Hyperbolic Positioning is also known as “Multilateration.” It is the triangulation process designed to locate an object by accurately computing the time difference of arrival (TDOA) of a signal emitted from the object to three or more receivers. The U.S. military and select civil agencies already use transponder multilateration in surveillance operations for locating stationary objects, vehicles, and aircraft.
In the words of the Federal Aviation Administration (FAA), “Multilateration is a surveillance technology that works by employing multiple small remote sensors throughout an area to compensate for terrain obstructions, and is another tool the SBS program uses to enhance air traffic surveillance. The data from multilateration sensors is fused to determine aircraft position and identification. This data is then transmitted to air traffic control for use in providing surveillance separation services.”
Multilateration can also be used by a single receiver to locate itself, by measuring the TDOA of signals emitted from three or more synchronized transmitters at known locations. This can be used by navigation systems. In fact LORAN-C functioned using TDOA of signals from multiple synchronized transmitters. GPS can function in a similar manner.
Quite simply, Multilateration uses the signals transmitted from the aircraft, accurately pinpointing the aircraft’s position. The system utilizes existing aircraft transmission systems including Mode A, Mode C, and Mode S transponders. This essentially eliminates the need for aircraft owners to purchase new or additional equipment for their aircraft to effectively use the system.
Using the same type of (ground) equipment and procedures, larger areas can be monitored to cover enroute and approach phases of flight. This system is called Wide Area Multilateration (WAM). One advantage of WAM is that controllers will see a smooth progression of the target without the target jumps, which were caused by the 4-12 second radar sweep and refresh.
Another advantage of WAM is that it can provide surveillance coverage over areas where current radars may be inadequate, non-existent, or not available below a base altitude of several thousand feet above ground level. This is a situation caused by natural terrain, and sometimes even man-made obstructions.
WAM is being used throughout Europe and the United Kingdom with economy and effectiveness. Several countries chose WAM as a replacement for their “Legacy†radars. They have discovered that WAM installations can be provided at significantly lower costs of installation and maintenance and it provides more accurate tracking even with stations located 100 km apart!
One might ask if WAM and ADS-B systems are similar, and the answer is, they are similar, but are differentiated by several factors. Both systems listen for an aircraft’s transponder signal and both send data to an ATC center. But an ADS-B (single) station utilizes the GPS position information transmitted within the aircraft’s ADS-B message, while the WAM system triangulates between several known ground stations and the aircraft, to determine the aircraft’s position. This is done by computing the time difference of arrival (TDOA) of the signal.
Multilateration is finding different use potentials. Airports are exploring the possibility of using it for surface management to increase airport efficiency, resource utilization, and safety. Its use, particularly at larger air service airports, could help to significantly reduce runway incursions and on-field accidents by ground vehicles and aircraft.
