Using the receive times to determine a plane's location is one method (multilateration), but it's also becoming common for planes to broadcast their position (A

Using the receive times to determine a plane's location is one method (multilateration), but it's also becoming common for planes to broadcast their position (ADS-B), determined with a high degree of accuracy using their onboard GPS. This ADS-B transmission is what sites like FlightRadar24 use as their main source, although they're now also able to use MLAT in areas with good base-station coverage.