Estimation of GPS signal delays in the presence of multipath.

Summary Gps is a satellite positioning system developed by the United States in the 1960s. Originally designed for military purposes, this system is now present in many civil applications (aerial radio navigation, maritime, cell phone positioning). Even if the system is very successful because of its diverse capabilities, it remains a source of errors (positioning imprecisions) due to the presence of secondary paths in certain configurations where the direct signal can reflect on the earth's surface, buildings, etc. The objective of the thesis is to remedy the positioning errors generated by these multipaths. The originality of the work done is to present in a first step the usual positioning techniques in the context without multipath. The studies published on the problems of delay estimation of gps signals usually present the methods (algorithm, architecture,) of resolution without justifying them. This paper aims at showing that the choices made are based on a maximum likelihood study and that the selected estimator is, in fact, optimal in the case without multipath. Moreover, we show the limitations (measurement imprecisions) of this estimator in the multipath case. In a second step, we analyze the physical properties of the reflected signals thanks to the experimental records of our industrial collaborators (thomson-csf detexis), in order to model the reflections as well as possible and to know the laws governing such physical phenomena. Finally, from the previous studies and especially based on the characterization of multipaths, the objective is to design an estimation method robust to both the absence and the presence of these secondary paths. The choice was made to focus on a maximum likelihood study (dedicated to the multipath context and therefore based on a multipath model) to obtain the optimal estimator in the studied context. It has been shown that this allows to keep a certain continuity of the treatments (usually used in the absence of multipath) and thus to provide a simple and easy to implement (patented) architecture for the set of treatments performed for positioning in the presence of multipath.