AMERICAN MOBILE SATELLITE CORP
S-3/A, 1999-07-26
COMMUNICATIONS SERVICES, NEC
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<PAGE>   1


     AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JULY 26, 1999.


                                                      REGISTRATION NO. 333-81459
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                            ------------------------


                               AMENDMENT NO. 2 TO


                                    FORM S-3
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------

                           AMERICAN MOBILE SATELLITE

                                  CORPORATION
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)

<TABLE>
<S>                                                 <C>
                     DELAWARE                                           93-0976127
          (STATE OR OTHER JURISDICTION OF                 (I.R.S. EMPLOYEE IDENTIFICATION NUMBER)
          INCORPORATION OR ORGANIZATION)
</TABLE>

                           10802 PARKRIDGE BOULEVARD
                          RESTON, VIRGINIA 20191-5416
                                 (703) 758-6000
         (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING
            AREA CODE, OF REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)
                            ------------------------

                                 RANDY S. SEGAL
                         SENIOR VICE PRESIDENT, GENERAL
                             COUNSEL AND SECRETARY
                     AMERICAN MOBILE SATELLITE CORPORATION
                           10802 PARKRIDGE BOULEVARD
                          RESTON, VIRGINIA 20191-5416
                                 (703) 758-6000
           (NAME, ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER,
                   INCLUDING AREA CODE, OF AGENT FOR SERVICE)
                            ------------------------

<TABLE>
<S>                                                 <C>
                                              COPIES TO:
                 ROBERT H. WINTER                                    GREGORY A. EZRING
                  ARNOLD & PORTER                                    LATHAM & WATKINS
               555 12TH STREET, N.W.                                 885 THIRD AVENUE
             WASHINGTON, DC 20004-1202                              NEW YORK, NY 10022
                   (202)942-5000                                       (212)906-1200
</TABLE>

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

     APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC:  As soon
as practicable after the effective date of this Registration Statement.

     If the only securities being registered on this Form are being offered
pursuant to dividend or interest reinvestment plans, please check the following
box.  [ ]

     If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933 (as defined below), other than securities offered only in connection with
dividend or interest reinvestment plans, check the following box.  [ ]

     If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering.  [ ]
- ---------------

     If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering.  [ ]
- ---------------

     If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box.  [ ]
                            ------------------------

     THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR
DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL
FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION
STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(a) OF
THE SECURITIES ACT OR UNTIL THIS REGISTRATION STATEMENT SHALL BECOME EFFECTIVE
ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID
SECTION 8(a), MAY DETERMINE.
- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>   2

                                    PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION.

     The following are the estimated expenses to be incurred in connection with
the issuance and distribution of the securities being registered.

<TABLE>
<S>                                                           <C>
SEC registration fee........................................  $ 41,000
Printing and engraving expenses.............................   200,000
Legal fees and expenses.....................................   250,000
Blue Sky fees and expenses..................................    50,000
NASD listing and filing fees................................    32,500
Accounting fees and expenses................................   200,000
Transfer agent fees.........................................    11,500
Total.......................................................  $785,000
                                                              ========
</TABLE>

ITEM 15. INDEMNIFICATION OF DIRECTORS AND OFFICERS.

     The Company's Bylaws provide that the Company will indemnify its directors
and officers to the fullest extent permitted by Delaware law. The Company may be
required to advance litigation expenses in the case of stockholder derivative
actions or other actions, against an undertaking by the indemnified party to
repay such advances if it is ultimately determined that the indemnified party is
not entitled to indemnification.

     In addition, the Company's Certificate of Incorporation provides that,
pursuant to Delaware law, its directors shall not be liable for monetary damages
for breach of the directors' fiduciary duty of care to the corporation and its
stockholders. This provision in the Certificate of Incorporation does not
eliminate the duty of care, and in appropriate circumstances equitable remedies
such as injunctive or other forms of nonmonetary relief will remain available
under Delaware law. In addition, each director will continue to be subject to
liability for breach of the director's duty of loyalty to the Company for acts
or omissions not in good faith or involving intentional misconduct, for knowing
violations of law, for actions leading to improper personal benefit to the
director, and for payment of dividends or approval of stock repurchases or
redemptions that are unlawful under Delaware law. The provision also does not
affect a director's responsibilities under any other law, such as the federal
securities laws.

     At present, there is no pending litigation or proceeding involving an
officer or director of the Company as to which indemnification is being sought,
nor is the Company aware of any threatened litigation that may result in claims
for indemnification by any officer or director.

                                      II-1
<PAGE>   3

ITEM 16. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.

     (a) Exhibits


<TABLE>
<C>    <S>
   1   Form of Underwriting Agreement among American Mobile
       Satellite Corporation, Bear, Stearns & Co. Inc., Credit
       Suisse First Boston Corporation, Deutsche Bank Securities
       Inc. and SoundView Technology Group, Inc.
   3.1 Restated Certificate of Incorporation of the Company (as
       restated effective May 1, 1996) (incorporated by reference
       to Exhibit 3.1a to the Company's Quarterly Report on Form
       10-Q filed for the periods ending March 31, 1996 and June
       30, 1996 (File No. 0-23044)).
   3.2 Amended and Restated Bylaws of the Company (effective as of
       May 26, 1999).*
   5   Opinion of Arnold & Porter.*
  23.1 Consent of Arthur Andersen LLP, independent public
       accountants.*
  23.2 Consent of KPMG LLP, independent certified public
       accountants.*
  23.3 Consent of Arnold & Porter (included in Exhibit 5 to this
       registration statement).*
  23.4 Consent of The Strategis Group.*
  24   Powers of Attorney of directors and officers of the
       Company.*
  27.1 Financial Data Schedule (Year ended December 31, 1998).*
  27.2 Financial Data Schedule (Quarter Ended March 31, 1999).*
  99.1 Exchange Agreement, dated June 7, 1999, by and among
       American Mobile Satellite Corporation, XM Satellite Radio
       Holdings Inc. and WorldSpace, Inc.*
  99.2 Shareholders Agreement, dated as of July 7, 1999, by and
       among XM Satellite Radio Holdings Inc., American Mobile
       Satellite Corporation, Baron Asset Fund, Clear Channel
       Investments, Inc., Columbia XM Radio Partners, LLC, DIRECTV
       Enterprises, Inc., General Motors Corporation, Madison
       Dearborn Capital Partners III, L.P., Special Advisors Fund
       I, LLC, Madison Dearborn Special Equity III, L.P., and
       Telcom-XM Investors, L.L.C.*
  99.3 Registration Rights Agreement, dated July 7, 1999, by and
       among XM Satellite Radio Holdings Inc., American Mobile
       Satellite Corporation, the Baron Asset Fund series of Baron
       Asset Fund, and the holders of Series A subordinated
       convertible notes of XM Satellite Radio Holdings Inc.*
  99.4 Note Purchase Agreement, dated June 7, 1999, by and between
       XM Satellite Radio Holdings Inc., XM Satellite Radio Inc.,
       Clear Channel Communications, Inc., DIRECTV Enterprises,
       Inc., General Motors Corporation, Telcom-XM Investors,
       L.L.C., Columbia XM Radio Partners, LLC, Madison Dearborn
       Capital Partners III, L.P., Madison Dearborn Special Equity
       III, L.P., and Special Advisors Fund I, LLC (including form
       of Series A subordinated convertible note of XM Satellite
       Radio Holdings Inc. attached as Exhibit A thereto).*
</TABLE>


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


* Previously filed.


ITEM 17. UNDERTAKINGS.

     The undersigned registrant hereby undertakes that:

     (1) For purposes of determining any liability under the Securities Act of
         1933, the information omitted from the form of prospectus filed as part
         of this registration statement in reliance upon Rule 430A and contained
         in a form of prospectus filed by the registrant pursuant to Rule
         424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to
         be part of this registration statement as of the time it was declared
         effective.

                                      II-2
<PAGE>   4

     (2) For the purpose of determining any liability under the Securities Act
         of 1933, each post-effective amendment that contains a form of
         prospectus shall be deemed to be a new registration statement relating
         to the securities offered therein, and the offering of such securities
         at that time shall be deemed to be the initial bona fide offering
         thereof.

     The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) that is incorporated by reference in the registration statement shall be
deemed to be a new registration statement related to securities offered therein,
and the offering of such securities at that time shall be deemed to be the
initial bona fide offering thereof.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
registrant pursuant to the General Corporation Law of the State of Delaware, the
Restated Certificate of Incorporation, as amended, or the Amended and Restated
Bylaws of registrant, indemnification agreements entered into between registrant
and its officers and directors, or otherwise, the registrant has been advised
that in the opinion of the Securities and Exchange Commission such
indemnification is against public policy as expressed in the Act and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the registrant of expenses incurred
or paid by a director, officer, or controlling person of the registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Act and will be governed by the final adjudication of
such issue.

                                      II-3
<PAGE>   5

                                   SIGNATURES


     Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this Amendment No. 2 to
the registration statement to be signed on its behalf by the undersigned,
thereunto duly authorized in the County of Fairfax, Commonwealth of Virginia, on
the 26th day of July, 1999.


                                          AMERICAN MOBILE SATELLITE
                                            CORPORATION


                                          By:      /s/ RANDY S. SEGAL

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

                                              Name:  Randy S. Segal


                                              Title: Senior Vice President and
                                                     General Counsel



     Pursuant to the requirements of the Securities Act of 1933, this Amendment
No. 2 to the Registration Statement has been signed below by the following
persons in the capacities and on the dates indicated:



<TABLE>
<CAPTION>
                     SIGNATURE                                     TITLE                    DATE
                     ---------                                     -----                    ----
<C>                                                  <S>                                <C>

                         *                           President and Chief Executive      July 26, 1999
- ---------------------------------------------------  Officer, and Director (Principal
              Walter V. Purnell, Jr.                 Executive Officer)

                         *                           Senior Vice President and Chief    July 26, 1999
- ---------------------------------------------------  Financial Officer (Principal
                 W. Bartlett Snell                   Financial and Accounting Officer)

                         *                           Chairman of the Board of           July 26, 1999
- ---------------------------------------------------  Directors
                  Gary M. Parsons

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                Douglas I. Brandon

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                  Pradeep P. Kaul

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                 Billy J. Parrott

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                Andrew A. Quartner
</TABLE>


                                      II-4
<PAGE>   6


<TABLE>
<CAPTION>
                     SIGNATURE                                     TITLE                    DATE
                     ---------                                     -----                    ----
<C>                                                  <S>                                <C>
                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                   Jack A. Shaw

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
             Roderick M. Sherwood, III

                         *                           Director                           July 26, 1999
- ---------------------------------------------------
                 Michael T. Smith

              *By: /s/ RANDY S. SEGAL
   ---------------------------------------------
               Name: Randy S. Segal
             Title:  Attorney-in-fact
</TABLE>


                                      II-5

<PAGE>   1
                                                                       EXHIBIT 1

                        7,000,000 SHARES OF COMMON STOCK

                      AMERICAN MOBILE SATELLITE CORPORATION

                             UNDERWRITING AGREEMENT

                                                                  July ___, 1999

Bear, Stearns & Co. Inc.
Credit Suisse First Boston Corporation
Deutsche Bank Securities Inc.
SoundView Technology Group, Inc.
c/o  Bear, Stearns & Co. Inc.
     245 Park Avenue
     New York, New York 10167

Ladies and Gentlemen:

              American Mobile Satellite Corporation, a Delaware corporation (the
"Company"), proposes, subject to the terms and conditions stated herein, to
issue and sell to the several underwriters named in Schedule I hereto
(collectively, the "Underwriters") an aggregate of 7,000,000 shares (the "Firm
Shares") of its common stock, par value $0.01 per share (the "Common Stock"). In
addition, for the sole purpose of covering over-allotments in connection with
the sale of the Firm Shares, Motorola, Inc., a Delaware corporation
("Motorola"), proposes, at the option of the Underwriters, to sell to the
Underwriters up to an additional 1,050,000 shares (the "Additional Shares") of
Common Stock owned by Motorola. The Firm Shares and any Additional Shares
purchased by the Underwriters are referred to herein as the "Shares". The Shares
are more fully described in the Registration Statement referred to below.

              1.     Representations and Warranties of the Company and Motorola.

              (a) The Company represents and warrants to, and agrees with, each
       of the Underwriters that:

              (i) The Company has filed with the Securities and Exchange
       Commission (the "Commission") a registration statement on Form S-3 (No.
       333-81459), and one or more amendments thereto, and related preliminary
       prospectuses for the registration under the Securities Act of 1933, as
       amended (the "Securities Act"), of shares of Common Stock, which
       registration statement, as so amended, has been declared effective by the
       Commission and copies of which have heretofore been delivered to the
       Underwriters. The registration statement, as amended at the time it
       became effective, including the exhibits and information (if any) deemed
       to be part of the registration statement at the time of effectiveness
       pursuant to Rule 430A under the Securities Act and the documents
       incorporated by reference therein pursuant to Item 12 of

<PAGE>   2

       Form S-3 which were filed under the Securities Exchange Act of 1934, as
       amended (the "Exchange Act"), is hereinafter referred to as the
       "Registration Statement". If the Company has filed or is required
       pursuant to the terms hereof to file a registration statement pursuant to
       Rule 462(b) under the Securities Act registering additional shares of
       Common Stock (a "Rule 462(b) Registration Statement"), then, unless
       otherwise specified, any reference herein to the term "Registration
       Statement" shall be deemed to include such Rule 462(b) Registration
       Statement. Other than a Rule 462(b) Registration Statement, which became
       effective upon filing, no other document with respect to the Registration
       Statement has heretofore been filed with the Commission (other than
       prospectuses filed pursuant to Rule 424(b) of the rules and regulations
       of the Commission under the Securities Act (the "Securities Act
       Regulations"), each in the form heretofore delivered to the
       Underwriters). No stop order suspending the effectiveness of either the
       Registration Statement or the Rule 462(b) Registration Statement, if any,
       has been issued and no proceeding for that purpose has been initiated or,
       to the Company's knowledge, threatened by the Commission. The Company, if
       required by the Securities Act Regulations, proposes to file the
       Prospectus (as defined below) with the Commission pursuant to Rule 424(b)
       of the Securities Act Regulations. The Prospectus, in the form in which
       it is to be filed with the Commission pursuant to Rule 424(b) of the
       Securities Act Regulations, is hereinafter referred to as the
       "Prospectus", except that, subject to Section 4(a) below, if any revised
       prospectus or prospectus supplement shall be provided to the Underwriters
       by the Company for use in connection with the offering and sale of the
       Shares (the "Offering") which differs from the Prospectus (whether or not
       such revised prospectus or prospectus supplement is required to be filed
       by the Company pursuant to Rule 424(b) of the Securities Act
       Regulations), the term "Prospectus" shall refer to such revised
       prospectus or prospectus supplement, as the case may be, from and after
       the time it is first provided to the Underwriters for such use. Any
       preliminary prospectus or prospectus subject to completion included in
       the Registration Statement or filed with the Commission pursuant to Rule
       424 under the Securities Act is hereafter called a "Preliminary
       Prospectus". All references in this Agreement to the Registration
       Statement, the Rule 462(b) Registration Statement, a Preliminary
       Prospectus and the Prospectus, or any amendments or supplements to any of
       the foregoing, shall be deemed to refer to and include (i) any document
       which is incorporated therein by reference, (ii) the filing of any
       document under the Exchange Act after the effective date of the
       Registration Statement, the date of such Preliminary Prospectus or the
       date of the Prospectus, as the case may be, which is incorporated therein
       by reference, and (iii) any copy thereof filed with the Commission
       pursuant to its Electronic Data Gathering, Analysis and Retrieval System
       ("EDGAR").

              (ii) The Registration Statement and the Prospectus, at the time
       the Registration Statement became effective and as of the Closing Date
       referred to in Section 2 hereof, complied and will comply in all material
       respects with the requirements of the Securities Act and the Exchange Act
       and the respective rules and regulations thereunder, and did not and as
       of the Closing Date will not contain any 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, in the light of the
       circumstances under which they were made, not misleading. The Prospectus,
       as of the date hereof (unless the term "Prospectus" refers to a
       prospectus which has been provided to the Underwriters by the Company for
       use in connection with the offering of the Shares which differs from the
       Prospectus filed with the Commission pursuant to Rule 424(b) of the
       Securities Act Regulations, in which case at the time it is first
       provided to the Underwriters for such use) and on the Closing Date, does
       not and will not include any untrue statement of a material fact or omit
       to state a material fact necessary to make the statements therein, in the
       light of the circumstances under which they were made, not misleading;
       provided, however, that the representations and



                                       2
<PAGE>   3

       warranties in this Section (1)(ii) shall not apply to statements in or
       omissions from the Registration Statement or Prospectus made in reliance
       upon and in conformity with information relating to any Underwriter
       furnished to the Company in writing by any Underwriter (as described in
       Section 12 hereof) expressly for use in the Registration Statement or the
       Prospectus. The information contained in the Preliminary Prospectus and
       Prospectus filed as part of the Registration Statement, as part of any
       amendment thereto or pursuant to Rule 424 under the Securities Act
       Regulations, if filed by electronic transmission pursuant to Regulation
       S-T under the Securities Act, was identical to the information contained
       in each copy thereof delivered to the Underwriters for use in connection
       with the Offering (except as may be permitted by Regulation S-T under the
       Securities Act). There are no contracts or other documents required to be
       described in the Prospectus or to be filed as exhibits to the
       Registration Statement under the Securities Act or the Exchange Act that
       have not been described or filed therein as required, and there are no
       business relationships or related-party transactions involving the
       Company or any of its subsidiaries or any other person required to be
       described in the Prospectus that have not been described therein as
       required.

              (iii) Each of the Company and its material subsidiaries listed in
       Schedule II hereto (A) has been duly incorporated and is validly existing
       as a corporation in good standing under the laws of its respective
       jurisdiction of incorporation, (B) has all requisite corporate power and
       authority to carry on its business as it is currently being conducted and
       as described in the Prospectus and to own, lease and operate its
       properties, and (C) is duly qualified and in good standing as a foreign
       corporation authorized to do business in each jurisdiction in which the
       nature of its business or its ownership or leasing of property requires
       such qualification except where the failure to be so qualified could not
       reasonably be expected to (x) result, individually or in the aggregate,
       in a material adverse effect on the properties, business, results of
       operations, condition (financial or otherwise) or affairs of the Company
       and its subsidiaries, taken as a whole, (y) interfere with or materially
       adversely affect the issuance or sale of the Shares pursuant hereto or
       (z) in any manner materially adversely affect the validity of this
       Agreement or the transactions described in the Prospectus under the
       captions "How We Intend to Use the Proceeds from the Offering" or "The XM
       Radio Transactions" (any of the events set forth in clauses (x), (y) or
       (z), a "Material Adverse Effect").

              (iv) All of the outstanding shares of capital stock of the Company
       have been duly authorized, validly issued, and are fully paid and
       nonassessable and were not issued in violation of any preemptive or
       similar rights. The Shares, when issued, delivered and sold in accordance
       with this Agreement, will be duly authorized and validly issued, fully
       paid and nonassessable, and will not have been issued in violation of or
       subject to any preemptive or similar rights. At March 31, 1999, after
       giving effect to the issuance and sale of the Shares pursuant hereto and
       the application of the net proceeds from the sale thereof, the Company
       had the pro forma consolidated capitalization as set forth in the
       Prospectus under the caption "Capitalization".

              (v) Except as disclosed in the Prospectus, as of the dates
       indicated therein and as of the Closing Date, all of the outstanding
       capital stock of, or other ownership interests in, the Company's
       subsidiaries is owned by the Company, free and clear of any security
       interest, claim, lien, limitation on voting rights or encumbrance; and
       all such securities have been duly authorized, validly issued, and are
       fully paid and nonassessable and were not issued in violation of any
       preemptive or similar rights.



                                       3
<PAGE>   4

              (vi) Except as disclosed in the Prospectus, there are not
       currently, and will not be as a result of the Offering, any outstanding
       subscriptions, rights, warrants, calls, commitments of sale or options to
       acquire or instruments convertible into or exchangeable for, any capital
       stock or other equity interest of the Company or any of its subsidiaries.

              (vii) The Common Stock (including the Shares) is registered
       pursuant to Section 12(g) of the Exchange Act and is listed for quotation
       on the Nasdaq National Market System ("Nasdaq"), and the Company has
       taken no action designed to, or likely to have the effect of, terminating
       the registration of the Common Stock under the Exchange Act or delisting
       the Common Stock from Nasdaq, nor has the Company received any
       notification that the Commission or Nasdaq is contemplating terminating
       such registration or listing.

              (viii) The Company has all requisite corporate power and authority
       to execute, deliver and perform its obligations under this Agreement and
       to consummate the transactions contemplated hereby, including, without
       limitation, the corporate power and authority to issue, sell and deliver
       the Shares as provided herein and the power to effect the Use of Proceeds
       as described in the Prospectus.

              (ix) The statistical and market-related data included in the
       Prospectus are based on or derived from sources which the Company
       believes to be reliable and accurate in all material respects.

              (x) This Agreement has been duly and validly authorized, executed
       and delivered by the Company and, assuming due and valid authorization,
       execution and delivery by Motorola and the Underwriters, is the legal,
       valid and binding agreement of the Company, enforceable against the
       Company in accordance with its terms, except as such enforceability may
       be limited by bankruptcy, insolvency, reorganization, moratorium and
       other similar laws relating to or affecting creditor's rights generally,
       by general equitable principles (regardless of whether such
       enforceability is considered in a proceeding in equity or at law) and as
       limited by the unenforceability under certain circumstances under law or
       court decisions of provisions providing for the indemnification of or
       contribution to a party with respect to a liability where such
       indemnification or contribution is contrary to public policy.

              (xi) Neither the Company nor any of its subsidiaries is, nor after
       giving effect to the Offering will be, (A) in violation of its charter or
       bylaws, (B) in default in the performance of any bond, debenture, note,
       indenture, mortgage, deed of trust or other agreement or instrument to
       which it is a party or by which it is bound or to which any of its
       properties is subject which, singly or in the aggregate, could reasonably
       be expected to have a Material Adverse Effect, or (C) in violation of any
       local, state or federal law, statute, ordinance, rule, regulation,
       requirement, judgment or court decree (including, without limitation, the
       Communications Act of 1934 (the "Communications Act") and the rules and
       regulations of the Federal Communications Commission (the "FCC"), and
       environmental laws, statutes, ordinances, rules, regulations, judgments
       or court decrees) applicable to the Company or any of its subsidiaries or
       any of their assets or properties (whether owned or leased) which, singly
       or in the aggregate, could reasonably be expected to have a Material
       Adverse Effect. To the best knowledge of the Company, there exists no
       condition that, with notice, the passage of time or otherwise, would
       constitute a default under any such document or instrument which could
       reasonably be expected to have a Material Adverse Effect.



                                       4
<PAGE>   5

              (xii) Except as disclosed in the Prospectus, none of (a) the
       execution, delivery or performance by the Company of this Agreement, (b)
       the issuance and sale of the Shares and (c) the consummation by the
       Company of the transactions contemplated hereby and in the Prospectus
       under the heading "The XM Radio Transactions", violate, conflict with or
       constitute a breach of any of the terms or provisions of, or a default
       under (or an event that with notice or the lapse of time, or both, would
       constitute a default), or require consent (other than consents that have
       already been obtained) under, or result in the imposition of a lien on
       any properties of the Company or any of its subsidiaries, or an
       acceleration of any indebtedness of the Company or any of its
       subsidiaries pursuant to, (1) the charter or bylaws of the Company or any
       of its subsidiaries, (2) any bond, debenture, note, indenture, mortgage,
       deed of trust or other agreement or instrument to which the Company or
       any of its subsidiaries is a party or by which the Company or its
       subsidiaries or their properties is or may be bound, (3) any material
       statute, rule or regulation applicable to the Company or any of its
       subsidiaries or any of their assets or properties or (4) any judgment,
       order or decree of any court or governmental agency or authority having
       jurisdiction over the Company or any of its subsidiaries or any of their
       assets or properties. No consent, approval, authorization or order of, or
       filing, registration, qualification, license or permit of or with, (A)
       any court or governmental agency, body or administrative agency or (B)
       any other person is required for (1) the execution, delivery and
       performance by the Company of this Agreement or (2) the issuance and sale
       of the Shares and the transactions contemplated hereby and thereby,
       except such as have been or will be obtained and made on or prior to the
       Closing Date under the Securities Act, the Exchange Act and state
       securities or Blue Sky laws and regulations or such as may be required by
       the National Association of Securities Dealers, Inc. (the "NASD").

              (xiii) There is (A) no action, suit or proceeding before or by any
       court, arbitrator or governmental agency, body or official, domestic or
       foreign, now pending or, to the best knowledge of the Company, threatened
       or contemplated to which the Company or any of its subsidiaries is a
       party or to which the business or property of the Company or any of its
       subsidiaries may be subject, (B) no statute, rule, regulation or order
       that has been enacted, adopted or issued by any governmental agency or
       that has been proposed by any governmental body and (C) no injunction,
       restraining order or order of any nature by a federal or state court or
       foreign court of competent jurisdiction to which the Company or any of
       its subsidiaries is or may be subject or to which the business, assets,
       or property of the Company or any of its subsidiaries are or may be
       subject, that, in the case of clauses (A), (B) and (C) above, (1) is
       required to be disclosed in the Prospectus and that is not so disclosed,
       or (2) could reasonably be expected to result in a Material Adverse
       Effect.

              (xiv) To the Company's knowledge, no action has been taken and no
       statute, rule, regulation or order has been enacted, adopted or issued by
       any governmental agency that prevents the issuance of the Shares or
       prevents or suspends the use of the Prospectus; no injunction,
       restraining order or order of any nature by a federal or state court of
       competent jurisdiction has been issued that prevents the issuance of the
       Shares or prevents or suspends the sale of the Shares in any jurisdiction
       referred to in Section 4(d) hereof; and every request made to the Company
       by any securities authority or agency of any jurisdiction for additional
       information has been complied with in all material respects.

              (xv) The Company has delivered to the Underwriters or otherwise
       made available to them true and correct copies of all material documents
       and agreements, including all amendments. modifications or waivers
       thereto, related to its subsidiaries, including XM Satellite



                                       5
<PAGE>   6

       Radio Holdings, Inc. and XM Satellite Radio, Inc., and all such documents
       and agreements are in effect, except as could not reasonably be expected
       to have a Material Adverse Effect.

              (xvi) The entities listed on Schedule II hereto are the only
       material subsidiaries of the Company.

              (xvii) To the best knowledge of the Company, no collective
       bargaining organizing activities are taking place with respect to the
       Company or any of its subsidiaries. None of the Company or any of its
       subsidiaries has violated (A) any federal, state or local law or foreign
       law relating to discrimination in hiring, promotion or pay of employees,
       (B) any applicable wage or hour laws or (C) any provision of the Employee
       Retirement Income Security Act of 1974, as amended ("ERISA"), or the
       rules and regulations thereunder, except those violations, in the case of
       (A), (B) or (C) that could not reasonably be expected to have a Material
       Adverse Effect.

              (xviii) None of the Company or any of its subsidiaries has
       violated any foreign, federal, state or local law or regulation relating
       to the protection of human health and safety, the environment or
       hazardous or toxic substances or wastes, pollutants or contaminants
       ("Environmental Laws") except those violations that could not reasonably
       be expected to have a Material Adverse Effect.

              (xix) Each of the Company and its subsidiaries has such permits,
       licenses, franchises and authorizations of governmental or regulatory
       authorities ("permits"), including, without limitation, under any
       applicable Environmental Laws, as are necessary to own, lease and operate
       their respective properties and to conduct their businesses except where
       the failure to have such permits could not reasonably be expected to
       result in a Material Adverse Effect; except as set forth in the
       Prospectus, each of the Company and its subsidiaries has fulfilled and
       performed all of its obligations with respect to such permits and no
       event has occurred which allows, or after notice or lapse of time would
       allow, revocation or termination thereof or results in any other material
       impairment of the rights of the holder of any such permit; and, except as
       described in the Prospectus, such permits contain no restrictions that
       are materially burdensome to the Company or such subsidiary, as the case
       may be.

              (xx) Each of the Company and its subsidiaries has (A) title to all
       of the properties and assets described in the Prospectus as owned by it,
       free and clear of all liens, charges, encumbrances and restrictions
       (except for Permitted Liens (as defined in the indenture governing the
       Company's 12 1/4% Senior Notes due 2008)), taxes not yet payable, and
       general regulatory restrictions) except as could not reasonably be
       expected to result in a Material Adverse Effect and (B) peaceful and
       undisturbed possession under all material leases to which any of them is
       a party as lessee and each of which lease is valid and binding and no
       default exists thereunder, except for defaults that could not reasonably
       be expected to have a Material Adverse Effect. All material leases to
       which the Company or any of its subsidiaries is a party are valid and
       binding and no default by the Company or such subsidiary, as the case may
       be, has occurred and is continuing thereunder and, to the best knowledge
       of the Company, no material defaults by the landlord are existing under
       any such lease, except as could not reasonably be expected to have a
       Material Adverse Effect.

              (xxi) Except as disclosed in the Prospectus, each of the Company
       and its subsidiaries owns, possesses or has the right to employ all
       material patents, patent rights, licenses (including all FCC, state,
       local or other jurisdictional regulatory licenses), inventions,
       copyrights, know-



                                       6
<PAGE>   7

       how (including trade secrets and other unpatented and/or unpatentable
       proprietary or confidential information, software, systems or
       procedures), trademarks, service marks and trade names, inventions,
       computer programs, technical data and information (collectively, the
       "Intellectual Property") presently employed by it in connection with the
       businesses now operated by it or that are proposed to be operated by it
       free and clear of and without violating any right, claimed right, charge,
       encumbrance, pledge, security interest, restriction (other than
       regulatory restrictions) or lien of any kind of any other person, and
       none of the Company or any of its subsidiaries has received any notice of
       infringement of or conflict with asserted rights of others with respect
       to any of the foregoing, except as disclosed in the Prospectus. The use
       of the Intellectual Property in connection with the business and
       operations of the Company or any of its subsidiaries does not infringe on
       the rights of any person, except as could not reasonably be expected to
       have a Material Adverse Effect.

              (xxii) All material tax returns required to be filed by the
       Company or any of its subsidiaries in all jurisdictions have been so
       filed, except where failure to so file could not reasonably be expected
       to have a Material Adverse Effect. All taxes, including withholding
       taxes, penalties and interest, assessments, fees and other charges due or
       claimed to be due from such entities or that are due and payable have
       been paid, other than those being contested in good faith and for which
       adequate reserves have been provided or those currently payable without
       penalty or interest. To the knowledge of the Company, there are no
       material proposed additional tax assessments against the Company or any
       of its subsidiaries, or the assets or property of the Company or any of
       its subsidiaries, except those tax assessments for which adequate
       reserves have been established.

              (xxiii) Except as disclosed in the Prospectus, there are no
       holders of securities of the Company or any of its subsidiaries who, by
       reason of the execution by the Company of this Agreement to which it is a
       party or the consummation by the Company or any of its subsidiaries of
       the transactions contemplated hereby, have the right to request or demand
       that the Company or any of its subsidiaries register under the Securities
       Act or analogous foreign laws and regulations securities held by them.

              (xxiv) Each of the Company and its subsidiaries maintains a system
       of internal accounting controls sufficient to provide reasonable
       assurance that: (A) transactions are executed in accordance with
       management's general or specific authorizations; (B) transactions are
       recorded as necessary to permit preparation of financial statements in
       conformity with generally accepted accounting principles and to maintain
       accountability for assets; (C) access to assets is permitted only in
       accordance with management's general or specific authorization and (D)
       the recorded accountability for assets is compared with the existing
       assets at reasonable intervals and appropriate action is taken with
       respect thereto.

              (xxv) The Company carries in-orbit insurance with respect to
       MSAT-2. Except with respect to MSAT-2, each of the Company and its
       subsidiaries maintains insurance covering its properties, operations,
       personnel and businesses, insuring against such losses and risks as are
       consistent with industry practice to protect the Company and its
       subsidiaries and their respective businesses. None of the Company or any
       of its subsidiaries has received notice from any insurer or agent of such
       insurer that substantial capital improvements or other expenditures will
       have to be made in order to continue such insurance.



                                       7
<PAGE>   8

              (xxvi) None of the Company or any of its subsidiaries has (A)
       taken, directly or indirectly, any action designed to, or that might
       reasonably be expected to, cause or result in stabilization or
       manipulation of the price of any security of the Company or any of its
       subsidiaries to facilitate the sale or resale of the Common Stock or (B)
       since the date of the Preliminary Prospectus (1) sold, bid for, purchased
       or paid any person any compensation for soliciting purchases of the
       Common Stock or (2) paid or agreed to pay to any person any compensation
       for soliciting another to purchase any other securities of the Company or
       any of its subsidiaries.

              (xxvii) Subsequent to the respective dates as of which information
       is given in the Prospectus and up to the Closing Date, except as set
       forth in the Prospectus, (A) none of the Company or any of its
       subsidiaries has incurred any liabilities or obligations, direct or
       contingent, which are material, individually or in the aggregate, to the
       Company and its subsidiaries, taken as a whole, nor entered into any
       transaction not in the ordinary course of business, (B) there has not
       been any change or development which, singly or in the aggregate, could
       reasonably be expected to result in a Material Adverse Effect and (C)
       there has been no dividend or distribution of any kind declared, paid or
       made by the Company or its subsidiaries on any class of their capital
       stock.

              (xxviii) To the Company's knowledge, the accountants who have
       certified or will certify the financial statements included or to be
       included as part of the Prospectus are independent accountants as
       required by the Securities Act. Except as expressly stated in the letters
       delivered pursuant to Section 6(e) hereof, the historical financial
       statements, including those incorporated by reference, together with
       related schedules and notes thereto, comply as to form in all material
       respects with the requirements applicable to registration statements on
       Form S-3 under the Securities Act and under the Exchange Act, as
       applicable and present fairly in all material respects the financial
       position and results of operations of the Company and its subsidiaries,
       at the dates and for the periods indicated. Such financial statements
       have been prepared in accordance with generally accepted accounting
       principles applied on a consistent basis throughout the periods
       presented. The pro forma financial statements included in the Prospectus
       have been prepared on a basis consistent with such historical statements
       of the Company, except for the pro forma adjustments specified therein,
       and give effect to assumptions made on a reasonable basis and present
       fairly in all material respects the historical and proposed transactions
       described in the Prospectus; and such pro forma financial statements
       comply as to form in all material respects with the requirements
       applicable to pro forma financial statements included in registration
       statements on Form S-3 under the Securities Act, except as expressly
       stated therein. The other financial and statistical information and data
       included in the Prospectus, historical and pro forma, are accurately
       presented in all material respects and prepared on a basis consistent
       with the financial statements, historical and pro forma, included in the
       Prospectus and the books and records of the Company and its subsidiaries.

              (xxix) Except pursuant to this Agreement, there are no contracts,
       agreements or understandings between the Company and any other person
       that would give rise to a valid claim against the Company or any of the
       Underwriters for a brokerage commission, finder's fee or like payment in
       connection with the issuance, purchase and sale of the Shares.

              (xxx) Each of the Company and its subsidiaries has implemented
       Year 2000 compliance programs designed to ensure that its computer
       systems and applications will function properly beyond 1999. The Company
       believes that adequate resources have been allocated for



                                       8
<PAGE>   9

       this purpose and expects the Company's and its subsidiaries' Year 2000
       date conversion programs to be completed on a timely basis.

              (xxxi) Each certificate signed by any officer of the Company and
       delivered to the Underwriters or counsel for the Underwriters shall be
       deemed to be a representation and warranty by the Company to the
       Underwriters as to the matters covered thereby.

                 (b) Motorola represents and warrants to, and agrees with the
       several Underwriters that:

                 (i) The execution, delivery and performance of this Agreement
       by Motorola and the consummation of the transactions contemplated hereby
       and thereby will not (A) conflict with or result in the breach of any of
       the terms and provisions of, or constitute a default (or an event which
       with notice or lapse of time, or both, would constitute a default) or
       require consent under, or result in the creation or imposition of any
       lien, charge or encumbrance upon any property or assets of Motorola
       pursuant to the terms of, any agreement, instrument, franchise, license
       or permit to which Motorola is a party or by which Motorola or any of its
       property or assets may be bound, including, without limitation, the terms
       of any trust or similar agreement under which Motorola was formed, or (B)
       violate or conflict with any judgment, decree, order, statute, rule or
       regulation of any court or any public, governmental or regulatory agency
       or body having jurisdiction over Motorola or its properties or assets.

                 (ii) Motorola has, and will have at the time of delivery of the
       Shares to be sold by it, full legal right, power, authority and capacity,
       and, except as required under the Securities Act and state securities and
       Blue Sky laws, all necessary consents, approvals, authorizations, orders,
       registrations, filings, qualifications, licenses and permits of and from
       all public, regulatory or governmental agencies and bodies, as are
       required for the execution, delivery and performance of this Agreement
       and the consummation of the transactions contemplated hereby and thereby,
       including the sale, assignment, transfer and delivery of the Shares to be
       sold, assigned, transferred and delivered by Motorola hereunder.

                 (iii) This Agreement has been duly and validly authorized,
       executed and delivered by Motorola and, assuming the execution and
       delivery by the other parties hereto and thereto, is the valid and
       binding obligation of Motorola, enforceable against Motorola in
       accordance with its terms, except as such enforceability may be limited
       by bankruptcy, insolvency, reorganization, moratorium and other similar
       laws relating to or affecting creditor's rights generally, by general
       equitable principles (regardless of whether such enforceability is
       considered in a proceeding in equity or at law) and as limited by the
       unenforceability under certain circumstances under law or court decisions
       of provisions providing for the indemnification of or contribution to a
       party with respect to a liability where such indemnification or
       contribution is contrary to public policy.

                 (iv) Motorola has good, valid and marketable title to the
       Shares to be sold by it pursuant to this Agreement, free and clear of all
       liens, encumbrances, adverse claims, security interests, restrictions on
       transfer, shareholders' agreements, voting trusts, options and other
       defects in title whatsoever, with full power to deliver such Shares
       hereunder, and, upon the delivery of and payment for such Shares as
       herein contemplated, each of the Underwriters will receive good, valid
       and marketable title to the Shares purchased by it from Motorola, free
       and clear of all liens, encumbrances, adverse claims, security interests,
       restrictions on transfer, shareholders' agreements, voting trusts,
       options and other defects in title whatsoever.



                                       9
<PAGE>   10

                 (v) None of Motorola or any of its subsidiaries has (A) taken,
       directly or indirectly, any action designed to, or that might reasonably
       be expected to, cause or result in stabilization or manipulation of the
       price of any security of the Company or any of its subsidiaries to
       facilitate the sale or resale of the Common Stock or (B) since the date
       of the Preliminary Prospectus (1) sold, bid for, purchased or paid any
       person any compensation for soliciting purchases of the Common Stock or
       (2) paid or agreed to pay to any person any compensation for soliciting
       another to purchase any other securities of the Company or any of its
       subsidiaries.

                 (vi) Except as disclosed in the Prospectus, Motorola (A) does
       not have any preemptive right, co-sale right or right of first refusal or
       other similar right to purchase any of the Shares that are to be sold by
       to the Underwriters pursuant to this Agreement, and (B) does not own any
       warrants, options or similar rights to acquire, and does not have any
       right or arrangement to acquire, any capital stock, rights, warrants,
       options or other securities from the Company.

                 (vii) Except as disclosed in the Prospectus, Motorola does not
       possess any registration rights with respect to any securities of the
       Company or its subsidiaries.

                 The Company and Motorola acknowledge that each of the
Underwriters and, for purposes of the opinions to be delivered to the
Underwriters pursuant to Section 6 hereof, counsel to the Company, counsel to
Motorola and counsel to the Underwriters, will rely upon the accuracy and truth
of the foregoing representations and hereby consents to such reliance.

              2. Purchase, Sale and Delivery of the Shares.

                 (a) On the basis of the representations, warranties, covenants
and agreements herein contained, but subject to the terms and conditions herein
set forth, the Company agrees to sell to the Underwriters and the Underwriters,
severally and not jointly, agree to purchase from the Company, at a purchase
price per share of $__, the number of Firm Shares set forth opposite the
respective names of the Underwriters in Schedule I hereto.

                 (b) Payment of the purchase price for, and delivery of
certificates for, the Firm Shares shall be made at the office of Latham &
Watkins, 885 Third Avenue, Suite 1000, New York, New York, 10022, or at such
other place as shall be agreed upon by the Underwriters and the Company, at
10:00 A.M. on July ___, 1999 (unless postponed in accordance with the provisions
of Section 9 hereof) after the determination of the public offering price of the
Firm Shares, or such other time not later than ten business days after such date
as shall be agreed upon by the Underwriters and the Company (such time and date
of payment and delivery being herein called the "Closing Date"). Payment shall
be made to the Company by wire transfer in same day funds, against delivery to
the Underwriters of certificates for the Shares to be purchased by them.
Certificates for the Firm Shares shall be registered in such name or names and
in such authorized denominations as the Underwriters may request in writing at
least two full business days prior to the Closing Date. The Company will permit
the Underwriters to examine and package such certificates for delivery at least
one full business day prior to the Closing Date.

                 (c) In addition, Motorola hereby grants to the Underwriters the
option to purchase up to 1,050,000 Additional Shares at the same purchase price
per share to be paid by the Underwriters to the Company for the Firm Shares as
set forth in this Section 2, for the sole purpose of covering over-allotments in
the sale of Firm Shares by the Underwriters. This option may be exercised



                                       10
<PAGE>   11

at any time, in whole or in part, on or before the thirtieth day following the
date of the Prospectus, by written notice by the Underwriters to Motorola. Such
notice shall set forth the aggregate number of Additional Shares as to which the
option is being exercised and the date and time, as reasonably determined by the
Underwriters, when the Additional Shares are to be delivered (such date and time
being herein sometimes referred to as the "Additional Closing Date"); provided,
however, that the Additional Closing Date shall not be earlier than the Closing
Date or earlier than the second full business day after the date on which the
option shall have been exercised nor later than the eighth full business day
after the date on which the option shall have been exercised (unless such time
and date are postponed in accordance with the provisions of Section 9 hereof).
Certificates for the Additional Shares shall be registered in such name or names
and in such authorized denominations as the Underwriters may request in writing
at least two full business days prior to the Additional Closing Date. Motorola
will permit the Underwriters to examine and package such certificates for
delivery at least one full business day prior to the Additional Closing Date.

                 (d) The number of Additional Shares to be sold to each
Underwriter shall be the number which bears the same ratio to the aggregate
number of Additional Shares being purchased as the number of Firm Shares set
forth opposite the name of such Underwriter in Schedule I hereto (or such number
increased as set forth in Section 9 hereof) bears to the total number of Firm
Shares being purchased from the Company, subject, however, to such adjustments
to eliminate any fractional shares as the Underwriters in their sole discretion
shall make.

                 (e) Payment for the Additional Shares shall be made by wire
transfer in same day funds payable to the order of Motorola at the office of
Latham & Watkins, 885 Third Avenue, Suite 1000, New York, New York, 10022, or
such other location as may be mutually acceptable, against delivery of the
certificates for the Additional Shares to the Underwriters.

              3. Offering. Upon the Underwriters' authorization of the release
of the Firm Shares, the Underwriters propose to offer the Shares for sale to the
public upon the terms set forth in the Prospectus.

              4. Covenants of the Company. The Company covenants and agrees with
each of the Underwriters that:

                 (a) The Company will notify the Underwriters as soon as
reasonably practicable (and, if requested by the Underwriters, will confirm such
notice in writing) (i) when any post-effective amendment to the Registration
Statement becomes effective, (ii) of any request by the Commission for any
amendment of or supplement to the Registration Statement or the Prospectus or
for any additional information, (iii) of the mailing or the delivery to the
Commission for filing of the Prospectus or any amendment of or supplement to the
Registration Statement or the Prospectus or any document to be filed pursuant to
the Exchange Act during any period when the Prospectus is required to be
delivered under the Securities Act, (iv) of the issuance by the Commission of
any stop order suspending the effectiveness of the Registration Statement or any
post-effective amendment thereto or of the initiation, or the threatening, of
any proceedings therefor, (v) of the receipt of any comments or inquiries from
the Commission, and (vi) of the receipt by the Company of any notification with
respect to the suspension of the qualification of the Shares for sale in any
jurisdiction or the initiation or threatening of any proceeding for that
purpose. If the Commission shall propose or enter a stop order at any time, the
Company will make every reasonable effort to prevent the issuance of any such
stop order and, if issued, to obtain the lifting of such order as soon as
possible. The Company will not file any post-effective



                                       11
<PAGE>   12

amendment to the Registration Statement or any amendment of or supplement to the
Prospectus (including any revised prospectus which the Company proposes for use
by the Underwriters in connection with the offering of the Shares which differs
from the prospectus filed with the Commission pursuant to Rule 424(b) of the
Securities Act Regulations, whether or not such revised prospectus is required
to be filed pursuant to Rule 424(b) of the Securities Act Regulations) to which
the Underwriters or Underwriters' Counsel (as hereinafter defined) shall
reasonably object, will furnish the Underwriters with copies of any such
amendment or supplement a reasonable amount of time prior to such proposed
filing or use, as the case may be, and will not file any such amendment or
supplement or use any such prospectus to which the Underwriters or counsel for
the Underwriters shall reasonably object.

                 (b) If any event shall occur as a result of which the
Prospectus would, in the judgment of the Underwriters or the Company, include 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, in the light
of the circumstances under which they were made, not misleading, or if it shall
be necessary at any time to amend or supplement the Prospectus or the
Registration Statement to comply with the Securities Act or the Securities Act
Regulations, the Company will notify the Underwriters promptly and prepare and
file with the Commission an appropriate amendment or supplement (in form and
substance satisfactory to the Underwriters) which will correct such statement or
omission or which will effect such compliance.

                 (c) The Company has delivered to the Underwriters five signed
copies of the Registration Statement as originally filed, including exhibits,
and all amendments thereto, and the Company will promptly deliver to each of the
Underwriters, from time to time during the period that the Prospectus is
required to be delivered under the Securities Act, such number of copies of the
Prospectus and the Registration Statement, and all amendments of and supplements
to such documents, if any, as the Underwriters may reasonably request.

                 (d) The Company will endeavor in good faith, in cooperation
with the Underwriters, to qualify the Shares for offering and sale under the
securities laws relating to the offering or sale of the Shares of such
jurisdictions as the Underwriters may designate and to maintain such
qualification in effect for so long as required for the distribution thereof;
except that in no event shall the Company be obligated in connection therewith
to qualify as a foreign corporation or to execute a general consent to service
of process.

                 (e) The Company will make generally available (within the
meaning of Section 11(a) of the Securities Act) to its security holders and to
the Underwriters as soon as practicable, but not later than 45 days after the
end of its fiscal quarter in which the first anniversary date of the effective
date of the Registration Statement occurs (or if such fiscal quarter is the
Company's fourth fiscal quarter, not later than 90 days after the end of such
quarter), an earnings statement (in form complying with the provisions of Rule
158 of the Regulations) covering a period of at least twelve consecutive months
beginning after the effective date of the Registration Statement (as defined in
Rule 158(c) under the Securities Act).

                 (f) Except as disclosed in the Prospectus, during the period of
120 days from the date of the Prospectus, the Company will not, directly or
indirectly, without the prior written consent of Bear, Stearns & Co. Inc. ("Bear
Stearns"), offer, sell, contract to sell, grant any option to purchase, pledge
or otherwise dispose (or announce any offer, sale, contract to sell, grant of an
option to purchase, pledge or other disposition) of any shares of Common Stock
of the



                                       12
<PAGE>   13

Company or any securities convertible into or exercisable or exchangeable for
such Common Stock, except that the Company may issue (i) shares of Common Stock
and options to purchase Common Stock under its 1989 Stock Option Plan (as such
term is defined in the Prospectus), (ii) shares of Common Stock upon exercise of
warrants to purchase Common Stock that were issued and outstanding on the date
of the Prospectus, (iii) shares to be issued to XM Ventures as described in the
Prospectus under the heading "The XM Radio Transactions" or (iv) shares of
Common Stock in connection with strategic relationships and acquisitions of
businesses, technologies and products complementary to those of the Company, so
long as the recipients of such shares agree to be bound by a lock-up agreement
substantially in the form of Exhibit E hereto (which shall provide that any
transferees and assigns of such recipients shall be bound by the lock-up
agreement) for the remainder of the 120-day lock-up period.

                 (g) During a period of three years from the date of the
Prospectus, the Company will furnish to the Underwriters copies of (i) all
reports to its stockholders; and (ii) all reports, financial statements and
proxy or information statements filed by the Company with the Commission or any
national securities exchange.

                 (h) The Company will apply the proceeds from the sale of the
Shares as set forth under "How We Intend to Use the Proceeds from the Offering"
in the Prospectus.

                 (i) If the Company elects to rely upon Rule 462(b), the Rule
462(b) Registration Statement shall have become effective by 10:00 P.M., New
York City time, on the date of this Agreement, no stop order suspending the
effectiveness of the Registration Statement or any part thereof shall have been
issued and no proceeding for that purpose shall have been initiated or
threatened by the Commission, and all requests for additional information on the
part of the Commission shall have been complied with to the Underwriters'
reasonable satisfaction.

                 (j) The Company, during the period when the Prospectus is
required to be delivered under the Securities Act or the Exchange Act, will file
all documents required to be filed with the Commission pursuant to Sections 13,
14 or 15 of the Exchange Act within the time periods required by the Exchange
Act and the rules and regulations thereunder.

              5. Payment of Expenses. Whether or not the transactions
contemplated in this Agreement are consummated or this Agreement is terminated,
the Company hereby agrees to pay all costs and expenses incident to the
performance of the obligations of the Company hereunder, including those in
connection with (i) preparing, printing, duplicating, filing and distributing
the Registration Statement, as originally filed and all amendments thereto
(including all exhibits thereto), any Preliminary Prospectus, the Prospectus and
any amendments or supplements thereto (including, without limitation, fees and
expenses of the Company's accountants and counsel) and all other documents
related to the public offering of the Shares (including those supplied to the
Underwriters in quantities as hereinabove stated), (ii) the issuance, transfer
and delivery of the Shares to the Underwriters, including any transfer or other
taxes payable thereon, (iii) the qualification of the Shares under state or
foreign securities or Blue Sky laws, including the costs of printing and mailing
a preliminary and final "Blue Sky Memorandum" and the fees of counsel in
connection therewith and such counsel's disbursements in relation thereto, in an
amount not to exceed $7,500, (iv) listing of the Shares for quotation on the
Nasdaq, (v) filing fees of the Commission and the NASD, (vi) the cost of
printing certificates representing the Shares, and (vii) the cost and charges of
any transfer agent or registrar.



                                       13
<PAGE>   14

              6. Conditions of Underwriters' Obligations. The obligations of the
Underwriters to purchase and pay for the Firm Shares and the Additional Shares,
as provided herein, shall be subject to the accuracy of the representations and
warranties of the Company and Motorola herein contained, as of the date hereof
and as of the Closing Date (for purposes of this Section 6, "Closing Date" shall
refer to the Closing Date for the Firm Shares and any Additional Closing Date,
if different, for the Additional Shares), to the absence from any certificates,
opinions, written statements or letters furnished to the Underwriters or to
Latham & Watkins ("Underwriters' Counsel") pursuant to this Section 6 of any
material misstatement or omission, to the performance by the Company and
Motorola of their obligations hereunder, and to the following additional
conditions:

                 (a) The Registration Statement shall have become effective not
       later than 5:00 P.M., New York time, on the date of this Agreement or at
       such later time and date as shall have been consented to by the
       Underwriters, and shall continue to be effective; no stop order
       suspending the effectiveness of the Registration Statement shall have
       been issued under the Securities Act or proceedings therefor initiated
       or, to the Company's knowledge, threatened by the Commission; the
       Prospectus shall have been filed or transmitted for filing with the
       Commission pursuant to Rule 424(b) of the Securities Act Regulations
       within the prescribed time period; and prior to the Closing Date the
       Company shall have provided evidence satisfactory to the Underwriters of
       such timely filing or transmittal.

                 (b) All of the representations and warranties of the Company
       and Motorola contained in this Agreement shall be true and correct on the
       date hereof and on the Closing Date with the same force and effect as if
       made on and as of the date hereof and the Closing Date, respectively. The
       Company and Motorola shall have performed or complied with all of the
       agreements herein contained and required to be performed or complied with
       by them at or prior to the Closing Date.

                 (c) The Prospectus shall have been printed and copies
       distributed to the Underwriters not later than 10:00 a.m., New York City
       time, on the second business day following the date of this Agreement or
       at such later date and time as to which the Underwriters may agree, and
       no stop order suspending the qualification or exemption from
       qualification of the Shares in any jurisdiction referred to in Section
       4(d) shall have been issued and no proceeding for that purpose shall have
       been commenced or shall be pending or threatened.

                 (d) No action shall have been taken and no statute, rule,
       regulation or order shall have been enacted, adopted or issued by any
       governmental agency which would, as of the Closing Date, prevent the
       issuance of the Shares; no action, suit or proceeding shall have been
       commenced and be pending against or affecting or, to the best knowledge
       of the Company, threatened against, the Company or any of its
       subsidiaries before any court or arbitrator or any governmental body,
       agency or official that, if adversely determined, could reasonably be
       expected to result in a Material Adverse Effect.

                 (e) Since the dates as of which information is given in the
       Prospectus and except as contemplated by the Prospectus, (i) there shall
       not have been any material adverse change, or any development that is
       reasonably likely to result in a material adverse change, in the capital
       stock or the long-term debt, or material increase in the short-term debt,
       of the Company or any of its subsidiaries from that set forth in the
       Prospectus, (ii) no dividend or distribution of any kind shall have been
       declared, paid or made by the Company or any of its subsidiaries on any
       class of its capital stock, (iii) neither the Company nor any of its
       subsidiaries shall have incurred any



                                       14
<PAGE>   15

       liabilities or obligations, direct or contingent, that are material,
       individually or in the aggregate, to the Company and its subsidiaries,
       taken as a whole, and that are required to be disclosed on a balance
       sheet or notes thereto in accordance with generally accepted accounting
       principles and are not disclosed on the latest balance sheet or notes
       thereto included in the Prospectus. Since the date hereof and since the
       dates as of which information is given in the Prospectus, there shall not
       have occurred any material adverse change in the business, financial
       condition or results of operations of the Company and its subsidiaries,
       taken as a whole.

                 (f) The Underwriters shall have received (I) a certificate,
       dated the Closing Date, signed on behalf of the Company by each of the
       Company's Chief Executive Officer and Chief Financial Officer in form and
       substance reasonably satisfactory to the Underwriters, confirming, as of
       the Closing Date, the matters set forth in paragraphs (a) through (e) of
       this Section 6 and that, as of the Closing Date, the obligations of the
       Company to be performed hereunder on or prior thereto have been duly
       performed in all material respects and (ii) a certificate, dated the
       Closing Date, signed on behalf of the Company's Secretary, in form and
       substance reasonably satisfactory to the Underwriters.

                 (g) The Underwriters shall have received on the Closing Date an
       opinion, dated the Closing Date, in form and substance reasonably
       satisfactory to the Underwriters and counsel to the Underwriters, of
       Arnold & Porter, counsel for the Company, to the effect set forth in
       Exhibit A hereto.

                 (h) The Underwriters shall have received on the Closing Date an
       opinion, dated the Closing Date, in form and substance reasonably
       satisfactory to the Underwriters and counsel to the Underwriters, of
       Fisher Wayland Cooper Leader & Zaragoza L.L.P., counsel for the Company,
       to the effect set forth in Exhibit B hereto.

                 (i) The Underwriters shall have received on the Closing Date an
       opinion, dated the Closing Date, in form and substance reasonably
       satisfactory to the Underwriters and counsel to the Underwriters, of
       Randy S. Segal, General Counsel for the Company, to the effect set forth
       in Exhibit C hereto.

                 (j) The Underwriters shall have on the Closing Date an opinion,
       dated the Closing Date, in form and substance reasonably satisfactory to
       the Underwriters and counsel to the Underwriters, of Carol Forsyte,
       Senior Counsel to Motorola, to the effect set forth in Exhibit D hereto.

                 (k) The Underwriters shall have received an opinion, dated the
       Closing Date, in form and substance reasonably satisfactory to the
       Underwriters, of Latham & Watkins, counsel to the Underwriters, covering
       such matters as are customarily covered in such opinions.

                 (l) Latham & Watkins shall have been furnished with such
       documents, in addition to those set forth above, as they may reasonably
       require for the purpose of enabling them to review or pass upon the
       matters referred to in this Section 6 and in order to evidence the
       accuracy, completeness or satisfaction in all material respects of any of
       the representations, warranties or conditions herein contained.

                 (m) At the time this Agreement is executed and at the Closing
       Date the Underwriters shall have received from each of Arthur Andersen
       LLP, independent public



                                       15
<PAGE>   16

       accountants for the Company and its subsidiaries, and KPMG LLP,
       independent public accountants for XM Satellite Radio Holdings, Inc. and
       XM Satellite Radio, Inc., dated as of the date of this Agreement and as
       of the Closing Date, customary comfort letters addressed to the
       Underwriters and in form and substance satisfactory to the Underwriters
       and counsel to the Underwriters with respect to the financial statements
       and certain financial information of the Company and its subsidiaries
       contained in the Prospectus.

                 (n) At the time this Agreement is executed, the Underwriters
       shall have received a "lock-up" agreement, substantially in the form
       attached as Exhibit F hereto, from each of the officers and directors of
       the Company identified on Exhibit G hereto.

                 (o) At the time this Agreement is executed, the Underwriters
       shall have received a "lock-up" agreement, substantially in the form
       attached as Exhibit H hereto, from each of Hughes Communications
       Satellite Services, Inc. and AT&T Wireless Services, Inc.

                 (p) At the time this Agreement is executed, the Underwriters
       shall have received a "lock-up" agreement, substantially in the form
       attached as Exhibit I hereto, from Motorola.

                 (q) At the Closing Date, the Shares shall have been approved
       for quotation on the Nasdaq.

                 (r) At the time this Agreement is executed and at the Closing
       Date, the NASD shall not have withdrawn, or given notice of an intention
       to withdraw, its approval of the fairness of the underwriting terms and
       arrangements of the Offering of the Shares by the Underwriters.

                 (s) All opinions, certificates, letters and other documents
       required by this Section 6 to be delivered by the Company and Motorola
       will be in compliance with the provisions hereof only if they are
       reasonably satisfactory in form and substance to the Underwriters. The
       Company and Motorola will furnish the Underwriters with such conformed
       copies of such opinions, certificates, letters and other documents as
       Bear Stearns shall reasonably request. Prior to the Closing Date, the
       Company and Motorola shall have furnished to the Underwriters such
       further information, certificates and documents as the Underwriters may
       reasonably request.

       If any of the conditions specified in this Section 6 shall not have been
fulfilled when and as required by this Agreement, or if any of the certificates,
opinions, written statements or letters furnished to the Underwriters or to
Underwriters' Counsel pursuant to this Section 6 shall not be in all material
respects reasonably satisfactory in form and substance to the Underwriters and
to Underwriters' Counsel, all obligations of the Underwriters hereunder may be
canceled by the Underwriters at, or at any time prior to, the Closing Date and
the obligations of the Underwriters to purchase the Additional Shares may be
canceled by the Underwriters at, or at any time prior to, the Additional Closing
Date. Notice of such cancellation shall be given to the Company and Motorola in
writing, or by telephone, telecopy, telex or telegraph, confirmed in writing.

              7. Indemnification.

              (a) The Company agrees to indemnify and hold harmless (i) each of
the Underwriters, (ii) each person, if any, who controls any of the Underwriters
within the meaning of Section 15 of the Securities Act or Section 20(a) of the
Exchange Act and (iii) the respective officers,



                                       16
<PAGE>   17

directors, partners, employees, representatives and agents of each of the
Underwriters or any controlling person to the fullest extent lawful, from and
against any and all losses, liabilities, claims, damages and expenses whatsoever
as incurred (including but not limited to reasonable attorneys' fees and any and
all expenses whatsoever incurred in investigating, preparing or defending
against any investigation or litigation, commenced or threatened, or any claim
whatsoever, and any and all amounts paid in settlement of any claim or
litigation), joint or several, to which they or any of them may become subject
under the Securities Act, the Exchange Act or otherwise, insofar as such losses,
liabilities, claims, damages or expenses (or actions in respect thereof) arise
out of or are based upon any untrue statement or alleged untrue statement of a
material fact contained in the Registration Statement, as originally filed or
any amendment thereof, or any related Preliminary Prospectus or the Prospectus,
or in any supplement thereto or amendment thereof, or arise out of or are based
upon the omission or alleged omission to state therein a material fact required
to be stated therein or necessary to make the statements therein, in the light
of the circumstances under which they were made, not misleading; provided,
however, that the Company will not be liable in any such case (i) to the extent
but only to the extent that any such loss, liability, claim, damage or expense
arises out of or is based upon any such untrue statement or alleged untrue
statement or omission or alleged omission made therein in reliance upon and in
conformity with written information furnished to the Company by or on behalf of
any Underwriter expressly for use therein and (ii) with respect to any
Preliminary Prospectus or Preliminary Prospectus supplement to the extent that
any such loss, claim, damage or liability results from the fact that an
Underwriter sold Shares to a person as to whom it shall be established that
there was not sent or given, at or prior to written confirmation of such sale, a
copy of the Prospectus or Prospectus supplement as then amended or supplemented
in any case where such delivery is required by the Securities Act if the Company
has previously furnished copies thereof in sufficient quantity to such
Underwriter and with sufficient time to effect a recirculation pursuant to Rule
461 under the Securities Act and the loss, claim, damage or liability of the
Underwriters results from an untrue statement or omission of a material fact
contained in the Preliminary Prospectus or Preliminary Prospectus supplement
which was identified in writing prior to the effective date of the registration
statement to such underwriter and corrected in the Prospectus or Prospectus
supplement as then amended, and such correction would have cured the defect
giving rise to such loss, claim, damage or liability. The foregoing indemnity
shall be in addition to any liability which the Company may otherwise have.

              (b) Motorola agrees to indemnify and hold harmless each
Underwriter, its directors, its officers and each person, if any, who controls
any Underwriter within the meaning of 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 each Underwriter, but only with reference to information
relating to Motorola furnished in writing by or on behalf of Motorola expressly
for use in the Registration Statement, the Prospectus or any Preliminary
Prospectus. Notwithstanding the foregoing, the aggregate liability of Motorola
pursuant to the provisions of this paragraph shall be limited to an amount equal
to the total proceeds (before deducting underwriting discounts and commissions
and expenses) received by Motorola from the sale of Shares owned by it
hereunder.

              (c) Each Underwriter, severally and not jointly, agrees to
indemnify and hold harmless (i) the Company and Motorola, (ii) each person, if
any, who controls the Company and Motorola within the meaning of Section 15 of
the Securities Act or Section 20(a) of the Exchange Act, and (iii) the
respective officers, directors, partners, employees, representatives and agents
of the Company and Motorola, against any losses, liabilities, claims, damages
and expenses whatsoever (including but not limited to reasonable attorneys' fees
and any and all expenses whatsoever incurred in investigating, preparing or
defending against any investigation or litigation, commenced or threatened, or
any claim whatsoever and any and all amounts paid in settlement of any claim or
litigation), joint or



                                       17
<PAGE>   18

several, to which they or any of them may become subject under the Securities
Act, the Exchange Act or otherwise, insofar as such losses, liabilities, claims,
damages or expenses (or actions in respect thereof) arise out of or are based
upon any untrue statement or alleged untrue statement of a material fact
contained in the Registration Statement, as originally filed or any amendment
thereof, or any related preliminary prospectus, preliminary prospectus
supplement or prospectus, or in any amendment thereof or supplement thereto, or
arise out of or are based upon the omission or alleged omission to state therein
a material fact required to be stated therein or necessary to make the
statements therein, in the light of the circumstances under which they were
made, not misleading, in each case to the extent, but only to the extent, that
any such loss, liability, claim, damage or expense arises out of or is based
upon any such untrue statement or alleged untrue statement or omission or
alleged omission made therein in reliance upon and in conformity with written
information furnished to the Company by or on behalf of any Underwriter
expressly for use therein; provided, however, that in no case shall any
Underwriter be liable or responsible for any amount in excess of the
underwriting discount applicable to the Shares purchased by such Underwriter
hereunder. This indemnity will be in addition to any liability which any
Underwriter may otherwise have, including under this Agreement.

                 (d) Promptly after receipt by an indemnified party under
subsection (a), (b) or (c) above of notice of the commencement of any action,
such indemnified party shall, if a claim in respect thereof is to be made
against the indemnifying party under such subsection, notify each party against
whom indemnification is to be sought in writing of the commencement thereof (but
the failure so to notify an indemnifying party shall not relieve it from any
liability which it may have under this Section 7 except to the extent that it
has been prejudiced in any material respect by such failure or from any
liability which it may otherwise have). In case any such action is brought
against any indemnified party, and it notifies an indemnifying party of the
commencement thereof, the indemnifying party will be entitled to participate
therein, and to the extent it may elect by written notice delivered to the
indemnified party promptly after receiving the aforesaid notice from such
indemnified party, to assume the defense thereof with counsel reasonably
satisfactory to such indemnified party. Notwithstanding the foregoing, the
indemnified party or parties shall have the right to employ its or their own
counsel in any such case, but the fees and expenses of such counsel shall be at
the expense of such indemnified party or parties unless (i) the employment of
such counsel shall have been authorized in writing by the indemnifying parties
in connection with the defense of such action, (ii) the indemnifying parties
shall not have employed counsel to take charge of the defense of such action
within a reasonable time after notice of commencement of the action, or (iii)
such indemnified party or parties shall have reasonably concluded that there may
be defenses available to it or them which are different from or additional to
those available to one or all of the indemnifying parties (in which case the
indemnifying party or parties shall not have the right to direct the defense of
such action on behalf of the indemnified party or parties), in any of which
events such fees and expenses of counsel shall be borne by the indemnifying
parties; provided, however, that the indemnifying party under subsection (a),
(b) or (c) above, shall only be liable for the legal expenses of one counsel (in
addition to any local counsel) for all indemnified parties in each jurisdiction
in which any claim or action is brought. Anything in this subsection to the
contrary notwithstanding, an indemnifying party shall not be liable for any
settlement of any claim or action effected without its prior written consent;
provided, however, that such consent was not unreasonably withheld.

              8. Contribution. In order to provide for contribution in
circumstances in which the indemnification provided for in Section 7 hereof is
for any reason held to be unavailable from any indemnifying party or is
insufficient to hold harmless a party indemnified thereunder, the Company and
Motorola (the "Sellers"), on the one hand, and each Underwriter, on the other
hand, shall contribute to the aggregate losses, claims, damages, liabilities and
expenses of the nature contemplated by such



                                       18
<PAGE>   19

indemnification provision (including any investigation, legal and other expenses
incurred in connection with, and any amount paid in settlement of, any action,
suit or proceeding or any claims asserted, but after deducting in the case of
losses, claims, damages, liabilities and expenses suffered by the Sellers any
contribution received by the Sellers from persons, other than the Underwriters,
who may also be liable for contribution, including persons who control the
Company within the meaning of Section 15 of the Securities Act or Section 20(a)
of the Exchange Act, officers of the Company who signed the Registration
Statement and directors of the Company) as incurred to which the Sellers and one
or more of the Underwriters may be subject, in such proportions as is
appropriate to reflect the relative benefits received by the Sellers, on the one
hand, and such Underwriter, on the other hand, from the offering of the Shares
or, if such allocation is not permitted by applicable law or indemnification is
not available as a result of the indemnifying party not having received notice
as provided in Section 7 hereof, in such proportion as is appropriate to reflect
not only the relative benefits referred to above but also the relative fault of
the Sellers, on the one hand, and the Underwriters, on the other hand, in
connection with the statements or omissions which resulted in such losses,
claims, damages, liabilities or expenses, as well as any other relevant
equitable considerations. The relative benefits received by the Sellers, on the
one hand, and each Underwriter, on the other hand, shall be deemed to be in the
same proportion as (x) the total proceeds from the offering (net of underwriting
discounts and commissions but before deducting expenses) received by the Sellers
and (y) the underwriting discounts and commissions received by the Underwriters,
respectively, in each case as set forth in the table on the cover page of the
Prospectus. The relative fault of the Sellers, on the one hand, and each
Underwriter, on the other hand, 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 the Sellers or the Underwriters and the parties' relative intent,
knowledge, access to information and opportunity to correct or prevent such
statement or omission. The Sellers and the Underwriters agree that it would not
be just and equitable if contribution pursuant to this Section 8 were determined
by pro rata allocation (even if the Underwriters were treated as one entity for
such purpose) or by any other method of allocation which does not take account
of the equitable considerations referred to above. Notwithstanding the
provisions of this Section 8, (i) in no case shall any Underwriter be liable or
responsible for any amount in excess of the underwriting discount applicable to
the Shares purchased by such Underwriter hereunder, and (ii) 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. Notwithstanding the provisions of
this Section 8 and the preceding sentence, no Underwriter shall be required to
contribute any amount in excess of the amount by which the total price at which
the Shares underwritten by it and distributed to the public were offered to the
public exceeds the amount of any damages that such Underwriter has otherwise
been required to pay by reason of such untrue or alleged untrue statement or
omission or alleged omission. For purposes of this Section 8, (A) each person,
if any, who controls either of the Underwriters within the meaning of Section 15
of the Securities Act or Section 20(a) of the Exchange Act and (B) the
respective officers, directors, partners, employees, representatives and agents
of each of the Underwriters or any controlling person shall have the same rights
to contribution as such Underwriter, and (A) each person, if any, who controls
any of the Sellers within the meaning of Section 15 of the Securities Act or
Section 20(a) of the Exchange Act and (B) the respective officers, directors,
partners, employees, representatives and agents of the Sellers shall have the
same rights to contribution as the Sellers, subject in each case to clauses (i)
and (ii) of this Section 8. Any party entitled to contribution will, promptly
after receipt of notice of commencement of any action, suit or proceeding
against such party in respect of which a claim for contribution may be made
against another party or parties, notify each party or parties from whom
contribution may be sought, but the omission to so notify such party or parties
shall not relieve the party or parties from whom contribution may be sought from
any obligation it or they may have under this Section 8 or otherwise. No party
shall be liable for contribution with respect to any action or claim



                                       19
<PAGE>   20

settled without its prior written consent; provided, however, that such written
consent was not unreasonably withheld. Notwithstanding the foregoing, the
aggregate liability of Motorola pursuant to the provisions of this paragraph
shall be limited to an amount equal to the total proceeds (before deducting
underwriting discounts and commissions and expenses) received by Motorola from
the sale of its Shares hereunder.

              9. Default by an Underwriter.

                 (a) If any Underwriter or Underwriters shall default in its or
their obligation to purchase Firm Shares or Additional Shares hereunder, and if
the Firm Shares or Additional Shares with respect to which such default relates
do not (after giving effect to arrangements, if any, made by the Underwriters
pursuant to Subsection (b) below) exceed in the aggregate 10% of the number of
Firm Shares or Additional Shares, the Firm Shares or Additional Shares which
such defaulting Underwriter or Underwriters agreed but failed or refused to
purchase shall be purchased by the non-defaulting Underwriters in proportion to
the respective proportions which the numbers of Firm Shares set forth opposite
their respective names in Schedule I hereto bear to the aggregate number of Firm
Shares set forth opposite the names of the non-defaulting Underwriters.

                 (b) In the event that such default relates to more than 10% of
the Firm Shares or Additional Shares, as the case may be, the Underwriters may
in their discretion arrange for themselves or for another party or parties
(including any non-defaulting Underwriter or Underwriters who so agree) to
purchase such Firm Shares or Additional Shares, as the case may be, to which
such default relates on the terms contained herein. In the event that within
five calendar days after such a default the Underwriters do not arrange for the
purchase of the Firm Shares or Additional Shares, as the case may be, to which
such default relates as provided in this Section 9, this Agreement, or in the
case of a default with respect to the Additional Shares, the obligations of the
Underwriters to purchase and of the Company to sell the Additional Shares, shall
thereupon terminate, without liability on the part of the Company with respect
thereto (except in each case as provided in Section 5, 7(a) and 8 hereof) or the
Underwriters, but nothing in this Agreement shall relieve a defaulting
Underwriter or Underwriters of its or their liability, if any, to the other
Underwriters and the Company for damages occasioned by its or their default
hereunder.

                 (c) In the event that the Firm Shares or Additional Shares to
which the default relates are to be purchased by the non-defaulting
Underwriters, or are to be purchased by another party or parties as aforesaid,
the Underwriters or the Company shall have the right to postpone the Closing
Date or Additional Closing Date, as the case may be, for a period not exceeding
five business days, in order to effect whatever changes may thereby be made
necessary in the Registration Statement or the Prospectus or in any other
documents and arrangements, and the Company agrees to file promptly any
amendment or supplement to the Registration Statement or the Prospectus which,
in the opinion of Underwriters' Counsel, may thereby be made necessary or
advisable. The term "Underwriter" as used in this Agreement shall include any
party substituted under this Section 9 with like effect as if it had originally
been a party to this Agreement with respect to such Firm Shares or Additional
Shares.

              10. Survival of Representations and Agreements. All
representations and warranties, covenants and agreements of the Underwriters,
the Company and Motorola contained in this Agreement, including the agreements
contained in Section 5, the indemnity agreements contained in Section 7 and the
contribution agreements contained in Section 8, shall remain operative and in
full force and effect regardless of any investigation made by or on behalf of
any Underwriter or any controlling person thereof or by or on behalf of the
Company or Motorola, any of their officers and directors, or any



                                       20
<PAGE>   21

of their controlling persons, and shall survive delivery of and payment for the
Shares to and by the Underwriters. The representations contained in Section 1
and the agreements contained in Sections 5, 7, 8, 11(d) and 12 hereof shall
survive the termination of this Agreement, including termination pursuant to
Section 9 or 11 hereof.

              11. Effective Date of Agreement; Termination.

                 (a) This Agreement shall become effective upon the execution
and delivery of a counterpart hereof by each of the parties hereto.

                 (b) The Underwriters shall have the right to terminate this
Agreement at any time prior to the Closing Date or the obligations of the
Underwriters to purchase the Additional Shares at any time prior to the
Additional Closing Date, as the case may be, if on or prior to such date, (i)
either of the Company or Motorola shall have failed, refused or been unable to
perform in any material respect any agreement on its part to be performed
hereunder, (ii) any other condition to the obligations of the Underwriters
hereunder as provided in Section 6 is not fulfilled when and as required in any
material respect, (iii) in the judgment of the Underwriters any changes of
circumstance shall have occurred since the respective dates as of which
information is given in the Prospectus which could have a Material Adverse
Effect, other than as set forth in the Prospectus, or (iv) (A) any domestic or
international event or act or occurrence has materially adversely effected, or
in the opinion of the Underwriters will in the immediate future materially
adversely effect, the market for the Company's securities or for securities in
general; or (B) trading in securities generally on the New York Stock Exchange
("NYSE") or quotations on the Nasdaq shall have been suspended or materially
limited, or minimum or maximum prices for trading shall have been established,
or maximum ranges for prices for securities shall have been required, on such
exchange, or by such exchange or other regulatory body or governmental authority
having jurisdiction; or (C) a banking moratorium shall have been declared by
federal or state authorities, or a moratorium in foreign exchange trading by
major international banks or persons shall have been declared; or (D) there is
an outbreak or escalation of armed hostilities involving the United States on or
after the date hereof, or if there has been a declaration by the United States
of a national emergency or war, the effect of which shall be, in the
Underwriters' judgment, to make it inadvisable or impracticable to proceed with
the offering, sale and delivery of the Firm Shares or the Additional Shares, as
the case may be, on the terms and in the manner contemplated by the Prospectus;
or (E) there shall have been such a material adverse change in general economic,
political or financial conditions or if the effect of international conditions
on the financial markets in the United States shall be such as, in the
Underwriters' judgment, makes it inadvisable or impracticable to proceed with
the offering, sale and delivery of the Firm Shares or the Additional Shares, as
the case may be, on the terms and in the manner contemplated by the Prospectus.

                 (c) Any notice of termination pursuant to this Section 11 shall
be by telephone, telecopy, telex, or telegraph, confirmed in writing by letter.

                 (d) If this Agreement shall be terminated pursuant to any of
the provisions hereof (other than pursuant to Section 9(b) or 11(b) hereof), or
if the sale of the Shares provided for herein is not consummated because any
condition to the obligations of the Underwriters set forth herein is not
satisfied or because of any refusal, inability or failure on the part of the
Company to perform any agreement herein or comply with any provision hereof, the
Company will, subject to demand by the Underwriters, reimburse the Underwriters
for all out-of-pocket expenses (including the reasonable fees and expenses of
their counsel) incurred by the Underwriters in connection herewith.



                                       21
<PAGE>   22

                 (e) If the sale of the Additional Shares provided for herein is
not consummated because any condition to the obligations of the Underwriters set
forth herein is not satisfied or because of any refusal, inability or failure on
the part of Motorola to perform any agreement herein or comply with any
provision hereof, Motorola will, subject to demand by the Underwriters,
reimburse the Underwriters for all out-of-pocket expenses (including the
reasonable fees and expenses of their counsel) incurred by the Underwriters in
connection therewith.

              12. Underwriters' Information. The Company, Motorola and the
Underwriters severally acknowledge that the statements set forth in (i) the last
paragraph of the outside front cover of the Prospectus concerning the delivery
of the shares of Common Stock to the Underwriters and the offering of such
shares by the Underwriters; (ii) the fourth paragraph under the caption
"Underwriting" in the Prospectus concerning the proposed public offering price,
discount and concession; and (iii) the seventh and eighth paragraphs under the
caption "Underwriting" in the Prospectus concerning transactions that stabilize,
maintain, or otherwise affect the price of the Common Stock, constitute the only
information furnished in writing by or on behalf of any Underwriter expressly
for use in the Registration Statement, as originally filed or in any amendment
thereof, any related Preliminary Prospectus or preliminary prospectus supplement
or the Prospectus or in any amendment thereof or supplement thereto, as the case
may be.

              13. Notices. All communications hereunder, except as may be
otherwise specifically provided herein, shall be in writing and, if sent to the
Underwriters shall be mailed, delivered, telegraphed or telecopied to the
Underwriters, c/o Bear, Stearns & Co. Inc., 245 Park Avenue, New York, New York
10167, Attention: Corporate Finance Department, telecopy number: (212) 272-3092,
with a copy to Latham & Watkins, 885 Third Avenue, Suite 1000, New York, New
York 10022, Attention: Gregory A. Ezring, telecopy number: (212) 751-4864; and
if sent to the Company, shall be mailed, delivered, telexed, telegraphed or
telecopied to American Mobile Satellite Corporation, 10802 Parkridge Boulevard,
Reston, Virginia 20191, Attention: Randy S. Segal, Senior Vice-President,
General Counsel and Secretary, telecopy number: (703) 758-6134, with a copy to
Arnold & Porter, 555 Twelfth Street, Washington, D.C. 20004, Attention: Richard
E. Baltz, telecopy number: (202) 942-5999; and if sent to Motorola, shall be
mailed, delivered, telexed, telegraphed or telecopied to Motorola, Inc., 1303
East Algonquin Road, Schaumburg, Illinois 60196, Attention: General Counsel,
telecopy number: (847) 576-3628; provided, however, that any notice pursuant to
Sections 7 or 8 shall be mailed, delivered, telegraphed or telecopied and
confirmed in writing.

              14. Parties. This Agreement shall inure solely to the benefit of,
and shall be binding upon, the Underwriters, the Company, Motorola and the
controlling persons, directors, officers, employees and agents referred to in
Section 7 and 8, and their respective successors and assigns, and no other
person shall have or be construed to have any legal or equitable right, remedy
or claim under or in respect of or by virtue of this Agreement or any provision
herein contained. The term "successors and assigns" shall not include a
purchaser, in its capacity as such, of Shares from any of the Underwriters.

              15. Construction. This Agreement shall be construed in accordance
with the internal laws of the State of New York applicable to agreements made
and to be performed within New York, without giving any effect to any provisions
thereof relating to conflicts of law. TIME IS OF THE ESSENCE IN THIS AGREEMENT.

              16. Captions. The captions included in this Agreement are included
solely for convenience of reference and are not to be considered a part of this
Agreement.



                                       22
<PAGE>   23

              17. Counterparts. This Agreement may be executed in various
counterparts which together shall constitute one and the same instrument.



                                       23
<PAGE>   24

       If the foregoing correctly sets forth the understanding among the
Underwriters, the Company and Motorola, please so indicate in the space provided
below for that purpose, whereupon this letter shall constitute a binding
agreement among us.

                                Very truly yours,

                                AMERICAN MOBILE SATELLITE CORPORATION

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

                                MOTOROLA, INC.

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

Accepted as of the date first above written

BEAR, STEARNS & CO. INC.
            Acting on behalf of itself and the several
            Underwriters named in Schedule I hereto

BEAR, STEARNS & CO. INC.

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


<PAGE>   25

                                   SCHEDULE I

<TABLE>
<CAPTION>
                                                                                        Number of Firm
Name of Underwriter                                                             Shares to be Purchased
- ------------------------------------------------------------------------------------------------------
<S>                                                                             <C>
Bear, Stearns & Co. Inc. ...........................................................................
Credit Suisse First Boston Corporation..............................................................
Deutsche Bank Securities Inc........................................................................
SoundView Technology Group..........................................................................
[others]............................................................................................
 ....................................................................................................
 ....................................................................................................

     Total..........................................................................................
</TABLE>


<PAGE>   26

                                   SCHEDULE II

                              Material Subsidiaries

AMSC Acquisition Company, Inc.

AMSC Subsidiary Corporation

XM Satellite Radio Holdings, Inc.


<PAGE>   27

                                                                       Exhibit A

                       FORM OF OPINION OF ARNOLD & PORTER

       1. The Company has all requisite corporate power and authority to
execute, deliver and perform its obligations under the Underwriting Agreement
and to consummate the transactions contemplated thereby and in the Prospectus
under the heading "The XM Radio Transactions," including, without limitation,
the corporate power and authority to issue, sell and deliver the Shares as
provided therein.

       2. The Underwriting Agreement has been duly and validly authorized,
executed and delivered by the Company, and is the valid and binding obligation
of the Company, enforceable against the Company in accordance with its terms.

       3. The Registration Statement, Preliminary Prospectus and Prospectus
comply as to form in all material respects with the requirements for
registration statements on Form S-3 under the Securities Act and the Securities
Act Regulations; it being understood, however, that such counsel need not
express an opinion with respect to the financial statements, schedules and other
financial and market data included in the Registration Statement, Preliminary
Prospectus or Prospectus.

       4. To such counsel's knowledge, there are no contracts or other
agreements of a character required to be filed as an exhibit to the Registration
Statement that are not so filed.

       5. The Registration Statement has become effective under the Securities
Act and, to the best knowledge of such counsel, no stop order suspending the
effectiveness of the Registration Statement has been issued and no proceedings
therefor have been initiated or threatened by the Commission and all filings
required by Rule 424(b) of the Regulations have been made.

       6. None of (A) the execution, delivery or performance by the Company of
the Underwriting Agreement, (B) the issuance and sale of the Shares or (C) the
consummation by the Company of the transactions described in the Prospectus
under the caption "How We Intend to Use the Proceeds from the Offering" and "The
XM Radio Transactions" violates, conflicts with or constitutes a breach of any
of the terms or provisions of, or a default under (or an event that with notice
or the lapse of time, or both, would constitute a default), or requires consent
under, or results in the imposition of a lien or encumbrance on any properties
of the Company or any of its subsidiaries, or an acceleration of any
indebtedness of the Company or any of its subsidiaries pursuant to, (i) the
charter or bylaws of the Company or any of its subsidiaries, (ii) any bond,
debenture, note, indenture, mortgage, deed of trust or other agreement or
instrument to which the Company or any of its subsidiaries is a party or by
which any of them or their property is or may be bound that has been filed or
incorporated by reference as an exhibit to any filing by the Company with the
Securities and Exchange Commission, (iii) and statute, rule or regulation
applicable to the Company or any of its subsidiaries or any of their assets or
properties or (iv) to the best of our knowledge, any judgment, order or decree
of any court or governmental agency or authority having jurisdiction over the
Company or any of its subsidiaries or any of their assets or properties, except
as would not, individually or in the aggregate, have a material adverse effect.
Assuming compliance with applicable state securities and Blue Sky Laws and
Federal Communications Commission law, as to which we express no opinion, no
consent, approval, authorization or order of, or filing, registration or
qualification of or with, (a) any court or governmental agency, body or
administrative agency or (b) any other person is required for (1) the execution,
delivery and performance by the Company of the Underwriting Agreement, (2) the
issuance and sale of the Shares or (3)


<PAGE>   28

consummation by the Company of the transactions described in the Prospectus
except such as have been obtained and made or have been disclosed in the
Prospectus, and except as would not, individually or in the aggregate, have a
material adverse effect

       7. The statements contained in the Prospectus under the captions "Certain
Relationships and Related Transactions" and "Description of Capital Stock", in
each case insofar as such statements constitute statements of law, descriptions
of statutes, rules or regulations or legal conclusions, summarize fairly in all
material respects the matters required to be disclosed therein.

       8. None of the Company or any of its subsidiaries is an "investment
company" or a company "controlled" by an investment company within the meaning
of the Investment Company Act.

       Such counsel shall state in a separate letter:

       We have participated in conferences with officers and other
representatives of the Company, representatives of the independent certified
public accountants of the Company and the Underwriters and their representatives
at which the contents of the Registration Statement, Preliminary Prospectus and
Prospectus and related matters were discussed and, although we have not
undertaken to investigate or verify independently, and do not assume any
responsibility for, the accuracy, completeness or fairness of the statements
contained in the Registration Statement, Preliminary Prospectus or Prospectus
(except as indicated above), on the basis of the foregoing, no facts have come
to our attention which led us to believe that the Registration Statement,
Preliminary Prospectus and Prospectus, as of its date or the Closing Date,
contained an untrue statement of a material fact or omitted to state any
material fact required to be stated therein or necessary to make the statements
therein, in the light of the circumstances under which they were made, not
misleading (except as to financial statements and related notes, the financial
statement schedules and other financial and statistical data included therein).


<PAGE>   29

                                                                       Exhibit B

        FORM OF OPINION OF FISHER WAYLAND COOPER LEADER & ZARAGOZA L.L.P.

       1. Except as set forth in the Prospectus, the (i) the FCC has granted the
Company all material authorizations needed to construct, launch, and operate a
mobile satellite system and offer, on a common carrier basis, a variety of
domestic and international mobile satellite communications services, including
aeronautical, maritime and land mobile services pursuant to such authorization,
(ii) the FCC has assigned the Company an orbital location at 101 degrees W.L.
for the Company's first satellite and frequency bands 1544 to 1559 MHz and
1645.5 to 1660.5 MHz to provide further authorized mobile satellite services,
(iii) the FCC has granted XM Satellite Radio Holdings, Inc. ("XM") all material
authorizations needed to construct, launch, and operate a satellite radio
service and offer a subscription service on a non-common carrier basis, and (iv)
the FCC has assigned XM two orbital locations at _____ and _____ to provide such
satellite radio service.

       2. Except as set forth in the Prospectus, the FCC has granted the Company
all material authorizations needed to construct and operate a two-way wireless
data network and offer, on a common carrier basis, a variety of two-way wireless
data services.

       3. Except for such FCC consents, approvals, authorizations or orders that
have already been obtained, no material consent, approval, authorization or
order of the FCC is required to be obtained by the Company or its subsidiaries
under the Communications Laws for the consummation of the transactions
contemplated under the Underwriting Agreement, except that, from time to time,
the Company and its subsidiaries may be required to obtain certain
authorizations that would be required in the ordinary course of business.

       4. The execution and delivery of the Underwriting Agreement, and the
consummation of the transactions contemplated thereunder, by the Company do not
and will not violate any material provision of the Communications Laws.

       5. The statements set forth in the Prospectus under the captions "RISK
FACTORS--Our wireless business is subject to domestic and international
regulation which could impose significant costs or otherwise harm our business,
"RISK FACTORS--XM Radio's business involves significant risks and these risks
may impair the value of our investment in XM Radio--Oversight by the FCC and
other regulatory bodies involves costs and risks" and "REGULATION," insofar as
such statements constitute a summary of material (a) legal matters, (b)
documents, or (c) proceedings under the Communications laws, fairly present the
information contained under such captions in light of the circumstances in which
they were made.

       6. Except as set forth in the Prospectus, to our knowledge, there is no
investigation or complaint before the FCC pending or threatened in writing that
is specifically directed against or in respect of the Company or its
subsidiaries (including XM Satellite Radio Holdings, Inc.), or any of the FCC
licenses held by the Company or its subsidiaries (including XM Satellite Radio
Holdings, Inc.) that would reasonably be expected to result in the revocation of
any material FCC authorizations or otherwise to materially impair the operations
of the Company or its subsidiaries (including XM Satellite Radio Holdings,
Inc.).


<PAGE>   30

                                                                       Exhibit C

FORM OF OPINION OF RANDY S. SEGAL, GENERAL COUNSEL TO AMERICAN MOBILE SATELLITE
CORPORATION

       1. Each of the Company and its subsidiaries is duly organized and validly
existing as a corporation in good standing under the laws of its jurisdiction of
incorporation, and has all requisite corporate power and authority to carry on
its business as it is being conducted and as described in the Prospectus and to
own, lease and operate its properties, and is duly qualified and in good
standing as a foreign corporation authorized to do business in each jurisdiction
in which the nature of its business or its ownership or leasing of property
requires such qualification, except where the failure to be so qualified or in
good standing would not, singly or in the aggregate, have a Material Adverse
Effect.

       2. All of the outstanding shares of capital stock of the Company have
been duly authorized, validly issued, and are fully paid and nonassessable and
were not issued in violation of any preemptive or similar rights under the
Delaware General Corporation Law. The authorized, issued and outstanding capital
stock of the Company conforms in all respects to the description thereof set
forth in the Prospectus.

       3. Except as disclosed in the Prospectus, all of the outstanding capital
stock of each subsidiary of the Company is owned by the Company, free and clear
of any security interest, claim, lien, limitation on voting rights or
encumbrance; and all such securities have been duly authorized, validly issued,
and are fully paid and nonassessable and were not issued in violation of any
preemptive or similar rights.

       4. The Company has all requisite corporate power and authority to
execute, deliver and perform its obligations under the Underwriting Agreement
and to consummate the transactions contemplated thereby and in the Prospectus,
including, without limitation, the corporate power and authority to issue, sell
and deliver the Shares as provided in the Underwriting Agreement and in the
Prospectus.

       5. The Underwriting Agreement has been duly and validly authorized,
executed and delivered by the Company and, assuming due execution by the other
parties thereto, is the legally valid and binding agreement of the Company.

       6. Except as described in the Prospectus, none of (A) the execution,
delivery or performance by the Company of the Underwriting Agreement, (B) the
issuance and sale of the Shares or (C) the consummation by the Company of the
transactions described in the Prospectus under the caption "How We Intend to Use
the Proceeds from the Offering" violates, conflicts with or constitutes a breach
of any of the terms or provisions of, or a default under (or an event that with
notice or the lapse of time, or both, would constitute a default), or requires
consent under, or results in the imposition of a lien or encumbrance on any
properties of the Company or any of its subsidiaries, or an acceleration of any
indebtedness of the Company or any of its subsidiaries pursuant to, (i) the
charter or bylaws of the Company or any of its subsidiaries, (ii) any bond,
debenture, note, indenture, mortgage, deed of trust or other agreement or
instrument to which the Company or any of its subsidiaries is a party or by
which any of them or their property is or may be bound (assuming all of such
agreements are governed by New York law), (iii) any statute, rule or regulation
applicable to the Company or any of its subsidiaries or any of their assets or
properties or (iv) to the best of my knowledge, any judgment, order or decree of
any court or governmental agency or authority having jurisdiction over the
Company or any of its subsidiaries or any of their assets or properties.
Assuming compliance with applicable state securities


<PAGE>   31

and Blue Sky laws, as to which I express no opinion, and the filing of a
registration statement under the Securities Act of 1933, no consent, approval,
authorization or order of, or filing, registration, qualification, license or
permit of or with, any court or governmental agency, body or administrative
agency is required for (1) the execution, delivery and performance by the
Company of the Underwriting Agreement, (2) the issuance and sale of the Shares
or (3) the consummation by the Company of the transactions contemplated by the
Underwriting Agreement and in the Prospectus under the heading "The XM Radio
Transactions," except such as have been obtained and made or have been disclosed
in the Prospectus.

       7. To the best of my knowledge, there is (a) no action, suit,
investigation or proceeding before or by any court, arbitrator or governmental
agency, body or official, domestic or foreign, now pending, or threatened or
contemplated to which any of the Company or any of its subsidiaries is or may be
a party or to which the business or property of any of the Company or any of its
subsidiaries is or may be subject, (b) no statute, rule, regulation or order
that has been enacted, adopted or issued by any governmental agency or that has
been proposed by any governmental body, or (c) no injunction, restraining order
or order of any nature by a federal or state court of competent jurisdiction to
which any of the Company or any of its subsidiaries is or may be subject or to
which the business, assets or property of the Company or any of its subsidiaries
is or may be subject, that, in the case of clauses (a), (b) and (c) above, is
required to be disclosed in the Prospectus and that is not so disclosed.

       8. To the best of my knowledge, except as set forth in the Prospectus,
there are no holders of any securities of the Company or any of its subsidiaries
who, by reason of the execution by the Company of the Underwriting Agreement or
the consummation by the Company of the transactions contemplated thereby, have
the right to request or demand that the Company register under the Securities
Act securities held by them.

       9. To the best of my knowledge, no stop order preventing the use of the
Preliminary Prospectus or the Prospectus, or any amendment or supplement
thereto, has been issued.


<PAGE>   32

                                                                       Exhibit D

                     FORM OF OPINION OF COUNSEL TO MOTOROLA

       1. Motorola is duly organized and validly existing as a corporation in
good standing under the laws of its jurisdiction of incorporation.

       2. The Underwriting Agreement has been duly and validly authorized,
executed and delivered by Motorola and is a valid and binding obligation of
Motorola, enforceable against Motorola in accordance with its terms.

       3. Motorola has all requisite power and authority, and all necessary
consents, approvals, authorizations, orders, registrations, filings,
qualifications, licenses and permits of and from all courts and all public,
governmental or regulatory agencies and bodies as are required for the
execution, delivery and performance of the Underwriting Agreement and the
consummation of the transactions contemplated hereby and thereby, except for (1)
such as may be required under state securities or Blue Sky laws in connection
with the purchase and distribution of the Shares by the Underwriters (as to
which such counsel need express no opinion) and (2) such as have been made or
obtained under the Securities Act.

       4. Upon the delivery of and payment for the Shares to be sold by Motorola
pursuant to the Underwriting Agreement as herein contemplated, each of the
Underwriters will receive all rights and interests in and to the Shares
purchased by it from Motorola, free of any adverse claim.

       5. The execution, delivery and performance of the Underwriting Agreement
by Motorola and the consummation of the transactions contemplated hereby and
thereby will not violate or conflict with, to the knowledge of such counsel, any
judgment, decree, order, statute, rule or regulation of any court or any public,
governmental or regulatory agency or body having jurisdiction over Motorola or
any of its properties or assets.


<PAGE>   33

                                                                       Exhibit E

                      American Mobile Satellite Corporation
                            10802 Parkridge Boulevard
                             Reston, Virginia 20191

                                LOCK-UP AGREEMENT

Bear, Stearns & Co. Inc.
245 Park Avenue
New York, New York 10167

Dear Sirs:

       The undersigned is a holder of securities of American Mobile Satellite
Corporation, a Delaware corporation (the "Company"), and wishes to facilitate
the public offering (the "Offering") of the Company's common stock, par value
$.01 per share (the "Common Stock"), pursuant to a Registration Statement on
Form S-3 (File No. 333-81549). The undersigned recognizes that the Offering will
be of benefit to the undersigned.

       In consideration of the foregoing and in order to induce you to act as
Underwriter in connection with the Offering, the undersigned hereby agrees, for
the benefit of the Company and the Underwriter, that during the period beginning
from the date hereof and continuing to and including the date 120 days after the
date of the final prospectus relating to the Offering, the undersigned will not,
directly or indirectly, without the prior written consent of Bear, Stearns & Co.
Inc., offer, sell, contract to sell, swap, make any short sale, pledge,
establish an open "put equivalent position" within the meaning of Rule 16a-1(h)
under the Securities Exchange Act of 1934, as amended (the "Exchange Act"),
grant any option to purchase or otherwise dispose (or publicly announce the
undersigned's intention to do any of the foregoing) of, directly or indirectly,
any shares of Common Stock or other capital stock of the Company, or any
securities convertible into, or exercisable or exchangeable for, any shares of
Common Stock or other capital stock of the Company that the undersigned
currently beneficially owns (within the meaning of Rule 13d-3 under the Exchange
Act), directly or indirectly, or may beneficially own, directly or indirectly,
in the future.

       The undersigned confirms that the undersigned understands that the
Underwriter and the Company will rely upon the representations set forth in this
Agreement in proceeding with the Offering. The undersigned further confirms that
the agreements of the undersigned are irrevocable and shall be binding upon the
undersigned's heirs, legal representatives, successors and assigns. The
undersigned agrees and consents to the entry of stop transfer instructions with
the Company's transfer agent against the transfer of securities of the Company
held by the undersigned except in compliance with this Agreement.

                                        Very truly yours,
                                        By:
                                            ---------------------------
                                            Name:
                                            Title:

Dated:  July ___, 1999


<PAGE>   34

                                                                       Exhibit F

                      American Mobile Satellite Corporation
                            10802 Parkridge Boulevard
                             Reston, Virginia 20191

                                LOCK-UP AGREEMENT

Bear, Stearns & Co. Inc.
245 Park Avenue
New York, New York 10167

Dear Sirs:

       The undersigned is a holder of securities of American Mobile Satellite
Corporation, a Delaware corporation (the "Company"), and wishes to facilitate
the public offering (the "Offering") of the Company's common stock, par value
$.01 per share (the "Common Stock"), pursuant to a Registration Statement on
Form S-3 (File No. 333-81549). The undersigned recognizes that the Offering will
be of benefit to the undersigned.

       In consideration of the foregoing and in order to induce you to act as
Underwriter in connection with the Offering, the undersigned hereby agrees, for
the benefit of the Company and the Underwriter, that during the period beginning
from the date hereof and continuing to and including the date 90 days after the
date of the final prospectus relating to the Offering, the undersigned will not,
directly or indirectly, without the prior written consent of Bear, Stearns & Co.
Inc., offer, sell, contract to sell, swap, make any short sale, pledge,
establish an open "put equivalent position" within the meaning of Rule 16a-1(h)
under the Securities Exchange Act of 1934, as amended (the "Exchange Act"),
grant any option to purchase or otherwise dispose (or publicly announce the
undersigned's intention to do any of the foregoing) of, directly or indirectly,
any shares of Common Stock or other capital stock of the Company, or any
securities convertible into, or exercisable or exchangeable for, any shares of
Common Stock or other capital stock of the Company that the undersigned
currently beneficially owns (within the meaning of Rule 13d-3 under the Exchange
Act), directly or indirectly, or may beneficially own, directly or indirectly,
in the future; provided, however, that the foregoing 90-day restriction shall
not apply to 401(k) benefit plan transfers and shall only be applicable for so
long as the undersigned is an employee of the Company.

       The undersigned confirms that the undersigned understands that the
Underwriter and the Company will rely upon the representations set forth in this
Agreement in proceeding with the Offering. The undersigned further confirms that
the agreements of the undersigned are irrevocable and shall be binding upon the
undersigned's heirs, legal representatives, successors and assigns. The
undersigned agrees and consents to the entry of stop transfer instructions with
the Company's transfer agent against the transfer of securities of the Company
held by the undersigned except in compliance with this Agreement.

                                        Very truly yours,
                                        By:
                                            ---------------------------
                                            Name:
                                            Title:

Dated:  July ___, 1999


<PAGE>   35

                                                                       Exhibit G

         PERSONS DELIVERING A LOCK-UP AGREEMENT PURSUANT TO SECTION 6(k)

Carson E. Agnew
Thomas P. Bowen
Richard J. Burnheimer
Daniel Croft
Michael D. Fabbri
Robert L. Goldsmith
Paul M. Gremaud
Lon C. Levin
Susan Byrd Lubert
Dennis W. Matheson
Brian E. Nimmo
Gary M. Parsons
Deborah L. Peterson
Walter V. Purnell, Jr.
Randy S. Segal
W. Bartlett Snell
Patricia C. Tikkala
Matthew J. Whitehead, II


<PAGE>   36

                                                                       Exhibit H

                                LOCK-UP AGREEMENT

Bear, Stearns & Co. Inc.
245 Park Avenue
New York, New York 10167

Dear Sirs:

       The undersigned is a holder of securities of American Mobile Satellite
Corporation, a Delaware corporation (the "Company"), and wishes to facilitate
the public offering (the "Offering") of the Company's common stock, par value
$.01 per share (the "Common Stock"), pursuant to a Registration Statement on
Form S-3 (File No. 333-81549). The undersigned recognizes that the Offering will
be of benefit to the undersigned.

       In consideration of the foregoing and in order to induce you to act as
Underwriter in connection with the Offering, the undersigned hereby agrees, for
the benefit of the Company and the Underwriter, that during the period beginning
from the date hereof and continuing to and including the date 120 days after the
date of the final prospectus relating to the Offering, the undersigned will not,
directly or indirectly, without the prior written consent of Bear, Stearns & Co.
Inc., offer, sell, contract to sell, swap, make any short sale, pledge,
establish an open "put equivalent position" within the meaning of Rule 16a-1(h)
under the Securities Exchange Act of 1934, as amended (the "Exchange Act"),
grant any option to purchase or otherwise dispose (or publicly announce the
undersigned's intention to do any of the foregoing) of, directly or indirectly,
any shares of Common Stock or other capital stock of the Company, or any
securities convertible into, or exercisable or exchangeable for, any shares of
Common Stock or other capital stock of the Company that the undersigned
currently beneficially owns (within the meaning of Rule 13d-3 under the Exchange
Act), directly or indirectly, or may beneficially own, directly or indirectly,
in the future; provided that the outstanding Registration Statement on Form S-3
shall not be deemed to violate this agreement.

       The undersigned confirms that the undersigned understands that the
Underwriter and the Company will rely upon the representations set forth in this
Agreement in proceeding with the Offering. The undersigned further confirms that
the agreements of the undersigned are irrevocable and shall be binding upon the
undersigned's heirs, legal representatives, successors and assigns. The
undersigned agrees and consents to the entry of stop transfer instructions with
the Company's transfer agent against the transfer of securities of the Company
held by the undersigned except in compliance with this Agreement.

                                        Very truly yours,
                                        By:
                                            ---------------------------
                                            Name:
                                            Title:

Dated:  July ___, 1999


<PAGE>   37

                                                                       Exhibit I

                           AMERICAN MOBILE CORPORATION

                                LOCK-UP AGREEMENT

Bear, Stearns & Co. Inc.
245 Park Avenue
New York, New York 10167

Dear Sirs:

       The undersigned is a holder of securities of American Mobile Satellite
Corporation, a Delaware corporation (the "Company"), and wishes to facilitate
the public offering (the "Offering") of the Company's common stock, par value
$.01 per share (the "Common Stock"), pursuant to a Registration Statement on
Form S-3 (File No. 333-81549). As a shareholder, the undersigned recognizes that
the Offering will be of benefit to the undersigned and other shareholders of the
Company.

       In consideration of the foregoing and in order to induce you to act as
Underwriter in connection with the Offering, the undersigned hereby agrees, for
the benefit of the Company and the Underwriter, that during the period beginning
from the date hereof and continuing to and including the date 120 days after the
date of the final prospectus relating to the Offering, the undersigned will not,
directly or indirectly, without the prior written consent of Bear, Stearns & Co.
Inc., offer, sell, contract to sell, swap, make any short sale, pledge,
establish an open "put equivalent position" within the meaning of Rule 16a-1(h)
under the Securities Exchange Act of 1934, as amended (the "Exchange Act"),
grant any option to purchase or otherwise dispose (or publicly announce the
undersigned's intention to do any of the foregoing) of, directly or indirectly,
any shares of Common Stock or other capital stock of the Company, or any
securities convertible into, or exercisable or exchangeable for, any shares of
Common Stock or other capital stock of the Company that the undersigned
currently beneficially owns (within the meaning of Rule 13d-3 under the Exchange
Act), directly or indirectly, or may beneficially own, directly or indirectly,
in the future; provided that neither the offer or sale of the Company's common
stock as part of the exercise of the Underwriters' over-allotment option nor the
pendency of the outstanding Registration Statement on Form S-3 (No. 333-71423)
alone shall be deemed to violate this agreement.

       The undersigned confirms that the undersigned understands that the
Underwriter and the Company will rely upon the representations set forth in this
Agreement in proceeding with the Offering. The undersigned further confirms that
the agreements of the undersigned are irrevocable and shall be binding upon the
undersigned's heirs, legal representatives, successors and assigns. The
undersigned agrees and consents to the entry of stop transfer instructions with
the Company's transfer agent against the transfer of securities of the Company
held by the undersigned except in compliance with this Agreement.

                                 Very truly yours,

                                 MOTOROLA, INC.

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

Dated:  July ___, 1999



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