Multilateration’s Alpine Leap Forward

Multilateration’s ability to improve the control of aircraft and vehicle movements on the ground at Europe’s busier airports has earned it respect on the continent. Faced with relentless air traffic growth, European authorities have been seeking ways to maximize airport throughput and efficiency, maintain operations in low visibility and ensure safety, as well as prevent runway incursions. Multilateration is a key component of the solution. It expands the coverage provided by traditional surface movement radar and adds the ability to identify aircraft unambiguously.

But some forward-looking air navigation service providers (ANSPs) are seeking even more from the technology. Austria’s Austro Control GmbH, for instance, has extended multilateration coverage beyond airport boundaries, into the third spatial dimension, and applied it to air traffic control (ATC) within the terminal area and en route. As a result, wide area multilateration (WAM), sometimes augmented by automatic dependent surveillance-broadcast (ADS-B), could soon take off.

ADS-B is regarded as a complementary technology, providing wide area coverage and compensating for ATC radar’s shortcomings. ANSPs are enthused about ADS-B’s global momentum and how the GPS-based system might complement technologies such as multilateration.

Where It Began

First, a brief retrospective. Three years ago, London-Heathrow claimed to be the first airport to make multilateration operational for ATC, albeit for ground control only. At the time, Graeme Henderson, manager of surveillance and display systems for the UK’s National Air Traffic Services (NATS), summed up the technology’s significance, commenting, "For the first time air traffic controllers have an accurate, labeled display of all aircraft on the airport surface in all weather conditions. I’m confident that multistatic dependent surveillance will give major safety and efficiency benefits." One of multilateration’s pioneers and the supplier of the Heathrow system, Sensis Corp., DeWitt, N.Y., calls its system multistatic dependent surveillance (MDS).

By tagging transponder-equipped aircraft targets on a screen with their identities, multilateration provides that vital nugget of information that primary radar misses. Moreover, they can "see" such aircraft even when they are hidden from conventional radar by buildings, terrain or other obstructions. Multilateration also can track surface vehicles that are fitted with the necessary transponders. The technique therefore provides many of the benefits of secondary surveillance radar (SSR), but does so close in, at substantially lower cost, and with greater precision.

Like several other European airports, Heathrow saw multilateration as complementing ground radar within an existing advanced surface monitoring and ground control system (A-SMGCS). The Sensis MDS can operate with SSR transponders of Mode A, C and S types, plus transponders associated with military identification friend or foe (IFF) systems and also ADS-B.

At Heathrow eight MDS ground sensors are placed around the airport and another seven are within the central terminal area. The sensors feed their data to a central display processor, where it is fused with surface movement radar data for presentation on a controller workstation.

Another key European airport, Schiphol International, at Amsterdam, also chose MDS for an A-SMGCS program. Providing oversight of all the airport’s maneuvering areas, the system helps to enhance the control of surface movements for the Netherlands ATC, managed by Luchtverkeersleiding Nederland (LVNL). Sensis additionally supplied LVNL with a number of its Veelo units–reduced-cost Mode S transponders designed for surface vehicles–for vehicle tracking trials.

A similar system has gone to Paris’ Charles de Gaulle airport. According to Aeroports de Paris, the system demonstrated that it could determine aircraft position within 25 feet (7.5 m), compared with about 40 feet (12 m) with the ground radar. This helps the busy 24/7 airport maintain its throughput capacity at night and in low visibility. The MDS contributes to ground surveillance for some 1,370 movements daily, updating aircraft position and identity once per second on the ground movement display.

Swiss ATC authority Skyguide first tried MDS for advanced surface surveillance at Zurich airport. Heinz Schmid, program manager for Skyguide’s SAMAX project to provide an A-SMGCS at both the Zurich and Geneva airports, was gratified by the fact that installing a test system–six sensors and a reference transponder–on Zurich airport’s north side took less than two months. Subsequently, Skyguide extended coverage at Zurich to the entire airport and ordered an additional system for Geneva.

An MDS ordered by Belgocontrol is part of its A-SMGCS program at Brussels International, an airport much frequented by personnel from the European Commission and Eurocontrol.

But the country making the biggest waves with ATC multilateration in Europe, and arguably the world, is Austria. Austro Control, the country’s ANSP, first commissioned a system for Vienna International airport. In the first six months of 2004, the airport saw a 14.3 percent rise in flight movements, putting it on course to break its 2003 record of 12 million passengers.

According to Johann Zemsky, Austro Control’s chief operating officer, Vienna airport can expect "an additional 4 percent traffic growth in the next few years and 8 percent this year." The airport is undertaking a major terminal expansion project over the next four years and anticipates a near doubling of passenger numbers when the project is complete. "MDS enables us to meet our current surveillance needs and has the flexibility to grow with our airport," Zemsky comments.

However, the jewel in the multilateration crown is a new surveillance system at Austria’s Innsbruck airport, nestled in the towering Austrian Alps. Here Austro Control is taking the technique a massive step beyond airport surface monitoring and applying it to aircraft operating in three spatial dimensions in the terminal movement area (TMA). Deployed in the surrounding mountainous terrain, this WAM-based surveillance system is intended to overcome problems that would have been experienced with conventional radar as aircraft make approaches rendered difficult by high terrain and frequent bad weather.

According to Werner Langhans, Austro Control’s head of technology development, Innsbruck poses unique challenges. Because the airport resides in an alpine valley, aircraft must approach and depart in a confined east-west cleft bounded to the north and south by mountain ranges reaching 9,850 feet (3,000 m) high. The Inn Valley also experiences much inclement weather, with frequent visibility degradation. Its soaring conditions are attractive to glider pilots, as well, and this creates visual flight rules (VFR) traffic. Aircraft movements in the valley average some 200 per day, but can exceed 360 at peak times. Difficulties for airliners are compounded by an off-centerline ILS approach. Pilots need special training before they are allowed to make approaches to and departures from Innsbruck’s airport in anything but the highest minima conditions.

A Practical Decision

Austro Control decided that, in the future, the airport will transition to new RNP 0.3 (0.3 mile required navigation performance) procedures, which are now in place and have been validated with Austrian carrier Lauda Air. The agency also decided to install a surveillance system for the TMA.

To provide the latter, Austro Control first considered upgrading its radar infrastructure. But radar is problematic in mountainous regions, being subject to multipath reflections and performance degradation through precipitation. Studies revealed a need for two radar stations, which would be difficult to maintain and make cost of ownership extremely high.

"Optimally sited radar stations would have to be accessed by ski during the winter season, which is about half the year," Langhans explains. "A simple maintenance or repair action could easily become an all-day event. Another issue was that of obtaining permission from the authorities to site two high-power radar transmitters."

Instead, Austro Control began considering wide area multilateration, perhaps augmented by ADS-B, to provide the necessary surveillance and tracking capability. The reliable, non-rotating sensors incorporated in MDS are quickly installed, simple to maintain and use relatively little power. Life cycle costs, therefore, are relatively low.

Most commercial aircraft already carry the necessary transponders. An exhaustive series of computer simulations indicated that the MDS/ADS-B concept would be viable. In response, Sensis made the best commercial offer. Austro Control also asked that the Mode A-, C- and S-capable MDS system be integrated with an ADS-B system, so that a complementary channel would be available for validation purposes.

Originally, Austro Control intended to use the system for test purposes only. But its performance proved to be so beneficial that the system has achieved full operational status. Austro Control collaborated with Sensis and other contractors in developing a system that would integrate with radar (for surveillance above FL110), VHF digital link Mode 4 (VDL-4), ADS-B sensors, flight plan data, an Avibit ASTOS data fusion and tracking system, a parallel Eurocontrol-standard air traffic management (ATM) surveillance tracker and server (ARTAS) for validation purposes, a traffic information service-broadcast (TIS-B) server, an Asterix archiver, a display processor, and an air situation display in the tower.

Austro Control’s setup includes an ADS-B system from CNS Systems and Terma A/S. It fuses data from a VDL-4 data link and the 1090-MHz extended squitter ADS-B link that was added as a spin-off from Sensis’ MDS system. The ARTAS tracker is the 7.0 version that can accept ADS-B inputs. Ground vehicles are fitted with transponders that communicate with a central ground station via the VDL-4 data link. The ground station is linked to the Innsbruck local surveillance system. Vehicles also have their own displays.

The MDS itself includes reference transmitters, a target processor, maintenance display, portable display, and nine remote sensor units, six of which are receive-only. The target processor and other major units are dual-redundant. The receive-only sensors, each with a fixed omnidirectional antenna, are located at various sites in the Inn Valley. They are positioned at heights ranging from the valley floor to the top of the highest mountain, all carefully chosen to secure the best possible multilateration geometry. The geometry remains acceptable even when a single sensor fails.

Sensis included dual-reference transmitters at two of the stations, to provide high-precision time synchronization. This timing configuration satisfies Austro Control’s wish to be independent from GPS. Each sensor requires a few hundred watts of power and a 64-Kbit/s telecommunications or microwave link for data transfer.

Everyone’s Pleased

Results from flight tests carried out in late 2003 proved highly encouraging, validating predictions from the simulation studies and pleasing pilots and controllers alike with the system’s accuracy and stability. Some 97 percent of all measured positions were shown to be within 230 feet (70 m) of actual position and accuracies as high as 23 feet (7 m) were achieved on the airport surface.

Mode A, C and S transponder returns provided stable results and proved that aircraft well outside the coverage area can be monitored. For example, the system successfully tracked a small private aircraft departing the airport and maintaining 2,000 feet, as well as other aircraft that were well over the FL110 threshold at which radar surveillance is expected to take over.

The update rate averaged at least once per second, and even squitter signals from aircraft overflying the TMA at 20,000 feet were received sufficiently often for tracking updates at a rate of 1.55 times per second. Although Austro Control has no immediate plans to use WAM-derived altitude data for anything other than a means to check barometric altitudes, the agency viewed the fact that the system returned satisfactory determinations as a potential future asset.

Overall, says Langhans, WAM has shown that it can improve accuracy and performance while also reducing Austro Control’s surveillance costs. Illustrating the latter point, he said that a new radar system providing required coverage would have cost some $13 million, about five times the cost of the multilateration system. Langhans explains that his agency sees WAM as a complement to ADS-B and likely to prove invaluable in securing successful surveillance in terminal areas, especially in high-density airspace and at busy airports.

Spreading the Coverage

Austro Control is now fired with WAM ambition. First it wants to integrate the Innsbruck WAM sensors into its existing ATC data processing system. Second it intends to work with German provider Deutsche Flugsicherung GmbH (DFS) and Skyguide in Switzerland to extend WAM coverage from Innsbruck towards Munich and Zurich. Germany’s DFS already is advancing the technology in Frankfurt. Austro Control wants to expand multilateration coverage at Vienna airport from the surface into the surrounding TMA and then to the rest of Austria.

With partner service providers, Austro Control aims to expand coverage still further into central Europe, integrating with the Central European Air Traffic System. The agency seeks partners to join in promoting the technology and entities such as Eurocontrol and the International Civil Aviation Organization to standardize it. Langhans said he would like to see standards to support the system’s use both with radar and as a stand-alone surveillance tool.

Like other technologies, multilateration has drawbacks. For example, it relies on cooperation and cannot detect targets that do not have transponders fitted or have their transponders switched off. It may be necessary to caution pilots not to turn off their transponders too early on approaching gates. Parked aircraft that are not powered up will not be detected, nor will vehicles without transponders. Vehicles used for unlawful purposes may avoid detection.

These are, of course, good reasons for retaining radar. Another danger is that compact traffic displays on aircraft and vehicles will do little to discourage the "heads-down syndrome." Operating procedures will have to be framed to ensure that adequate attention is paid to the outside world on, for example, a fire truck racing across the airport. Also, display clutter may become an issue in dense traffic situations.

Still, European authorities appear ready to accept multilateration as a useful new weapon in the surveillance armory, applicable both at the airport surface and in the wider terminal area. At least, Austria’s example gives them food for thought.

Meanwhile, In Frankfurt…

Germany seems set to follow the example of its neighbor, Austria, in extending the benefits of multilateration into the wider area and the airborne environment. Fraport, the air navigation service provider for Frankfurt airport, intends to develop its Mode S multilateration-based, cooperative area precision tracking system (CAPTS) to provide approach surveillance. Currently, a Sensis Corp. multistatic dependent surveillance (MDS) system is at the heart of CAPTS, providing oversight of all aircraft movements on the airport’s surface.

In cooperation with German air traffic control (ATC) provider Deutsche Flugsicherung GmbH (DFS) and Lufthansa German Airlines, Fraport has developed a precision approach monitoring prototype designed to provide accurate coverage in alignment with runway 25. If the results of a recently completed evaluation phase are considered sufficiently positive, Fraport and DFS want to integrate the system into the existing ATC system. They also intend to trial Mode S transmitters for airport vehicle applications, and use multilateration data to determine runway occupancy and taxi times.

A Matter of Timing

Multilateration is a child of the 21st century. Among the pioneers have been Sensis Corp. and Rannoch Corp., both from the United States, and ERA Co. from the Czech Republic. In 2000 FAA ordered Sensis’ version of the technology, called multistatic dependent surveillance (MDS), as part of the agency’s airport surface detection equipment X (ASDX) program. (The "X" refers to the X-band operation of the system’s surface detection radar, which gives a sharper, more detailed surface picture than previous airport radars).

The reason multilateration was not viable prior to the 21st century is that the system relies on precise timing. Small, non-rotating ground sensors located on and around the airport emit signals at slightly different times, and from those differences a target’s spatial location is computed. The required precision in timing hasn’t been available until recent years.

Now that it is, multilateration systems can triangulate on the basis of time differences of (signal) arrival (TDOA). In addition to locating an asset in two-dimensional space, the technology can tag target returns with their identities and encode details such as altitude. Multilateration ground sensors monitor returns from airborne secondary surveillance radar (SSR) transponders, including Modes A, C and S types transmitting at 1090 MHz, and military identification friend/foe (IFF) emitters. In addition, they can track automatic dependent surveillance-broadcast (ADS-B) and traffic alert collision avoidance system (TCAS) transponders. Equipping ground vehicles with transponders, which can be simplified versions of those on aircraft, enable these to be tracked, too.

Multilateration is more accurate than SSR, has a higher update rate, and is less expensive to acquire and operate. Although the system can be functionally independent of air traffic control radars, its "view" may be superimposed on the screen of the controller’s airport surface monitoring (ASM) radar where this is present. At airports lacking such radar, systems can operate alone.

Multilateration in Europe

The three main developers of multilateration technology-Sensis Corp., Rannoch Corp. and ERA-have provided multilateration systems for airports at the following European cities: