Modelli statistici degli orologi e problemi di sincronizzazione nei sistemi di sorveglianza cooperativa distribuita

The choice of a stable time reference common to all the components of a distributed system (i.e. made up by stations or sensors placed in different sites), is a topic which applies to several telecommunication and electronic problems. In a distributed system, in fact, it is fundamental that all elements have a unique time reference so that they can exchange data in unambiguous way and for a possible subsequent data elaboration. In this work a distributed system, the multilateration (in short MLAT) will be considered, with particular attention to the generation of a common time reference and to the keeping of synchronization between the different elements. In a MLAT system the ground stations receive Mode S signals (reply/squitter) from transponders installed on board the cooperative targets (en route aircrafts, aircrafts and vehicles on airport surface) and determine the position of targets using the TDOA (Time Difference of Arrival) technique. In the first part of the thesis (chapters I – IV) the aspects related to the generation of a stable time reference by a local oscillator will be investigated. In particular the fluctuations of a generic oscillator will be examined, which can be of two types: systematic fluctuations, main cause of divergence from the “real” time and the “real” frequency in the long term (days or years) and the random fluctuations, observed in the short term (fractions of second or minutes). For the oscillator random fluctuations a mathematical model will be introduced, in order to have a simulation method of the oscillator behavior. The model is different and more complex than the well-known additive gaussian noise frequently assumed in telecommunications. Finally some oscillator types on the market will be presented with a comparison between the performances declared by the manufacturers. In the second part of the thesis (chapters V – VII) the operation of a multilateration system studied by the Radarlab at Tor Vergata University in Rome together with Selex-Sistemi Integrati company, which has realized it, will be described; the system has been successfully deployed at Milano Malpensa for the airport surveillance. In particular the study that led to the selection of the oscillator for the system stations will be reported and the synchronization procedure designed to maintain a common time reference will be described. Besides an innovative multilateration architecture will be presented which allows the extension of surveillance, in addition to aircrafts, also to vehicles moving on airport surface (ground surveillance), with reduced update costs. Finally a multilateration architecture for aircrafts surveillance in wider areas (WAM, Wide Area MLAT) such as en route and airport approach areas will be presented, an application whom many manufacturers are looking at with interest, as an alternative to classic radar systems.