Exhibit 10.24d
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                                 Addendum No. 3
                                       to
                            VOLUME PURCHASE AGREEMENT

         This Addendum No. 3 to VOLUME PURCHASE  AGREEMENT (this  "Addendum") is
hereby  entered  into  by  AMSC  Subsidiary  Corporation  ("AMSC")  and  TRIMBLE
NAVIGATION LIMITED ("TRIMBLE") on the date last indicated below.

                                    RECITALS

A.       TRIMBLE and AMSC entered into a certain VOLUME PURCHASE AGREEMENT dated
         March 10,  1995,  as amended on December  19, 1995 and later on January
         29, 1997 (collectively, the "Agreement").

B.       TRIMBLE and AMSC intend to further  amend the  Agreement  to reduce the
         shipment rate of Galaxy Production Units (as defined in the Agreement).

C. TRIMBLE and AMSC therefore hereby modify the Agreement as follows:

                                    ADDENDUM

1.       REDUCTION OF SHIPMENT RATE

         Beginning  August  1997,  Trimble  will  reduce  the  number  of Galaxy
         Production  Units  shipped  to AMSC  from  five  hundred  (500)  Galaxy
         Production Units per month to two hundred fifty (250) Galaxy Production
         Units per month, and AMSC shall purchase such units at such rate, until
         AMSC has purchased the number of such Galaxy  Production Units required
         by the Agreement.

2.       PRICES

         Prices for Units  shipped to AMSC  through  November  1997 shall remain
         unchanged  at one  thousand  six  hundred  seventy  five  U.S.  Dollars
         ($1,675). Beginning with the Galaxy Production Units shipped to AMSC in
         December,  1997 the price per unit will increase to one thousand  seven
         hundred fifty U.S. Dollars ($1,750).

3.       PREPAYMENT

         3.1      In accordance with the Agreement, AMSC has prepaid Trimble one
                  million  six  hundred  seventy  five  thousand  U.S.   Dollars
                  ($1,675,000)  for  Galaxy  Production  Units  scheduled  to be
                  shipped  through  at the  rate of five  hundred  (500)  Galaxy
                  Production  Units  per month  for the  months  of  August  and
                  September,  1997.  Due to the  reduction  in shipment  rate as
                  provided  above,  this prepaid  amount shall be  sufficient to
                  prepay for shipment through November, 1997.

         3.2      The prepayment due for December, 1997 shipment of  two hundred
                  fifty (250)  Galaxy  Production  Units  shall  be  prepaid  as
                  follows:

                  3.2.1    Two hundred  eighteen  thousand  seven  hundred fifty
                           U.S.  Dollars  ($218,750)  shall be paid on or before
                           October 15, 1997, and

                  3.2.2    Two hundred  eighteen  thousand  seven  hundred fifty
                           U.S.  Dollars  ($218,750)  shall be paid on or before
                           November 1, 1997.

                  Trimble's  obligation   to  ship  the  December,  1997  Galaxy
Production Units is conditioned upon





                  Trimble's timely receipt of such prepayments.

         3.3      Beginning  November  15,  1997,  AMSC will  continue to prepay
                  Trimble for Galaxy  Production  Units on or before the 15th of
                  each  month for the  shipment  two  months  later  (e.g.,  the
                  payment for the two  hundred  fifty  (250)  Galaxy  Production
                  Units to be shipped January,  1997 shall be due and payable on
                  or before November 15, 1997.) Trimble's obligation to ship the
                  Galaxy  Production  Units is conditioned upon Trimble's timely
                  receipt of such prepayments.

4.       INCREASE OF SHIPMENT RATE

         4.1      Upon  forty-five  (45) days  prior  written  notice,  AMSC may
                  instruct  Trimble to resume shipping at a rate of five hundred
                  (500) Galaxy  Production Units per month on a sustained basis.
                  Such instruction shall be irrevocable.

         4.2      Upon such  instruction  to increase the shipment  rate to five
                  hundred  (500)  Galaxy  Production  Units  per  month and as a
                  condition  of  Trimble's   obligation   to  ship  such  Galaxy
                  Production Units:

                  4.2.1    At least  forty-five  days  prior to the first day of
                           the month in which Trimble makes the first  increased
                           shipment.  AMSC shall make a prepayment to Trimble of
                           eight hundred thirty seven thousand five hundred U.S.
                           Dollars ($837,500);

                  4.2.2    AMSC shall make  prepayments  of eight hundred thirty
                           seven thousand five hundred U.S. Dollars  (:$837,500)
                           on or before the 15th of each month; and

                  4.2.3    The price of each Galaxy Production Unit shall be one
                           thousand  six  hundred  seventy  five  U.S.   Dollars
                           ($1,675).

5.       RATIFICATION OF THE AGREEMENT

         In all other  respects,  TRIMBLE and AMSC hereby  ratify and affirm the
Agreement.

WITH INTENT TO BE BOUND,  TRIMBLE and AMSC have  signed  this  Amendment  on the
dates written below.

              TRIMBLE NAVIGATION LIMITED          AMSC SUBSIDIARY CORPORATION

Signature:    /s/Tom Ellis                        /s/Stephen D. Peck
              ------------                        ------------------
Typed
Name:         Tom Ellis                           Stephen D. Peck

Title:        Vice President, Mobile Position     Chief Financial Officer
              & Communications 

Date:         8/28/97                             8/29/97
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