What is MLAT (Multilateration) and How Does It Track Aircraft?

The problem MLAT solves

Some aircraft don't transmit ADS-B Out — older airframes, military aircraft, or ones with malfunctioning equipment. They still respond to secondary surveillance radar (SSR) interrogation with Mode S transponder data, but without a position.

MLAT figures out where they are.

How it works

Multiple ground receivers listen to the same Mode S transponder reply. Tiny differences in the time of arrival of the same signal at different receivers reveal the geometry. With four or more receivers, position can be calculated.

The math is the same as GPS, just inverted — instead of one receiver listening to many satellites, many receivers listen to one transmitter.

Why it's important

• Fills in coverage for non-ADS-B aircraft.
• Useful for flight tracking near airports where lots of receivers are clustered.
• Often the only way to track GA aircraft in the US under 18,000 ft.

Limitations

• Requires 3–4 receivers with line-of-sight.
• Less accurate than ADS-B (typically 100–500 m vs 5–25 m).
• Doesn't work over open ocean.
• High altitudes need very wide receiver geometry.

What you'll see in a tracker

FlightyFlow and other modern trackers fuse ADS-B and MLAT data automatically. The flight icon doesn't usually distinguish between the two — you just see the plane.