Thanks both, very helpful, I have installed graphs1090.
I have looked at the gain optimization scripts (I am impressed at all that in bash!), but I don’t think the script’s logic will work well for me. I am in a very sparsely over-flown area with very few contacts and mostly at extreme range, but a handful of flights per day operate from an airstrip a couple of Km away, and fly right over my head at a few 100 feet. It looks like the script tries to adjust the gain according to the fraction of messages with strong signals, so it would wind-up the gain to max for all those very weak and distant signals.
I will have a play with the config file (can anyone tell the values allowed for rtlsdr-gain ? The manual says a numeric gain value in dB, or “max” for AGC , what numeric range is allowed ?
Perhaps I will also try to deconstruct the gain scripts and customize something. I am surprised there seems to be no way to adjust gain on the fly in response to excess signal.