ATM Modernization

European Air Traffic Surveillance: A Combination Approach

By by George Marsh | September 1, 2006
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Once air traffic surveillance meant "Mk 1 eyeballs" scanning the airspace from a tall tower. Then came radar, which extended oversight well beyond visual range, becoming the surveillance mainstay for six decades. Recently, however, new automatic dependent surveillance (ADS) and multilateration (MLAT) technologies have challenged radar's dominance. Air navigation service providers (ANSPs) contemplating upgrades now have to decide which sensors to use. In Europe, an emerging answer is to combine all three to provide the broadest possible traffic awareness with the highest possible integrity.

Austria is a case in point. Picture an alpine valley bounded on both sides by snow-capped mountains. The Inn Valley is frequented by gliders and light aircraft, as well as airliners, and is known for bad weather. That's quite a challenge for air traffic control (ATC) personnel trying to assure safe movements in and out of Innsbruck Airport near the head of the valley. Pilots, who receive special training and clearance to use the one-way-in, one-way out airport in anything but the highest minima conditions, must cope with an off-centerline ILS approach as well as weather and airborne hazards. Capacity was, until recently, limited by the need to apply procedural separation, down to only one aircraft at a time transiting the 30-nautical mile valley on the approach in instrument flight rules (IFR) conditions.

A few years ago, in order to safely increase traffic throughput, ANSP Austro Control GmbH decided to transition to required navigation performance (RNP 0.3) procedures and improved surveillance standards for the terminal area (TMA). It first planned to add two new radar stations. But installing and maintaining radar on mountains is expensive, coverage below about 10,000 feet would still be limited--and accompanied by high multipath effects--and there were issues with siting permissions. Austro Control therefore decided to try out another type of system, multilateration, and possibly augment this eventually with automatic dependent surveillance-broadcast (ADS-B), together with traffic information service-broadcast (TIS-B) uplinks.

Like secondary surveillance radar (SSR), multilateration can provide identity-tagged, 3D aircraft positional data. But MLAT is a simpler system that can work with any type of vehicle or asset fitted with a transponder that emits a given omnidirectional radio frequency signal. It operates via triangulation, measuring the time differences of arrival (TDOA) of a transponder signal at multiple ground receivers sited at different locations. An algorithm uses these time differences to compute each transponder's two- or three-dimensional position in space.

MLAT Evolution

MLAT ground stations are compact and can be mounted on existing towers or structures. The units, which employ nonrotating antennas, require minimal maintenance and can operate unattended. Power consumption is low. The latest receivers have more easily upgradeable, "soft" architectures, are self-calibrating and less prone than earlier generations to terrain and other effects. Receivers are data-linked to a central processor, which carries out the necessary computations and delivers the identity-tagged position data to air traffic or other displays in the appropriate formats. This simplicity of design equates to low acquisition and support costs for systems like the Vigilance series from Siemens, the multistatic dependent surveillance (MDS) system from the Sensis Corp. and Rannoch Corp.'s AirScene. Airborne infrastructure is minimal, as many aircraft already have the necessary transponders. MLAT systems are also compatible with ADS-B.

Thanks to technical evolution over the last decade, MLAT has become a fairly precise positioning system. Although accuracy depends on several factors, including the positional geometry of the receivers and transponder response rate, the absolute key is the precise determination of the differential times, according to Timothy Quilter, head of air traffic management (ATM) infrastructure at Roke Manor Research, UK, a Siemens company. Good timing accuracy means fewer ground sites and lower transponder interrogation rates are needed, or the accuracy can be traded for operation over a wider area. Achieving the necessary level of accuracy requires the precise measurement of signal times of arrival at the various ground receivers plus, crucially, the close synchronization of all the receivers to a common time base.

For some years timing and synchronization were a major challenge, but high-speed electronics and new timing architectures have provided the solution. Companies use their own, patented timing techniques. Roke Manor Research, for instance, has developed a common-view GNSS (global navigation satellite system) synchronization solution that can deliver timing accuracies of better than 1 nanosecond, enabling positional accuracies down to a few meters. This method compares time signals from several different satellites in common view. Any differences must be due to atmospheric and ionospheric effects. Since the satellites are in known positions, these errors can be determined, and hence corrections can be applied to obtain the precise time. Other solutions use reference transmitters and are fully independent of GPS/GNSS.

Wide Area MLAT

Fourth-generation products available since about 2000 have made MLAT a realistic option for surveillance over a wide area. No longer is the technology limited to tracking aircraft and service vehicles on an airport's surface, the initial application at European airports. It also can be used to locate aircraft flying within the TMA or beyond. Austria, Germany and the Czech Republic were instrumental in extending the technique vertically and increasing the range. Today MLAT coverage can extend from the apron to the ramp and even to en-route airspace. This broad coverage, along with ability to track surface as well as airborne vehicles, makes multilateration a versatile surveillance tool.

In the Austrian application, soon to be extended across a swath of central Europe, the aircraft use 1090-MHz, Mode S-capable transponders installed under Europe's mandate for commercial aircraft to carry equipment that interoperates with secondary surveillance radar. These aircraft require no additional equipage. The MLAT system can operate with other transponders, such as Modes A and C, or more affordable, reduced-specification units intended for light aircraft and surface vehicles.

Multilateration timing precision translates to accuracies around 10 to 23 feet (3 to 7 m) at the airport surface, and 33 to 164 feet (10 to 50 m) in en-route airspace, but still exceeding normal SSR capabilities. Moreover, MLAT easily beats radar on costs. According to Werner Langhans, head of strategic development at Austro Control, the ANSP invested some ?1.5 million ($1.9 million) in planning, implementing and testing its wide-area multilateration (WAM) system. This is based on a Sensis MDS comprising nine remote ground stations, a fully redundant central processing system, and an interconnecting data link. A comparable SSR solution would have cost about ?13 million ($16.7 million), even assuming that permission for two extra radar stations could have been obtained, says Langhans. The MLAT annual operating costs in Innsbruck are likewise much less than SSR.

Operating successfully in the TMA and surrounding en-route airspace since November 2004, the Innsbruck multilateration system has been approved for operation as a "radar-like surveillance service" for monitoring instrument flight rules (IFR) aircraft, providing traffic information and aircraft separation. A 5-nm separation standard is in force, though MLAT can safely provide less than 3-nm separation. Operating independently of the aircraft's navigation equipment, the system provides 1.5 updates per second, a detection probability of 99.4 percent and a wide area positional accuracy of 43 feet (13 m) mean error and 72 feet (22 m) root mean square (RMS), considerably better than an SSR can deliver. It extends surveillance below existing radar coverage and remains effective regardless of weather. For the system to be fully effective, however, all targets eventually will have to be equipped with transponders.

Austro Control's technology manager, Michael Loeffler, told delegates at Helios Information Systems' recent SurTech 2006 conference that the ANSP intends to implement multilateration in the terminal area for Vienna Airport next year, as a second source to remotely located secondary surveillance radar. A 14-ground station MLAT is already in use for the airport's surface.

Moves to further extend the system over southern Germany, eastern Switzerland and western Austria are proceeding as the Central European WAM (CEWAM) project, sponsored by ANSPs, Austro Control, Skyguide (Switzerland) and DFS (Germany). Managed by Skyguide, this project aims to analyze technical, safety and commercial aspects and to establish a common infrastructure for supporting WAM. Operation is intended in more TMAs, particularly in Switzerland, and to higher altitudes in Germany. An initial study phase has been completed and technical specifications and operational plans are being prepared.

On present indications, MLAT constitutes an accurate apron-to-en-route, surface and air vehicle, glider-to-widebody surveillance system that can be implemented quickly at modest cost. As such, it would be useful for small visual-only airfields and other airports lacking radar cover, as well as in sophisticated ATC environments like Innsbruck and Vienna. In surface-only applications, it can usefully complement surface movement radars within existing systems such as the advanced surface monitoring and ground control system (A-SMGCS) at Vienna.

Another MLAT advantage, according to Austro Control, is its ability to support Mode S elementary through enhanced surveillance functions, as well as ADS-B, without any change to aircraft systems. This preserves ANSPs' Mode S secondary surveillance radar investments but allows the opportunity to consider future investments in MLAT and ADS-B.

Radar Remains

Despite wishful thinking to the contrary, MLAT cannot replace radar completely since it can detect only cooperative targets, i.e., those that have transponders and have them switched on. That is not good enough for military and security authorities, who are interested in threats, or for air traffic control authorities concerned to see every airborne platform in their airspace, whether transponding or not. Because primary radar can detect both cooperative and non-cooperative targets, it will not be retired any time soon. But secondary radar will have to compete with technologies such as wide area multilateration to provide value to airspace users.

Unlike ADS-B and one form of MLAT, radar also is fully independent of GNSS, whose signals can be disrupted by deliberate or unintended interference. Because it is entirely ground-based and has a dissimilar mode of operation, it is a natural complement to satellite-based systems.

ADS-B

Nevertheless, radar's near-monopoly in ATC surveillance is threatened not only by MLAT, but also by another evolving technology, automatic dependent surveillance. ADS requires minimal incremental infrastructure because it depends on aircraft relaying their own navigation and flight path data to entities intended to have it, whether by agreement as in ADS-contract (ADS-C), or simply because they are equipped to receive it, as in ADS-broadcast, or ADS-B. Essentially, nothing is needed in the air except a link to the existing navigation source, typically the flight management system, and a communications data link. By 2008, 1090-MHz Mode S Extended Squitter will be mandatory for aircraft using European airspace. Happily, this data link can serve ADS-B purposes, too. Ground provision comprises ADS-B-capable receivers with a processor able to provide the data for a separate ADS display or to fuse it with other sensor data for presentation on an ATC display. Sensis today fuses ADS-B and other sensor data at Indira Gandhi International Airport in New Delhi, India, as part of its complete advanced surface movement guidance and control system, according to the company.

Broadcasting one's identity, location and other parameters to all receivers within range also raises the possibility that ADS-B data can be used by other aircraft, enabling pilots to monitor the air traffic situation on their own flight deck displays. This could allow pilots to negotiate trajectory changes with ground controllers cooperatively. Additionally, they could maintain separation during pre-landing maneuvers such as merging and sequencing, which would reduce controller workload and associated delays. Thus ADS-B, along with airborne separation assistance systems (ASAS) currently under development, could return to pilots some of the autonomy they have lost over the years. As a result, air traffic capacity should increase.

ADS-B compares well with both radar and multilateration. Granted, primary radar has the major advantage of not requiring target cooperation, relying on an aircraft's inherent ability to reflect incident radar signals. On the debit side, radar is expensive, difficult to site and requires periodic parametric adjustment for optimum performance. Its update rate is limited by the rotation rate of its antennas. Because radar is a beam system, accuracy declines as a function of distance from the emitting source. Targets masked by larger or nearer objects can go undetected. Target identity can be positively established only by interrogation, as with secondary surveillance radar.

ADS-B, a technology that effectively sidesteps radar, avoids these disadvantages. In fact, as Michael Sharples, a principal scientist with UK research organization Qinetiq put it in a presentation given at Surtech 2006, if today's microprocessing power had been available years ago, "all cooperative surveillance would now be being done this [the ADS-B] way." ADS-B is an automatically operating, passive system whose accuracy is limited only by that of the aircraft navigational system to which it is linked. It can therefore significantly surpass the accuracy of radar. Update rates are higher, terrain masking problems are avoided and coverage is maintained below radar cut-off heights in mountainous terrain.

On the other hand, overall system integrity--a combination of the integrity of the aircraft's navigation source and its data link--will be less than for the GPS or other navigation system it is "reporting." Undetected errors are more difficult to identify than with SSR, which, unlike ADS, tends to supply "garbage" in the event of a malfunction. ADS-B accuracy can alter with aircraft change of direction, as well, due to masking effects.

Furthermore, the dependence of ADS-B on the aircraft's navigation system has the disadvantage that GNSS-based navigation sources can be jammed or spoofed. This is an issue that will have to be addressed as ADS-B evolves into the future. ADS-B reports also can include a statement of accuracy, along with integrity information, expressed in terms of a containment radius.

Combinations

In the future surveillance may well involve combinations of radar, multilateration, ADS-B and maybe other sensors. Austro Control, which sees multilateration as an affordable substitute for SSR, is not about to dispense with radar. A combination of secondary surveillance radar and multilateration will, it believes, provide the equivalent of double SSR coverage inside the flight information region (FIR) and single coverage out to the forward FIR boundaries. ADS-B for en-route airspace applications eventually may be added, though not before 2010, Loeffler says. This GNSS-based system is considered less vital for Austria than for other countries whose surveillance infrastructure is less developed, he explains, adding that airlines are not yet obliged to equip with it. Austro Control already is using ADS-B at the Vienna and Innsbruck airports for vehicle detection, he adds. But initially the ANSP intends to concentrate on independent surveillance, introducing dependent systems later.

The high precision of combined surveillance systems will, Austro Control expects, support the use of four-dimensional (4D) trajectory data on a prototype basis at Vienna, starting in mid-2007. This will ultimately deliver such benefits as continuous descent ("green") approaches and information technology-supported departure sequences.

CRISTAL Program

Meanwhile, a program in the UK is evaluating ADS-B's ability to enhance service in a highly evolved radar environment. National Air Traffic Services (NATS), in collaboration with Helios Technology Ltd., Cranfield University, Raytheon and SITA, is engaged in Eurocontrol's CRISTAL (CoopeRative ValIdation of Surveillance Techniques and AppLications) activities, intended to determine the role of passive surveillance in future surveillance infrastructure. Qinetiq and Eurocontrol are involved, as well. A trial implementation in southeast England led to the initial conclusion that ADS-B, though probably unable to meet integrity requirements alone, can usefully supplement radar and that the combination of the two potentially could permit reduced separations.

In Phase 2 of the UK's CRISTAL program, NATS and its partners are investigating the comparative benefits of a wide area multilateration, ADS-B, and a radar/ADS-B combination. A trial in the London terminal area is helping to validate the ADS/radar combination. The hope is that the inclusion of a passive surveillance system (PSS) could enable London Terminal Control to dispense with one of its contingency radars. This would demonstrate the possibility of cutting infrastructure costs by reducing levels of overlapping radar coverage in southeast England and Scotland.

Sharples argues, however, that the future for cooperative ATM could be a partnership of ADS-B, not with radar, but with multilateration. Describing these technologies as naturally complementary, he points out that both exploit the same physical signals and are compatible with Mode S/1090-MHz technology. And, while ADS-B is resistant to signal and path distortion, multilateration resists spoofing. Acting together, Sharples declares, the duo can improve radar's accuracy, while potentially equaling the older system's integrity. The combination can be completely passive, use limited communication bandwidth, and lower sensor density and costs.

It seems that the combination approach to air traffic surveillance may well suit Europe, with its mixed ATM infrastructure. Like other regions globally, it will be able to mix and match available techniques to best suit circumstances in particular areas. 

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