99                    10,000                $4.25

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

         On April 1, 1999, as consideration for Hughes Electronics' agreement
(along with the agreement of the other guarantors of the restructured long-term
financing) to eliminate certain financial covenants applicable to the Issuer and
the Issuer's subsidiary, the two warrants for shares of Common Stock held by
Hughes Electronics (totaling 4,875,000 shares of Common Stock) were each amended
so as to change the exercise price from $12.51 per share to $7.50 per share.

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

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

(e) Not applicable.
<PAGE>

                                                                              11

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

         Item 6 is amended and restated only with respect to the subject
headings listed below:

                                * * * * * * * * *

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

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

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

         The Original Warrant also provided that the number of warrant shares
and the exercise price were to be adjusted under certain conditions, including
stock splits and asset distributions to holders of Common Stock. (Section 10 of
the Warrant)

         In addition to the restrictions upon exercise of the Original Warrant
described above, the number of shares of Common Stock for which the Original
Warrant could be exercised was limited to the extent that certain financial
performance tests restricted the ability of the Issuer's subsidiary to borrow
fully under the long-term loan agreements. (Section 15 of the Warrant)

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

         On March 27, 1997, pursuant to the amendment to the Guaranty Issuance
Agreement described above, the Original Warrant was amended (as amended, the
"Amended Warrant") (a) to increase the number of shares of Common
<PAGE>

                                                                              12

Stock issuable upon exercise thereof to 687,500, (b) to change the exercise
price to $13 per share, and (c) to delete Section 15. On March 31, 1998,
pursuant to the Second Guaranty Issuance Agreement (described below), the
Amended Warrant was further amended so as to (a) change the exercise price to
$12.51 per share and (b) change the expiration date to March 31, 2005. On April
1, 1999, pursuant to Amendment No. 2 to the Second Guaranty Issuance Agreement,
the Amended Warrant was again amended, so as to change the exercise price to
$7.50 per share.

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

                                * * * * * * * * *

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

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

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

         In addition, in consideration for Guarantors' willingness to guarantee
the Issuer's obligations under the restructured long-term financing beyond five
years, and up to a maximum of eight years, the Second Guaranty Issuance
Agreement provided that the Issuer would pay each Guarantor certain specified
fees for each one-
<PAGE>

                                                                              13

year extension of the credit facility beyond the fifth year. The Issuer also
agreed to (a) reimburse each Guarantor for all reasonable expenses associated
with the negotiation, preparation, administration, and enforcement of the
Agreement, the guaranties, and the related documents, and (b) to execute a
security and pledge agreement granting the Guarantors a lien and security
interest in all of the Issuer's assets to secure the Issuer's obligations under
the Agreement, including the Issuer's obligation to reimburse any Guarantor that
is required to make any payment under its guaranties. (Sections 1, 2, and 4)

         The Second Guaranty Issuance Agreement also contains a limited
intercreditor agreement among the Guarantors. If any Guarantor makes any payment
under its guaranties or acquires any notes or obligations under the long-term
loan agreements, thereafter all decisions to act or to refrain from acting with
respect to the enforcement of such notes or obligations or the reimbursement
obligations contained in Section 13 of the Agreement against the Issuer or AMSC
Acquisition (including enforcement with respect to any collateral security
therefor) must first be approved in writing by Guarantors having Pro Rata Shares
greater than 87%. In addition, if any Guarantor does not make a required payment
under one of its guaranties and such payment is made by any other Guarantor,
then the defaulting Guarantor is obligated to reimburse the paying Guarantor for
such payment on demand, and any amounts which would otherwise be payable to the
defaulting Guarantor by the Issuer or AMSC Acquisition or with respect to any
collateral therefor shall first be paid to the paying Guarantor until such
payment obligation of the defaulting Guarantor has been fully satisfied.
(Section 14)

         On January 15, 1999, the Second Guaranty Issuance Agreement was amended
(Amendment No. 1) so as to clarify the reimbursement obligations contained in
Section 13. On March 29, 1999, the Second Guaranty Issuance Agreement was
further amended (Amendment No. 2) so as to eliminate certain financial covenants
applicable to the Issuer and AMSC Acquisition that were contained in Section 3.
Amendment No. 2 also provided that, as consideration for the elimination of the
financial covenants, the warrants held by the Guarantors would be amended,
effective April 1, 1999, so as to change the exercise price to $7.50 per share.

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

         Pursuant to the terms of the Second Guaranty Issuance Agreement, the
Issuer issued to Singapore Telecom the warrant dated March 31, 1998 ("Warrant
No. 2"). Warrant No. 2 entitled Singapore Telecom to purchase from the Issuer
125,000 shares of Common Stock at an exercise price of $12.51 per share. Warrant
No. 2 was exercisable in full as of March 31, 1998, has an expiration date of
March 31, 2005, and otherwise contains rights and conditions similar to those
contained in the Amended Warrant as amended. On April 1, 1999, pursuant to
Amendment No. 2 to the Second Guaranty Issuance Agreement, Warrant No. 2 was
amended so as to change the exercise price to $7.50 per share.

         On March 31, 1998, the Issuer and each of the Guarantors entered into
the Amended and Restated Registration Rights Agreement ("Amended Registration
<PAGE>

                                                                              14

Rights Agreement"). The Amended Registration Rights Agreement provides certain
demand and piggyback registration rights with respect to shares of Common Stock
issuable upon the exercise of the old warrants and the new warrants, as well as
to other restricted securities held by the Guarantors. By letter agreement dated
March 22, 1999, the Issuer and Hughes Electronics agreed, with the concurrence
of Singapore Telecom and Baron, to amend the Amended and Restated Registration
Rights Agreement so as to clarify and expand certain of the registration rights
accorded to the Guarantors under the Agreement.

Item 7.  Material to be Filed as Exhibits

         Item 7 is amended and restated in its entirety as follows:

Exhibit I -- Joint Filing Agreement dated April 13, 1999

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

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

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

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

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

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

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

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

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

Exhibit XI -- Amendment No. 1 to Guaranty Issuance Agreement dated March 27,
1997 (previously filed)

Exhibit XII -- Amendment No. 1 to 1996 Warrant Certificates dated March 27, 1997
(previously filed)

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

Exhibit XIV -- Guaranty Issuance Agreement dated March 31, 1998 (previously
filed)
<PAGE>

                                                                              15

Exhibit XV -- Amendment No. 2 to 1996 Warrant Certificates dated March 31, 1998
(previously filed)

Exhibit XVI -- Warrant dated March 31, 1998 (previously filed)

Exhibit XVII -- Amended and Restated Registration Rights Agreement dated March
31, 1998 (previously filed)

Exhibit XVIII -- Amendment No. 1 to Guaranty Issuance Agreement dated January
15, 1999

Exhibit XIX -- Letter Agreement dated March 22, 1999

Exhibit XX -- Amendment No. 2 to Guaranty Issuance Agreement dated March 29,
1999

Exhibit XXI -- Amendment No. 3 to 1996 Warrant Certificates dated April 1, 1999

Exhibit XXII -- Amendment No. 1 to 1998 Warrant Certificates dated April 1, 1999
<PAGE>

                                                                              16

                                   SIGNATURES


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

                                    SINGAPORE TELECOMMUNICATIONS LTD.


                                    By: /s/ Hoh Wing Chee
                                        -----------------
                                        Name:  Hoh Wing Chee (Mr.)
                                        Title: VP (International Network)

Dated:  April 13, 1999


                                    TEMASEK HOLDINGS (PRIVATE) LTD.


                                    By: /s/ Ng Kin Meng
                                        ---------------
                                        Name:  Ng Kin Meng (Mrs.)
                                        Title: Senior Vice President/
                                               Company Secretary

Dated:  April 13, 1999
<PAGE>

                                                                              17

                                  EXHIBIT INDEX

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


Exhibit I        Joint Filing Agreement dated April 13, 1999

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

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

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

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

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

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

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

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

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

Exhibit XI       Amendment No. 1 to Guaranty Issuance Agreement dated March 27, 
                 1997 (previously filed)

Exhibit XII      Amendment No. 1 to 1996 Warrant Certificates dated March 27, 
                 1997 (previously filed)

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

Exhibit XIV      Guaranty Issuance Agreement dated March 31, 1998 (previously
                 filed)

Exhibit XV       Amendment No. 2 to 1996 Warrant Certificates dated March 31, 
                 1998 (previously filed)
<PAGE>

                                                                              18

Exhibit XVI      Warrant dated March 31, 1998 (previously filed)

Exhibit XVII     Amended and Restated Registration Rights Agreement dated March 
                 31, 1998 (previously filed)

Exhibit XVIII    Amendment No. 1 to Guaranty Issuance Agreement dated January 
                 15, 1999

Exhibit XIX      Letter Agreement dated March 22, 1999

Exhibit XX       Amendment No. 2 to Guaranty Issuance Agreement dated March 29, 
                 1999

Exhibit XXI      Amendment No. 3 to 1996 Warrant Certificates dated April 1,
                 1999

Exhibit XXII     Amendment No. 1 to 1998 Warrant Certificates dated April 1,
                 1999
<PAGE>

                               SCHEDULES I AND II

                                   SCHEDULE I

                      SINGAPORE TELECOMMUNICATIONS LIMITED
                         DIRECTORS & EXECUTIVE OFFICERS

<TABLE>
<CAPTION>
                                                                         PRESENT PRINCIPAL
NAME                     POSITION              BUSINESS ADDRESS        OCCUPATION/EMPLOYMENT            CITIZENSHIP
- ----                     --------              ----------------        ---------------------            -----------                 
<S>                      <C>                   <C>                     <C>                              <C>
- -------------------------------------------------------------------------------------------------------------------
Mr. Koh Boon Hwee        Board Member          Singapore Telecom       Executive Chairman               Singapore
                         Chairman              31 Exeter Road          Wuthelam Holdings Pte Ltd.
                                               Comcentre               298 Tiong Bahru Road
                                               Singapore 239732        #08-00 Tiong Bahru Plaza
                                                                       Singapore 168730
- -------------------------------------------------------------------------------------------------------------------
Mr. Wong Hung Khim       Board Member          Singapore Telecom       Chairman                         Singapore
                         Deputy Chairman       31 Exeter Road          Del Gro Corporation Limited
                                               Comcentre               205 Braddell Road
                                               Singapore 239732        Singapore 579701
- -------------------------------------------------------------------------------------------------------------------
BG Lee Hsien Yang        Board Member          Singapore Telecom       President & CEO                  Singapore
                         President & CEO       31 Exeter Road          Singapore Telecom
                                               Comcentre               31 Exeter Road
                                               Singapore 239732        Comcentre
                                                                       Singapore 239732
- -------------------------------------------------------------------------------------------------------------------
Dr. Hong Hai             Board Member          Singapore Telecom       President & CEO                  Singapore
                                               31 Exeter Road          Haw Par Corporation
                                               Comcentre               180 Clemenceau Avenue
                                               Singapore 239732        #08-00 Haw Par Glass Tower
                                                                       Singapore 239722
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                         PRESENT PRINCIPAL
NAME                     POSITION              BUSINESS ADDRESS        OCCUPATION/EMPLOYMENT            CITIZENSHIP
- ----                     --------              ----------------        ---------------------            -----------
<S>                      <C>                   <C>                     <C>                              <C>
- -------------------------------------------------------------------------------------------------------------------
Mr. Lim Ho Kee           Board Member          Singapore Telecom       Director                         Singapore
                                               31 Exeter Road          Cypress Woods Pte Ltd.
                                               Comcentre               80 Raffles Place
                                               Singapore 239732        #24-21 UIB Plaza 2
                                                                       Singapore 048624
- -------------------------------------------------------------------------------------------------------------------
Mr. Quek Poh Huat        Board Member          Singapore Telecom       President                        Singapore
                                               31 Exeter Road          Temasek Holdings (Private)
                                               Comcentre                 Limited
                                               Singapore 239732        8 Shenton Way #38-03
                                                                       Temasek Tower
                                                                       Singapore 068811
- -------------------------------------------------------------------------------------------------------------------
BG Lim Chuan Poh         Board Member          Singapore Telecom       Chief of Army                    Singapore
                                               31 Exeter Road          Ministry of Defence
                                               Comcentre               303 Gombak Drive
                                               Singapore 239732        Singapore 669645
- -------------------------------------------------------------------------------------------------------------------
Mr. Keith Tay Ah Kee     Board Member          Singapore Telecom       Asia Quest Associates            Singapore
                                               31 Exeter Road            Pte Ltd.
                                               Comcentre               161A Telok Ayer Street
                                               Singapore 239732        Singapore 068615
- -------------------------------------------------------------------------------------------------------------------
Mrs. Yu-Foo Yee Shoon    Board Member          Singapore Telecom       Deputy Secretary General         Singapore
                                               31 Exeter Road          National Trades Union
                                               Comcentre               Congress
                                               Singapore 239732        Trade Union House
                                                                       Shenton Way
                                                                       Singapore 068810
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                                 PRESENT PRINCIPAL
NAME                     POSITION                      BUSINESS ADDRESS        OCCUPATION/EMPLOYMENT            CITIZENSHIP
- ----                     --------                      ----------------        ---------------------            -----------
<S>                      <C>                           <C>                     <C>                              <C>
- ---------------------------------------------------------------------------------------------------------------------------
Mr. Lim Toon             Chief Operating Officer       Singapore Telecom       Chief Operating Officer          Singapore
                                                       31 Exeter Road          Singapore Telecom
                                                       Comcentre               31 Exeter Road
                                                       Singapore 239732        Comcentre
                                                                               Singapore 239732
- ---------------------------------------------------------------------------------------------------------------------------
Mr. Lee Shin Koi         Executive Vice President      Singapore Telecom       Executive Vice President         Singapore
                         (Consumer Business)           31 Exeter Road          (Consumer Business)
                                                       Comcentre               Singapore Telecom
                                                       Singapore 239732        31 Exeter Road
                                                                               Comcentre
                                                                               Singapore 239732
- ---------------------------------------------------------------------------------------------------------------------------
Mr. Lim Chuan Poh        Executive Vice President      Singapore Telecom       Executive Vice President         Singapore
                         (Corporate Business)          31 Exeter Road          (Corporate Business)
                                                       Comcentre               Singapore Telecom
                                                       Singapore 239732        31 Exeter Road
                                                                               Comcentre
                                                                               Singapore 239732
- ---------------------------------------------------------------------------------------------------------------------------
Mr. Lim Shyong           Executive Vice President      Singapore Telecom       Executive Vice President         Singapore
                         (Global Business)             31 Exeter Road          (Global Business)
                                                       Comcentre               Singapore Telecom
                                                       Singapore 239732        31 Exeter Road
                                                                               Comcentre
                                                                               Singapore 239732
- ---------------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                                 PRESENT PRINCIPAL
NAME                     POSITION                      BUSINESS ADDRESS        OCCUPATION/EMPLOYMENT            CITIZENSHIP
- ----                     --------                      ----------------        ---------------------            -----------
<S>                      <C>                           <C>                     <C>                              <C>
- ---------------------------------------------------------------------------------------------------------------------------
Ms. Chua Sock Koong      Chief Financial Officer       Singapore Telecom       Chief Financial Officer          Singapore
                                                       31 Exeter Road          Singapore Telecom
                                                       Comcentre               31 Exeter Road
                                                       Singapore 239732        Comcentre
                                                                               Singapore 239732
- ---------------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>

                                   SCHEDULE II

                       TEMASEK HOLDINGS (PRIVATE) LIMITED
                         DIRECTORS & EXECUTIVE OFFICERS

<TABLE>
<CAPTION>
                                                                                         PRESENT PRINCIPAL
NAME                     POSITION                      BUSINESS ADDRESS                OCCUPATION/EMPLOYMENT             CITIZENSHIP
- ----                     --------                      ----------------                ---------------------             -----------
<S>                      <C>                           <C>                             <C>                               <C>
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. S. Dhanabalan        Chairman                      Temasek Holdings (Private)      Chairman                          Singapore
                                                         Limited                       Development Bank of Singapore Ltd.
                                                       8 Shenton Way #38-03            46th Floor, DBS Building Tower One
                                                       Temasek Tower                   Singapore 068809
                                                       Singapore 068811
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Ngiam Tong Dow       Deputy Chairman               Temasek Holdings (Private)      Permanent Secretary               Singapore
                                                         Limited                       Ministry of Finance
                                                       8 Shenton Way #38-03            100 High Street #10-01
                                                       Temasek Tower                   The Treasury
                                                       Singapore 068811                Singapore 179434
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Kwa Chong Seng       Deputy Chairman               Temasek Holdings (Private)      Chairman/Managing Director        Singapore
                                                         Limited                       Esso Singapore Pte Ltd.
                                                       8 Shenton Way #38-03            1 Raffles Place
                                                       Temasek Tower                   OUB Centre 38th Floor
                                                       Singapore 068811                Singapore 048616
- ------------------------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                                         PRESENT PRINCIPAL
NAME                     POSITION                      BUSINESS ADDRESS                OCCUPATION/EMPLOYMENT             CITIZENSHIP
- ----                     --------                      ----------------                ---------------------             -----------
<S>                      <C>                           <C>                             <C>                               <C>
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Fock Siew Wah        Director                      Temasek Holdings (Private)      Chairman                          Singapore
                                                         Limited                       Land Transport Authority
                                                       8 Shenton Way #38-03            460 Alexandra Road
                                                       Temasek Tower                   PSA Building #28-00
                                                       Singapore 068811                Singapore 119963
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Lim Siong Guan       Director                      Temasek Holdings (Private)      Second Permanent Secretary        Singapore
                                                         Limited                       Ministry of Finance
                                                       8 Shenton Way #38-03            100 High Street #06-01
                                                       Temasek Tower                   The Treasury
                                                       Singapore 068811                Singapore 179434

                                                                                       AND

                                                                                       Permanent Secretary
                                                                                       Ministry of Education
                                                                                       Kay Siang Road
                                                                                       Singapore 248922
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Koh Boon Hwee        Director                      Temasek Holdings (Private)      Executive Chairman                Singapore
                                                         Limited                       Wuthelam Holdings Pte Ltd.
                                                       8 Shenton Way #38-03            177 River Valley Road
                                                       Temasek Tower                   #04-13 Liang Court
                                                       Singapore 068811                Singapore 179030
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Kua Hong Pak         Director                      Temasek Holdings (Private)      President & CEO                   Singapore
                                                         Limited                       Times Publishing Group
                                                       8 Shenton Way #38-03            Times Publishing Limited
                                                       Temasek Tower                   1 New Industrial Road
                                                       Singapore 068811                Singapore 536196
- ------------------------------------------------------------------------------------------------------------------------------------
</TABLE>
<PAGE>
<TABLE>
<CAPTION>
                                                                                         PRESENT PRINCIPAL
NAME                     POSITION                      BUSINESS ADDRESS                OCCUPATION/EMPLOYMENT             CITIZENSHIP
- ----                     --------                      ----------------                ---------------------             -----------
<S>                      <C>                           <C>                             <C>                               <C>
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Khaw Boon Wan        Director                      Temasek Holdings (Private)      Permanent Secretary               Singapore
                                                         Limited                       Ministry of Trade and Industry
                                                       8 Shenton Way #38-03            100 High Street #09-01
                                                       Temasek Tower                   The Treasury
                                                       Singapore 068811                Singapore 179434
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Quek Poh Huat        President                     Temasek Holdings (Private)      President                         Singapore
                                                         Limited                       Temasek Holdings (Private) Limited
                                                       8 Shenton Way #38-03            8 Shenton Way #38-03
                                                       Temasek Tower                   Temasek Tower
                                                       Singapore 068811                Singapore 068811
- ------------------------------------------------------------------------------------------------------------------------------------
Mr. Peter Ong Boon       Executive Vice                Temasek Holdings (Private)      Executive Vice President          Singapore
Kwee                     President                       Limited                       Temasek Holdings (Private) Limited
                                                       8 Shenton Way #38-03            8 Shenton Way #38-03
                                                       Temasek Tower                   Temasek Tower
                                                       Singapore 068811                Singapore 068811
- ------------------------------------------------------------------------------------------------------------------------------------
Mrs. Ng Kin Meng         Company Secretary/            Temasek Holdings (Private)      Company Secretary/                Singapore
                         Senior Vice President           Limited                       Senior Vice President
                                                       8 Shenton Way #38-03            Temasek Holdings (Private) Limited
                                                       Temasek Tower                   8 Shenton Way #38-03
                                                       Singapore 068811                Temasek Tower
                                                                                       Singapore 068811
- ------------------------------------------------------------------------------------------------------------------------------------
</TABLE>



                                    EXHIBIT I

                             JOINT FILING AGREEMENT


         In accordance with Rule 13d-1(k) under the Securities Exchange Act of
1934, as amended, the persons named below agree to the joint filing on behalf of
each of them of a statement on Schedule 13D dated April 13, 1999 with respect to
the Common Stock, par value $.01 per share, of American Mobile Satellite
Corporation, a Delaware corporation. This Joint Filing Agreement shall be
included as an Exhibit to such joint filing. In evidence thereof each of the
undersigned, being duly authorized, hereby executes this Agreement this 13th day
of April 1999.

                                    SINGAPORE TELECOMMUNICATIONS LTD.


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


                                    TEMASEK HOLDINGS (PRIVATE) LTD.


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


                                  EXHIBIT XVIII


         AMENDMENT, dated as of January 15, 1999 (this "Amendment"), by and
among HUGHES ELECTRONICS CORPORATION, a Delaware corporation ("Hughes"),
SINGAPORE TELECOMMUNICATIONS LTD., a Singapore corporation ("SingTel"), BARON
CAPITAL PARTNERS, L.P., a Delaware limited partnership ("Baron; and Hughes,
SingTel and Baron, collectively, the Guarantors"), AMERICAN MOBILE SATELLITE
CORPORATION, a Delaware corporation ("AMSC Parent"), and AMSC ACQUISITION
COMPANY, INC., a Delaware corporation and a wholly-owned subsidiary of AMSC
Parent ("AMSC Acquisition"), to the Guaranty Issuance Agreement dated as of
March 31, 1998 (said Agreement, as the same may be amended, supplemented or
otherwise modified from time to time, being the "Guaranty Issuance Agreement,"
and the terms defined therein being used herein as therein defined unless
otherwise defined herein) by and among each of the Guarantors, AMSC Parent and
AMSC Acquisition.

                              W I T N E S S E T H:

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

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

         SECTION 1. AMENDMENT TO THE GUARANTY ISSUANCE AGREEMENT. The Guaranty
Issuance Agreement is, subject to Section 2 hereof, hereby amended as follows:

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

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

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

         SECTION 3. MISCELLANEOUS.

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

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

         (c) The execution, delivery and effectiveness of this Amendment shall
not, except as expressly provided herein, operate as a waiver of any right,
power, or remedy which AMSC Parent, AMSC Acquisition or any Guarantor hereto may
have under the Guaranty Issuance Agreement.

         (d) This Amendment may be executed in any number of counterparts and by
different parties hereto in separate counterparts, each of which when so
executed and delivered, shall be deemed to be an original and all of which taken
together shall constitute but one and the same instrument.

         (e) Each of AMSC Parent and AMSC Acquisition jointly and severally
agrees that it will, upon demand, pay to each Guarantor the amount of any and
all reasonable expenses, including, without limitation, the reasonable fees and
expenses of such Guarantor's counsel and of any experts and agents, which such
Guarantor may incur in connection with the negotiation, preparation or
administration of this Amendment.

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

                                        2
<PAGE>

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


AMSC ACQUISITION                          SINGAPORE
COMPANY, INC.                             TELECOMMUNICATIONS, LTD.


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


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


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

HUGHES ELECTRONICS
CORPORATION


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

                                        3


                                   EXHIBIT XIX

     American Mobile

     Randy Segal                                            Direct: 703-758-6130
     Senior Vice President, General Counsel                 Fax:    703-758-6134
     and Secretary


March 22, 1999

Mr. Brian R. Winikoff
Hughes Electronics Corporation
200 North Sepulveda Boulevard
P. O. Box 956
MSA 148
El Segundo, CA 90245-0956

Dear Brian:

Reference is made to the Amended and Restated Registration Rights Agreement
dated as of March 31, 1998 (the "Registration Rights Agreement") among American
Mobile Satellite Corporation ("American Mobile"), Hughes Electronics Corporation
("Hughes"), Singapore Telecommunications, Ltd. ("SingTel") and Baron Capital
Partners, L.P. (collectively, the "Guarantors"). Capitalized terms used herein
without definition shall have the respective meanings set forth in the 
Registration Rights Agreement.

In consideration of Hughes' agreement with American Mobile not to exercise its
rights to "piggyback" on the Registration Statement on Form S-3 anticipated to
become effective March 31, 1999, American Mobile agrees with Hughes to amend the
Registration Rights Agreement as follows:

1.       To extend the expiration date for exercise of the demand registration
         rights in Section 2.1(a) by two years, i.e. from March 31, 2005 to
         March 31, 2007.

2.       To clarify and confirm that the rights provided in the Registration
         Rights Agreement are assignable by the Guarantors provided such
         assignee agrees to become a party to the Registration Rights Agreement.
<PAGE>

Mr. Brian R. Winikoff
March 22, 1999
Page Two

3.       To provide for one additional Demand Registration that may be exercised
         only by Hughes or its assignee. This additional Demand Registration
         Right may be exercised only after the two Demand Registration Rights
         presently contemplated under the Registration Rights Agreement has been
         exercised or if the Guarantors other than Hughes (or assignees thereof)
         no longer own any Registrable Securities. If this additional demand
         registration right is exercised, the Guarantors other than Hughes (or
         assignees thereof) will have piggyback registration rights for any
         Registrable Securities they own in accordance with Section 2.2 of the
         Registration Rights Agreement.

Very truly yours,

AMERICAN MOBILE SATELLITE CORPORATION


/s/ Randy S. Segal
- ------------------
By:     Randy S. Segal
Title:  Senior Vice President

Acknowledged and Agreed as of this 22nd day of March, 1999


SINGAPORE TELECOMMUNICATIONS, LTD.


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

BARON CAPITAL PARTNERS, L.P.

By:     Baron Capital Management, Inc.,
        a General Partner


/s/ Morty Schaja
- ----------------
By:     Morty Schaja
Title:  Senior Vice President & Chief Operating Officer


                                   EXHIBIT XX

                               AMENDMENT NO. 2 TO
                           GUARANTY ISSUANCE AGREEMENT

         AMENDMENT, dated as of March 29, 1999 (this "Amendment"), by and among
HUGHES ELECTRONICS CORPORATION, a Delaware corporation ("Hughes"), SINGAPORE
TELECOMMUNICATIONS LTD., a Singapore corporation ("SingTel"), BARON CAPITAL
PARTNERS, L.P., a Delaware limited partnership ("Baron," and collectively with
Hughes and SingTel, the Guarantors"), AMERICAN MOBILE SATELLITE CORPORATION, a
Delaware corporation ("AMSC Parent"), and AMSC ACQUISITION COMPANY, INC., a
Delaware corporation and a wholly-owned subsidiary of AMSC Parent ("AMSC
Acquisition"), to the Guaranty Issuance Agreement dated as of March 31, 1998
(said Agreement, as the same may be amended, supplemented or otherwise modified
from time to time, being the "Guaranty Issuance Agreement", and the terms
defined therein being used herein as therein defined unless otherwise defined
herein), by and among each of the Guarantors, AMSC Parent and AMSC Acquisition.

                              W I T N E S S E T H:

         WHEREAS, AMSC Parent and AMSC Acquisition wish to eliminate the
financial covenants contained in the Guaranty Issuance Agreement and the
Guarantors are willing to accept the elimination of such covenants, subject to
the terms and conditions set forth herein;

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

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

         SECTION 2. AMENDMENTS TO SECTION 3 OF THE GUARANTY ISSUANCE AGREEMENT.
Section 3 of the Guaranty Issuance Agreement is hereby amended as follows:

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

         (b) AMENDMENT TO SECTION 3(b). Section 3(b) thereof is amended by
deleting the phrase ", Section 3(a)" from Section 3(b)(2).
<PAGE>

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

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

         SECTION 3. EFFECTIVENESS. This Amendment shall become effective as of
the date first set forth above upon the execution of a counterpart hereof by
each of the Guarantors, AMSC Parent and AMSC Acquisition.

         SECTION 4. MISCELLANEOUS.

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

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

         (c) The execution, delivery and effectiveness of this Amendment shall
not, except as expressly provided herein, operate as a waiver of any right,
power, or remedy which AMSC Parent, AMSC Acquisition or any Guarantor hereto may
have under the Guaranty Issuance Agreement.

         (d) This Amendment may be executed in any number of counterparts and by
different parties hereto in separate counterparts, each of which when so
executed and delivered, shall be deemed to be an original and all of which taken
together shall constitute but one and the same instrument.

         (e) Each of AMSC Parent and AMSC Acquisition acknowledges its joint and
several obligation, under Section 4 of the Guaranty Issuance Agreement, to pay,
upon demand, to each Guarantor the amount of any and all reasonable expenses,
including, without limitation, the reasonable fees and expenses of such
Guarantor's counsel and of any experts and agents, which such Guarantor has
incurred or may incur in connection with the negotiation, preparation or
administration of this Amendment.

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

                                        2
<PAGE>

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

AMSC ACQUISITION COMPANY, INC.          

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


AMERICAN MOBILE SATELLITE CORPORATION   
                                        
By: /s/ Randy Segal                     
    ---------------                     
    Name:  Randy Segal                  
    Title: Senior Vice President        
                                        

HUGHES ELECTRONICS CORPORATION

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


SINGAPORE TELECOMMUNICATIONS, LTD.    
                                      
By: /s/ Hoh Wing Chee                 
    -----------------                 
    Name:  Hoh Wing Chee              
    Title: CEO (International Network)
                                      
                                      
BARON CAPITAL PARTNERS, L.P.,         
a Delaware limited partnership        
                                      
By: Baron Capital Management, Inc.,   
    a General Partner                 
                                      
By: /s/ Ronald Baron                  
    ----------------                  
    Name:  Ronald Baron               
    Title: Chief Executive Officer &  
           Chairman                   

                                        3


                                   EXHIBIT XXI

                                 AMENDMENT NO. 3
              TO WARRANT CERTIFICATES FOR THE PURCHASE OF SHARES OF
              COMMON STOCK OF AMERICAN MOBILE SATELLITE CORPORATION


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

                              W I T N E S S E T H:

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

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

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

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

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

         NOW THEREFORE, the undersigned parties hereto agree as follows:

         SECTION 1. AMENDMENT.

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

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

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

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

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


AMERICAN MOBILE SATELLITE                 SINGAPORE
CORPORATION                               TELECOMMUNICATIONS LTD.

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


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

By:  Baron Capital Management, Inc.       By: /s/ Mark A. McEachen
     a General Partner                        -------------------- 
                                              Name:  Mark A. McEachen
                                              Title: Corporate V.P. & Treasurer
By: /s/ Ronald Baron
    ----------------
    Name:  Ronald Baron
    Title: Chief Executive Officer
           & Chairman

                                        2


                                  EXHIBIT XXII

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


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

                              W I T N E S S E T H:

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

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

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

         NOW THEREFORE, the undersigned parties hereto agree as follows:

         SECTION 1. AMENDMENT.

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

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

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

         SECTION 3. ISSUANCE OF REPLACEMENT WARRANT. Upon the request of any
Holder, the Company promptly shall issue a new Warrant, incorporating the
amendments effected hereby to replace the presently outstanding Warrant held by
such Holder.
<PAGE>

                                                                               2

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


AMERICAN MOBILE SATELLITE                 SINGAPORE
CORPORATION                               TELECOMMUNICATIONS LTD.

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


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

By:  Baron Capital Management, Inc.       By: /s/ Mark A. McEachen
     a General Partner                        -------------------- 
                                              Name:  Mark A. McEachen
                                              Title: Corporate V.P. & Treasurer
By: /s/ Ronald Baron
    ----------------
    Name:  Ronald Baron
    Title: Chief Executive Officer
           & Chairman

                                        2


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