John Sheridan

One of the air traffic management systems least well known to pilots is multilateration, sometimes called MLat, or multilat, or WAM (for wide-area multilateration). At any time of the day or night, and at airports and along air routes around the world, controllers routinely use MLat to monitor safe aircraft approaches to small airports in mountainous regions, or to track airliners as they taxi around major airports, or to ensure accurate separation between adjacent aircraft approaching on closely separated parallel runways, or to monitor aircraft positions over water or over large uninhabited areas where radar surveillance would be cost prohibitive. 
MLat is an extraordinarily flexible and highly accurate controller tool enjoying growing installation numbers every year. Eventually significantly lower-cost MLat might replace ATC surveillance radar, whose services it duplicates and, in many applications, exceeds.
How does MLat work? The principle is similar to that used by DME and ATC secondary surveillance radar (SSR), with one big difference. In DME and SSR, a DME transmitter in the aircraft or an airport’s SSR sends out interrogating signals to which the aircraft’s DME and radar transponder respectively reply with uniquely coded responses. 
Then, the aircraft’s DME unit and the airport’s SSR precisely measure the time it took for their signals to go out and return. Applying half that roundtrip time to the speed of light (186,000 miles per second, which is also the speed of radio signals) gives the aircraft’s distance from the DME station and, separately, the aircraft’s distance from the radar. The big difference is that MLat need not transmit any signals itself, but merely hitch hikes on the return half of the SSR’s roundtrip signals. (Airport SSR radars use the long, slim horizontal antennas rotating below the large, football shaped antennas of the long-range primary radars that simply “skin paint” aircraft targets as “blips” on the controllers’ screens, since they cannot process transponder returns.)
In MLat, a number of small, suitcase-size receiver stations–from five to as many as 20 or more, depending on the size and nature of the surrounding terrain–are spread around an airport or other designated area at various distances out to many tens of miles apart (which is where the W in WAM comes in) and each acts as an individual, passive “listening post.” When an aircraft’s transponder is interrogated by an ATC SSR, its unique reply goes back not only to the radar but is also picked up by all the MLat listening posts within reception range, with each post precisely logging each incoming signal’s time of arrival. Since the MLat listening posts are at different distances from the aircraft, the different times of arrival of the transponder’s signals from their different locations allow the MLat’s central data processor to calculate the aircraft’s position with great accuracy. From there the aircraft position is forwarded to the controller’s display, tagged with the aircraft’s ID and other data. 
In high-density traffic operations, an MLat ground listening post receiver can handle more than 100 incoming mode-A/C, mode-S and ADS-B signals simultaneously, since each incoming signal carries the unique coded ID of its aircraft’s transmitter, with the MLat’s central processor separately assembling each transponder’s or ADS-B unique message sets. 
Originally developed by Czech engineers seeking non-radiating techniques to track adversary aircraft under conditions where radar interrogation could not be used, multilateration has, from a slow start, become an essential element in civil ATC today. Yet while MLat plays a key role in the FAA’s ASDE-X and its overseas equivalents for airport surface movement and ground control (SMGC), its rapidly expanding adoption for both surface and airspace surveillance has occurred primarily outside the U.S. and Europe, whose overlapping radar networks normally duplicate, often triplicate and sometimes even quadruplicate, tracking coverage. 
So far, however, the ATC equipment manufacturing giants have not entered the MLat market, with the result that several midsize specialist companies are offering MLat equipment, with ERA a.s. of the Czech Republic, a subsidiary of the Czech Omnipol conglomerate, standing as the apparent leader in Eastern Europe and Asia.
MLat in Operation
While the open landing areas of an airport allow conventional monitoring of taxiing aircraft, they can frequently slip out of the surface ASDE-X’s line of sight as they move behind terminal buildings and other obstructions. In those cases, MLat stations are positioned at strategic locations to cover those “blind spots.” 
Similarly, the radar “shielding” presented by local high ground and mountainous areas has often limited flight operations in the approach and terminal areas, creating impractical IFR decision heights for scheduled and unscheduled operations. This was the case in Innsbruck, Austria, where steep mountainous terrain either side of the valley airport made the installation and regular maintenance of full radar coverage financially prohibitive. But small MLat stations positioned along the mountainsides now ensure full, radar-equivalent coverage over the approach, the runway and the missed approach airspace.
Flight access to the New Zealand ski and mountain resort city of Queenstown was also severely restricted by surrounding mountains until RNP approaches, weaving around the high ground to its single runway, were developed. But ATC safety requirements also demanded an independent flight-path-monitoring capability, resulting in a widespread network of MLat stations that was further expanded to cover almost the whole of the country’s South Island. 
In a slightly different application, access to several Colorado ski resort area airports was limited to a time-consuming one-in, one-out VFR procedure, to accommodate the area’s location behind a mountain range that placed it in the “shadow” of the Denver ARTCC’s SSR. Here, MLat stations were installed covering each airport’s operating area, with each area’s MLat configuration including dual mode stations that acted as listening posts and were also configured to transmit SSR interrogation signals identical to, and synchronized with, Denver’s SSR, thereby ensuring continuous surveillance by Denver ARTCC controllers, all the way to the airport surface. Suddenly these tricky airports became as accessible as wide-open, flat-terrain airports. 
At all airports with MLat service, airport vehicles can be equipped with small MLat transponders using unique response IDs, providing controllers with a complete picture of airport surface movements. Currently, Amsterdam Schiphol Airport has 407 vehicles equipped with ERA MLat transponders.
In the area of en route and terminal operations MLat has what some feel is an unlimited future, based on its extreme flexibility in adapting to new coverage requirements as traffic steadily increases and, perhaps even more important to air navigation service providers (ANSPs), its substantially lower costs than radar. When the price is totaled for its equipment, site access and preparation, installation, provision of diesel power, fuel and ongoing maintenance, basic radar can cost more than twice as much as a complete multi-station MLat configuration that usually covers more airspace. As a result, MLat is becoming the preferred replacement for aging current radars and for new sites where earlier a full surveillance radar would have been the only option. The UK and the Czech Republic have already selected MLat to replace radar at existing and new sites, with significant cost savings.
MLat also provides a benefit in the area of runway monitoring. As urban and industrial zoning gradually restricts airport expansion, a common solution to maintain or increase flight operations is to build new runways within an airport’s boundaries. Often, this will result in close parallel runways, where landing aircraft must be closely monitored. MLat has now been adopted at a number of major airports–most recently by China’s Beijing International–for this function. 
Called the precision runway monitor (PRM), the system employs a number of MLat units to cover the approach area with highly accurate signals whose data are presented on large displays monitored by dedicated controllers. The displays show both runways and their approach areas with a clearly marked no transgression zone (NTZ) between them. Should an aircraft drift off a runway’s centerline towards the NTZ, it receives a prompt controller instruction to return to the centerline. Should it continue toward the NTZ, the wandering aircraft is instructed to execute a missed approach. As it happens, San Francisco Runways 28L and 28R are PRM equipped, and information on PRM procedures is published with the runways’ special procedures. But had the <a href="http://www.ainonline.com/aviation-news/ain-air-transport-perspective/2013-07-08/asiana-777-approached-sfo-too-slowly">Asiana Boeing 777</a> been holding the centerline before its accident, the PRM–which does not monitor descent rates–would not have tracked its vertical flight path. It is conceivable that the FAA is investigating the need to include vertical-path PRM monitoring, despite the low probability of a recurrence of that event.
Offshore MLat
The earliest offshore MLat applications were probably those used to monitor helicopters serving the UK and European oil rigs in the North Sea. MLat was found to be an indispensable ATC aid in those areas since conventional shore-based radars suffer from surface sea clutter at the helicopters’ low altitudes, particularly in high sea states. Like the small Colorado airports not served by a conventional SSR, all the drilling rigs spread across the oilfield are now equipped with MLat receivers, including a number that carry dual-mode interrogator/receiver units, with all being datalinked back to shore.
Over the next few years, pilots may notice the gradual disappearance of the rotating secondary-radar antennas–known affectionately as “hog troughs” by early radar engineers–but they need have no concern. Their replacement by much less conspicuous MLat stations will bring equal or better performance, with a commensurate reduction in airport charges.

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