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                                                                    Exhibit 4.16

                                 FIRST AMENDMENT

                  FIRST AMENDMENT, dated as of August 5, 2002 (this
"Amendment"), to the Investment Agreement, dated as of December 27, 2001 (as
amended, supplemented or otherwise modified, the "Investment Agreement"), among
Acterna Corporation, a Delaware corporation (together with its successors and
assigns, the "Parent"), Acterna LLC, a Delaware limited liability company wholly
owned and controlled by the Parent (together with its successors and assigns,
the "Company") and Clayton, Dubilier & Rice Fund VI Limited Partnership, a
Cayman Islands exempted limited partnership (together with its successors and
assigns, the "Investor").

                              W I T N E S S E T H:

                  WHEREAS, pursuant to the Investment Agreement, the Investor
has purchased certain 12% Senior Secured Convertible Notes due December 31, 2007
of the Company;

                  WHEREAS, in connection with a proposed purchase by CD&R VI
(Barbados), Ltd. of the Senior Secured Convertible Notes due December 31, 2007,
Series 2 of the Company, the Company and the Parent have requested the Investors
to agree to amend the Investment Agreement; and

                  WHEREAS, the Investor has agreed to the requested amendments
but only on the terms and conditions contained in this Amendment;

                  NOW, THEREFORE, the parties hereto hereby agree as follows:

                  Section 1. Defined Terms. Terms defined in the Investment
Agreement and used herein shall have the meanings given to them in the
Investment Agreement.

                  Section 2. Amendment to Subsection 6.6 (Mandatory Offer to
Repurchase the CD&R Notes Upon Disposition of Assets). Section 6.6(a) of the
Investment Agreement is hereby amended by deleting the parenthetical "(other
than to the Company or any of its Restricted Subsidiaries)" where it appears in
clause (i) of the definition of Mandatory 6.6(a) Repurchase Event and inserting
in lieu thereof the parenthetical "(other than to the Company or any of its
Restricted Subsidiaries, and other than any sale, transfer or other disposition
described in subsection 14.5(m) of the Credit Agreement as amended by the Third
Credit Agreement Amendment, to the extent the proceeds thereof are used in
accordance with the Credit Agreement as so amended)".

                  Section 3. Amendment to Section 7.10 (Liens). Section 7.10 of
the Investment Agreement is hereby amended by inserting the phrase "other than
Airshow Sale Proceeds Cash Collateral Account (as defined in the Credit
Agreement as amended by the Third Credit Agreement Amendment)" immediately after
the phrase "properties or assets" in such Section 7.10.

                  Section 4. Amendment to Section 8.2 (Limitation on Restricted
Payments). Section 8.2(a) of the Investment Agreement is hereby amended by
inserting at the end of the parenthetical in clause (iii), and at the end of the
first parenthetical in clause (iv), of such Section

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8.2(a) the phrase "and other than as permitted by Section 14.12 of the Credit
Agreement as amended by the Third Credit Agreement Amendment".

                  Section 5. Amendment to Section 8.7 (Limitation on Optional
Payments and Modifications of Debt Instruments and other Material Agreements).
Section 8.7 of the Investment Agreement is hereby amended by inserting at the
beginning of clause (a) of such Section 8.7 the phrase "except as permitted by
Section 14.12(a) of the Credit Agreement as amended by the Third Credit
Agreement Amendment,".

                  Section 6. Amendment to Section 8.8 (Limitation on Negative
Pledge Clauses). Section 8.8 of the Investment Agreement is hereby amended by
inserting in clause (a) of such Section 8.8 immediately after the phrase "this
Agreement and the other Note Financing Documents" the phrase ", the Barbados
Investment Agreement and any Note Financing Documents as defined in the Barbados
Investment Agreement".

                  Section 7. Amendment to Section 10.1 (Events of Default).
Section 10.1(f) of the Investment Agreement is hereby amended by deleting the
phrase "or (ii)" in such Section 10.1(f) and inserting in lieu thereof the
phrase ", (ii) any Barbados Default Event shall occur or exist or (iii)".

                  Section 8. Amendment to Article XII (Definitions). (a) Article
XII of the Investment Agreement is hereby amended by adding the following new
definitions in the appropriate alphabetical order:

                           "Barbados Default Event" means a "Default" or "Event
                  of Default" as those terms are defined in the Barbados
                  Investment Agreement, or any other default or event of default
                  under the Barbados Investment Agreement.

                           "Barbados Investment Agreement" means the Investment
                  Agreement, dated as of August 5, 2002, among the Company, the
                  Parent and CD&R VI (Barbados), Ltd., as the same may be
                  amended, supplemented, waiver or otherwise modified from time
                  to time.

                           "Barbados Notes" means the "Notes" as defined in the
                  Barbados Investment Agreement.

                           "Third Credit Agreement Amendment" means the Third
                  Amendment, dated as of August 7, 2002, to the Credit
                  Agreement.

                  (b) Article XII of the Investment Agreement is hereby further
amended by deleting the phrase "or (4)" where it occurs in clause (k) of the
definition of "Permitted Liens" and inserting in lieu thereof the phrase ", (4)
securing the Barbados Notes or (5)" in clause (p) of the definition of
"Permitted Liens."

                  (c) Article XVIII of the Investment Agreement is hereby
amended by adding the following provision as Section 13.17 of the Investment
Agreement:

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                           "Limitation on Certain Transactions. The Investor
                  hereby agrees that, from and after the date hereof, neither
                  the Investor nor any other investment vehicle managed by
                  Clayton, Dubilier & Rice, Inc. shall engage in any Rule 13e-3
                  transaction, as such term is defined in Rule 13e-3 of the
                  Exchange Act, without the prior written consent of the Special
                  Committee (as such term is defined in the Barbados Investment
                  Agreement). The agreements in this Section 13.17 shall survive
                  repayment of the Notes and all other amounts payable hereunder
                  and the issuance of the Warrants."

                  Section 9. Amendments to Guarantee and Collateral Agreement.
The definition of "Credit Agreement" in Section 1.1 of the Guarantee and
Collateral Agreement is hereby amended by deleting "2001" and inserting in lieu
thereof "2000".

                  Section 10. Consent. The Investor hereby consents to the
amendment contained in the Third Credit Agreement Amendment to Section 14.12(e)
of the Credit Agreement.

                  Section 11. Waiver. In furtherance of the foregoing, the
Investor consents to the Company's and the Parent's execution, delivery and
performance of the Investment Agreement, dated as of the date of this Amendment,
among the Company, the Parent and CD&R VI (Barbados), Ltd., and hereby waives
any default or event of default under the Investment Agreement or the other Note
Financing Documents that may result from such execution, delivery or
performance.

                  Section 12. Representation and Warranties; No Defaults or
Events of Default. Each of the Company and the Parent represents and warrants to
the Administrative Agent and the Lenders that as of the date hereof, after
giving effect to this Amendment, no Default or Event of Default has occurred and
is continuing, and the representations and warranties made by the Company and
the Parent in or pursuant to the Investment Agreement or any other Note
Financing Document are true and correct in all material respects on and as of
the date hereof as if made on such date (except to the extent of changes
resulting from transactions contemplated or permitted by this Amendment, and
except to the extent that any such representations and warranties expressly
relate to an earlier date, in which case such representations and warranties
were true and correct in all material respects on and as of such earlier date).

                  Section 13. Payment of Expenses. The Company agrees to pay or
reimburse the Investor for all of its reasonable out-of-pocket costs and
expenses incurred in connection with this Amendment, any other documents
prepared in connection herewith and the transactions contemplated hereby,
including, without limitation, the reasonable fees and disbursements of counsel
to the Investor.

                  Section 14. Continuing Effect of the Investment Agreement.
This Amendment shall not constitute an amendment or waiver of or consent to any
provision of the Investment Agreement not expressly referred to herein and shall
not be construed as an amendment, waiver or consent to any action on the part of
the Parent or the Company or any of its Subsidiaries that would require an
amendment, waiver or consent of the Investor except as expressly stated herein.
Except as expressly amended hereby, the provisions of the Investment Agreement
are and shall remain in full force and effect in accordance with its terms.

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                  Section 15. Counterparts. This Amendment may be executed by
one or more of the parties to this Amendment on any number of separate
counterparts (including by facsimile), and all of said counterparts taken
together shall be deemed to constitute one and the same instrument.

                  Section 16. GOVERNING LAW. THIS AMENDMENT AND THE RIGHTS AND
OBLIGATIONS OF THE PARTIES UNDER THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED AND INTERPRETED IN ACCORDANCE WITH, THE LAW OF THE STATE OF NEW YORK.

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                  IN WITNESS WHEREOF, the parties hereto have caused this
Amendment to be duly executed and delivered by their respective proper and duly
authorized officers as of the day and year first above written.

                                    ACTERNA LLC



                                    By: __________________________________
                                          Name:
                                          Title:

                                    ACTERNA CORPORATION



                                    By: ____________________________________
                                          Name:
                                          Title:



                                    CLAYTON, DUBILIER & RICE FUND VI
                                     LIMITED PARTNERSHIP

                                    By: CD&R Associates VI Limited Partnership,
                                     its general partner

                                    By: CD&R Investment Associates VI, Inc., its
                                     managing general partner


                                    By: ____________________________________
                                          Name:
                                          Title:


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                           ACKNOWLEDGMENT AND CONSENT

         Each of the undersigned as guarantors under the Guarantee and
Collateral Agreement, dated as of December 27, 2001, made by the undersigned in
favor of the Investor and the other Secured Parties referred to therein, hereby
(a) consents to the transactions contemplated by this Amendment and (b)
acknowledges and agrees that the guarantees (and grants of collateral security
therefor) contained in such Guarantee and Collateral Agreement are, and shall
remain, in full force and effect after giving effect to this Amendment.

                                    ACTERNA BUSINESS TRUST



                                    By: ____________________________________
                                          Name:
                                          Title:


                                    Airshow, Inc.



                                    By: ____________________________________
                                          Name:
                                          Title:


                                    APPLIED DIGITAL ACCESS, INC.

                                    By: ____________________________________
                                          Name:
                                          Title:


                                    DA VINCI SYSTEMS, INC.



                                    By: ____________________________________
                                          Name:
                                          Title:


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                                    ITRONIX CORPORATION

                                    By: ____________________________________
                                          Name:
                                          Title: