Document:

<PAGE>   1
                                                                 EXHIBIT 10.1.13
                                                                [EXECUTION COPY]

                                AMENDMENT No. 13
                                       TO
                           THIRD AMENDED AND RESTATED
                       CREDIT AND REIMBURSEMENT AGREEMENT

         AMENDMENT No. 13 dated as of February 23, 2001 among ORBITAL SCIENCES
CORPORATION (the "COMPANY"), the BANKS listed on the signature pages hereof and
MORGAN GUARANTY TRUST COMPANY OF NEW YORK, as Administrative Agent and as
Collateral Agent.

                                   WITNESSETH:

         WHEREAS, the parties hereto have heretofore entered into a Third
Amended and Restated Credit and Reimbursement Agreement dated as of December 21,
1998 (as amended prior to the date hereof, the "EXISTING AGREEMENT"); and

         WHEREAS, the Company and the Banks wish to amend and restate the Credit
Agreement as set forth herein;

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

         SECTION 1. Definitions; References . Unless otherwise specifically
defined herein, each term used herein shall have the meaning assigned to such
term in Exhibit A hereto. Each reference to "hereof", "hereunder", "herein" and
"hereby" and each other similar reference contained in the Credit Agreement
shall from and after the Effective Date (as defined in Section 4 below) refer to
the Credit Agreement as amended and restated hereby.

         SECTION 2. Amendment and Restatement of the Credit Agreement . The
Existing Agreement is hereby amended and restated in its entirety to read as set
forth in Exhibit A (as so amended and restated, the "CREDIT AGREEMENT"):

         SECTION 3. Representations and Warranties . The Company hereby
represents and warrants that the representations and warranties set forth in the
Credit Agreement are true and correct.

         SECTION 4. Effectiveness . This Amendment shall become effective on the
date (the "EFFECTIVE DATE") on which the following conditions shall have been
<PAGE>   2

satisfied (except Sections 2.13 and 10.03 of the Credit Agreement, which shall
become effective when the Administrative Agent shall have received the documents
specified in paragraph (a) below):

             (a)      The Administrative Agent shall have received counterparts
hereof signed by each of the parties hereto (or, in the case of any party as to
which an executed counterpart shall not have been received, receipt by the
Administrative Agent in form satisfactory to it of telegraphic, telex, facsimile
or other written confirmation from such party of execution of a counterpart
hereof by such party);

             (b)      The Administrative Agent shall have received an opinion of
Hogan & Hartson L.L.P., special counsel for the Borrowers, substantially in the
form of Exhibit B hereto and covering such additional matters relating to the
transactions contemplated by the Financing Documents as the Required Banks may
reasonably request;

             (c)      The Collateral and Guarantee Requirement shall have been
satisfied;

             (d)      The New Agreement shall have become effective in
accordance with its terms;

             (e)      The Administrative Agent shall have received an amendment
fee for the account of each Bank from which the Administrative Agent shall have
received a signed counterpart hereof (or satisfactory confirmation of its
signing a counterpart hereof) not later than the date of satisfaction of the
condition in paragraph (a) in an amount equal to 0.60% of such Bank's
Commitment; and

             (f)      The Administrative Agent shall have received all documents
it may reasonably request relating to the existence of the Borrowers, the
corporate authority for and the validity of the Financing Documents, and any
other matters reasonably relevant hereto, all in form and substance reasonably
satisfactory to the Administrative Agent;

provided that this Agreement shall not become effective or be binding on any
party hereto unless the foregoing conditions are satisfied not later than
February 23, 2001. On the Effective Date the Credit Agreement will be
automatically amended and restated in its entirety to read as set forth in
Exhibit A. On and after the Effective Date the rights and obligations of the
parties hereto shall be governed by the Credit Agreement. Without limiting the
generality of the foregoing, on and after the Effective Date the provisions of
prior amendments of and waivers under the Existing Agreement (including
Amendment No. 4 thereto which by its terms has not become effective) shall be of
no further force and effect except insofar as the same are reflected in the
terms of the Credit Agreement. The Administrative Agent shall

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<PAGE>   3

promptly notify the Company and the Banks of the effectiveness of this
Agreement, and such notice shall be conclusive and binding on all parties
hereto.

         SECTION 5. Governing Law . This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

         SECTION 6. Counterparts . 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.

                                     - 3 -
<PAGE>   4

         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., f/k/a
                                       NATIONSBANK, N.A.

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

                                   FIRST UNION COMMERCIAL CORPORATION

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

                                     - 4 -
<PAGE>   5

                                   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:

                                   WACHOVIA BANK, N.A.

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

                                     - 5 -
<PAGE>   6

                                   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:

ORBITAL COMMERCIAL SYSTEMS, INC.

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

                                     - 6 -
<PAGE>   7

ORBITAL INTERNATIONAL, INC.

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

ORBITAL SERVICES CORPORATION

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

ORBITAL NAVIGATION CORPORATION

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

ORBLINK LLC

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

                                     - 7 -
<PAGE>   8
                                TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                        Page
                                                                        ----

<S>             <C>                                                     <C>
      SECTION 1.  Definitions; References.................................1
      SECTION 2.  Amendment and Restatement of the Credit Agreement.......1
      SECTION 3.  Representations and Warranties..........................1
      SECTION 4.  Effectiveness...........................................1
      SECTION 5.  Governing Law...........................................3
      SECTION 6.  Counterparts............................................3
</TABLE>

                                     - i -<PAGE>   1

                                                                 EXHIBIT 10.1.14

                                                                [EXECUTION COPY]

                           AMENDMENT NO. 14 AND WAIVER

        AMENDMENT No. 14 and WAIVER ("THIS AMENDMENT") dated as of April 12,
2001 relating to the Third Amended and Restated Credit and Reimbursement
Agreement dated as of December 21, 1998 (as the same has been amended and
restated by Amendment No. 13 and as the same may hereafter be amended from time
to time, the "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 Credit Agreement
has the meaning assigned to such term in the 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 Credit Agreement shall, after this Amendment becomes effective,
refer to the 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 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 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 to the Definitions. (a) The definition of
"REDUCTION PERCENTAGE" set forth in Section 1.01 of the Credit Agreement is
amended to read in its entirety as follows:

        "REDUCTION PERCENTAGE" means (i) with respect to any Debt Issuance,
100%, (ii) with respect to any Equity Issuance, 55%, and (iii) with respect to
any Asset Sale, (x) for any Asset Sale that occurs on or prior to September 30,
2001, (A) to the extent the aggregate Net Cash Proceeds of such Asset Sales
since the Effective Date (excluding Net Cash Proceeds of any Asset Sale applied
to prepay Loans under, or reduce the Commitments under, the New

<PAGE>   2

Agreement) do not exceed $20,000,000, 100%, (B) to the extent the aggregate Net
Cash Proceeds of such Asset Sales since the Effective Date (excluding Net Cash
Proceeds of any Asset Sale applied to prepay Loans under, or reduce the
Commitments under, the New Agreement) exceed $20,000,000 but do not exceed
$100,000,000, 43.75%, and (C) to the extent the aggregate Net Cash Proceeds of
such Asset Sales since the Effective Date (excluding Net Cash Proceeds of any
Asset Sale applied to prepay Loans under, or reduce the Commitments under, the
New Agreement) exceed $100,000,000, 70%, and (y) 70% for any Asset Sale that
occurs after September 30, 2001.

        SECTION 4. Amendment of a Certain Information Covenant. Section 5.01(o)
of the Credit Agreement is amended by deleting the words "simultaneously with"
and replacing them with the words "within five days after".

        SECTION 5. Amendment of the Minimum Consolidated Net Worth Covenant.
Section 5.08 of the 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 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 6. Amendment of the Consolidated Leverage Ratio Covenant.
Section 5.09(a) of the 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 7. Amendment of the Senior Leverage Ratio Covenant. Section
5.09(b) of the 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 8. Amendment of the Minimum Operating Cash Flow Covenant.
Section 5.10 of the 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

<PAGE>   3

        (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 9. Amendment of the Subsidiary Debt Covenant. Section 5.15 of
the 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 10. Governing Law. This Amendment shall be governed by and
construed in accordance with the laws of the State of New York.

        SECTION 11. 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 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 10.03 of the Credit Agreement.

<PAGE>   4

        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:

<PAGE>   5

                                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:

<PAGE>   6

                                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:

<PAGE>   7

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:

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