The PockeTracker incorporates a TinyTrak PIC IC and a VHF Dual-frequency 270 mW transmitter. It was a product of Byonics, but is no longer available.
PocketTracker has an option in the later model to change the frequency of the transmitter by talking directly to a divide-by network.
In the previous model the frequency of operation was fixed between two different known frequencies via a jumper.
The main advantage of the PocketTracker is its low power consumption and its size. It can run for something like 100 hours on a 9Volt battery. It was housed inside an Altoids tin, requiring only an external antenna and a GPS connection for operation.