What is ADS-B flight tracking?

ADS-B stands for Automatic Dependent Surveillance–Broadcast. An equipped aircraft derives its own position from GNSS and broadcasts it roughly once a second — along with altitude, velocity and identity — on 1090 MHz. Nothing interrogates it and nothing acknowledges it: the aircraft simply says where it is, continuously.

Those broadcasts are unencrypted, so anyone within range with a receiver can hear them. Networks of volunteer ground stations aggregate that reception into the live picture TallyAir draws from, which is why the map is only as good as the receivers nearby: aircraft drop out over water and empty terrain, and an aircraft not broadcasting ADS-B never appears at all.