Closed form localization algorithms for mode S wide area multilateration

Multilateration (MLAT) is an increasingly important location and identification system for surface traffic surveillance, is in use in large airports such as London-Heathrow, Frankfurt/Main and Rome-Fiumicino. Target location can be extended to approach and terminal area surveillance (wide area multilateration, WAM), with a system coverage to some hundred kilometers around the airport. Practically used MLAT systems exploit signals transmitted by aircraft equipped with SSR (or ADS-B) transponders for target location. Multilateration algorithms locate targets using hyperbolic time difference of arrival (TDOA) techniques. Under the special conditions imposed by the geometry in the WAM case, high-performance algorithms are required for targets far outside the polygon enclosing the receiver stations. This paper presents algorithms formulated in TOA (time of arrival) representation directly, without resorting to the TDOA. Their performance is compared with other present-day algorithms that are mostly formulated in terms of TDOA.