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.
Waterfall Plot showing button presses of three different remotes
PushStack 
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