Precise positioning of a geostationary data relay using LEO satellites

Earth observation uses a large number of Low Earth Orbiting (LEO) satellites (ca. 400). The increasing resolution of their instruments has inflated the data volumes that need to be transmitted. The low altitudes limit the contact times, which is challenging both with respect to the data volumes and delay until data can be transmitted. Geostationary satellite (GEO) relaying is thus a promising alternative. In order to allow pre-compensation of Doppler Shift at the LEO side and to synchronize the transmission from the LEOs to the GEO, the positions and relative velocities of the satellites have to be known. LEOs can be positioned today with centimetre accuracy by GPS. GEOs orbit on higher altitudes than GPS satellites and therefore face problems using GPS signals. Conventional methods allow GEO positioning from ground with accuracies in the km range. In this paper, a new concept of GEO precise positioning using communication channels of LEO satellites is presented. Simulations show that Kalman filtered pseudoranges lead to positioning errors in cm range. A new method based on Newton algorithm allows to determine the Keplerian parameters and their linear drifts in order to predict the GEO position.