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                                                                     Exhibit 4.1






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                          @TRACK COMMUNICATIONS, INC.

                                      AND

                           THE CHASE MANHATTAN BANK,

                                    TRUSTEE



                               _________________


                         FIRST SUPPLEMENTAL INDENTURE

                           Dated as of June 20, 2001

                                 TO INDENTURE

                        Dated as of September 23, 1997

                              ___________________

                                 $125,000,000


                  13 3/4% Senior Subordinated Notes due 2005

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                          FIRST SUPPLEMENTAL INDENTURE
                          ----------------------------


     This FIRST SUPPLEMENTAL INDENTURE dated as of June 20, 2001, is among
@Track Communications, Inc., a Delaware corporation (the "Company"), as
successor-in-interest to HighwayMaster Communications, Inc., a Delaware
corporation ("HighwayMaster") and as successor-in-interest to HighwayMaster
Corporation, a Delaware corporation ("Subsidiary Guarantor") and The Chase
Manhattan Bank, a national banking association, as Trustee (the "Trustee"), as
successor-in-interest to Chase Bank of Texas, N.A., as successor-in-interest to
Texas Commerce Bank National Association, and, relative to the modification of
the Indenture (as defined below) contained in Article Two hereof, has been
consented to by Holders (as defined in the Indenture) of at least a majority in
aggregate principal amount of the outstanding Notes (as defined in the
Indenture).


                                R E C I T A L S:
                                - - - - - - - -


     1.   HighwayMaster, the Subsidiary Guarantor and the Trustee previously
entered into that certain Indenture dated as of September 23, 1997, (the
"Indenture"), pursuant to which up to $125,000,000 of the Company's 13 3/4%
Senior Notes due 2005, Series A, and 13 3/4% Senior Notes due 2005, Series B
were issued.

     2.   The Company proposes to sell to Minorplant Systems PLC, a public
limited company organized under the laws of the United Kingdom ("MPS"),
30,000,000 shares of the Company's common stock (the "Proposed Change of Control
Transaction").  In addition, pursuant to that certain Exchange Offer Circular,
the Company has offered to purchase all of the issued and outstanding Notes upon
the terms and conditions described therein.

     3.   The parties hereto desire to modify and amend the Indenture to permit
the consummation of the Proposed Change of Control Transaction without
triggering the Change of Control Offer contained in Section 4.14 of the
Indenture and as otherwise hereinafter set forth.

     NOW THEREFORE, in consideration of the matters hereinabove recited and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto do hereby modify and amend the Indenture
and agree as follows:

                                  ARTICLE ONE

                                  DEFINITIONS
                                  -----------

     Capitalized terms not otherwise defined herein shall have the meanings
given in the Indenture as supplemented hereby.

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                                  ARTICLE TWO

                           MODIFICATION OF INDENTURE
                           -------------------------

     Section 2.1  Amendment to Definitions.
                  ------------------------

     (a)  Effective as of the date hereof, clause (i) of the definition of
"Permitted Holders" contained in Article 1 of the Indenture is hereby amended to
add "Minorplanet Systems PLC" thereto; and

     (b)  Effective as of the date hereof, the definition of "Change of Control"
contained in Article 1 of the Indenture is hereby amended to read in its
entirety as follows:

     "Change of Control" means the occurrence of any of the following: (i) the
sale, lease, transfer, conveyance or other disposition (other than by way of
merger or consolidation), in one or a series of related transactions, of all or
substantially all of the assets of the Company and its Restricted Subsidiaries
taken as a whole to any "person" (as such term is used in Section 13(d)(3) of
the Exchange Act) other than a Restricted Subsidiary; (ii) the adoption of a
plan relating to the liquidation or dissolution of the Company; (iii) the first
day on which the Permitted Holders in the aggregate hold less than 35% of the
voting stock of the Company.

     Section 2.2  Deletion of Covenants.  Effective as of the date hereof, each
                  ---------------------
of the following sections of the Indenture (each, a "Deleted Covenant"),
together with all references to any such section set forth in the Indenture, are
hereby deleted in their entireties:

          Section 4.5   Taxes.

          Section 4.7   Limitation of Restricted Payments.

          Section 4.8   Limitation on Dividend and Other Payment Restrictions
                        Affecting Restricted Subsidiaries.

          Section 4.9   Limitation on Indebtedness.

          Section 4.10  Limitation on Asset Sales.

          Section 4.11  Limitation on Transactions with Affiliates.

          Section 4.12  Limitation on Liens.

          Section 4.13  Limitation on Issuance and Sale of Capital Stock of
                        Restricted Subsidiaries.

          Section 4.15  Limitation on Sale and Leaseback Transactions.

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          Section 4.16  Covenant with respect to Business Activities.

     Section 2.3  Covenant Concerning Compliance Certificates and Notices of
                  ----------------------------------------------------------
Default.  Effective as of the date hereof, subsection (b) of Section 4.4 of the
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Indenture, is hereby deleted in its entirety.

     Section 2.4  Provision Concerning When Company May Merge, Etc.  Effective
                  -------------------------------------------------
as of the date hereof, subsections (i) and (iv) of Section 5.1 of the Indenture
are hereby deleted in their entireties.

     Section 2.5  Deletion of Definitions.  Effective as of the date hereof, all
                  -----------------------
definitions set forth in Article One of the Indenture which relate solely to the
Deleted Covenants or to Section 5.1 of the Indenture are hereby deleted in their
entireties from the Indenture.

     Section 2.6  Continuing Directors.  For the purposes of the definition of
                  --------------------
"Continuing Directors" contained in Article 1 of the Indenture, the parties
hereto acknowledge and agree that the directors to be nominated to the Board of
Directors by MPS shall be elected with the approval of a majority of the
Continuing Directors and shall constitute Continuing Directors under the
Indenture for all purposes.

                                 ARTICLE THREE

                                 MISCELLANEOUS
                                 -------------

     Section 3.1  Continuation.  Except as amended by this First Supplemental
                  ------------
Indenture, the Indenture remains in full force and effect in accordance with its
terms.

     Section 3.2  Governing Law.  THIS FIRST SUPPLEMENTAL INDENTURE SHALL BE
                  -------------
GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK,
AS APPLIED TO CONTRACTS MADE AND PERFORMED WITHIN THE STATE OF NEW YORK, WITHOUT
REGARD TO PRINCIPALS OF CONFLICTS OF LAW.  Each of the parties hereto agrees to
submit to the jurisdiction of the courts of the State of New York in any action
or proceeding arising out of or relating to this First Supplemental Indenture.

     Section 3.3  Successors.  All agreements of the Company and the Subsidiary
                  ----------
Guarantor in this First Supplemental Indenture shall bind their respective
successors.  All agreements of the Trustee in this First Supplemental Indenture
shall bind its successors.

     Section 4.4  Duplicate Originals.  All parties may sign any number of
                  -------------------
copies of this First Supplemental Indenture.  Each signed copy shall be an
original, but all of them together shall represent the same agreement.

     Section 4.5  Severability.  In case any one or more of the provisions of
                  ------------
this First Supplemental Indenture shall be held invalid, illegal or
unenforceable in any respect for any reason, the validity, legality and
enforceability of any such provision in every other respect and

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of the remaining provisions, shall not in any way be affected or impaired
thereby, it being intended that all of the provisions hereof shall be
enforceable to the full extent permitted by law.


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                                   SIGNATURES
                                   ----------

     IN WITNESS WHEREOF, the parties hereto have caused this First Supplemental
Indenture to be duly executed, and their respective corporate seals to be
hereunto affixed, all as of the date first written above.

                              COMPANY:
                              -------

                              @TRACK COMMUNICATIONS, INC.


                              By:  /s/ Jana Ahlfinger Bell
                                   --------------------------------------
                                    Name:  Jana Ahlfinger Bell
                                    Title: President and CEO

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                              TRUSTEE:
                              -------

                              THE CHASE MANHATTAN BANK, solely in its capacity
                              as Trustee



                              By:  /s/ John G. Jones
                                   ------------------------------------------
                                    Name:  John G. Jones
                                    Title: Vice President and Trust Officer

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