Command line option explanations
I’ll collect some explanation on the command line option for airspy_adsb in this post:
-r Reduced IF bandwidth
This option limits the bandwidth of the signal visible to the ADC to about 3.5 MHz instead of the default 9 MHz. This can be useful in some situations when you have some broadband noise right over the 1090 MHz band. But mind you, reducing the bandwidth of a signal also smooths the transitions between 0 and 1 levels in the time domain. This makes the detection harder or even impossible in some cases.
(written by prog the developer of airspy_adsb here: Thoughts on optimizing gain - #73 by prog)
-x Enable DX mode
-x is unrelated to oversampling. It simply tells the decoder to trust more frames that are likely to be good.
(written by prog here: Planes but not feeding - #25 by prog)
-m <mlat_freq> MLAT frequency in MHz: 12 or 20
This option name is a little misleading, more accurate would be: Sample Rate.
Basically it determines how often the ADC is read. Setting this to 20 will use a lot more USB bandwidth and CPU power for decoding. On a Raspberry Pi using 20 therefore might not work.
-p Enable Bit Packing
In short if -m20 is not working properly you can try adding -p to the options (actually i included it by default when you use my configuration file)
Also, what exactly does bit packing do and is it ever a bad thing?
Bit packing tells the hardware to pack every 2 x 12bit samples into 3 bytes instead of sending every samples as a 16bit integer. This saves 25% of the USB bandwidth, but leaves less headroom in the MCU for synchronizing the USB with the ADC. In general, it helps with some old computers or SBCs.
(Answer by prog HOWTO: Airspy mini and Airspy R2: Piaware / dump1090-fa configuration - #49 by prog)