Multilateration (MLAT)


Multilateration (MLAT) is a surveillance system consisting of three or more ground stations placed in strategic locations which listen to transponder signal replies. A sophisticated computer processor then uses a method of hyperbolic positioning from time difference calculations to pinpoint the precise 3D position of each aircraft, and provides this information to ATC via the situation display.


The advantages of MLAT over conventional SSR (secondary surveillance radar) is that it is cheaper to install and maintain, is more accurate and works well in situations where conventional radar (with it’s large rotating antenna) has previously been problematic.

MLAT in New Zealand

MLAT is currently used at two locations within New Zealand:

1. Auckland airport for surface movement surveillance

The incorporation of MLAT into Auckland airport’s surface movement surveillance system has helped create a safer operating environment and improved the situational awareness of Tower controllers, particularly during low visibility operations.

2. Queenstown basin area wide area multilateration (WAM) surveillance coverage.

Queenstown WAM has provided a high accuracy surveillance tool in an area with previously little or no radar coverage. Queenstown ATC are now better equipped to cater for the growth in jet operations in complex Class D airspace without being quite so constrained by procedural limitations. The accuracy of traffic information passed between IFR and VFR flights is much improved with the controllers having access to accurate MLAT-derived information on their radar displays.

The way of the future?

MLAT requires no additional avionics equipment to operate as it utilizes existing SSR transponder technology.
It is also “future-proofed” as being compatible with ADS-B transponders, with the two complementary systems expected to eventually replace SSR and form the basis of global air traffic surveillance of the future.


  1. Nolan, M. (2004). Fundamentals of Air Traffic Control. Brooks/Cole
  2. ICAO. (2010) Aeronautical Telecommunications - Surveillance Radar and Collision Avoidance Systems. ANNEX 10 Volume IV Amendment. 85.
  3. James, C. (2007). Multilateration: Radar’s Replacement? Avionics Magazine. Retrieved 15 August 2011 from

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