John Sheridan

For many, multilateration (sometimes abbreviated Mlat) is one of those vague ATC terms that is always hard to define. Put simply, it is how a spread-out group of small, unmanned, ground-based “listening posts” continuously monitors aircraft transponder signals, and then collectively triangulates them to derive individual aircraft positions. Following that, they send those positions plus their idents, altitudes and other data to ATC. It’s a pretty straightforward concept. Consequently, in the early 1990s, airport ground controllers at Dallas/Fort Worth, Atlanta, Heathrow, Frankfurt, Amsterdam and elsewhere acquired multilateration systems from Sensis of Syracuse, N.Y., to monitor surface traffic movements. 
Today, that technology has expanded into so many diverse applications that it can be truly called ATC’s Swiss army knife. With its faster-than-radar one-second position update rate, its GPS-like accuracy, its lower cost and its extraordinary flexibility, multilateration has become an essential element in a growing number of U.S. and international ATC and airport systems as we move into the coming age of air traffic management. 
While Sensis is well known in world ATC and airport circles, it doesn’t flaunt its successes in the wider aviation community. In fact, when the FAA issues its expected final ADS-B carriage rule next month, few pilots will recall that in 2000, the operational practicality of ADS-B was already being proved daily in the agency’s Alaska Capstone program, using a widespread network of Sensis ground stations. 
One result of that pioneering work is Nav Canada’s first major ADS-B network, now operational over 250,000 square miles of Hudson Bay. Further expansion is planned for Canada’s northeast coast and, eventually, across the country’s barren north, with its reduced separations bringing time and fuel savings, and safety, to regions where radar is impractical.
The ground stations receive and process both conventional transponder and ADS-B signals, a key attribute during the transition to ADS-B. In doing so, they also provide a vital backup against onboard or ground-system ADS-B failures or GPS outages. In fact, industry observers have long suggested that wide-area multilateration (Wam) networks throughout the NAS could provide superior service, at much lower cost, than today’s radars. That point was proved in 2005 at the Innsbruck, Austria, mountain valley airport, where the cost of complete radar coverage of the terminal area and approaches was prohibitive. But the small, suitcase-size Sensis units, some located high up the mountain slopes, provided full coverage and at greatly reduced cost. 
Improved Accuracy This process has been repeated at other terrain-challenged locations, such as Rifle and Hayden in Colorado; Juneau in Alaska; Vancouver and Fort St. John in British Columbia; and through the mountains of Tasmania. In Colorado, IFR 
<link news>access to winter resort airports has traditionally been a “one in, one out” procedure, since the FAA’s Denver radar cannot reliably track targets below 13,000 feet. With Wam, surveillance coverage allows five-mile en route separation and coverage down to traffic-pattern height and below at the four airports around Rifle and Hayden. And when aircraft descend below Denver radar’s line of sight, the multilateration ground stations automatically transmit radar-identical interrogations to the aircraft transponders. 
On flatter terrain at Detroit, the FAA has certified the Sensis high-accuracy multilateration precision runway monitor to guide aircraft on simultaneous approaches to closely spaced parallel runways. That is the first such installation; Sydney, Australia, will follow later this year. Last year the NextGen Transition Task Force recommended multilateration PRM for closely spaced parallels, to increase throughput and capacity. In addition, increasing pressure from airport neighbors means that few future runways will be built outside today’s airport boundaries, resulting in more– and more closely spaced–runways inside them, with increasing need for PRM’s safety assurance.
But the flagship of the Sensis portfolio is the Asde-X airport surface detection equipment, now being installed at 35 of the FAA’s busiest airports. Here, data from multilateration stations around the airport is “fused” with the returns from the system’s radar, producing six-meter, high-integrity position accuracy. Major airports worldwide have purchased a similar configuration, under ICAO’s name of the advanced surface movement guidance and control system.
Asde-X has itself also “fathered” several diverse applications, including its Safety Logic program, which in an innovative test project with Honeywell provided crews with instant voice alerts about potential surface collisions. Separately, several airports, including JFK and Newark, and airlines such as Continental and Delta, currently take data feeds from FAA Asde-X units to their individual Sensis Aerobahn systems, to obtain updates on real-time arrivals, departures, surface movement densities, taxiway backups, de-icing wait times and other causes of delay. While no FBOs have yet adopted Aerobahn, it could offer useful benefits to corporate operators.
Sensis is beginning installation at Orlando of the first of 22 FAA runway safety light systems, where Asde-X automatically illuminates a string of bright red centerline lights upon detecting a runway incursion. The system also illuminates similar bars as aircraft approach taxiway entrances to runways.
Sensis v-p and general manager for air traffic systems John Jarrell told AIN, “As we move toward NextGen, we are seeing more and more potential applications for our multilateration technology, both here and overseas.” Jarrell, a West Pointer, former Army VIP pilot and an ATP holder, added, “We also see increasing benefits for the total aviation community, including corporate and general aviation.”

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