A350-900 - MSN93 (CX #14) B-LRO

In some regions with coverage from several FR24-receivers we also calculate positions of non-ADS-B equipped aircraft with the help of Multilateration (MLAT), by using a method known as Time Difference of Arrival (TDOA). By measuring the the time it takes to receive the signal from aircraft with an older ModeS-transponder, it's possible to calculate the position of these aircraft. Four FR24-receivers or more, receiving signals from the same aircraft, are needed to make MLAT work. MLAT coverage can only be achieved above about 3,000-10,000 feet as the probability that four or more receivers can receive the transponder signal increases with increased altitude.

An aircraft’s ADS-B transponder is transmitting the position from the on board flight computer, which in most modern aircraft comes from GPS data and is very accurate. On some older aircraft (for example some older A300, B737, B757, B767 and RJ100) the position is calibrated on ground before take off and after that the current position is calculated based on speed and direction of the aircraft. When flying for some time, in wind or in circles, the calculation becomes increasingly worse, which at landing can give position calculations that are up to 10 km incorrect.