[amsat-bb] PicSat demodulator?
daniel at destevez.net
Wed Jan 17 22:12:57 UTC 2018
El 17/01/18 a las 17:56, Scott escribió:
> Despite the documentation, PicSat beacons so far have been 1k2 BPSK.
> There is a 1k2 BPSK software modem in:
> ... and the demod on that would be USB. Of course the bandwidth &
> centering of the audio output is critical for this scheme; what works well
> for me is to output the audio (I use HDSDR into VB Audio virtual cable) at
> 4 KHz with the signal centered on the 2000 Hz mark. If you match that 2000
> Hz center on the 1k2 BPSK software modem screen, it decodes very nicely.
As Scott said, the complete description for PicSat's mode is 1k2 BPSK
AX.25 with G3RUH scrambler.
UZ7HO's soundmodem should works nicely with this, but for the
adventurous, there is also a GNU Radio based decoder in gr-satellites.
For this, the signal should be centered at the 1500 Hz mark (though you
can change this setting).
The integrated telemetry decoder is underway (at least some form of it).
I've yet to optimize the gr-satellites decoder, but I've noted that
PicSat's signal almost fills a typical 2700Hz bandwidth (it's nominally
2400Hz wide), so no wonder that tuning is critical. Perhaps I'll do
another decoder that works on "wide SSB" (up to 24kHz width, with the
signal on the 12kHz mark), as is typically used for the wider 9k6 BPSK
modes. Using the "FLL Band edge" in GNU Radio, it should be possible to
correct automatically around 500Hz of mistuning.
Also, there is a decoder from the PicSat team that runs on Windows and
Linux. I haven't downloaded it, and I'm not even sure if it has been
released in public (other than in its Github repo), but I've heard that
it even searches for the signal (like the FUNcube dashboard or FOXtelem)
so you don't even have to worry about tuning.
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