Packet radio from the ISS with GNU Radio and USRP

On July 10, 2010, I was listening to the linear transponder downlink of VO-52 when I suddenly noticed a very strong peak about 50 kHz below. I checked Gpredict and found out that it was the APRS downlink from the International Space Station RS0ISS on 145.825 MHz. Thanks to the flexible GNU Radio framework and the USRP, I could easily receive both satellites at the same time as shown on the video below.

I used an Arrow II antenna connected directly to the WBX transceiver on the USRP. The IQ data from the USRP was recorded to a binary file as described in this article. Having the recorded data, which is roughly 1.5 GB for a 15 minute pass sampled at the lowest possible rate of the USRP 250 ksps, I can always replay the pass and fine tune the decoding process.

I used the multimon software modem for decoding the AFSK packets. It’s a great app that uses either the soundcard or an audio file for input. I used the latter and even managed to decode in real time by using a named pipe between the GNU Radio software defined radio receiver and the multimon decoder. A tutorial showing how to do that is available here.

Watch video on YouTube.

Author: Alexandru Csete

Embedded software engineer in the satcom industry during the day. Radio amateur and SDR hacker during the night.