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EXHIBIT 10.58
                      EXHIBIT E - FORM OF CONVERTIBLE NOTE

THIS NOTE AND THE LIMITED PARTNERSHIP UNITS THAT MAY BE RECEIVED UPON CONVERSION
OF THIS NOTE HAVE BEEN  ACQUIRED  FOR  INVESTMENT  AND HAVE NOT BEEN  REGISTERED
UNDER THE  SECURITIES  ACT OF 1933,  AS AMENDED  (THE  "ACT") OR UNDER ANY STATE
SECURITIES LAWS (THE "STATE ACTS") AND CANNOT BE OFFERED,  SOLD,  TRANSFERRED OR
OTHERWISE  DISPOSED  OF IN THE  ABSENCE OF  REGISTRATION  OR AN  EXEMPTION  FROM
REGISTRATION  UNDER  THE  ACT,  THE  STATE  ACTS  AND  REGULATIONS   PROMULGATED
THEREUNDER.

                          MOBILE SATELLITE VENTURES LP

No. ___                                                         U.S.$___________

               Convertible  Note  at 10%  Interest  (calculated  and  compounded
          semi-annually)  Maturity  Date:  [date  which is five years  after the
          First Closing], or earlier as determined herein

FOR VALUE RECEIVED, Mobile Satellite Ventures LP, a Delaware limited partnership
(the  "Issuer"),  hereby  acknowledges  itself  indebted  and promises to pay to
________________,  or its permitted  assigns (the  "Noteholder"),  the principal
amount of  ______________  UNITED STATES  DOLLARS  (U.S.$_____________),  on the
Maturity Date (as hereinafter  defined in Schedule A), at the head office of the
Noteholder or at such other place as directed in writing by the  Noteholder.  In
addition,  the Issuer agrees to pay interest (computed on the basis of a year of
365 or 366 days, as the case may be), in like money on the outstanding principal
amount from the date hereof until the Maturity Date, or until such later date as
all  obligations  hereunder  have been paid in full,  or  earlier  as  otherwise
provided  herein,  at the rate of TEN PERCENT  (10%) per annum,  calculated  and
compounded semi-annually and payable in arrears on the Maturity Date, or earlier
as otherwise  provided herein, and should the Issuer at any time make default in
the  payment of any  principal  or  interest on this  Convertible  Note,  to pay
interest  on the  amount in  default at the same  rate,  in like  money,  all in
accordance  with the terms and conditions set out in Schedule A. If at any time,
the  payment  of the  principal  of or  interest  on  this  Convertible  Note is
rescinded  or must  otherwise  be  restored  or  returned  upon the  insolvency,
bankruptcy,   or  reorganization  of  the  Issuer  or  otherwise,  the  Issuer's
obligations  under this  Convertible  Note shall be  reinstated  at such time as
though such payment had been due but not made at such time.

This  Convertible  Note is one of the  "Convertible  Notes"  as  defined  in the
Investment  Agreement  (as defined in Schedule A hereto),  and is subject to the
terms and  conditions of the Investment  Agreement and the Ancillary  Agreements
(as defined in the Investment Agreement).

The Terms and  Conditions  set out on  Schedule  A  attached  hereto  are hereby
incorporated by reference into this  Convertible Note and constitute an integral
part hereof.  THIS  CONVERTIBLE NOTE IS SUBORDINATED TO OTHER DEBT OF THE ISSUER
AS SET FORTH IN SECTION 2 OF SCHEDULE A.

IN WITNESS  WHEREOF,  the Issuer has caused this Convertible Note to be executed
on its behalf by the signatures of its officers duly authorized in that regard.

Issued as of _______ __, 2001             MOBILE SATELLITE VENTURES LP

                                          By: Mobile Satellite Ventures GP Inc.,
                                                 its general partner

                                          By:________________________________
                                          Name:
                                          Title:

<PAGE>

                                   SCHEDULE A
                        TERMS AND CONDITIONS OF THE NOTE

1.        Terms  not  otherwise  defined  herein  shall  have  their  respective
          meanings  set forth in the  Investment  Agreement  or, if not  defined
          therein,  in the Newco LP Agreement.  In this  Convertible  Note,  the
          following capitalized terms shall have the meanings set out below:

(a)            "Affiliate" means with respect to a specified Person,  any Person
               that directly or  indirectly  controls,  is controlled  by, or is
               under common control with, the specified  Person. As used in this
               definition, the term "control" means the possession,  directly or
               indirectly,  of the power to direct or cause the direction of the
               management and policies of a Person, whether through ownership of
               voting securities, by contract or otherwise;

(b)            "Business Day" means any day on which banks are open for business
               in the  place  where  this  Convertible  Note  is  scheduled  for
               payment;

(c)            "Canadian  Licenses" means the licenses issued by Industry Canada
               to TMI and which were  transferred to Mobile  Satellite  Ventures
               (Canada)  Inc.  pursuant  to the TMI Asset  Sale  Agreements  (as
               defined in the Investment Agreement);

(d)            "Capital Lease Obligations"  means all monetary  obligations of a
               Person  under  any  leasing  or  similar  arrangement  which,  in
               accordance  with GAAP,  is required to be classified as a capital
               lease;

(e)            "Cash   Equivalents"   means:  (i)  securities  issued  or  fully
               guaranteed  or  insured by the United  States  Government  or any
               agency  thereof  and  backed by the full  faith and credit of the
               United  States  having  maturities of not more than twelve months
               from the date of acquisition;  (ii) certificates of deposit, time
               deposits,  Eurodollar  time  deposits,  or  banker's  acceptances
               having in each case a tenor of not more than six  months,  issued
               by any U.S.  commercial bank having combined  capital and surplus
               of not less than  $500,000,000  whose short term  securities  are
               rated both A-1 or higher by Standard & Poor's Corporation and P-1
               or higher by Moody's Investors  Services,  Inc.; (iii) commercial
               paper of an issuer rated either at least A-1 by Standard & Poor's
               Ratings  Group,  a division of  McGraw-Hill,  Inc.  and/or P-1 by
               Moody's  Investors Service Inc. and in either case having a tenor
               of not more than three months;  (iv repurchase  agreements  fully
               collateralized by securities issued by U.S. government  agencies;
               and (v) money  market  mutual funds  invested in the  instruments
               permitted by clauses (i), (ii), (iii) and (iv) above;

(f)            "Class A Preferred  Units" shall have the meaning given such term
               in the Newco LP Agreement;

(g)            "Contingent  Obligation"  means,  as applied to any  Person,  any
               direct or indirect  liability  of that Person with respect to any
               Debt, lease, dividend,  letter of credit or other obligation (the
               "primary obligations") of another Person (the "primary obligor"),
               including,  without  limitation,  any  obligation of that Person,
               whether  or  not  contingent,  (a)  to  purchase,  repurchase  or
               otherwise  acquire  such  primary  obligations  or  any  property
               constituting  direct or  indirect  security  therefor,  or (b) to
               advance or provide  funds (i) for the payment or discharge of any
               such primary  obligation,  or (ii) to maintain working capital or
               equity  capital of the primary  obligor or  otherwise to maintain
               the net worth or  solvency or any  balance  sheet item,  level of
               income or financial  condition of the primary obligor,  or (c) to
               purchase  property,  securities  or  services  primarily  for the
               purpose of assuring the owner of any such primary  obligation  of
               the  ability  of the  primary  obligor  to make  payment  of such
               primary  obligation,  or (d) otherwise to assure or hold harmless
               the holder of any such primary obligation against loss in respect
               thereof, or (e) to purchase or otherwise acquire, or otherwise to
               assure a  creditor  against  loss in  respect  of any  Debt.  For
               purposes  of  this  definition,  the  amount  of  any  Contingent
               Obligation  shall be deemed to be an amount  equal to the maximum
               reasonably anticipated liability in respect thereof;

(h)            "Conversion Amount" means, the portion of the principal amount of
               this  Convertible Note elected by the Noteholder to be converted,
               which amount may be all or any portion of the remaining principal
               amount of this Convertible Note;

(i)            "Conversion  Date"  means  the date  specified  in the  Notice of
               Conversion, or if no date is specified therein, the next Business
               Day  following the date that the Notice of Conversion is received
               by the  Issuer  (unless  a filing  under  the HSR Act was made in
               connection  with such  conversion,  in which case the  conversion
               shall  be  deemed  effective  on  the  Business  Day  immediately
               following the expiration or termination of the applicable waiting
               period under the HSR Act); provided, however, that the Conversion
               Date shall be no sooner than one (1)  Business Day after the date
               of  delivery  (by  facsimile  or  otherwise)  of  the  Notice  of
               Conversion  and any Notice of Conversion  delivered to the Issuer
               on a day which is not a Business Day shall be deemed delivered as
               of the next following Business Day;

(j)            "Conversion Price" means U.S.$6.4465405156, subject to adjustment
               in accordance with Section 14;

(k)            "Convertible  Note" means this  Convertible Note and "Convertible
               Notes" mean,  collectively,  this  Convertible Note and the other
               Convertible  Notes of like  tenor  all  issued  on the same  date
               totalling $55,000,000 in original principal amount;

(l)            "Debt"  means:  (i)  all  indebtedness  of  the  Issuer  and  its
               Subsidiaries   for  borrowed  money,   including   borrowings  of
               commodities,  bankers' acceptances,  letters of credit or letters
               of  guarantee;  (ii)  all  indebtedness  of the  Issuer  and  its
               Subsidiaries  for the  deferred  purchase  price of  property  or
               services  other  than for goods  and  services  purchased  in the
               ordinary  course  of  business  and paid for in  accordance  with
               customary practice and not represented by a note, bond, debenture
               or other evidence of indebtedness; (iii) all indebtedness created
               or arising under any  conditional  sale or other title  retention
               agreement with respect to property acquired by the Issuer and its
               Subsidiaries  (even  though the rights and remedies of the seller
               or  lender  under  such  agreement  in the event of  default  are
               limited  to  repossession  or sale of such  property);  (iv)  all
               current   liabilities   of  the  Issuer   and  its   Subsidiaries
               represented  by a note,  bond,  debenture  or other  evidence  of
               indebtedness; (v) all obligations under leases which have been or
               should be, in  accordance  with U.S.  GAAP,  recorded  as capital
               leases in respect of which the  Issuer and its  Subsidiaries  are
               liable as lessee; and (vi) all indebtedness of the kinds referred
               to in (i)  through  (v) above  which is  directly  or  indirectly
               guaranteed by the Issuer and its Subsidiaries or which the Issuer
               and its Subsidiaries  have agreed  (contingently or otherwise) to
               purchase or otherwise acquire,  or in respect of which the Issuer
               and its Subsidiaries  have otherwise  assured a creditor or other
               Person against loss;

(m)            "Existing System Equipment Financing" means Debt, up to a maximum
               principal  amount of  U.S.$4,000,000,  incurred by the Issuer for
               the  purpose  of  enabling  the  Issuer to  acquire  hardware  or
               software,  to replace or upgrade  hardware and software  owned by
               the  Issuer  as at the date of this  Note but only to the  extent
               that the  proceeds  of such  Debt are  applied  to  acquire  such
               hardware or software;

(n)            "Event of Default" means any one or more of the following events:

               (i)  if the  Issuer  defaults  in payment  of the  principal  of,
                    and/or  interest  on,  this  Convertible  Note when the same
                    becomes  due, and  continuance  of such failure for five (5)
                    Business  Days  after the date on which  such  principal  or
                    interest is due;

               (ii) if  a  decree,   judgement   or  order  by  a  court  having
                    jurisdiction  shall have been entered adjudging the Issuer a
                    bankrupt or  insolvent,  or  approving  as properly  filed a
                    petition seeking liquidation or reorganization of the Issuer
                    under any applicable  bankruptcy,  reorganization or similar
                    law,  and  such  decree,   judgement  or  order  shall  have
                    continued undismissed, undischarged or unstayed for a period
                    of 60 calendar days;

               (iii)if the Issuer shall institute  proceedings to be adjudicated
                    a voluntary  bankrupt,  or shall  consent to the filing of a
                    bankruptcy  proceeding  against it, or shall file a petition
                    or  answer  or  consent  seeking  reorganization  under  any
                    applicable  bankruptcy,  liquidation  or  reorganization  or
                    similar  law,  or shall  consent  to the  filing of any such
                    petition,  or shall consent to the appointment of a receiver
                    or  liquidator  or  trustee or  assignee  in  bankruptcy  or
                    insolvency  of it or of its  property or of any  substantial
                    part thereof, or shall make an assignment for the benefit of
                    creditors or a proposal under any  applicable  bankruptcy or
                    other law, or shall admit in writing  its  inability  to pay
                    its debts generally as they become due;

               (iv) if there  occurs a default in the payment of any  principal,
                    premium,  interest or other  amount due on (or for) any Debt
                    (other than this  Convertible  Note),  satellite  insurance,
                    satellite construction costs or in any payment due under any
                    obligation,  guarantee  or  indemnity  of the  Issuer  in an
                    aggregate amount in excess of U.S.$1,000,000 ("Obligations")
                    beyond any period of grace provided with respect thereto, or
                    if any other default occurs in the  performance of any other
                    term,   condition   or  covenant   contained   in  any  such
                    Obligations  or any agreement  under which such  Obligations
                    are created,  the result of which is to cause any  holder(s)
                    of such Obligations (or a trustee or agent on behalf of such
                    holder(s)) to accelerate such Obligations, provided that the
                    resulting  default  under  this  Convertible  Note  shall be
                    deemed  to be cured or  waived if such  other  defaults  are
                    cured or waived;

               (v)  if the Issuer  shall fail to carry out or observe  any other
                    covenant  or  condition  of  this  Convertible  Note  or any
                    covenant  contained in the Investment  Agreement on its part
                    to be observed or performed  and after notice in writing has
                    been given to the Issuer by the Noteholder  specifying  such
                    default and  requiring the Issuer to put an end to the same,
                    the Issuer shall fail to remedy or cure such default  within
                    a period of 30 calendar days;

               (vi) if any representation,  warranty, certification or statement
                    made  by the  Issuer  (or its  predecessor)  or  Parent  (as
                    defined  in the  Investment  Agreement)  in  the  Investment
                    Agreement or any other  Ancillary  Agreement  (as defined in
                    the Investment Agreement) to which the Issuer or Parent is a
                    party or in any  certificate,  financial  statement or other
                    document  delivered  pursuant  to the  Investment  Agreement
                    shall prove to have been  incorrect in any material  respect
                    when made (or deemed made); or

               (vii)if the Issuer or any  Subsidiary  of the Issuer or  Canadian
                    License  Co.  shall  have  lost  ownership  of or  rights in
                    respect of the FCC Licenses or the Canadian Licenses, except
                    for such losses of  ownership or rights that would not cause
                    a Material Adverse Effect on the Issuer and its Subsidiaries
                    taken as a whole.

(o)            "FCC  Licenses"   means  the  licenses   issued  by  the  Federal
               Communications  Commission to MSI and which were  transferred  to
               the Issuer or a Subsidiary of the Issuer  pursuant to the Amended
               and  Restated  Sub  Asset  Sale  Agreement  (as  defined  in  the
               Investment Agreement);

(p)            "GAAP" means generally accepted  accounting  principles set forth
               in the opinions and  pronouncements of the Accounting  Principles
               Board and the American  Institute of Certified Public Accountants
               and statements  and  pronouncements  of the Financial  Accounting
               Standards Board (or agencies with similar functions of comparable
               stature and authority  within the accounting  profession),  or in
               such other  statements  by such other entity as may be in general
               use by significant  segments of the U.S.  accounting  profession,
               which  are  applicable  to the  circumstances  as of the  date of
               determination;

(q)            "General Partner" means the general partner of the Issuer;

(r)            "Governmental  Authority"  means any  nation or  government,  any
               state or other political  subdivision  thereof,  any central bank
               (or similar monetary or regulatory authority) thereof, any entity
               exercising  executive,   legislative,   judicial,  regulatory  or
               administrative functions of or pertaining to government,  and any
               corporation or other entity owned or controlled, through stock or
               capital ownership or otherwise, by any of the foregoing;

(s)            "Investment" means any investment in any Person, whether by means
               of  share  purchase,   capital  contribution,   loan,  Contingent
               Obligation, time deposit or otherwise;

(t)            "Investment  Agreement" means the Amended and Restated Investment
               Agreement dated October 12, 2001 among Mobile Satellite  Ventures
               LLC (the predecessor to the Issuer),  Motient  Corporation,  TMI,
               and the other parties  identified  therein as  "Investors"  as in
               effect  on the  Issue  Date,  and  without  giving  effect to any
               amendment,  modification,  supplement,  or restatement thereto or
               waiver  thereunder  except to the  extent  that the same has been
               approved in writing by the Lead Note  Investor (as defined in the
               Investment Agreement);

(u)            "Issuer" means Mobile Satellite Ventures LP;

(v)            "Issue Date" means [the date of the First Closing];

(w)            "Lien" means any mortgage, deed of trust, pledge,  hypothecation,
               assignment,  charge or  deposit  arrangement,  encumbrance,  lien
               (statutory or other) or  preference,  priority or other  security
               interest  or  preferential  arrangement  of any  kind  or  nature
               whatsoever  (including,  without  limitation,  those  created by,
               arising under or evidenced by any conditional sale or other title
               retention  agreement,  the  interest of a lessor  under a Capital
               Lease  Obligation,  any financing lease having  substantially the
               same economic  effect as any of the  foregoing,  or the filing of
               any  financing  statement  naming the owner of the asset to which
               such lien relates as debtor, under the Uniform Commercial Code or
               any  comparable  law) and any  contingent  or other  agreement to
               provide any of the foregoing;

(x)            "Limited Partner" and "Limited  Partners" shall have the meanings
               given such term in the Newco LP Agreement;

(y)            "Material  Adverse Effect" means a material adverse change in, or
               a  material  adverse  effect  upon,  any of (1)  the  operations,
               business,  properties,  condition (financial or otherwise) of the
               Issuer and its Subsidiaries  taken as a whole; (2) the ability of
               the  Issuer or any of its  Subsidiaries  to  perform  under  this
               Convertible  Note,  the  Investment  Agreement,  or  any  of  the
               agreements or  instruments  executed in  connection  therewith to
               which the Issuer or any of its  Subsidiaries  is a party;  or (3)
               the legality,  validity, binding effect or enforceability of this
               Convertible  Note,  the  Investment  Agreement,  or  any  of  the
               agreements or  instruments  executed in  connection  therewith to
               which the Issuer or any of its Subsidiaries is a party;

(z)            "Maturity Date" means the earliest of the following dates:

               (i)  [date which is five years after the First Closing]; and

               (ii) the date,  if any,  on which the  Noteholder  declares  this
                    Convertible  Note due and  payable  pursuant  to  Section  8
                    below;

(aa)           "MSI" means Motient Services Inc.;

(bb)           "MSI Note" means the $15 Million Note of even date  herewith made
               by the Issuer payable to MSI, as in effect on the Issue Date, and
               without giving effect to any amendment, modification, supplement,
               or restatement  thereto or waiver thereunder except to the extent
               that the  same has been  approved  in  writing  by the Lead  Note
               Investor (as defined in the Investment Agreement);

(cc)           "Newco GP Agreement" means the Stockholders'  Agreement of Mobile
               Satellite  Ventures GP Inc.,  the general  partner of the Issuer,
               dated as of the date hereof,  among the parties specified therein
               as in effect on the Issue Date,  and without giving effect to any
               amendment,  modification,  supplement,  or restatement thereto or
               waiver  thereunder  except to the  extent  that the same has been
               approved in writing by the Lead Note  Investor (as defined in the
               Investment Agreement);

(dd)           "Newco LP Agreement" means the Limited  Partnership  Agreement of
               the  Issuer,  dated  as of the date  hereof,  among  the  parties
               specified  therein as in effect on the Issue  Date,  and  without
               giving  effect to any  amendment,  modification,  supplement,  or
               restatement  thereto  or waiver  thereunder  except to the extent
               that the  same has been  approved  in  writing  by the Lead  Note
               Investor (as defined in the Investment Agreement);

(ee)           "Next Generation Financing" means Debt, up to a maximum principal
               amount of U.S.$5,000,000,  incurred by the Issuer for the purpose
               of enabling the Issuer to: (1) acquire  hardware or software;  or
               (2)  pay for  legal,  regulatory  or  consulting  costs  directly
               related to the development of the Issuer's next-generation mobile
               satellite system and its associated terrestrial segment, but only
               to the  extent  that the  proceeds  of such Debt are  applied  to
               acquire  such  hardware  or  software,  or to pay for such legal,
               regulatory and consulting costs;

(ff)           "Notice of Conversion" means the notice of conversion in the form
               attached hereto;

(gg)           "Person"   means  any   individual,   corporation,   association,
               partnership (general or limited),  joint venture,  trust, estate,
               limited liability company, or other legal entity or organization;

(hh)           "Purchase Money  Indebtedness" means any Debt incurred or assumed
               by the  Issuer  to  finance  the  cost  (including  the  cost  of
               construction) of an item of real or personal property  (including
               the Existing System  Equipment  Financing and the Next Generation
               Financing) or the  improvement  to such  property,  the aggregate
               principal  amount of which  Debt does not  exceed  the sum of (i)
               100% of such cost and (ii)  reasonable  fees and  expenses of the
               Issuer incurred in connection therewith;  provided that such Debt
               is incurred or assumed within 90 days of the  acquisition  of, or
               improvement to, such property;

(ii)           "Reduction Event" means any one or more of the following:

                (i) the sale, disposition,  transfer, assignment or lease of any
                    or all of the assets  and/or  business  of the Issuer  other
                    than Permitted Transfers (as defined in Section 12 below);

                (ii)the issuance of Debt (other than  Permitted Debt (as defined
                    in Section 12 below)), where the lesser of:

                    (A) 50% of the total value of the consideration given to the
                    Issuer (or to such other  party as the Issuer may direct) as
                    a result of such Debt,  after deduction for the direct costs
                    incurred to consummate such Debt; and

                    (B) an  amount  of cash  equal to the  total  principal  and
                    interest on this Note outstanding as at the creation of such
                    Debt. is paid to the  Noteholder in cash pursuant to Section
                    10 below;

               (iii)the issuance of the equity of, or an ownership  interest in,
                    the Issuer,  other than issuance of equity (A) at the Second
                    Closing (as defined in the Investment Agreement) pursuant to
                    the  Investment  Agreement,  or  (B)  as  a  result  of  the
                    conversion  of  the  Convertible  Notes  or  (C)  where  the
                    proceeds of such  issuance  consist of  consideration  other
                    than cash;

(jj)           "Reduction   Event  Net  Proceeds"  means,  in  relation  to  any
               Reduction  Event, the total value of the  consideration  given to
               the Issuer (or to such other party as the Issuer may direct) for,
               or as a consequence of, such Reduction Event,  after deduction of
               the  direct  costs  incurred  by the  Issuer to  consummate  such
               Reduction Event;

(kk)           "Required Note  Majority"  means the holders of a majority of the
               aggregate  outstanding principal amounts outstanding under all of
               the Convertible Notes;

(ll)           "Subsidiary"  means,  as to any Person,  any corporation or other
               entity of which  securities or other ownership  interests  having
               ordinary  voting  power  to  elect a  majority  of the  board  of
               directors or other persons  performing  similar  functions are at
               the time directly or indirectly owned by such Person;

(mm)           "TMI" means TMI Communications and Company, Limited Partnership;

(nn)           "TMI Note"  means the $11.5  Million  Note of even date  herewith
               made by the Issuer payable to TMI as in effect on the Issue Date,
               and  without  giving  effect  to  any  amendment,   modification,
               supplement, or restatement thereto or waiver thereunder; and

(oo)           "Units"  shall have the  meaning  given such term in the Newco LP
               Agreement.

2.   This Convertible  Note is a direct and unsecured  senior  obligation of the
     Issuer.  The Noteholder  agrees that the Note is  subordinated  in right of
     payment  to the prior  payment in full of the  Issuer's  Debt under the TMI
     Note.  The  provisions  of this  paragraph  may not be  modified or amended
     without  the  prior  consent  of  TMI  for  so  long  as the  TMI  Note  is
     outstanding.  TMI shall be a  third-party  beneficiary  of the  immediately
     preceding sentence for so long as the TMI Note is outstanding.

     The foregoing  provisions of this Section 2 are and are intended solely for
     the purpose of defining the relative  rights of the  Noteholder  on the one
     hand and the holders of the TMI Note on the other hand.  Nothing  contained
     in  this  Section  2 or  elsewhere  in  this  Convertible  Note  or in  the
     Investment  Agreement  is  intended  to or shall (a)  impair,  as among the
     Issuer,  its  creditors  other  than  holders  of the  TMI  Note,  and  the
     Noteholder,   the   obligation  of  the  Issuer,   which  is  absolute  and
     unconditional,  to pay to the  Noteholder the principal of, and interest on
     this  Convertible Note as and when the same shall become due and payable in
     accordance  with its terms;  or (b) affect the relative  rights against the
     Issuer of the Noteholder and creditors of the Issuer other than the holders
     of the TMI Note; or (c) prevent the Noteholder from exercising all remedies
     otherwise permitted by applicable law upon a default or an Event of Default
     under this Convertible Note.

     The failure to make a payment on account of  principal  of, or interest on,
     this  Convertible  Note by reason of the first  paragraph  of this  section
     shall not be  construed as  preventing  the  occurrence  of a default or an
     Event of Default hereunder.

3.   The principal amounts outstanding under this Convertible Note bear interest
     (both before and after  maturity,  default and  judgement  with interest on
     overdue  interest  at the same rate) from the Issue Date at the rate of ten
     percent  (10%) per  annum,  calculated  and  compounded  semi-annually  and
     payable in arrears on the Maturity  Date, or earlier as otherwise  provided
     herein.  The principal amounts owing under this Convertible Note will cease
     to  bear  interest  on the  date  on  which  all  principal  owed  on  this
     Convertible  Note  are  fully  repaid,   unless  payment  of  principal  is
     improperly withheld or refused by the Issuer.  Interest shall not accrue on
     accrued  but  unpaid  interest  unless and until  such  accrued  but unpaid
     interest  becomes  overdue,  in which case such overdue interest shall bear
     interest at the rate per annum set forth above  commencing on the first day
     that such interest  becomes overdue until such overdue  interest is paid in
     full.

4.   The principal of this Convertible Note and interest thereon will be payable
     in United States Dollars, at the option of the Noteholder:  (i) at the head
     office of the  Noteholder;  (ii) at such other place as directed in writing
     by the  Noteholder;  or (iii) by  electronic  funds  transfer to an account
     maintained by the Noteholder with a bank in the United States.

5.   If the date for payment of any amount of  principal  or interest in respect
     of this Convertible  Note is not, at the place of payment,  a Business Day,
     the Noteholder shall not be entitled to payment of the amount due until the
     next  following  Business  Day at the  place of  payment  and  shall not be
     entitled to any further interest or payment in respect of such delay.

6.   Any monies  paid on this  Convertible  Note shall be applied,  firstly,  on
     account of accrued interest, and secondly, on account of principal.

7.   Unless  previously  converted,  redeemed or purchased  by the Issuer,  this
     Convertible  Note will mature at its principal  amount then  outstanding on
     the Maturity Date.

8.   If an Event of Default other than those in (ii) and (iii) of the definition
     of Event of Default occurs the Noteholder may, at its option,  by notice in
     writing to the Issuer, declare this Convertible Note to be due and payable,
     and upon such declaration,  unless all Events of Default have been cured by
     the  Issuer  prior  to  receipt  by the  Issuer  of such  declaration,  all
     principal  of, and interest  on, this  Convertible  Note shall  immediately
     become  due and  payable.  In the  case of the  occurrence  of an  Event of
     Default  referred to in clause (ii) or (iii) of the definition of "Event of
     Default" the principal amount then outstanding of, and the accrued interest
     on any amounts  owing under this  Convertible  Note,  and all other amounts
     payable by the Issuer hereunder shall become automatically  immediately due
     and  payable  without  presentment,  demand,  diligence,  protest  or other
     formalities  of any kind, all of which are hereby  expressly  waived by the
     Issuer.

9.   The Issuer agrees to pay to Noteholder and reimburse Noteholder for any and
     all  reasonable  costs and expenses,  including  attorney's  fees and court
     costs,  if  any,   incurred  by  the  Noteholder  in  connection  with  the
     enforcement or collection hereof, both before and after the commencement of
     any action to enforce or collect this Convertible  Note, but whether or not
     any such action is commenced by the  Noteholder.  The Issuer  hereby waives
     presentment,  demand, protest or other notice of any kind, and all defenses
     and pleas on the grounds of  extension of the time of payments or due dates
     of this Convertible  Note, in the enforcement of this Convertible Note, and
     expressly agrees that this Convertible Note or any payment hereunder may be
     extended  from time to time without in any way  affecting  the liability of
     the Issuer hereunder.  The rights and remedies of the Noteholder  hereunder
     shall  be  cumulative  and  concurrent  and  may  be  pursued   singularly,
     successively or together at the sole discretion of the Noteholder,  and may
     be exercised as often as occasion  therefor shall occur, and the failure to
     exercise  any such  right or  remedy  shall in no event be  construed  as a
     waiver or release of the same or any other right or remedy.

10.  Within five Business Days  following the  occurrence of a Reduction  Event,
     the Issuer  shall give  written  notice  thereof to the  Noteholder,  which
     notice shall set forth in  reasonable  detail the terms of the  transaction
     constituting such Reduction Event. After the TMI Note has been paid in full
     and until such time as this  Convertible  Note has been repaid in full, the
     Noteholder  may,  within 10 days after receipt of the notice of a Reduction
     Event referred to above,  elect that the Issuer pay to the  Noteholder,  in
     cash,  its pro rata  portion  of 50% of all  Reduction  Event Net  Proceeds
     immediately  upon receipt by the Issuer of the same. The  Noteholder's  pro
     rata portion  shall be  determined  by reference  to the  principal  amount
     outstanding  under  this  Convertible  Note,  on the one hand,  and the (i)
     aggregate  principal amount  outstanding under all of the Convertible Notes
     (without regard to whether any other holders of the Convertible  Notes have
     elected to have the Issuer pay their  respective  portions of the Reduction
     Event Net Proceeds in connection  with such Reduction  Event) plus (ii) the
     principal amount  outstanding under the MSI Note, on the other hand. If the
     Noteholder  exercises such option, the Noteholder may require the Issuer to
     direct the person to whom assets or securities are sold in connection  with
     such  Reduction  Event to pay such monies owed to the  Noteholder  pursuant
     this Section 10 directly to the Noteholder.

11.  If, for any given  Reduction  Event,  the cash  payment  to the  Noteholder
     described in Section 10 above does not repay this Convertible Note in full,
     the Noteholder will have the option,  but not the  obligation,  to exchange
     the remaining  portion of this  Convertible  Note for a pro-rata portion of
     the  debt  or  equity  instruments,   if  any,  issued  by  the  Issuer  as
     consideration for such Reduction Event. Upon such exchange,  the Noteholder
     shall  be  entitled  to  receive   accrued  and  unpaid  interest  on  this
     Convertible Note through the date of such exchange, which interest shall be
     payable by the Issuer in cash on the earlier of the Second  Closing Date or
     the Maturity Date. 12. Except for any transaction,  agreement, undertaking,
     activity  or other  action  required in order for the Issuer to perform its
     obligations  specifically  set  forth in the  Investment  Agreement  or any
     Ancillary  Agreement  (as defined in the  Investment  Agreement) or for the
     Issuer to exercise any of its rights  specifically  set forth therein,  the
     Issuer  agrees that,  for so long as 25% or more of the original  aggregate
     principal amount of the Convertible Notes remain  outstanding,  without the
     prior consent of the Required Note Majority:

(a)  neither  the Issuer nor any  Subsidiary  of the Issuer  shall,  directly or
     indirectly, make, create, incur, assume or suffer to exist any Lien upon or
     with  respect to any part of its  property or assets,  whether now owned or
     hereafter  acquired,  or offer or agree to do so, other than the  following
     ("Permitted Liens"):

        (i)    any Lien in favor of the Noteholder;

        (ii)   Liens to  secure  Purchase  Money  Indebtedness  permitted  under
               Section  12(g)(i) below,  provided that such Liens cover only the
               assets acquired with such Purchase Money Indebtedness.

        (iii)  Liens for taxes, fees,  assessments or other governmental charges
               which are not  delinquent  or  remain  payable  without  penalty,
               unless the same are being  contested in good faith by appropriate
               proceedings  and adequate  reserves in  accordance  with GAAP are
               maintained;

        (iv)   carriers', warehousemen's, mechanics', landlords', materialmen's,
               repairmen's or other similar Liens arising in the ordinary course
               of business  which do not secure Debt and are not  delinquent  or
               remain payable without penalty;

        (v)    Liens (other than any Lien imposed by Employee  Retirement Income
               Security  Act of 1974,  as amended) on the property of the Issuer
               incurred,  or pledges or deposits  required,  in connection  with
               workmen's  compensation,  unemployment insurance and other social
               security legislation;

        (vi)   Liens on the property of the Issuer  securing (1) the performance
               of bids,  trade contracts (other than for borrowed money or other
               Debt),  leases,  statutory  obligations,  and (2)  obligations on
               surety  and appeal  bonds,  and (3) other  obligations  of a like
               nature  incurred in the ordinary  course of business which do not
               secure Debt,  provided that all such Liens in the aggregate could
               not cause a Material Adverse Effect;

        (vii)  easements,   rights-of-way,   restrictions   and  other   similar
               encumbrances  incurred in the ordinary  course of business which,
               in the aggregate, are not substantial in amount, and which do not
               in any case  materially  detract  from the value of the  property
               subject  thereto or interfere  with the  ordinary  conduct of the
               businesses of the Issuer;

        (viii) Liens on contract rights under subscriber  equipment leases sold,
               pledged  or  otherwise  transferred  pursuant  to any  bona  fide
               financing of such leases; and

        (ix)   Liens to secure Debt permitted under Section 12(g)(vii); provided
               that all such Liens in the  aggregate  could not cause a Material
               Adverse Effect.

(b)  neither  the Issuer nor any  Subsidiary  of the Issuer  shall,  directly or
     indirectly,  (i) sell, assign, lease, convey, transfer or otherwise dispose
     of (whether in one or a series of transactions) any of its assets, business
     or property,  (including  accounts and notes  receivable)  (with or without
     recourse),  or enter into  equipment  sale-leaseback  transactions  or (ii)
     merge or consolidate with any other Person,  or enter into any agreement to
     do any of the  foregoing  described  in clauses  (i) or (ii) except for the
     following ("Permitted Transfers"):

        (i)    sales,  transfers,  or other dispositions of inventory,  or used,
               worn-out  or surplus  property,  or property of no further use to
               the Issuer, all in the ordinary course of business;

        (ii)   sales,  transfers,  or other  dispositions  of  equipment  in the
               ordinary  course of business to the extent that such equipment is
               exchanged  for  credit  against  the  purchase  price of  similar
               replacement equipment or the proceeds of such sale are reasonably
               promptly  applied  to the  purchase  price  of  such  replacement
               equipment;

        (iii)  sales,   transfers,   or  other  dispositions  of  communications
               services,   capacity  or  equipment   pursuant  to  the  customer
               contracts  providing  for the  sale of  communications  services,
               capacity or equipment in the ordinary course of business;

        (iv)   sales, transfers, or other dispositions of assets in the ordinary
               course of  business  having a fair  market  value  not  exceeding
               $500,000 per item or  $1,000,000  in the  aggregate in any fiscal
               year (excluding  sales,  transfers and  dispositions  theretofore
               approved  in  accordance  with the terms  hereof  in such  fiscal
               year);

        (v)    sales,  transfers or other  dispositions of contract rights under
               subscriber  equipment  leases pursuant to any bona fide financing
               of such leases permitted under Section 12(g) below;

        (vi)   non-exclusive licenses of technology and other intangible assets,
               excluding spectrum licenses;

        (vii)  sales of mobile earth terminals and related equipment,  and other
               inventory;

        (viii) sales,  transfers, or other dispositions of assets as a result of
               the combination and integration of the business of MSI and TMI as
               contemplated by the Operations Plan, including without limitation
               the sale,  transfer or other  disposition  of: (y) duplicative or
               unnecessary  assets, or (z) assets in connection with the closing
               of facilities  and  reductions  in workforce;  provided that such
               disposition does not include a disposition of the FCC Licenses or
               the Canadian Licenses;

        (ix)   any wholly-owned Subsidiary of the Issuer may merge,  consolidate
               or  combine  with or into,  or  transfer  (for no  consideration)
               assets  to the  Issuer;  provided  that the  Issuer  shall be the
               continuing or surviving corporation; and

        (x)    any  wholly-owned  Subsidiary  of the  Issuer  may  sell,  lease,
               transfer or  otherwise  dispose of any or all of its assets (upon
               voluntary  liquidation  or  otherwise),  to another  wholly-owned
               Subsidiary.

               For  certainty,  "Permitted  Transfers"  shall not  include:  (1)
               sales,  assignments,  leases,  conveyances,  transfers  or  other
               dispositions of the Issuer or any of its Subsidiaries  outside of
               the  ordinary  course of  business  other  than as  permitted  in
               paragraphs  (viii),  (ix)  and  (x)  above;  and  (2)  the  sale,
               assignment,  lease,  conveyance,  transfer  or other  disposition
               (whether  in  one  or  a  series  of   transactions)  of  all  or
               substantially  all of the  assets of any of the  Issuer or any of
               its Subsidiaries or Canadian License Co.

(c)  neither  the Issuer nor any  Subsidiary  of the Issuer  shall,  directly or
     indirectly,  make or acquire any  Investment  in any Person  other than the
     following:

        (i)    Investments in Persons which are wholly-owned Subsidiaries of the
               Issuer  (including  Newco  Sub  (as  defined  in  the  Investment
               Agreement)) on the date hereof;

        (ii)   Investments in Mobile Satellite Ventures (Canada) Inc. and Mobile
               Satellite  Ventures  (Canada)  Holdings  Inc.  as required by the
               Capacity Lease  Agreement,  the Rights and Services  Agreement or
               the Canadian Shareholders Agreement (as such terms are defined in
               the Investment Agreement);

        (iii)  Cash Equivalents; and

        (iv)   any Investment not otherwise  permitted by the foregoing  clauses
               (i), (ii) and (iii) if, immediately after such Investment is made
               or acquired,  the  aggregate  net book value (or  aggregate  cost
               thereof, if greater than book value at the time of determination)
               of all Investments permitted by this clause (iii) does not exceed
               $2,000,000.

(d)  neither  the  Issuer  nor any  Subsidiary  of the  Issuer  shall  repay any
     outstanding Debt to holders of equity securities of the Issuer prior to the
     Second  Closing  (as such term is  defined  in the  Investment  Agreement),
     except for repayments under the Convertible Notes, repayments under the TMI
     Note and  repayments  under the MSI Note,  each in  accordance  with  their
     respective terms.

(e)  other than Investments  permitted by Section 12(c)(ii),  neither the Issuer
     nor any Subsidiary of the Issuer shall, directly or indirectly, (i) pay any
     funds to or for the account of any  Affiliate  which is not a  wholly-owned
     Subsidiary of the Issuer,  (ii) make any investment in any Affiliate  which
     is not a wholly-owned  Subsidiary of the Issuer  (whether by acquisition of
     stock or indebtedness, by loan, advance, transfer of property, guarantee or
     other agreement to pay,  purchase or service,  directly or indirectly,  any
     Debt, or otherwise),  (iii) lease,  sell,  transfer or otherwise dispose of
     any  assets,  tangible  or  intangible,  to any  Affiliate  which  is not a
     wholly-owned  Subsidiary  of the  Issuer,  or (iv)  except in the  ordinary
     course of  business  consistent  with  past  practice,  participate  in, or
     effect,   any  other   transaction  with  any  Affiliate  which  is  not  a
     wholly-owned   Subsidiary  of  the  Issuer,  except  in  each  case  on  an
     arm's-length  basis on terms at least as  favorable  to the Issuer as could
     have been  obtained  from a third  party  that was not an  Affiliate  or as
     otherwise expressly approved in writing by the Noteholder.

(f)  neither the Issuer nor any  Subsidiary  of the Issuer  shall (i) declare or
     make any  dividend  payment or other  distribution  of assets,  properties,
     cash,  rights,  obligations  or  securities  on account of any Units of any
     class of its equity,  or (ii) redeem,  repurchase or otherwise  acquire for
     value any Units of its equity or any warrants, rights or options to acquire
     such  Units,  now or  hereafter  outstanding  except,  in either  case,  as
     provided in the Convertible Notes or in the Newco LP Agreement.

(g)  neither the Issuer nor any  Subsidiary of the Issuer shall  create,  incur,
     assume,  guaranty,  suffer to exist, or otherwise become or remain directly
     or  indirectly  liable with respect to, any Debt,  other than the following
     (the "Permitted Debt"):

        (i)    Purchase Money  Indebtedness not to exceed  U.S.$9,000,000 at any
               one time outstanding;

        (ii)   accounts  payable to trade  creditors  for goods and services and
               current operating liabilities (not the result of the borrowing of
               money) incurred in the ordinary course of the Issuer's  business,
               as the case may be, in accordance  with customary  terms and paid
               within the  specified  time,  unless  contested  in good faith by
               appropriate proceedings and reserved for in accordance with GAAP;

        (iii)  income taxes payable and deferred taxes;

        (iv)   accrued expenses and deferred income;

        (v)    Debt  outstanding  on the  Issue  Date  that  is  identified  and
               quantified on Schedule B hereto;

        (vi)   Debt  incurred  to finance  in-orbit  insurance  in an  aggregate
               amount outstanding at any time not to exceed $5,000,000;

        (vii)  any other Debt incurred after the date hereof;  provided that the
               aggregate outstanding principal amount of all such Debt shall not
               at any time exceed $2,000,000 outstanding; and

        (viii) other Debt that satisfies the following five conditions: (1) such
               Debt is  subordinated  to this  Convertible  Note  pursuant  to a
               written  subordination  agreement  satisfactory to the Noteholder
               which  provides that no payment shall be made on or in respect of
               such Debt until the Debt  evidenced by this  Convertible  Note is
               paid in full in cash,  (2) the  maturity  date of such  Debt is a
               date  subsequent to the first  anniversary  of the Maturity Date,
               (3) such Debt is not mandatorily redeemable pursuant to a sinking
               fund or  otherwise,  and does not have a scheduled  principal  or
               similar  payment  due,  prior  to the  first  anniversary  of the
               Maturity  Date,  (4) such Debt is not redeemable at the option of
               the holder thereof, in whole or in part, upon the happening of an
               event,  passage  of  time,  or  otherwise,  prior  to  the  first
               anniversary  of the  Maturity  Date,  and  (5)  such  Debt is not
               convertible  into or exercisable or  exchangeable  for any equity
               securities,   warrants,  rights  or  options  to  acquire  equity
               securities of the Issuer or any Subsidiary of the Issuer,  nor is
               such Debt issued with any such warrants, rights or options or any
               agreement to issue such warrants, rights or options of the Issuer
               or any Subsidiary of the Issuer.

(h)  neither the Issuer nor any Subsidiary of the Issuer shall (i) amend, alter,
     repeal or otherwise modify, or consent to the amendment, alteration, repeal
     or other modification of, the FCC Licenses or (ii) amend,  alter, repeal or
     otherwise modify, or consent to the amendment,  alteration, repeal or other
     modification of, any agreements relating to the Canadian Licenses, or waive
     any  contractual   rights  thereto;   except  in  each  case,   amendments,
     alterations,  repeals,  or other  modifications or waivers (y) which do not
     have  Material  Adverse  Effect on the Issuer,  or (z) required by any law,
     rule, regulation or order of any Government Authority.

(i)  prior to the Second Closing, the Issuer shall not permit the removal of the
     General Partner or the appointment of a replacement General Partner.

13.  This  Convertible Note shall be convertible into Class A Preferred Units of
     the Issuer as follows:

(a)  Subject to the limitations on conversions contained in this Section 13, the
     Noteholder  may,  at any time and from time to time  until the  outstanding
     principal  amount of this  Convertible Note is paid in full (and whether or
     not an Event of Default exists under this  Convertible  Note),  convert (an
     "Optional  Conversion")  all or a portion of the  principal  amount of this
     Convertible  Note (but not,  for the  avoidance  of  doubt,  any  interest,
     including  without  limitation any interest which may have been  compounded
     pursuant to Section 3 above)  into a number of Class A  Preferred  Units of
     the Issuer equal to the number determined by dividing the Conversion Amount
     by the Conversion  Price in effect on the Conversion  Date. To convert this
     Convertible  Note,  the  Noteholder  must (a) complete and sign a Notice of
     Conversion  substantially in the form attached  hereto,  (b) surrender this
     Convertible  Note to the Issuer,  (c) furnish  appropriate  endorsements or
     transfer  documents  if  required by the  Issuer,  (d) pay any  transfer or
     similar  tax,  if  required,  and (e)  execute  and  deliver  documentation
     required by the Issuer in order for the Noteholder to join and become bound
     by the terms of the Newco LP Agreement. The outstanding principal amount of
     this  Convertible  Note shall be reduced  by the  portion of the  principal
     amount  converted  into Class A Preferred  Units.  The converted  principal
     amount of this Convertible Note shall cease to accrue interest. The accrued
     but unpaid interest on the converted  principal  amount of this Convertible
     Note shall be due and  payable on the earlier of (x) the  Maturity  Date or
     (y) the Second Closing.

(b)  Subject  to Section  13(g)  below,  as soon as  practicable  following  the
     receipt of the deliveries required by Section 13(a), the Issuer shall issue
     and deliver to the  Noteholder  (x) that number of Class A Preferred  Units
     issuable  upon  conversion  of the portion of this  Convertible  Note being
     converted in the Optional  Conversion and (y) a new Convertible Note in the
     form hereof  representing  the balance of the  principal  amount hereof not
     being  converted,  if any.  Delivery  under this Section  13(b) may be made
     personally  or by  reputable  overnight  courier.  The  person  or  persons
     entitled to receive Class A Preferred  Units issuable upon such  conversion
     shall be treated  for all  purposes  as the  record  holder of such Class A
     Preferred  Units at the close of business on the  Conversion  Date and such
     Class A Preferred Units shall be issued and outstanding as of such date.

(c)  Upon the  consummation  of the Second Closing (as defined in the Investment
     Agreement),  the entire  outstanding  principal  amount of this Convertible
     Note (the "Automatic Conversion Amount") shall automatically convert into a
     number  of Class A  Preferred  Units  of the  Issuer  equal  to the  number
     determined by dividing the Automatic  Conversion  Amount by the  Conversion
     Price in effect on the Second  Closing  Date (as defined in the  Investment
     Agreement).  Upon such  conversion  against  issuance  and delivery of such
     Class A  Preferred  Units free and clear of any  encumbrances  (other  than
     those arising pursuant to applicable  federal and state  securities  laws),
     all  payment  obligations  of the  Issuer  in  respect  to the  outstanding
     principal  amount  of this  Convertible  Note  hereunder  shall  be  deemed
     satisfied.  Upon such conversion of this Convertible  Note, the accrued and
     unpaid interest on this Convertible Note shall  immediately  become due and
     payable in cash.  Upon  receipt of  written  notice  from the Issuer of the
     occurrence of the Second Closing,  the Noteholder  shall (i) surrender this
     Convertible Note to the Issuer,  (ii) furnish  appropriate  endorsements or
     transfer  documents  if required by the Issuer,  (iii) pay any  transfer or
     similar  tax, if  required,  and (iv)  execute  and  deliver  documentation
     required by the Issuer in order for the Noteholder to join and become bound
     by the terms of the Newco LP Agreement.  Subject to Section 13(g) below, as
     promptly as  practicable  after the receipt of the  deliveries  required by
     this Section 13(c),  the Issuer shall issue and deliver to the  Noteholder,
     that number of Class A Preferred  Units  issuable  upon  conversion of this
     Convertible  Note. From and after the Second Closing,  the registered owner
     of this  Convertible  Note  shall  cease  to have  any  rights  under  this
     Convertible  Note  except the right to receive  payment of the  accrued and
     unpaid interest on this Convertible Note and those other things to which it
     is entitled  pursuant to this Section 13(c),  the principal  amount of this
     Convertible Note shall be deemed no longer  outstanding and shall be deemed
     satisfied  in full,  and this  Convertible  Note  shall not  thereafter  be
     transferred  (except  with the  consent of the  Issuer) on the books of the
     Issuer and shall not be deemed outstanding for any purpose whatsoever.

(d)  No  fractional  interest  in a Class A  Preferred  Unit will be issued upon
     conversion but a cash adjustment will be made for any fractional interest

(e)  The  Noteholder  shall pay any and all taxes  (other than  transfer  taxes)
     which may be imposed with respect to the issuance and delivery of the Class
     A Preferred Units upon the conversion of this Convertible Note.

(f)  In the case of any dispute with respect to a  conversion,  the Issuer shall
     promptly  issue such number of Class A Preferred  Units as are not disputed
     in  accordance  with this  Section 13. If such  dispute  only  involves the
     calculation of the Conversion  Price,  the Issuer shall submit the disputed
     calculations  to  an  independent  accounting  firm  of  national  standing
     (reasonably  acceptable to the Noteholder) within five (5) Business Days of
     receipt of the Notice of Conversion.  The  accounting  firm shall audit the
     calculations  and notify the Issuer and the  Noteholder  of the  results no
     later  than  fifteen  (15)  Business  Days  from the date it  receives  the
     disputed  calculations.  The accounting firm's  calculation shall be deemed
     conclusive. As soon as possible thereafter, the Issuer shall then issue the
     appropriate  number  of Class A  Preferred  Units in  accordance  with this
     Section 13.

(g)  Prior to any  conversion,  the Noteholder and the Issuer shall cooperate in
     good  faith  (A)  to   determine   if  a  filing  is  required   under  the
     Hart-Scott-Rodino  Antitrust Improvements Act of 1976 or any successor law,
     and  regulations and rules issued pursuant to that Act or any successor law
     (the "HSR Act") in  connection  with such  conversion,  and (B) to make the
     filing under the HSR Act as promptly as practicable  after a  determination
     is made that such a filing is necessary.  The Issuer shall take all actions
     reasonably  requested by the  Noteholder to cause the early  termination of
     any applicable waiting period under the HSR Act.  Notwithstanding  anything
     in  this  Convertible  Note to the  contrary,  if a  determination  is made
     pursuant to this Section 13(g) that a filing under the HSR Act is required,
     the Issuer  shall not be  obligated  to issue any Class A  Preferred  Units
     pursuant to this  Section 13 until the date that is two (2)  Business  Days
     following the expiration or  termination  of the applicable  waiting period
     under the HSR Act

14.  In order to prevent dilution of the conversion rights granted under Section
     13, the Conversion  Price shall be subject to adjustment  from time to time
     pursuant  to this  Section  14. In the  event  that any  adjustment  of the
     Conversion  Price as required  herein results in a fraction of a cent, such
     Conversion  Price shall be rounded to the nearest  .00001.  For purposes of
     determining  the  adjusted  Conversion  Price  under this  Section  14, the
     following provisions shall be applicable:

(a)  If the  Issuer  shall  declare  or  make  any  distribution  of its  assets
     (including  cash) to holders of any Units (except for  distributions to pay
     taxes in  accordance  with  Section 8.2 of the LP  Agreement)  as a partial
     liquidating dividend, by way of return of capital or otherwise, then, after
     the date of  record  for  determining  holders  of Units  entitled  to such
     distribution,  but prior to the date of such  distribution,  the Noteholder
     shall be  entitled  to receive  the amount of such  assets  which,  if this
     Convertible   Note  had  been  converted  into  Class  A  Preferred   Units
     immediately  prior to such time,  would have been  payable to the holder of
     such Class A Preferred  Units on the record date for the  determination  of
     partners entitled to such distribution.  Notwithstanding anything herein to
     the contrary,  no adjustment  in the  Conversion  Price shall be made under
     this  Section  14(a) to the  extent  the  Noteholder  participates  in such
     Noteholder's  capacity as a holder of Class A  Preferred  Units in any such
     distribution of assets in accordance with this Section 14(a).

(b)  If  the  Issuer  at  any  time   subdivides   (by  any   split,   dividend,
     recapitalization,   reorganization,   reclassification  or  otherwise)  the
     outstanding  Units into a greater number of Units,  then, after the date of
     record for  effecting  such  subdivision,  the  Conversion  Price in effect
     immediately prior to such subdivision shall be proportionately  reduced. If
     the  Issuer  at any time  combines  (by  reverse  split,  recapitalization,
     reorganization, reclassification or otherwise) the outstanding Units into a
     smaller number of Units,  then, after the date of record for effecting such
     combination,  the  Conversion  Price in  effect  immediately  prior to such
     combination shall be proportionately increased.

(c)  If the Issuer shall, at any time or from time to time after the Issue Date,
     issue any Units  (other  than an  issuance  of Units as a dividend  or in a
     split of or subdivision in respect of which the adjustment  provided for in
     Section  14(b)  applies),  options to purchase or rights to  subscribe  for
     Units,  securities  by their terms  convertible  into or  exchangeable  for
     Units,  or options to purchase or rights to subscribe for such  convertible
     or exchangeable  securities without  consideration or for consideration per
     share less than the Conversion  Price in effect  immediately  prior to such
     issuance,  then such Conversion Price shall forthwith be lowered to a price
     equal to the price obtained by multiplying:

        (i)    the Conversion Price in effect  immediately prior to the issuance
               of such Units, options, rights or securities by

        (ii)   a fraction  of which (x) the  denominator  shall be the number of
               Units outstanding on a fully-diluted basis immediately after such
               issuance and (y) the numerator shall be the sum of (a) the number
               of Units  outstanding on a fully-diluted  basis immediately prior
               to such issuance and (b) the number of additional Units which the
               aggregate  consideration for the number of Units so offered would
               purchase at the Conversion Price.

     For purposes of this Section 14, "fully  diluted basis" shall be determined
in accordance with the treasury stock method of computing fully diluted earnings
per Unit in accordance with GAAP.

(d)  If the Issuer shall, at any time or from time to time after the Issue Date,
     directly or indirectly,  redeem,  purchase or otherwise  acquire any Units,
     options to purchase or rights to subscribe  for Units,  securities by their
     terms  convertible  into or exchangeable  for Units (other than Convertible
     Notes which are redeemed  according to their terms), or options to purchase
     or rights to subscribe for such convertible or exchangeable securities, for
     a consideration  per share greater than the Conversion  Price (plus, in the
     case of such options,  rights, or securities,  the additional consideration
     required to be paid to the Issuer upon  exercise,  conversion  or exchange)
     immediately  prior to such event, then the Conversion Price shall forthwith
     be lowered to a price equal to the price obtained by multiplying:

        (i)    the Conversion Price in effect immediately prior to such event by

        (ii)   a fraction of which (x) the  denominator  shall be the Conversion
               Price immediately prior to such event and (y) the numerator shall
               be the result of dividing:

               (1) the  product  of (A) the  number  of Units  outstanding  on a
               fully-diluted  basis and (B) the Conversion  Price,  in each case
               immediately  prior  to  such  event,   minus  (2)  the  aggregate
               consideration paid by the Issuer in such event (plus, in the case
               of  such  options,   rights,   or  convertible  or   exchangeable
               securities,  the aggregate additional consideration to be paid by
               the Issuer upon  exercise,  conversion or  exchange),  by (2) the
               number of Units outstanding on a fully-diluted  basis immediately
               after such event.

(e)  For the  purposes  of any  adjustment  of a  Conversion  Price  pursuant to
     paragraphs (c) and (d) of this Section 14, the following  provisions  shall
     be applicable:

        (i)    In the  case of the  issuance  of  Units  for  cash  in a  public
               offering or private placement,  the consideration shall be deemed
               to be the  amount  of cash paid  therefor  before  deducting  any
               customary discounts, commissions or placement fees payable by the
               Issuer to any  underwriter or placement  agent in connection with
               the issuance and sale thereof.

        (ii)   In the case of the issuance of Units for a consideration in whole
               or in part  other than cash,  the  consideration  other than cash
               shall be deemed to be the fair market value thereof as determined
               in good faith by the board of directors of the General Partner.

        (iii)  In the case of the  issuance  of options to purchase or rights to
               subscribe for Units,  securities by their terms  convertible into
               or  exchangeable  for Units,  or options to purchase or rights to
               subscribe for such convertible or exchangeable securities:

               (A) the  aggregate  maximum  number  of  Units  deliverable  upon
               exercise of such options to purchase or rights to  subscribe  for
               Units  shall be  deemed  to have  been  issued  at the time  such
               options or rights  were issued and for a  consideration  equal to
               the   consideration   (determined  in  the  manner   provided  in
               subparagraphs (i) and (ii) above), if any, received by the Issuer
               upon the  issuance  of such  options or rights  plus the  minimum
               purchase  price  provided in such options or rights for the Units
               covered thereby;

               (B) the  aggregate  maximum  number  of  Units  deliverable  upon
               conversion  of  or  in  exchange  for  any  such  convertible  or
               exchangeable  securities  or upon  the  exercise  of  options  to
               purchase  or  rights  to  subscribe  for  such   convertible   or
               exchangeable  securities  and  subsequent  conversion or exchange
               thereof  shall be  deemed  to have  been  issued at the time such
               securities,   options,   or  rights   were   issued   and  for  a
               consideration  equal to the consideration  received by the Issuer
               for any such securities and related options or rights  (excluding
               any cash  received  on  account of  accrued  interest  or accrued
               dividends),  plus the  additional  consideration,  if any,  to be
               received  by the Issuer upon the  conversion  or exchange of such
               securities or the exercise of any related  options or rights (the
               consideration  in  each  case  to be  determined  in  the  manner
               provided in paragraphs (i) and (ii) above);

               (C) on any  change  in  the  number  of  Units  deliverable  upon
               exercise  of any such  options  or  rights or  conversions  of or
               exchanges for such securities, other than a change resulting from
               the anti-dilution  provisions thereof, the applicable  Conversion
               Price shall forthwith be readjusted to such  Conversion  Price as
               would  have  been  obtained  had the  adjustment  made  upon  the
               issuance of such  options,  rights or  securities  not  converted
               prior  to such  change  or  options  or  rights  related  to such
               securities not converted  prior to such change been made upon the
               basis of such change; and

               (D) no further  adjustment of the Conversion  Price adjusted upon
               the issuance of any such options, rights,  convertible securities
               or  exchangeable  securities  shall  be made as a  result  of the
               actual  issuance  of Units on the  exercise of any such rights or
               options or any conversion or exchange of any such securities.

(f)  If the Issuer has  issued or issues  any  securities  on or after the Issue
     Date containing provisions protecting the holder or holders thereof against
     dilution in any manner  more  favorable  to such holder or holders  thereof
     than  those set forth in this  Section  14,  such  provisions  (or any more
     favorable portion thereof) shall be deemed to be incorporated  herein as if
     fully set forth herein and, to the extent  inconsistent  with any provision
     herein, shall be deemed to be substituted therefor.

(g)  Notwithstanding  anything contained in this Section 14 to the contrary,  no
     adjustment to the  Conversion  Price shall be made upon the issuance of any
     interests (or securities  convertible or exercisable into interests) of the
     Issuer  issued  (i)  pursuant  to the  Investment  Agreement  or any of the
     transactions  contemplated thereby, (ii) in connection with the acquisition
     of the  business  of  another  entity,  whether by the  purchase  of equity
     securities,  assets  or  otherwise,  (iii)  as  part of an  initial  public
     offering or other registered  underwritten  public offering of the Issuer's
     securities,  (iv)  under an  employee  compensation  plan  approved  by the
     General  Partner,  and (v) upon conversion or exchange of this  Convertible
     Note in accordance with its terms.

(h)  In case of any  consolidation  of the Issuer with,  or merger of the Issuer
     with or into, any other entity, or in case of any sale or conveyance of all
     or substantially all of the assets of or interests in the Issuer other than
     in connection with a plan of complete  liquidation of the Issuer, then as a
     condition  of such  consolidation,  merger,  sale or  conveyance,  adequate
     provision  shall be made  whereby  the  Noteholder  shall have the right to
     acquire and receive upon conversion of this  Convertible Note in lieu of or
     in addition to (as the case may be) the Class A Preferred Units immediately
     theretofore  acquirable upon the conversion of this Convertible  Note, such
     shares of capital  stock,  securities or assets as may be issued or payable
     with  respect to or in exchange  for the number of Class A Preferred  Units
     immediately  theretofore  acquirable and receivable upon conversion of this
     Convertible Note.

(i)  Within 10 Business Days after the  occurrence  of any event which  requires
     any  adjustment  of the  Conversion  Price,  the Issuer  shall give written
     notice  thereof to the  Noteholder.  Such notice shall state the Conversion
     Price  resulting  from such  adjustment  and shall set forth in  reasonable
     detail the method of calculation and the facts upon which such  calculation
     is based. Such calculation shall be certified by an appropriate  officer of
     the general partner of the Issuer.

(j)  No  adjustment of the  Conversion  Price shall be made in an amount of less
     than 1% of the  Conversion  Price in effect at the time such  adjustment is
     otherwise  required  to be made,  but any such lesser  adjustment  shall be
     carried  forward and shall be made at the time and  together  with the next
     subsequent  adjustment  which,  together  with any  adjustments  so carried
     forward, shall amount to not less than 1% of such Conversion Price.

15.  This  Convertible  Note is being  executed and  delivered  in, and shall be
     governed by and construed in accordance  with the laws of, the State of New
     York,  without regard to its principles of conflicts of law that would give
     effect to the application of the law of another  jurisdiction.  Each of the
     parties hereto hereby irrevocably and unconditionally consents to submit to
     the  non-exclusive  jurisdiction of the courts of the State of New York and
     of the United States of America,  in each case having jurisdiction over the
     County of New York, for any  litigation  arising out of or relating to this
     Convertible Note and the transactions  contemplated hereby and thereby (and
     agrees not to  commence  any  litigation  relating  thereto  except in such
     courts  unless  such courts have  declined to exercise  jurisdiction),  and
     further agrees that service of any process,  summons, notice or document by
     U.S.  registered  mail to it shall be effective  service of process for any
     litigation brought against it in any such court. Each of the parties hereto
     hereby irrevocably and  unconditionally  waives any objection to the laying
     of venue of any  litigation  arising  out of this  Convertible  Note or the
     transactions  contemplated hereby in the courts of the State of New York or
     the United  States of America,  in each case having  jurisdiction  over the
     County of New York,  and hereby  further  irrevocably  and  unconditionally
     waives  and  agrees  not to plead or claim in any such  court that any such
     litigation brought in any such court has been brought in an inconvenient or
     improper forum.

16.  This  Convertible  Note shall inure to the benefit of and be enforceable by
     the Noteholder and the Noteholder's  successors and permitted assigns,  and
     shall be binding  and  enforceable  against  the  Issuer  and the  Issuer's
     successors and assigns.  The Noteholder  shall not be permitted to transfer
     this  Convertible  Note to any Person except in accordance with the Newo GP
     Agreement.  Any attempted transfer of this Convertible Note in violation of
     the  provisions  thereof  shall be void and of no  effect  and shall not be
     registered  on the books of the  Issuer.  Any  permitted  transfer  of this
     Convertible Note in accordance with the Newco GP Agreement shall be subject
     to and  conditioned  upon such  permitted  transferee  having  executed and
     delivered documentation reasonably required by the Issuer in order for such
     permitted  assign  to join and  become  bound by the  terms of the Newco GP
     Agreement.

17.  Issuer,  by  execution  of  this  Convertible  Note,  and  Noteholder,   by
     acceptance  of this  Convertible  Note,  agree  that any suit,  action,  or
     proceeding,  whether  claim or  counterclaim,  brought or  instituted by or
     against the Issuer or  Noteholder,  or any successor or assign of Issuer or
     Noteholder,  on or with respect to this  Convertible  Note, or which in any
     way  relates,  directly  or  indirectly,  to the  obligations  of Issuer to
     Noteholder under this Convertible Note, or the dealings of the parties with
     respect thereto,  shall be tried only by a court and not by a jury.  ISSUER
     AND NOTEHOLDER  HEREBY  EXPRESSLY WAIVE ANY RIGHT TO A TRIAL BY JURY IN ANY
     SUCH SUIT,  ACTION OR  PROCEEDING.  Issuer and Noteholder  acknowledge  and
     agree  that  this  provision  is a  specific  and  material  aspect  of the
     agreement  between the parties and that the Noteholder would not enter into
     the transaction  contemplated hereby with Issuer if this provision were not
     part of their agreement.

18.  No provision of this Convertible Note may be amended or otherwise  modified
     or waived without the prior written consent of the Noteholder.

<PAGE>

                              NOTICE OF CONVERSION

The  undersigned  hereby  irrevocably  elects  to  convert  $__________  of  the
outstanding principal amount of Convertible Note No. __ (the "Convertible Note")
plus $________ of the accrued and unpaid interest on such principal  amount into
Class A Preferred Units of Mobile Satellite  Ventures LP (the "Issuer) according
to the conditions of the Convertible Note, as of the date written below.

The undersigned  represents and warrants to the Issuer that all offers and sales
by the undersigned of the securities issuable to the undersigned upon conversion
of this  Convertible  Note shall be made pursuant to  registration  of the Units
under the  Securities  Act of 1933,  as amended (the  "Act"),  or pursuant to an
exemption from registration under the Act.

In the event of partial exercise, please reissue an appropriate Convertible Note
for the principal balance which shall not have been converted.

                           Conversion Date:
                                           -------------------------------------

                                 Signature:
                                           -------------------------------------

                                      Name:
                                           -------------------------------------

                                   Address:
                                           -------------------------------------

ACKNOWLEDGED AND AGREED:

MOBILE SATELLITE VENTURES LP

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

Date:
         --------------------------------------------EXHIBIT 10.59

                            FORM OF PROMISSORY NOTE

THIS PROMISSORY NOTE HAS BEEN ACQUIRED FOR INVESTMENT AND HAS NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT") OR UNDER ANY
STATE SECURITIES LAWS (THE "STATE ACTS") AND CANNOT BE OFFERED, SOLD,
TRANSFERRED OR OTHERWISE DISPOSED OF IN THE ABSENCE OF REGISTRATION OR AN
EXEMPTION FROM REGISTRATION UNDER THE ACT, THE STATE ACTS AND REGULATIONS
PROMULGATED THEREUNDER.

                   MOBILE SATELLITE VENTURES LP (the "Issuer")
U.S.$15,000,000.00   Note   at   10%   Interest   (calculated   and   compounded
semi-annually)

Maturity Date:  [NTD:  insert date which is five years after First Closing],  or
earlier as determined herein

FOR  VALUE  RECEIVED,  the  Issuer,  a  Delaware  limited  partnership,   hereby
acknowledges  itself  indebted and promises to pay to MOTIENT  SERVICES INC., or
its  permitted  assigns  (the  "Noteholder"),  the  principal  amount of FIFTEEN
MILLION  UNITED STATES  DOLLARS  (U.S.$15,000,000.00),  on the Maturity Date (as
hereinafter  defined in Schedule A), at the head office of the  Noteholder or at
such other place as  directed in writing by the  Noteholder.  In  addition,  the
Issuer  agrees to pay  interest  (computed  on the basis of a year of 365 or 366
days,  as the case may be), in like money on the  outstanding  principal  amount
from the date hereof  until the Maturity  Date,  or until such later date as all
obligations  hereunder  have been paid in full, at the rate of TEN PERCENT (10%)
per annum, calculated and compounded semi-annually and payable in arrears on the
Maturity Date, or earlier as otherwise provided herein, and should the Issuer at
any time make default in the payment of any  principal or interest on this Note,
to pay interest on the amount in default at the same rate, in like money, all in
accordance  with the terms and conditions set out in Schedule A. If at any time,
the payment of the  principal  of or interest on this Note is  rescinded or must
otherwise  be  restored  or  returned  upon  the  insolvency,   bankruptcy,   or
reorganization of the Issuer or otherwise,  the Issuer's  obligations under this
Note shall be  reinstated  at such time as though such  payment had been due but
not made at such time.

This Note is the "MSI Note" as defined in the  Investment  Agreement (as defined
in  Schedule  A  hereto),  and is  subject  to the terms and  conditions  of the
Investment  Agreement and the Ancillary Agreements (as defined in the Investment
Agreement).

The Terms and  Conditions  set out on  Schedule  A  attached  hereto  are hereby
incorporated by reference into this Note and constitute an integral part hereof.
THIS NOTE IS  SUBORDINATED TO OTHER DEBT OF THE ISSUER AS SET FORTH IN SECTION 2
OF SCHEDULE A.

IN WITNESS WHEREOF, the Issuer has caused this Note to be executed on its behalf
by the signatures of its officers duly authorized in that regard.

Issued as of _______ __, 2001             MOBILE SATELLITE VENTURES LP

                                          By: Mobile Satellite Ventures GP Inc.,
                                                 its general partner

                                          By:________________________________
                                          Name:
                                          Title:

<PAGE>

                                   SCHEDULE A
                        TERMS AND CONDITIONS OF THE NOTE

1.   Terms not otherwise defined herein shall have their respective meanings set
     forth in the Investment  Agreement or, if not defined therein, in the Newco
     LP Agreement.  In this Note, the following capitalized terms shall have the
     meanings set out below:

(a)  "Business  Day" means any day on which  banks are open for  business in the
     place where this Note is scheduled for payment;

(b)  "Canadian License Co." means Mobile Satellite Ventures (Canada) Inc.

(c)  "Canadian Licenses" means the licenses issued by Industry Canada to TMI and
     which were  transferred to Canadian  License Co.  pursuant to the TMI Asset
     Sale Agreements (as defined in the Investment Agreement);

(d)  "Convertible  Notes"  means  the  Convertible  Notes  (as  defined  in  the
     Investment  Agreement)  of even date herewith made by Issuer in the initial
     aggregate  principal amount of U.S.$55  million,  as in effect on the Issue
     Date, and without giving effect to any amendment, modification, supplement,
     or restatement  thereto or waiver  thereunder except to the extent that the
     same has been approved in writing by the Noteholder;

(e)  "Debt" means:  (i) all  indebtedness of the Issuer and its subsidiaries for
     borrowed money, including borrowings of commodities,  bankers' acceptances,
     letters of credit or letters of  guarantee;  (ii) all  indebtedness  of the
     Issuer and its subsidiaries for the deferred  purchase price of property or
     services other than for goods and services purchased in the ordinary course
     of business  and paid for in  accordance  with  customary  practice and not
     represented by a note,  bond,  debenture or other evidence of indebtedness;
     (iii) all  indebtedness  created or arising under any  conditional  sale or
     other title  retention  agreement with respect to property  acquired by the
     Issuer and its  subsidiaries  (even  though the rights and  remedies of the
     seller or lender  under such  agreement in the event of default are limited
     to repossession or sale of such property);  (iv) all current liabilities of
     the Issuer and its subsidiaries  represented by a note, bond,  debenture or
     other evidence of indebtedness; (v) all obligations under leases which have
     been or should be, in accordance with U.S. GAAP, recorded as capital leases
     in respect of which the  Issuer and its  subsidiaries  is liable as lessee;
     and (vi) all indebtedness of the kinds referred to in (i) through (v) above
     which  is  directly  or  indirectly   guaranteed  by  the  Issuer  and  its
     subsidiaries  or  which  the  Issuer  and  its   subsidiaries   has  agreed
     (contingently or otherwise) to purchase or otherwise acquire, or in respect
     of which the Issuer and its subsidiaries  has otherwise  assured a creditor
     or other Person against loss;

(f)  "Existing System Equipment Financing" means Debt, up to a maximum principal
     amount  of  U.S.$4,000,000,  incurred  by the  Issuer  for the  purpose  of
     enabling the Issuer to acquire hardware or software,  to replace or upgrade
     hardware and  software  owned by the Issuer as at the date of this Note but
     only to the extent  that the  proceeds  of such Debt are applied to acquire
     such hardware or software;

(g)  "Event of Default" means any one or more of the following events:

        (i)    if the Issuer  defaults in payment of the  principal  of,  and/or
               interest on, this Note when the same becomes due, and continuance
               of such  failure  for five (5)  Business  Days  after the date on
               which such principal or interest is due;

        (ii)   if a decree,  judgement or order by a court  having  jurisdiction
               shall  have been  entered  adjudging  the  Issuer a  bankrupt  or
               insolvent,  or  approving  as properly  filed a petition  seeking
               liquidation or  reorganization of the Issuer under any applicable
               bankruptcy,  reorganization  or  similar  law,  and such  decree,
               judgement or order shall have continued undismissed, undischarged
               or unstayed for a period of 60 calendar days;

        (iii)  if the Issuer shall  institute  proceedings  to be  adjudicated a
               voluntary  bankrupt,   or  shall  consent  to  the  filing  of  a
               bankruptcy  proceeding  against  it, or shall file a petition  or
               answer or consent  seeking  reorganization  under any  applicable
               bankruptcy,  liquidation  or  reorganization  or similar  law, or
               shall  consent  to the  filing  of any  such  petition,  or shall
               consent to the appointment of a receiver or liquidator or trustee
               or assignee in  bankruptcy or insolvency of it or of its property
               or of any substantial  part thereof,  or shall make an assignment
               for the benefit of creditors or a proposal  under any  applicable
               bankruptcy  or other law, or shall admit in writing its inability
               to pay its debts generally as they become due;

        (iv)   if there  occurs  a  default  in the  payment  of any  principal,
               premium, interest or other amount due on (or for) any Debt (other
               than this  Note),  satellite  insurance,  satellite  construction
               costs or in any payment due under any  obligation,  guarantee  or
               indemnity  of the  Issuer  in an  aggregate  amount  in excess of
               U.S.$1,000,000.00  ("Obligations")  beyond  any  period  of grace
               provided with respect thereto,  or if any other default occurs in
               the  performance  of  any  other  term,   condition  or  covenant
               contained in any such  Obligations  or any agreement  under which
               such Obligations are created, the result of which is to cause any
               holder(s) of such Obligations (or a trustee or agent on behalf of
               such holder(s)) to accelerate such Obligations, provided that the
               resulting  default under this Note shall be deemed to be cured or
               waived if such other defaults are cured or waived;

        (v)    if the  Issuer  shall  fail to carry  out or  observe  any  other
               covenant or condition of this Note or the Investment Agreement on
               its part to be observed or performed  and after notice in writing
               has been given to the Issuer by the  Noteholder  specifying  such
               default and requiring  the Issuer to put an end to the same,  the
               Issuer shall fail to remedy or cure such default  within a period
               of 30 calendar days;

        (vi)   if any representation,  warranty, certification or statement made
               by the Issuer (or its predecessor) in the Investment Agreement or
               any other  Ancillary  Agreement  (as  defined  in the  Investment
               Agreement) to which the Issuer is a party or in any  certificate,
               financial  statement or other document  delivered pursuant to the
               Investment  Agreement  shall prove to have been  incorrect in any
               material respect when made (or deemed made); or

        (vii)  if the Issuer or any Subsidiary of the Issuer or Canadian License
               Co. shall have lost  ownership of or rights in respect of the FCC
               Licenses  or the  Canadian  Licenses,  except for such  losses of
               ownership  or rights  that  would not  cause a  Material  Adverse
               Effect on the Issuer and its Subsidiaries taken as a whole.

(h)  "FCC  Licenses"  means the  licenses  issued by the Federal  Communications
     Commission to MSI and which were  transferred to the Issuer or a Subsidiary
     of the Issuer pursuant to the Amended and Restated Sub Asset Sale Agreement
     (as defined in the Investment Agreement);

(i)  "Investment  Agreement" means the Amended and Restated Investment Agreement
     dated October **, 2001 among Mobile Satellite Ventures LLC (the predecessor
     to the  Issuer),  Motient  Corporation,  Motient  Services  Inc.,  TMI, and
     various other parties identified therein as "Investors" as in effect on the
     Issue  Date,  and without  giving  effect to any  amendment,  modification,
     supplement,  or  restatement  thereto  or waiver  thereunder  except to the
     extent that the same has been approved in writing by the Noteholder;

(j)  "Issue Date" means [NTD: insert date of the First Closing];

(k)  "Issuer" means Mobile Satellite Ventures LP;

(l)  "Material Adverse Effect" means a material adverse change in, or a material
     adverse  effect  upon,  any of (1) the  operations,  business,  properties,
     condition (financial or otherwise) of the Issuer and its Subsidiaries taken
     as a whole;  (2) the  ability of the Issuer or any of its  Subsidiaries  to
     perform under this Note, the Investment Agreement, or any of the agreements
     or instruments  executed in connection therewith to which the Issuer or any
     of its  Subsidiaries  is a party;  or (3) the legality,  validity,  binding
     effect or enforceability of this Note, the Investment Agreement,  or any of
     the agreements or instruments executed in connection therewith to which the
     Issuer or any of its Subsidiaries is a party;

(m)  "Maturity Date" means the earliest of the following dates:

        (i)    [NTD: insert date which is five years after First Closing];

        (ii)   the date, if any, on which the Noteholder  declares this Note due
               and payable pursuant to Section 11 below; and

        (iii)  the Second Closing Date (as defined in the Investment Agreement);

(n)  "Newco LP  Agreement"  means the Limited  Partnership  Agreement  of Mobile
     Satellite  Ventures LP,  dated as of the date  hereof,  between the parties
     specified therein as in effect on the Issue Date, and without giving effect
     to any  amendment,  modification,  supplement,  or  restatement  thereto or
     waiver  thereunder  except to the extent that the same has been approved in
     writing by the Noteholder;

(o)  "Next Generation Financing" means Debt, up to a maximum principal amount of
     U.S.$5,000,000,  incurred  by the Issuer for the  purpose of  enabling  the
     Issuer  to:  (1)  acquire  hardware  or  software;  or (2) pay  for  legal,
     regulatory or consulting  costs directly  related to the development of the
     Issuer's   next-generation  mobile  satellite  system  and  its  associated
     terrestrial  segment, but only to the extent that the proceeds of such Debt
     are applied to acquire such hardware or software, or to pay for such legal,
     regulatory and consulting costs;

(p)  "Note" means this note;

(q)  "Person"  means  any  individual,  corporation,   association,  partnership
     (general or limited),  joint  venture,  trust,  estate,  limited  liability
     company, or other legal entity or organization;

(r)  "Reduction Event" means any one or more of the following:

        (i)    the sale,  disposition,  transfer,  assignment or lease of any or
               all of the assets  and/or  business  of the Issuer and other than
               Permitted Transfers (as defined below);

        (ii)   the  issuance of Debt (other than  Permitted  Debt (as defined in
               the TMI Note)), where the lesser of:

               (A) 50% of the  total  value  of the  consideration  given to the
               Issuer  (or to such other  party as the  Issuer may  direct) as a
               result  of  such  Debt,  after  deduction  for the  direct  costs
               incurred to consummate such Debt; and

               (B) an amount of cash equal to the total  principal  and interest
               on this Note outstanding as at the creation of such Debt;

               is paid to the Noteholder in cash pursuant to Section 10 below;

        (iii)  the issuance of the equity of, or an  ownership  interest in, the
               Issuer,  other than issuance of equity (A) at the Second  Closing
               (as  defined  in  the  Investment   Agreement)  pursuant  to  the
               Investment  Agreement,  (B) as a result of the  conversion of the
               Convertible Notes (as defined in the Investment Agreement) or (C)
               where the  proceeds  of such  issuance  consist of  consideration
               other than cash;

(s)  "Reduction  Event Net Proceeds"  means, in relation to any Reduction Event,
     the total value of the consideration  given to the Issuer (or to such other
     party as the Issuer may direct) for or as a consequence  of, such Reduction
     Event,  after  deduction  of the  direct  costs  incurred  by the Issuer to
     consummate such Reduction Event;

(t)  "Subsidiary"  means,  as to any Person,  any corporation or other entity of
     which securities or other ownership  interests having ordinary voting power
     to elect a majority of the board of directors or other  persons  performing
     similar  functions  are at the time  directly or  indirectly  owned by such
     Person.

(u)  "TMI" means TMI Communications and Company Limited Partnership; and

(v)  "TMI Note" means the $11.5  Million Note of even date  herewith made by the
     Issuer  payable to TMI as in effect on the Issue Date,  and without  giving
     effect to any amendment,  modification,  supplement, or restatement thereto
     or waiver  thereunder  except to the extent that the same has been approved
     in writing by the Noteholder;

2.   This Note is a direct and unsecured  senior  obligation of the Issuer.  The
     Issuer  agrees  that this Note is  subordinated  in right of payment to the
     prior  payment  in full of the  Issuer's  Debt  under  the  TMI  Note.  The
     provisions  of this  paragraph  may not be modified or amended  without the
     prior consent of TMI for so long as the TMI Note is outstanding.  TMI shall
     be a third-party  beneficiary of the immediately  preceding sentence for so
     long as the TMI Note is outstanding.

     The foregoing  provisions of this Section 2 are and are intended solely for
     the purpose of defining the relative  rights of the  Noteholder  on the one
     hand and the holders of the TMI Note on the other hand.  Nothing  contained
     in this Section 2 or elsewhere in this Note or in the Investment  Agreement
     is  intended to or shall (a) impair,  as among the  Issuer,  its  creditors
     other than holders of the TMI Note, and the  Noteholder,  the obligation of
     the Issuer,  which is absolute and unconditional,  to pay to the Noteholder
     the  principal  of,  and  interest  on this Note as and when the same shall
     become due and  payable  in  accordance  with its terms;  or (b) affect the
     relative  rights  against the Issuer of the Noteholder and creditors of the
     Issuer  other  than  the  holders  of the  TMI  Note;  or (c)  prevent  the
     Noteholder from exercising all remedies  otherwise  permitted by applicable
     law upon a default or an Event of Default under this Note.

     The failure to make a payment on account of  principal  of, or interest on,
     this Note by reason of the first  paragraph  of this  section  shall not be
     construed as preventing  the occurrence of a default or an Event of Default
     hereunder.

3.   The  principal  amounts  outstanding  under this Note bear  interest  (both
     before and after  maturity,  default and judgement with interest on overdue
     interest  at the same rate) from the Issue Date at the rate of ten  percent
     (10%) per annum,  calculated  and compounded  semi-annually  and payable in
     arrears on the Maturity Date, or earlier as otherwise  provided herein. The
     amounts  owing  under this Note will cease to bear  interest on the date on
     which all such  amounts are fully  repaid,  unless  payment of principal or
     interest is improperly withheld or refused by the Issuer.

4.   The  principal of this Note and interest  thereon will be payable in United
     States Dollars, at the option of the Noteholder:  (i) at the head office of
     the  Noteholder;  (ii) at such other  place as  directed  in writing by the
     Noteholder;  or (iii) by electronic funds transfer to an account maintained
     by the Noteholder with a bank in the United States.

5.   If the date for payment of any amount of  principal  or interest in respect
     of this  Note is  not,  at the  place  of  payment,  a  Business  Day,  the
     Noteholder  shall not be  entitled  to  payment of the amount due until the
     next  following  Business  Day at the  place of  payment  and  shall not be
     entitled to any further interest or payment in respect of such delay.

6.   Any monies paid on this Note,  including without limitation  payments under
     Section 8, Section 9 or Section 10, shall be applied,  firstly,  on account
     of accrued interest, and secondly, on account of principal.

7.   Unless  previously  redeemed or  purchased  by the  Issuer,  this Note will
     mature at its principal amount then outstanding on the Maturity Date.

8.   The Issuer may at any time or times prepay this Note,  in whole or in part,
     together  with  interest  accrued to the date of such  prepayment,  without
     premium or penalty,  upon providing the  Noteholder  with two Business Days
     prior written notice.

9.   This Note shall  become due and payable  upon the Second  Closing  Date (as
     defined in the Investment Agreement), and shall be paid out of the Investor
     Purchase Price (as defined in the Investment  Agreement) in priority to any
     other payment  except for the TMI Note;  provided that there shall not then
     be an Event of Default under the  Convertible  Notes  pursuant to which the
     holders thereof have declared the Convertible Notes due and payable.

10.  Within five Business Days  following the  occurrence of a Reduction  Event,
     the Issuer  shall give  written  notice  thereof to the  Noteholder,  which
     notice shall set forth in  reasonable  detail the terms of the  transaction
     constituting such Reduction Event. After the TMI Note has been paid in full
     and until such time as this Note has been  repaid in full,  the  Noteholder
     may,  within 10 days  after  receipt  of the  notice of a  Reduction  Event
     referred to above,  elect that the Issuer pay to the  Noteholder,  in cash,
     its pro rata portion of 50% of all Reduction Event Net Proceeds immediately
     upon receipt by the Issuer of the same. The  Noteholder's  pro rata portion
     shall be determined by reference to the principal amount  outstanding under
     this Note, on the one hand, and (i) the principal amount  outstanding under
     the MSI Note plus (ii) the aggregate principal amount outstanding under all
     of the  Convertible  Notes (without  regard to whether any other holders of
     the Convertible  Notes have elected to have the Issuer pay their respective
     portions  of the  Reduction  Event Net  Proceeds  in  connection  with such
     Reduction  Event),  on the other hand.  If the  Noteholder  exercises  such
     option,  the Noteholder may require the Issuer to direct the person to whom
     assets or securities are sold in connection  with such  Reduction  Event to
     pay such monies owed to the Noteholder pursuant this Section 10 directly to
     the Noteholder.

11.  If an  Event of  Default  (a)  other  than  those in (ii) and  (iii) of the
     definition of Event of Default occurs the Noteholder may, at its option, by
     notice in writing to the Issuer,  declare  this Note to be due and payable,
     and upon such declaration,  unless all Events of Default have been cured by
     the  Issuer  prior  to  receipt  by the  Issuer  of such  declaration,  all
     principal of, and interest on, this Note shall  immediately  become due and
     payable;  (a) in the case of the occurrence of an Event of Default referred
     to in clause  (ii) or (iii) of the  definition  of "Event of  Default"  the
     principal  amount  then  outstanding  of, and the  accrued  interest on any
     amounts owing under this Note, and all other amounts  payable by the Issuer
     hereunder  shall become  automatically  immediately due and payable without
     presentment,  demand, diligence,  protest or other formalities of any kind,
     all of which are hereby expressly waived by the Issuer.

12.  The Issuer agrees to pay to Noteholder and reimburse Noteholder for any and
     all  reasonable  costs and expenses,  including  attorney's  fees and court
     costs,  if  any,   incurred  by  the  Noteholder  in  connection  with  the
     enforcement or collection hereof, both before and after the commencement of
     any action to enforce or  collect  this Note,  but  whether or not any such
     action  is  commenced  by  the   Noteholder.   The  Issuer   hereby  waives
     presentment,  demand, protest or other notice of any kind, and all defenses
     and pleas on the grounds of  extension of the time of payments or due dates
     of this Note, in the  enforcement of this Note,  and expressly  agrees that
     this  Note or any  payment  hereunder  may be  extended  from  time to time
     without in any way  affecting the  liability of the Issuer  hereunder.  The
     rights and remedies of the  Noteholder  hereunder  shall be cumulative  and
     concurrent and may be pursued  singularly,  successively or together at the
     sole  discretion  of the  Noteholder,  and may be  exercised  as  often  as
     occasion  therefor shall occur,  and the failure to exercise any such right
     or remedy shall in no event be construed as a waiver or release of the same
     or any other right or remedy.

14.  Except  for any  transaction,  agreement,  undertaking,  activity  or other
     action  required  in  order  for the  Issuer  to  perform  its  obligations
     specifically  set  forth  in the  Investment  Agreement  or  any  Ancillary
     Agreement  (as defined in the  Investment  Agreement)  or for the Issuer to
     exercise  any of its rights  specifically  set forth  therein),  the Issuer
     agrees  that,  without  the prior  consent of the  Noteholder,  neither the
     Issuer nor any Subsidiary of the Issuer shall, directly or indirectly,  (i)
     sell, assign,  lease, convey,  transfer or otherwise dispose of (whether in
     one or a series of transactions)  any of its assets,  business or property,
     (including  accounts and notes receivable) (with or without  recourse),  or
     enter  into  equipment   sale-leaseback   transactions  or  (ii)  merge  or
     consolidate with any other Person, or enter into any agreement to do any of
     the  foregoing  described  in clauses (i) or (ii) except for the  following
     ("Permitted Transfers"):

(a)  sales,  transfers, or other dispositions of inventory, or used, worn-out or
     surplus property,  or property of no further use to the Issuer,  all in the
     ordinary course of business;

(b)  sales, transfers, or other dispositions of equipment in the ordinary course
     of business  to the extent  that such  equipment  is  exchanged  for credit
     against the purchase price of similar replacement equipment or the proceeds
     of such sale are reasonably  promptly applied to the purchase price of such
     replacement equipment;

(c)  sales,  transfers,  or  other  dispositions  of  communications   services,
     capacity or equipment pursuant to the customer contracts  providing for the
     sale of  communications  services,  capacity or  equipment  in the ordinary
     course of business;

(d)  sales, transfers, or other dispositions of assets in the ordinary course of
     business  having a fair market  value not  exceeding  $500,000  per item or
     $1,000,000 in the aggregate in any fiscal year (excluding sales,  transfers
     and dispositions  theretofore  approved in accordance with the terms hereof
     in such fiscal year);

(e)  sales,  transfers or other dispositions of contract rights under subscriber
     equipment  leases  pursuant  to any  bona  fide  financing  of such  leases
     permitted under Section 2 of the TMI Note;

(f)  non-exclusive licenses of technology and other intangible assets, excluding
     spectrum licenses;

(g)  sales of mobile earth terminals and related equipment, and other inventory;

(h)  sales,  transfers,  or other  dispositions  of  assets  as a result  of the
     combination  and integration of the business of MSI and TMI as contemplated
     by the Operations Plan,  including without limitation the sale, transfer or
     other disposition of: (y) duplicative or unnecessary  assets, or (z) assets
     in connection  with the closing of facilities  and reductions in workforce;
     provided that such  disposition  does not include a disposition  of the FCC
     Licenses or the Canadian Licenses;

(i)  any wholly-owned Subsidiary of the Issuer may merge, consolidate or combine
     with or into,  or  transfer  (for no  consideration)  assets to the Issuer;
     provided that the Issuer shall be the continuing or surviving  corporation;
     and

(j)  any  wholly-owned  Subsidiary  of the Issuer may sell,  lease,  transfer or
     otherwise  dispose of any or all of its assets (upon voluntary  liquidation
     or otherwise), to another wholly-owned Subsidiary.

     For  certainty,   "Permitted  Transfers"  shall  not  include:  (1)  sales,
     assignments,  leases,  conveyances,  transfers or other dispositions of the
     Issuer  or any of its  Subsidiaries  outside  of  the  ordinary  course  of
     business other than as permitted in paragraphs (h), (i) and (j) above;  and
     (2) the sale, assignment, lease, conveyance,  transfer or other disposition
     (whether in one or a series of transactions) of all or substantially all of
     the  assets of any of the  Issuer or any of its  Subsidiaries  or  Canadian
     License Co.

15.  This Note is being  executed and delivered in, and shall be governed by and
     construed in  accordance  with the laws of, the State of New York,  without
     regard to its  principles of conflicts of law that would give effect to the
     application of the law of another jurisdiction.  Each of the Parties hereto
     hereby   irrevocably  and   unconditionally   consents  to  submit  to  the
     non-exclusive  jurisdiction  of the  courts of the State of New York and of
     the United  States of America,  in each case having  jurisdiction  over the
     County of New York, for any  litigation  arising out of or relating to this
     Note and the transactions  contemplated  hereby and thereby (and agrees not
     to commence any  litigation  relating  thereto except in such courts unless
     such courts have  declined to exercise  jurisdiction),  and further  agrees
     that service of any process, summons, notice or document by U.S. registered
     mail to it shall be effective service of process for any litigation brought
     against it in any such court. Each of the parties hereto hereby irrevocably
     and  unconditionally  waives  any  objection  to the laying of venue of any
     litigation arising out of this Note or the transactions contemplated hereby
     in the courts of the State of New York or the United States of America,  in
     each case  having  jurisdiction  over the  County of New York,  and  hereby
     further irrevocably and  unconditionally  waives and agrees not to plead or
     claim in any such court that any such litigation  brought in any such court
     has been brought in an inconvenient or improper forum.

16.  Except with the prior written consent of the Noteholder,  this Note may not
     be  assigned by the Issuer.  Except with the prior  written  consent of the
     Issuer,  this Note may not be assigned by the  Noteholder  to any person or
     entity other than an Affiliate of the Noteholder.  This Note shall inure to
     the benefit of and be enforceable  by the  Noteholder and the  Noteholder's
     successors  and  permitted  assigns,  and shall be binding and  enforceable
     against the Issuer and the Issuer's successors and assigns.

17.  Issuer,  by execution of this Note, and  Noteholder,  by acceptance of this
     Note,  agree  that  any  suit,  action,  or  proceeding,  whether  claim or
     counterclaim, brought or instituted by or against the Issuer or Noteholder,
     or any successor or assign of Issuer or  Noteholder,  on or with respect to
     this Note,  or which in any way  relates,  directly or  indirectly,  to the
     obligations of Issuer to Noteholder under this Note, or the dealings of the
     parties with respect  thereto,  shall be tried only by a court and not by a
     jury.  ISSUER AND NOTEHOLDER HEREBY EXPRESSLY WAIVE ANY RIGHT TO A TRIAL BY
     JURY  IN ANY  SUCH  SUIT,  ACTION  OR  PROCEEDING.  Issuer  and  Noteholder
     acknowledge and agree that this provision is a specific and material aspect
     of the  agreement  between the parties  and that the  Noteholder  would not
     enter  into  the  transaction  contemplated  hereby  with  Issuer  if  this
     provision were not part of their agreement.

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