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

The Global Positioning System (GPS) Block IIR satellites, which will r eplace the current Block II/IIIA satellites, will have satellite-to-sa tellite communication capabilities that will allow intersatellite rang ing between the Block IIR satellites. The cross-link pseudorange measu rements will be used by onboard computers to update the stored navigat ion messages, which are based on trajectories predicted over an extend ed period of time, by ground-based processing of tracking data, During normal operations, the cross-link pseudorange measurements will provi de improved satellite states, which then can be broadcast to the users , One of the error sources in the updated navigation messages, which c annot be corrected by cross-link measurements, is found in the Earth o rientation parameter (EOP) errors, ks 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 th e Block II GPS autonomous navigation accuracy is described. This work is based on simulated cross-link pseudorange measurements. Realistic f orce, measurement, and reference frame models are used in the analysis to account for additional major error sources. Cross-link measurement s for a period of one day, generated at the end of each of three diffe rent prediction intervals, are used to update the predicted trajectory . The estimated solutions then are compared to true solutions to evalu ate the effect of prediction errors. With the current EOP prediction e rrors, the user range errors (URE), computed from improved trajectorie s and clock differences for a 90-day prediction, exceed 9 m, and for a 130-day prediction, they exceed 17 m. Finally, results of processing measurements from ground stations, instead of cross links, are discuss ed wherein the URE for the 180-day prediction case are shown to be abo ut 3.1 m.