Research On Satellites Selection And Clock Modeling For Multi-GNSS Precise Point Positioning

Precise point positioning is a high precision absolute positioning technology which based on a single receiver.The traditional single-system precise single-point positioning is limited by less visual satellites and simple geometrical layout of satellite constellation,which leads to too long convergence time.In the environment of severe signal occlusion,high precision positioning can not be completed,which seriously restricts the expansion of the application field of PPP.In recent years,with the continuous construction and improvement of satellite navigation systems such as GLONASS,GALILEO,BDS,users can obtain more observations,thus making the multi-GNSS PPP possible.However,multi-GNSS PPP is not a simple combination of systems,and there are still many problems to be solved.Aiming to further improve the positioning accuracy and convergence rate of PPP,the positioning performance of multi-GNSS PPP,satellites selection algorithm and the high-performance receiver atomic clock modeling are studied systematically in the paper.The main contents and conclusions of this paper are as follows:(1)The unified method of time system and coordinate system of each satellite system are discussed in this paper.Meanwhile,the error source of multi-GNSS PPP and the strategy for error elimination are analyzed.(2)The mathematical model of multi-GNSS PPP and data processing strategy are determined.And the positioning performance of single-GNSS(GPS)as well as multi-GNSS combination(GPS/GLONASS,GPS/GLONASS/GALILEO,GPS/GLONASS/GALILEO/BDS)are compared and analyzed.Experimental results show that,compared with GPS PPP,the combined GPS/GLONASS PPP and the combined GPS/GLONASS/GALILEO PPP have significantly improved the convergence rate and positioning accuracy,while the combined GPS/GLONASS/GALILEO/BDS PPP has further improved convergence rate and positioning accuracy.(3)Based on the geometrical layout of satellites constellation,a satellites selection algorithm is constructed by using three-dimensional convex hull.At the same time,the signalto-noise ratio(SNR)is introduced to measure the quality of observation,the paper proposes the idea of three-dimensional convex hull satellites selection algorithm considering the quality of observation.The experimental results show that the number of satellites is greatly reduced and the selected satellites can maintain a reasonable geometrical layout after the three-dimensional convex hull satellites selection algorithm.Compared with the maximum volume satellites selection algorithm and two-dimensional convex hull satellites selection algorithm,the convergence rate and positioning accuracy of PPP are improved to a certain extent.And the convergence rate and positioning accuracy of PPP are further improved by three-dimensional convex hull satellites selection algorithm considering the quality of observation.(4)The traditional PPP takes the receiver clock error as the white noise for parameter estimation,which ignoring the strong correlation between the receiver clock error and elevation parameters,resulting in the reduction of elevation accuracy.In order to solve this problem,the high-performance receiver atomic clock is adopted,and the multi-GNSS PPP receiver clock modeling algorithm is constructed by Kalman filter.Experimental results show that the convergence time of elevation direction is shortened from tens of minutes to about three minutes by using the algorithm,and the convergence rate and positioning accuracy of PPP are greatly improved.(5)Aiming to further analyze the performance of fusion algorithm in multi-GNSS PPP,the satellites selection algorithms are combined with receiver clock modeling algorithm.The experimental results show that,compared with the maximum volume satellite selection algorithm and two-dimensional convex hull satellites selection algorithm,the convergence rate and positioning accuracy of PPP which using three-dimensional convex hull satellites selection algorithm considering the quality of observation are improved.And the receiver clock modeling algorithm further improves the convergence rate and positioning accuracy of PPP.