What Is MLAT (Multilateration)?

MLAT (multilateration) is a technique that finds an aircraft’s position by measuring exactly when the same transponder signal arrives at several ground receivers. It lets trackers show aircraft that do not broadcast their own position.

The idea

Radio signals travel at a known speed. If an aircraft sends a signal, a receiver nearer to it hears it slightly earlier than one farther away. With the precise arrival times at several receivers, a computer can calculate the position that fits all the time differences. Four or more receivers are normally needed to get a position and altitude, and the receivers must have synchronised clocks.

Why it is needed

Many aircraft still carry older Mode S transponders. These reply to radar interrogations but do not broadcast GPS positions. Without MLAT they would be invisible to a tracker that relies only on ADS-B. See Mode S transponders explained.

Strengths and weaknesses

• Works for aircraft without ADS-B.
• Needs several receivers close together, so it works best in areas with dense coverage, usually near busy airports.
• Less precise than ADS-B and sometimes shows jumps or errors.

What it means for Pakistan

Where receivers are few, MLAT is unlikely to work, so the number of aircraft you see depends on volunteers in each area. Adding receivers helps; see become a feeder.

General information only, not legal or safety advice. Details change; check with your airline and the relevant authority. See our editorial policy.