Satellite clock estimation at 1 Hz for realtime kinematic PPP applications

Abstract

Realtime kinematic precise point positioning (PPP) requires 1 Hz GPS satellite clock corrections. An efficient clock estimation approach is presented. It applies a combined dual-thread algorithm consisting of an undifferenced (UD) and epoch-differenced (ED) engine. The UD engine produces absolute clock values every 5 s, and the ED engine produces relative clock values between neighboring epochs at 1-s interval. A final 1-Hz satellite clock can be generated by combining the UD absolute clock and ED relative clock efficiently and accurately. Forty stations from a global tracking network are used to estimate the realtime 1-Hz clock with the proposed method. Both the efficiency and accuracy of the resultant clock corrections are validated. Efficiency test shows that the UD processing thread requires an average time of 1.88 s on a 1-GHz CPU PC for one epoch of data, while ED processing requires only 0.25 s. Accuracy validation test shows that the estimated 1-Hz clock agrees with IGS final clock accurately. The RMS values of all the available GPS satellite clock bias are less than 0.2 ns (6 cm), and most of them are less than 0.1 ns (3 cm). All the RMS values of Signal in Space Range Error (SISRE) are at centimeter level. Applying the accurate and realtime clock to realtime PPP, an accuracy of 10 cm in the horizontal and 20 cm in the vertical is achieved after a short period of initialization.