Method Of Fast Real-time GPS Precise Clock Estimation

Real-time satellite orbits and clocks are essential for real-time precise point posi-tioning(PPP)applications.Currently,the real-time orbits' accuracy provided by IGS is better than 5 cm,however the clocks' accuracy is about 3 ns,which can not meet the requirments for PPP.Therefore,it becomes a hot topic to estimate satellite clocks with high precision and short latency.Because a large number of ambiguity parameters need to be estimated,the pro-cedure becomes highly time-consuming with undifferenced model.In order to improve the efficiency,the epoch-differenced model for carrier observations is used to obtain high-rate relative clocks between adjacent epochs and then the low-rate absolute clocks are introduced to reform the new observation equations for estimating the high-rate absolute clocks.Obviously,the undifferenced model should be used to provide the low-rate absolute clocks and it's very necessary to study how to estimate clocks with high efficiency using undifferenced model.In this paper,we have summarized the principle and method of real-time satellite clockes estimation,which include the data preprocessing,the method for parameter estimation and clockes quality evaluation etc.The main contents can be listed as following:(1)The errors and correction models are listed in detail in this paper and a ro-bust scheme,known as the Institute of Geodesy and Geophysics(IGG)III function,is introduced to inhibit the effect of gross errors on the results.(2)The single Observation Kalman filter is proposed to avoid inverting the high dimensional matrix,which is very time-consuming and effects the precision of results.In order to improve the efficiency of updating covariance matrix,the OpenMP technique is used and the algorithm is proposed in this paper.(3)Two indices called the availability and the Maximum loss are defined in this paper to evaluate the analysis center's real-time data stream's availability from different time scales.Then,Comparing the real-time orbits and clocks from different analysis center with the IGS final orbits and clocks,the result shows that the real-time orbits'accuracy is better then 5 cm and the STD for real-time clocks is better than 0.3 ns(4)The software for real-time satellite clocks estimation is implemented in this paper and the estimated clocks are compared with IGS final clocks as well as CNES real-time clocks with double-differenced method,the result shows that the STD is better than 0.2 ns(5)The experiments of static real-time PPP and kinematic real-time PPP are carried out in this paper by selecting 10 refernce stations,the result shows that the esti-mated clocks' accuracy equals with the analysis center's real-time clocks,the horizontal accuracy is better than 1 cm and vertical accuracy is better than 3 cm in static mode,the kinematic mode's horizontal accuracy is better than 4cm and vertical accuracy is better than 10 cm.