Document:

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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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                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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               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,

<PAGE>   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.

<PAGE>   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:

<PAGE>   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

<PAGE>   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

<PAGE>   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

<PAGE>   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.

<PAGE>   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

<PAGE>   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.

<PAGE>   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: ________________________

<PAGE>   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<PAGE>   1

                                                                   EXHIBIT 10.4

                      AMENDED AND RESTATED OPTION AGREEMENT

                 entitling the holders to acquire common shares
                                in the capital of

                    MACDONALD, DETTWILER AND ASSOCIATES LTD.

                  (amalgamated pursuant to the laws of Canada)

         WHEREAS the Optionholders and Holdco entered into an amended and
restated secondary option agreement (the "ORIGINAL OPTION AGREEMENT") dated June
29, 2000, pursuant to which Holdco granted the Optionholders an option to
purchase certain common shares in the capital of the Corporation;

         AND WHEREAS in accordance with the terms of the Letter Agreement (as
defined below), the parties thereto agreed, among other things, to amend and
restate the terms of the Original Option Agreement (which are hereby repealed
and replaced in their entirety) on the terms and conditions set out herein;

         In consideration of the mutual agreements contained herein (the receipt
and adequacy of which are acknowledged), the parties agree as follows:

1.       DEFINITIONS. As used in this Option Agreement, the following terms have
         the following meanings:

         (a)      "AMENDED AND RESTATED REGISTRATION RIGHTS AGREEMENT" means the
                  amended and restated registration rights agreement dated of
                  even date herewith among the CAI Entities, CAI Managers & Co.,
                  L.P, Numberco, the Institutional Investor, Holdco and Orbital;

         (b)      "ARM'S LENGTH" has the meaning specified in the Income Tax Act
                  (Canada);

         (c)      "BUSINESS DAY" means any day of the year, other than a
                  Saturday, a Sunday and any statutory holiday in Vancouver,
                  British Columbia, Toronto, Ontario or New York, New York;

         (d)      "CAI ENTITIES" means CAI Capital Partners and Company II,
                  L.P., CAI Partners and Company II, L.P., and CAI Capital
                  Partners and Company II-C, L.P., and "CAI ENTITY" means any
                  one of them;

         (e)      "CAPITAL REORGANIZATION EVENT" has the meaning ascribed
                  thereto in Section 8(a);

<PAGE>   2

                                      -2-

         (f)      "COMMON SHARES" means common shares of the Corporation;

         (g)      "CORPORATION" means MacDonald, Dettwiler and Associates Ltd.;

         (h)      "CUSTODIAL AGENT" has the meaning ascribed thereto in Section
                  18(a);

         (i)      "EXERCISE PRICE" has the meaning ascribed thereto in Section
                  2;

         (j)      "HOLDCO" means MDA Holdings Corporation;

         (k)      "HOLDCO FLOOR PRICE" has the meaning ascribed thereto in
                  Section 11(d);

         (l)      "HOLDCO SALE PRICE" has the meaning ascribed thereto in
                  Section 11(d);

         (m)      "INSOLVENCY OPTION EVENT" means

                  (i)      If Orbital shall become subject to Chapter 11
                           proceedings or other proceedings under U.S.
                           bankruptcy or similar laws and, in the event of an
                           involuntary proceeding, the continuance of such
                           proceeding for a period of 30 consecutive calendar
                           days;

                  (ii)     If there shall be a material breach of the covenants
                           set forth in Section 9 or Section 3.7 of the Amended
                           and Restated Registration Rights Agreement; or

                  (iii)    If there shall be any filing under Canadian, U.S. or
                           other bankruptcy or similar laws in respect of Holdco
                           and, in the event of an involuntary proceeding, the
                           continuance of such proceeding for a period of 30
                           consecutive calendar days.

         (n)      "INSTITUTIONAL INVESTOR" means Ontario Teachers Pension Plan
                  Board;

         (o)      "LETTER AGREEMENT" means the letter agreement dated April 12,
                  2001 among the CAI Entities, Numberco, the Institutional
                  Investor, Holdco and Orbital;

         (p)      "NUMBERCO" means 597858 B.C. Ltd.;

         (q)      "OFFER" has the meaning ascribed thereto in Section 11(a);

         (r)      "OPTIONS" has the meaning ascribed thereto in Section 2;

         (s)      "OPTION AGREEMENT" means this amended and restated option
                  agreement, including any schedules attached hereto;

<PAGE>   3

                                      -3-

         (t)      "OPTIONED SHARES" has the meaning ascribed in Section 2;

         (u)      "OPTIONHOLDERS" has the meaning ascribed thereto in Section 2;

         (v)      "OPTIONHOLDER FLOOR PRICE" has the meaning ascribed thereto in
                  Section 11(e);

         (w)      "OPTIONHOLDER SALE PRICE" has the meaning ascribed thereto in
                  Section 11(e);

         (x)      "OPTIONS" has the meaning ascribed thereto in Section 2;

         (y)      "ORBITAL" means Orbital Sciences Corporation;

         (z)      "PERSON" means a natural person, partnership, limited
                  partnership, limited liability company, body corporate, joint
                  stock company, trust, unincorporated association, joint
                  venture or other entity or a governmental entity, and pronouns
                  have a similarly extended meaning;

         (aa)     "PLEDGE AND CUSTODIAL AGREEMENT" has the meaning ascribed
                  thereto in Section 18(a);

         (bb)     "PURCHASED SHARES" has the meaning ascribed thereto in the
                  Letter Agreement;

         (cc)     "PURCHASE PRICE" means the purchase price per Common Share as
                  set out in the Letter Agreement, which, prior to any
                  adjustments in accordance with the Letter Agreement, shall be
                  Cdn $14.00 per Common Share;

         (dd)     "PURCHASERS" means the CAI Entities and Numberco, and
                  "PURCHASER" means any one of them;

         (ee)     "REMAINING SHARES" has the meaning ascribed thereto in Section
                  11(a);

         (ff)     "SOURCING AND STRUCTURING FEE" has the meaning ascribed
                  thereto in Section 7(a);

         (gg)     "SUBSIDIARY" means a subsidiary of the Corporation within the
                  meaning of the Canada Business Corporations Act;

         (hh)     "TERMINATION DATE" means May 31, 2001 or in the event that any
                  Optionholder or any transferee or assignee is unable to obtain
                  financing or all necessary regulatory approvals in connection
                  with any intended exercise of an Option prior to such time,
                  such date not later

<PAGE>   4

                                      -4-

                  than June 30, 2001 as specified by such Optionholder in
                  writing on or prior to May 31, 2001; and

         (ii)     "TRANSFER" has the meaning ascribed thereto in Section 9(a).

2.       GRANT OF OPTIONS. Holdco hereby grants the Purchasers (the Purchasers
         and any subsequent assignee or transferee being called the
         "OPTIONHOLDERS" or "HOLDERS" and any one of them an "OPTIONHOLDER" or
         "HOLDER") the irrevocable right and option (the "OPTIONS"), exercisable
         at any time or from time to time on or prior to the Termination Date,
         to acquire from Holdco an aggregate of 5,650,000 Common Shares (as
         adjusted in accordance with Section 8 hereof) (the "OPTIONED SHARES")
         free of any and all liens or encumbrances, for an exercise price (the
         "EXERCISE PRICE") of Cdn $14.25 per Optioned Share (as adjusted in
         accordance with Sections 8 and 27 hereof and subject to Section 7
         hereof), upon the terms and conditions set out in this Option
         Agreement.

3.       TERMINATION OF OPTION. Notwithstanding anything to the contrary
         contained herein, if an Insolvency Option Event occurs on or prior to
         the Termination Date, each Optionholder shall be entitled, in its sole
         discretion at any time prior to the Termination Date, to immediately
         terminate this Agreement with respect to such Optionholder and by
         notice in writing to Holdco, whereupon the Option represented hereby
         shall be deemed to be automatically exercised for an exercise price per
         Optioned Share equal to the Exercise Price; provided, however, that in
         no event shall any of the Optioned Shares subject to such exercise be
         transferred to any Optionholder prior to the payment of the Exercise
         Price therefor in accordance with Sections 5, 7 and 27; and provided
         further, that the obligations of the Purchasers pursuant to Section
         11(e) shall survive any termination of this Agreement pursuant to this
         Section 3.

4.       NO EXERCISE AFTER EXPIRY. If an Optionholder has not given notice of
         exercise of the Option to Holdco in full or the Option has not
         otherwise been exercised on or prior to the Termination Date, the
         unexercised portion of the Option shall thereafter be void.

5.       EXERCISE OF OPTION.

         (a)      An Optionholder may exercise the Options by giving notice of
                  exercise to Holdco, at any time, in whole or in part, on or
                  prior to the Termination Date. Such notice shall set out the
                  number or percentage of Common Shares the Optionholder is
                  acquiring. Within 3 Business Days of the date of the delivery
                  of a notice of exercise, payment in accordance with Section 13
                  by the Optionholder of the Exercise Price of the Common Shares
                  which may be acquired pursuant to such exercise

<PAGE>   5

                                      -5-

                  (prior to taking into account the payment set out in Section
                  7) shall be delivered to Holdco and simultaneously therewith
                  the Common Shares shall be transferred by Holdco and a
                  certificate for the Common Shares purchased shall be delivered
                  by Holdco to the Optionholder either endorsed in blank or
                  accompanied by a duly completed, transferable stock power of
                  attorney (with signatures "medallion" guaranteed and together
                  with any other documents required by MDA's transfer agent in
                  connection with such purchase). In addition, in connection
                  with any exercise of the Option, the provisions of Section
                  12(d) shall apply mutatis mutandis with respect to the
                  completion of any such exercise.

         (b)      At any time prior to the Termination Date, an Optionholder
                  may, in lieu of exercising any of the Options that it is
                  entitled to exercise in accordance with this Option Agreement,
                  provide written notice to Holdco requesting Holdco to sell any
                  or all of the Optioned Shares with respect to those Options
                  pursuant to a public offering in respect of which the price at
                  which the Optioned Shares to be sold pursuant to such public
                  offering as set out in the applicable underwriting agreement
                  (the "PUBLIC OFFERING SALE PRICE") is expected to be
                  determined prior to the Termination Date. If an Optionholder
                  gives such notice, Holdco and Orbital agree to co-operate in
                  full with the Optionholder in connection with such public
                  offering, including without limitation, participating in the
                  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 Corporation's
                  initial public offering. In addition, on the closing date of
                  such public offering, Holdco agrees to pay (or, at the written
                  request of the Optionholder, irrevocably direct the
                  underwriters to pay) to the Optionholder, by certified cheque,
                  bank draft or wire transfer, (A) an amount equal to 50% of the
                  amount by which (i) the proceeds of the public offering with
                  respect to the sale of the Optioned Shares (net of applicable
                  underwriting commissions and all non-reimbursed and reasonable
                  expenses (provided that such expenses shall not exceed US
                  $100,000 in the aggregate) incurred by them in connection
                  therewith), exceed (ii) the product of (x) Cdn. $14.25 per
                  share, and (y) the total number of Optioned Shares sold
                  pursuant to the public offering, plus (B) a sourcing and
                  structuring fee in an amount equal to 2% of the product of
                  $14.25 and the number of Optioned Shares sold in the public
                  offering (net of applicable underwriting commissions incurred
                  by Holdco and Orbital in connection therewith). If, for any
                  reason, the Public Offering Sale Price cannot be determined on
                  or prior to the

<PAGE>   6

                                      -6-

                  Termination Date, the Optionholder shall continue to have all
                  rights with respect to the Optioned Shares and the Remaining
                  Shares set out in this Option Agreement (including, without
                  limitation, the right to exercise the Option on or prior to
                  the Termination Date), notwithstanding any delivery of a
                  notice in accordance with this Section 5(b). Further, if, for
                  any reason, the Public Offering Sale Price cannot be
                  determined prior to the Termination Date, the Optionholder(s)
                  that exercised its rights pursuant to this Section 5(b) shall
                  promptly pay Holdco and Orbital all non-reimbursed and
                  reasonable expenses (provided that such expenses shall not
                  exceed US $100,000 in the aggregate) incurred by Holdco and
                  Orbital in connection with such public offering. For greater
                  certainty, the parties acknowledge and agree that in no event
                  shall Holdco be required under this Section 5(b) to enter into
                  any underwriting agreement in which the Public Offering Sale
                  Price, net of applicable underwriting commissions incurred by
                  Holdco and Orbital, is less than $14.25 per share.

6.       RIGHT OF EACH OPTIONHOLDER. The Optionholders shall be entitled to
         purchase the following number of Optioned Shares: (i) as to the CAI
         Entities, 2,650,000 Optioned Shares, proportionate to their respective
         holdings of common shares of the Corporation on the date of the Letter
         Agreement and prior to the completion of the transactions contemplated
         therein, and (ii) as to Numberco, 3,000,000 Optioned Shares (or in such
         other proportions as any of the CAI Entities and Numberco may otherwise
         agree among themselves and jointly notify to Holdco).

7.       FEES.

         (a)      In connection with any exercise of the Option by an
                  Optionholder in accordance with Section 5(a), or upon any
                  deemed exercise thereof in accordance with Section 3 and upon
                  the delivery of a notice of exercise together with the
                  aggregate amount payable by the Optionholder as set out in
                  Section 3 or Section 5(a) in connection with such exercise,
                  Holdco shall pay the Purchaser that was the original holder of
                  such Option (and, for greater certainty, not to any assignee
                  of the Option), or as it may direct, a sourcing and
                  structuring fee (the "SOURCING AND STRUCTURING FEE") in the
                  amount of 2% of the aggregate amount payable by the
                  Optionholder to Holdco as set out in Section 3 or Section
                  5(a), as applicable, in connection with such exercise. The
                  Sourcing and Structuring Fee shall be paid to such Purchaser
                  in the manner set out in Section 7(b).

<PAGE>   7

                                      -7-

         (b)      If the Optionholder exercising the Option is the Purchaser
                  that was the original holder of such Option (and, for greater
                  certainty, is not an assignee of the Option) or if, at the
                  time of any deemed exercise thereof in accordance with Section
                  3, the Optionholder is the Purchaser that was the original
                  holder of such Option (and, for greater certainty, is not an
                  assignee of the Option), the Sourcing and Structuring Fee
                  shall be offset against the amounts otherwise payable to
                  Holdco by the exercising Optionholder. If, on the other hand,
                  the Optionholder exercising the Option is not the Purchaser
                  that was the original holder of such Option (an assignee of
                  the Option), or if, at the time of any deemed exercise thereof
                  in accordance with Section 3, the Optionholder is not the
                  Purchaser that was the original holder of such Option (an
                  assignee of the Option), Holdco shall pay (or, at such
                  Purchaser's written request, irrevocably direct the
                  Optionholder to pay from the amounts otherwise payable by the
                  Optionholder to Holdco) the Sourcing and Structuring Fee to
                  such Purchaser by certified cheque, bank draft or wire
                  transfer.

8.       ANTI-DILUTION.

         (a)      If and whenever at any time prior to the expiry date there
                  occurs any of the following events (each a "CAPITAL
                  REORGANIZATION EVENT"):

                  (i)      a reclassification of the Common Shares or a capital
                           reorganization of the Corporation or a subdivision or
                           consolidation of the Common Shares; or

                  (ii)     an amalgamation, merger or other combination of the
                           Corporation with or into any other Person or any plan
                           of arrangement to which the Corporation is a party;
                           or

                  (iii)    the issuance to all or substantially all of the
                           holders of Common Shares of any rights, options,
                           warrants or other similar securities to acquire
                           Common Shares or securities exchangeable or
                           convertible into Common Shares; or

                  (iv)     the declaration of a dividend on the Common Shares
                           payable in Common Shares or any right, options,
                           warrants or other similar securities to acquire
                           Common Shares or securities exchangeable or
                           convertible into Common Shares to all or
                           substantially all of the holders of Common Shares; or

                  (v)      the making or payment of any other distribution of
                           property or assets, including cash dividends, to all
                           or substantially all of the

<PAGE>   8

                                      -8-

                           holders of Common Shares (excluding normal course
                           cash dividends, in the aggregate not to exceed 10% of
                           the Corporation's annual net profits in any year),

                  then, thereafter if an Optionholder is entitled to acquire
                  Common Shares hereunder at any date subsequent to the record
                  date or the effective date of such Capital Reorganization
                  Event, as appropriate, the Optionholder shall be entitled to
                  receive and shall accept, in lieu of the number of Common
                  Shares then sought to be obtained by him hereunder, the number
                  of Common Shares or other securities or property of the
                  Corporation or of the Person resulting from such Capital
                  Reorganization Event that such Optionholder would have been
                  entitled to receive on such Capital Reorganization Event if,
                  on the record date or the effective date thereof, as the case
                  may be, the Optionholder had already been the registered
                  holder of the number of additional Common Shares then sought
                  to be obtained by him pursuant to the provisions hereof. The
                  entitlement of an Optionholder to receive any rights, options
                  or warrants or other securities referred to in this Section 8
                  shall, if appropriate, be subject to the payment by the
                  Optionholder of the purchase price thereof and the
                  satisfaction of all other appropriate conditions precedent to
                  the issuance thereof, such entitlement to be determined as
                  though the date for such payment and the fulfilment of such
                  conditions were the date of delivery to the Optionholder of
                  any Common Shares or other securities or property it is
                  entitled to with respect to such Capital Reorganization Event.

         (b)      The adjustments provided for herein and the classes of
                  securities or property of the Corporation or any Person which
                  are to be received by an Optionholder hereunder are
                  cumulative. After any adjustment pursuant hereto, the term
                  "Common Shares" where used in this Option Agreement shall be
                  interpreted to mean securities of any class or classes and
                  property that, as a result of all prior adjustments pursuant
                  hereto, an Optionholder is entitled to receive hereunder, and
                  the number of Common Shares to be issued to the Optionholder
                  hereunder shall be interpreted to mean the number and classes
                  of securities and property of the Corporation or any Person
                  which, as a result of all prior adjustments pursuant hereto,
                  the Optionholder is entitled to receive hereunder.

         (c)      In addition to the adjustment of the number of Common Shares
                  to which Optionholder would be entitled in (a) above, the
                  Exercise Price, as contemplated elsewhere in this Option
                  Agreement, and for greater certainty, the floor prices set
                  forth in Sections 11(d) and 11(e), shall also be adjusted
                  accordingly to preserve the same economic result.

<PAGE>   9

                                      -9-

9.       RESTRICTIONS ON TRANSFER BY HOLDCO AND ORBITAL.

         (a)      During the term of this Option Agreement, Holdco shall not
                  sell, assign, transfer, pledge, encumber or otherwise deal
                  with or dispose of (collectively, a "TRANSFER") any of the
                  Optioned Shares owned by it except in the manner expressly
                  permitted in this Option Agreement, without the prior express
                  written consent of each of the Optionholders.

         (b)      During the term of this Option Agreement, Orbital shall ensure
                  that at all times Holdco is directly or indirectly
                  wholly-owned by Orbital and no other Persons and shall not
                  Transfer any of the securities of Holdco owned by it or permit
                  Holdco to issue any securities to any other Person, wind up,
                  consolidate or dissolve or to amalgamate or merge with any
                  other Person, without the prior express written consent of
                  each of the Optionholders. Notwithstanding the foregoing,
                  Orbital shall be entitled to pledge any or all of the
                  securities of Holdco held by it to a bona fide arm's length
                  lender or lenders (the "LENDER") to Orbital from time to time
                  as security for indebtedness of Orbital owed to such Lender
                  provided that such Lender acknowledges in writing to the
                  Optionholders that (i) until a default under the pledge, the
                  pledged securities shall continue to be registered in the name
                  of Orbital and Orbital shall be entitled to exercise all
                  rights in respect thereof (including voting rights and
                  director nomination rights), and (ii) if the Lender commences
                  enforcement proceedings or otherwise realizes on the pledged
                  securities following default, the Lender shall immediately
                  notify the Optionholders and its interest in the pledged
                  securities shall be subject to the provisions of this Option
                  Agreement applicable thereto as and to the same extent as
                  though the Lender were Orbital.

10.      PERMITTED TRANSFERS BY HOLDCO.

         (a)      Holdco may transfer all or any of its Optioned Shares to a
                  direct or indirect wholly-owned Subsidiary of Holdco which
                  executes and delivers to the parties and the Custodial Agent,
                  as applicable, a counterpart copy of this Option Agreement and
                  the Pledge and Custodial Agreement or a written agreement in
                  form and substance satisfactory to the other parties and the
                  Custodial Agent, as applicable, agreeing to be bound by the
                  terms and conditions of this Option Agreement and the Pledge
                  and Custodial Agreement as if it were Holdco, provided that no
                  such Subsidiary shall cease to be wholly-owned (directly or
                  indirectly) by Holdco without first transferring all of such
                  Optioned Shares back to Holdco, but in the event of any such
                  transfer Holdco shall continue to be liable for any default in
                  the

<PAGE>   10

                                      -10-

                  performance of any obligations of such Subsidiary under this
                  Option Agreement and the Pledge and Custodial Agreement, as
                  applicable.

         (b)      Holdco shall be entitled to transfer Optioned Shares pursuant
                  to the provisions of Section 5(b) and Section 11 hereof.

11.      RIGHT OF FIRST REFUSAL; RESALES BY PURCHASERS, ETC.

         (a)      If at any time after the Termination Date and on or prior to
                  June 30, 2002, Holdco is the holder of any Optioned Shares
                  (collectively, including any shares into which such Optioned
                  Shares 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, the
                  "REMAINING SHARES") and Holdco desires to sell all or any of
                  the Remaining Shares to a third party with whom Holdco is
                  dealing at Arm's Length, Holdco shall obtain from the third
                  party a bona fide offer (an "OFFER") in writing which Offer
                  shall be irrevocable for a period ending not sooner than the
                  expiration of the 30-day period during which the Optionholders
                  may exercise their purchase rights as set out in Section
                  11(b), and which Offer it is ready and willing to accept, to
                  purchase all or that portion of the Remaining Shares that it
                  desires to sell on the terms and conditions and for the amount
                  set forth in the Offer by certified cheque, bank draft or wire
                  transfer, and shall give notice in writing to the
                  Optionholders of the receipt of the Offer within 10 days
                  thereof together with a copy thereof.

         (b)      The Optionholders shall have the irrevocable right,
                  exercisable by written notice given to Holdco (with a copy to
                  the other Optionholders) within 30 days after the giving of
                  the notice by Holdco, to purchase the Remaining Shares that
                  are the subject matter of the Offer for the amount set forth
                  in the Offer, in the same proportions as the Optionholders are
                  entitled to purchase Optioned Shares as set out in Section 6
                  (or in such other proportions as any of them may agree among
                  themselves and jointly notify Holdco), and except as set out
                  in this Section 11(b) and Section 11(f), on the terms of the
                  Offer. Subject to Section 11(f), following an exercise of the
                  purchase rights set forth in this Section 11(b) with respect
                  to all (but not less than all) of the Remaining Shares which
                  are subject to the Offer, each Optionholder exercising such
                  rights and Holdco shall complete the purchase and sale of such
                  Remaining Shares (including the making of all payments
                  required in connection with such exercise in accordance with
                  Section 13, and all deliveries set out in Section 12) on the
                  date specified by the exercising Optionholder that is not
                  later than 30 days following the

<PAGE>   11

                                      -11-

                  earlier of (i) the expiration of the 30-day period during
                  which the Optionholders may exercise their purchase rights as
                  set out in this Section 11(b) and (ii) the date on which the
                  exercise notice is given pursuant to this Section 11(b). If
                  the consideration for the Remaining Shares as set out in the
                  Offer involves a transfer of property other than cash, then
                  the Optionholders shall be entitled, in lieu of transferring
                  such non-cash property as a part of the purchase, to pay the
                  cash equivalent of such property, which shall be the fair
                  market value thereof as at the date of the exercise by the
                  Optionholder(s) of its purchase rights hereunder and as is
                  reasonably determined by the exercising Optionholder(s) and
                  Holdco in good faith, or if they are unable to agree on such
                  fair market value, by Arthur Anderson (or if it is unable or
                  unwilling to act with respect to such determination, by Ernst
                  and Young, or if both Arthur Anderson and Ernst and Young are
                  unable or unwilling to act, such other firm as they shall
                  jointly designate). The costs of any such determination shall
                  be shared equally between the exercising Optionholders on the
                  one hand and Holdco on the other hand. Unless the purchase
                  rights set forth in this Section 11(b) are exercised with
                  respect to all of the Remaining Shares that are the subject of
                  the Offer, Section 11(c) shall apply.

         (c)      If following compliance with Section 11(b) there shall remain
                  Remaining Shares that are the subject matter of the Offer
                  which no Optionholder has elected to purchase, the exercise by
                  any Optionholder of its rights pursuant to Section 11(b) shall
                  be null and void, and Holdco may accept the Offer and complete
                  the transaction with the said third party, in each case, only
                  with respect to the Remaining Shares that are the subject of
                  the Offer in accordance with the terms and conditions of such
                  third party's Offer, provided that if the sale of such
                  Remaining Shares to the third party is not completed within 90
                  days following the expiration of the 30-day period during
                  which the Optionholders may exercise their purchase rights as
                  set out in Section 11(b), the provisions of Section 11(b)
                  shall again apply to any proposed sale of such Remaining
                  Shares. The Optionholders, before instructing the Custodial
                  Agent to release the Remaining Shares to be transferred to the
                  third party, shall be entitled to require proof that the sale
                  is taking place in accordance with the third party's Offer and
                  Holdco agrees that it shall not be entitled to have such
                  shares released before it has furnished such proof and made
                  arrangements with the Optionholders satisfactory to them for
                  the immediate payment, of the amounts, if any, payable by
                  Holdco to them pursuant to Section 11(d) with respect to any
                  such sale.

<PAGE>   12

                                      -12-

         (d)      If at any time on or prior to June 30, 2002, Holdco completes
                  the sale of any Remaining Shares to a third party in
                  accordance with Section 11(c) for a net price per Remaining
                  Share (the "HOLDCO SALE PRICE") that exceeds Cdn. $15.00 (as
                  the same may be adjusted as set out below, the "HOLDCO FLOOR
                  PRICE"), then immediately following the completion of any such
                  sale, Holdco shall pay to the Purchasers (and, for greater
                  certainty, not to any assignee hereof) pro rata to their
                  respective holdings of Common Shares on the date of the Letter
                  Agreement and prior to the completion of the transactions
                  contemplated therein (or in such other proportions as they may
                  agree among themselves and jointly notify Holdco) by certified
                  cheque, bank draft or wire transfer, an amount per remaining
                  share sold that is equal to 50% of the amount by which the
                  Holdco Sale Price exceeds the Holdco Floor Price. For purposes
                  hereof, if there occurs one or more Capital Reorganization
                  Events on or prior to June 30, 2002, the Holdco Floor Price
                  shall be adjusted with respect to each such Capital
                  Reorganization Event in the manner set out in Section 8(c),
                  mutatis mutandis. This Section 11(d) shall, for greater
                  certainty, also not apply to any sale of shares by Holdco
                  pursuant to a public offering as described in Section 5(b).

         (e)      If at any time on or prior to June 30, 2002, a Purchaser has
                  acquired any Common Shares pursuant to the exercise of the
                  Options and such Purchaser completes the sale of any of such
                  Common Shares to a third party on or before June 30, 2002 for
                  a net price per Common Share (the "OPTIONHOLDER SALE PRICE")
                  that exceeds Cdn. $14.25 (as the same may be adjusted as set
                  forth below, the "OPTIONHOLDER FLOOR PRICE"), then immediately
                  following the completion of any such sale, the Purchaser shall
                  pay to Holdco in accordance with Section 13, as additional
                  consideration for the exercise of the Options, an amount per
                  each such Common Share sold by the Purchaser in such third
                  party sale, that is equal to 50% of the amount by which the
                  Optionholder Sale Price exceeds the Optionholder Floor Price.
                  For purposes hereof, if there occurs one or more Capital
                  Reorganization Events on or prior to June 30, 2002, the
                  Optionholder Floor Price shall be adjusted with respect to
                  each such Capital Reorganization Event in the manner set out
                  in Section 8(c), mutatis mutandis. In the alternative, in the
                  event any Purchaser (and, for greater certainty, not any
                  subsequent Optionholder) sells any Option to a third party
                  prior to the exercise of such Option, such Purchaser shall pay
                  to Holdco in accordance with Section 13 an amount that is
                  equal to 50% of all net proceeds from such sale. The
                  provisions of this Section 11(e) shall not apply to any
                  subsequent exercise of any such Option or to any sale by a
                  third party of such Option or of Common Shares which are
                  acquired pursuant to

<PAGE>   13

                                      -13-

                  the exercise of any Option which was sold to such third party
                  in accordance with this Section 11(e). This Section 11(e)
                  shall, for greater certainty, also not apply to any sale of
                  shares by Holdco pursuant to a public offering as described in
                  Section 5(b).

         (f)      Notwithstanding Section 11(b), the parties acknowledge and
                  agree that (i) an Optionholder's obligation to complete any
                  purchase under this Section 11 shall be subject, in any event,
                  to its receipt of all necessary third party consents required
                  under any contracts to which the Corporation or any subsidiary
                  is a party and all governmental and regulatory consents and
                  approvals in connection therewith, and (ii) the date on which
                  an Optionholder is required to complete any purchase of
                  Remaining Shares in accordance with Section 11(b) shall be
                  extended, as is necessary, for the Optionholder to obtain such
                  consents and approvals for a period of not more than 60 days
                  from the expiry of the 30 day notice period set out in Section
                  11(b). In the event that any such third party, governmental or
                  regulatory consents or approvals are needed, the
                  Optionholders, agree to use their reasonable efforts to obtain
                  such consents or approvals as promptly as possible and Orbital
                  and Holdco agree to fully co-operate with the Optionholders
                  with respect to obtaining such consents.

         (g)      In connection with any sale of Common Shares by Holdco or any
                  Purchaser which is subject to Sections 11(d) or (e), the
                  seller of the Common Shares shall deliver a certificate to the
                  other parties to this Option Agreement certifying that the
                  sale of such Common Shares was made pursuant to an arm's
                  length transaction and that the seller of such Common Shares
                  is not entitled to any consideration in respect of such sale
                  (including, without limitation, any undisclosed fees or rights
                  to share in, or receive any other compensation in respect of,
                  any future proceeds resulting from resales of such shares by
                  the third party buyer) other than the Optionholder Sale Price
                  or the Holdco Sale Price, as applicable, and any other
                  consideration disclosed in such certificate. Any such
                  consideration disclosed in the certificate shall, if and when
                  received, be treated as part of the Optionholder Sale Price or
                  the Holdco Sale Price, as applicable, and appropriate payments
                  shall be made with respect thereto as contemplated in Section
                  11(d) and 11(e), as the case may be. This Section 11(g) shall
                  survive the termination of this Agreement.

12.      SALE PROVISIONS.

         At or prior to the completion of any sale of Remaining Shares pursuant
to Section 11(b), Holdco shall:

<PAGE>   14

                                      -14-

         (a)      assign and transfer to the purchaser the Remaining Shares to
                  be sold and deliver the share certificate(s) representing such
                  Remaining Shares either endorsed in blank or accompanied by a
                  duly completed, transferable stock power of attorney (with
                  signatures "medallion" guaranteed and together with any other
                  documents required by MDA's transfer agent in connection with
                  such purchase);

         (b)      do all other things required in order to deliver good and
                  marketable title to the Remaining Shares to be sold to the
                  purchaser free and clear of any encumbrances whatsoever;

         (c)      deliver to the parties hereto and the purchaser all necessary
                  documents (which documents shall be in form and substance
                  reasonably satisfactory to the purchaser) required to comply
                  fully with the intent of this Option Agreement; and

         (d)      if at completion, the Remaining Shares to be sold by Holdco
                  are not free and clear of all encumbrances, the purchaser may,
                  without prejudice to any other rights which it may have,
                  purchase the Remaining Shares from Holdco subject to such
                  encumbrances and, in such event, the purchaser shall, at
                  completion, assume all obligations and liabilities with
                  respect to such encumbrances and the purchase price payable by
                  the purchaser for the Remaining Shares shall be satisfied, in
                  whole or in part, as the case may be, by such assumption or
                  payment and the amount so assumed or paid, as determined by
                  the purchaser acting reasonably and upon prior written notice
                  to Holdco, shall be deducted from the purchase price payable
                  to Holdco at completion; provided, however, that if Holdco has
                  given notice to the purchaser that it is contesting any such
                  encumbrance in good faith, then the purchaser will not make
                  any such payment until such contest is resolved, unless it is
                  required to do so in accordance with any applicable law.

13.      PAYMENTS. Upon the written request of Holdco given to an Optionholder
         not less than two Business Days prior to the date that any amount is
         required to be paid to Holdco in accordance with this Agreement, the
         Optionholder shall pay when such amount is due, the U.S. dollar
         equivalent (based on a rate of exchange available to the Optionholder
         at its bank on the Business Day prior to the date such amount is due)
         of such amount, net of any foreign exchange costs actually incurred by
         it. All payments to Holdco hereunder shall be made by certified check,
         bank draft or wire transfer, as Holdco may designate in writing at
         least two Business Days' prior to the date that each payment is due.

<PAGE>   15

                                      -15-

14.      TRANSFERS. This Option Agreement and all rights hereunder may be
         transferred or assigned in whole or in part by a Purchaser or an
         Optionholder without the prior written consent of Holdco, provided that
         if such assignment includes an assignment of rights under Section
         11(b), such assignment shall not result in the applicability of
         additional third party, governmental consents or regulatory consents or
         approvals which would reasonably be expected to delay the completion of
         the purchase and sale of such shares under such Section for a period in
         excess of 60 days.

15.      LIABILITY OF HOLDCO. Subject to the provisions of applicable law, the
         Optionholders shall be entitled to the rights and privileges attaching
         to the Options and the Remaining Shares set forth in this Option
         Agreement, and the transfer of Common Shares by Holdco upon the
         exercise of Options or the rights set out in Sections 11(b) or (d) by
         the Optionholders in accordance with the terms and conditions herein
         contained, shall discharge the responsibilities of Holdco to such
         extent.

16.      SHAREHOLDING. Unless otherwise agreed to in writing by the
         Optionholders, Holdco shall be the registered and beneficial owner of
         Optioned Shares and the Remaining Shares free and clear of all liens or
         encumbrances (other than pursuant to the arrangements described in
         Section 18) so as to permit the Optionholders to obtain Optioned Shares
         and the Remaining Shares and other securities and/or rights which they
         are entitled to acquire upon the exercise of the rights conferred
         hereunder.

17.      TRANSFER OF OPTIONS BY OPTIONHOLDERS. The Optionholders may transfer
         and assign their respective rights under this Option Agreement to a
         U.S. commercial bank or stockbroker of their choice as their respective
         agent at the time that the pledge of shares referred to below is
         entered into. In connection with this transfer, Orbital, Holdco and the
         Optionholders agree (i) to make any reasonable non-economic amendments
         which may be requested by the U.S. commercial bank or stockbroker, (ii)
         to fully indemnify, in connection with its custodial obligations, the
         U.S. commercial bank or stockbroker (except for wilful default of its
         obligations), and (iii) that any transfer pursuant to this Section 17
         shall not be deemed a sale to a third party for purposes of Section
         11(e) and the provisions of Section 11(e) shall apply, mutatis
         mutandis, to any sale of Common Shares or the Option by the transferee
         under this Section 17 as if such transferee were an Optionholder.

18.      PLEDGE OF SHARES.

         (a)      To secure the performance of the obligations of Holdco
                  hereunder, Holdco agrees to immediately pledge and grant a
                  security interest in the Optioned Shares and the Remaining
                  Shares and shall deliver,

<PAGE>   16

                                      -16-

                  pledge and grant a security interest in form and substance
                  satisfactory to the Optionholders to the U.S. commercial bank
                  or stockbroker selected above as agent for the Optionholders
                  (the "CUSTODIAL AGENT"), and place in the custody of the
                  Custodial Agent, in accordance with the provisions of the
                  pledge and custodial agreement (as the same may be modified by
                  any supplemental direction, amended or replaced from time to
                  time, the "PLEDGE AND CUSTODIAL AGREEMENT") dated as of June
                  12, 2000 among Holdco, the Optionholders, and RBC Dominion
                  Securities Corporation (a copy of which is attached hereto as
                  Schedule "A"), certificates representing the Optioned Shares
                  (and, as applicable, the Remaining Shares).

         (b)      Upon the execution of this Option Agreement, Holdco and the
                  Optionholders shall execute and deliver irrevocable
                  instructions to the Custodial Agent substantially in the form
                  of Schedule "B" hereto. Holdco and the Optionholders further
                  agree that, effective as of the date of this Option Agreement,
                  (i) all references in the Pledge and Custodial Agreement to
                  the Secondary Option Agreement shall be deemed to refer to
                  this Agreement, (ii) all references in the Pledge and
                  Custodial Agreement to "Pledged Shares" shall be deemed to
                  refer to the Optioned Shares and, as applicable, the Remaining
                  Shares, (iii) the provisions of Section 7 of the Pledge and
                  Custodial Agreement shall be of no force and effect, (iv) in
                  the event of any inconsistency between any provision of, on
                  the one hand, the Pledge and Custodial Agreement and, on the
                  other hand, any provision of this Option Agreement or the
                  Letter Agreement, the terms of this Option Agreement or the
                  Letter Agreement, as applicable, shall be determinative, and
                  (v) Holdco and the Optionholders will provide any instructions
                  to the Custodial Agent which are necessary or appropriate to
                  effectuate the terms of this Section 18(b).

19.      AMENDMENT. The terms of this Option Agreement may be amended, subject
         to all required regulatory approvals, by agreement among Orbital (as
         applicable), Holdco and the Optionholders.

20.      ASSIGNMENT. This Option Agreement shall, subject to compliance with the
         provisions hereof, enure to the benefit of the Optionholders and their
         respective successors and assigns and shall be binding upon Orbital and
         Holdco and their successors and permitted assigns (whether by way of
         amalgamation, merger, plan of arrangement or divestiture). Orbital and
         Holdco may only assign their respective rights and obligations
         hereunder with the prior written consent of the Optionholders.

<PAGE>   17

                                      -17-

21.      NOTICE. Except as otherwise expressly provided in this Agreement, all
         notices and other communications provided for hereunder shall be in
         writing and shall be deemed to have been given or made when delivered
         by hand or sent by facsimile (if promptly confirmed by mail):

         (a)      If to Orbital, if delivered by hand or sent by facsimile to
                  its offices at 21700 Atlantic Blvd., Dulles, Virginia 20166,
                  Attention: Legal Department, Fax: (703) 406-5572 or to such
                  other address as Orbital may from time to time hereafter
                  designate by written notice given hereunder;

         (b)      If to the CAI Entities, if delivered by hand or sent by
                  facsimile to them c/o CAI Managers & Co., L.P., 767 5th
                  Avenue, 5th Floor, New York, New York 10153 U.S.A., Attention:
                  Manfred Yu, Fax: (212) 319-0232 or such other address as the
                  CAI Entities may from time to time hereafter designate by
                  written notice given hereunder;

         (c)      If to Numberco, if delivered by hand or sent by facsimile to
                  it c/o McCullough O'Connor Irwin 1100-888 Dunsmuir Street,
                  Vancouver, British Columbia V6C 3K4, Attention: Jonathan
                  McCullough, Fax: (604) 687-7099 or such other address as
                  Numberco may from time to time hereafter designate by written
                  notice given hereunder; and

         (d)      If to Holdco, if delivered by hand or sent by facsimile to MDA
                  Holdings Corporation c/o Orbital Sciences Corporation, 21700
                  Atlantic Boulevard, Dulles, Virginia, 20166, Attention: Legal
                  Department, Fax (703) 406-5572 or to such other address as
                  Holdco may from time to time hereafter designate by written
                  notice given hereunder.

22.      GOVERNING LAW. This agreement and the Options granted hereunder shall
         be governed by and construed in accordance with the laws of the
         Province of Ontario and the laws of Canada applicable therein.

23.      COMPLIANCE. Orbital shall cause Holdco to comply with its obligations
         hereunder and shall be responsible for any failure by Holdco to do so.

24.      NUMBERCO. Numberco acknowledges and agrees that it is acting 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 the
         investor's Common Shares and the rights under this Agreement for the
         benefit of any other person without the prior written consent of the
         other parties, such consent not to be unreasonably withheld. Such
         investor shall be entitled to the rights and subject to the obligations
         of Numberco hereunder directly. Numberco

<PAGE>   18

                                      -18-

         shall at all times be controlled by such investor. Nothing in this
         Section 24 shall prejudice Numberco's rights under Section 20.

25.      OPERATIVE DATE. If the date for the happening of an event shall fall on
         a day which is not a Business Day, then that event shall occur on the
         next day following which is a Business Day.

26.      TERMINATION. This Agreement shall terminate on July 1, 2002. The
         termination of this Agreement at such time shall have no effect upon
         any obligation of any party to satisfy any obligations owing by it
         under this Agreement prior to the date of such termination.

27.      REPRICING. The parties acknowledge and agree that it is intended that
         the price per share for the Optioned Shares and the Remaining 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 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, Holdco
         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 and any
         termination of this Agreement.

28.      ENTIRE AGREEMENT. This Option Agreement, the Pledge and Custodial
         Agreement and the Letter Agreement constitute the entire agreement
         between the parties to this Option Agreement with respect to the
         matters referred to herein and supersede all prior agreements,
         understandings, negotiations and discussions, whether oral or written,
         of such parties with respect thereto. There are no representations,
         warranties, conditions or other agreements, express or implied,
         statutory or otherwise, between the parties to this Option Agreement in
         connection with the subject matter of this Option Agreement except as
         specifically set forth herein and therein and none of such parties has
         relied or is relying on any other information, discussion or
         understanding in entering into and completing the transactions
         contemplated in this Option Agreement, the Pledge and Custodial
         Agreement and the Letter Agreement. In the event that any provision of
         this Option Agreement is inconsistent with any provision of the Pledge
         and Custodial Agreement or

<PAGE>   19

                                      -19-

         the Letter Agreement, the provisions of this Option Agreement shall
         supersede and take precedence over any such other provision.

29.      COUNTERPARTS. This Option Agreement may be executed in one or more
         counterparts and all such counterparts taken together shall be deemed
         to constitute one and the same instrument.

<PAGE>   20

                                      -20-

IN WITNESS WHEREOF Holdco, Orbital and the Optionholders have caused this Option
Agreement to be signed by their respective duly authorized officers as of May
30, 2001.

<TABLE>
<S>                                                   <C>
                                                       CAI CAPITAL PARTNERS AND COMPANY II, L.P. BY ITS GENERAL
                                                       PARTNER CAI CAPITAL PARTNERS GP & CO., L.P., BY ONE OF
                                                       ITS GENERAL PARTNERS MANTHING II CO.

                                                       By:
                                                               ---------------------------------------------------
                                                               Name:       Manfred Yu
                                                               Title:      President

                                                       CAI PARTNERS AND COMPANY II,  L.P. BY ITS GENERAL PARTNER
                                                       CAI PARTNERS GP & CO., L.P., BY ONE OF ITS GENERAL
                                                       PARTNERS, MANTHING II CO.

                                                       By:
                                                               ---------------------------------------------------
                                                               Name:       Manfred Yu
                                                               Title:      President

                                                       CAI CAPITAL PARTNERS AND COMPANY II-C, L.P., BY ITS
                                                       GENERAL PARTNER CAI CAPITAL PARTNERS GP II-C INC.

                                                       By:
                                                               ---------------------------------------------------
                                                               Name:       Manfred Yu
                                                               Title:      Assistant Secretary

                                                       597858 B.C. LTD., AS AGENT

                                                       By:
                                                               ---------------------------------------------------
                                                               Name:       Randall Mullan
                                                               Title:      Director
</TABLE>

<PAGE>   21

                                      -21-

<TABLE>
<S>                                                 <C>
                                                       MDA HOLDINGS CORPORATION

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

                                                       ORBITAL SCIENCES CORPORATION (AS TO SECTIONS
                                                       5(b), 9(b), 11(f), 19, 20, 21, 23 AND 28 ONLY)

                                                       By:
                                                               ---------------------------------------------------
                                                               Name:
                                                               Title:
</TABLE>

<PAGE>   22

                               TRANSFER OF OPTION

         FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers
unto ______________________________________, Options held by the undersigned to
acquire __________________________________ Common Shares of MacDonald, Dettwiler
and Associates Ltd. represented hereby and irrevocably constitutes and appoints
____________________________the attorney of the undersigned to transfer the said
Options with full power of substitution hereunder.

<TABLE>
<S>                                          <C>
         DATED the                           day of                    ,                         .
                   -------------------------        -------------------  ------------------------

----------------------------------------------       ----------------------------------------------------
Witness                                                     Authorized Signatory of Optionholder
</TABLE>

<PAGE>   23

                                  EXERCISE FORM

TO:      MACDONALD, DETTWILER AND ASSOCIATES LTD. AND MDA HOLDINGS CORPORATION

         The undersigned hereby exercises the right to acquire       of      the
Common Shares of MacDonald, Dettwiler and Associates Ltd. (the "Corporation")
(or such number of other securities, money or property to which the Option
Agreement entitles the undersigned, in lieu thereof or in addition thereto,
according to the terms thereof) from MDA Holdings Corporation or its permitted
assignee:

Such securities, money or property are to be transferred as follows:

Name:
(Print Clearly)
               -----------------------------------------------------------------

Address in Full
               -----------------------------------------------------------------

Number or percentage of Common Shares:

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

<TABLE>
<S>                                          <C>
         DATED the                           day of                    ,                         .
                   -------------------------       --------------------  ------------------------
</TABLE>

                                            ------------------------------------
                                            Authorized Signatory of Optionholder

                                            ------------------------------------
                                            Print full name

<PAGE>   24

                                  SCHEDULE "A"
                     FORM OF PLEDGE AND CUSTODIAL AGREEMENT

<PAGE>   25

                                  SCHEDULE "B"
                              IRREVOCABLE DIRECTION

TO:               RBC DOMINION SECURITIES CORPORATION (THE "CUSTODIAL AGENT")
                  ONE LIBERTY PLAZA, 165 BROADWAY, NEW YORK, NY  10006-1404
                  ATTENTION:  ROGER A. BLISETT, TELECOPIER NUMBER (212) 858-7468
                  WITH A COPY TO MICHAEL FROMMER (212) 858-7435

RE:               PLEDGE AND CUSTODIAL AGREEMENT - HOLDING OF PLEDGED SECURITIES

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

         We refer to the Pledge and Custodial Agreement (the "PLEDGE AND
CUSTODIAL AGREEMENT") dated July 12, 2000 among MDA Holdings Corporation
("HOLDCO"), the Custodial Agent, CAI Capital Partners and Company, II, L.P., CAI
Partners and Company II L.P., CAI Capital Partners and Company II-C, L.P. and
597858 B.C. Limited (collectively, the "OPTIONHOLDERS") pursuant to which the
Custodial Agent agreed to act as agent of the Optionholders for the purpose of
holding certain common shares (the "PLEDGED SHARES") in the capital of MacDonald
Dettwiler and Associates Ltd. that were pledged to it in accordance with the
terms of a secondary option agreement dated June 29, 2000 between the same
parties, as the same was amended and restated on the date hereof (the "OPTION
AGREEMENT").

         In accordance with the Option Agreement, 5,650,000 of the Pledged
Shares will continue to be pledged by Holdco to the Optionholders and are hereby
delivered to you to be held by you in accordance with the Pledge and Custodial
Agreement. Each of the undersigned hereby directs you to hold such Pledged
Shares in accordance with the terms of the Pledge and Custodial Agreement.

         This shall be your good, sufficient and irrevocable authority for doing
the foregoing.

<PAGE>   26

                                      -3-

         This irrevocable direction may be executed in one or more counterparts
and all such counterparts taken together shall be deemed to constitute one and
the same instrument.

         DATED this ____ day of May, 2001.

<TABLE>
<S>                                                        <C>
                                                            CAI CAPITAL PARTNERS AND COMPANY II, L.P.

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

                                                            CAI PARTNERS AND COMPANY II, L.P.

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

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

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

                                                            597858 B.C. LTD.

                                                            By:

                                                                   -----------------------------------------------
                                                                   Name:
                                                                   Title:
</TABLE>

<PAGE>   27

                                      -4-

<TABLE>
<S>                                                       <C>
                                                            MDA HOLDINGS CORPORATION

                                                            By:
                                                                   -----------------------------------------------
                                                                   Name:
                                                                   Title:
</TABLE>

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00028-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00028-of-00352.parquet"}]]