Modern flight tracking rests on ADS-B (Automatic Dependent Surveillance–Broadcast). “Automatic” because it transmits without being asked; “dependent” because it depends on the aircraft’s own navigation, typically GPS; “surveillance” because it reports a position; and “broadcast” because that report goes out to everyone, not to a single recipient. Each equipped aircraft fixes its position from satellites and broadcasts it on 1090 MHz, usually about twice a second.
Those broadcasts are short digital messages picked up by ground stations — antennas and software-defined radios run by airports, agencies and ordinary enthusiasts. No single station sees the whole sky; each hears aircraft within radio range, roughly to the horizon. Community networks such as OpenSky and ADSB.lol pool receivers, so an aircraft is tracked as long as participating coverage can hear it.
The tracker bundles each aircraft’s latest messages into a “state vector”: a snapshot of position (latitude and longitude), barometric and geometric altitude, ground speed, track or heading, and vertical rate (climbing or descending). This is the unit the map renders — one row per aircraft, refreshed as new broadcasts arrive.
Because every aircraft computes and broadcasts its own data, a flight tracker is fundamentally a listening device. It does not interrogate aircraft or instruct them; it simply collects what they are already transmitting. That makes the picture wonderfully open, but it also means coverage, accuracy and completeness depend entirely on what gets broadcast and who is listening.