EXHIBIT 10.50a

                              AMENDED AND RESTATED
                               DOCUMENT STANDSTILL
                            AND TERMINATION AGREEMENT

     This Amended and Restated  Document  Standstill and  Termination  Agreement
(this  "Agreement")  is made  as of  October  12,  2001  by and  among  Motient
Corporation  ("Motient"),   Motient  Services  Inc.  ("Sub"),  Mobile  Satellite
Ventures LLC (formerly known as Motient  Satellite  Ventures  LLC)("Newco")  and
each of the  Investors  (the  "Investors")  listed  on  Schedule  I of the  June
Investment Agreement (as defined below).

     WHEREAS,  Motient,  Newco  and the  Investors  entered  into an  Investment
Agreement,  dated June 22,  2000 (the "June  Investment  Agreement";  which term
shall also be deemed to include the Parent Transfer Letter Agreement (as defined
in the June Investment  Agreement) and the Section 2.1 Letter (as defined in the
June Investment  Agreement)) whereby the Investors invested in Newco and Motient
and the Investors  entered into a First Amended and Restated  Limited  Liability
Company Agreement of Newco dated June 29, 2000 (the "LLC Agreement");

     WHEREAS,  the  parties  hereto  entered  into the January  2001  Investment
Agreement  dated as of January 8, 2001 (the "January  Investment  Agreement") by
and among Motient, TMI Communications and Company,  Limited Partnership ("TMI"),
Newco,  the Investors and the other parties named therein to provide for,  among
other things, the addition of TMI as a limited partner of Newco;

     WHEREAS, in connection with the January Investment  Agreement,  the parties
hereto entered into a Document Standstill and Termination  Agreement dated as of
January 8, 2001 (the "First Document Standstill and Termination Agreement");

     WHEREAS, the parties hereto entered into an Amended and Restated Investment
Agreement,  dated as of the date hereof (the "October Investment  Agreement") by
and among Motient, TMI, Newco, the Investors and the other parties named therein
which amends and restates the January  Investment  Agreement  and provides  for,
among other  things,  the addition of TMI as a limited  partner of Newco and the
sale of the Convertible Notes (as defined in the October Investment Agreement);

     WHEREAS, the parties hereto desire for the terms of this Agreement to amend
and  restate  the  terms  of  the  First  Document  Standstill  and  Termination
Agreement;  provided,  however,  that if the  October  Investment  Agreement  is
terminated,  the parties hereto desire that this Agreement terminate and further
desire  to  continue  thereafter  with the  terms  and  conditions  of the First
Document  Standstill and  Termination  Agreement as if this Agreement were never
executed; and

     WHEREAS,  any capitalized term used but not otherwise  defined herein shall
have the meaning ascribed to such term in the October Investment Agreement.

     NOW  THEREFORE,  for good and valuable  consideration,  the  sufficiency of
which is hereby acknowledged, the parties hereto hereby agree as follows:

1. The  parties  hereby  consent to the  execution  and  delivery of the October
Investment  Agreement which amends and restates the January Investment Agreement
and provides for, among other things,  the addition of TMI as a limited  partner
of Newco and the sale of the Convertible Notes at the First Closing thereunder.

2. Subject to Section 3 below,  this  Agreement  hereby  amends and restates the
First Document Standstill and Termination Agreement in its entirety.

3. This Agreement shall terminate upon the termination of the October Investment
Agreement, and all of the terms of the First Document Standstill and Termination
Agreement shall continue in full force and effect (without giving effect to this
Agreement) as if this Agreement were never executed.

4. Subject to and effective upon the occurrence of the consummation of the First
Closing under the October Investment Agreement (a) the June Investment Agreement
shall be deemed terminated, and upon such termination all rights and obligations
of the parties thereunder,  other than the rights and obligations under Sections
7.9, 12, 17 and 18,  shall  terminate  and (b) the Newco LP  Agreement  shall be
deemed to supersede the LLC Agreement.

5. Until the  occurrence  of the  earlier of (a) the  consummation  of the First
Closing under the October  Investment  Agreement and (b) the  termination of the
October  Investment  Agreement,  the  parties  shall not  exercise  any of their
respective rights under the June Investment  Agreement (other than those arising
in connection  with a breach of the June  Investment  Agreement by another party
thereto arising prior to the date of this Agreement).

6. Whenever an agreement is said to be  terminated  or amended  pursuant to this
Agreement, this Agreement shall constitute the written consent of the parties to
such agreement to such termination or amendment.

7. This  Agreement is being  executed and delivered in, and shall be governed by
and construed according to the laws of, the State of New York (but not including
any choice of law rules  thereof  which would lead to a contrary  result).  This
Agreement shall be binding upon, and inure to the benefit of, the parties hereto
and their  respective  successors and permitted  assigns.  This Agreement may be
assigned  by a party  hereto  only to a person or  entity to whom such  party is
concurrently  assigning  its rights  under the October  Investment  Agreement in
accordance with the terms of the October  Investment  Agreement.  This Agreement
may be executed in any number of counterparts,  each of which shall be deemed an
original  and   enforceable   against  the  parties   actually   executing  such
counterpart,  and all of  which,  when  taken  together,  shall  constitute  one
instrument.

                         [Signatures on Following Pages]








     IN WITNESS WHEREOF,  the undersigned  indicate their agreement to the above
as of the day and year first above written.



                               MOTIENT CORPORATION


                               By: /s/Gary M. Parsons
                                  ----------------------------------------------
                                  Gary M. Parsons, Chairman


                               MOTIENT SERVICES INC.



                               By: /s/Gary M. Parsons
                                  ----------------------------------------------
                                  Gary M. Parsons, Chairman


                               MOBILE SATELLITE VENTURES LLC



                               By: /s/Gary M. Parsons
                                  ----------------------------------------------
                                  Gary M. Parsons, Chairman


                               TELCOM SATELLITE VENTURES INC.


                               By: /s/Rahul Prakash
                                  ----------------------------------------------
                                  Rahul Prakash, President


                               COLUMBIA SPACE (QP), INC.


                               By: /s/James B. Fleming, Jr.
                                  ----------------------------------------------
                                  James B. Fleming, Jr.
                                  Title: President










                               COLUMBIA SPACE (AI), INC.


                               By: /s/James B. Fleming, Jr.
                                  ----------------------------------------------
                                  James B. Fleming, Jr.
                                  Title: President


                               COLUMBIA SPACE PARTNERS, INC.


                               By: /s/James B. Fleming, Jr.
                                  ----------------------------------------------
                                  James B. Fleming, Jr.
                                  Title: President



                               SPECTRUM SPACE EQUITY
                               INVESTORS IV, INC.


                               By: /s/Kevin J. Maroni
                                  ----------------------------------------------
                                  Kevin J. Maroni
                                  Title:  Chairman and CEO


                               SPECTRUM SPACE IV PARALLEL, INC.


                               By: /s/Kevin J. Maroni
                                  ----------------------------------------------
                                  Kevin J. Maroni
                                  Title:  Chairman and CEO


                               SPECTRUM SPACE IV MANAGERS, INC.


                               By: /s/Kevin J. Maroni
                                  ----------------------------------------------
                                  Kevin J. Maroni
                                  Title:  Chairman and CEO