AVAILABILITY, ACCURACY AND GLOBAL COVERAGE ANALYSIS FOR A HYBRID GPS/GALILEO SATELLITE CONSTELLATION USINGA GLOBAL NAVIGATION SATELLITE SYSTEM SIMULATOR

摘要

The advent of the European Galileo navigation system and the modernization of the American GPS will lead to an improved Global Navigation Satellite System (GNSS). Availability, reliability and accuracy are key parameters in evaluating GNSS performances. In order to completely understand the benefits that will bring the European Satellite Navigation System, Galileo, it is necessary to evaluate the performance improvement using a hybrid GPS/Galileo Satellite Constellation simulator. A brief review of the different approaches considered in the literature is presented, in order to better situate our approach. As the availability and accuracy may have several different definitions, a brief presentation of these parameters as used in the paper is also done. In this paper, the constellations parameters of the GPS and Galileo systems are briefly reviewed. The Galileo satellite constellation will at least double the number of the existing GPS navigation satellites. The increased number of satellites will minimize also the lack of visibility of satellites in urban canyons. The latest available values of RAAN and mean anomalies have been considered for the Galileo satellite navigation system, even if the problem of the initial offsets of GPS and Galileo orbital planes is still open. Worldwide simulated availability and accuracy are presented for the three possible satellite navigation system configurations: GPS, Galileo and hybrid GPS/Galileo. The following parameters have been considered in order to evaluate the performances of the GPS/Galileo navigation system compared to the existing GPS one: visibility, Geometric Dilution Of Precision (GDOP), Position Dilution Of Precision (PDOP), Horizontal Dilution Of Precision (HDOP), Vertical Dilution Of Precision (VDOP) and Time Dilution Of Precision (TDOP). Before considering temporal variation performances of the hybrid GPS/Galileo system, the spatial variation performances are analyzed. Hence, the evaluations and analysis are performed for all the geographical positions on the Earth (all latitudes and longitudes with a step of 2°). In order to take into account a variety of environments, 2 elevation masks have been considered for the accuracy analysis (10° and 20°). Temporal variation performances are then presented and compared, for the three configurations. The gain obtained with the Galileo satellite navigation system is analyzed. The benefits of the Galileo satellite system are obvious, especially for important values of the elevation mask. The addition of the Galileo system can significantly increase the urban availability (high elevation masks). It is shown in this paper that an integrated use of the GPS signals and the ones of the upcoming Galileo will improve the performances of the Global Navigation Satellite Systems (GNSS) from the availability and accuracy point of view. The analysis presented in this paper will quantify those gains.