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                                                                   EXHIBIT 10.56

                          SUPPLEMENTAL INDENTURE TO BE
                     DELIVERED BY GUARANTEEING SUBSIDIARIES

     Supplemental Indenture (this "Supplemental Indenture"), dated as of
February 25, 2004, among L-3 Communications Corporation (or its permitted
successor), a Delaware corporation (the "Company"), each subsidiary of the
Company signatory hereto (each, a "Guaranteeing Subsidiary", and collectively,
the "Guaranteeing Subsidiaries"), and The Bank of New York, as trustee under the
indenture referred to below (the "Trustee").

                              W I T N E S S E T H:
                              - - - - - - - - - -

     WHEREAS, the Company has heretofore executed and delivered to the Trustee
an indenture (as amended, the "Indenture"), dated as of December 11, 1998
providing for the issuance of an aggregate principal amount of up to
$300,000,000 of 8% Senior Subordinated Notes due 2008 (the "Notes");

     WHEREAS, the Indenture provides that under certain circumstances the
Guaranteeing Subsidiaries shall execute and deliver to the Trustee a
supplemental indenture pursuant to which the Guaranteeing Subsidiaries shall
unconditionally guarantee all of the Company's Obligations under the Notes and
the Indenture on the terms and conditions set forth herein (the "Subsidiary
Guarantee"); and

     WHEREAS, pursuant to Sections 4.13 and 9.01 of the Indenture, the Trustee
is authorized to execute and deliver this Supplemental Indenture.

     NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the
Guaranteeing Subsidiaries and the Trustee mutually covenant and agree for the
equal and ratable benefit of the Holders of the Notes as follows:

     1. CAPITALIZED TERMS. Capitalized terms used herein without definition
shall have the meanings assigned to them in the Indenture.

     2. AGREEMENT TO GUARANTEE. Each Guaranteeing Subsidiary hereby agrees as
follows:

         (a)   Each Guaranteeing Subsidiary, jointly and severally with all
               other current and future guarantors of the Notes (collectively,
               the "Guarantors" and each, a "Guarantor"), unconditionally
               guarantees to each Holder of a Note authenticated and delivered
               by the Trustee and to the Trustee and its successors and assigns,
               regardless of the validity and enforceability of the Indenture,
               the Notes or the Obligations of the Company under the Indenture
               or the Notes, that:

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               (i)  the principal of, premium and interest on the Notes will be
                    promptly paid in full when due, whether at maturity, by
                    acceleration, redemption or otherwise, and interest on the
                    overdue principal of, premium and interest on the Notes, to
                    the extent lawful, and all other Obligations of the Company
                    to the Holders or the Trustee thereunder or under the
                    Indenture will be promptly paid in full, all in accordance
                    with the terms thereof; and

               (ii) in case of any extension of time for payment or renewal of
                    any Notes or any of such other Obligations, that the same
                    will be promptly paid in full when due in accordance with
                    the terms of the extension or renewal, whether at stated
                    maturity, by acceleration or otherwise.

         (b)   Notwithstanding the foregoing, in the event that this Subsidiary
               Guarantee would constitute or result in a violation of any
               applicable fraudulent conveyance or similar law of any relevant
               jurisdiction, the liability of each Guaranteeing Subsidiary under
               this Supplemental Indenture and its Subsidiary Guarantee shall be
               reduced to the maximum amount permissible under such fraudulent
               conveyance or similar law.

     3. EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTEES.

         (a)   To evidence its Subsidiary Guarantee set forth in this
               Supplemental Indenture, each Guaranteeing Subsidiary hereby
               agrees that a notation of such Subsidiary Guarantee substantially
               in the form of Exhibit F to the Indenture shall be endorsed by an
               officer of such Guaranteeing Subsidiary on each Note
               authenticated and delivered by the Trustee after the date hereof.

         (b)   Notwithstanding the foregoing, each Guaranteeing Subsidiary
               hereby agrees that its Subsidiary Guarantee set forth herein
               shall remain in full force and effect notwithstanding any failure
               to endorse on each Note a notation of such Subsidiary Guarantee.

         (c)   If an Officer whose signature is on this Supplemental Indenture
               or on the Subsidiary Guarantee no longer holds that office at the
               time the Trustee authenticates the Note on which a Subsidiary
               Guarantee is endorsed, the Subsidiary Guarantee shall be valid
               nevertheless.

         (d)   The delivery of any Note by the Trustee, after the authentication
               thereof under the Indenture, shall constitute due delivery of the
               Subsidiary Guarantee set forth in this Supplemental Indenture on
               behalf of each Guaranteeing Subsidiary.

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         (e)   Each Guaranteeing Subsidiary hereby agrees that its obligations
               hereunder shall be unconditional, regardless of the validity,
               regularity or enforceability of the Notes or the Indenture, the
               absence of any action to enforce the same, any waiver or consent
               by any Holder of the Notes with respect to any provisions of the
               Notes or the Indenture, the recovery of any judgment against the
               Company, any action to enforce the same or any other circumstance
               which might otherwise constitute a legal or equitable discharge
               or defense of a guarantor.

         (f)   Each Guaranteeing Subsidiary hereby waives diligence,
               presentment, demand of payment, filing of claims with a court in
               the event of insolvency or bankruptcy of the Company, any right
               to require a proceeding first against the Company, protest,
               notice and all demands whatsoever and covenants that its
               Subsidiary Guarantee made pursuant to this Supplemental Indenture
               will not be discharged except by complete performance of the
               obligations contained in the Notes and the Indenture.

         (g)   If any Holder or the Trustee is required by any court or
               otherwise to return to the Company or any Guaranteeing
               Subsidiary, or any Custodian, Trustee, liquidator or other
               similar official acting in relation to either the Company or such
               Guaranteeing Subsidiary, any amount paid by either to the Trustee
               or such Holder, the Subsidiary Guarantee made pursuant to this
               Supplemental Indenture, to the extent theretofore discharged,
               shall be reinstated in full force and effect.

         (h)   Each Guaranteeing Subsidiary agrees that it shall not be entitled
               to any right of subrogation in relation to the Holders in respect
               of any obligations guaranteed hereby until payment in full of all
               obligations guaranteed hereby. Each Guaranteeing Subsidiary
               further agrees that, as between such Guaranteeing Subsidiary, on
               the one hand, and the Holders and the Trustee, on the other hand:

               (i)  the maturity of the obligations guaranteed hereby may be
                    accelerated as provided in Article 6 of the Indenture for
                    the purposes of the Subsidiary Guarantee made pursuant to
                    this Supplemental Indenture, notwithstanding any stay,
                    injunction or other prohibition preventing such acceleration
                    in respect of the obligations guaranteed hereby; and

               (ii) in the event of any declaration of acceleration of such
                    obligations as provided in Article 6 of the Indenture, such
                    obligations (whether or not due and payable) shall forthwith
                    become due and payable by such Guaranteeing

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                    Subsidiary for the purpose of the Subsidiary Guarantee made
                    pursuant to this Supplemental Indenture.

         (i)   Each Guaranteeing Subsidiary shall have the right to seek
               contribution from any other non-paying Guaranteeing Subsidiary so
               long as the exercise of such right does not impair the rights of
               the Holders or the Trustee under the Subsidiary Guarantee made
               pursuant to this Supplemental Indenture.

     4. GUARANTEEING SUBSIDIARIES MAY CONSOLIDATE, ETC. ON CERTAIN TERMS.

         (a)   Except as set forth in Articles 4 and 5 of the Indenture, nothing
               contained in the Indenture, this Supplemental Indenture or in the
               Notes shall prevent any consolidation or merger of any
               Guaranteeing Subsidiary with or into the Company or any other
               Guarantor or shall prevent any transfer, sale or conveyance of
               the property of any Guaranteeing Subsidiary as an entirety or
               substantially as an entirety, to the Company or any other
               Guarantor.

         (b)   Except as set forth in Article 4 of the Indenture, nothing
               contained in the Indenture, this Supplemental Indenture or in the
               Notes shall prevent any consolidation or merger of any
               Guaranteeing Subsidiary with or into a corporation or
               corporations other than the Company or any other Guarantor (in
               each case, whether or not affiliated with such Guaranteeing
               Subsidiary), or successive consolidations or mergers in which a
               Guaranteeing Subsidiary or its successor or successors shall be a
               party or parties, or shall prevent any sale or conveyance of the
               property of any Guaranteeing Subsidiary as an entirety or
               substantially as an entirety, to a corporation other than the
               Company or any other Guarantor (in each case, whether or not
               affiliated with such Guaranteeing Subsidiary) authorized to
               acquire and operate the same; provided, however, that each
               Guaranteeing Subsidiary hereby covenants and agrees that (i)
               subject to the Indenture, upon any such consolidation, merger,
               sale or conveyance, the due and punctual performance and
               observance of all of the covenants and conditions of the
               Indenture and this Supplemental Indenture to be performed by such
               Guaranteeing Subsidiary, shall be expressly assumed (in the event
               that such Guaranteeing Subsidiary is not the surviving
               corporation in the merger), by supplemental indenture
               satisfactory in form to the Trustee, executed and delivered to
               the Trustee, by the corporation formed by such consolidation, or
               into which such Guaranteeing Subsidiary shall have been merged,
               or by the corporation which shall have acquired such property and
               (ii)

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               immediately after giving effect to such consolidation, merger,
               sale or conveyance no Default or Event of Default exists.

         (c)   In case of any such consolidation, merger, sale or conveyance and
               upon the assumption by the successor corporation, by supplemental
               indenture, executed and delivered to the Trustee and satisfactory
               in form to the Trustee, of the Subsidiary Guarantee made pursuant
               to this Supplemental Indenture and the due and punctual
               performance of all of the covenants and conditions of the
               Indenture and this Supplemental Indenture to be performed by each
               Guaranteeing Subsidiary, such successor corporation shall succeed
               to and be substituted for such Guaranteeing Subsidiary with the
               same effect as if it had been named herein as the Guaranteeing
               Subsidiary. Such successor corporation thereupon may cause to be
               signed any or all of the Subsidiary Guarantees to be endorsed
               upon the Notes issuable under the Indenture which theretofore
               shall not have been signed by the Company and delivered to the
               Trustee. All the Subsidiary Guarantees so issued shall in all
               respects have the same legal rank and benefit under the Indenture
               and this Supplemental Indenture as the Subsidiary Guarantees
               theretofore and thereafter issued in accordance with the terms of
               the Indenture and this Supplemental Indenture as though all of
               such Subsidiary Guarantees had been issued at the date of the
               execution hereof.

     5. RELEASES.

         (a)   Concurrently with any sale of assets (including, if applicable,
               all of the Capital Stock of a Guaranteeing Subsidiary), all
               Liens, if any, in favor of the Trustee in the assets sold thereby
               shall be released; provided that in the event of an Asset Sale,
               the Net Proceeds from such sale or other disposition are treated
               in accordance with the provisions of Section 4.10 of the
               Indenture. If the assets sold in such sale or other disposition
               include all or substantially all of the assets of a Guaranteeing
               Subsidiary or all of the Capital Stock of a Guaranteeing
               Subsidiary, then the Guaranteeing Subsidiary (in the event of a
               sale or other disposition of all of the Capital Stock of such
               Guaranteeing Subsidiary) or the Person acquiring the property (in
               the event of a sale or other disposition of all or substantially
               all of the assets of such Guaranteeing Subsidiary) shall be
               released from and relieved of its obligations under this
               Supplemental Indenture and its Subsidiary Guarantee made pursuant
               hereto; provided that in the event of an Asset Sale, the Net
               Proceeds from such sale or other disposition are treated in
               accordance with the provisions of Section 4.10 of the Indenture.
               Upon delivery by the Company to the Trustee of an Officers'
               Certificate to the effect that such sale or other disposition was
               made by the Company or

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               the Guaranteeing Subsidiary, as the case may be, in accordance
               with the provisions of the Indenture and this Supplemental
               Indenture, including without limitation, Section 4.10 of the
               Indenture, the Trustee shall execute any documents reasonably
               required in order to evidence the release of the Guaranteeing
               Subsidiary from its obligations under this Supplemental Indenture
               and its Subsidiary Guarantee made pursuant hereto. If the
               Guaranteeing Subsidiary is not released from its obligations
               under its Subsidiary Guarantee, it shall remain liable for the
               full amount of principal of and interest on the Notes and for the
               other obligations of such Guaranteeing Subsidiary under the
               Indenture as provided in this Supplemental Indenture.

         (b)   Upon the designation of a Guaranteeing Subsidiary as an
               Unrestricted Subsidiary in accordance with the terms of the
               Indenture, such Guaranteeing Subsidiary shall be released and
               relieved of its obligations under its Subsidiary Guarantee and
               this Supplemental Indenture. Upon delivery by the Company to the
               Trustee of an Officers' Certificate and an Opinion of Counsel to
               the effect that such designation of such Guaranteeing Subsidiary
               as an Unrestricted Subsidiary was made by the Company in
               accordance with the provisions of the Indenture, including
               without limitation Section 4.07 of the Indenture, the Trustee
               shall execute any documents reasonably required in order to
               evidence the release of such Guaranteeing Subsidiary from its
               obligations under its Subsidiary Guarantee. Any Guaranteeing
               Subsidiary not released from its obligations under its Subsidiary
               Guarantee shall remain liable for the full amount of principal of
               and interest on the Notes and for the other obligations of any
               Guaranteeing Subsidiary under the Indenture as provided herein.

     6. NO RECOURSE AGAINST OTHERS. No past, present or future director,
officer, employee, incorporator, stockholder or agent of any Guaranteeing
Subsidiary, as such, shall have any liability for any obligations of the Company
or any Guaranteeing Subsidiary under the Notes, any Subsidiary Guarantees, the
Indenture or this Supplemental Indenture or for any claim based on, in respect
of, or by reason of, such obligations or their creation. Each Holder of the
Notes by accepting a Note waives and releases all such liability. The waiver and
release are part of the consideration for issuance of the Notes. Such waiver may
not be effective to waive liabilities under the federal securities laws and it
is the view of the Commission that such a waiver is against public policy.

     7. SUBORDINATION OF SUBSIDIARY GUARANTEES; ANTI-LAYERING. No Guaranteeing
Subsidiary shall incur, create, issue, assume, guarantee or otherwise become
liable for any Indebtedness that is subordinate or junior in right of payment to
any Senior Debt of a Guaranteeing Subsidiary and senior in any respect in right
of payment to any of the Subsidiary Guarantees. Notwithstanding the foregoing
sentence, the Subsidiary Guarantee of each Guaranteeing Subsidiary shall be
subordinated to the prior payment in full of

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all Senior Debt of that Guaranteeing Subsidiary (in the same manner and to the
same extent that the Notes are subordinated to Senior Debt), which shall include
all guarantees of Senior Debt.

     8. THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     9. COUNTERPARTS. The parties may sign any number of copies of this
Supplemental Indenture. Each signed copy shall be an original, but all of them
together represent the same agreement.

     10. EFFECT OF HEADINGS. The Section headings herein are for convenience
only and shall not affect the construction hereof.

     11. THE TRUSTEE. The Trustee shall not be responsible in any manner
whatsoever for or in respect of the validity or sufficiency of this Supplemental
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Guaranteeing Subsidiaries and the Company.

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     IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed, all as of the date first above written.

Dated: February 25, 2004                     L-3 COMMUNICATIONS CORPORATION

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

Dated: February 25, 2004       APCOM, INC., a Maryland corporation
                               BROADCAST SPORTS INC., a Delaware corporation
                               L-3 COMMUNICATIONS CSI, INC., a California
                                   corporation
                               ELECTRODYNAMICS, INC., an Arizona corporation
                               HENSCHEL INC., a Delaware corporation
                               HYGIENETICS ENVIRONMENTAL SERVICES, INC., a
                                   Delaware corporation
                               INTERSTATE ELECTRONICS CORPORATION, a California
                                   corporation
                               KDI PRECISION PRODUCTS, INC., a Delaware
                                   corporation
                               L-3 COMMUNICATIONS AEROMET, INC., an Oregon
                                   corporation
                               L-3 COMMUNICATIONS VERTEX AEROSPACE LLC, a
                                   Delaware limited liability company
                               L-3 COMMUNICATIONS AIS GP CORPORATION, a
                                   Delaware corporation
                               L-3 COMMUNICATIONS AVIONICS SYSTEMS, INC., a
                                   Delaware corporation
                               L-3 COMMUNICATIONS AYDIN CORPORATION, a Delaware
                                   corporation
                               L-3 COMMUNICATIONS ESSCO, INC., a Delaware
                                   corporation
                               L-3 COMMUNICATIONS FLIGHT INTERNATIONAL AVIATION
                                   LLC, a Delaware limited liability company
                               L-3 COMMUNICATIONS FLIGHT CAPITAL LLC, a Delaware
                                   limited liability company
                               L-3 COMMUNICATIONS GOVERNMENT SERVICES, INC., a
                                   Virginia corporation
                               L-3 COMMUNICATIONS ILEX SYSTEMS, INC., a
                                   Delaware corporation
                               L-3 COMMUNICATIONS INTEGRATED SYSTEMS L.P., a
                                   Delaware limited partnership
                               L-3 COMMUNICATIONS INVESTMENTS INC., a Delaware
                                   corporation
                               L-3 COMMUNICATIONS KLEIN ASSOCIATES, INC., a
                                   Delaware corporation
                               L-3 COMMUNICATIONS MAS (US) CORPORATION, a
                                   Delaware corporation
                               L-3 COMMUNICATIONS SECURITY AND DETECTION SYSTEMS
                                   CORPORATION CALIFORNIA, a California
                                   corporation
                               L-3 COMMUNICATIONS SECURITY AND DETECTION SYSTEMS
                                   CORPORATION DELAWARE, a Delaware corporation
                               L-3 COMMUNICATIONS SECURITY SYSTEMS CORPORATION,
                                   a Delaware corporation

                               L-3 COMMUNICATIONS STORM CONTROL SYSTEMS, INC.,
                                   a California corporation
                               L-3 COMMUNICATIONS VECTOR INTERNATIONAL AVIATION
                                   LLC, a Delaware limited liability company
                               L-3 COMMUNICATIONS WESTWOOD CORPORATION, a Nevada
                                   corporation
                               MCTI ACQUISITION CORPORATION, a Maryland
                                   corporation
                               MICRODYNE COMMUNICATIONS TECHNOLOGIES
                                   INCORPORATED, a Maryland corporation
                               MICRODYNE CORPORATION, a Maryland corporation
                               MICRODYNE OUTSOURCING INCORPORATED, a Maryland
                                   corporation
                               MPRI, INC., a Delaware corporation
                               PAC ORD INC., a Delaware corporation
                               POWER PARAGON, INC., a Delaware corporation
                               SHIP ANALYTICS, INC., a Connecticut corporation
                               SHIP ANALYTICS INTERNATIONAL, INC., a Delaware
                                   corporation
                               SHIP ANALYTICS USA, INC., a Connecticut
                                   corporation
                               SPD ELECTRICAL SYSTEMS, INC., a Delaware
                                   corporation
                               SPD SWITCHGEAR INC., a Delaware corporation
                               SYCOLEMAN CORPORATION, a Florida corporation
                               TROLL TECHNOLOGY CORPORATION, a California
                                   corporation
                               WESCAM AIR OPS INC., a Delaware corporation
                               WESCAM AIR OPS LLC, a Delaware limited liability
                                   company
                               WESCAM HOLDINGS (US) INC., a Delaware corporation
                               WESCAM INCORPORATED, a Florida corporation
                               WESCAM LLC, a Delaware limited liability company
                               WESCAM SONOMA INC., a California corporation
                               WOLF COACH, INC., a Massachusetts corporation
                                   As Guaranteeing Subsidiaries

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

Dated:  February 25, 2004                    THE BANK OF NEW YORK,
                                             as Trustee

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