MLAT explained: how aircraft without an ADS-B position get located

A gray military transport appears on the live map, with a small “MLAT” next to it. A few minutes later its track jumps, then disappears altogether. Anyone who regularly uses flight tracker services knows such moments – and wonders where the position actually comes from if the aircraft doesn’t even transmit it itself. The answer is multilateration, MLAT for short. This article explains how the method calculates, what it requires, where it hits its limits and how to interpret MLAT tracks correctly when spotting.

MLAT in one sentence

Multilateration determines an aircraft’s position by having several ground receivers pick up the same transponder signal while a server works backward from the tiny differences in arrival times to where the signal was sent from. The aircraft doesn’t have to reveal anything about its location for this. It is enough that it transmits at all.

The technical term for this principle is Time Difference of Arrival, TDOA for short. It is the key to understanding: what is measured is not the distance to the aircraft, but only how much later one receiver hears the signal than another.

Why MLAT is needed at all

Most aircraft you see on flight tracker maps supply their position themselves. They regularly transmit their GPS coordinates, altitude, speed and identifier via ADS-B. Receivers on the ground only have to pick up these messages and forward them – how that works in detail is described in the guide Flight tracking with ADS-B for spotters.

But there are aircraft that have a transponder switched on yet send no position along with it. Typical candidates:

• Military aircraft that operate without an ADS-B position for operational reasons but often have their Mode S transponder active.
• Older business and private aircraft with a Mode S transponder and no connected GPS source.
• Light aircraft and helicopters in general aviation that only have the mandatory equipment on board.
• Aircraft whose ADS-B output is faulty or temporarily provides no valid position.

These aircraft still transmit their unique identifier and usually their barometric altitude via their Mode S transponder. A single receiver then knows: the aircraft is somewhere within range, at this altitude. But not where. That is exactly the gap MLAT closes.

The principle: time differences instead of distances

Imagine an aircraft sends a short signal. Receiver A hears it first, receiver B a few microseconds later. It follows that the aircraft is closer to A than to B, by exactly the distance the signal covers in that time difference.

All points for which this difference in distance holds lie on a curved surface – mathematically a hyperbola in the plane or a hyperboloid in space. A pair of receivers therefore yields not a point but a whole curve of possible locations. Only when several such curves from different receiver pairs are overlaid do they intersect at one place: the aircraft’s position.

Why four receivers?

For a position in three-dimensional space, the method needs three independent time differences – and for that, four receivers. Because Mode S signals usually contain the barometric altitude, one dimension can be saved; then three stations are theoretically enough. In practice, the networks still insist on at least four: FlightAware names that explicitly as a requirement, because the additional station evens out measurement errors and makes the position more stable.

Why the clock decides everything

Radio waves are fast: about 300 meters per microsecond, 30 centimeters per nanosecond. If one receiver’s timing is off by just one microsecond, the calculated position already shifts by hundreds of meters. MLAT therefore stands or falls with how precisely the clocks of the receivers involved match each other.

Professional air traffic control systems work with precisely synchronized ground stations. Hobby networks made up of thousands of Raspberry Pi receivers don’t have that – and solve the problem elegantly via the sky itself.

The feeder networks’ trick: ADS-B aircraft as timekeepers

The open-source server mlat-server, on which many community solutions are built, describes the approach openly: it uses ADS-B aircraft that transmit their position as reference beacons. Because it is known where these aircraft are, it can be calculated when their signal should have arrived at each receiver. The deviation between expected and actual timestamp reveals how each individual receiver’s clock runs – how fast it ticks and how far it is offset.

With this clock model, the server then calculates backward for the aircraft that don’t transmit a position. That has two consequences you should know as a spotter:

• No ADS-B traffic, no MLAT. In regions with little position-reporting traffic, the references are missing and synchronization gets worse.
• The antenna position has to be exact. If a feeder enters its coordinates or height wrongly, it distorts the calculation for everyone. Networks therefore often exclude such receivers from MLAT.

How an MLAT calculation works

Behind the little abbreviation on the map is a chain of several steps that runs through in a few seconds.

FlightAware gives a typical delay of 4 to 6 seconds for this process and shows up to six MLAT positions per minute and aircraft on its website. In the local map program SkyAware, MLAT targets appear in light orange. Other services mark them with their own colors or the note “MLAT” in the source data field – what the individual symbols mean is explained in the article Symbols and colors on flight tracker maps.

Where the 24-bit address comes from – and who falls through

For the server to know that four receivers are really hearing the same aircraft, it needs a unique identifier. Mode S supplies it: every Mode S transponder transmits the ICAO 24-bit address, the so-called hex code, which is permanently assigned to one aircraft. Via this address, the registration can usually also be looked up in databases – how that works is shown in the guide Tracking an aircraft by registration or hex code.

Older transponders in Mode A/C, by contrast, only transmit a four-digit squawk code and altitude but no unique address. For the feeder networks, they are practically impossible to match up and drop out of multilateration. Aircraft with the transponder switched off remain entirely invisible – they show up on no civil flight tracker, no matter how many receivers are nearby.

What the aircraft transmits	Position on the flight tracker	Typical source
ADS-B with GPS position (Mode S Extended Squitter)	direct, usually accurate to the second	Airliners, many modern business and light aircraft
Mode S without position, with altitude	only via MLAT, if enough receivers are in range	Some military, business and GA aircraft
Mode A/C only	practically none in hobby networks	Older transponder equipment
Transponder off	none at all	Military on operations, gliders without a transponder

For gliders, by the way, there is a route of their own: they can often be tracked via FLARM and the Open Glider Network, which the article Receiving FLARM and OGN introduces.

The limits of MLAT

MLAT is a great tool, but not a radar replacement with seamless coverage. Three factors limit the method.

Line of sight and flight altitude

Signals at 1090 MHz propagate almost in a straight line. A receiver only hears an aircraft as long as there is no curvature of the earth, mountain or building between the two. At cruise altitude, many stations see the same aircraft; on approach and on the ground, only a few. That’s why MLAT tracks often drop out just when it gets exciting for spotters: on final approach to the runway.

Receiver geometry

If all receivers stand roughly in a line, the calculated curves intersect at a very shallow angle, and small measurement errors turn into large position errors. FlightAware therefore recommends spreading receivers about 10 to 150 kilometers apart in a triangle, square or circle arrangement rather than along a line. An aircraft flying outside the area spanned by the receivers is also calculated less accurately.

Network density

In metropolitan areas with many feeders, MLAT coverage is good; over sparsely populated regions and over the sea, it is barely present. Over open oceans, MLAT doesn’t help at all; there, space-based ADS-B plays to its strengths – though also only for aircraft that transmit a position themselves.

MLAT in air traffic control

What hobby networks do with Raspberry Pis, air traffic control uses in professional form. There, the method is called Wide Area Multilateration (WAM). A network of ground stations receives the transponders’ replies and calculates positions from them that appear on controllers’ screens. WAM works with Mode A/C, Mode S and ADS-B without anything having to be retrofitted on board.

In performance terms, WAM is considered comparable to classic secondary radar but is often cheaper and can be set up in mountains, where radar stations would be shadowed by peaks. Named areas of use include the Innsbruck region in Austria and parts of Colorado and Juneau, Alaska in the US. The same principle is also used on airport aprons to monitor taxiing aircraft.

An important difference from the hobby variant: professional systems are designed for precise time synchronization and, depending on the design, can also interrogate transponders themselves. Feeder networks, by contrast, only listen passively. If you’re interested in the organization behind it, you’ll find more in the article on DFS Deutsche Flugsicherung (Germany’s air navigation service provider).

Contributing to the MLAT network yourself

If you run your own ADS-B receiver, you can directly improve MLAT coverage in your region. The big networks support this with their feeder programs; which software is responsible for which network is compared in the article ADS-B feeder programs compared.

At FlightAware, MLAT works from PiAware 2.0 or FlightFeeder 7.0 onward; you’ll find the instructions under Setting up PiAware. For active participation in the calculation, FlightAware cites an upload requirement of about 50 kbit/s – no problem for a normal internet connection. Community networks such as ADS-B Exchange, adsb.fi or airplanes.live also run their own MLAT servers; how to feed them is shown in Setting up a feeder for ADS-B Exchange. Note that some networks place their MLAT results under their own license terms and restrict redistribution.

The most important lever is the location: a high-mounted antenna with a clear horizon sees lower-flying aircraft for longer and delivers more common targets with the neighboring stations. Tips on that are in Finding the best antenna location. Whether you’re allowed to receive and share ADS-B data at all is covered in the article on the legal situation of ADS-B reception.

Reading MLAT tracks correctly when spotting

At the fence, the math matters less than putting things in the right context. A few rules help:

• MLAT means “calculated”. The position is an estimate with a delay of a few seconds. For the question “Is it about to arrive?” that’s enough, for the exact second of landing it isn’t.
• Jumpy tracks are normal. If the receivers involved change, the position can appear briefly offset. A single outlier doesn’t mean a change of course.
• A drop-out on approach is not a go-around. If an MLAT target vanishes shortly before the airport, it has usually just sunk below the receivers’ common line of sight.
• If the track is missing, the aircraft hasn’t gone. Many reasons why an aircraft doesn’t appear at all are explained in the article Why an aircraft doesn’t show up on Flightradar.
• Compare several services. Each network has its own receivers and thus its own MLAT coverage. A look in a second app can fill the gap – differences are shown in the flight tracker app comparison.

MLAT gets especially exciting for anyone looking specifically for military or business aviation. You’ll find pointers in Tracking military aircraft online and Spotting business jets. Rescue and police helicopters also often show up only via MLAT, as the guide Finding helicopters on the flight tracker shows.

Conclusion

Multilateration is the reason aircraft that don’t reveal their position at all also appear on flight tracker maps. The method uses only the arrival times of ordinary Mode S signals at several receivers, aligns their clocks via ADS-B aircraft and calculates a location from that. The prerequisites are at least four receivers with a clear view, a sensible distribution on the ground and a Mode S transponder on board. Where these conditions are missing – near the ground, at the network edge or with plain Mode A/C – the track ends.