Geosynchronous Satellite Use of GPS

The purpose of this study was to determine the accuracy of GPS use for a geosynchronous (GEO) satellite. Current missions at GEO altitude mainly use traditional ranging for orbit determination. With changing mission requirements and the increase in the number of GEO missions, utilizing GPS signals is becoming an increasingly attractive alternative for position and timing determination. A previous ION paper discusses the use of GPS data for a particular geosynchronous satellite mission and is included here as a reference. GPS use at GEO is primarily limited by the availability of the spillover from the GPS earth coverage signal. The availability of the GPS signal at GEO is determined by the GPS block specific antenna patterns and the GEO satellite’s receiver antenna. This analysis specifically examined the effects of the GPS constellation availability, 122 ION GNSS 18th International Technical Meeting of the Satellite Division, 13-16 September 2005, Long Beach, CA antenna gain patterns, and GPS receiver clock stability on position and timing accuracies at GEO. INTRODUCTION GPS missions at GEO synchronous altitude use the spillover of the earth coverage GPS signals. The main beam of the GPS antenna varies from approximately 23° to 26° depending on the Block and frequency of the GPS vehicle. In addition to the main earth coverage beam, the GPS vehicles have side-lobe signals which have enough power to reach a user at GEO altitude. Unlike a Terrestrial User or a Low Earth Orbit (LEO) user of GPS, the GPS satellites a GEO satellite will use are beneath the GEO satellite. An illustration of this is given in Figure 1. Figure 1 – GPS at GEO diagram If a GEO satellite is restricted to the main beam of the GPS signal, a significant reduction in the availability of a single GPS satellite is seen. Outages of a couple of hours can be encountered. These outages are a function of the current constellation size and location of the GPS satellites and also of the Block of GPS satellites that are Main beam Side lobe GEO Satellite GPS Side lobe