This PR adds basic adaptive gain support, which adjusts SDR gain on the fly based on the noise & signal levels seen.
There are two control mechanisms:
Dynamic range control is enabled by the --adaptive-range option. This adjusts SDR gain to try to achieve a minimum dynamic range, regardless of the exact hardware in the RF path.
Burst (loud message) control is enabled by the --adaptive-burst option. This decreases SDR gain when undecodable loud messages are heard, allowing for better reception of nearby aircraft at the expense of range.
This is only the basic implementation - see the PR for remaining work to do.
Oliver Jowett