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                                                                    EXHIBIT 10.3

April 12, 2001

MDA Holdings Corporation
21700 Atlantic Boulevard
Dulles, Virginia 20166
Attention: Legal Dept.

Fax 703-406-5572

Dear Sirs:
              RE: MACDONALD, DETTWILER AND ASSOCIATES LTD. ("MDA")

CAI Capital Partners and Company II, L.P., CAI Capital Partners and Company
II-C, L.P., CAI Partners and Company II, L.P. (collectively, the "CAI
Entities"), 597858 B.C. Ltd. ("BC"), and the Ontario Teachers' Pension Plan
Board (the "Institutional Investor" and, collectively with the CAI Entities and
BC, the "Purchasers") understand that MDA Holdings Corporation ("Holdco" or the
"Seller"), a wholly-owned subsidiary of Orbital Sciences Corporation
("Orbital"), owns 18,000,000 common shares of MacDonald, Dettwiler & Associates
Ltd. ("MDA" or the "Corporation"). The Purchasers wish to purchase 12,350,000 of
such common shares (the "Purchased Shares") and obtain additional rights in
respect of the balance of 5,650,000 of such common shares (the "Remaining
Shares" and, together with the Purchased Shares, the "Shares").

1.      SALE OF THE PURCHASED SHARES.

        (a)     Subject to the terms and conditions hereof, the Purchasers agree
                to purchase from the Seller and the Seller agrees to sell to the
                Purchasers the Purchased Shares for a purchase price of Cdn.
                $14.00 per share, or Cdn. $172.9 million in the aggregate,
                before deducting all fees to the Purchasers and expenses of the
                Purchasers payable by Holdco pursuant hereto, and on and subject
                to the following terms and conditions. The Purchased Shares are
                proposed to be purchased in the following respective amounts:
                CAI Capital Partners and Company II, L.P.: 783,696 shares; CAI
                Capital Partners and Company II-C, L.P.: 96,242 shares; CAI
                Partners and Company II, L.P.: 470,062 shares; BC: 1,000,000
                shares; and the Institutional Investor: 10,000,000 shares;
                provided that such amounts may be re-allocated by giving written
                notice to the Seller at least two business days before the
                Closing without changing the total if so agreed among the CAI
                Entities, BC and the Institutional Investor. In no event will
                the Seller be obliged to
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                                      -2-


                complete the Closing if less than all of the Purchased Shares
                are proposed to be purchased, however.

        (b)     Subject to the terms and conditions hereof, the purchase price
                will be paid and satisfied by a wire transfer of immediately
                available funds in accordance with wire instructions to be
                provided in writing by the Seller to each of the Purchasers no
                less than two (2) business days prior to Closing. All dollar
                amounts referred to in this letter agreement are in Canadian
                dollars. Despite the foregoing, at the written request of the
                Seller made no less than two (2) business days prior to
                Closing, the Purchasers shall pay the U.S. dollar equivalent
                (based on a rate of exchange available either on the Closing
                date or the one or two prior business days, as determined by
                each of the Purchasers) of any amount otherwise payable, net
                of any foreign exchange costs actually incurred by them.

        (c)     At Closing, the Seller shall pay to the CAI Entities and BC a
                sourcing and structuring fee of Cdn. $3.458 million
                (representing 2.0% of the purchase price), with one-half of
                such amount payable to each of them, and shall pay or
                reimburse the Purchasers for their reasonable fees and
                expenses incurred in connection herewith (for greater
                certainty including those referred to in section 4.2), up to
                an aggregate amount of Cdn. $500,000.

2.      REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE SELLER.

        The Seller represents, warrants and covenants as follows to the
Purchasers now and as at the Closing:

        (a)     The Seller is and will at Closing be the legal and beneficial
                owner of the Purchased Shares, which will at Closing be
                delivered to the Purchasers free and clear of any and all
                adverse claims, security interests or encumbrances whatsoever.
                At the date hereof, the Purchased Shares are subject to the
                Shareholders' Agreement (as defined herein), the Amended and
                Restated Secondary Option Agreement, the Reverse Option
                Agreement, the Pledge and Custodial Agreement, and an
                underwriting agreement and resale restrictions agreement
                entered into at the time of the initial public offering of MDA
                (the "Underwriters' Covenants"). A waiver and release of the
                Underwriters' Covenants in respect of both the Purchased
                Shares and the Remaining Shares will be obtained by the Seller
                prior to Closing.

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        (b)     The Seller has all power and authority necessary to execute
                and deliver this letter agreement and to perform and complete
                its obligations hereunder, and such actions will not violate
                its articles or by-laws, any applicable laws, or any agreement
                by which it, Orbital or the Shares are bound, except to the
                extent set forth in section 2(a) or as contemplated by the
                following sentence. If the consent of Orbital's lenders is
                required hereunder, the Seller will obtain such consent prior
                to Closing.

        (c)     Immediately following the Closing, the Seller (i) will be
                solvent, (ii) will not be engaged in a business or a
                transaction, or about to be engaged in a business or a
                transaction, for which any property remaining with the Seller
                would be an unreasonably small capital, and (iii) has not
                incurred, and does not intend to incur, or believe that it
                would incur, debts that would be beyond the Seller's ability
                to pay as such debts matured.

        (d)     Orbital and the Seller have complied with and will continue to
                comply with sections 2.4 and 2.5 of the Amended and Restated
                Shareholders' Agreement (the "Shareholders' Agreement") dated
                as of June 29, 2000 among the CAI Entities, BC, Orbital and
                Holdco, and the Seller has complied with and will continue to
                comply with its charter documents (including for at least 2
                years after Closing). For greater certainty, this provision
                shall survive the consummation of the transactions
                contemplated hereby.

        (e)     To the best knowledge of the Seller and Orbital, after due
                enquiry, neither the Seller nor Orbital is aware of any
                material fact or material change with respect to MDA or its
                shares that has not been disclosed publicly by MDA or
                disclosed to all of the Purchasers in writing by the Seller
                (or, in the case of the CAI Entities and BC only, disclosed to
                their nominees in their capacities as directors of MDA), and
                neither the Seller nor Orbital is aware of any
                misrepresentation in any of the Corporation's public
                disclosure documents filed on SEDAR.

        (f)     No person has any agreement, right or privilege for the
                purchase, acquisition or transfer from the Seller of any of
                the Purchased Shares or any of the Remaining Shares or any
                interest therein except (i) the Purchasers pursuant hereto,
                and (ii) pursuant to the Shareholders' Agreement (as defined
                herein), the Amended and Restated Secondary Option Agreement,
                the Reverse Option Agreement and the Pledge and Custodial
                Agreement. The Seller has the right to sell the Purchased



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                Shares and the Remaining Shares to the Purchasers except that,
                as at the date hereof, the Shares are also subject to the
                Underwriters' Covenants.

        (g)     The Seller is a non-resident of Canada within the meaning of
                the Income Tax Act (Canada) and, for greater certainty, its
                address in the records of the Corporation is not in Canada.

        (h)     The Seller is acting as principal hereunder.

        (i)     The Seller has held the Purchased Shares since at least July
                31, 2000.

        (j)     Neither the Seller nor any affiliate of or person acting
                jointly or in concert with the Seller has purchased or sold
                any common shares of MDA or entered into any derivative or
                similar transactions with respect thereto at any time since
                July 31, 2000.

        (k)     This letter agreement is a legal, valid and binding obligation
                of the Seller enforceable against the Seller in accordance
                with its terms, subject however to limitations with respect to
                the enforcement of remedies, to bankruptcy, reorganization,
                insolvency, moratorium and other laws relating to or affecting
                creditors' rights generally and subject to the availability of
                equitable remedies such as specific performance and
                injunctions.

        (l)     The Seller's and Orbital's agreement with any and all
                prospective purchasers of the Shares other than the Purchasers
                has been terminated in accordance with its terms.

3.      REPRESENTATIONS AND WARRANTIES OF THE PURCHASERS.

        Each of the Purchasers represents and warrants as follows to the
Seller now and as at the Closing (for greater certainty, with each Purchaser
warranting as to itself only, severally and not jointly or jointly and
severally):

        (a)     Such Purchaser has all power and authority necessary to
                execute and deliver this agreement and to perform and complete
                its obligations hereunder, and such actions will not violate
                its articles or by-laws, any applicable laws, or any agreement
                by which it is bound.

        (b)     This letter agreement is a legal, valid and binding obligation
                of such Purchaser enforceable against such Purchaser in
                accordance with its terms, subject however to limitations with
                respect to the enforcement



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                of remedies, to bankruptcy, reorganization, insolvency,
                moratorium and other laws relating to or affecting creditors'
                rights generally and subject to the availability of equitable
                remedies such as specific performance and injunctions.

        (c)     Other than CAI Partners and Company II, L.P., which is a "WTO
                investor", such Purchaser is not a "non-Canadian" for purposes
                of the Investment Canada Act (Canada).

4.      COVENANTS.

4.1     The Seller covenants and agrees with the Purchasers that it will do or
        cause to be done, in addition to its covenants pursuant to section 2,
        the following:

        (a)     The Seller shall use all reasonable efforts to satisfy all
                requirements and conditions contained in this letter agreement
                in order to complete the transactions contemplated hereby and
                to comply with, satisfy and fulfil promptly all legal and
                regulatory requirements applicable to the Seller with respect
                to the consummation of the transactions contemplated hereby.
                Despite the foregoing, the Seller shall not be required to
                execute any additional agreements referred to in section 7(c)
                which are not satisfactory to it, acting reasonably (without
                prejudice to the Purchasers' rights under section 5).

        (b)     The Seller shall cause two of its three nominees to the board
                of directors of the Corporation to resign at such time and in
                such manner (which may include voting for their successors) as
                is requested by the Purchasers at or following Closing, and
                shall cause its remaining nominee to resign at such time as it
                no longer holds at least 5% of the outstanding common shares
                of MDA, and shall use its reasonable efforts to cause its
                remaining nominee to support the appointment in their stead of
                such individuals as the Purchasers shall identify in writing
                to the Seller. For greater certainty, this provision shall
                survive the consummation of the transactions contemplated
                hereby.

        (c)     In the period prior to the Closing, the Seller shall consult
                with the Purchasers prior to exercising any voting rights
                attached to the Purchased Shares.  In the event that the
                Seller and the Purchasers, each acting reasonably, do not
                agree as to how the voting rights are to be exercised on a
                matter which, in the opinion of the Purchasers, acting
                reasonably, is material to the value of the Purchasers'
                current investment or proposed investment in the Corporation,
                the Purchasers



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                may terminate this letter agreement, which shall, without
                prejudice to any other remedy of any Purchaser hereunder or
                for breach of any other obligation of the Seller hereunder,
                and subject to the requirement that the Seller reimburse all
                of the Purchasers' reasonable fees and expenses incurred in
                connection herewith (which obligation shall survive any
                termination of this letter agreement), shall be the exclusive
                remedy of the Purchasers hereunder for a breach of this
                section 4.1(c) by the Seller. Notwithstanding the foregoing
                terms of this section 4.1(c), the Purchasers acknowledge that
                at all times prior to the Closing the Seller shall retain sole
                decision-making authority with respect to the exercise of the
                voting rights attached to the Shares.

        (d)     Until the Closing, the Seller will co-operate with the
                Purchasers and MDA in making all requisite regulatory filings
                and seeking all regulatory and third party consents.

        (e)     Without the express prior written consent of the Purchasers,
                the Seller and its affiliates shall maintain in strict
                confidence and not use for any purpose whatsoever any
                non-public information of which they are aware at the date
                hereof or of which they subsequently become aware relating to
                MDA, except as required by law and following prior written
                notice and consultation with MDA and the Purchasers. The
                foregoing shall not apply to disclosures pursuant to a
                reasonable written confidentiality agreement for the purpose
                of seeking offers for the Remaining Shares subsequent to June
                30, 2001. The following information shall not be considered
                non-public information:  (i)  information that is or becomes
                publicly available through no fault of the Seller or its
                affiliates; (ii)  information that was known by the Seller or
                Orbital (other than on a confidential basis) prior to the
                receipt of such information from MDA; or (iii)  information
                that becomes available to the Seller or Orbital on a
                non-confidential basis from MDA (following Closing) or from a
                source other than MDA which source, to the knowledge of the
                Seller or Orbital, is not bound by a confidentiality
                obligation with MDA or otherwise prohibited from disclosing
                such information by a contractual, legal or fiduciary
                obligation. For greater certainty, this provision shall
                survive the consummation of the transactions contemplated
                hereby and any termination of this letter agreement.

        (f)     The Seller and its affiliates agree to use all reasonable
                efforts to satisfy all terms and conditions hereof, and also
                to co-operate in full with any reasonable request made by the
                Purchasers in connection with this



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                transaction and/or the Remaining Shares, including without
                limitation in connection with the Remaining Shares by
                assisting in the process of obtaining financing for the
                purchase of the Remaining Shares, participating in any
                prospectus process, assisting with access for due diligence
                purposes, and signing any requested underwriting or
                indemnification agreements on terms not materially more
                adverse to them than those applicable in the IPO, provided
                that the Seller shall be reimbursed by the CAI Entities and/or
                BC or otherwise for all commissions payable by it related
                thereto and all reasonable expenses (not to exceed US
                $100,000) incurred by it in connection therewith.  For greater
                certainty, this provision shall survive the consummation of
                the transactions contemplated hereby.

        (g)     If the Seller dividends or distributes any of the net proceeds
                hereof to Orbital, then Orbital (which is a party to this
                agreement solely for the purposes of this section 4(g)) will
                ensure that it forthwith repays to MDA the amount of U.S.
                $3,889,810 (or the reasonable Canadian dollar equivalent
                thereof) (or such other amount as is acceptable to the
                Purchasers, acting reasonably, based upon the letter agreement
                between Orbital and MDA dated January 16, 2001) on behalf of
                certain of Orbital's or its subsidiaries' (other than
                Orbimage's) liabilities to MDA, which amount is net of amounts
                owed to Orbital by MDA.

        (h)     Until the Closing or the termination of this letter agreement,
                whichever occurs first, the Seller shall not enter into any
                other agreement in respect of the Shares and shall refrain
                from any purchases of or offers to purchase additional shares
                of MDA, any material transactions with MDA, and any proxy
                solicitations of MDA shareholders, in each case without the
                express prior written consent of the Purchasers.

        (i)     Until the Closing or termination of this letter agreement,
                whichever occurs first, the Seller and its affiliates shall
                co-ordinate all public announcements or statements and filings
                in respect of the transactions contemplated by this letter
                agreement with the Purchasers and shall consult with the
                Purchasers in respect thereof and make any changes reasonably
                requested by them.

        (j)     The Seller shall use its reasonable efforts to cause MDA to be
                included in any release obtained by Orbital or the Seller in
                respect of Orbcomm, provided that MDA cooperates and agrees to
                subrogate its claims against Orbcomm. For greater certainty,
                this provision shall survive the consummation of the
                transactions contemplated hereby.



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4.2     The Purchasers (for greater certainty, each severally as to itself and
        not jointly or jointly and severally with the other Purchasers)
        covenant and agree with the Seller that they will use all reasonable
        efforts to satisfy all requirements and conditions applicable to them
        contained in this letter agreement in order to complete the
        transactions contemplated hereby and to comply with, satisfy and
        fulfil promptly all legal and regulatory requirements applicable to
        them with respect to the consummation of the transactions contemplated
        hereby. Without limiting the generality of the foregoing, the
        Purchasers (for greater certainty, each severally as to itself and not
        jointly or jointly and severally with the other Purchasers) agree that
        they shall as soon as practicable make and diligently prosecute:

        (a)    if necessary, an application to the Commissioner of Competition
               for an advance ruling certificate under section 102 of the
               Competition Act (Canada) and/or a pre-notification filing under
               Part IX of the Competition Act (Canada) with respect to the
               purchase by the Purchasers of the Purchased Shares (and, if
               considered appropriate, the Remaining Shares); and

        (b)    if necessary, a Hart-Scott-Rodino Antitrust Improvements Act of
               1976 filing with respect to the purchase by the Purchasers of
               the Purchased Shares (and, if considered appropriate, the
               Remaining Shares).

        The Purchasers shall pay the filing fees in respect thereof, subject
        to reimbursement as provided herein by the Seller. Despite the
        foregoing, the CAI Entities and BC shall not be required to execute
        any additional agreements referred to in section 7(c) which are not
        satisfactory to them, acting reasonably (without prejudice to their
        rights under section 5), and provided that if the Seller refuses to
        sign any agreements contemplated by section 7(c), it shall be required
        to reimburse the Purchasers their reasonable fees and expenses
        incurred in connection herewith (which obligation shall survive any
        termination of this letter agreement).

5.      CONDITIONS OF CLOSING IN FAVOUR OF THE PURCHASERS.

        The obligations of the Purchasers to purchase the Purchased Shares
under this letter agreement are subject to the following conditions for the
exclusive benefit of the Purchasers to be fulfilled and/or performed prior to
the Closing:

        (a)     The representations and warranties of the Seller contained in
                this letter agreement shall be true and correct in all
                respects as of the date of this letter agreement and shall
                also be true and correct in all respects on


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                and as of the Closing with the same force and effect as if
                made on and as of such date.

        (b)     The Seller shall have performed or complied in all respects
                with all covenants to be performed or complied with by the
                Seller in this letter agreement at or prior to the Closing.

        (c)     The Corporation shall have confirmed at Closing to the
                satisfaction of the Purchasers, acting reasonably: (i) the
                absence of any consents (other than those that have been
                obtained on terms satisfactory to the Purchasers, acting
                reasonably) that are required under the terms of any
                agreements to which MDA or its subsidiaries are a party or
                bound in order to avoid default, termination or other adverse
                effects (including without limitation under the Landamerica,
                Radarsat II, BC Online and UK agreements, to the extent
                required); and (ii) the absence of any material fact or
                material change with respect to MDA or its shares that has not
                been disclosed publicly by MDA.

        (d)     The Purchasers shall be satisfied that MDA and its
                subsidiaries and their respective assets and operations have
                insurance in place or are covered by insurance to the extent
                deemed necessary by the Purchasers (following consultation
                with MDA) and that all amounts paid or to be paid by MDA or
                its subsidiaries to Orbital until Closing on account of
                premiums or contributions on all insurance policies, employee
                benefit plans, 401K plans and similar plans maintained at the
                date hereof by Orbital on behalf of MDA and its subsidiaries
                have been paid or made in full.

        (e)     The Purchasers shall have received a certificate dated the
                Closing in form and on terms satisfactory to the Purchasers,
                acting reasonably, signed by two senior officers of the Seller
                on behalf of the Seller, to the effect that the conditions
                precedent specified in sections 5(a) and 5(b) have been
                complied with.

        (f)     If a filing under the Competition Act (Canada) is required,
                the Commissioner of Competition (the "Commissioner") appointed
                under the Competition Act (Canada) (the "Act") shall have
                issued an advance ruling certificate under section 102 of the
                Act in respect of the transactions contemplated hereby; (ii)
                the Commissioner shall have advised the Purchasers that he
                does not intend at the current time to apply to the
                Competition Tribunal for an order under section 92 of the Act
                in respect of such transactions; or (iii) the applicable
                waiting



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                period under section 123 of the Act shall have expired without
                the Commissioner having notified the Purchasers that he
                intends to apply to the Competition Tribunal for an order
                under section 92 of the Act in respect of such transactions;
                and no proceedings shall have been taken or threatened under
                the merger provisions of Part VIII or under section 45 of the
                Act in respect of such transactions.

        (g)     If a filing under the Hart-Scott-Rodino Antitrust Improvements
                Act of 1976 is required in the United States, any applicable
                pre-merger filing requirements under that Act shall have been
                fulfilled, any waiting period requirements under that Act
                shall have expired or been terminated, and there shall be no
                pending or threatened actions or proceedings to restrain the
                transactions contemplated hereby or asserting its illegality.

        (h)     Other than any court or regulatory proceedings referred to in
                section 9(e) to the extent that they shall oblige the parties
                hereto to adjust the purchase price, no action or proceeding
                which is outstanding and not resolved shall have been taken
                before or by any domestic or foreign court, governmental
                authority or other regulatory authority, no order, decision or
                ruling shall have been made by any such court, governmental
                authority or other regulatory authority, and no law or
                regulation shall have been passed since the date hereof which
                in any of the foregoing cases would have the effect of:

                (i)   ceasing trading or prohibiting the purchase or resale
                      by the Purchasers of any of the Purchased Shares;

                (ii)  imposing on the Purchasers material burdensome
                      obligations, restrictions, limitations or conditions on
                      their ownership of or exercise of rights of ownership
                      over the Purchased Shares which could reasonably be
                      expected to have a materially adverse effect on the
                      value of the Purchased Shares; or

                (iii) requiring the Purchasers to make an offer to purchase
                      any additional common shares of MDA held by other
                      shareholders;

               it being acknowledged and agreed that the Purchased Shares are
               not at the date hereof freely tradeable.

        (i)    There shall not exist any prohibition at law against the
               Purchasers acquiring the Purchased Shares, and the Seller shall
               have obtained any necessary consents from its lenders and
               pursuant to the Underwriters'



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                Covenants to complete the transactions contemplated herein,
                including in section 7(c).

        (j)     MDA shall not have suffered an adverse material change in its
                business subsequent to the date hereof (or an undisclosed
                adverse material change in its business prior to the date
                hereof).

        (k)     The arrangements set forth in section 7(c) hereof shall have
                been executed, delivered and completed to the satisfaction of
                the Purchasers, acting reasonably.

        (l)     The Seller shall have received a fairness opinion from an
                independent advisor in form and substance satisfactory to the
                Purchasers, acting reasonably.

        (m)     The Purchasers shall have received a legal opinion from the
                Seller's counsel in form and substance satisfactory to the
                Purchasers, acting reasonably, with respect to the due
                authorization and execution of all agreements entered into in
                connection with the transactions contemplated herein.

        (n)     The Institutional Investor shall have been provided by the
                Corporation with two demand and unlimited piggy-back
                registration rights on terms substantially similar to those
                currently granted to the CAI Entities and BC, which shall be
                confirmed on substantially the same terms to reflect necessary
                changes and the grant of one further additional demand
                registration right to the Purchasers in aggregate.

        (o)     The Purchasers shall be satisfied that Orbital shall have
                withdrawn its request to MDA to seek the release of Orbital's
                guarantee of MDA's obligations to the Canadian Space Agency.

        In the event that any of such conditions shall not have been satisfied
or waived on or prior to the Closing, the Purchasers or any of them may in
their discretion terminate the agreement to purchase the Purchased Shares by
notice in writing to the Seller, without any party having any further
obligation hereunder (except that the Seller shall be required to reimburse
the Purchasers their reasonable fees and expenses incurred in connection
herewith, which obligation shall survive any termination of this letter
agreement), but without prejudice to liability for any prior breach hereunder,
and, for greater certainty, the terms of the Shareholders' Agreement, the
Amended and Restated Secondary Option Agreement, the Reverse Option Agreement,
the Pledge and Custodial Agreement, the Exchange and Registration Rights
Agreement and any other agreements among the CAI Entities,




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                                     -12-


BC, the Seller and/or Orbital shall remain in full force and effect,
unaffected thereby.

6.      CONDITIONS OF CLOSING IN FAVOUR OF THE SELLER.

        The obligations of the Seller under this agreement are subject to the
following conditions for the exclusive benefit of the Seller to be fulfilled
and/or performed prior to the Closing:

        (a)    The representations and warranties of the Purchasers contained
               in this letter agreement shall be true and correct in all
               respects as of the date of this letter agreement and shall also
               be true and correct in all respects on and as of the Closing,
               with the same force and effect as if made on and as of such
               date.

        (b)    The Purchasers shall have performed or complied in all respects
               with all covenants to be performed or complied with by the
               Purchasers in this letter agreement at or prior to the Closing.

        (c)    There shall not exist any prohibition at law against the Seller
               selling the Purchased Shares to the Purchasers.

        (d)    The Seller shall have obtained any necessary consents from its
               lenders and pursuant to the Underwriters' Covenants to complete
               the transactions contemplated herein, including in section
               7(c).

        (e)    The Seller shall have received certificates dated the Closing
               in form and on terms satisfactory to the Seller, acting
               reasonably, from the Purchasers to the effect that the
               conditions precedent specified in sections 6(a) and 6(b) have
               been complied with.

        (f)    If a filing under the Competition Act (Canada) is required, the
               Commissioner of Competition (the "Commissioner") appointed
               under the Competition Act (Canada) (the "Act") shall have
               issued an advance ruling certificate under section 102 of the
               Act in respect of the transactions contemplated hereby; (ii)
               the Commissioner shall have advised the Purchasers that he does
               not intend at the current time to apply to the Competition
               Tribunal for an order under section 92 of the Act in respect of
               such transactions; or (iii) the applicable waiting period under
               section 123 of the Act shall have expired without the
               Commissioner having notified the Purchasers that he intends to
               apply to the Competition Tribunal for an order under section 92
               of the Act in respect of such transactions; and no proceedings
               shall have been taken





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                                     -13-

               or threatened under the merger provisions of Part VIII or under
               section 45 of the Act in respect of such transactions.

        (g)    If a filing under the Hart-Scott-Rodino Antitrust Improvements
               Act of 1976 is required in the United States, any applicable
               pre-merger filing requirements under that Act shall have been
               fulfilled, any waiting period requirements under that Act shall
               have expired or been terminated, and there shall be no pending
               or threatened actions or proceedings to restrain the
               transactions contemplated hereby or asserting its illegality.

        In the event that any of such conditions shall not have been satisfied
or waived on or prior to the Closing, the Seller may in its discretion
terminate the agreement to sell the Purchased Shares by notice in writing to
the Purchasers, without any party having any further obligation hereunder, but
without prejudice to liability for any prior breach hereunder, and, for
greater certainty, the terms of the Shareholders' Agreement, the Amended and
Restated Secondary Option Agreement, the Reverse Option Agreement, the Pledge
and Custodial Agreement, the Exchange and Registration Rights Agreement and
any other agreements among the CAI Entities, BC, the Seller and/or Orbital
shall remain in full force and effect, unaffected thereby. Despite the
foregoing, in the event that the condition in section 6(d) is not satisfied or
waived, and the Seller terminates this letter agreement, then the Seller shall
be required to reimburse the Purchasers their reasonable fees and expenses
incurred in connection herewith, which obligation shall survive any
termination of this letter agreement.

7.      CLOSING ARRANGEMENTS; TERMINATION.

        (a)    The closing of the purchase and sale of the Purchased Shares
               shall take place at 10:00 a.m. (Toronto time) at the offices of
               Hogan & Hartson LLP in New York on the second business day
               following the date on which the Purchasers shall have notified
               the Seller that all of the conditions referred to in sections
               5(c)(i), (d), (f), (g) and (i) hereof have been satisfied or
               waived, or at such other time and date as may be mutually
               agreed upon between either of the Purchasers and the Seller
               (the "Closing").  Subject to the following sentence, the
               Purchasers or the Seller may terminate this letter agreement if
               the Closing does not occur on or before May 31, 2001 and in
               such event, for greater certainty, the terms of the
               Shareholders' Agreement, the Amended and Restated Secondary
               Option Agreement, the Reverse Option Agreement, the Pledge and
               Custodial Agreement, the Exchange and Registration Rights
               Agreement and any other agreements among the CAI Entities,




   14
                                     -14-





               BC, the Seller and/or Orbital shall remain in full force and
               effect, unaffected thereby. The Purchasers shall use their
               reasonable efforts to seek to satisfy such conditions to enable
               a closing as close to April 13, 2001 as is reasonably possible.

        (b)    At the Closing, the Seller agrees to deliver to the Purchasers
               definitive certificates representing the Purchased Shares duly
               endorsed for transfer (with signatures guaranteed and other
               documents required by MDA's transfer agent to process such a
               transfer) in the names of the Purchasers or as they may direct,
               against payment by the Purchasers of the purchase price for the
               relevant Purchased Shares.

        (c)    Contemporaneously with the Closing of the purchase and sale of
               the Purchased Shares, the Seller shall enter into binding
               agreements on terms and conditions (including as to
               representations, warranties and covenants) satisfactory to the
               CAI Entities and BC, acting reasonably, whereby the Seller
               grants them (pro rata to their shareholdings in MDA on the date
               hereof and prior to the transactions contemplated herein, or in
               such other proportions as they shall determine), an irrevocable
               and transferable option (the "Option"), in whole or in part, to
               elect to purchase the Remaining Shares at any time on or before
               May 31, 2001 (which may be extended by them for financing or
               regulatory reasons until June 30, 2001) at Cdn. $14.25 per
               share (subject to a 2% sourcing and structuring fee, a price
               adjustment provision as in section 9(e) hereof and subject to
               the requirement of the CAI Entities and BC to share as to 50%
               with the Seller all net proceeds from the sale of the Option or
               the Remaining Shares by them to any third party in excess of
               Cdn. $14.25 per share). The shares would be pledged with and
               held by a commercial bank or broker in the U.S. in a manner
               similar to that today, for purposes of assuring the Option and
               the following rights.  In addition, the CAI Entities and BC
               would have an irrevocable and transferable right of first
               refusal (exerciseable for a period of 30 days) on any sale of
               any of the Remaining Shares by the Seller on or prior to June
               30, 2002 to the extent that the Option is not exercised, and
               the right to receive 50% of the net proceeds in excess of Cdn.
               $15.00 per share in the event of any sale of any of the
               Remaining Shares by the Seller to any third party on or prior
               to such date to the extent that the right of first refusal is
               not exercised, in each case in accordance with the same
               pro-ration mechanism.




   15

                                     -15-



8.      SURVIVAL OF REPRESENTATIONS AND WARRANTIES.

        All representations and warranties of each party made herein, in any
certificate or other document delivered by it or on its behalf pursuant to the
provisions hereof or otherwise with respect to this letter agreement and the
transactions contemplated hereby shall survive the Closing of the purchase and
sale of the Purchased Shares and, notwithstanding such Closing, nor any
investigation made by or on behalf of such party, shall continue in full force
and effect without any limitation as to time (except that the representations
and warranties of the Seller in sections 2(e) and 2(l) shall only survive for
a period of 18 months following Closing if no claim in writing is made in
respect thereof prior to such time).

9.      GENERAL PROVISIONS.

(a)     Any notice, direction or other instrument required or permitted to be
        given or made hereunder shall be made in writing and shall be
        sufficiently given or made if delivered in person to the address set
        forth below or if telecopied or sent by other means of recorded
        electronic communication and confirmed by delivery as soon as
        practicable thereafter.

               Notices to the Seller shall be addressed as follows:

               MDA Holdings Corporation
               c/o Orbital Sciences Corporation
               21700 Atlantic Boulevard
               Dulles, Virginia USA 20166-6801



                Attention:         Legal Department

                Telephone:         (703) 406-5000
                Facsimile:         (703) 406-5572

               with a copy (which shall not constitute notice) to:

               Hogan & Hartson LLP
               555 Thirteen Street, NW
               Washington, DC USA 20004
               Attention: Gordon Wilson
               Telephone: (202) 537-5627
               Facsimile: (202) 537-5910

               Notices to the CAI Entities shall be addressed as follows:



   16





                                     -16-


                c/o CAI Managers & Co., L.P.

                767 Fifth Avenue, 5th Floor
                New York, New York
                10153

                Attention:         Manfred Yu

                Telephone:         212-319-3023
                Facsimile:         212-319-0232


               with a copy (which shall not constitute notice) to: Simon
               Romano and Roberta Carano, Stikeman Elliott, Suite 5300,
               Commerce Court West, Toronto, Ontario M5L 1B9, Telephone
               416-869-5500, Fax:  416-947-0866.

               Notices to BC shall be addressed as follows:




                Randall Mullan
                c/o McCullough O'Connor Irwin
                1100 - 888 Dunsmuir Street
                Vancouver,  British Columbia  Canada
                V6C 3K4

                Attention:         Jonathan McCullough

                Telephone:         604-687-7077
                Facsimile:         604-687-7099


               Notices to the Institutional Investor shall be addressed as
               follows:

               Ontario Teachers' Pension Plan Board
               5650 Yonge Street
               Toronto, Ontario, Canada M2M 4H5



                Attention:         Roy Graydon

                Telephone:         416-730-6167
                Facsimile:         416-730-5143


   17


                                     -17-


               with a copy (which shall not constitute notice) to: Jonathan
               Lampe and Francesca Guolo, Goodmans, Suite 2400, 250 Yonge
               Street, Toronto, Ontario M5B 2M6, Telephone 416-979-2211, Fax:
               416-979-1234.

               Any notice, direction or other communication so given or so
               made shall be deemed to have been given or made and to have
               been received on the day of delivery, if delivered, or on the
               day of sending if sent by telecopier or other means of recorded
               electronic communication (provided such day of delivery or
               sending is a business day and if not then on the first business
               day thereafter). Any party may change its address for notice by
               notice given to the other parties in the manner aforesaid.

        (b)    Except as expressly provided herein, all costs and expenses
               incurred in connection with this letter agreement and the
               transactions contemplated hereby shall be paid by the party
               incurring such expenses. The Seller covenants and agrees to
               indemnify and save harmless the Purchasers from and against any
               claims whatsoever for any commissions or remuneration payable
               or alleged to be payable to any broker, agent or other
               intermediary who has acted for the Seller or Orbital in
               connection with the sale of the Purchased Shares or the
               Remaining Shares.  For greater certainty, this provision shall
               survive the consummation of the transactions contemplated
               hereby and any termination of the letter agreement.

        (c)    This letter agreement shall be construed, interpreted and
               enforced in accordance with the laws of the Province of Ontario
               and the federal laws of Canada applicable therein (without
               regard to conflicts of law principles). Any disputes arising
               under this letter agreement shall be subject to the
               non-exclusive jurisdiction of the courts of the Province of
               Ontario, and each party irrevocably attorns to the courts of
               such jurisdiction.

        (d)    Time shall be of the essence of this letter agreement.

        (e)    The parties acknowledge and agree that it is intended that the
               price per share for the Purchased Shares does not exceed 115%
               of the "market price" of the Common Shares determined in
               accordance with the regulations under the Securities Act
               (Ontario) and that any recalculation of "market price" would be
               inappropriate in the circumstances.  However, the parties agree
               that if any regulatory authority or court of competent
               jurisdiction decides at any time to





   18

                                     -18-




               recalculate the "market price" and as a result determines that
               the price per share exceeds such 115% of the "market price",
               the price per share will be reduced to that price which is
               either (a) determined by such court or regulatory authority to
               be 115% of the "market price", or (b) the price which the
               parties, acting in good faith, believe to be 115% of the
               "market price".  If necessary, the Seller agrees to refund such
               amount to the Purchasers as is required to comply with the
               foregoing. For greater certainty, this provision shall survive
               the consummation of the transactions contemplated hereby.

        (f)    This agreement is without prejudice to the offer (the "ROFO")
               dated March 14, 2001 by the CAI Entities and BC pursuant to the
               terms of section 5.1 of the Shareholders' Agreement. Upon
               Closing, the Shareholders' Agreement, the Amended and Restated
               Secondary Option Agreement, the Reverse Option Agreement, the
               Pledge and Custodial Agreement and the Exchange and
               Registration Rights Agreement shall terminate, and the ROFO
               shall be of no force and effect. If Closing does not occur for
               any reason whatsoever, the ROFO shall remain in full force and
               effect (in accordance with its terms), as, for greater
               certainty, shall the Shareholders' Agreement, the Amended and
               Restated Secondary Option Agreement, the Reverse Option
               Agreement, the Pledge and Custodial Agreement and the Exchange
               and Registration Rights Agreement. For greater certainty, this
               provision shall survive the consummation of the transactions
               contemplated hereby and any termination of this letter
               agreement.

        (g)    Each of the parties to any agreement resulting herefrom shall
               execute all further documents and do all further acts and
               things as may be necessary for the purpose of implementing the
               terms of such agreement.

        (h)    The covenants contained herein shall, to the extent applicable,
               survive any purchase and sale in accordance herewith.

        (i)    The obligations of the Purchasers hereunder are several, and
               not joint nor joint and several.

        (j)    The Seller acknowledges that the purchase price and the price
               of the Option has been determined at arms' length, that the
               purchase price and the price of the Option represent reasonably
               equivalent value for the Shares, and that the purchase price
               and the price of the Option are fair and reasonable in the
               circumstances, and the Seller waives any




   19


                                     -19-




               right to claim that the purchase price or the Option price is
               not at arms' length, adequate and fair. For greater certainty,
               this provision shall survive the consummation of the
               transactions contemplated hereby.

        (k)    This letter agreement constitutes the entire agreement among
               the parties hereto pertaining to the subject matter hereof, and
               supersedes all prior agreements, understandings, negotiations
               and discussions, whether oral or written, of the parties with
               respect thereto, except as set forth in section 9(f) hereof.
               Despite the foregoing, the Purchasers acknowledge their intent
               to enter into agreements among themselves concerning certain
               matters. No supplement, modification or termination of this
               letter agreement shall be binding unless executed in writing by
               each of the parties hereto.

        (l)    Except as otherwise provided herein, no party may assign this
               letter agreement without the prior written consent of the other
               parties hereto.  The Purchasers may assign their rights under
               this letter agreement to any other person or entity in whole or
               in part without the prior written consent of the Seller,
               provided that such assignment shall not result in the
               applicability of additional regulatory approvals which would
               reasonably be expected to materially delay the Closing.  The
               Seller may not assign any of its rights or obligations under
               this letter agreement to any other person or entity in whole or
               in part without the prior written consent of the Purchasers,
               which may be unreasonably withheld. This letter agreement shall
               enure to the benefit of and be binding upon the parties hereto
               and their respective successors and permitted assigns.

        (m)    Each of the parties recognizes and acknowledges that a breach
               by a party of any covenants or other commitments contained in
               this letter agreement will cause the other parties to sustain
               injury for which they would not have an adequate remedy at law
               for money damages.  Therefore, each of the parties agrees that
               in the event of any such breach, the aggrieved party or parties
               shall be entitled to the remedy of specific performance of such
               covenants or commitments and preliminary and permanent
               injunctive and other equitable relief in addition to any other
               remedy to which it may be entitled, at law or in equity, and
               the parties further agree to waive any requirement for the
               securing or posting of any bond in connection with the
               obtaining of any such injunctive or other equitable relief.



   20

                                     -20-


        (n)    The Seller shall indemnify and save harmless the Purchasers of
               and from all losses whatsoever (including without limitation
               expenses, costs, damages, penalties, fines, charges, claims,
               demands, liabilities, interest, and any and all legal fees and
               disbursements) suffered by, imposed upon or asserted against
               the Purchasers (or against any of their directors, officers,
               employees, agents, representatives or shareholders) in respect
               of, as a result of, connected with or arising out of, under or
               pursuant to any inaccuracy in any representation or warranty
               provided by the Seller herein.  If for any reason the above
               indemnity shall be unenforceable, the Seller shall contribute
               to the losses incurred in such amount as is appropriate to
               reflect all equitable considerations. For greater certainty,
               this provision shall survive the consummation of the
               transactions contemplated hereby and any termination of this
               letter agreement.

        (o)    For the purpose of this letter agreement, the terms:

               (i)    "material fact", "material change", "misrepresentation"
                      and "subsidiary" are defined to have the definitions
                      thereof under the Securities Act (Ontario);

               (ii)   "business day" shall mean any day on which banks in the
                      cities of Toronto, Vancouver and New York are open for
                      business; and

               (iii)  references to "Shares", "Purchased Shares" or "Remaining
                      Shares" include any shares into which the foregoing may
                      be reclassified, sub-divided, consolidated or converted
                      and any rights and benefits arising therefrom, including
                      any dividends or distributions of securities or property
                      which may be declared in respect thereof.

        (p)    This letter agreement may be executed in one or more
               counterparts which together shall be deemed to constitute one
               valid and binding agreement, and delivery of the counterparts
               may be effected by means of a telecopier transmission.

        (q)    BC acknowledges and agrees that it is proposing to acquire its
               portion of the Purchased Shares and if applicable the Remaining
               Shares as bare trustee and agent for an investor, the identity
               of which has been disclosed to the other parties hereto, and
               that it will not hold such Shares for the benefit of any other
               person or entity without the prior




   21

                                     -21-



               written consent of the other parties, such consent not to be
               unreasonably withheld. BC shall at all times be controlled by
               such investor.

                            ---------------------

If you wish to accept this offer, please confirm your acceptance by signing
and returning this offer or a counterpart thereof to the CAI Entities, BC and
the Institutional Investor prior to 5:00 p.m. (Toronto time) on April 13,
2001, failing which it shall be null and void. For greater certainty, this
offer is without prejudice to the ROFO.



   22

                                     -22-


We hope that you will accept this offer and that the result will be a mutually
satisfactory transaction.

                             -------------------

Yours very truly,


CAI CAPITAL PARTNERS AND COMPANY II, L.P.

By: ______________________________________
 (an authorized signatory of its general partner)


CAI CAPITAL PARTNERS AND COMPANY II-C, L.P.

By: ______________________________________
 (an authorized signatory of its general partner)


CAI  PARTNERS AND COMPANY II, L.P.

By: ______________________________________
 (an authorized signatory of its general partner)


597858 B.C. LTD.

By: ______________________________________
 (an authorized signatory)


THE ONTARIO TEACHERS' PENSION PLAN BOARD

By: ______________________________________
 (an authorized signatory)


Accepted and agreed to at _______________this ____ day of _______________,
2001.


MDA HOLDINGS CORPORATION

By: ________________________



   23

                                     -23-

By: _________________________


ORBITAL SCIENCES CORPORATION (SOLELY FOR THE PURPOSES OF SECTION 4.1(g) ABOVE)

By: ________________________


By: _________________________



cc.     Peter Restler, Manfred Yu, Randall Mullan, Jonathan McCullough,
        Jonathan Lampe