Study of the impact of the troposphere on the Galileo navigation signals

This work consists in a validation exercise for the tropospheric delays model that will be used in Galileo. A comparison between several models of the troposphere has been performed; the models have been evaluated using existing experimental measurements or maps generated from assimilation of experimental data. The first chapter is a small general overview of the Galileo project with a particular regard to tropospheric effects on the propagation of the signals.In the second chapter, a description and a comparison of different ways of determining the tropospheric delays (vertical integration, models based on surface measurements and blind models) are exposed. In this chapter, an updated discussion on the refractivity coefficients is also presented.The third and fourth chapters expose how two validation exercises have been performed. The sources used for this work come from the British Atmospheric Data Center as well as data gathered during the validation campaign of Ra2, the RADAR altimeter of ESA's Envisat Earth observation satellite. A critical analysis of the existing procedures has been performed and new ones have been developed. As far as radiosonde data are concerned, some serious improvements in the processing of the measurements have been found. The results o this work have led to an evaluation of the performances of the models in several sites of the world. Concerning the second data set, we focused on the Numerical Weather Prediction data in order to assess the ESA model on a worldwide basis. The procedures used to derive the delay from these data have been studied in depth. Finally, an assessment of the models during the Ra2 validation campaign is exposed. The last chapter is consecrated to the conclusion of these analyses and validations, knowing that some points underlined during this study will be useful for the newt versions of the Galileo tropospheric model