Improved method for the GPS high-precision real-time satellite clock error service

The real-time service (RTS) of satellite clock offsets is one of the main routine tasks for real-time precise point positioning (PPP). The quadratic polynomial coefficients of the GPS satellite clock corrections in the International GNSS Service (IGS) RTS are updated every 5 s. This frequent updating of the satellite clock correction coefficients means that the quadratic polynomial adopted in IGS RTS is inadequate if a lack of connectivity necessitates extrapolation over minutes instead of seconds. An improved RTS, using a polynomial and harmonic-based function, is proposed to address this concern. Its functions are constructed based on the periodic variations in the GPS satellite clock corrections observed from the differences between the estimated satellite clock corrections and those of the broadcast ephemeris over one year. The eighth-order harmonic coefficients are updated in the improved RTS every 6 h. Extrapolation results show that the combined results of a quadratic polynomial plus an eighth-order harmonic-based function are better than those of a quadratic polynomial plus a fourth-order harmonic-based function. The 6 h extrapolation accuracy of the combined quadratic polynomial and eighth-order harmonic-based function reaches 0.81 ns and is significantly improved over that of the current IGS ultra-rapid products. Under good connectivity condition and eighth-order harmonic coefficients at 6 h intervals, which transmits linear polynomial coefficients at 1 min intervals, is better than the IGS RTS product, with a 5-day averaged root mean square of 0.10 ns. Although the update interval of the improved RTS is 12 times larger than that of the current IGS RTS, its 5-day static and kinematic PPP performances are better at all 55 IGS stations tested. Even if connectivity is interrupted for several minutes, the improved method can reach centimeter and even millimeter accuracy.