The GPS Tracker page covers the background for the tracker unit, while this page provides specific technical details for anyone wanting to create something similiar. As the original unit was somewhat of a lash-up, there's not a lot of documentation but the actual electronics is quite simple so it shouldn't be too hard to replicate.
The core of the system is a PIC microcontroller (specifically, a PIC16F690) running a minimal program that interrogates the GPS unit for the current position and which then sends the position to the GSM module for transmission to the webserver. Both the GPS unit and GSM module have RS232 interfaces and the PIC has a built-in UART so interfacing between these elements is in theory just a case of hooking them together (with the appropriate level translation chippery).
It is, of course, a touch more complex than that - the PIC only has a single UART, but needs to communicate with two different devices. However, it only needs to speak to one device at a time - so the solution was to switch the PIC TX and RX lines between the two devices using a multiplexer (the original circuit uses a CD4011 and the NAND function in the RS232 interface chips, controlled by an output pin of the PIC).
In the picture above the Li-Ion battery can be seen mounted into the case, with the GSM module above it (the SIMM card can just be seen at the lower edge of the module). The PIC and RS232 circuitry is built on 0.1" stripboard and mounted on the case lid. On the left side of the board the coloured ribbon cable is the RS232 interface to the GSM module while the 4-core cable on the right is the RS232 interface and power for the GPS unit (upper right).
The original design was centred around the parts that were to hand at the time of construction. If more time had been available, a cleaner solution would have been to use a Maxim RS232 interface chip with integrated charge pump and a better arrangement for the serial port mux (one benefit would be a far simple power supply design - the original has 5 rails, +3.7V VBatt, +/-7V for RS232, +5V for the RS232 receiver and +3.25V power out for the GPS.
It's also worth noting that the GSM module used is also available with an integrated GPS receiver - for a pure tracker that would almost certainly be a better choice, however in the case of the original the GPS unit was required anyway, for navigation and for route logging.
At the time of writing, no additional info or documentation is available other that what is presented on these pages. Further information may be published in the future, particularly if a 'MK2' device is constructed.
If you have comments or questions regarding this project, please use the Feedback page.