Multilateration is a proven technology that has been in use for many decades. It was developed for military purposes to accurately locate aircraft — many of which did not wish to be “seen” — by using a method known as Time Difference of Arrival (TDOA).

Multilateration employs a number of ground stations, which are placed in strategic locations around an airport, its local terminal area or a wider area that covers the larger surrounding airspace.

These units listen for “replies,” typically to interrogation signals transmitted from a local SSR or a multilateration station. Since individual aircraft will be at different distances from each of the ground stations, their replies will be received by each station at fractionally different times. Using advanced computer processing techniques, these individual time differences allow an aircraft’s position to be precisely calculated.

Multilateration requires no additional avionics equipment, as it uses replies from Mode A, C and S transponders, as well as military IFF and ADS-B transponders. Furthermore, while the radar and multilateration “targets” on a controller’s screen are identical in appearance, the very high update rate of the multilateration-derived targets makes them instantly recognizable by their smooth movement across the screen. A screen displaying multilateration information can be set to update as fast as every second, compared with the 4 - 12 second position “jumps” of the radar-derived targets.

MLAT in Action

1. Mode A/C/S Interrogation
2. Mode A/C/S Reply, ADS-B, IFF
3. Time Difference of Arrival (TDOA) Processing
4. Hyperbolic Positioning
5. Aircraft Position Display

MLAT ground stations receive replies from all transponder-equipped aircraft, including legacy radar and ADS-B avionics, and determine aircraft position based on the time difference of arrival (TDOA) of the replies.

“Multilateration offers ANSPs the possibility of providing a surveillance service at a potentially much lower cost, greater reliability and higher levels of accuracy than conventional SSR.”

Alexander ter Kuile, CANSO Secretary General