Pseudolite-Based Position and Attitude Control of Satellites in High Earth Orbits

The potential of positioning at the geostationary orbit (GEO) using the Global Positioning System (GPS) has been investigated in several studies. They predict an orbit determination accuracy in the range of state-of-the- art tracking methods for GEO satellites. The first measurements of GPS signals at a geostationary altitude were taken by the GPS experiment on board the German scientific satellite Equator-S showing the feasibility of navigation solutions. Both simulations and measurements reveal the same problems: First a limited availability due to shadowing of GPS signals by the Earth and second a limited precision due to selective availability (SA) and ionospheric refraction. These disadvantages can be solved or improved by using pseudolites, which are located on the ground and transmit GPS-like signals. Onboard a geostationary satellite a slightly modified GPS receiver will be used to receive both GPS satellite and pseudolite signals. Thus, the availability of positioning service is tremendously increased. A better accuracy of positioning can be achieved with a stand-alone pseudolite system using four or more pseudolites. This will also improve the attitude determination performance which is sufficiently accurate for coarse attitude control. This paper describes the pseudolite system as well as the receiver for autonomous on-board orbit and attitude determination. Simulation results for positioning, orbit and attitude determination will be shown.