The expanding use of Multilateration

Dec. 3, 2009
When multilateration entered the civil air traffic control (ATC) marketplace in the 1990s, it largely was to provide surface movement surveillance. The technology has since been tasked to do much more, and additional applications are being explored.

By David Jensen

When multilateration entered the civil air traffic control (ATC) marketplace in the 1990s, it largely was to provide surface movement surveillance. The technology has since been tasked to do much more, and additional applications are being explored.

Multilateration, or MLat, is both backward compatible with secondary surveillance radar (SSR) and can be forward compatible with automatic dependent surveillance-broadcast (ADS-B). ADS-B is a developing technology viewed as a significant component in the Federal Aviation Administration's (FAA's) NextGen and Eurocontrol's Single European Sky ATM Research (SESAR) modernization programs. Radar, MLat, and ADS-B all utilize transponder transmissions to perform the corresponding function of pinpointing aircraft positions.

MLat, therefore, is often viewed as a fitting technological bridge between surveillance radar and ADS-B. "It's as accurate [as ADS-B] yet doesn't require ADS-B avionics," says Bill Colligan, vice president and general manager of Era Systems Corp. in Fairfax, Va.

Multiple applications

MLat has come a long way since the 1990s largely because wide area multilateration (WAM) has been developed to provide surveillance well beyond airports. In one application or another MLat is in use in locations as diverse as Tasmania, Mongolia, South Africa, India, Chile and, of course, Europe and North America. Developing countries that find SSR too pricey to purchase and maintain, have "leapfrogged" technologically to the relatively less expensive and more advanced MLat.

Multilateration's applications, too, are diverse. Consider the following established, or proposed, installations:
* In Innsbruck, Austria, MLat provides terminal area surveillance where radar is unable to accommodate full coverage between the towering mountains flanking the Innsbruck Valley. Since 2005, the 11 MLat ground stations there have provided coverage up to 8,000 feet over a 350-square-nautical-mile area.

* In the North Sea oil patch, MLat will be monitoring offshore helicopter traffic beyond SSR's 80-nm range. Ground stations (in this case, "over water" stations) on 16 deep-water oilrigs are slated to be fully operational by summer 2010.

* At the U.S. Marine Corps' 29 Palms Air Ground Combat Center, the 32 MLat ground stations provide aircraft-positioning accuracy in an area where considerable live ammunition is fired.

* In Santa Clara, Calif., MLat is used for both airport operations management and aircraft noise monitoring. Unlike radar, it is geo referenced and, therefore, can accurately determine an aircraft's position relative to the ground, an important factor in noise monitoring.

* At Detroit Metro Wayne County Airport, multilateration provides precision runway monitoring that allows simultaneous parallel approaches to closely spaced (between 3,400 and 4,300 feet) runways.

* In Namibia, a nationwide surveillance system is planned using MLat. The African country has no radar.

* At Beijing, China, multilateration is used for enhanced terminal surveillance.

* In Frankfurt, the first WAM installed in Germany provides surveillance for one of Europe's busiest airspaces. It was added to existing surface-monitoring system, and like nearly all ATC MLat systems, the WAM will process ADS-B messages.

* And in Sydney, Australia, MLat will monitor precision approaches and, starting in 2010, will provide wide area surveillance. The technology serves Airservices Australia as an interim surveillance system until it fully establishes its ADS-B network.

While surface movement radar must serve solely in an ATC capacity, MLat can determine landing-fee billings, as well as monitor movements of aircraft on the ground. On average, landing fees represent at least one-third of an airport's revenue.

In the U.S., United Kingdom, Austria, and other countries, MLat also serves as a height-monitoring tool for certifying aircraft operating in a reduced vertical separation minima (RVSM) environment. Certification authorities require that onboard altimetry must be verified every two years.

MLat's roots

Like many advancement in aviation, MLat has its roots in the military, dating back to the 1960s. One of its earliest applications was passively tracking "bad guys" bearing devices that emit electromagnetic signals.

Today there remains a strong military market for the technology, in air defense, range control and, still, enemy detection, among other missions. However, worldwide, a majority of the MLat systems serve a civil use, says Colligan.

The Czech Republic's air navigation service provider (ANSP) was a pioneer in MLat's civil use. When Czechoslovakia was part of the Soviet Union, the Kremlin designated the country as a "pocket of excellence" for surveillance technology. Emphasis in advancing the technology continued after the Soviet Union was dissolved.

Prague-Ruzyne International Airport was first to employ multilateration as an advanced surface movement guidance and control system (A-SMGCS), in 2001. (The first FAA-approved A-SMGCS, certified in 2003, is at Gen. Mitchell International Airport in Milwaukee, Wis.)

In 2002, the first WAM became operational in Ostrava, to assure aircraft are separated by at least 3 nautical miles. In 2007, the Czech ANSP was first to add wide area surveillance to A-SMGCS, in Prague. In all cases, MLat's accuracy met the International Civil Aviation Organization's (ICAO's) and Eurocontrol's radar certification standards.

Era was instrumental in the Czech Republic reaching its milestones. The U.S. company, Rannoch Corp., acquired Czech-based Era in 2006 and maintained its internationally familiar name. Prior to the buy-out, the Czech company's business was primarily outside the U.S.

In 2008, Fairfax-based SRA International Inc., a government contractor with deep roots in the IT field, chose to enter a product-oriented business by acquiring Era.

The company's primary competitors are Sensis Corp., Syracuse, N.Y., and France's Thales. Thales North America is teamed with ITT to pursue the multilateration and ADS-B markets in the U.S. and Canada.

How it works

Multilateration can be a passive surveillance system. It accurately determines aircraft positions by receiving Mode A, C or S transponder signals that are responses to SSR interrogation. With four receivers, a central processing unit can calculate an aircraft's position in three dimensions by computing the time difference of arrival (TDOA).

A transponder's emitted pulse will reach the receivers at difference times, and calculation of those differences will determine an aircraft's position. The system does not need to know exactly when the pulse was emitted, only the difference in time it is received.

TDOA distinguishes MLat from ADS-B, though they both use ground stations that listen to transponder signals. With ADS-B, the ground station receives a transmission of an aircraft's GPS position, altitude and ID from an onboard Mode S transponder.

MLat computes the signal-receiving-time differences of a transmission coming from a Mode A (ID only) or Mode C (ID and altitude) transponders, as well as from Mode S.

MLat proponents believe multilateration would be an ideal backup to ADS-B. "It can carry you through in case of a GPS outage," suggests Marc Viggiano, Sensis Corp.'s chief operating officer.

Sensis, Era, and Thales all manufacture dual-use, MLat/ADS-B sensors. "Multilateration can be used to verify ADS-B tracks," says Colligan.

Regardless the segment of flight under surveillance – en-route, terminal area or approach and after touchdown – the ground-based MLat sensors function the same, allowing for flexibility and uncomplicated expansion.

"You can have sensors for airport surface monitoring, and then you can add more sensors for precision approach, and more sensors yet for a radar substitute," Viggiano says.

MLat ground systems are more dependable and require less maintenance that SSR with its moving parts. It can withstand lightning strikes, gale-force winds, torrential rain, sand and dust storms, and extreme temperatures.

When MLat's positioning data is delivered to a ATC screen, the air traffic controller will notice no difference from radar – except he or she may perceive the aircraft designations tracking smoother. Secondary surveillance radar, with its rotating antenna, updates an aircraft's position every four to five seconds, thus creating tracks with a "jumpy" movement.

Conversely, the stationary multilateration (and ADS-B) sensors and central processor deliver updates every second, so the aircraft designation on the controller's screen moves smoothly.

A day will no doubt come when all controller screens will show smooth aircraft tracking. But not quite yet. In the case of a WAM system Sensis installed in the Colorado Rockies, the Denver area control center chose not to modify its software to accommodate the faster updates. Hence, its MLat updates have been slowed to radar speed.

Certified in September, the Colorado system is the first WAM to receive FAA approval for aircraft separation. Like the Innsbruck installation, the 26 ground stations around Garfield County Regional Airport, Steamboat Springs Airport, Craig-Moffat County Airport, and Yampa Valley Airport furnish terminal area surveillance in mountain valleys, where radar contact is lost.

"These airports are so small they can't justify having radar," says Viggiano. Now aircraft approaching the four airports, which serve a bustling snow skiing area, can be safely separated in the terminal control area (TCA) and not have to wait until one lands and files its arrival before another can enter the TCA.

Like SSR, MLat also can be an active surveillance system. In fact, multilateration installations used for air traffic control have a rotating interrogator, operating on 1030 MHz frequency, collocated with the sensor.

An active MLat system "is best," according to Viggiano, "because you can be assured of the update rate and of knowing the aircraft's altitude and ID." Unless an aircraft is ADS-B equipped, a passive MLat system will not receive that input.

A promising market

More multilateration applications are being explored. Although MLat manufacturers keep their cards close to their chests, Era's Colligan did say that "from a business perspective, we see multilateration moving into airport surface management, to maximize airport efficiency and attain better gate and ramp management and resource utilization."

Sensis sees a growing WAM market. "Multilateration offers service providers an additional surveillance option, and an opportunity for lower costs, expanded coverage and transition to ADS-B," says Viggiano. "Wide area multilateration is being selected to replace or augment older rotating radars.

"It provides coverage in geographically constrained areas where rotating radars are not practical," he adds. "Since WAM independently calculates an aircraft's position with accuracy on a par with ADS-B, it offers an ideal backup to ADS-B and the ability to coexist with other surveillance technologies.

Seeking a quieter Washington, D.C.

The Metropolitan Washington Airports Authority (MWAA) recently installed Era Systems Corp.'s noise and operations monitoring system (NOMS) at Ronald Reagan/Washington National Airport and Washington Dulles International Airport.

NOMS was installed, in large part, because Reagan National is located near downtown, where aircraft noise can be a sensitive issue, and Washington Dulles, which also is near a population area, is the capital city's busiest airport. is an integrated airport operations management tool that can be used for revenue management and gate allocation, as well as for noise monitoring and other applications. With tracking data from ADS-B and multilateration surveillance, the system provides an airport with real-time access to all flight track information.

In addition, according to Era vice president Bill Colligan, "we are providing MWAA with their first public portal to allow concerned citizens to file noise complaints over the web. This will help airport staff continue their pioneering noise mitigation work."

The interactive web portal includes features such as online complaint entry, report viewing, address location, noise abatement information and historic replay.

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