Most aircraft on a flight tracker are there because they broadcast their own GPS position over ADS-B. But not every aircraft does — plenty of older ones have a transponder that replies with its identity and altitude and no position at all. So how do those still show up on the map? The answer is MLAT — multilateration — a clever trick that locates an aircraft using nothing but the tiny differences in when several ground receivers hear the same radio signal. Here's how it works.
The problem MLAT solves
An ADS-B-equipped aircraft is easy: it figures out where it is with GPS and broadcasts that position, so a single receiver hearing it knows exactly where it is. But a Mode S transponder without ADS-B replies to interrogations with its identity and pressure altitude — useful, but it never says where it is. One receiver hearing that reply knows the aircraft exists and is somewhere in range, but not where. MLAT is how a network of receivers turns that positionless signal into a location.
How multilateration works
The whole thing rests on one fact: radio waves travel at a fixed, known speed (the speed of light). When an aircraft's transponder transmits, that signal reaches nearby ground receivers at slightly different times, because each receiver is a different distance away. Measure those differences precisely and you can work backward to the aircraft's position.
Here's the geometry. Take two receivers. The difference in the signal's arrival time between them tells you the difference in distance from the aircraft to each — and the set of all points with a fixed difference in distance to two fixed points is a hyperbola. So one pair of receivers narrows the aircraft down to a curve. Add a third receiver and you get another hyperbola; where they cross is the position. In practice you want four or more receivers hearing the same signal at once — three to pin down latitude and longitude, and a fourth for altitude and to make the solution robust. This is called TDOA — Time Difference Of Arrival.
It's essentially GPS run in reverse. With GPS, the satellite positions are known and your receiver solves for its own location; with MLAT, the receiver positions are known and the network solves for the aircraft's location. Both are the same distance-from-known-points math.
What MLAT needs to work
Multilateration is powerful but demanding, which shapes exactly where it works:
- At least four receivers hearing the aircraft simultaneously. No dense cluster of stations, no fix.
- Extremely precise, synchronized clocks. Since the position comes from time differences measured in nanoseconds, every receiver's clock must be tightly synchronized (typically GPS-disciplined). A clock error of a microsecond is a positional error of hundreds of metres.
- Good geometry and altitude. The aircraft has to be high enough for line-of-sight to several spread-out receivers. Down low, or over a sparse region, too few stations hear it and MLAT drops out.
MLAT vs ADS-B vs radar
Three ways to know where an aircraft is, and MLAT sits neatly between the other two:
- Primary radar bounces a pulse off the aircraft — fully independent, needs no cooperation from the aircraft, but expensive and gives no identity.
- ADS-B has the aircraft broadcast its own GPS position — cheap and precise, but dependent on the aircraft's own navigation and on it being equipped.
- MLAT computes position from the transponder's timing — it's dependent on the transponder replying, but independent of the aircraft knowing or sending its own position. It's how trackers cover the Mode S aircraft that ADS-B misses.
The trade-off: MLAT is less precise than ADS-B's GPS fix and needs that dense receiver network, so it fills gaps rather than replacing ADS-B. As ADS-B mandates roll out worldwide, fewer aircraft rely on it — but it remains a key part of how a complete tracking picture gets built, which we cover in how Flightradar24 actually works.
Getting the data (positions, not timing)
If you're building on flight data, the good news is you don't implement any of this — a tracking API hands you positions, already solved from whatever combination of ADS-B and MLAT saw the aircraft. SkyLink API's ADS-B endpoint returns each aircraft's position, altitude, and identity keyed on its ICAO 24-bit hex address, so you consume the finished fix rather than nanosecond timestamps. For the broadcast underneath it all, What Is ADS-B? is the companion read.
SkyLink API gives you a free tier of 1,000 requests/month to build a tracker against, with paid plans from $19/mo for production. It's available through the free trial — sign up, grab a key, and get positions without running a receiver network.