问题
I am having some difficulties in understanding how to decode a Bell 202 encoded signal. How do I do that?
I know Bell202 encodes binary data with ones and zeros as 1200 Hz and 2200 Hz. I recorded a radio signal on 145.825 Mhz with a 2Mhz sampling rate when the International Space Station (ISS) transmitted APRS.
I uploaded a Python Numpy NPY file here https://drive.google.com/drive/folders/0B-6iySZt3rrjQVdpRHBOc2lyQTQ and it has 4 data streams in it. Don't be confused, I just recorded the same signal at 4 location (more here https://twitter.com/andreashornig/status/897435897414791168) Each stream is the IQ-data as a complex array of the same peak you see in the twitter graph. I bandpass filter the signal in the vicinity of the center, because the ISS APRS transmits on 145.825 and I recorded exactly on it. If the bandpass is +-5KHz and you plot it as a waterfall, you can even see the dopplershift.
With the bandpass filter in mind, I do an FFT with 2m kernel, multiply the fft with the filter and then I can do IFFT and receive the complex signal back, as seen on the twitter graph.
So with that, I would now run an FFT kernel over the newly filter complex signal for find the two twones on 1200 and 2200 Hz.I also did and found some phases of frequencies around 1200 and other phases of frequencies around 2200 Hz. But So far, I couldn't find the starting flag of 01111110.
What would you advice me to do now? I am a very visual person but so far I couldn't find any tutorial with nice graphs I could easily understand.
Thank you in advance or any help or tipp!
Andreas
来源：https://stackoverflow.com/questions/45699913/how-to-decode-bell-202-signal-aprs-data-from-international-space-station