Multilateration (MLAT) locates an aircraft by measuring the tiny differences in the time its transponder signal reaches several ground receivers with synchronized clocks. Four receivers are enough to solve a 3D position. Three are enough if the altitude is known from another source. Because the position is computed on the ground, MLAT is independent of what the aircraft says about itself.
§01How does multilateration work?
A multilateration system is a set of antennas plus a central processor. The processor computes the aircraft’s position from the time difference of arrival (TDOA) of the same signal at different antennas. Mathematically, each TDOA between two antennas puts the aircraft on a hyperboloid. With four antennas the intersection gives a 3D position. With three, a position can still be calculated if altitude is known from another source, such as the Mode C reply.1Source 1 · National Aerospace Laboratory NLR for EUROCONTROL, 2005Wide Area Multilateration: Report on EATMP TRS 131/04 (NLR-CR-2004-472)eurocontrol.int When receivers are spread across a wide region rather than one airport, the technique is called wide area multilateration (WAM).
§02Example: why timing matters
Radio covers about 300 m per microsecond. If two receivers’ clocks disagree by just one microsecond, the range difference is off by about 300 m before any geometry is applied. That is why MLAT networks invest heavily in synchronization (GNSS-disciplined clocks or reference transmitters) and why positions far outside the polygon formed by the receivers degrade quickly.
§03How is MLAT different from ADS-B?
| ADS-B | Multilateration | |
|---|---|---|
| Who computes the position | The aircraft (from its own navigation) | The ground network (from signal timing) |
| Needs | ADS-B Out equipment | Any transmitting transponder, plus 3–4+ synchronized receivers |
| Affected by aircraft GNSS problems | Yes | No (receivers still need good timing) |
| Typical use | Primary position source | Coverage for non-ADS-B aircraft; independent cross-check |
The OpenSky researchers describe WAM as independent of the aircraft, unlike ADS-B, though it still requires the aircraft’s cooperation by transmitting.2Source 2 · Schäfer, Strohmeier, Lenders, Martinovic, Wilhelm (IPSN 2014), 2014Bringing up OpenSky: A Large-scale ADS-B Sensor Network for Researchcs.ox.ac.uk That independence makes MLAT a natural cross-check on ADS-B positions, for example during GNSS interference.
§04Common misconceptions
- “MLAT can see any aircraft.” Only aircraft whose transponders are on and transmitting, and only where enough receivers hear them.
- “More receivers always means better positions.” Geometry matters as much as count. Research on crowdsourced networks found classic multilateration struggles with random, unplanned receiver layouts and proposed alternatives that improved coverage by up to 2.5×.3Source 3 · Strohmeier, Martinovic, Lenders, arXiv, 2016A Localization Approach for Crowdsourced Air Traffic Communication Networksarxiv.org
- “MLAT replaces radar.” Operational systems fuse it with radar and ADS-B. EUROCONTROL’s ARTAS tracker, for one, combines WAM with primary and secondary radar, Mode S and ADS-B.4Source 4 · EUROCONTROLAir traffic management surveillance tracker and server (ARTAS)eurocontrol.int
§05How Kimo uses MLAT
Kimo keeps the position source on every point of the tracks model, so the Airspace map can style MLAT and ADS-B differently and compare them where both exist. A persistent gap between an aircraft’s broadcast position and its MLAT position goes to the review queue. Feeds come in through the ADS-B and OpenSky connectors; see the ingestion guide.
§06Related terms
- ADS-B: self-reported positions that MLAT can verify.
- Mode S: the transmissions MLAT receivers time.
- Data fusion: merging MLAT, ADS-B and radar into one track.
Frequently asked questions
How many receivers does MLAT need?
Is MLAT affected by GNSS jamming?
Why do some flight trackers label positions “MLAT”?
Sources
4 references- Wide Area Multilateration: Report on EATMP TRS 131/04 (NLR-CR-2004-472) (opens in a new tab)National Aerospace Laboratory NLR for EUROCONTROL2005eurocontrol.int
Principle of multilateration: TDOA, hyperboloids, four vs three antennas.
- Bringing up OpenSky: A Large-scale ADS-B Sensor Network for Research (opens in a new tab)Schäfer, Strohmeier, Lenders, Martinovic, Wilhelm (IPSN 2014)2014cs.ox.ac.uk
WAM independent of the aircraft unlike ADS-B; TDOA with four or more sensors.
- A Localization Approach for Crowdsourced Air Traffic Communication Networks (opens in a new tab)Strohmeier, Martinovic, Lenders, arXiv2016arxiv.org
Limits of multilateration with unplanned receiver geometry; coverage up to 2.5× with k-NN.
- Air traffic management surveillance tracker and server (ARTAS) (opens in a new tab)EUROCONTROLeurocontrol.int
Multi-sensor tracking using PSR, SSR, Mode S, WAM and ADS-B.
External sources were accessed at the time of writing. Kimo product details, customers and figures in examples are illustrative unless a source is cited.
