GPS-Based Tracking System For Tom Orbit Determination

About three years ago, NASA's Oceanic Processes Branch requested that we evaluate alternate tracking concepts for sub-decimeter positioning of earth orbiting spacecraft such as TOPEX. Current high precision techniques, specifically TRANET and ground based laser ranging, yielded at best about 40 cms orbit accuracies on Seasat which flew at an 800 km altitude. Improved versions of these techniques, when coupled with projected improvements in knowledge of the gravitational field of the earth and with the higher altitude (1300 km) of 'IOPEX, appear capable of achieving about a factor of 3 improvement in orbit accuracy. Even at this level orbit errors dominate the overall system error budget. Our study led us to adopt a new tracking system concept based on utilization of the constellation of Naystar satellites in the Global Positioning System (GPS). In simplest terms, this concept involves simultaneous and continuous metric tracking of the signals from all visible Naystar satellites by approximately six globally distributed ground terminals and by the TOPEX spacecraft (see Figure 1). Error studies indicate that this system could be capable of obtaining decimeter position accuracies and, most importantly, around 5 cm in the radial component which is key to exploiting the full accuracy potential of the altimetic measurements for ocean topography. A series of proof-of-concept demonstrations has been completed with a pair of recently developed precision GPS receivers. This paper provides brief background discussions of the GPS, the precision mode for utilization of the system, past JPL research for using the GPS in precision applications, the present tracking system concept for high accuracy satellite positioning, and results from the demonstration.