Multi-Lateration (MLAT) | ADAPT aeroNAV

MULTILATERATION (MLAT)

Description

Multilateration (MLAT), known as Hyperbolic Positioning, is when an object (aircraft) is located by calculating the Time Difference of Arrival (TDOA) of a signal transmitted from that object to at least 3 receivers distributed over a certain geographic area.

Having an airport under MLAT coverage requires the installation of strategically located ground stations (called remote units) around that airport. These units listen for aircraft replies to interrogations initiated by a near-by Secondary Surveillance Radar (SSR) or by another MLAT unit. Replies could be generated by a Mode A, Mode C, Mode S, or even an ADS-B transponder, therefore no additional avionics must be installed in aircrafts to communicate with MLAT ground stations (units), and this is a major advantage.

Once MLAT units receive replies from the same aircraft, a central processing unit computes the TDOA of the same aircraft signal to the different remote units and triangulates its position in 3D with high precision. The calculated 3D coordinates of the aircraft are then displayed on the ATC radar screen. A simple, yet ingenious technology, at a fractional cost of what a typical SSR would cost, while getting rid of all the radar disadvantages like reflected and deflected signals, obstructed signals, blind areas, weather interference, installation and maintenance costs, and others like Label Drops and Label Swaps on airport surface. A MLAT unit could weight around 25 kg; it can be installed on any high structure like GSM antennas, around the airport, at a mountain top, in the middle of a desert, or on a remote island. A MLAT unit has proved to endure sever weather conditions, and it requires minimal maintenance.

MLAT Applications

MLAT has proven to be highly reliable and efficient in aircraft detection and surveillance on Airport Surface, in Terminal Area, in Wide Area, in Precision Runway Monitoring, as a Height Monitoring Unit, in Environmental Management, and in Airport Operations & Revenue Management.


	Airport Surface Surveillance
	MLAT can be an essential element of Advanced Surface Movement Guidance and Control Systems (A-SMGCS) replacing the legacy Airport Surface Detection Equipment (ASDE) or the Surface Movement Radar (SMR), a rapidly rotating radar which signal deteriorates in heavy rain, or can be blocked by airport structures like hangars and terminals. A-SMGCS, also known as Local Area MLAT (LAM), guarantees complete coverage of the airport surface and high accuracy in target identification. MLAT enabled A-SMGCS is foreseen to improve surveillance, control, guidance, and planning under all weather and visibility conditions, ie in poor visibility and even at night.


	Terminal Area Surveillance
	In several airports around the world, lower altitude operations are restricted due to the airport proximity of terrain elevations like hills or mountains. The rotating radar has limited coverage over areas behind these elevations which prevented local controllers from using and monitoring such areas. By installing a MLAT remote unit at the top of these elevations, areas once restricted become available for usage by local controllers, resulting in a more efficient usage of Terminal Area.


	Wide Area Surveillance
	Wide Area Multilateration, also known as WAM, functions under the same concept of Airport Surface and Terminal Area Monitoring but with further extended remote units. A MLAT receiver can be installed in any remote area to cover airspaces that were never seen before by a rotating radar. A whole country can be covered with several remote MLAT units at distances of up to 100 Km between each others, and all that at a fraction of a single SSR cost.


	Precision Runway Monitoring
	MLAT is so precise that landings at closely spaced parallel runway can occur simultaneously without the need for in-trail spacing of approaching aircraft. This Precision in runway monitoring increases the airport throughput mainly during peak times and adverse weather conditions.


	Height Monitoring Unit
	MLAT is used to measure and verify the accuracy of Reduced Vertical Separation Minima (RVSM) equipment above FL 290 where vertical separation is reduced from 2,000 ft to 1,000 ft due to the introduction of RVSM altimeter technology.


	Environmental Management
	MLAT can record the aircraft information and its exact path over the ground, thus allowing further analysis related to noise and emissions reduction.


	Airport Operations and Revenue Management
	MLAT information being recorded, this will support further analysis and measurement of aircraft surface movement to achieve a more efficient usage of airport surface resources like ramps, taxiways, gates, turn-around equipment, etc...

Reference used: Multilateration & ADS-B Executive Reference Guide, a guide supported by ERA Corporation.