In October 2000, the U.S. Federal Aviation Administration (FAA) awarded a Raytheon Co./Sensis Corp. team a $200-million contract to develop and build airport surface detection equipment, Mode X (ASDE-X). Commencing in 2004, ASDE-X units will be progressively commissioned at 25 U.S. airports. These facilities rank in traffic density just behind the nation’s 34 busiest hubs, which already are equipped with earlier ASDE-3 systems.
When FAA Administrator Jane Garvey first announced the ASDE-X project at the agency’s Runway Safety Summit in June 2000, many assumed that the system was simply a newer version of the current ASDE-3. In fact, an FAA news release at the time appeared to confirm this.
But ASDE-X introduces an entirely new airport surveillance technology to the FAA’s arsenal. And it allows the agency to play catch up with European, Canadian and Japanese aviation authorities, which are investigating advanced surface management tools.
ASDE-X comprises two basic elements. The first is a fairly conventional, rapid scanning, 360ï¿½, airport surface detection radar. The main difference from its ASDE-3 predecessor is its use of an X-band radar frequency–hence ASDE-X–which gives controllers a sharper, clearer picture of airport surface activity. But this picture still only shows aircraft or vehicles moving on the airport surface as several different sized "blips" on the controller’s radar screen.
Consequently, while controllers can use ASDE to follow individual aircraft movements around the airport, they still must make frequent radio exchanges with the pilots to confirm aircraft locations. Thus, considerable controller concentration is required to track the identities of all the objects moving across their radar screens. And ASDE makes it virtually impossible to differentiate between aircraft in close proximity, such as when they are lined up awaiting takeoff clearance.
Enter ASDE-X’s second element, which brings the system truly into the 21st century. This element, a multilateration system, places an identification "tag"–for example, UA437–against each individual target (blip) on the controller’s radar screen.
A U.S. Navy Requirement
What is multilateration? Once called triangulation, the aviation application is said to have stemmed from a U.S. Navy requirement following the accidental shooting down in 1988 of an IranAir Airbus 300 over the Arabian Gulf by the USS Vincennes. In that incident, the ship’s radar was reportedly able to interrogate only military IFF (identification, friend or foe) radar transponders, which were not carried by the A300.
As a result, the aircraft was tracked just by its basic "skin paint" radar return and mistakenly assumed to be an attacking aircraft. The results were tragic, and the subsequent military development led to what are now called multilateration or multistatic surveillance systems.
An Unobstructed View
In a typical civil installation today, four or more multilateration ground stations are positioned around an airport. Their locations provide an unobstructed view of the airport surface and the surrounding airspace. Each station continuously monitors returns from the following:
Mode S (including traffic alert collision avoidance system, or TCAS);
Automatic dependent surveillance-broadcast (ADS-B); and
Military IFF transponder.
The returns come from every aircraft, as well as from appropriately equipped vehicles, on the airport surface.
The ground stations are completely independent of the air traffic control (ATC) radar, but their outputs are superimposed on the controllers’ ASDE radar screens, providing the ASDE system’s vital missing link. With the addition of multilateration, controllers will see the identification of each aircraft, as it moves to and from the airport’s gates, ramps, taxiways and runways.
Multilateration systems offer three other key advantages:
They can plot aircraft positions with an accuracy that radar just can’t equal.
Their update rates are typically once per second, almost five times faster than secondary radar, and this rate can be greatly increased, if required.
And the ground stations for multilateration are small, unmanned units, which can be easily installed. And they cost a fraction of what the very sophisticated surface surveillance radars cost.
Multilateration systems, therefore, provide a worthy complement to ATC airport surface radars. FAA insiders suggest that eventually they will be in widespread use across the nation. In fact, at many locations it seems likely that, because of their relative lower cost, multilateration systems could be installed as stand-alone surveillance units. They then could operate as "pseudo radars," with their signal monitoring coverage areas extending well beyond the local airport surface to include the total surrounding airspace, possibly out to 100 nautical miles or more.
This similarity to radar’s basic functions has suggested the use of multilateration systems in both temporary and permanent applications, roles that formerly could be performed only by conventional radar. For example, under Nav Canada’s planned national radome refurbishment program, multilateration equipment reportedly could be used as a short-term replacement, as individual radars are switched off.
Other potential uses have been proposed, based on the system’s low cost, unmanned characteristics. As an air traffic monitor located in the remote areas of some nations, it could provide accurate data on aircraft entering or exiting that country’s airspace, for the calculation of user charges. In the airport environment, airline interest exists in using multilateration’s high accuracy and rapid update rate for ramp and gate control in low visibility. And some airport authorities are considering multilateration’s application in local noise monitoring programs. By an odd quirk of U.S. law, records from multilateration systems appear to be admissible as legal evidence in noise-complaint cases, whereas FAA radar data may not be.
A Choice of Two
Currently, two major providers of airport multilateration systems exist: Sensis Corp. of DeWitt, N.Y., and Rannoch Corp. of Alexandria, Va. So far, both firms’ airport installations have been a mix of outright purchases and evaluation lease arrangements. Sensis has systems delivered to or destined for London-Heathrow, Frankfurt, Toronto, Dallas/Fort Worth and Memphis. Rannoch’s contracts cover three systems to Japan, including Tokyo’s Haneda Airport, plus Detroit, Calgary and the municipal airport at Barnstaple/Hyannis, Mass. The Hyannis system reportedly will be used for noise monitoring purposes.
Of note, none of the U.S. installations are FAA funded. They are the result of local airport initiatives, with the exception of Detroit, which is a National Aeronautics and Space Administration (NASA) supported technology evaluation.
Will the earlier ASDE-3s installed at the nation’s top 34 airports now be upgraded to incorporate multilateration technology? That’s an awkward question for FAA.
Several years ago, the agency launched the development of the Airport Movement Area Safety System (AMASS). This advanced data processing program was to identify surface targets on the ASDE-3 radar screens and warn controllers of potential runway incursions or similar situations.
But the AMASS program has developed a voracious appetite for money and little ability to meet delivery dates. At the FAA’s National Runway Summit meeting last year, Kenneth Mead, inspector general of FAA’s parent, the Department of Transportation, stated that AMASS was six years behind schedule and $92 million over its originally estimated $60 million development cost. "AMASS has been delivered to 33 airports, but is not yet operational anywhere," he said. FAA tests of AMASS continue at its San Francisco airport "flagship" installation, but test data released in January 2001, were not encouraging. Some agency officials are recommending privately that a backup multilateration strategy be developed for ASDE-3.
Not Just for Controllers
Controllers are not alone in requiring a complete picture of movements on the airport surface. Pilots, too, have an equally pressing need, both in poor visibility conditions and particularly at night when, as most pilots will attest, the taxiway lighting at a large airport becomes "a sea of blue." As part of its wide-ranging Aviation Safety Program, NASA has several projects underway to develop flight deck displays that would provide pilots with a synthetic "virtual reality" picture of the runway and taxiway environment ahead.
One of these projects calls for Rannoch Corp. to provide collision avoidance advisories and alerts directly to the pilots of taxiing aircraft. The reports would use data from various sources, including a multilateration system. However, besides showing potential conflict situations with other aircraft and vehicles, the system, called PathProx, would also provide valuable assistance to pilots using unfamiliar airports by showing the required turnoffs, taxi routes and runway crossing points from the runway to the terminal, and vice versa.
‘Last Unfulfilled Safety Net’
Airport managers now give surface surveillance and the prevention of runway incursions a high priority. At a U.S. Air Traffic Control Association conference last year, 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 U.S. airports and many overseas airports would have multilateration systems in everyday operation. With the FAA and its ASDE-X now following comparable efforts by overseas airport administrations, that prediction appears likely to become true.
Rannoch Corp. recently entered into an agreement, with the Holland Institute of Traffic Technology BV (HITT), in the Netherlands, which will result in creating an aircraft tracking system comparable to the ASDE-X jointly developed by Sensis Corp. and Raytheon. The agreement, penned in mid-April, has Rannoch Corp.’s AirScene technology integrated with HITT’s Advanced Surface Movement Guidance and Control System (A-SMGCS) technology. The result will be a system that tracks aircraft on the ground using multilateration and automatic dependent surveillance (ADS), from AirScene, with HITT’s radar-based surveillance system.
Rannoch also announced in April that the Japanese government formerly accepted the AirScene system after undergoing trials at Haneda Airport and around Tokyo Bay. Here, instead of ground movements, the system has been employed for "wide area operations," tracking aircraft during instrument departures and arrivals, as well as overflights in the Tokyo Bay area.