Community / MLAT
How MLAT works
Multilateration (MLAT) is the technique that allows tracking aircraft that do not transmit GPS position — including many military aircraft, helicopters and aircraft with Mode-A/C transponders.
The principle: TDOA
Every Mode-S transponder — even without ADS-B — responds to secondary interrogations with a 1090 MHz radio signal. This signal travels at the speed of light and arrives at different receivers at slightly different times. The Time Difference Of Arrival (TDOA) depends on the aircraft's position relative to the receivers.
With two receivers, the aircraft position lies on a hyperbola in space. Three receivers give two hyperbolas — their intersection narrows to two points. With four or more receivers the solution is unique and three-dimensional: latitude, longitude and altitude are recovered.
Receiver synchronization
TDOA only works if receivers have nanosecond-synchronized clocks. The FlyItalyADSB MLAT server does not use GPS for clock synchronization: instead, it uses aircraft with verified ADS-B position as a reference.
When two receivers detect the same aircraft with a known GPS position, the server can compute the clock offset between the two receivers. This self-calibration process is continuous and ensures precision even without dedicated GPS hardware on the receivers.
Why it matters
Italian military aircraft
Fighters, patrol aircraft, SAR helicopters — most fly with basic Mode-S transponders without ADS-B. MLAT is the only way to track them on the civilian map.
General aviation
Many light aircraft, ultralights and gliders carry only Mode-A/C transponders. MLAT makes them visible even at low altitude.
Meteorological data
For the weather-32 research project, aircraft tracked via MLAT also transmit Mode-S messages — enabling BDS50/BDS60 wind data extraction.
Safety
MLAT coverage reduces surveillance "blind spots", useful for traffic analysis and safety in mountainous areas with limited radar coverage.