I’ve finally managed to capture some Somfy RTS frames using a RTL2838 based DVB-T dongle. By demodulating the signal using rtl_fm in AM mode and feeding the output through a simple threshold detector, the binary data can be obtained.
rtl_fm -M am -f 433.42M -s 270K | ./am_to_ook -d 10 -t 2000 - | ./decode_somfy
During testing at first I only managed to receive one of my remotes. Taking a look with GQRX quickly showed the problem. Seems like the crystal in the other remote isn’t very accurate causing it to send on a slightly higher frequency of about 433.5 MHz. Given the low sample rate of 27 KHz I was using, the signal fell outside the tuned range. Increasing the sample rate fixed this.
All tools are now available from github.