European complement to GPS: Orbit determination strategies and results of preliminary experiments

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The reference mission of the European Complement to GPS (CE-GPS) is destined to insure the integrity of the navigation message of the U.S. Global Positioning System (GPS) for the Civil Aviation community. CE-GPS is based on a geostationary payload capable of transmitting a GPS-like signal to ground users (civil aviation extended to maritime and terrestrial transport). This signal is broadcasted from a ground station (the Feeder Link Station) towards the geostationary which repeats it. The signal includes both a navigation message (GPS satellites ephemerides) and a health status of the GPS constellation (integrity message). An Operational Orbitography Center capable of processing the orbits of the geostationary and GPS satellites elaborates the navigation and integrity messages. The measurements required for the computation of the orbits are collected by a network of about six Monitoring Stations (in the equatorial area) and transmitted to the Feeder Link station. Experiments are under way to test the different new techniques involved in the development of the ground stations, and to select the optimal strategies for the geostationary orbit determination process and for station synchronization. This paper presents the different orbit determination strategies developed for the geostationary satellite and preliminary results obtained with data collected during two campaigns: autumn 1993 with two ground stations (Paris and Toulouse) and Spring 1994 with three ground stations (Kourou, Hartebeeshoek and Toulouse). The paper is organized in three parts: first, the CE-GPS concept is presented; second, the experimental system designed by CNES to assess the CE-GPS technical performances is described; and last, the experimental results are presented.