Multilateration — The newest airport surveillance technology

What does the small airport in Barnstable/Hyannis, MA have in common with international hubs like Heathrow, Frankfurt, Toronto, Dallas, and Calgary? Until recently, not very much. But by early 2001, Barnstable will join the others in being among the first in the world to install a multilateration aircraft surveillance system, which many authorities now feel is the next wave in airport safety technology.

Multilateration systems are designed to track all aircraft equipped with Mode A/C, Mode S (including TCAS), ADS-B and military IFF transponders, at selectable ranges out to more than 200 nm from the airport. Controllers see the results on a dedicated display screen, with each aircraft tagged with its identification, altitude, groundspeed, intent (climbing, descending or level), plus a variety of other selectable data, against a background depiction of the local ATC environment, including airways, radio aids, reporting points, and runway approach paths. At a much larger scale, the airport surface presentation shows the terminal area, gates, ramps, taxiways and runways.

But surely, one asks, doesn't radar already do this? It does, but a multilateration system complements the local radar by doing it differently and, proponents claim, better, in three important areas. First, the system plots aircraft positions, on the airport surface as well as in the air, with pinpoint accuracy which radar just can't equal. As a result, and second, it allows controllers to instantly pick out individual aircraft parked on the ramp, moving along taxiways, lining up at the runway, or taking off and landing: clearly a valuable aid in preventing runway incursion accidents. Third, the system's update rate is once per second, almost five times faster than secondary radar. FAA officials see multilateration systems as valuable complements to surface radars at the nation's major airports, and also capable of providing, as "stand alone" systems, lower cost alternative at less busy locations.

The multilateration concept is said to have stemmed from a US Navy requirement following the accidental shooting down in 1988 of an IranAir Airbus 300 over the Arabian Gulf. In that incident, the ship's radar was reportedly only able to interrogate military IFF (Identification, Friend or Foe) transponders and, as a result, the aircraft was tracked just by its basic "skin paint" radar return, with tragic results. The subsequent military development led eventually to what, in today's air traffic jargon, are called multi-static or multilateration surveillance systems - terms which, in simpler English, describe the method of fixing the position of an object by observing it from several, or multi, locations. In earlier times, this was called triangulation.

In a typical airport installation, four or more multilateration ground stations are positioned around the airport, at locations providing an unobstructed view of the airport surface and the surrounding airspace, and each station continuously monitors the transponder returns from every aircraft in the area. The stations are completely independent of the local ATC radars, and the signals they monitor are aircraft responses to civil or military ATC secondary radar interrogations, or replies to airliner TCAS interrogations, or autonomous ADS-B transmissions Each station processes the incoming data from all aircraft and then forwards it to a central computer, where triangulation software determines the position of each aircraft (and vehicle, in the case of surface surveillance), which is then fed to the controllers' display screen.

Multilateration systems therefore complement, rather than replace, ATC secondary radars. Yet the similarity of their displayed data has suggested their potential as temporary or emergency radar backups. For example, Nav Canada plans a nationwide radome refurbishment program where, as individual radars are switched off, the multilateration equipment could be used as a short term replacement. Several other potential applications have been proposed, based on the system's low cost, unmanned, characteristics. As a remotely located air traffic monitor, it could provide very accurate data on aircraft entering or exiting a nation's airspace, for the calculation of user charges. In the airport environment, airline interest is reported in using its high accuracy for ramp and gate control in low visibility, while airport authorities are considering its application in local noise monitoring programs. By an odd quirk of US law, multilateration system records appear to be admissible as legal evidence in such cases, whereas FAA radar data may not be.

Currently, there are two major providers of aviation multilateration systems. These are Sensis Corporation, of Syracuse, NY, and Rannoch Corporation of Alexandria, VA. So far, both firms' airport installations have been a mix of outright purchases and evaluation lease arrangements. Sensis has systems delivered or destined for Heathrow, Frankfurt, Toronto, and Dallas/Fort Worth, while Rannoch has provided or will provide systems to Tokyo's Haneda airport and at two other Japanese locations, plus Detroit, Calgary, and Barnstable/Hyannis. Interestingly, Barnstable/Hyannis was the first airport in North America to buy a system. European interest in multilateration is growing rapidly, with Swisscontrol planning to purchase systems for Geneva and Zurich in the Spring of 2001, and as many as 20 other leading airports reportedly considering the purchase of systems.

All these activities are, however, aimed at providing controllers with a complete picture of air and surface movements. For pilots, the US National Aeronautics and Space Administration (NASA) has several projects underway to develop flight deck synthetic vision displays where, in low visibility, pilots would have a "virtual reality " picture of the runway and taxiway environment ahead. This view would show potential conflict situations with other aircraft and vehicles, plus the useful feature of showing the required turnoffs, taxi routes and other runway crossing points at unfamiliar airports. One of NASA's projects calls for Rannoch to provide collision avoidance advisories and alerts directly to the pilots of taxiing aircraft, using multilateration and other data. Under a separate NASA contract, Rannoch is developing a low cost Traffic Information Service (TIS) which will provide general aviation pilots with a cockpit picture of the traffic around them in an airport's terminal area.

Surface surveillance and the prevention of runway incursions is now becoming a high priority for airport administrations. At last year's US Air Traffic Control Association international conference, several speakers referred to heightened interest in this area, with one describing it as "the last unfulfilled safety net" in the aviation system. It was predicted that by 2005, all major US airports, and many overseas airports would have multilateration systems in everyday operation.