Wayne Farley's Aviation Blog - Air Traffic Control | Air Traffic Management | Safety Management System

Multilateration techniques were developed by the military decades ago and have been successfully employed for airport surveillance more recently. Today, these same techniques are being used for larger areas to cover the en-route and approach phases of flight. Such systems are called Wide Area Multilateration (WAM) systems.

Multilateration is a surveillance system which makes use of signals transmitted by an aircraft to calculate the aircraft’s position. It uses currently existing aircraft transmissions systems (eg. Mode A, Mode C and Mode S transponders), thus requiring no changes to the airborne infrastructure.

Principle of Multilateration

A multilateration system consists of a number of antennas which are placed in strategic locations around an airport, its local terminal area or a wider area that covers the larger surrounding airspace. These antennas listen for “replies from aircraft,” typically to interrogation signals transmitted from a local SSR or a multilateration station. A central processing unit connected to the antennas precicely calculates the aircraft’s position from the time difference of arrival (TDOA) of the signal at the different antennas.

The TDOA between two antennas corresponds, mathematically speaking, with a hyperboloid (in 3D) on which the aircraft is located. When four antennas detect the aircraft’s signal, it is possible to estimate the 3D-position of the aircraft by calculating the intersection of the resulting hyperbolas.

When only three antennas are available, a 3D-position cannot be estimated directly, but if the target altitude is known from another source (e.g. from Mode C or in an SMGCS environment) then the target position can be calculated. This is usually referred to as a 2D solution. It should be noted that the use of barometric altitude (Mode C) can lead to a less accurate position estimate of the target, since barometric altitude can differ significantly from geometric height. With more than four antennas, the extra information can be used to either verify the correctness of the other measurements or to calculate an average position from all measurements which should have an overall smaller error.

Radar and multilateration “targets” on a controller’s screen are identical in appearance, however, the very high update rate of the multilateration-derived targets makes them instantly recognizable by their smooth movement across the screen. A screen displaying multilateration information can be set to update as fast as every second, compared with the 4 - 12 second position “jumps” of the radar-derived targets.

WAM Application

As the need for traffic surveillance expands over areas not presently covered by conventional secondary radar, many ANSPs are taking advantage of the cost benefits of multilateration versus new radar installations.

In wide area multilateration (WAM), the stations are spread much further apart, at distances of up to 100 km between each other. WAM installations in places such as Tasmania and the Czech Republic provide superior range over secondary radar, more accurate tracking, significantly lower costs, and significantly earlier operational readiness following contract award. Armenia has chosen WAM as a replacement over their existing “legacy” secondary radar because cost and performance analyses has showed the clear advantages of multilateration.

In addition, in the North Sea, between the UK and northern Europe and Scandinavia, the small, lightweight and low powered multilateration units will be mounted on offshore drilling platforms to provide “better than radar” performance down to the surface, in locations where secondary radar would have been impractical.

WAM and ADS-B

WAM and ADS-B systems are similar, but different. Both use non-radar ground stations to listen for aircraft transponder transmissions, and send those data to the ATC center, and then to the controllers’ screens. WAM determines an aircraft’s position by triangulation from several stations, while a single ADS-B station uses the GPS position transmitted in the aircraft’s ADS-B message.

The two systems are complementary to each other, since WAM stations can be upgraded to ADS-B ground stations as the user aircraft community gradually equips with ADS-B.

Similar Aviation Applications

• Decca Navigator System - A system used from the end of World War II to the year 2000, employing the phase-difference of multiple transmitters to locate on the intersection of hyperboloids
• OMEGA Navigation System - A worldwide system similar to Decca, shut down in 1997
• LORAN-C - navigation system using TDOA of signals from multiple synchronised transmitters