"RSSI offset" default of 0 is not useful
The "rssi offset" value that can be configured via the VTY and command line arguments is initialized to a default of 0.
This does not work out at all, at the very least it's proven to be bogus on the popular USRP B2xx hardware in DCS 1800 (See #4467). I would acually be surprised if it's correct on any hardware at all.
In an ideal world, every GPSDR vendor would ship every unit together with a proper calibration table over frequency, so that a power level as seen in the I/Q samples can be translated into an absolute value in dBm.
However, the world is far from ideal, and the best we can do is try to measure this offset for the few commonly used devices we have available (B200, B210, N200, LimeSDR-mini, LimeSDR USB) and then use that value instead of '0'.
We may need a value per band (particularly on LMS where different bands go thorugh different RF paths), and it will of course also change depending on how the hardware receive gains/LNAs are configured. Also, it will drift over frequency within the band, and it will of course have a spread across different units of a given device type.
However, I guess anything is better than "0" at this point.