The topic of gain comes up often enough and unlike some i don’t like the gain optimization script.
Aircraft fading in and out of view make short term number gathering not that useful.
The metric i like most for optimizing gain is the percentage of what dump1090 calls “strong_signals”.
Those “strong_signals” are messages that get close to getting clipped.
If you don’t care about some background you can scroll down to the next post for the HOWTO
This is a clipped signal, the waveform coming into the analog to digital converter gets its largest and lowest values cut off.
(courtesy of wikipedia: https://en.wikipedia.org/wiki/Clipping_(audio)#/media/File:Clipping.svg)
If too much is clipped it is harder to recognize the waveform. With dump1090 it’s not critical that we don’t clip at all though, because of how the 1s and 0s are encoded in the Mode-S downlink and the way dump1090 detects the bits.
Anyway suffice it to say if your gain is set too high and an airplane flies by relatively close and at low altitude, the messages might not be decoded anymore. (At high altitude due to the reception pattern of most antennas the plane signal actually gets much weaker as it flies directly overhead)
There is an intermediate stage where only some message of a plane get decoded because the others are too strong, so position updates might get slower before no position is detected anymore.
This phenomenon can be observed on your local map if your gain is set really high or a plane flies by really close.
Adjusting gain is mostly a topic for people with an LNA or the yellow or blue FA dongle.
Without amplification the rtl-sdr dongles typically won’t be sensitive enough for the maximum gain setting of -10 (AGC) to be a problem. So without amplification you can just use -10. But it doesn’t hurt to check anyway if you are curious.
I might add some more explanation on RSSI/dBFS here later.
Now in the next post on to checking the percentage of strong signals and changing gain.