This small AVR based APRS tracker is designed to be used in balloon. Tracker can be interfaced with GPS receiver; also it has several analog and digital I/O where various sensors can be attached. WhereAVR has ability to decode ax.25 packets, what allows reception of remote commands like real TNC does.
Board itself is built around ATmega8 microcontroller which has all necessary functions built in. Tracker is able to generate 1200-baud audio signal which is used to modulate carrier radio signal. I also have a zero-crossing audio signal detector. And of course what tracker would be without GPS module.
Module was put on a test where balloon flew above 90000ft. It worked fine … but its gone somewhere over Atlantic. Nice project to learn from. Author provides all necessary files in case you want to repeat his experiment.