Accurate absolute GPS positioning through satellite clock error estimation

An algorithm for very accurate absolute positioning through Global Position ing System (GPS) satellite clock estimation has been developed. Using Inter national GPS Service (IGS) precise orbits and measurements, GPS clock error s were estimated at 30-s intervals. Compared to values determined by the Je t Propulsion Laboratory, the agreement was at the level of about 0.1 ns (3 cm). The clock error estimates were then applied to an absolute positioning algorithm in both static and kinematic modes. For the static case, an IGS station was selected and the coordinates were estimated every 30 s. The est imated absolute position coordinates and the known values had a mean differ ence of up to 18 cm with standard deviation less than 2 cm. For the kinemat ic case, data obtained every second from a GPS buoy were tested and the res ult from the absolute positioning was compared to a differential GPS (DGPS) solution. The mean differences between the coordinates estimated by the tw o methods are less than 40 cm and the standard deviations are less than 25 cm. It was Verified that this poorer standard deviation on 1-s position res ults is due to the clock error interpolation from 30-s estimates with Selec tive Availability (SA). After SA was turned off, higher-rate clock error es timates (such as 1 s) could be obtained by a simple interpolation with negl igible corruption. Therefore. the proposed absolute positioning technique c an be used to within a few centimeters' precision at any rate by estimating 30-s satellite clock errors and interpolating them.