Document:

EXHIBIT 10.51b

                               Motient Corporation
                              MR Acquisition Corp.
                              10802 Parkridge Blvd.
                           Reston, Virginia 20191-5416

                                               October 1, 2001

Rare Medium Group, Inc.
565 Fifth Avenue
29th Floor
New York, NY 10017
Attention:  General Counsel

                RE: Merger Agreement and Note Purchase Agreement

Ladies and Gentlemen:

     Reference is hereby made to the  Agreement  and Plan of Merger by and among
Motient Corporation  ("Acquiror"),  MR Acquisition Corp. ("Merger Sub") and Rare
Medium Group, Inc. (the "Company," which, along with Acquiror and Merger Sub, is
sometimes  hereinafter  referred to collectively as the "Parties"),  dated as of
May 14,  2001 (as  amended  through the date  hereof,  the "Merger  Agreement").
Reference  is also hereby  made to the Note  Purchase  Agreement  by and between
Acquiror  and the  Company,  dated as of  April  2,  2001  (the  "Note  Purchase
Agreement").  All capitalized  terms used but not otherwise defined herein shall
have the meaning set forth in the Merger Agreement.

     The  Parties  hereby  acknowledge  and agree that the Merger  Agreement  is
terminated  pursuant to Section  8.01(a)  thereof  effective as of the Company's
acceptance  of this  agreement by signing this  agreement in the space  provided
below and that, as a result of such termination, none of the Parties (nor any of
their respective directors, officers, employees or advisors in their capacity as
such) shall have any further  obligations or liabilities  owing to the others in
connection with or arising out of the Merger Agreement or its termination except
as  specifically  set forth in the first  sentence of Section 8.02 of the Merger
Agreement.  Nothing  in this  paragraph  shall  have any effect on the rights or
obligations of any party in respect of the Note Purchase  Agreement or any other
document executed or delivered in connection therewith.

     In addition,  the Company and Acquiror hereby agree that effective upon the
Company's  acceptance of this  agreement by signing this  agreement in the space
provided below, the Maturity Date (as defined in the Note Purchase Agreement) of
the Tranche A Note and the Tranche B Note (as such terms are defined in the Note
Purchase  Agreement) is hereby extended to October 8, 2001, provided that if, on
or prior to the Maturity  Date,  Acquiror  shall repay the  outstanding  amounts
under the Tranche A Note and the Tranche B Note  utilizing XM Shares (as defined
in the Note Purchase  Agreement),  3,000,000 XM Shares shall first be applied to
repay amounts outstanding under the Tranche A Note and 2,000,000 XM Shares shall
first be applied  to repay  amounts  outstanding  under the  Tranche B Note.  In
consideration  of the extension,  Acquiror  hereby agrees,  from the date hereof
through  and  including  October  8,  2001,  (i) not to,  and  shall  cause  its
subsidiaries  (other than Mobile  Satellite  Ventures,  LLC ("MSV")),  officers,
directors,  employees,  investment  bankers,  attorneys,  accountants  and other
agents and affiliates (other than MSV) not to, directly or indirectly, initiate,
solicit,   encourage,   negotiate,  have  discussions  regarding,  or  otherwise
facilitate  the  submission  by a third party of, or negotiate or enter into any
agreement  with a third  party with  respect  to: (a) any  proposal  to acquire,
directly or indirectly, any equity interest or other securities (whether debt or
equity  securities)  of MSV  (or any  successor  entity  thereto)  or any of its
subsidiaries  (or any successor  entity  thereto) or any or all of the assets of
Motient  Services  Inc.  (or  any  successor  entity  thereto)  or  any  of  its
subsidiaries  (or any successor  entity thereto) (each, a "MSV  Transaction") or
(b) any  proposal  to transfer or  otherwise  dispose of or pledge or  otherwise
encumber  any  equity  interest  or other  securities  (whether  debt or  equity
securities) of MSV (or any successor  entity thereto) or any of its subsidiaries
(or any successor entity thereto) held by Acquiror,  directly or indirectly, and
(ii) in the  event  that the  Company  or one of its  subsidiaries  indicates  a
willingness  to execute and deliver on or prior to October 8, 2001 an  agreement
on terms  substantially  consistent with the draft  memorandum of  understanding
relating to a possible  investment by the Company or one of its  subsidiaries in
MSV (or any successor  entity  thereto)  previously  reviewed by the parties (as
such terms have been modified in discussions by the parties), Acquiror agrees to
support  such a  transaction,  including,  without  limitation,  by signing  any
documentation  signed by the Company and using reasonable best efforts to obtain
the signature of any other required  parties.  The Acquiror's  obligations under
the Note  Purchase  Agreement  shall  otherwise  remain in  effect  as  provided
therein.

     For the  avoidance  of  doubt,  the  parties  hereto  acknowledge  that the
immediately  preceding  paragraph does not apply to any member of MSV other than
Acquiror  and does not apply to MSV or the  subsidiaries,  officers,  directors,
employees,  investment  bankers,  attorneys,  accountants  and other  agents and
affiliates  of MSV in their  capacity as such (all such persons and entities are
hereinafter  sometimes referred to collectively as the "Excluded  Persons").  No
failure  by any  Excluded  Person  to comply  with the terms of the  immediately
preceding  paragraph  shall be a default by Acquiror under this  agreement.  The
Parties further  acknowledge that the immediately  preceding  paragraph does not
prohibit or limit Acquiror or the subsidiaries,  officers, directors, employees,
investment  bankers,  attorneys,  accountants and other agents and affiliates of
Acquiror from complying with their obligations under the January 2001 Investment
Agreement  dated as of  January  8, 2001 as in effect on the date  hereof  among
Acquiror, MSV and the other parties thereto (the "January Investment Agreement")
or the Ancillary  Agreements (as defined in the January Investment  Agreement) ,
as all such agreements are in effect on the date hereof.

     This  agreement may be executed and delivered in two or more  counterparts,
and by the different Parties hereto in separate counterparts, each of which when
executed and delivered  shall be deemed to be an original but all of which taken
together shall constitute one and the same agreement.

     This agreement shall be governed by, and construed in accordance  with, the
laws of the State of Delaware  without  regard to any principles of conflicts of
law.

<PAGE>

Please  acknowledge  your  understanding  of and agreement with the foregoing by
signing this agreement in the space provided below.

                                             Sincerely,

                                             MOTIENT CORPORATION

                                             By:/s/Walter V. Purnell, Jr.
                                                -------------------------
                                             Name:  Walter V. Purnell, Jr.
                                             Title: President and
                                                      Chief Executive Officer

                                             MR ACQUISITION CORP.

                                             By:/s/Walter V. Purnell, Jr.
                                                -------------------------
                                             Name:  Walter V. Purnell, Jr.
                                             Title: President

AGREED TO AND ACCEPTED
This 1st day of October, 2001

RARE MEDIUM GROUP, INC.

By:/s/Glenn Meyers
   ---------------
Name:  Glenn Meyers
Title:  Chairman and
         Chief Executive OfficerEXHIBIT 10.54b

                               Motient Corporation
                              MR Acquisition Corp.
                              10802 Parkridge Blvd.
                           Reston, Virginia 20191-5416

                                 October 8, 2001

Rare Medium Group, Inc.
565 Fifth Avenue
29th Floor
New York, NY 10017
Attention:  General Counsel

                           RE:      Letter Agreement

Ladies and Gentlemen:

     Reference is hereby made to the letter  agreement  dated October 1, 2001 by
and among Motient Corporation ("Acquiror"),  MR Acquisition Corp. ("Merger Sub")
and Rare Medium  Group,  Inc.  (the  "Company,"  which,  along with Acquiror and
Merger Sub, is sometimes  hereinafter referred to collectively as the "Parties")
(the "Letter Agreement").

     The Parties  hereby  acknowledge  and agree that the date  "October 8, 2001
wherever  it appears in the Letter  Agreement  is amended to read  "October  12,
2001."

     Except as expressly  modified by this  amendment,  the Letter  Agreement is
hereby  ratified and confirmed  and shall remain in full force and effect.  This
amendment may be executed and delivered in two or more counterparts,  and by the
different Parties hereto in separate  counterparts,  each of which when executed
and delivered  shall be deemed to be an original but all of which taken together
shall constitute one and the same amendment.

     This amendment shall be governed by, and construed in accordance  with, the
laws of the State of Delaware  without  regard to any principles of conflicts of
law.

     Please  acknowledge your  understanding of and agreement with the foregoing
by signing this amendment in the space provided below.

                          Sincerely,

                          MOTIENT CORPORATION

                          By:/s/Walter V. Purnell, Jr.
                             -------------------------
                          Name:  Walter V. Purnell, Jr.
                          Title:  President and
                                  Chief Executive Officer

                          MR ACQUISITION CORP.

                          By:/s/Walter V. Purnell, Jr.
                          Name:  Walter V. Purnell, Jr.
                                 ----------------------
                          Title:  President

AGREED TO AND ACCEPTED
This 8th day of October, 2001

RARE MEDIUM GROUP, INC.

By:/s/Glenn Meyers
------------------
Name:  Glenn Meyers
Title:  Chairman and
        Chief Executive Officer

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