Document:

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                                                                   EXHIBIT 10.28

                      FOURTH AMENDMENT TO CREDIT AGREEMENT

              This FOURTH  AMENDMENT TO CREDIT  AGREEMENT (this  "Amendment") is
entered into as of March 9, 2000,  by and among  SpectraSite  Communications,
Inc., a Delaware  corporation (the ABorrower@),  SpectraSite  Holdings,  Inc., a
Delaware  corporation  ("Holdco"),  CIBC World Markets  Corp.  and Credit Suisse
First Boston,  as arrangers (the  "Arrangers"),  Credit Suisse First Boston,  as
syndication agent (the "Syndication Agent"), Canadian Imperial Bank of Commerce,
as administrative agent (the "Administrative Agent"),  Canadian Imperial Bank of
Commerce,  as  collateral  agent (the  "Collateral  Agent") and the other Credit
Parties signatory hereto (the "Credit Parties").

                              W I T N E S S E T H:

              WHEREAS,  the Borrower,  Holdco,  the Arrangers,  the  Syndication
Agent, the Administrative Agent, the Collateral Agent and the Credit Parties are
parties to that certain Credit  Agreement dated as of April 20, 1999, as amended
by that certain First Amendment to Credit Agreement dated as of August 23, 1999,
as further amended by that certain Second Amendment to Credit Agreement dated as
of December 22, 1999,  as further  amended by that  certain  Third  Amendment to
Credit  Agreement  dated as of  February  14,  2000 (as the same may be  further
amended,  restated,  supplemented  or otherwise  modified from time to time, the
ACredit Agreement@); and

              WHEREAS,   the  Borrower  and  Holdco  have  requested,   and  the
Arrangers, the Syndication Agent, the Administrative Agent, the Collateral Agent
and the Credit Parties have agreed,  to amend the Credit Agreement as and to the
extent set forth herein;

              NOW THEREFORE,  in  consideration of the premises set forth above,
the  terms  and  conditions   contained  herein  and  other  good  and  valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties  hereby  agree that all  capitalized  terms used  herein  shall have the
meanings ascribed thereto in the Credit Agreement,  as amended hereby, except as
otherwise  defined  or  limited  herein,  and  further  agree,  subject  to  the
conditions precedent to this Amendment hereinafter set forth, as follows:

              1.      Amendments to Article 1.

                      (a)      Article 1 of the Credit Agreement, Definitions,
is hereby modified and amended by adding the following new definitions to be
placed in appropriate alphabetical order:

                      "Additional  Telecorp Towers" shall mean up to 250 Towers
              (such  number  being  subject to  increase  or  decrease  based on
              negotiations  between the Borrower and Telecorp) to be constructed
              or  acquired  by the  Borrower,  from  time to time,  for lease

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              to Telecorp  pursuant to the future tower commitment set forth in
              the Telecorp Purchase Documents.

                      "Fourth  Amendment"  shall mean the Fourth  Amendment  to
              this Agreement dated as of March ___, 2000.

                      "Holdco  2010  Notes"  shall  mean  shall mean the Senior
              Notes and the Senior  Discount Notes, in each case due 2010 issued
              by Holdco  pursuant to the terms and  conditions  of the Indenture
              For 2010 Notes.

                      "Holdco Equity Offering" shall mean the issuance and sale
              by Holdco of  25,645,000  shares of its common  Capital Stock that
              was consummated on or about February 1, 2000.

                     "Indenture For 2010 Notes" shall mean, collectively, that
              certain  Senior Notes  Indenture to be entered into by Holdco,  as
              issuer,  and United  States Trust Company of New York, as trustee,
              in respect of the  Holdco  2010  Notes,  and that  certain  Senior
              Discount Notes Indenture to be entered into by Holdco,  as issuer,
              and United  States  Trust  Company  of New York,  as  trustee,  in
              respect of the Holdco 2010 Notes,  which Indentures shall provide,
              among other things,  that (a) interest  payable on the Holdco 2010
              Notes may be paid at a coupon no greater than, (i) with respect to
              the  Senior  Notes,  12.5%,  and (ii) with  respect  to the Senior
              Discount  Notes,  14.5%,  (b)  interest  shall not be payable with
              respect to more than  $200,000,000  of the Holdco 2010 Notes prior
              to the  expiration  of the Five  Year  PIK  Period  except  from a
              Pre-Funded  Interest Account,  and (c) the Holdco 2010 Notes shall
              have no required cash redemptions  (other than customary change of
              control  redemption  provisions) or principal  maturities prior to
              the Tranche B Maturity Date.

                      "Target  Acquisition" shall mean that certain Acquisition
              of Tower Assets in conjunction  with up to 110 Tower Sites,  which
              shall be  consummated  by the  Borrower  on terms  and  conditions
              reasonably  satisfactory  to  the  Administrative  Agent,  for  an
              aggregate Purchase Price not to exceed $180,000,000.

                    "Telecorp"  shall mean Telecorp PCS,  Inc., a [_________]
              corporation.

                      "Telecorp  Acquisition" shall mean the Acquisition by the
              Borrower  of  certain   Tower  Assets  from   Telecorp   including
              approximately  205 Tower Sites for an aggregate  Purchase Price of
              approximately  $61,500,000,  and the  related  lease to  Telecorp,
              under a master lease  agreement,  of space on such Towers (and any
              Additional  Telecorp  Towers) and ground  space on the land in the
              vicinity  of such  Towers (and any  Additional  Telecorp  Towers),
              pursuant  to the terms and  conditions  of the  Telecorp  Purchase
              Documents.

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                      "Telecorp  Purchase  Documents"  shall mean the documents
              relating to the Telecorp  Acquisition,  which shall  contain terms
              and  conditions  reasonably  satisfactory  to  the  Administrative
              Agent,  as such  documents  may be  amended  from  time  to  time,
              together  with  all  schedules  and  exhibits  thereto  and  other
              documents executed in connection therewith.

                      "UK Investment" shall mean the Investment  (including the
              purchase of  approximately  fifty  percent (50%) of the issued and
              outstanding  equity  interests  in such joint  venture) by Holdco,
              directly  or  indirectly,  in  a  joint  venture  with  an  entity
              organized under the laws of the United Kingdom and having a market
              capitalization  in excess of  $5,000,000,000,  for the  purpose of
              engaging   in  the   business   of  owning,   leasing,   managing,
              Abuilding-out@  and constructing  Towers in the United Kingdom and
              Europe, in an aggregate amount not to exceed,  (a) with respect to
              stock  consideration,  $100,000,000  in  common  Capital  Stock of
              Holdco  (or  assets  of Holdco  acquired  in  connection  with the
              issuance  of such  Capital  Stock),  and (b) with  respect to cash
              consideration, $100,000,000 in cash.

                      "UK  Acquisition  Documents"  shall  mean  the  documents
              relating to the UK  Investment,  as amended from time to time, and
              all schedules and exhibits thereto and other documents executed in
              connection therewith."

                      (b)      Article 1 of the Credit Agreement, Definitions,
is hereby further modified and amended by deleting the existing definition of
AIndentures@ and by substituting the following in lieu thereof:

                      "Indentures" shall mean, collectively,  the Indenture For
              2008 Notes,  the  Indenture  For 2009 Notes and the  Indenture For
              2010 Notes."

                      (c)      Article 1 of the Credit Agreement, Definitions,
is hereby further modified and amended by deleting the existing definition of
APre-Approved Transactions@ and by substituting the following in lieu thereof:

                      "Pre-Approved Transactions= shall mean, collectively, the
              Apex   Acquisition,   the  DigiPH   Acquisition,   the   UbiquiTel
              Investment,  the  AirTouch  Acquisition,  the UK  Investment,  the
              Nextel  2   Acquisition,   the   Telecorp   Acquisition   and  the
              Acquisitions described in Sections 8.5(xiii) and (xv) hereof."

                      (d)      Article 1 of the Credit Agreement, Definitions,
is hereby further modified and amended by deleting the existing definition of
"Pre-Approved Transaction Documents" and by substituting the following in lieu
thereof:

                      "Pre-Approved    Transaction   Documents=   shall   mean,
              collectively,  the Westower Merger  Agreement,  the NTA Investment
              Documents,  the Apex  Acquisition  Documents,  the DigiPH Purchase
              Documents,   the  UbiquiTel  Investment  Documents,

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              the  AirTouch Purchase Documents, the Nextel 2 Purchase
              Documents, UK Investment Documents, the Telecorp Purchase
              Documents, the documents relating to the  acquisition of
              Additional  Nextel Towers and the documents relating to the
              Acquisitions  described in Section  8.5(xiii) and (xv) hereof
             (including all schedules and exhibits thereto)."

                      (e)      Article 1 of the Credit Agreement, Definitions,
is hereby further modified and amended by adding the following sentence at the
end of the existing definition of "Subsidiary":

                      "Notwithstanding  anything  contained in the  foregoing to
              the contrary,  neither the UK Investment nor any entity formed for
              the purpose of making the UK Investment that is owned, directly or
              indirectly,  by Holdco shall be deemed a  Subsidiary  of Holdco or
              the Borrower for purposes of this Agreement."

              2. Amendment to Section 2.7. Section 2.7 of the Credit  Agreement,
Mandatory   Repayments,   is  hereby   modified  and  amended  by  deleting  the
parenthetical beginning in the second line of subsection (c) and by substituting
the following in lieu thereof:

                      "(other  than the  issuance of the Holdco 2009 Notes in an
              amount not to exceed  $340,003,656  and the issuance of the Holdco
              2010 Notes in an amount not to exceed $500,000,000)"

              3.  Amendment  to  Section  5.1.  Section  5.1(h)  of  the  Credit
Agreement,  Title to Assets, is hereby modified and amended by deleting the last
sentence therefrom and by substituting the following in lieu thereof:

              "Tower  Sub does not own any  material  Assets  other  than  Tower
              Assets  comprising  the Nextel  Collateral  and Tower  Space Lease
              Agreements  with  Co-Locators  on  Towers  comprising  the  Nextel
              Collateral, and Holdco does not own any material Assets other than
              (i) the Capital Stock of the Borrower,  (ii) the UK Investment (or
              the Capital  Stock of any entity  formed for the purpose of making
              the  UK  Investment),   (iii)  the  Capital  Stock  of  any  shell
              Subsidiary formed by Holdco solely for the purpose of consummating
              any  Permitted  Acquisition,  and (iv) on the closing  date of any
              Permitted  Acquisition,  the Capital  Stock of any target  company
              acquired in connection with such Permitted Acquisition immediately
              prior to the contribution of such Capital Stock to the Borrower."

              4. Amendment to Section 6.2. Section 6.2 of the Credit  Agreement,
Business;  Compliance  with  Applicable  Law, is hereby  amended by deleting the
second sentence therefrom and substituting the following in lieu thereof:

                      "Holdco will engage  solely in the business of holding the
              Capital Stock of (a) the Borrower,  (b) the UK Investment  (or the
              Capital  Stock of any entity  formed for

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              the purpose of making the UK Investment),  (c) any shell
              Subsidiary  formed by Holdco solely for the purpose of
              consummating any Permitted Acquisition, and (d) on the  closing
              date of any  Permitted  Acquisition,  any  target company
              acquired in connection  with such  Permitted  Acquisition
              immediately prior to the contribution of such Capital Stock to the
              Borrower."

              5. Amendments to Section 6.16.

                      (a)      Section 6.16 of the Credit Agreement, Covenants
Regarding Formation of Subsidiaries and the Making of Investments  and
Acquisitions,  is hereby  modified and amended by deleting the parenthetical
beginning  after the words Asuch  Subsidiary or Person@ in clause (b) of the
first sentence and by substituting the following in lieu thereof:

              "(other than (i) the Foreign Westower Subsidiaries,  in which case
              the pledge shall be of sixty-five percent (65%) (or in the case of
              Westower Highlight Do Brazil,  sixty percent (60%)) of the Capital
              Stock  of such  Subsidiary  and  (ii)  the UK  Investment  (or the
              Capital  Stock of any entity  formed for the purpose of making the
              UK Investment))"

                      (b)  Section  6.16  of  the  Credit  Agreement,  Covenants
Regarding Formation of Subsidiaries and the Making of
Investments  and  Acquisitions,  is  hereby  further  modified  and  amended  by
inserting  A(other than the UK  Investment  (or the Capital  Stock of any entity
formed for the purpose of making the UK Investment))@  immediately following the
words Aafter the Agreement Date@ in the second sentence thereof.

              6.  Amendment  to  Article 6.  Article 6 of the Credit  Agreement,
Affirmative  Covenants,  is hereby  modified and amended by adding the following
new Section 6.17 at the end thereof:

                      "Section 6.17 Special Purpose Vehicles. The Borrower shall
              form one or more special purpose Subsidiaries (the ASPVs") for the
              purpose  of  holding  and  making  Acquisitions  of  Tower  Assets
              permitted  under Section 8.5 hereof,  and with respect to all such
              Acquisitions  completed on and after March ___, 2000, the Borrower
              shall, and shall cause each of its Subsidiaries to, contribute all
              Towers  and  Tower  Assets   acquired  by  the  Borrower  or  such
              Subsidiary to an SPV. The Borrower  shall comply with Section 6.16
              hereof in respect  of each SPV.  Notwithstanding  anything  to the
              contrary  contained in this  Agreement,  the following  provisions
              shall apply with respect to SPVs:  (a) any SPV may transfer  Tower
              Assets to Tower Sub to the  extent  that such Tower  Assets  shall
              constitute  Future  Nextel  Towers;  (b) none of the SPVs may make
              Investments  otherwise permitted to be made by Subsidiaries of the
              Borrower pursuant to Section 8.2 or 8.5(v) hereof; (c) none of the
              SPVs may liquidate or dissolve  itself (or suffer any  liquidation
              or dissolution),  or enter into any merger, consolidation or other
              business   combination   (except  with  another  SPV),   otherwise
              permitted to be made by Subsidiaries  of the Borrower  pursuant to
              Section
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              8.5(ii)   hereof;   (d)   none  of  the  SPVs  may  form
              Subsidiaries;   (e)  SPVs  may  transfer   Assets  freely  amongst
              themselves, but not to the Borrower or any other Subsidiary of the
              Borrower  that does not  constitute  an SPV;  (f) none of the SPVs
              shall create,  assume, incur or remain obligated in respect of, or
              permit to be  outstanding,  any  Indebtedness  except for trade or
              accounts  payable and other  obligations,  and  accrued  expenses,
              incurred  in the  ordinary  course  of  business,  other  than for
              borrowed  money;  and (g) none of the SPVs shall be  obligated  in
              respect of any  Guaranty  of the  performance  obligations  of the
              Borrower  or any of its  Subsidiaries  (other  than under the Loan
              Documents).  Neither Tower Sub nor  California Sub shall be deemed
              SPVs for purposes of this Agreement."

              7. Amendments to Section 8.1.

                      (a)    Section 8.1 of the Credit Agreement, Indebtedness,
is hereby modified and amended by deleting clause (i) from subsection (h) and by
substituting the following in lieu thereof:

              "(i) Indebtedness represented by the Holdco 2008 Notes, the Holdco
2009 Notes and the Holdco 2010 Notes,"

                      (b) Section 8.1 of the Credit Agreement,  Indebtedness, is
hereby further modified and amended by deleting
clause (iii) from subsection (h) and by substituting the following in lieu
thereof:

              "(iii) accrual of interest, accrual of dividends, the accretion of
              accreted value,  the payment of interest in the form of additional
              Indebtedness   and  the  payment  of  dividends  in  the  form  of
              additional  shares in respect of the Holdco 2008 Notes, the Holdco
              2009  Notes,   the  Holdco  2010  Notes  or  any  other  Permitted
              High-Yield Securities,"

                      (c) Section 8.1 of the Credit Agreement,  Indebtedness, is
hereby further modified and amended by deleting
the phrase Aeither of the Indentures"  from clause (iv) of subsection (h) and by
substituting Aany of the Indentures" in lieu thereof.

              8. Amendments to Section 8.5.

                      (a) Section 8.5 of the Credit Agreement, Liquidation;
Merger; Acquisition or Disposition of Assets, is hereby modified and amended by
deleting  existing  subsection  (xii) thereof and substituting the following in
lieu thereof:

                      "(xii)  so long as no  Default  or Event of  Default  then
              exists or would be caused  thereby,  subject  to  compliance  with
              Sections   6.10  and  6.16  hereof,   (A)  the  Borrower  and  its
              Subsidiaries  (other  than  Tower Sub and  California  Tower)  may
              consummate each of the Pre-Approved  Transactions  (other than the
              Nextel  2
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              Acquisition,  the UK  Investment  and  the  transaction described
              in  clause  (xiii)  below),  and (B) the  Borrower  may construct
              or purchase the Additional  Telecorp  Towers;  provided,  however,
              that  with  respect  to the  Telecorp  Acquisition,  the Borrower
              shall provide the Arrangers with evidence satisfactory to them
              that  Holdco  shall have  received at least  $400,000,000  of
              proceeds in connection with the issuance of the 2010 Notes (or, in
              the   alternative,   provide   the   Arrangers   and  the  Lenders
              calculations demonstrating pro forma compliance with the Financial
              Covenants after giving effect to the Telecorp Acquisition)."

                      (b)   Section 8.5 of the Credit Agreement, Liquidation;
Merger; Acquisition or Disposition of Assets, is hereby  further  modified and
amended by adding the  following  new  subsections (xiv) and (xv) at the end
thereof:

                      "(xiv)  so long as no  Default  or Event of  Default  then
              exists or would be caused thereby, and notwithstanding anything to
              the contrary contained herein or in any Loan Document, Holdco may,
              directly or indirectly,  make and hold the UK Investment,  so long
              as the cash portion of the Purchase Price is from the Net Proceeds
              received by Holdco in  connection  with the issuance of the Holdco
              2010 Notes or the Holdco Equity Offering; and

                      "(xv) so long as (A) no Default  or Event of Default  then
              exists or would be caused thereby,  (B) the Borrower shall provide
              the Arrangers with evidence satisfactory to them that Holdco shall
              have received at least $400,000,000 of proceeds in connection with
              the  issuance of the 2010 Notes (or, in the  alternative,  provide
              the Arrangers and the Lenders calculations demonstrating pro forma
              compliance  with the  Financial  Covenants  after giving effect to
              such  Acquisition),   and  (C)  the  Borrower  shall  provide  the
              Administrative  Agent with  evidence  satisfactory  to it that the
              revenue and cash flow acquired by the Borrower in connection  with
              the Target Acquisition shall have no material negative  deviations
              from the  projected  revenue and cash flow  relating to the Target
              Acquisition as set forth in the Borrower=s  Projections  delivered
              to  the  Administrative   Agent  in  connection  with  the  Fourth
              Amendment,  the Borrower may,  subject to compliance with Sections
              6.10, 6.16 and 6.17 hereof, consummate the Target Acquisition."

              9. Amendments to Section 8.7.

                      (a)      Section 8.7 of the Credit Agreement, Restricted
Payments and Purchases, is hereby modified and amended by deleting clause (a)
and by substituting the following in lieu thereof:

             "(a) (i)  interest  payments  on the Holdco  2008 Notes and on the
              Holdco  2009  Notes,  (ii)  interest  payments  on the Holdco 2010
              Notes, and (iii) interest or dividend payments, as applicable,  on
              Permitted High-Yield  Securities issued on or before the

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              Agreement Date, in the case of clauses (i) and (iii),  following
              expiration of  the  Five  Year  PIK  Period  or  from a
              Pre-Funded  Interest Account,"

                      (b)  Section  8.7  of  the  Credit  Agreement,  Restricted
Payments and Purchases, is hereby further modified and
amended by deleting clause (e) and by substituting the following in lieu thereof

           "(e) payments of >Additional Interest= (as that term is defined in
              the Registration Rights Agreements entered into in connection with
              the Holdco 2008  Notes,  the Holdco 2009 Notes and the Holdco 2010
              Notes)  and any other  comparable  payments  in  respect  of other
              Permitted High-Yield Securities"

              10. Amendment to Section 8.9. Section 8.9 of the Credit Agreement,
Corporate Name;  Corporate  Structure;  Business,  is hereby amended by deleting
subsection (b) therefrom and by substituting the following in lieu thereof:

                      "(b) with respect to the  Borrower  and its  Subsidiaries,
              engage in any businesses  other than the Tower  Operations and the
              Other Operations and activities  related or incident thereto,  and
              with respect to Holdco,  engage in any business other than that of
              holding  the  Capital  Stock  of (i)  the  Borrower,  (ii)  the UK
              Investment  (or the  Capital  Stock of any  entity  formed for the
              purpose of making the UK Investment),  (iii) any shell  Subsidiary
              formed  by Holdco  solely  for the  purpose  of  consummating  any
              Permitted  Acquisition,  and  (iv)  on  the  closing  date  of any
              Permitted  Acquisition,  any target company acquired in connection
              with  such  Permitted   Acquisition   immediately   prior  to  the
              contribution of such Capital Stock to the Borrower"

              11.  Amendment  to  Section  10.1.  Section  10.1  of  the  Credit
Agreement,  Events of  Default  (Senior  Obligations),  is hereby  modified  and
amended by deleting the phrase Aeither of the  Indentures"  from  subsection (m)
and by substituting Aany of the Indentures@ in lieu thereof.

              12. No Other  Amendments,  Waivers  or  Consents.  Except  for the
amendments,  waivers  and  consents  set  forth  above,  the text of the  Credit
Agreement and the other Loan Documents shall remain  unchanged and in full force
and effect, and the Arrangers,  the Syndication Agent, the Administrative Agent,
the Collateral  Agent and the Credit Parties hereby reserve the right to require
strict  compliance  with the terms of the  Credit  Agreement  and the other Loan
Documents in the future.

              13. Conditions to Effectiveness. This Amendment shall be effective
as of the date first written above (the AEffective Date@) upon the following:

                      (a)      The Administrative Agent=s receipt of a
counterpart hereof duly executed by the Borrower and Holdco, and by the Majority
Lenders;
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                      (b) With respect to issuance of the Holdco 2010 Notes, the
              Arrangers" receipt of calculations  demonstrating  the Borrower=s
              pro forma compliance with the Credit Agreement through the Tranche
              B Maturity Date; and

                      (c) All of the  representations  and  warranties of Holdco
              and the  Borrower  set  forth  in the  Credit  Agreement  and this
              Amendment,  other  than  those  that  are  expressly  made as of a
              specific date, are true and correct in all material  respects with
              the same effect as though such  representations and warranties had
              been made on and as of the Effective Date as though made on and as
              of such date.

              14.  Amendment  Fee.  The  Borrower  hereby  agrees to pay, on the
Effective  Date, to each Lender that delivers such Lender=s  written  consent to
this Amendment to counsel for the  Administrative  Agent prior to 5:00 p.m. (New
York time) on Thursday,  March 9, 2000,  an amendment  fee in an amount equal to
one-tenth of one percent (0.10%) of such Lender=s Revolving  Commitment and Term
Loans then outstanding.

              15.  Conditions  Subsequent.  As a  condition  subsequent  to  the
amendments set forth in this  Amendment,  the Borrower shall perform or cause to
be performed the  following  (the failure by the Borrower to so perform or cause
to be performed for any reason constituting an Event of Default under the Credit
Agreement):

                      (a)  Collateral.  Promptly  following the closing date for
              each of the Telecorp  Acquisition and the Target Acquisition,  the
              Borrower  shall  execute and deliver to the  Collateral  Agent all
              agreements,   instruments  and  other  items  required  to  be  so
              delivered  pursuant  to  Section  6.16  of the  Credit  Agreement,
              including,  without  limitation,  a collateral  assignment  of the
              Telecorp Purchase Documents in favor of the Collateral Agent; and

                      (b) Definitive Documentation. Promptly upon the closing of
              each of the Telecorp  Acquisition and the Target Acquisition,  the
              Borrower  shall  deliver to the  Arrangers a full set of copies of
              the documents  executed in connection with each such  Acquisition.
              Promptly upon  consummation  of the  transactions  contemplated in
              connection  with  the  issuance  of the  Holdco  2010  Notes,  the
              Borrower  shall  deliver to the  Arrangers a full set of copies of
              the  Indenture  For 2010  Notes,  the  Holdco  2010  Notes and all
              documents executed in connection therewith.

              16.  Representations  and  Warranties.  Each of the  Borrower  and
Holdco, for itself and on behalf of each of its Subsidiaries, agrees, represents
and  warrants  in  favor  of  the  Arrangers,   the   Syndication   Agent,   the
Administrative Agent, the Collateral Agent and the Credit Parties that:

                      (a) This Amendment has been executed and delivered by duly
              authorized  representatives  of the Borrower  and Holdco,  and the
              Credit  Agreement,  as  modified  and  amended by this  Amendment,
              constitutes a legal,  valid and binding obligation of

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              the Borrower and Holdco and is  enforceable  against the Borrower
              and Holdco in accordance with its terms, except as enforceability
              may be limited by  bankruptcy,   insolvency,   reorganization   or
              similar  laws affecting  creditors'  rights  generally and by the
              application of general equitable principles;

                      (b) Before and after giving effect to this  Amendment,  no
              Default or Event of Default with respect to the Borrower or Holdco
              has occurred and is continuing;

                      (c) As of the date hereof and after  giving  effect to the
              issuance  of the  Holdco  2010  Notes,  (i)  the  property  of the
              Borrower,  at a fair  valuation  on a going  concern  basis,  will
              exceed  its debt;  (ii) the  capital of the  Borrower  will not be
              unreasonably small to conduct its business; and (iii) the Borrower
              will not have  incurred  debts,  or have  intended to incur debts,
              beyond its ability to pay such debts as they mature; and

                      (d) No event  contemplated in connection with the issuance
              of the Holdco 2010 Notes shall occur, which has not been consented
              to or waived,  the  occurrence of which  constitutes,  or with the
              passage  of time or giving of notice or both would  constitute,  a
              material  default  by  Holdco,   the  Borrower  or  any  of  their
              respective Subsidiaries under any material indenture, agreement or
              other  instrument,  including,  without  limitation,  the material
              Necessary  Authorizations  and the  Indentures,  or any  judgment,
              decree or order,  to which  Holdco,  the  Borrower or any of their
              respective  Subsidiaries  is a  party  or  by  which  Holdco,  the
              Borrower or any of their  respective  Subsidiaries or any of their
              respective properties may be bound or affected.

              17.  Effect  on  the  Credit  Agreement.  Except  as  specifically
provided herein, the Credit Agreement shall remain in full force and effect, and
is hereby ratified,  reaffirmed and confirmed. This Amendment shall be deemed to
be a Loan Document for all purposes.

              18. Counterparts.  This Amendment may be executed in any number of
separate   counterparts  and  by  the  different   parties  hereto  on  separate
counterparts,  each of which shall be deemed an original and all of which, taken
together, shall be deemed to constitute one and the same instrument.  In proving
this Amendment in any judicial proceedings, it shall not be necessary to produce
or account for more than one such  counterpart  signed by the party against whom
such  enforcement is sought.  Any  signatures  delivered by a party by facsimile
transmission shall be deemed an original signature hereto.

              19. Law of Contract. THIS CONSENT SHALL BE CONSTRUED IN ACCORDANCE
WITH AND GOVERNED BY THE INTERNAL  LAWS OF THE STATE OF NEW YORK  APPLICABLE  TO
CONTRACTS MADE AND TO BE PERFORMED WHOLLY WITHIN SUCH STATE.

                [REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                       10

<PAGE>   11

                      IN WITNESS WHEREOF,  this Amendment has been duly executed
as of the day and year first written above.

BORROWER:                      SPECTRASITE COMMUNICATIONS, INC.

                                        By:      /s/ David P. Tomick
                                           -----------------------------------
                                        Name:         David P. Tomick
                                             ---------------------------------
                                        Title:        Executive Vice President
                                              --------------------------------

                                    Attest:
                                             --------------------------------
                                     Name:
                                             --------------------------------
                                     Title:
                                             --------------------------------

HOLDCO:                        SPECTRASITE HOLDINGS, INC.

                                        By:      /s/ David P. Tomick
                                             --------------------------------
                                        Name:         David P. Tomick
                                             --------------------------------
                                        Title:        Executive Vice President
                                             --------------------------------

                                    Attest:
                                             --------------------------------
                                     Name:
                                             --------------------------------
                                     Title:
                                             --------------------------------

<PAGE>   12

ADMINISTRATIVE                 CANADIAN IMPERIAL BANK OF
AGENT:                                  COMMERCE

                                        By:      /s/  Colleen Risorto
                                             --------------------------------
                                        Name:          Colleen Risorto
                                             --------------------------------
                                        Title:         Executive Director
                                             --------------------------------
                                             CIBC World Markets Corp. as Agent

<PAGE>   13

ARRANGERS:                               CIBC WORLD MARKETS CORP.
                                        (f/k/a CIBC Oppenheimer Corp.)

                                        By:      /s/  Colleen Risorto
                                             --------------------------------
                                        Name:          Colleen Risorto
                                             --------------------------------
                                        Title:         Executive Director
                                             --------------------------------
                                             CIBC World Markets Corp. as Agent

                                        CREDIT SUISSE FIRST BOSTON

                                        By:      /s/  James P. Moran
                                             --------------------------------
                                        Name:          James P. Moran
                                             --------------------------------
                                        Title:              Director

                                        By:      /s/  Kristin Lepri
                                             --------------------------------
                                        Name:          Kristin Lepri
                                             --------------------------------
                                        Title:         Associate
                                             --------------------------------

<PAGE>   14

COLLATERAL AGENT:     CANADIAN IMPERIAL BANK OF COMMERCE

                                        By:      /s/  Colleen Risorto
                                             --------------------------------
                                        Name:          Colleen Risorto
                                             --------------------------------
                                        Title:         Executive Director
                                             --------------------------------
                                             CIBC World Markets Corp. as Agent

<PAGE>   15

SYNDICATION AGENT:    CREDIT SUISSE FIRST BOSTON

                                        By:      /s/  James P. Moran
                                             --------------------------------
                                        Name:          James P. Moran
                                             --------------------------------
                                        Title:              Director
                                             --------------------------------

                                        By:      /s/  Kristin Lepri
                                             --------------------------------
                                        Name:          Kristin Lepri
                                             --------------------------------
                                        Title:         Associate
                                             --------------------------------

<PAGE>   16

MANAGING AGENTS:               BANK OF MONTREAL, CHICAGO BRANCH

                                        By:      /s/  Karen Klapper
                                             --------------------------------
                                        Name:          Karen Klapper
                                             --------------------------------
                                        Title:         Director
                                             --------------------------------

                                        THE BANK OF NOVA SCOTIA

                                        By:      /s/  P.A. Weissenberger
                                             --------------------------------
                                        Name:       P.A. Weissenberger
                                             --------------------------------
                                        Title:         Authorized Signatory
                                             --------------------------------

                                   FLEET NATIONAL BANK (f/k/a BankBoston, N.A.)

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

                                   DRESDNER BANK AG, NEW YORK AND GRAND
                                   CAYMAN BRANCHES

                                        By:      /s/  Patrick A. Kelehe
                                             --------------------------------
                                        Name:       Patrick A. Kelehe
                                             --------------------------------
                                        Title:         Vice President
                                             --------------------------------

                                        By:      /s/  Brian E. Haughne
                                             --------------------------------
                                        Name:       Brian E. Haughney
                                             --------------------------------
                                        Title:         Assistant Vice President
                                             --------------------------------

                                        TORONTO DOMINION (TEXAS), INC.

                                        By:      /s/  Ann Slanis
                                             --------------------------------
                                        Name:          Ann Slanis
                                             --------------------------------
                                        Title:          Vice President
                                             --------------------------------

<PAGE>   17

                                        UNION BANK OF CALIFORNIA, N.A.

                                        By:      /s/  Peter C. Connoy
                                             --------------------------------
                                        Name:       Peter C. Connoy
                                             --------------------------------
                                        Title:         Vice President
                                             --------------------------------

<PAGE>   18

CO-AGENT:                      CREDIT LYONNAIS NEW YORK BRANCH

                                        By:      /s/  Patrick McCarthy
                                             --------------------------------
                                        Name:          Patrick McCarthy
                                             --------------------------------
                                        Title:         Authorized Signature
                                             --------------------------------

<PAGE>   19

LENDERS:                       CIBC INC.

                                        By:      /s/  Colleen Risorto
                                             --------------------------------
                                        Name:          Colleen Risorto
                                             --------------------------------
                                        Title:           Executive Director
                                             --------------------------------
                                             CIBC World Markets Corp. as Agent

<PAGE>   20

                                        CREDIT SUISSE FIRST BOSTON

                                        By:      /s/ James P. Moran
                                             --------------------------------
                                        Name:         James P. Moran
                                             --------------------------------
                                        Title:             Director
                                             --------------------------------

                                        By:      /s/  Kristin Lepri
                                             --------------------------------
                                        Name:          Kristin Lepri
                                             --------------------------------
                                        Title:
                                             --------------------------------

<PAGE>   21

                                        BANK OF MONTREAL, CHICAGO BRANCH

                                        By:      /s/  Karen Klapper
                                             --------------------------------
                                        Name:          Karen Klapper
                                             --------------------------------
                                        Title:             Director
                                             --------------------------------

<PAGE>   22

                                        THE BANK OF NOVA SCOTIA

                                        By:      /s/  P.A. Weissenberger
                                             --------------------------------
                                        Name:          P.A. Weissenberger
                                             --------------------------------
                                        Title:            Authorized Signatory
                                             --------------------------------

<PAGE>   23

                                    FLEET NATIONAL BANK (f/k/a BankBoston, N.A.)

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

<PAGE>   24

              DRESDNER BANK AG, NEW YORK AND GRAND CAYMAN BRANCHES

                                        By:      /s/  Patrick A. Keleher
                                             --------------------------------
                                        Name:          Patrick A. Keleher
                                             --------------------------------
                                        Title:            Vice President
                                               --------------------------------

                                        By:      /s/  Brian E. Haughney
                                             --------------------------------
                                        Name:          Brian E. Haughney
                                             --------------------------------
                                        Title:         Assistant Vice President
                                                 -------------------------------

<PAGE>   25

                                        TORONTO DOMINION (TEXAS), INC.

                                        By:      /s/  Ann S. Slanis
                                             --------------------------------
                                        Name:          Ann S. Slanis
                                             --------------------------------
                                        Title:            Vice President
                                               --------------------------------

<PAGE>   26

                                        UNION BANK OF CALIFORNIA, N.A.

                                        By:      /s/  Peter C. Connoy
                                             --------------------------------
                                        Name:          Peter C. Connoy
                                             --------------------------------
                                        Title:               Vice President
                                             --------------------------------

<PAGE>   27

                                        CREDIT LYONNAIS NEW YORK BRANCH

                                        By:      /s/  Patrick McCarthy
                                             --------------------------------
                                        Name:          Patrick McCarthy
                                             --------------------------------
                                        Title:           Authorized Signature
                                             --------------------------------

<PAGE>   28

                                        THE BANK OF NEW YORK

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

<PAGE>   29

                                        CAISSE DE DEPOT ET PLACEMENT DU QUEBEC

                                        By:      /s/  Louie Lavoie
                                             --------------------------------
                                        Name:          Louie Lavoie
                                             --------------------------------
                                        Title:              Manager
                                             --------------------------------

                                        By:      /s/  Lucie Rousseau
                                             --------------------------------
                                        Name:          Lucie Rousseau
                                               --------------------------------
                                        Title:                  Vice President
                                               --------------------------------

<PAGE>   30

                                        CANADIAN IMPERIAL BANK OF COMMERCE

                                        By:      /s/  Colleen Risorto
                                             --------------------------------
                                        Name:          Colleen Risorto
                                             --------------------------------
                                        Title:           Executive Director
                                             --------------------------------
                                              CIBC World Markets Corp. as Agent

<PAGE>   31

                                        THE CIT GROUP/EQUIPMENT FINANCING, INC.

                                        By:      /s/  Daniel E. A. Nichols
                                             --------------------------------
                                        Name:          Daniel E. A. Nichols
                                             --------------------------------
                                        Title          Assistant Vice President
                                             --------------------------------

<PAGE>   32

                                        CYPRESSTREE INVESTMENT FUND, LLC

                                        By:      CypressTree Investment
                                                 Management Company, Inc., its
                                                 Managing Member

                                        By:      /s/  Peter K. Merrill
                                             --------------------------------
                                        Name:          Peter K. Merrill
                                             --------------------------------
                                        Title:              Managing Director
                                             --------------------------------

<PAGE>   33

                                        NORTHAMERICAN SENIOR FLOATING RATE FUND

                                        By:      CypressTree Investment
                                                 Management Company, Inc., its
                                                 Managing Member

                                        By:      /s/  Peter K. Merrill
                                             --------------------------------
                                        Name:          Peter K. Merrill
                                             --------------------------------
                                        Title:              Managing Director
                                             --------------------------------

<PAGE>   34

                                CYPRESSTREE INVESTMENT MANAGEMENT COMPANY, INC.

                                        As:      Attorney-in-Fact and on behalf
                                                 of First Allmerica Financial
                                                 Life Insurance Company as
                                                 Portfolio Manager

                                        By:      /s/  Peter K. Merrill
                                             --------------------------------
                                        Name:          Peter K. Merrill
                                             --------------------------------
                                        Title:              Managing Director
                                             --------------------------------

<PAGE>   35

                                        KZH CYPRESSTREE-1 LLC

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

<PAGE>   36

                                        FREMONT INVESTMENT & LOAN

                                        By:      /s/  Kannika Viravan
                                             --------------------------------
                                        Name:       Kannika Viravan
                                             --------------------------------
                                        Title:         Vice President
                                             --------------------------------

<PAGE>   37

                                        HELLER FINANCIAL, INC.

                                        By:      /s/  Scott Ziemke
                                             --------------------------------
                                        Name:          Scott Ziemke
                                             --------------------------------
                                        Title:      Assistant Vice President
                                             --------------------------------

<PAGE>   38

                                        MERRILL LYNCH SENIOR FLOATING RATE FUND,
                                         INC.

                                        By:      /s/  Joseph Matteo
                                             --------------------------------
                                        Name:          Joseph Matteo
                                             --------------------------------
                                        Title:
                                             --------------------------------

<PAGE>   39

                                        PPM AMERICA, INC., as attorney in fact,
                                        on behalf of Jackson National Life
                                        Insurance Company

                                        By:      /s/  John Walding
                                             --------------------------------
                                        Name:          John Walding
                                             --------------------------------
                                        Title:             Managing Director
                                             --------------------------------

<PAGE>   40

                                        Fleet  National Bank (f/k/a  BankBoston,
                                        N.A.),   as  Trust   Administrator   for
                                        LONGLANE MASTER TRUST IV

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

<PAGE>   41

                                        GALAXY CLO 1999-1, LTD.

                                        By:      SAI Investment Advisor, Inc.,
                                                 its Collateral Manager

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

<PAGE>   42

                                        OXFORD STRATEGIC INCOME FUND

                                        By:      Eaton Vance Management, as
                                                 Investment Advisor

                                        By:      /s/  Payson F. Swaffield
                                             --------------------------------
                                        Name:          Payson F. Swaffield
                                             --------------------------------
                                        Title:           Vice President
                                             --------------------------------

<PAGE>   43

                                        SENIOR DEBT PORTFOLIO

                                        By:      Boston Management and Research,
                                                 as Investment Advisor

                                        By:      /s/  Payson F. Swaffield
                                             --------------------------------
                                        Name:          Payson F. Swaffield
                                             --------------------------------
                                        Title:              Vice President
                                             --------------------------------

<PAGE>   44

                                        J.H. WHITNEY MARKET VALUE FUND, L.P.

                                        By:      J.H. Whitney Value GP, Ltd.,
                                                 its General Partner

                                        By:      /s/  Daniel J. O'Brien
                                             --------------------------------
                                        Name:       Daniel J. O'Brien
                                             --------------------------------
                                        Title:
                                             --------------------------------

<PAGE>   45

                                        J.H. WHITNEY MEZZANINE FUND, L.P.

                                  By:         Whitney GP, Ltd. - General Partner
                                               -------------------------------

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

<PAGE>   46

                                        BANK OF AMERICA, N.A.

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

<PAGE>   47

                                        DEBT STRATEGIES FUND I, INC.

                                        By:      /s/  Joseph Matte
                                             --------------------------------
                                        Name:          Joseph Matteo
                                             --------------------------------
                                        Title:
                                             --------------------------------

<PAGE>   48

                                        DEBT STRATEGIES FUND II, INC.

                                        By:      /s/  Joseph Matte
                                             --------------------------------
                                        Name:          Joseph Matteo
                                             --------------------------------
                                        Title:
                                             --------------------------------

<PAGE>   49

                                        THE FUJI BANK, LIMITED, NEW YORK BRANCH

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

<PAGE>   50

                                        VAN KAMPEN SENIOR INCOME TRUST

                                        By:      /s/  Darvin D. Pierce
                                             --------------------------------
                                        Name:       Darvin D. Pierce
                                             --------------------------------
                                        Title:         Vice President
                                             --------------------------------

<PAGE>   51

                                        PROSPECT INTERNATIONAL DEBT
                                        STRATEGY FUND

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

<PAGE>   52

                                        EATON VANCE SENIOR INCOME TRUST

                                        By:      /s/ Payson F. Swaffield
                                             --------------------------------
                                        Name:         Payson F. Swaffield
                                             --------------------------------
                                        Title:                 Vice President
                                             --------------------------------

<PAGE>   53

                                 MERRILL LYNCH SENIOR FLOATING RATE FUNDII, INC.

                                        By:      /s/  Joseph Matteo
                                             --------------------------------
                                        Name:          Joseph Matteo
                                             --------------------------------
                                        Title:
                                              --------------------------------

<PAGE>   54

                                   MORGAN STANLEY DEAN WITTER PRIME INCOME TRUST

                                        By:      /s/  Sheila Finnerty
                                             --------------------------------
                                        Name:       Sheila Finnerty
                                             --------------------------------
                                        Title:         Senior Vice President
                                             --------------------------------

<PAGE>   55

                                        SAWGRASS TRADING LLC

                                        By:      /s/  Kelly C. Walker
                                             ----------------------------
                                        Name:          Kelly C. Walker
                                             ----------------------------
                                        Title:               Vice President
                                             ----------------------------

<PAGE>   56

                                        BANC OF AMERICA SECURITIES LLC

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

<PAGE>   57

                                        FIRST DOMINION FUNDING III

                                        By:      /s/  Andrew H. Marshak
                                             ------------------------
                                        Name:       Andrew H. Marshak
                                             ------------------------
                                        Title:      Managing Director
                                             ------------------------<PAGE>   1
                                                                     EXHIBIT 4.8

                                                                [EXECUTION COPY]

                          REGISTRATION RIGHTS AGREEMENT

                          DATED AS OF JANUARY 15, 2000

                                 By and Between

                          ORBITAL SCIENCES CORPORATION

                                       and

                   MORGAN GUARANTY TRUST COMPANY OF NEW YORK,
                             as Administrative Agent

<PAGE>   2

                          REGISTRATION RIGHTS AGREEMENT

       This REGISTRATION RIGHTS AGREEMENT (this "AGREEMENT") is made and entered
into as of January 15, 2000, by and between ORBITAL SCIENCES CORPORATION, a
Delaware corporation (the "COMPANY"), and MORGAN GUARANTY TRUST COMPANY OF NEW
YORK, as agent (the "ADMINISTRATIVE AGENT") for the Approving Banks (the
"APPROVING BANKS") referred to in Amendment No. 6 dated as of December 21, 1999
to the Third Amended and Restated Credit and Reimbursement Agreement dated as of
December 21, 1998 among Orbital, the Banks parties thereto and Morgan Guaranty
Trust Company of New York, as Administrative Agent and as Collateral Agent (as
amended from time to time, the "CREDIT AGREEMENT").

                              W I T N E S S E T H:

       WHEREAS, the Company has agreed to issue to the Approving Banks warrants
(the "WARRANTS") to purchase an aggregate of 100,000 shares (the "WARRANT
SHARES") of the Company's Common Stock, par value $.01 per share (the "COMMON
STOCK"), in accordance with the terms thereof;

       WHEREAS, in connection with the issuance of the Warrants, the Company has
agreed to provide to the holders of the Warrant and the Warrant Shares the
registration rights set forth herein;

       NOW, THEREFORE, in consideration of the mutual covenants and undertakings
contained herein, and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, and subject to and on the terms
and conditions herein set forth, the parties hereto hereby agree as follows:

                                    ARTICLE I

                               Certain Definitions

       As used in this Agreement, the following terms shall have the meanings
ascribed to them below:

<PAGE>   3

       1.1 "Commission" shall mean the Securities and Exchange Commission or any
federal agency at the time administering the Securities Act.

       1.2 "Exchange Act" shall mean the Securities Exchange Act of 1934, as
amended, or any federal statute then in effect which has replaced such statute.

       1.3 "Group" shall mean two or more Persons that would be deemed a "group"
for purposes of Rule 13d-5 under the Exchange Act.

       1.4 "Holder" means any Person who is a holder or beneficial owner of
Registrable Securities for so long as such Person owns any Registrable
Securities. For this purpose, the holder of any Warrant shall be deemed to be
the holder of the shares of Common Stock issuable upon exercise of such Warrant.

       1.5 "Person" shall mean an individual, corporation, limited liability
company, joint venture, partnership, trust, unincorporated organization,
government or any agency or political subdivision thereof or any other entity
that may be treated as a person under applicable law.

       1.6 "Registrable Securities" shall mean the Common Stock issued or
issuable upon exercise of the Warrants.

       As to any Registrable Securities, such securities shall cease to be
Registrable Securities when (i) a registration statement registering such
Registrable Securities under the Securities Act has been declared or becomes
effective and such Registrable Securities have been sold or otherwise
transferred by the Holder thereof pursuant to such effective registration
statement; (ii) such Registrable Securities are sold pursuant to Rule 144 under
circumstances in which any legend borne by such Registrable Securities relating
to restrictions on the transferability thereof, under the Securities Act or
otherwise, is removed by the Company or such Registrable Securities are
eligible to be sold pursuant to paragraph (k) of Rule 144; or (iii) such
Registrable Securities shall cease to be outstanding.

       1.7 "Rule 144" shall mean Rule 144 promulgated under the Securities Act.

       1.8 "Securities Act" shall mean Securities Act of 1933, as amended, or
any federal statute then in effect which has replaced such statute.

                                       2
<PAGE>   4

                                   ARTICLE II

                 Public Offering Pursuant to Registration Rights

       2.1 Piggyback Registration.

              (a) If the Company shall determine to register any equity
securities of the Company for its own account or for the account of other
holders of equity securities of the Company on any registration form (other than
Form S-4 or S- 8 or other successor forms) which permits the inclusion of
Registrable Securities held by any Holder (a "PIGGYBACK REGISTRATION"), the
Company will promptly give each Holder written notice thereof and, subject to
Section 2.1(c), shall include in such registration all Registrable Securities
requested to be included therein pursuant to the written requests of Holders
received within 10 business days after delivery of the Company's notice. Any
Piggyback Registeration may be withdrawn by the Company at any time.

              (b) If the Piggyback Registration relates to an underwritten
public offering, the Company shall so advise the Holders as part of the written
notice given pursuant to Section 2.1(a). In such event, the right of any Holder
to participate in such registration shall be conditioned upon such Holder's
participation in such underwriting in accordance with the terms and conditions
thereof. The Board shall have the right to select the managing underwriter(s)
for any underwritten Piggyback Registration. All Holders proposing to distribute
their Registrable Securities through such underwriting shall (together with the
Company) enter into an underwriting agreement in customary form.

              (c) If such proposed Piggyback Registration is an under written
offering and the managing underwriter for such offering advises the Company
that the securities requested to be included therein exceeds the amount of
securities that can be sold in such offering, any securities to be sold by the
Company or other holders of the Company's securities initiating such offering in
such offering shall have priority over any Registrable Securities held by
Holders, and the number of shares to be included by a Holder and other holders
of the Company's securities that did not initiate the offering in such
registration shall be reduced pro rata on the basis of the percentage of the
then outstanding Registrable Securities held by each such Holder and all other
holders exercising similar registration rights.

                                       3
<PAGE>   5

       2.2 Expenses of Registration. All expenses incurred in connection with
all Piggyback Registrations shall be borne by the Company, including without
limitation the reasonable cost of one counsel to all Holders reasonably
acceptable to the Company (the Company herein acknowledging that Davis Polk &
Wardwell is acceptable counsel). All underwriting discounts, selling commissions
and other similar fees relating to Registrable Securities included in any
Piggyback Registration shall be borne by the holders of such Registrable
Securities pro rata on the basis of the amount of Registrable Securities sold by
them.

       2.3 Registration Procedures. In the case of each registration effected by
the Company pursuant to this Article II, the Company will keep each Holder
advised in writing as to the initiation of such registration and as to the
completion thereof. At its expense, the Company will use its reasonable best
efforts, subject to Section 2.1(a), to:

              (a) cause such registration to be declared effective by the
Commission;

              (b) as soon as reasonably possible, prepare and file with the
Commission such amendments and supplements to such registration statement and
the prospectus included therein (including post-effective amendments, prospectus
supplements and pricing supplements) as may be necessary to effect and maintain
the effectiveness of such registration statement;

              (c) provide (A) the Holders of the Registrable Securities to be
included in such registration statement, (B) the underwriters (which term, for
purposes of this Agreement, shall include a person deemed to be an underwriter
within the meaning of Section 2(11) of the Securities Act) if any, thereof, (C)
the sales or placement agent therefor, if any, and (D) counsel for such
underwriters or agent, the opportunity to participate in the preparation of such
registration statement, each prospectus included therein or filed with the
Commission, and each amendment or supplement thereto;

              (d) (A) register or qualify the Registrable Securities to be
included in such registration statement under such securities laws or blue sky
laws of such jurisdictions as any Holder of such Registrable Securities and each
placement or sales agent, if any, therefor and underwriter, if any, thereof
shall reasonably request, and (B) take any and all other actions as may be
reasonably necessary or advisable to

                                       4
<PAGE>   6

enable each such Holder, agent, if any, and underwriter, if any, to consummate
the disposition in such jurisdictions of such Registrable Securities; provided,
however, that the Company shall not be required for any other purpose to (1)
qualify as a foreign corporation in any jurisdiction wherein it would not
otherwise be required to qualify but for the requirements of this Section
2.3(d) or (2) consent to general service of process or taxation in any such
jurisdiction;

              (e) furnish such number of prospectuses and other documents
incident thereto, including any amendment of or supplement to the prospectus, as
any selling Holder from time to time may reasonably request;

              (f) promptly notify the selling Holders of Registrable Securities,
the sales or placement agent, if any, therefor and the managing under writer or
underwriters, if any, thereof and confirm such advice in writing, (A) when such
registration statement or the prospectus included therein or any prospectus
amendment or supplement or post-effective amendment has been filed, and with
respect to such registration statement or any post-effective amendment, when the
same has become effective, (B) of any comments by the Commission, the Blue Sky
or securities commissioner or regulator of any state with respect thereto or any
request by the Commission for amendments or supplements to such registration
statement or prospectus or for additional information, (C) of the issuance by
the Commission of any stop order suspending the effectiveness of such
registration statement or the initiation or threatening of any proceedings for
that purpose, (D) if at any time the representations and warranties of the
Company contemplated by Section 2.3(m) or Section 3 cease to be true and correct
in all material respects, (E) of the receipt by the Company of any notification
with respect to the suspension of the qualification of the Registrable
Securities for the sale in any jurisdiction or the initiation or threatening of
any proceeding for such purpose, or (F) at any time when a prospectus is
required to be delivered under the Securities Act, that such registration
statement, prospectus, prospectus amendment or supplement or post-effective
amendment, or any document incorporated by reference in any of the foregoing,
contains an untrue statement of a material fact or omits to state any material
fact required to be stated therein or necessary to make the statements therein
not misleading in light of the circumstances then existing;

              (g) obtain the withdrawal of any order suspending the
effectiveness of such registration statement or any post-effective amendment
thereto at the earliest practicable date;

                                       5
<PAGE>   7

              (h) if requested by any managing underwriter or underwriters, any
placement or sales agent or any selling Holder of Registrable Securities,
promptly incorporate in a prospectus supplement or post-effective amendment such
information as is required by the applicable rules and regulations of the
Commission and as such managing underwriter or underwriters, such agent or such
holder specifies should be included therein relating to the terms of the sale of
such Registrable Securities, including, without limitation, information with
respect to the principal amount of Registrable Securities being sold by such
Holder or agent or to any underwriters, the name and description of such Holder,
agent or underwriter, the offering price of such Registrable Securities and any
discount, commission or other compensation payable in respect thereof, the
purchase price being paid therefor by such underwriters and with respect to any
other terms of the offering of the Registrable Securities to be sold by such
Holder or agent or to such underwriters;

              (i) furnish to each Holder of Registrable Securities included in
such registration statement, each placement or sales agent, if any, therefor,
each underwriter, if any, thereof an executed copy of such registration
statement, each such amendment and supplement thereto (in each case including
all exhibits thereto and documents incorporated by reference therein) and such
number of copies of such registration statement (excluding exhibits thereto and
documents incorporated by reference therein unless specifically and reasonably
so requested by such Holder, agent or underwriter, as the case may be) and of
the prospectus included in such registration statement (including each
preliminary prospectus and any summary prospectus), in conformity with the
requirements of the Securities Act; and the Company hereby consents to the use
of such prospectus (including such preliminary and summary prospectus) and any
amendment or supplement thereto by each such Holder and by any such agent and
underwriter, if any, in each case in the form most recently provided to such
party by the Company, in connection with the offering and sale of the
Registrable Securities covered by the prospectus (including such preliminary and
summary prospectus) or any supplement or amendment thereto;

              (j) cause all Registrable Securities covered by such registration
to be listed on each securities exchange or inter-dealer quotation system on
which similar securities issued by the Company are then listed;

              (k) provide a transfer agent and registrar for all Registrable
Securities covered by such registration and a CUSIP number for all such
Registrable Securities, in each case not later than the effective date of such
registration;

                                       6
<PAGE>   8

              (l) cooperate with the Holders of Registrable Securities and the
managing underwriters, if any, to facilitate the timely preparation and delivery
of certificates representing Registrable Securities to be sold, which
certificates shall not bear any restrictive legends;

              (m) in the event that any broker-dealer registered under the
Exchange Act shall underwrite any Registrable Securities or participate as a
member of an underwriting syndicate or selling group or "assist in the
distribution" (within the meaning of the Rules of Conduct (the "RULES OF
CONDUCT") of the National Association of Securities Dealers, Inc. ("NASD")
thereof, whether as a holder of such Registrable Securities or as an
underwriter, a placement or sales agent or a broker or dealer in respect
thereof, or otherwise use its reasonable best efforts to assist such
broker-dealer in complying with the requirements of such Rules of Conduct,
including, without limitation, by providing such information to such
broker-dealer as may be required in order for such broker-dealer to comply with
the requirements of the Rules of Conduct;

              (n) otherwise comply with all applicable rules and regulations of
the Commission and make available to its security holders, as soon as reasonably
practicable but in no event later than eighteen months after the effective date
of such registration statement, an earnings statement covering the period of at
least twelve months, but not more than 18 months, beginning with the first month
after the effective date of the registration statement, which earnings statement
shall satisfy the provisions of Section 11(a) of the Securities Act (including,
at the option of the Company, Rule 158 thereunder); and

       2.4 Delivery of Prospectus Supplement. In the event that the Company
would be required, pursuant to Section 2.3(f) above, to notify the selling
Holders of Registrable Securities, the placement or sales agent, if any,
therefor and the managing underwriters, if any, thereof, the Company shall as
soon as reasonably practicable prepare and furnish to each such Holder, to each
placement or sales agent, if any, and to each underwriter, if any, a reasonable
number of copies of a prospectus supplemented or amended so that, as thereafter
delivered to initial purchasers of Registrable Securities, such prospectus shall
not contain an untrue statement of a material fact or omit to state a material
fact required to be stated therein or necessary to make the statements therein
not misleading in light of the circumstances then existing. Each Holder of
Registrable Securities agrees that upon receipt of any notice from the Company
pursuant to Section 2.3(f) hereof, such Holder shall forthwith discontinue the
disposition of Registrable Securities pursuant to the

                                       7
<PAGE>   9

registration statement applicable to such Registrable Securities until such
Holder shall have received copies of such amended or supplemented prospectus,
and if so directed by the Company, such Holder shall deliver to the Company (at
the Company's expense) all copies, other than permanent file copies, then in
such Holder's possession of the prospectus covering such Registrable Securities
at the time of receipt of such notice.

       2.5 Furnishing Information by the Holders. The Company may require, as a
condition to the inclusion of any Registrable Securities of a Holder in any
registration statement, each Holder of Registrable Securities as to which any
registration is being effected to furnish to the Company such information
regarding such Holder and such Holder's intended method of distribution of such
Registrable Securities as the Company may from time to time reasonably request
in writing, but only to the extent that such information is required in order to
comply with the Securities Act. Each such Holder agrees to notify the Company as
promptly as practicable of any inaccuracy or change in information previously
furnished by such Holder to the Company or of the occurrence of any event in
either case as a result of which any prospectus relating to such registration
contains or would contain an untrue statement of a material fact regarding such
Holder or such Holder's intended method of distribution of such Registrable
Securities or omits to state any material fact regarding such Holder or such
Holder's intended method of distribution of such Registrable Securities required
to be stated therein or necessary to make the statements therein not misleading
in light of the circumstances then existing, and promptly to furnish information
so required so that such prospectus shall not contain, with respect to such
Holder or the distribution of such Registrable Securities, an untrue statement
of a material fact or omit to state a material fact required to be stated
therein or necessary to make the statements therein not misleading in light of
the circumstances then existing.

                                       8
<PAGE>   10

       2.6 Indemnification.

              (a) The Company will indemnify each Holder whose Registrable
Securities are to be included in a registration pursuant to this Article II,
each of such Holder's officers, directors, partners, agents, employees and
representatives and each person controlling such Holder within the meaning of
Section 15 of the Securities Act or Section 20 of the Exchange Act, against all
expenses, claims, losses, damages and liabilities (or actions, proceedings or
settlements in respect thereof) arising out of or based on any untrue statement
(or alleged untrue statement) of a material fact contained in any registration
statement, or document incorporated by reference therein, or any omission (or
alleged omission) to state therein a material fact required to be stated therein
or necessary to make the statements therein not misleading, and will reimburse
each such indemnified person for any legal and any other expenses reasonably
incurred in connection with investigating and defending or settling any such
claim, loss, damage, liability or action, or arising out of any untrue statement
or alleged untrue statement of a material fact contained in any prospectus (or
any amendment or supplement thereto) or the omission or alleged omission
therefrom of a material fact necessary in order to make the statement therein,
in the light of the circumstances under which they were made, not misleading;
provided, however, that the Company will not be liable in any such case to a
Holder to the extent that any such claim, loss, damage, liability or expense
arises out of or is based on any untrue statement or omission based upon written
information furnished to the Company by such Holder and provided for use in such
registration statement, prospectus, offering circular or other document or the
Holder delivered a registration or prospectus in violation of Section 2.4 hereof
after notice was provided by the Company as provided in Section 2.4. It is
agreed that the indemnity agreement contained in this Section 2.6(a) shall not
apply to amounts paid in settlement of any such loss, claim, damage, liability
or action if such settlement is effected without the consent of the Company
(which consent shall not be unreasonably withheld or delayed).

              (b) Each Holder whose Registrable Securities are included in any
registration effected pursuant to this Article II shall indemnify the Company,
each of its directors, officers, agents, employees and representatives, and each
Person who controls the Company within the meaning of Section 15 of the
Securities Act or Section 20 of the Exchange Act, each other such Holder and
each of their officers, directors, partners, agents, employees and
representatives and each person controlling such Holder, and each underwriter,
if any, of such Registrable Securities and each Person who controls any such
underwriter, against all expenses, claims, losses,

                                       9
<PAGE>   11

damages and liabilities (or actions, proceedings or settlements in respect
thereof) arising out of the indemnity as described in Section 2.6(a), but only
to the extent, that such untrue statement (or alleged untrue statement) or
omission (or alleged omission) is made in such registration statement,
prospectus, offering circular or other document based upon written information
furnished to the Company by such Holder and provided specifically for use
therein; provided, however, that (x) no Holder shall be liable hereunder for any
amounts in excess of the gross proceeds received by such Holder pursuant to such
registration, and (y) the obligations of such Holder hereunder shall not apply
to amounts paid in settlement of any such claims, losses, damages or liabilities
(or actions in respect thereof) if such settlement is effected without the
consent of such Holder (which consent shall not be unreasonably withheld).

              (c) Each party entitled to indemnification under this Section 2.6
(the "INDEMNIFIED PARTY") shall give notice to the party required to provide
indemnification (the "INDEMNIFYING PARTY") promptly after such Indemnified Party
has actual knowledge of any claim as to which indemnity may be sought and shall
permit the Indemnifying Party to assume the defense of any such claim or any
litigation resulting therefrom, provided that counsel for the Indemnifying
Party, who shall conduct the defense of such claim or any litigation resulting
therefrom, shall be approved by the Indemnified Party (whose approval shall not
unreasonably be withheld or delayed, the Company hereby approving Davis Polk &
Wardwell as counsel to the Holders for the purposes of this Section 2.6(c)), and
the Indemnified Party may participate in such defense with counsel reasonably
acceptable to and paid for by the Indemnifying Party but otherwise at the
Indemnified Party's expense, and provided, further, that the failure of any
Indemnified Party to give notice as provided herein shall not relieve the
Indemnifying Party of its obligations under this Section 2.6 to the extent such
failure is not materially prejudicial. No Indemnifying Party in the defense of
any such claim or litigation shall except with the consent of each Indemnified
Party, consent to entry of any judgment or enter into any settlement which does
not include an unconditional release of such Indemnified Party from all
liability in respect of such claim or litigation. Each Indemnified Party shall
furnish such information regarding itself or the claim in question as an
Indemnifying Party may reasonably request in writing and as shall be reasonably
required in connection with the defense of such claim and litigation resulting
therefrom.

              (d) If the indemnification provided for in this Section 2.6 is
held by a court of competent jurisdiction to be unavailable to an Indemnified
Party with respect to any loss, liability, claim, damage or expense referred to
therein, then the Indemnifying Party, in lieu of indemnifying such Indemnified
Party hereunder,

                                       10
<PAGE>   12

shall contribute to the amount paid or payable by such Indemnified Party as a
result of such loss, liability, claim, damage or expense in such proportion as
is appropriate to reflect the relative fault of the Indemnifying Party on the
one hand and of the Indemnified Party on the other in connection with the
statements or omissions which resulted in such loss, liability, claim, damage or
expense as well as any other relevant equitable considerations. The relative
fault of the Indemnifying Party and of the Indemnified Party shall be determined
by reference to, among other things, whether the untrue or alleged untrue
statement of a material fact or the omission to state a material fact relates to
information supplied by the Indemnifying Party or by the Indemnified Party and
the parties' relative intent, knowledge, access to information and opportunity
to correct or prevent such statement or omission. No person guilty of fraudulent
misrepresentation (within the meaning of section 11(f) of the Securities Act)
shall be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

              (e) Notwithstanding the foregoing, to the extent that the
provisions on indemnification and contribution contained in an underwriting
agreement entered into in connection with an underwritten public offering are in
conflict with the foregoing provisions, the provisions in the underwriting
agreement shall control.

       2.7 Other Obligations. With a view to making available the benefits of
certain rules and regulations of the Commission which may effectuate the
registration of Registrable Securities or permit the sale of Registrable
Securities to the public without registration, the Company agrees to:

              (a) exercise reasonable efforts to cause the Company to be
eligible to utilize Form S-3 (or any similar form) for the registration of
securities;

              (b) at such time as any Registrable Securities are eligible for
transfer under Rule 144(k), upon the request of the holder of such Registrable
Securities, remove any restrictive legend from the certificates evidencing such
Registrable Securities at no cost to such holder;

              (c) exercise reasonable efforts to make and keep available public
information as defined in Rule 144 under the Securities Act at all times;

              (d) exercise reasonable efforts to file with the Commission in a
timely manner all reports and other documents required of the Company under

                                       11
<PAGE>   13

the Securities Act and the Exchange Act at any time after it has become subject
to such reporting requirements; and

              (e) furnish any Holder upon request a written statement by the
Company as to its compliance with the reporting requirements of Rule 144 (at any
time from and after 90 days following the effective date of the first
registration statement filed by the Company for an offering of its securities to
the general public), and of the Securities Act and the Exchange Act (at any time
after it has become subject to such reporting requirements), a copy of the most
recent annual or quarterly report of the Company, and such other reports and
documents as a Holder may reasonably request in availing itself of any rule or
regulation of the Commission (including Rule 144A) allowing a holder of
Registrable Securities to sell any such Registrable Securities without
registration.

       2.8 Hold-Back Agreements. If requested by the Company or any underwriter
of securities of the Company, Holders shall not sell or otherwise transfer or
dispose of the Warrant or any Common Stock (other than pursuant to such
registration) during the period 15 days prior to and 90 days following the
effective date of registration statement relating to the offering of the
Company's securities for its own account or such longer period that the
underwriters may reasonably request. The obligations described in this Section
2.8 shall not apply to a registration on Form S-4 or Form S-8 or similar forms
which may be promulgated in the future and shall not apply to a Holder holding
less than 1% of the then-outstanding Common Stock (assuming exercise of the
Warrant for this purpose).

                                   ARTICLE III

                         Representations and Warranties

       The Company represents and warrants to, and agrees with, the
Administrative Agent and each of the Holders from time to time of Registrable
Securities that:

              (a) The compliance by the Company with all of the provisions of
this Agreement and the consummation of the transactions herein contemplated will
not conflict with or result in a breach of any of the terms or provisions of, or
constitute a default under, any indenture, mortgage, deed of trust, loan
agreement or other agreement or instrument to which the Company or any

                                       12
<PAGE>   14

subsidiary of the Company is a party or by which the Company or any subsidiary
of the Company is bound or to which any of the property or assets of the Company
or any subsidiary of the Company is subject nor will such action result in any
violation of the provisions of the certificate of incorporation or by-laws of
the Company or any statute or any order, rule or regulation of any court or
governmental agency or body having jurisdiction over the Company or any
subsidiary of the Company or any of their properties; and no consent, approval,
authorization, order, registration or qualification of or with any such court or
governmental agency or body is required for the consummation by the Company of
the transactions contemplated by this Agreement, except the registration under
the Securities Act of the Registrable Securities, and such consents, approvals,
authorizations, registrations or qualifications as may be required under State
securities or blue sky laws in connection with the offering and distribution of
the Registrable Securities.

              (b) This Agreement has been duly authorized, executed and
delivered by the Company.

                                   ARTICLE IV

                                   Termination

              This Agreement shall terminate immediately following the moment at
which there exist no securities of the Company that constitute Registrable
Securities; provided, however, that Section 2.6 hereof shall survive
indefinitely.

                                    ARTICLE V

                                  Miscellaneous

       5.1 Recapitalization, Exchanges, etc. Affecting the Common Stock. The
provisions of this Agreement shall apply to the full extent set forth herein
with respect to (a) the Registrable Securities and (b) any and all shares of
capital stock of the Company or any successor or assign of the Company (whether
by merger, consolidation, sale of assets or otherwise) which may be issued in
respect of, in exchange for, or in substitution for the Registrable Securities,
by reason of any stock dividend, split, reverse split, combination,
recapitalization, reclassification, merger, consolidation or otherwise. In the
event of any change in the capitalization

                                       13
<PAGE>   15

of the Company as a result of any stock split, stock dividend or stock
combination, the provisions of this Agreement shall be appropriately adjusted.

       5.2 Injunctive Relief. It is hereby agreed and acknowledged that it will
be impossible to measure in money the damages that would be suffered if the
parties fail to comply with any of the obligations herein imposed on them and
that in the event of any such failure, an aggrieved Person will be irreparably
damaged and will not have an adequate remedy at law. Any such Person shall,
therefore, in addition to any other remedies available under applicable law, be
entitled to injunctive relief, including specific performance, to enforce such
obligations, without the posting of any bond, and if any action should be
brought in equity to enforce any of the provisions of this Agreement, none of
the parties hereto shall raise the defense that there is an adequate remedy at
law.

       5.3 Parties in Interest. All the terms and provisions of this Agreement
shall be binding upon, shall inure to the benefit of and shall be enforce able
by the Company, the Administrative Agent, the Holders and their respective
successors and assigns. In the event that any transferee of any Holder of
Registrable Securities shall acquire Registrable Securities, in any manner,
whether by gift, bequest, purchase, operation of law or otherwise, such
transferee shall, without any further writing of any kind, be deemed a party
hereto for all purposes and such Registrable Securities shall be held subject to
all of the terms of this Agreement, and by taking and holding such Registrable
Securities such transferee shall be entitled to receive the benefits of and be
conclusively deemed to have agreed to be bound by and to perform all of the
terms and provisions of this Agreement.

       5.4 Survival. The respective indemnities, agreements, representations,
warranties and each other provision set forth in this Agreement or made pursuant
hereto shall remain in full force and effect regardless of any investigation (or
statements as to the results thereto) made by or on behalf of any Holder of
Registrable Securities, any director, officer or partner of such Holder, any
agent or underwriter or any director, officer or partner thereof, or any
controlling person of any of the foregoing, and shall survive delivery of and
payment for the Registrable Securities pursuant to the Warrant and the transfer
of Registrable Securities by such Holder.

                                       14
<PAGE>   16

       5.5 Amendment; Waiver.

              (a) This Agreement may be amended only by a written instrument
signed by the Company and by the Administrative Agent with the consent of
Holders holding more than 66% of the then outstanding Registrable Securities.

              (b) No provision of this Agreement may be waived orally, but only
by a written instrument signed by the party against whom enforcement of such
waiver is sought. Holders shall be bound from and after the date of the receipt
of a written notice from the Company setting forth such amendment or waiver,
whether or not the Registrable Securities shall have been marked to indicate
such amendment or waiver.

       5.6 Notices. Except as otherwise provided in this Agreement, notices and
other communications under this Agreement shall be in writing (including a
writing delivered by facsimile transmission) and shall be deemed to have been
duly given if delivered personally, or sent by either certified or registered
mail, return receipt requested, postage prepaid, or by overnight courier
guaranteeing next day delivery, or by telex or telecopier, at the following
addresses:

                           if to the Company:

                           21700 Atlantic Boulevard
                           Dulles, VA 20166
                           Attention: Legal Department
                           Telecopier: 703-406-5572

                           with a copy to

                           Hogan and Hartson, L.L.P.
                           555 13th Street NW
                           Washington, DC 20004
                           Attention: Eve N. Howard, Esq.
                           Telecopier: 202-637-5910

                           if to the Administrative Agent:

                           Morgan Guaranty Trust Company of New York

                                       15
<PAGE>   17

                           60 Wall Street
                           New York, New York 10260
                           Attention: Robert Vecsler
                           Telecopier: (212) 648-5005

                           with a copy to:

                           Davis Polk & Wardwell
                           450 Lexington Avenue
                           New York, New York 10017
                           Attention: Tiziana M. Tabucchi, Esq.
                           Telecopier: (212) 450-4800

                           if to any initial Holder, to it at its address
                           for notices under the Credit Agreement, with
                           a copy to the Administrative Agent

Any party may, by written notice given to the other parties in accordance with
this Section 5.6, change the address to which such notice or other
communications are to be sent to it. All such notices and communications shall
be deemed to have been given on the date of delivery thereof, if delivered by
hand, on receipt, if mailed, on the next day after the sending thereof, if by
overnight courier and when receipt is acknowledged, if telecopied.

       5.7 Inspection. So long as this Agreement shall be in effect, this
Agreement and any amendments hereto shall be made available for inspection by
any Holder at the principal offices of the Company.

       5.8 APPLICABLE LAW. THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT GIVING EFFECT TO THE
CONFLICTS OF LAW PRINCIPLES THEREOF.

       5.9 Headings. Article, section and paragraph headings are inserted for
convenience only and do not constitute a part of this Agreement.

       5.10 Integration. This Agreement and the documents referred to herein or
delivered pursuant hereto which form a part hereof contain the entire
understanding of the parties with respect to the subject matter hereof. There
are no

                                       16
<PAGE>   18

restrictions, agreements, promises, representations, warranties, covenants or
under takings with respect to the subject matter hereof other than those
expressly set forth or referred to herein. This Agreement supersedes all prior
agreements and under standings between the parties with respect to this subject
matter.

       5.11 Illegality. In case any provision in this Agreement shall be
declared or held invalid, illegal or unenforceable, in whole or in part, whether
generally or in any particular jurisdiction, such provision shall be deemed
amended to the extent, but only to the extent, necessary to cure such
invalidity, illegality or unenforceability, and the validity, legality and
enforceability of the remaining provisions, both generally and in every other
jurisdiction, shall not in any way be affected or impaired thereby.

       5.12 Counterparts. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which shall
constitute one and the same instrument, and it shall not be necessary in making
proof of this Agreement to produce or account for more than one such
counterpart.

                                       17
<PAGE>   19

       IN WITNESS WHEREOF, the undersigned have executed this Agreement as of
the date set forth above.

                                          ORBITAL SCIENCES CORPORATION

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

                                          Title:

                                          MORGAN GUARANTY TRUST COMPANY
                                            OF NEW YORK, as Administrative Agent

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

                                          Title:

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