Document:

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                                                                   EXHIBIT 10.29

                               SUBLEASE AGREEMENT

     This Sublease Agreement ("Sublease") is made and entered into as of the
19th day of May, 2000, between Orbital Sciences Corporation, a Delaware
corporation ("Sublandlord"), and ORBCOMM Global, L.P., a Delaware limited
partnership ("Subtenant").

                                  WITNESSETH:

     WHEREAS, pursuant to that certain Deed of Lease Agreement dated as of
April 5, 1999 (the "Prime Lease") by and between Boston Properties Limited
Partnership (the "Landlord") and Sublandlord, Sublandlord has leased a certain
building being constructed on certain land known as Lot 8A, The Corporate
Center at Steeplechase, Section I, Loudoun County, Virginia (all as more
particularly described in the Prime Lease, the "Premises"). A true and complete
copy of the Prime Lease is attached hereto as Exhibit A and incorporated herein
by reference and made a part hereof; and

     WHEREAS, Subtenant desires to sublet the Premises, and Sublandlord is
willing to sublet the same on the conditions hereinafter set forth.

     NOW, THEREFORE, Sublandlord, for and in consideration of the rents,
covenants and agreements hereinafter contained on the part of Subtenant to be
paid, kept and performed, does hereby sublet and demise unto Subtenant, and
Subtenant hereby takes and hires from Sublandlord, the Premises;

     TO HAVE AND TO HOLD the same unto Subtenant, its successors and assigns,
for the term described herein, subject to the terms and conditions of the Prime
Lease, and upon the rentals, terms, covenants, conditions and provisions
hereinafter set forth;

     AND Sublandlord and Subtenant hereby agree as follows:

     1.   Defined Terms. Except as specifically defined herein, all capitalized
terms shall have the definitions provided in the Prime Lease.

     2.   Premises. Upon the terms and conditions set forth herein, Sublandlord
hereby subleases to Subtenant, and Subtenant does hereby sublease from
Sublandlord, the Premises.

     3.   Term. Subject to Section 15 hereof, the term of this Sublease (the
"Sublease Term") shall commence as of the Lease Commencement Date and shall run
for a period of fifteen (15) years from such date, subject to termination as
described herein. The parties agree that the Lease Commencement Date occurred
on May 3, 2000. Sublandlord hereby agrees that, so long as Subtenant is not in

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default hereunder, Sublandlord shall not voluntarily terminate the Prime Lease
prior to the end of the Sublease Term, without first obtaining Subtenant's
written consent.

     4.   Rent.

          (a)  Annual Base Rental and Other Sums.  For and during the Sublease
Term, Subtenant covenants and agrees to pay Sublandlord one hundred percent
(100%) of all Annual Base Rent (subject to adjustment pursuant to the terms of
the Prime Lease and the Development Agreement) and other sums (including,
without limitation, Expenses) due from time to time under the Prime Lease.
Promptly following the execution of any amendment to the Prime Lease pursuant
to Section 3.1(c) thereof adjusting the Annual Base Rent, Sublandlord and
Subtenant shall execute a corresponding amendment to this Sublease, which will
be retroactive to the Lease Commencement Date (and any overpayment or
underpayment of Annual Base Rent previously made hereunder shall be paid or
credited in the same manner as set forth in Section 3.1(c) of the Prime Lease).
Sublandlord covenants and agrees, to the extent funds are actually received
from Subtenant, to remit such sums to Landlord as required by the terms of the
Prime Lease.

          (b)  Payments.  The Annual Base Rent shall be paid in immediately
available funds by Subtenant in advance in equal monthly installments at least
five (5) days prior to the first (1st) day of each and every month during the
Sublease Term, provided however, that the Annual Base Rent for the period May
3, 2000 through May 31, 2000 shall be paid on May 19, 2000. All other sums
payable under the Prime Lease shall be made in the same manner and at the same
time as required under the Prime Lease, provided that any payments of other
sums payable under the Prime Lease at regularly scheduled intervals shall be
paid by Subtenant at least five (5) days prior to the date Sublandlord is
required to make such payments under the Prime Lease. All payments of Annual
Base Rent and other sums shall be made to Sublandlord by wire transfer of
immediately available funds to the account of Sublandlord at Bank of America,
ABA Number, 051000017, Account Number 411330524599, or in accordance with other
written instructions as may be provided to Subtenant from time to time by
Sublandlord. All payments of Annual Base Rent and other sums due hereunder
shall be payable without demand (unless otherwise specified), set-off or
deduction. If Sublandlord fails to remit to Landlord sums actually received
from Subtenant, as required by the terms of the Prime Lease, Subtenant may
withhold further payments due hereunder, including Annual Base Rent and other
sums, until such time as Sublandlord has remitted to Landlord all such funds
previously received from Subtenant.

     5.   Delivery and Condition of Premises. Subtenant agrees to accept the
Premises from Sublandlord in their as-is condition upon delivery from Landlord
pursuant to the terms of the Prime Lease, subject to the Integrated Punchlist
(as

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defined in, and to be completed by Landlord in accordance with the terms of,
the Development Agreement). Sublandlord shall have no obligation to improve the
Premises in any manner. To the extent that any amounts are due to Landlord (and
not included in Annual Base Rent) in connection with the Leasehold Improvements
installed by Landlord in the Premises pursuant to the Prime Lease or the
Development Agreement, Subtenant shall pay all such amounts to Sublandlord
within ten (10) days following presentation of an invoice therefor. In
addition, Subtenant shall pay to Sublandlord, on the Lease Commencement Date, a
one-time charge equal to $115,459 for its pro rata share (calculated based upon
the anticipated completion of the third building being built by Landlord on
behalf of Sublandlord) of the costs to date for the design and construction of
the telecommunications ductbank connecting the telecommunications systems among
the Premises and the buildings located at 21829, 21700 and 21830 Atlantic
Boulevard, Dulles, Virginia. Upon completion of the third building being built
by Landlord on behalf of Sublandlord located at 21839 Atlantic Boulevard,
Dulles, Virginia, Subtenant shall pay to Sublandlord an additional amount equal
to 26.36% of the cost to complete construction of such ductbank (without any
additional upgrades thereto without Subtenant's written approval) after the
Lease Commencement Date to connect telecommunications systems with the building
at 21839 Atlantic Boulevard, Dulles, Virginia.

     6.   Alterations. Upon the prior written consent of the Landlord, as
required under the Prime Lease, Subtenant may cause to be made, at Subtenant's
sole cost and expense, alterations or improvements to the Premises. Prior to
undertaking any such alterations or improvements, Subtenant also must obtain
Sublandlord's consent thereto, which consent shall not be withheld or delayed
unreasonably, provided that Sublandlord may, in its reasonable discretion,
condition its approval of any alterations or improvements on Subtenant's
removal of same at the end of the Sublease Term or sooner termination of this
Sublease, and repair of any damage incident thereto. No other alterations or
improvements shall be allowed to the Premises.

     7.   Incorporation of Prime Lease. Except for Section 10.1, Section 10.2,
Sections 13.3(c) and (d), Article XXV, Article XXVII and Article XXVIII, and to
the extent not otherwise inconsistent or otherwise modified in accordance with
the agreements and understandings expressed in this Sublease or applicable only
to the original parties to the Prime Lease, the terms, provisions, covenants
and conditions of the Prime Lease are hereby incorporated herein by reference
and made a part hereof on the following understandings:

          (a)  To the extent not inconsistent with the agreements and
understandings expressed herein, the term "Landlord" as used in the Prime Lease
shall refer to Sublandlord hereunder (except with respect to Landlord's consent
rights under Article IX), and its successors and assigns, and the term "Tenant"
as used in the Prime Lease shall refer to Subtenant hereunder.

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          (b)  With respect to work, services, repairs, repainting and
restoration or the performance of other obligations required of the Landlord
under the Prime Lease, following Subtenant's written request to Sublandlord,
Sublandlord agrees to use reasonable, good faith efforts to obtain Landlord's
performance thereof; provided, however, that in no event shall Sublandlord have
any duty to expend any money to obtain such performance or have any liability
for Landlord's failure of performance. Notwithstanding the foregoing, to the
extent Sublandlord is entitled to an abatement of Annual Base Rent or any other
sums due under the Prime Lease upon a failure of Landlord to perform its
obligations under the Prime Lease, Subtenant shall be entitled to an abatement
of all such amounts otherwise payable hereunder.

          (c)  Subtenant shall comply with all rules and regulations of the
Building.

          (d)  Subtenant hereto agrees to perform and comply with all of the
terms, provisions, covenants and conditions of the Prime Lease applicable to
"Tenant" as they relate to the Premises.

          (e)  Sublandlord promptly shall provide to Subtenant copies of any
and all notices and other documents received from Landlord pursuant to the
Prime Lease.

          (f)  To the extent Sublandlord's approval is required under the Prime
Lease, Sublandlord shall not approve any Management Agreement proposed by
Landlord with a third party which would manage only the Premises and not also
those buildings located at 21839 Atlantic Boulevard and 21829 Atlantic
Boulevard (the "Other Buildings"), without first obtaining Subtenant's
approval, which shall not be withheld or delayed unreasonably. If, pursuant to
Section 13.3(c) of the Prime Lease, Sublandlord elects to assume responsibility
for the management of the Premises and the Other Buildings, Sublandlord shall
apportion the costs of such management equitably among the Premises and the
Other Buildings.

          (g)  Sublandlord acknowledges that it has reviewed the signage
Subtenant has ordered for the Premises and hereby gives its consent for
Subtenant to install and maintain such signage on the outside of the Premises
in accordance with the terms of the Prime Lease.

     8.   Use of Premises  Subtenant's use of the Premises shall be governed in
all respects by the Prime Lease.

     9.   Subleasing and Assignment.

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          (a)  Subtenant shall not assign, transfer or otherwise encumber
(collectively, "assignment") this Sublease or all or any of Subtenant's rights
hereunder or interest herein, whether by operation of law or otherwise, or
sublet or permit anyone to use or occupy (collectively, "sublet") the Premises
or any part thereof, without obtaining the prior written consent of
Sublandlord, which shall not be unreasonably withheld or delayed, and the
Landlord, as required under the terms and provisions of the Prime Lease.
Sublandlord agrees, on its own behalf (but not on behalf of Landlord), to
respond to any written request for consent to any assignment or subletting
within thirty (30) days following receipt of such request from Subtenant.

          (b)  If any sublease or assignment provides that the subtenant or
assignee thereunder is to pay any amount in excess of the rental and other
charges due under this Sublease (and if the subleased space does not constitute
the entire Premises, the existence of such excess shall be determined on a
pro-rata basis), Subtenant shall pay to Sublandlord one-half (1/2) of any such
excess or other premium applicable to the sublease or assignment.

     10.  Security Deposit.  Concurrently with the execution hereof, Subtenant
shall deposit with Sublandlord a security deposit in the amount of $200,000 to
be held by Sublandlord subject to all of the terms of Article V (including the
reduction or return thereof) of the Prime Lease.

     11.  No Default. Subtenant shall not cause or permit anything to be done
which would constitute a default or breach of Sublandlord's obligations as
"Tenant" under the Prime Lease, including, without limitation, any breach or
default which would cause the Prime Lease to be terminated or forfeited by
reason of any right of termination or forfeiture reserved or vested in the
Landlord under the Prime Lease. In the event of any default or breach by
Subtenant in the performance of the covenants and agreements contained herein,
Sublandlord shall have all of the rights as against Subtenant that the Landlord
under the Prime Lease has against the Sublandlord for a default under the Prime
Lease and all rights and remedies available at law and equity. Subtenant shall
indemnify, defend and hold Sublandlord harmless from and against all claims,
demands, damages, liabilities and expenses (including, without limitation,
reasonable attorneys' fees) of any kind whatsoever by reason of (a) Subtenant's
use and occupancy of the Premises or the business conducted by Subtenant
therein, or (b) any accident, injury or damage whatsoever caused to any person,
or to the property of any person, occurring on or about the Premises during the
Sublease Term; provided that Subtenant's obligations to indemnify and hold
Sublandlord harmless shall not include any costs, damages, claims, liabilities
or expenses suffered by or claimed against Sublandlord directly based on,
arising out of or resulting from Sublandlord's breach of, or default as to, any
of its covenants or other obligations under this Sublease or the negligence or
willful misconduct of Sublandlord or its employees or the gross negligence or
willful misconduct of Sublandlord's contractors

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or agents. So long as Subtenant fully and timely performs all of its
obligations under this Sublease, Sublandlord agrees that it shall not default
in the performance of any of its obligations under the Prime Lease.

     12.  Surrender of Possession. Subtenant covenants to remove all goods and
effects from the Premises not the property of Sublandlord or Landlord at the
expiration or other termination of this Sublease, and to yield up to
Sublandlord the Premises, together with all keys, locks, and other fixtures
connected therewith in good repair, order and condition in all respects,
reasonable wear and use thereof and damage by fire or other casualty covered by
insurance excepted.

     13.  Notices. All notices, demands, requests, or other communications
which may be or are required to be given, served, or sent by any party to any
other party pursuant to this Sublease shall be in writing and shall be mailed
by first-class, registered, or certified mail, return receipt requested,
postage prepaid, or transmitted by hand delivery, or recognized overnight
delivery service, addressed as follows:

     (i)  If to Sublandlord:

          Orbital Sciences Corporation
          21700 Atlantic Boulevard
          Dulles, Virginia  20166
          Attention:  Director, Corporate Finance and Real Estate

          with a copy to:

          Orbital Sciences Corporation
          21700 Atlantic Boulevard
          Dulles, Virginia  20166
          Attention:  General Counsel

     (ii) If to Subtenant:

          ORBCOMM Global, L.P.
          21819 Atlantic Boulevard
          Dulles, Virginia  20166
          Attn:  General Counsel

          with a copy to:

          Teleglobe Inc.
          1000 Rue de la Gauchetiere Ouest
          Montreal (Quebec)
          Canada, H3B 4X5

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          Attn:  Vice President & Chief Legal Officer

Each party may designate by notice in writing, in accordance with this Section
a new address to which any notice, demand, request, or communication may
thereafter be so given, served, or sent. Each notice, demand, request or
communication which shall be mailed, delivered, or transmitted in the manner
described above shall be deemed sufficiently given, served, sent and received
for all purposes at such time as it is delivered to the addressee, or at such
time as delivery is refused by the addressee upon presentation.

     14.  Binding Effect.  Subject to the provisions hereof restricting
transfer, assignment, or subleasing, this Sublease shall be binding upon and
shall inure to the benefit of the parties hereto and their respective
successors and assigns.

     15.  Consent.  This Sublease is subject to the Landlord's consent, as
provided in Article VII of the Prime Lease, and only shall become effective
upon the granting of such consent.

     16.  Subordination. This Sublease is subject and subordinate to the Prime
Lease, to all ground and underlying leases, and to all mortgages and deeds of
trust which may now or hereafter affect any of such leases, the leasehold
estate or estates thereby created or the real property of which the Premises
form a part, and to any and all renewals, modifications, consolidations,
replacements and extensions thereof. Sublandlord agrees that it shall not amend
or modify any provision of the Prime Lease which would materially adversely
affect Subtenant's use and enjoyment of the Premises. In the event, for any
reason whatsoever, the Prime Lease is terminated, then this Sublease shall also
terminate effective as of the same date of the Prime Lease termination. In such
event, neither Landlord nor Sublandlord shall have any liability to Subtenant.

     17.  Certain Miscellaneous Provisions.

          (a)  The section headings or titles appearing in this Sublease are
inserted and included solely for convenience and shall never be considered or
given any effect in construing this Sublease. All personal pronouns used in
this Sublease shall include the other genders whether used in the masculine or
feminine or neuter gender, and the singular shall include the plural whenever
and as often as may be appropriate. Subject to Section 9 hereof, the covenants
and agreements herein contained shall inure to the benefit of and be binding
upon Sublandlord, its successors and assigns, and Subtenant, and its permitted
and approved successors and assigns. Unless otherwise expressly agreed by
Sublandlord, if more than one party is named as Subtenant hereunder, each such
party shall be jointly and severally liable for each and every obligation of
Subtenant hereunder.

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          (b)  Neither payment of rent by Subtenant, nor acceptance of rent by
Sublandlord, nor failure by either party to complain of any action, non-action
or default of the other party shall constitute a waiver of any aggrieved
party's rights hereunder. No waiver or consent to a variation from any
provision of this Sublease shall be effective unless made in a written
instrument duly executed on behalf of both parties. Waiver by either party of
any right with respect to any default of the other party shall not constitute a
waiver of any right with respect to either a subsequent default of the same
obligation or for any other default.

          (c)  This Sublease represents the entire agreement between the
parties hereto with respect to the sublease of the Premises. As between
Sublandlord and Subtenant, the terms and conditions of this Sublease shall take
precedence with respect to any conflict between the terms and conditions
contained herein and the terms and conditions of the Prime Lease.

          (d)  If any term, covenant, condition or provision of this Sublease,
or the application thereof to any person or circumstance, shall ever be held to
be invalid or unenforceable, then, in each such event, such term, covenant,
condition or provision, or the application thereof, to any other person or any
other circumstance, shall not be affected, except to the extent held to be
invalid or unenforceable, and each term, covenant, condition and provision
hereof shall remain valid and enforceable to the fullest extent permitted by
law.

          (e)  No amendment, modification or discharge of this Sublease shall
be valid or binding unless set forth in writing and duly executed by or on
behalf of both parties.

          (f)  This Sublease, the rights and obligations of the parties hereto,
and any claims or disputes relating thereto, shall be governed by and construed
in accordance with the laws of the Commonwealth of Virginia (but not including
the choice of law rules thereof).

          (g)  Each party hereto represents and warrants that it is in good
standing in the jurisdiction where it is organized, that the execution of this
Sublease has been duly authorized by all necessary partnership or corporate
actions (as appropriate), and that the person executing this Sublease on behalf
of such party has the power and authority to execute it on behalf of such party
and that this Sublease is a valid and binding obligation of such party.

          (h)  This Sublease may be executed in any number of counterparts
which, when taken together, shall constitute a single, binding instrument.

                     [SIGNATURES APPEAR ON FOLLOWING PAGE]

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     IN WITNESS WHEREOF, the undersigned have caused this Sublease to be duly
executed on their behalf as of the date first above written.

Orbital Sciences Corporation                   ORBCOMM Global, L.P.

By:                                            By:
     ------------------------------                 ---------------------
Name:   Jeffrey V. Pirone                      Name:      Scott L. Webster
Title:  Executive Vice President               Title:     President and
        and Chief Financial Officer                 Chief Executive Officer<PAGE>   1

                                                                    EXHIBIT 10.1

                                                                    CONFIDENTIAL

                                                            __   Collins' Copy
                                                            __   Company's Copy

                              FOURTH AMENDMENT TO
                              EMPLOYMENT AGREEMENT

To WILLIAM L. COLLINS, III:

By signing below, you agree to an amendment (the "Amendment") to your Employment
Agreement with Metrocall, Inc. (the "Company") dated as of May 15, 1996 (the
"Employment Agreement") as previously amended. The Fourth Amendment is effective
as of January 1, 2000.

       New language in the Employment Agreement is shown as amended:

       Section 4, as amended, reads:

       4.     Salary, Bonus, and Expenses.

              (a)    In consideration for the Executive's services, the Company
                     shall pay to the Executive an annual base salary (the
                     "Base Salary") from January 1, 2000 equal to Five Hundred
                     Thirty Thousand Dollars ($530,000)....

              (b)    ...

              (f)    Executive will be eligible for a fifty percent (50%) of
                     Base Salary target bonus for 2000.

       The Employment Agreement, except as amended and modified above, remains
in effect.

                                                    METROCALL, INC.

Date: 4/19/00                             By: /s/ RICHARD M. JOHNSTON
     -------------------------                --------------------------------
                                                    Richard M. Johnston
                                                    Chairman

Date:                                     By: /s/ WILLIAM L. COLLINS, III
     -------------------------               ---------------------------------
                                                    William L. Collins, III

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