SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13G


                                 (Rule 13d-102)


             INFORMATION STATEMENT PURSUANT TO RULES 13d-1 AND 13d-2


                   Under the Securities Exchange Act of 1934
                                (Amendment No. )


                           MERCURY COMPUTER SYSTEMS INC
             -----------------------------------------------------
                                (Name of Issuer)
                                  Common Stock
              -----------------------------------------------------
                         (Title of Class of Securities)

                                   589378108
             -----------------------------------------------------
                                 (CUSIP Number)



Check the following box if a fee is being paid with this statement [ ].


- ----------
*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).



CUSIP No. 589378108                 13G                    Page 2  of 6 Pages



- --------------------------------------------------------------------------------
   1.  NAME OF REPORTING PERSON(S)
       S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON(S)

       Morgan  Stanley  Dean  Witter &  Co.
       IRS  #  39-314-5972
- --------------------------------------------------------------------------------
   2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                             (a)  [  ]
                                                             (b)  [  ]

- --------------------------------------------------------------------------------
   3.  SEC USE ONLY


- --------------------------------------------------------------------------------
   4.  CITIZENSHIP OR PLACE OF ORGANIZATION

       The state of organization is Delaware.
- --------------------------------------------------------------------------------
   NUMBER OF      5.   SOLE VOTING POWER
     SHARES            0
  BENEFICIALLY    --------------------------------------------------------------
    OWNED BY      6.   SHARED VOTING POWER
      EACH             1,143,156
   REPORTING      --------------------------------------------------------------
     PERSON       7.   SOLE DISPOSITIVE POWER
      WITH             0
                  --------------------------------------------------------------
                  8.   SHARED DISPOSITIVE POWER
                       1,145,759
- --------------------------------------------------------------------------------
   9.  AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

       1,145,759
- --------------------------------------------------------------------------------
  10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*


- --------------------------------------------------------------------------------
  11.  PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

       5.2157%
- --------------------------------------------------------------------------------
  12.  TYPE OF REPORTING PERSON*

       IA, CO
- --------------------------------------------------------------------------------
                     *SEE INSTRUCTIONS BEFORE FILLING OUT!




CUSIP No. 589378108                   13G                   Page 3  of 6 Pages





Item 1.     (a)   Name of Issuer:
                  MERCURY COMPUTER SYSTEMS INC
                  --------------------------------------------------------------
            (b)   Address of Issuer's Principal Executive Offices:
                  199 RIVERNECK RD
                  CHELMSFORD, MA 01824
                  --------------------------------------------------------------
Item 2.     (a)   Name of Person Filing:
                  Morgan Stanley Dean Witter & Co.
                  --------------------------------------------------------------
            (b)   Address of Principal Business Office, or if None, Residence:
                  1585 Broadway
                  New  York, New  York  10036
                  --------------------------------------------------------------
            (c)   Citizenship:

                  Incorporated by reference to Item 4 of the cover page
                  pertaining to each reporting person.
                  --------------------------------------------------------------
            (d)   Title of Class of Securities:
                  Common Stock
                  --------------------------------------------------------------
            (e)   CUSIP Number:
                  589378108
                  --------------------------------------------------------------

Item 3.           Morgan Stanley Dean Witter & Co. is a parent holding company.




CUSIP No. 589378108             13-G                   Page 4  of  6  Pages



Item 4.     Ownership.

            Incorporated by reference to Items (5) - (9) and (11) of the
            cover page.

            (a) Morgan Stanley Dean Witter & Co. is filing solely in its
                capacity as the parent company of, and indirect beneficial
                owner of securities held by, one of its business units.

Item 5.     Ownership of Five Percent or Less of a Class.

            Inapplicable

Item 6.     Ownership of More Than Five Percent on Behalf of Another Person.

            Accounts managed on a discretionary basis by Morgan Stanley
            Dean Witter & Co. are known to have the right to receive or the
            power to direct the receipt of dividends from, or the proceeds from,
            the sale of such securities. No such account holds more than 5
            percent of the class.

            See item 4 (a)

Item 7.     Identification and Classification of the Subsidiary which Acquired
            the Security Being Reported on By the Parent Holding Company.

Item 8.     Identification and Classification of Members of the Group.

Item 9.     Notice of Dissolution of Group.

Item 10.    Certification.

            By signing below I certify that, to the best of my knowledge and
            belief, the securities referred to above were acquired in the
            ordinary course of business and were not acquired for the purpose
            of and do not have the effect of changing or influencing the
            control of the issuer of such securities and were not acquired in
            connection with or as a participant in any transaction having
            such purpose or effect.


CUSIP No. 589378108             13-G                   Page 5  of  6 Pages


                                   Signature.


            After reasonable inquiry and to the best of my knowledge and belief,
            I certify that the information set forth in this statement is true,
            complete and correct.


Date:       January 31, 2002

Signature:  /s/ Dennine Bullard
            -----------------------------------------------------------------

Name/Title  Dennine Bullard /Vice President Morgan Stanley & Co. Incorporated
            -----------------------------------------------------------------
            MORGAN STANLEY DEAN WITTER & CO.




                       INDEX TO EXHIBITS                               PAGE
                       -----------------                               ----

EXHIBIT 1       Secretary's Certificate Authorizing Dennine Bullard       8
                to Sign on behalf of Morgan Stanley Dean Witter & Co.


* Attention. Intentional misstatements or omissions of fact constitute federal
  criminal violations (see 18 U.S.C. 1001).

(022597DTI)


                   EX-99.b
            SECRETARY'S CERTIFICATE



 CUSIP No. 589378108                  13-G                  Page 6 of 6 Pages

                                      EXHIBIT 1
                                ------------------

                 MORGAN STANLEY, DEAN WITTER, DISCOVER & CO.

                             SECRETARY'S CERTIFICATE


         I, Charlene R. Herzer, a duly elected and Assistant Secretary of Morgan
         Stanley Dean Witter & Co., a corporation organized and existing under
         the laws of the State of Delaware (the "Corporation"), certify that
         Stuart J.M. Breslow, Robert G. Koppenol and Dennine Bullard are
         authorized to sign reports to be filed under Sections 13 and 16 of the
         Securities Exchange Act of 1934 (the "Act") pursuant to the following:

             (1)      On May 31, 1997, the Board of Directors of the Corporation
                      granted authority to Stuart J.M. Breslow and Robert G.
                      Koppenol to sign reports to be filed under Sections 13 and
                      16 of the Act on behalf of the Corporation.

             (2)      On February 23, 2000, Donald G. Kempf, Jr., the Executive
                      Vice President, Chief Legal Officer and Secretary of the
                      Corporation, delegated authority to Dennine Bullard to
                      sign reports to be filed under Sections 13 and 16 of the
                      Act on behalf of the Corporation. Pursuant to Section 7.01
                      of the Bylaws of the Corporation and resolutions approved
                      by the Board of Directors of the Corporation on September
                      25, 1998, the Chief Legal Officer is authorized to enter
                      into agreements and other instruments on behalf of the
                      Corporation and may delegate such powers to others under
                      his jurisdiction.

          IN WITNESS WHEREOF, I have hereunto set my name and affixed the seal
          of the Corporation as of the 1st day of March, 2000.

                                                ------------------------------
                                                Charlene R. Herzer
                                                Assistant Secretary