How to decode Bell202 modem signal? APRS signals from the ISS at 4 stations

A project log for The Distributed Ground Station Network

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hornighornig 08/15/2017 at 17:240 Comments

Now, 4 DGSN stations receiving APRS from orbit by the ! Stations are now in Settrup (a lovely small village in Lower Saxony!), in Stuttgart, near Lake Constance and in Jena. Where should we put one next? If you have a suggestion, please tell me here >>

My task is now that I still want to know the content of APRS the sent here! Would anyone please help me decode? :) I uploaded a Python Numpy NPY file here. It incuded the Signal Peak you see in the graph above from those 4 stations

What I know is that APRS uses Bell202 coding with two frequencies of 1200 Hz and of 2200 Hz. Each represent either a 1 or 0. I just started to play with an FFT Kernel of 8K samples and I already found phases of those frequencies but so far I couldn't find the starting flag of the AX.25 frae 01111110.

As far as I know, Bell202 was also used in old internet modems. So I guess that there are people on Hackaday who play around with old modems and solve those problems while sleeping. If so, please comment below.

Until then, I will keep on playing with the 4 stations and trying to decode APRS signals.... :)