One of the rather surprising omissions of FAA’s Next Generation Air Transportation System (NextGen) Implementation Plan, released in March, was that, despite its full description of the agency’s automatic dependent surveillance-broadcast (ADS-B) program, the plan failed to mention the system that has been widely adopted by air navigation service providers (ANSP) around the world, and that is currently supporting the gradual transition to ADS-B and, separately, is starting to supplant ATC surveillance radar — multilateration.
Multilateration systems, manufactured by Era Systems, of Fairfax, Va., and Sensis Corp., of Syracuse, N.Y., are eloquently simple; they are essentially the 21st century versions of the ancient principle of trilateration where, for example, early land surveyors would measure the bearings of a distant point from three widely separated locations whose geographic positions were already accurately known. Carefully plotting the three bearings from their respective points of origin would reveal the geographic position of the distant point.
Today, multilateration’s “distant points” are aircraft ATC transponders, while the differing bearings of the past have been superseded by the fine differences in the times of arrival (TOA) of the transponders’ signals as they reach a network of widespread multilateration receivers on the surface. A central processor then converts the TOAs to produce aircraft position accuracies equal to, and often better than, modern ATC radars. These targets are then displayed on the air traffic controller’s screen, tagged with the aircraft’s identification, altitude and other data embedded in the Mode A/C, Mode-S, TCAS, ADS-B (both 1090ES and UAT) and even the military’s IFF transponder signals. This underlines an important aspect of multilateration operations: its transponder compatibility means there is no requirement for new avionics in the aircraft. Today’s multilateration tracking units are all ADS-B compatible, emphasizing the system’s benefits in the current transition to the full ADS-B environment of the future.
Normally, the multilateration receivers operate in a passive “listening” mode, and only become operational with the arrival of an aircraft’s transponder signals that have in turn been triggered by interrogations from a secondary surveillance radar (SSR) or from the TCAS unit in a passing aircraft. In some circumstances, however, the multilateration receiver network is beyond the range of the nearest SSR. In those cases, the multilateration unit can use its own built-in transmitter to send SSR-like interrogations either omni-directionally or over specific sectors of the local airspace. The multilateration unit also transmits SSR interrogations at a much higher rate of once per second, versus the 4.7- and 12-second intervals between consecutive sweeps of the interrogation beams of the terminal and long range surveillance radars, respectively. The higher rate is especially valuable in busy terminal airspace, particularly in sequencing and then maintaining safe separations between aircraft on their landing approaches.
MLat Applications
Most of the early applications of multilateration were centered on monitoring aircraft’s ground movements at airports, and with much less attention given to air traffic control. Indeed, some of those systems were partially justified for their value in identifying aircraft that had broken noise abatement procedures on departure. Other uses included the collection of landing fees from itinerant aircraft that would land, refuel and then depart without being manually logged in and out by the airport tower.
But possibly the major change of application came in 2005 when AustroControl certified a Sensis multilateration system for full arrival and departure operations at the Innsbruck airport, which is located in a narrow valley between mountain ridges up to 9,000 feet high. Where aircraft previously operated on a “one in, one out” sequence, multilateration’s positioning accuracy of 15 meters on the airport runway and 60 meters in the air allowed controllers to track multiple aircraft in the terminal area, with a dramatic increase in throughput.
The cost benefits at Innsbruck were also dramatic. First, the cost of the suitcase-size multilateration unit, its small antenna and, when required, a mounting mast totalled less than one third that of a terminal SSR. Second, the steep sides of the valley greatly restricted the number of locations where a SSR could be economically installed, thus increasing the number of SSRs that would have been required to provide gap-free coverage of the airport and its arrival and departure routes. Conversely, optimum locations could usually be found for the multilateration units.
Since that time, multilateration has become the virtual standard for surface movement control and monitoring at the world’s major airports, often integrated with Advanced Surface Movement Guidance and Control Systems (A-SMGCS) that incorporate local radar and data fusion technologies. Additionally, multilateration is being applied to increasing numbers of otherwise “difficult” locations, such as the mountainous ski and tourist resort areas of Colorado and the resource-rich areas of Western Canada, both of which make traditional surveillance radar impractical.
This is also true of the world’s flattest surfaces — the seas. There, far from land and at low altitudes, helicopters constantly ferry personnel and supplies between oil drilling rigs, with their movements closely monitored by shore stations linked to multilateration units strategically installed on several of the rigs to provide high accuracy coverage across the whole oil field. And, illustrating the competitive nature of the multilateration market, the equipment used in the two separate North Sea oil fields comes from the two separate manufacturers. In the south, just off the coast of the Netherlands, Era Systems is the exclusive supplier, while to the north, off the Scottish coast, Sensis has the contract.
Wide Area Multilateration
Those outside the industry are sometimes puzzled by the “wide area” prefix applied to multilateration, and its WAM acronym. Does it refer to the size of the area covered, or the number of multilateration units it accommodates, or perhaps even the average distance between individual units? There isn’t total agreement on just how wide, wide is. About the closest one can get is that, generally speaking, a multilateration configuration where the reporting units are employed in surface monitoring and are located within or close to the airport boundary, isn’t a wide area system.
On the other hand, there is no question that the system recently installed in Namibia by Era Systems certainly qualifies for the wide appellation. Covering the whole of the West African nation of just under 320,000 square miles, and with a network of 36 widely separated, individual multilateration units, the total installation is the largest multilateration system in the world. Yet Namibia’s program not only suggests an economical air traffic management system for other African nations presently lacking such a needed infrastructure, but it also appears to be part of a growing trend to nationwide WAM installations.
Austria, the Czech Republic and Sweden have each announced plans this year to augment their current, airport-centric multilateration installations with a country-wide WAM system that will actually extend signal acquisition coverage slightly beyond their borders. This was the basis of a Eurocontrol study several years ago that envisaged a linked net of border-to-border national WAM systems throughout Europe. With complete ADS-B equipage of the world’s entire civil aircraft fleet not expected by some industry observers until 2020 or even later, a global family of nationwide WAM systems could turn out to be a practical interim step for the foreseeable future.
It now appears to be generally accepted that even when the world’s civil air fleet is completely ADS-B equipped, there will still be a need for a backup system to cater to SSR or GPS failures. A FAA analysis of back-up candidates a few years ago assessed WAM as well qualified, but faulted it somewhat ambiguously on the basis of cost. It later turned out that the cost assessment seemed to have compared WAM acquisition costs against the very substantial “sunk costs” of the relatively new, and much more expensive, U.S. national SSR network. Consequently, the recommended backup for SSR was … SSR.
But in fact, many informed observers feel WAM is the only satisfactory backup for SSR, and Avionics has been informed that both Era and Sensis have been approached by ANSPs who operate legacy SSRs and are considering replacing them with WAM systems offering the same capabilities but with lower acquisition and support costs. The case of SSR or GPS failures is also interesting for one valuable WAM feature where, besides receiving and decoding the ADS-B message, the WAM processor independently calculates the aircraft position using the same signal, thereby providing a real-time check and an ADS-B backup.