| The Beidou satellite navigation system(BDS)is a global satellite navigation system independently developed by China.With the continuous improvement of the system construction,the Beidou-3 satellite system(BDS-3)will be fully completed in 2020,which indicates that China's Beidou system will completely transition from a regional satellite navigation system to a global satellite.Navigation System.With the increasing number of the observable satellites,signal types and the number of reference stations of the Beidou system,it means better satellite space configuration,rich observation information and diverse receiver types,which also put forward higher requirements for the performance of precise data processing based on Beidou system.This paper focuses on improving the precise positioning performance of the Beidou system.We maily study the observations haracteristics of the Beidou-2 and Beidou-3 satellite,on this basis,a pseudo-range bias correction model for Beidou-3 satellite is established.In addition,considering the hardware delay deviation between Beidou-2 and Beidou-3 satellites,different processing strategies are proposed and applied to real-time satellite clock estimation.Finally,based on the real-time orbit and clock products,we analyzed the real-time precise positioning performance of Beidou system with different regions,different combinations and different positioning processing modes.The main contributions of this paper are listed as follows:1)The characteristics of pseudorange observations in the Beidou satellite system are studied through the global tracking station network which equipped different types of receivers,and the differences between Beidou-2 and Beidou-3 are mainly analyzed.On this basis,a pseudorange deviation correction model related to the receiver types in the original frequency observations of the Beidou-3 satellite is established for the first time,and a data processing method which takes into account the hardware delay deviation between the Beidou-2 and Beidou-3 satellites is proposed.And by designing different satellite clock estimation schemes,the effectiveness of the models and methods proposed in this paper are verified.Firstly,when the pseudorange deviation correction model proposed in this paper is applied,the accuracy of the BDS-3 satellite clock estimation is improved by about 70.3%.And when considering the hardware delay deviation between the BDS-2 and BDS-3 satellites,the BDS-2 and BDS-3 satellites are treated as two systems and the satellite clock parameters are estimated separately.Compared with the processing method of one system,the clock accuracy increases by about 14.0% on average.2)Base on the method of integer recovery clock ambiguity resolution,the postprocessing data simitation real-time processing mode are used to estimate the Beidou satellite clock.Then we analyzed the results of different strategies from the aspects of UPD(Uncalibrated phase delays)stability,consistency,ambiguity resolution fixing rate and satellite clock accuracy.The experimental results show that,the widelane UPD value of BDS-3 satellite present good stability among one month,and can be calculated and predicted for a long time.And in terms of UPD consistency,the hardware delay deviation between BDS-2 and BDS-3 satellites will seriously destroy the consistency of the calculated UPD values,while BDS-2 and BDS2-3 satellites are used as two systems,and different benchmarks are selected,so the consistency of the estimated UPD values is higher.Then in the ambiguity resolution,with the pseudo-range deviation correction model proposed in this paper is applied,the success rate of wide-lane ambiguity can be improved by about 5.0%~6.0%.In addition,considering the hardware delay deviation between the BDS-2 and BDS-3 satellites,they are treated as two systems.Compared with the processing mode of one system,the success rate of wide-lane ambiguity can increased by an average of 26.8%.Finally,in the aspect of real-time satellite clock estimation,with the ambiguity resolution,the convergence speed and estimation accuracy of the BDS satellite clock are both improved.Among them,for the C21 satellite,the accuracy of the clock bias can be improved from 0.527 ns to 0.432 ns in the period of first 3h convergence time;for the C23 and C28 satellites,in the period of first 2h convergence time,theaccuracy of the clock bias can be improved from-0.554 ns to-0.436 ns and from-0.347 ns to-0.184 ns,respectively.For clock accuracy,according to the average STD value,the fixed solution clock accuracy of BDS-2 and BDS-3 satellites can be improved by 46.5% and 25.0%,respectively.3)Based on the above research,the dynamic PPP float solutions of different combination and different positioning modes are analyzed in detail.The experimental results show that the addition of BDS-3 satellite can significantly improve the PPP performance of BDS and the combination of GPS/BDS.In terms of convergence time and positioning accuracy,dynamic DFPPP/SFPPP based on combined BDS-3/BDS-2 is superior to BDS-2 only.Among all the combinations,the GPS/BDS-2/BDS-3 present the fastest convergence speed and the best positioning accuracy.When 68% confidence interval is selected,the convergence time for GPS/BDS-2/BDS-3 is reduced by 55.0%,51.9%,and 31.6%,27.1% over the GPS-only and GPS/BDS-2 in the horizontal and vertical direction respectivetly for dual-frequency PPP.As for the positioning accuracy,the results shows that compared with GPS-only,the combination of GPS/BDS-2/BDS-3 can improve 28.6%,33.8% and 36.2% in the three directions of up(U),north(N)and east(E)respectively,which is about 1.8 times as much as GPS/BDS-2.4)The “integer clock”estimated in this paper can be directly used for different combination of PPP ambiguity resolution,including GPS-only,BDS-3/BDS-2 and GPS/BDS-2/BDS-3.The experimental results show that the PPP ambiguity resolution can shorten convergence time and improve positioning accuracy.According to the average statistical level of all stations,the PPP fixed solution shows the most obvious improvement in E direction,which is about 9.1%.And in U and N direction,the improvement are 4.1% and 7.9%,respectively.Additionally,it is clear that the GPS/BDS-2/BDS-3 PPP fixed solution present the best performance in the first fixed time(TTFF)and positioning accuracy.For the dynamic dual frequency PPP,the average TTFF for the GPS/BDS-2/BDS-3 is about 82 min,and they are about 190 min and 156 min for BDS-2/BDS-3 and single GPS system. |
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