SUPPLEMENTAL INDENTURE

Exhibit 4.1

SUPPLEMENTAL INDENTURE

Supplemental Indenture (this “Supplemental Indenture”), dated as of April 1, 2011, among the parties identified in the signature page of this Supplemental Indenture as a Guaranteeing Subsidiary (each a “Guaranteeing Subsidiary”) of the Company (as defined herein), and Wilmington Trust FSB, as trustee under the Indenture referred to below (the “Trustee”).

W I T N E S S E T H

WHEREAS, Kratos Defense & Security Solutions, Inc., a Delaware corporation (the “Company”) has heretofore executed and delivered to the Trustee an indenture (the “Indenture”), dated as of May 19, 2010 providing for the issuance of 10% Senior Secured Notes due 2017 (the “Notes”);

WHEREAS, Section 4.16 of the Indenture provides that under certain circumstances each Guaranteeing Subsidiary shall execute and deliver to the Trustee a supplemental indenture and a Guarantee pursuant to which any newly-acquired or created Guarantor shall unconditionally guarantee all of the Company’s obligations under the Notes and the Indenture on the terms and conditions set forth therein and herein and in such Guarantee; and

WHEREAS, pursuant to Section 9.01 of the Indenture, the Trustee is authorized to execute and delivery this Supplemental Indenture.

NOW THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt of which is hereby acknowledged, each Guaranteeing Subsidiary 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.                                       Joinder to Indenture. Each of the Guaranteeing Subsidiaries hereby agree to become bound by the terms, conditions and other provisions of the Indenture with all attendant rights, duties and obligations stated therein, with the same force and effect as if originally named as a Guarantor therein and as if such Guaranteeing Subsidiary executed the Indenture on the date thereof.

3.                                       Agreement to Guarantee. Each Guarantor hereby fully, irrevocably and unconditionally, jointly and severally, unconditionally and irrevocably guarantees (such guarantee, as amended or supplemented from time to time, to be referred to herein as the “Guarantee”), to each of the Holders, the Trustee and the Collateral Agent and their respective successors and assigns that (i) the principal of, premium, if any and interest and Additional Interest, if any, on the Notes shall be promptly paid in full when due, subject to any applicable grace period, whether upon redemption pursuant to the terms of the Notes, by acceleration or otherwise, and interest on the overdue principal (including interest accruing at the then applicable rate provided in the

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Indenture Documents after the occurrence of any Event of Default set forth in Section 6.01(6) or 6.01(7) of the Indenture, whether or not a claim for post-filing or post-petition interest is allowed under applicable law following the institution of a proceeding under bankruptcy, insolvency or similar laws), if any, and interest on any interest and Additional Interest, if any, to the extent lawful, of the Notes and all other obligations of the Company to the Holders, the Trustee and the Collateral Agent hereunder, thereunder or under any Collateral Agreement shall be promptly paid in full or performed, all in accordance with the terms hereof, thereof and of the Collateral Agreements; and (ii) in case of any extension of time of payment or renewal of any of the Notes or of any such other obligations, the same shall be promptly paid in full when due or performed in accordance with the terms of the extension or renewal, subject to any applicable grace period, whether at stated maturity, by acceleration or otherwise, subject, however, in the case of clauses (i) and (ii) above, to the limitations set forth in Section 10.03 of the Indenture.

The obligations of each Guaranteeing Subsidiary to the Holders and to the Trustee pursuant to this Supplemental Indenture and the Indenture are expressly set forth in Article Ten of the Indenture and reference is hereby made to such Indenture for the precise terms of the Guarantee. No past, present or future director, officer, employee, incorporator, agent, stockholder or Affiliate of the Company or a Guarantor, as such, shall have any liability for any obligations of the Company or the Guarantors under the Notes, the Guarantees, the Indenture or the Collateral Agreements or for any claim based on, in respect of, such obligations or their creation. The Guarantee executed and delivered hereby is a continuing Guarantee and shall remain in full force and effect and shall be binding upon each Guarantor and its successors and assigns until full and final payment of all of the Company’s obligations under the Notes and Indenture or until released or legally defeased in accordance with the Indenture and shall inure to the benefit of the successors and assigns of the Trustee and the Holders, and, in the event of any transfer or assignment of rights by any Holder or the Trustee, the rights and privileges herein conferred upon that party shall automatically extend to and be vested in such transferee or assignee, all subject to the terms and conditions hereof. This is a Guarantee of payment and performance and not of collectibility.

The obligations of each Guaranteeing Subsidiary under its Subsidiary Guarantee shall be limited to the extent necessary to insure that it does not constitute a fraudulent conveyance under applicable law.

THE TERMS OF ARTICLE TEN OF THE INDENTURE ARE INCORPORATED HEREIN BY REFERENCE.

4.                                       GOVERNING LAW. THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK, AS APPLIED TO CONTRACTS MADE AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT REGARD TO PRINCIPLES OF CONFLICT OF LAWS. EACH OF THE PARTIES HERETO AGREES TO SUBMIT TO THE JURISDICTION OF THE COURTS OF THE STATE OF NEW YORK IN ANY ACTION OR PROCEEDING ARISING OUT OF OR RELATING TO THIS SUPPLEMENTAL INDENTURE.

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5.                                       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.

6.                                       Effect of Headings. The Section headings herein are for convenience only and shall not affect the construction hereof.

[signature page follows]

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

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