Ugifer wrote this instructable detailing the build of his Arduino based high-altitude balloon tracker:
The tracker is based upon the Atmel ATMeag328 Microcontroller which forms the heart of many of the popular “Arduino” boards. We are going to make an “Arduino Compatible” board which we can program using the Arduino IDE.
Because the GPS module and SD card both require 3.3v and we have plenty of computing power, we may as well make the whole tracker run on 3v3. That means that we can’t clock the ‘328 up to its full 16MHz but it will run happily at 8MHz on 3v3, and that’s plenty for our purposes.
Check out the video after the break.