Pseudolite augmented navigation for GEO communication satellite collocation

This paper presents some of the technical and economic incentives for developing satellite communication systems based on clusters of spacecraft located in a single GEO orbit slot. Collocating multiple satellites in the same slot will require improved techniques for relative vehicle sensing of both position and velocity. Current ground based tracking of satellite positions requires a high level of ground operations support to provide this information, a burden which will become unmanageable as the number of GEO satellites increases in the future. As a result, autonomous navigation of GEO orbiting satellites has become highly desirable, and GPS ranging measurements are a key enabling technology for achieving this goal. Two different methods for augmenting the GPS constellation with additional ranging measurements are examined in this paper. One method is based on placing ground-based pseudolite transmitters at widely separated locations on the Earth to provide constantly visible ranging signals to the GEO satellite. The second method uses pseudolite transmitters onboard each satellite in the GEO cluster to directly measure the relative spacecraft range and range rate. Results from a simulation study of the relative position and velocity determination performance of these two methods are compared with the performance using measurements from the unaugmented NAVSTAR satellites. Also, the benefits and drawbacks of these two methods, in terms of cost and potential for interference, are briefly addressed.