AUTONOMOUS NAVIGATION OF GLOBAL POSITIONING SYSTEM SATELLITES USING CROSS-LINK MEASUREMENTS

The Global Positioning System (GPS) Block IIR satellites, which will replace the current Block II/IIA satellites, will have satellite-to-satellite communication capabilities that will allow intersatellite ranging between the Block IIR satellites. The cross-link pseudorange measurements will be used by onboard computers to update the stored navigation messages, which are based on trajectories predicted over an extended period of time, by ground-based processing of tracking data. During normal operations, the cross-link pseudorange measurements will provideimproved satellite states, which then can be broadcast to theusers. One oftheerrorsourcesin the updated navigation messages, which cannot be corrected by cross-link measurements, is found in the Earth orientation parameter (EOP) errors. As a consequence, as the age of the Earth rotation parameter increases, the performance of the autonomous navigation system will degrade. The effect of this error source on the Block II GPS autonomous navigation accuracy is described. This work is based on simulated cross-link pseudorange measurements. Realistic force, measurement, and reference frame models are used in the analysis to account for additional major error sources. Cross-link measurementsfora period of oneday, generated at the end ofeach of three different prediction intervals, are used to update the predicted trajectory. The estimated solutions then are compared to true solutions to evaluate the effect of prediction errors. With the current EOP prediction errors, the user range errors (URE), computed from improved trajectories and clock differences for a 90-day prediction, exceed 9 m, and for a 180-day prediction, they exceed 17 m. Finally, results of processing measurements from ground stations, instead of cross links, are discussed wherein the URE for the 180-day prediction case are shown to be about 3.1 m.