The aim of this thesis, is to give an overview of the DORIS System ( Doppler Orbit determination and Radio-positioning Integrated on Satellite) and after introducing DORIS data, to perform a preliminary analysis on them. Doris system was designed to provide precise orbit determination (POD) of a satellite and it employs the Doppler Effect to determine the distance between ground stations and satellites orbiting the Earth. The ground segment consists of ground stations with known positions, well

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... distributed globally, while the space segment involves satellites carrying DORIS receivers (currently 8 missions, and 5 planned to be launched). DORIS/RINEX data and other DORIS products are overviewed, so that the Doppler equation be modified in a relativistic framework to be applied on the measurements. In this relativistic framework based on bibliography, the velocity of the on-board receiver is split into a theoretical and measured component, which are analysed separately. Thus, through modelling and via appropriate programming tools, a low accuracy estimate of the satellite's velocity is achieved, while also comparing the results for different epochs and data of different quality regarding the receiver's clock bias. The result of this preliminary analysis, estimating the satellite's velocity, agrees with the expected accuracy of a few km/s. Differences between the theoretical and measured component of the velocity are on the expected levels, considering the limitations on the parameters' estimate while complying with the behaviour on the other quantities such as elevation angle and Doppler count. Overviewing the results and conclusions of this analysis, proposals for future research and high quality analysis are made.