Tracking GPS above GPS satellite altitude: first results of the GPS experiment on the HEO mission Equator-S

In recent years the number of LEO satellites that carry a GPS receiver on-board has significantly increased. However, in orbits above the GPS satellites there is no experience about the reception of GPS signals. Especially in the Geostationary Orbit with its dense satellite population a precise orbit and attitude determination is needed. There is not only a demand for accuracy but also a demand for reducing costs with auto-navigation technologies. This would decrease the number of ground stations needed and the expense for equipment and commanding support. Both could be easily achieved using GPS. In order to assess the possibility of the use of GPS in such orbits, a GPS receiver was installed as an experiment on-board the HEO satellite Equator-S. The main goals of the GPS Experiment on-board Equator-S are to prove that the reception of GPS signals at this orbit is possible and to show that a reception of signals in the side lobes of the GPS antenna emission is also possible. The data will be evaluated regarding the quality of the physical signals and the data contents. Equator-S was launched in December 1998 into a Geostationary Transfer Orbit. The GPS receiver was switched on several hours after launch and tracked a GPS satellite only a few minutes after the initialization at a height of about 34000 km. The satellite is now in the final orbit which has an apogee at a height of 67000 km. In this orbit the reception of signals from the side lobes of the transmitting GPS antennas was proved and the tracking of GPS satellites at heights above 34000 km is expected. This paper describes the experiment and reports about the problems that have been encountered by using a receiver designed for a LEO orbit. Finally it presents the results and gives an outlook of the future operations.