AX.25 packet radio is currently the most popular data protocol for use in CubeSats.
Radio amateur Alejandro Santos LU4EXT is developing extmodem an open source APRS compatible AX.25 packet radio modem.
It is currently capable of both sending and receive packets at 1200 bps AFSK. The main feature of the software is that it is currently running three different demodulators in parallel, increasing the quality of reception. The first modem is Thomas Sailer HB9JNX / AE4WA‘s multimon, the other two are described by Sivan Toledo 4X6IZ in an article in the July/August 2012 issue of QEX.
Download extmodem for Windows from
QEX article – A High-Performance Sound-Card AX.25 Modem
APRS destination address for ham radio satellites
LU4EXT on Twitter https://twitter.com/alejolp