Gnu Radio integration experiment

[tomaszmrugalski]

The following PR introduces a bridge between Svarog and Satnogs. The whole Svarog infrastructure (station framework, cli, recipes, etc.) remains in place. There is a new Svarog recipe that calls satnogs_noaa_apt_decoder.py script. That script is generated by GNU Radio, when loading noaa_apt_decoder.grc. This is a bit complicated to set up, but has tons of advantages:

• Provides GNU Radio flexibility. Anything is possible there (doppler correction, waterfall, adjustments, advanced processing etc.)
• Uses Soapy component, which supports ALL available SDR hardware on market (this was tested on AirSpy)
• This does not replace the existing noaa_apt recipe, but rather provide an alternative noaa_apt_gr. It is possible to switch back and forth between then to do comparison, experiments etc.

This approach, if accepted, should address #39 (SoapySDR integration), #10 (GNU Radio integration), and would make #12 (Doppler correction) much easier to do and #4 (upgrade to noaa-apt 1.3.0) less urgent.

This is not a complete solution, just a step in what I think is the right direction. There are many more steps missing:

• the installation is currently manual
• there are no unit-tests
• the waterfall.png is generated, but the upload doesn't work
• there is no rating generated (that's essential for making comparisons between different radio settings)

[tomaszmrugalski]

Another problem: this does not turn bias-T on its own. If the NOAA LNA is not powered, it actually attenuates the signal and all we get is a noise.

The workaround for this is to do the following after pi reboot: cd programs/airspy_adsb/ && ./airspy_adsb -b and abort it using ctrl-\. This will bypass the shutdown routine and the bias-T will be kept on. Primitive, but seems to be working.

[tomaszmrugalski]

@fivitti, you made some comments about this PR when it was still a draft. Can I assume your review complete or should I wait for more?

[fivitti]

As an experiment this code is good. Probably we will need to refactor it later when we improve the GNU Radio integration.