EXHIBIT 4

          Each of the undersigned hereby acknowledges and agrees, in compliance
with the provisions of Rule 13d-1(k)(1) promulgated under the Securities
Exchange Act of 1934, as amended, that the Schedule 13D to which this Agreement
is attached as an Exhibit, and any amendments thereto, will be filed with the
Securities and Exchange Commission jointly on behalf of the undersigned.

          This agreement may be executed (including by facsimile transmission)
with counterpart signature pages or in one or more counterparts, each of which
shall be deemed an original and all of which together shall be considered one
and the same agreement.

                         L-3 COMMUNICATIONS CORPORATION


                          By: /s/ Christopher C. Cambria
                              --------------------------------------------------
                              Name: Christopher C. Cambria
                              Title: Senior Vice President, General Counsel and
                                    Secretary


                          L-3 COMMUNICATIONS HOLDINGS, INC.


                          By: /s/ Christopher C. Cambria
                              --------------------------------------------------
                              Name: Christopher C. Cambria
                              Title: Senior Vice President, General Counsel and
                                    Secretary


                          BLUE ACQUISITION CORP.


                          By: /s/ Christopher C. Cambria
                              --------------------------------------------------
                              Name: Christopher C. Cambria
                              Title: Vice President and Secretary

Date: As of August 15, 2002


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