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                                                                     Exhibit 4.3

            THIS SECOND SUPPLEMENTAL INDENTURE, effective as of October 14,
1999, among Anteon Corporation, a Virginia corporation (the "Company"),
Anteon-CITI LLC, a Delaware limited liability company ("CITI"), and IBJ
Whitehall Bank & Trust Company, as trustee (the "Trustee").

            WHEREAS, the Company, Vector Data Systems, Inc., a Virginia
corporation ("Vector Data"), Techmatics, Inc. ("Techmatics"), a Virginia
corporation, Analysis and Technology, Inc. ("A&T"), a Connecticut corporation,
and Interactive Media Corp. ("IMC"), a Delaware corporation, and the Trustee
entered into an Indenture dated as of May 11, 1999, as amended by the First
Supplemental Indenture dated as of June 23, 1999 (the "Indenture") to provide
for the issuance of the Company's 12% Senior Subordinated Notes due 2009;

            WHEREAS, on the date hereof, the Company has formed CITI as a
domestic subsidiary;

            WHEREAS, pursuant to Section 4.10 of the Indenture, CITI, as a new
Restricted Subsidiary, is required to enter into this Supplemental Indenture
(this "Supplemental Indenture") as a Subsidiary Guarantor;

            WHEREAS, the Company, CITI and the Trustee are authorized to enter
into this Supplemental Indenture;

            NOW, THEREFORE, for and in consideration of the premises and the
mutual covenants contained in this Supplemental Indenture and for other good and
valuable consideration, the receipt and sufficiency of which are herein
acknowledged, the Company, CITI and the Trustee hereby agree for the equal and
the ratable benefit of all Holders of the Securities as follows:

                                   ARTICLE ONE

                                   DEFINITIONS

            1.1 DEFINITIONS. For purposes of this Supplemental Indenture, the
terms defined in the recitals shall have the meanings therein specified; any
terms defined in the Indenture and not defined herein shall have the same
meanings herein as therein defined; and references to Articles or Sections
shall, unless the context indicates otherwise, be references to Articles or
Sections of the Indenture.
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                                   ARTICLE TWO

                  GUARANTIES OF SECURITIES AND OTHER PROVISIONS

            2.1 CITI GUARANTEE.

            (a) CITI hereby unconditionally and irrevocably guarantees, jointly
and severally, to each Holder and to the Trustee and its successors and assigns
(i) the full and punctual payment of principal of and interest on the Securities
when due, whether at maturity, by acceleration, by redemption or otherwise, and
all other monetary obligations of the Company under this Indenture and the
Securities and (ii) the full and punctual performance within applicable grace
periods of all other obligations of the Company under the Indenture and the
Securities (all the foregoing being hereinafter collectively called the
"Obligations"). CITI further agrees that the Obligations may be extended or
renewed, in whole or in part, without notice or further assent from CITI and
that CITI will remain bound under this Section notwithstanding any extension or
renewal of any Obligation.

            (b) CITI, the Trustee and each Holder by its acceptance of a
Security hereby agrees that the Subsidiary Guaranty of CITI provided hereunder
shall be subject to all terms, provisions and conditions in the Indenture that
relate to a Subsidiary Guaranty (including, without limitation, Articles 11 and
12 of the Indenture). CITI further agrees to be bound by, and to comply with,
all provisions of the Indenture and Subsidiary Guarantee that are applicable to
a Subsidiary Guarantor.

            2.2 EXECUTION AND DELIVERY OF SUBSIDIARY GUARANTIES.

            The delivery of any Security by the Trustee, after the
authentication thereof under the Indenture, shall constitute due delivery of the
Subsidiary Guaranty on behalf of CITI.

            2.3 NO PERSONAL LIABILITY.

            No stockholder, officer, director, employee or incorporator, past,
present or future, of CITI, as such, shall have any personal liability under the
Subsidiary Guaranty of CITI by reason of his, her or its status as such
stockholder, officer, director, employee or incorporator.
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                                  ARTICLE THREE

                                  MISCELLANEOUS

            3.1 EFFECT OF THE SUPPLEMENTAL INDENTURE. This Supplemental
Indenture supplements the Indenture and shall be a part and subject to all the
terms thereof. Except as supplemented hereby, the Indenture and the Securities
issued thereunder shall continue in full force and effect.

            3.2 COUNTERPARTS. This Supplemental Indenture may be executed in
counterparts, each of which shall be deemed an original, but all of which shall
together constitute one and the same instrument.

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


            IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed on this 14th day of October 1999.

                                 ANTEON CORPORATION


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


                                 ANTEON-CITI, LLC


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


                                 THE BANK OF NEW YORK
                                 As Trustee


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