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                                                                  EXHIBIT 10.3.1

                                                                [EXECUTION COPY]

                           AMENDMENT NO. 1 AND WAIVER


        AMENDMENT No. 1 and WAIVER ("THIS AMENDMENT") dated as of April 12, 2001
relating to the 364-Day Senior Credit Agreement dated as of February 23, 2001
(as the same may hereafter be amended from time to time, the "364-DAY SENIOR
CREDIT AGREEMENT") among ORBITAL SCIENCES CORPORATION (the "COMPANY"), the BANKS
party thereto (the "BANKS") and MORGAN GUARANTY TRUST COMPANY OF NEW YORK, as
Administrative Agent (the "ADMINISTRATIVE AGENT") and as Collateral Agent (the
"COLLATERAL AGENT").

        The parties hereto agree as follows:

        SECTION 1. Defined Terms; References. Unless otherwise specifically
defined herein, each term used herein which is defined in the 364-Day Senior
Credit Agreement has the meaning assigned to such term in the 364-Day Senior
Credit Agreement. Each reference to "hereof", "hereunder", "herein" and "hereby"
and each other similar reference and each reference to "this Agreement" and each
other similar reference contained in the 364-Day Senior Credit Agreement shall,
after this Amendment becomes effective, refer to the 364-Day Senior Credit
Agreement as amended hereby.

        SECTION 2. Limited Waiver of Compliance with Certain Information
Covenants. (a) The Required Banks hereby waive compliance by the Company with
Sections 5.01(a), 5.01(c) and 5.01(d) of the 364-Day Senior Credit Agreement,
and any Default arising from its failure to comply with such Sections, during
the period from and including April 2, 2001 to but not including April 18, 2001.

        (b) The waiver granted pursuant to subsection (a) shall be limited
precisely as written, shall not constitute a waiver of compliance with, or of a
Default arising under, any provision of the 364-Day Senior Credit Agreement
except Sections 5.01(a), 5.01(c) and 5.01(d) and shall not constitute a waiver
of compliance with, or of a Default under, Sections 5.01(a), 5.01(c) and 5.01(d)
at any time after such waiver ceases to be effective. Such waiver shall cease to
be effective at 12:01 A.M. (New York City time) on April 18, 2001.

        SECTION 3. Amendment of a Certain Information Covenant. (a) Section
5.01(o) of the 364-Day Senior Credit Agreement is amended by deleting the words
"simultaneously with" and replacing them with the words "within five days
after".

        SECTION 4. Amendment of the Minimum Consolidated Net Worth Covenant.
Section 5.08 of the 364-Day Senior Credit Agreement is amended by:

        (i) substituting the dollar amount "$140,000,000" for the dollar amount
        "$190,000,000" set forth in the table in Section 5.08 opposite


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        the fiscal quarter ended December 31, 2000; and

        (ii) substituting the dollar amount "$115,000,000" for the dollar amount
        "$175,000,000" set forth in the table in Section 5.08 opposite the
        fiscal quarter ended March 31, 2001.

        SECTION 5. Amendment of the Consolidated Leverage Ratio Covenant.
Section 5.09(a) of the 364-Day Senior Credit Agreement is amended by:

        (i) substituting the ratio "5.80:1" for the ratio "4.85:1" set forth in
        the table in Section 5.09(a) opposite the period beginning October 1,
        2000 and ending December 31, 2000; and

        (ii) substituting the ratio "9.50:1" for the ratio "4.50:1" set forth in
        the table in Section 5.09(a) opposite the period beginning January 1,
        2001 and ending March 31, 2001.

        SECTION 6. Amendment of the Senior Leverage Ratio Covenant. Section
5.09(b) of the 364-Day Senior Credit Agreement is amended by:

        (i) substituting the ratio "4.10:1" for the ratio "3.50:1" set forth in
        the table in Section 5.09(b) opposite the period beginning October 1,
        2000 and ending December 31, 2000; and

        (ii) substituting the ratio "6.60:1" for the ratio "3.35:1" set forth in
        the table in Section 5.09(b) opposite the period beginning January 1,
        2001 and ending March 31, 2001.

        SECTION 7. Amendment of the Minimum Operating Cash Flow Covenant.
Section 5.10 of the 364-Day Senior Credit Agreement is amended by:

        (i) adding the words "(in thousands)" immediately after the word
        "Amount" in the table in Section 5.10;

        (ii) adding the dollar amount "$(101,300)" in the table in Section 5.10
        opposite the period beginning January 2001 and ending July 2001; and

        (iii) adding the dollar amount "$(104,600)" in the table in Section 5.10
        opposite the period beginning January 2001 and ending August 2001.

        SECTION 8. Amendment of the Subsidiary Debt Covenant. Section 5.15 of
the 364-Day Senior Credit Agreement is amended by deleting "5%" and substituting
in lieu thereof the phrase "(x) prior to June 30, 2001, 7.5% of Consolidated Net
Worth and (y) on and after June 30, 2001, 5%".

        SECTION 9. Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

        SECTION 10. Counterparts, Effectiveness. This Amendment may be signed in
any number of counterparts, each of which shall be an original, with the same
effect as if the signatures thereto and hereto were upon the same instrument.
This Amendment shall become effective as of the date hereof on the date when the
following conditions are met:

        (a) the Administrative Agent shall have received from each of the
        Company and the Required Banks a counterpart hereof signed by such


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        party or facsimile or other written confirmation (in form satisfactory
        to the Administrative Agent) that such party has signed a counterpart
        hereof; and

        (b) the Administrative Agent shall have received payment in full of all
        fees and expenses payable by the Company in connection with this
        Amendment pursuant to Section 8.03 of the 364-Day Senior Credit
        Agreement.



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


                                ORBITAL SCIENCES CORPORATION


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


                                MORGAN GUARANTY TRUST COMPANY
                                  OF NEW YORK


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


                                THE BANK OF NOVA SCOTIA


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


                                BANK OF AMERICA, N.A.


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



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                                FIRST UNION COMMERCIAL CORPORATION


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


                                DEUTSCHE BANK AG, NEW YORK
                                  AND/OR CAYMAN ISLAND BRANCHES


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


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


                                KEYBANK NATIONAL ASSOCIATION


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


                                BANK OF TOKYO-MITSUBISHI TRUST COMPANY


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




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                                WACHOVIA BANK, N.A.


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


                                CHEVY CHASE BANK


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


                                MORGAN GUARANTY TRUST COMPANY
                                  OF NEW YORK, as Administrative Agent and as
                                  Collateral Agent


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

Acknowledged by:


ENGINEERING TECHNOLOGIES, INC.


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

ORBITAL SPACE SYSTEMS, INC.


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


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ORBITAL COMMERCIAL SYSTEMS, INC.


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


ORBITAL INTERNATIONAL, INC.


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


ORBITAL SERVICES CORPORATION


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


ORBITAL NAVIGATION CORPORATION


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


ORBLINK LLC


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