How does adsb.fi work? - adsbfi/adsb-wiki GitHub Wiki

adsb.fi mainly shows ADS-B and MLAT positions. ADS-B: ADS-B transponders broadcast the aircrafts location twice a second (and also send other data which includes everything a Mode S transponder sends).

MLAT: Mode S transponders are the older technology and do not broadcast location. To obtain the location the arrival time of a message received by at least 4 receivers is used to determine the location, this is less precise than ADS-B positions but large errors will usually be obvious looking at the flight path (jagged path).

Secondary radar in general: https://www.radartutorial.eu/13.ssr/sr04.en.html

Mode S: https://www.radartutorial.eu/13.ssr/sr20.en.html

ADS-B / Mode S message formats: https://mode-s.org/decode/

The ICAO 24-bit address for each aircraft, often called icao id / hex id or even shorter icao / hex: https://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes#ICAO_24-bit_address

Note that transponders can be misconfigured and will then transmit the wrong identification. Military planes will sometimes use a bogus hex id or will deactivate their civilian transponders altogether. Some older military planes only have a Mode C transponder installed, adsb.fi does not display Mode C data as getting a position via MLAT would be unreliable and not worth the effort as the plane cannot be identified (no unique 24-bit hex id like Mode S has).

And if you're into videos, this explains a bit: https://www.youtube.com/watch?v=KkowxHF8g3M

Around 1500 feeders all over the world use their SDR (software defined radio) and an antenna to receive 1090 MHz and send the data to adsb.fi