Harbinger
      Capital Partners Master Fund I, Ltd.

    Harbinger
      Capital Partners Special Situations Fund, L.P.

    Harbinger
      Capital Partners Fund I, L.P.

    Harbinger
      Co-Investment Fund, L.P.

    555
      Madison Avenue, 16th Floor

    New
      York,
      NY 10022

    


    


    


    August
      22, 2008

     

    SkyTerra
      Communications, Inc.

    Mobile
      Satellite Ventures Subsidiary LLC

    Mobile
      Satellite Ventures L.P.

    10802
      Parkridge Boulevard

    Reston,
      VA 20191

    Attention:
      General Counsel

    


    Gentlemen:

    


    Reference
      is made to that certain Master Contribution and Support Agreement, dated as
      of
      July 24, 2008 (the “MCSA”)
      by and
      among (i) HARBINGER CAPITAL PARTNERS MASTER FUND I, LTD, an exempted company
      organized under the Laws of the Cayman Islands ("Harbinger
      Master"),
      (ii)
      HARBINGER CAPITAL PARTNERS SPECIAL SITUATIONS FUND, L.P., a Delaware limited
      partnership ("Harbinger
      Special"),
      (iii)
      HARBINGER CAPITAL PARTNERS FUND I, L.P., a Delaware limited partnership
      ("Harbinger
      Fund"),
      (iv)
      HARBINGER CO-INVESTMENT FUND, L.P., a Delaware limited partnership (the
      "Harbinger
      Satellite Fund",
      and
      together with Harbinger Master, Harbinger Special and Harbinger Fund,
      "Harbinger"),
      (v)
      SKYTERRA COMMUNICATIONS, INC., a Delaware corporation (the "Company"),
      (vi)
      MOBILE SATELLITE VENTURES SUBSIDIARY LLC, a Delaware limited liability company
      ("MSV
      LLC"),
      and
      (vii) MOBILE SATELLITE VENTURES L.P., a Delaware limited partnership
      ("MSV").
      Capitalized terms used in this letter agreement (this “Letter
      Agreement”)
      and
      not otherwise defined shall have the meanings accorded them in the
      MCSA.

    


    In
      consideration of the mutual covenants and agreements contained herein and for
      other good and valuable consideration, the receipt and sufficiency of which
      are
      hereby acknowledged, each of the Parties hereby agrees as follows.

    


    The
      undersigned shall, as promptly as practicable, take all actions necessary to
      consummate the actions set forth herein, including to amend the MCSA in
      accordance with Section 21.10 thereof, and to amend the Registration Rights
      Agreement in accordance with Section 5.7(b) thereof, as applicable, in order
      to
      reflect the matters set forth in Sections 1-3 below.

    


    
      
         

      

      
         

        
          

        

      

      
         

      

       

    

    1. TVCC.
      The
      MCSA and all exhibits, schedules and other documents to be delivered in
      connection with the consummation of the transactions contemplated thereby,
      shall
      each be amended such that TVCC, the TVCC Contribution Closing, and all
      references, terms, conditions, representations, warranties, covenants,
      liabilities or other obligations of the Parties with respect thereto, including
      without limitation the transfer of all of the TVCC LLC Interests from Harbinger
      Master and Harbinger Special to the Company in exchange for shares of Voting
      Common Stock in accordance with Section 2.1(c) of the MCSA, shall be deleted
      from the MCSA and all exhibits, schedules and other documents to be delivered
      in
      connection with the consummation of the transactions contemplated thereby.
      

    


    2. Termination
      Fee.
      As
      consideration for the amendment to the MCSA as set forth in Section 1 of this
      Letter Agreement, on the Closing Date, the Company shall issue to Harbinger
      10,300,000 fully-paid, non-assessable shares of Voting Common
      Stock.

    


    3. Registration
      Rights Agreement.
      As
      promptly as practicable, the Parties shall take all actions necessary to amend
      the definition of “Registrable Shares” in the Registration Rights Agreement to
      include the shares of Voting Common Stock to be issued to Harbinger pursuant
      to
      Section 2 hereof. 

    


    Except
      as
      expressly amended and modified in accordance with the terms of this Letter
      Agreement, the MCSA and the Registration Rights Agreement shall each remain
      unchanged and in full force and effect and are hereby ratified and confirmed
      in
      all respects.

    


    This
      Letter Agreement, together with the MCSA, the Stock Purchase Agreement, the
      Securities Purchase Agreement, the Registration Rights Agreement, the Consulting
      Agreement, the Confidentiality Agreement and the Confidentiality Side Letter
      (together with the schedules and exhibits thereto and the other documents
      delivered or to be delivered in connection herewith and therewith), constitutes
      the entire agreement among the Parties hereto, and supersedes all prior
      agreements and contemporaneous, arrangements, covenants, promises, conditions,
      undertakings, inducements, representations, warranties and negotiations,
      expressed or implied, oral or written, between the Parties, with respect to
      the
      subject matter hereof.

    


    This
      Letter Agreement shall be governed by and construed in accordance with the
      laws
      of the State of New York applicable to contracts made and performed in such
      state and without regard to the conflicts or choice of law provisions thereof
      that would give rise to the application of the domestic substantive law of
      any
      other jurisdiction.

    


    This
      Letter Agreement may be executed in several counterparts, each of which shall
      be
      deemed an original, but all of which together shall constitute one and the
      same
      Letter Agreement. Facsimile and electronic signatures on this Letter Agreement
      shall be deemed original signatures.  

     

    
      
         

      

      
         

        
          

        

      

      
         

      

    

    
 

    IN
      WITNESS WHEREOF, the
      Parties have duly executed this Letter Agreement as of the date and year first
      above written.

     

     

    
      	 	
              HARBINGER
                CAPITAL PARTNERS MASTER FUND I, LTD.

              


              By:
                Harbinger Capital Partners Offshore Manager, LLC, as investment
                manager

              


              By:
                /s/
                William R. Lucas,
                Jr.                               

              


              Name:
                 William
                R. Lucas, Jr.

              Title: Executive
                Vice President

              


              HARBINGER
                CAPITAL PARTNERS SPECIAL SITUATIONS FUND, L.P.

              


              By:
                Harbinger Capital Partners Special Situations GP, LLC, as general
                partner

              


              By:
                /s/ William R. Lucas,
                Jr.                               

              


              Name: William
                R. Lucas, Jr.

              Title: Executive
                Vice President

              


              HARBINGER
                CAPITAL PARTNERS FUND I, L.P.

              


              By:
                Harbinger Capital Partners GP, LLC, as general partner

              


              By:
                /s/ William R. Lucas,
                Jr.                               

              


              Name: William
                R. Lucas, Jr.

              Title: Executive
                Vice President

              


              HARBINGER
                CO-INVESTMENT FUND, L.P.

              


              By:
                Harbinger Co-Investment GP, LLC, as general partner

              


              By: HMC
                - New York, Inc., as managing member

              


              By:
                /s/ William R. Lucas,
                Jr.                               

              


              Name: William
                R. Lucas, Jr.

              Title: Executive
                Vice President

            


    

     

    
      
         

      

      
         

        
          

        

      

      
         

      

       

      
        	 	
                SKYTERRA
                  COMMUNICATIONS, INC.

                


                By:
                  /s/
                  Scott
                  Macleod                                         
                  

                Name: Scott
                  Macleod

                Title: Chief
                  Financial Officer

                


                MOBILE
                  SATELLITE VENTURES SUBSIDIARY LLC

                


                By:
                  /s/ Scott
                  Macleod                                         
                  

                Name: Scott
                  Macleod

                Title: Chief
                  Financial Officer

                


                MOBILE
                  SATELLITE VENTURES L.P.

                


                By:
                  /s/ Scott
                  Macleod                                         
                  

                Name: Scott
                  Macleod

                Title: Chief
                  Financial Officer