Air based trackers:
I am currently on the 4th revision of my rocket trackers, although development of the current revision is still in its infancy. The 3rd revision tracker is almost to a usable state, however the board layout needs modifications, and I am still developing the code. Versions 1 and 2 had serious flaws and are not actively being worked on any more.
GPS Tracker Rev 1
GPS Tracker Rev 2
GPS Tracker Rev 3
Ground based trackers:
I only started working on ground based tracking recently, and only have had one revision thus far, rev0. The hardware is very similar to the the 3rd revision of my GPS tracker, and much of the code is works on both of them.
Vehicle Tracker rev 0
A History...
I have been developing both ground based and air based GPS tracking systems for several years now. Things started when my rockets started to get big enough that it was hard to find them after a launch, so a means of tracking them was needed. After evaluating a variety of radio direction finding methods it was decided that a more robust method was needed, so I decided on a system that uses a GPS module and a RF modem to transmit the coordinates of the rocket back to a base station. This eventually evolved into a system that includes a variety of other sensors and outputs to fire e-matches in the rocket to deploy parachutes, smoke charges, etc. I have also developed a ground based tracker using very similar technology for vehicle tracking.