EXHIBIT 8.1
 
                                 June 23, 1998



American Mobile Satellite Corporation
AMSC Acquisition Company, Inc.
10802 Parkridge Blvd.
Reston, VA  20191-5416

       Re:  Registration Statement on Form S-4
            File Nos. 333- 52777 and 333-52777-01 to -09


Ladies and Gentlemen:

       We have acted as special tax counsel to American Mobile Satellite
Corporation and AMSC Acquisition Company, Inc. (the "Company") in connection
with the preparation and filing with the Securities and Exchange Commission
under the Securities Act of 1933, as amended, of a Form S-4 Registration
Statement (the "Registration Statement"), relating to the offer (the "Exchange
Offer") by the Company to exchange all the Company's outstanding Series A 
12 1/4% Senior Notes due 2008 for the Company's Series B 12 1/4 % Senior Notes
due 2008 (the "Exchange Notes").

       This opinion letter relates to the principal material federal income tax
consequences of the Exchange Offer and the ownership and disposition of the
Exchange Notes.  All capitalized terms used in this opinion letter and not
otherwise defined herein are used as described in the Registration Statement.

       The opinion set forth herein is subject to the assumptions and conditions
set forth in the Registration Statement under the heading "Certain United States
Federal Income Tax Consequences" and is premised on the assumption that all of
the factual information, descriptions, representations and assumptions set forth
or referred to in this letter and in the Registration Statement, are accurate
and complete. We have not independently verified any factual matters relating to
the Exchange Notes in connection with or apart from our preparation of this
opinion and, accordingly, our opinion does not take into account any matters not
set forth herein which might have been disclosed by independent verification. We
have examined the Registration Statement and such other documents as we have
deemed necessary to render our opinion expressed below. In our examination of
such material, we have relied upon the current and continued accuracy of the
factual matters we have considered, and we have assumed the genuineness of all
signatures, the authenticity of all documents submitted to us as originals and
the

 
American Mobile Satellite Corp.
June 23, 1998
Page 2

conformity to original documents of all copies of documents submitted to us.
Should any of the facts, circumstances, or assumptions on which we have relied
subsequently be determined to be incorrect or inaccurate, our conclusions may
vary from those set forth below and such variance could be material.

       Based on and subject to the foregoing, and subject to further
qualifications set forth below, it is our opinion that the statements in the
Registration Statement under the caption "Certain Federal Income Tax
Consequences," to the extent such statements constitute a summary of applicable
United States federal income tax law, accurately describe the material United
States federal income tax consequences of the Exchange Offer and the ownership
and disposition of the Exchange Notes.

       This opinion is based upon the Internal Revenue Code of 1986, as amended,
the Treasury regulations promulgated thereunder and other relevant authorities
and law, all as in effect on the date hereof.  All of the above are subject to
change or modification by subsequent legislative, regulatory, administrative or
judicial decisions which could adversely affect our opinions.  Consequently,
future changes in the law, or in administrative or judicial interpretations
thereof, may cause the tax treatment of the transactions referred to herein to
be materially different from that described above.

       Other than the specific tax opinion set forth in this letter, no other
opinion has been rendered with respect to the tax treatment of the Exchange
Offer and the ownership and disposition of the Exchange Notes, including, but
not limited to, their tax treatment under other provisions of the Code and the
regulations, the tax treatment of any conditions existing at the time of, or
effects resulting from, the Exchange Offer and the ownership and disposition of
the Exchange Notes that are not specifically covered by the above opinions, or
the tax treatment under state, local, foreign or any other tax laws.

 
American Mobile Satellite Corp.
June 23, 1998
Page 3

       We hereby consent to the filing of this letter as an exhibit to the
Registration Statement.  In giving such consent, we do not concede that this
consent is required under Section 7 of the Securities Act of 1933.

                               Very truly yours,
 
                               /s/ ARNOLD & PORTER
 
                               Arnold & Porter