NEW TIME OF ARRIVAL ESTIMATION METHOD FOR MULTILATERATION TARGET LOCATION Gaspare Galati , Mauro Leonardi, Patrizio De Marco, Luca Menè Pierfrancesco Magarò,Maurizio Gasbarra Tor Vergata University, Selex Sistemi Integrati, DISP and Vito Volterra Centre, Via Tiburtina Km. 12 + 400, 00131, Via del Politecnico 1, 00133, Roma, ITALY, Roma , ITALY ABSTRACT The “Multilateration” (MLAT) system is a Surveillance and Identification element of the A-SMGCS that uses the SSR transponder (Mode S or even A/C) Reply/Squitter as received by a number (e.g. 15 or 20) of fixed stations where the time of arrival , TOA, is estimated. In a central processing unit, the target position is estimated by the TDOA technique (Time Difference Of Arrival), which exploits the TOA in one station taken as the reference station. Due to the (possibly large) Dilution of Precision, the final accuracy as specified by the international recommendations can be obtained only if the TOA accuracy is very good (i.e. sub - metric). In this work a new, patented [7] TOA (Time Of Arrival) estimation method, derived from the maximum likelihood estimation (MLE) method, is described and compared with the standard method (based on pulses edges and threshold crossing). The accuracy, by analysis and simulation, is one order of magnitude better than standard methods. Moreover a set of measurements and pre-operational trials with real signals in an airport environment (Tassignano, Tuscany) are reported and compared with analysis and simulations to verify the performances of this new TOA estimation method as embedded in a complete system. INTRODUCTION Multilateration of Mode S squitters/replies (MLAT) is becoming an important surveillance and identification system for large airports [1], [2]. A typical MLAT system as shown in Figure 1, is made up by a number (e.g. 12-15) of Measurement Stations capable of receiving, time-tagging and transmitting oven a redundant Local Area Network (LAN) the replies and the squitters to a Central Processing Station (CPS) in the airport. Moreover, one or more Reference Transponders permit synchronization and monitoring of the whole system; the Times of Arrival (TOA’s) of replies/squitters due to the SSR equipped aircraft and vehicles and to the reference Transponder(s) are processed in the Central Processing Station where multilateration algorithms locate the aircrafts and mobiles. Because of the increasing airport traffic, in particular on the 1090 MHz channel, both the MLAT accuracy/resolution and the robustness to interference have to be improved without increasing the number of MLAT stations too much (limitations are due to cost and installation problems). Different technical solutions have been implemented to locate the SSR transponder of an aircraft on the airport surface or of the “non-transponder device” of equipped vehicles. The “non transponder device” has the same functionalities as the SSR Mode S transponder without the “flyability” characteristics. The MLAT location error is affected by a significant Dilution of Precision (DOP), therefore in order to maintain the overall location r.m.s. error within 3.75 metres (in [2] it is specified an error below 7.5 m for 95% of time) the r.m.s. error of each station has to be of the order of 1 metre, i.e. the various contributions (whose root sum squares is to be below 1 metre) must be kept in the order of 0.3 metres, corresponding to about one ns of equivalent time error. The limitations of the state of the art in this context (TOA estimation) can be referred to the limited precision of the time measurements that allow localization of the transponder and therefore of the vehicle that carries it on board. In fact in the existing MLAT systems the measurements of the time of arrival of SSR signals in each Station are obtained getting the instant of time correspondent to the overcoming of an assigned threshold of amplitude by the leading edge (or trailing edge) of the first pulse of the signal, with a quantization error due to the fact that such time is found by reading, in correspondence to this event, a clock with which the Station is equipped. This procedure involves, for the measurement, a quantization step equal to the period of the clock (for example 33 nanoseconds - corresponding to 10 meters - for clock at frequency of 30 MHz) with a correspondent peak to peak error of the same order of magnitude.