| The last satellite of BDS-3 was launched and successfully networking on June 23,2020,marking the completion of global navigation satellite system for China.The research on BDS-3 will help to promote it to play a greater role in many fields such as national economy and national defense construction.This paper mainly studies the PPP algorithm of BDS-3,BDS-2and GPS,analyzes the quality of BDS and GPS observations,and tries to apply the measurement results to engineering practice.The main contents and conclusions of this paper are as follows:(1)The basic principle of PPP is discussed.This paper introduces the characteristics of GPS,BDS-2 and BDS-3,and the role of IGS(International GNSS services)in PPP;expounds the function model,stochastic model,parameter estimation method,data preprocessing and quality control method of PPP;introduces the error term to be considered in the process of PPP solution and common processing methods.(2)The data quality of GPS,BDS-2 and BDS-3 is analyzed.The satellite systems and signal types received by IGN(Institut Geographique National)sites are counted.the data integrity rate of GPS,BDS-2 and BDS-3,the number of global visible satellites,signal-to-noise ratio(SNR),multipath effect,the residual of pseudo range and carrier observation for zero baseline are analyzed.The results show that BDS-3 is similar to GPS but better than BDS-2 for data integrity rate,number of visible satellites,SNR level,and observation residual.(3)The model of accelerating PPP convergence speed is studied.Based on the observation residuals of GPS,BDS-2 and BDS-3,the stochastic model of exponential function based on altitude angle is constructed;the PPP positioning characteristics of GPS,BDS and combined positioning are studied,and the inter system bias between BDS-2,BDS-3 and GPS is analyzed The results show that the convergence time of combination of GPS and BDS is 16.22 minutes shorter than that of single GPS for PPP,and the plane accuracy is improved from 0.59 cm to0.44cm;the maximum change of ISB of BDS 3 in a single day is 0.8ns,and the maximum change of ISB of BDS 2 in a single day is 1.25ns;the convergence time of multi frequency BDS 3 is 13.71 minutes shorter than that of dual frequency for PPP The plane positioning accuracy is improved from 1.55 cm to 1.02 cm,and the elevation accuracy is improved from7.87 cm to 3.70 cm.(4)PPP ambiguity resolution of GPS is studied.This paper introduces the principle of PPP ambiguity resolution and FCB estimation,analyzes characteristics of the wide lane FCB and narrow lane FCB for School of Geodesy and Geomatics’ s products,and uses products to realize GPS ambiguity resolution.The results show that the plane positioning accuracy of fixed-PPP is improved from cm level to mm level compared with float-PPP at the 2-hour.(5)The fitting method of regional height anomaly in GNSS measurement is studied.GNSS can achieve high precision geodetic height,but there is elevation anomaly between geodetic height and normal height.The accuracy of elevation anomaly directly affects the accuracy of normal height from GNSS measurement.In this paper,226 known points of elevation anomaly are used to test the piecewise surface fitting method and moving surface fitting method.The mean square error of the former is 2.47 cm,and the latter is 1.86 cm.Compared with the measured data of the second-class leveling,the deviation of the normal height indirectly measured by GNSS of three points is less than 2cm.In the fitting area of this paper,assuming that the geodetic height error measured by GNSS is 1cm,taking twice the mean square error as the limit error,GNSS survey can replace the fourth-order leveling with leveling line greater than 4.5km. |
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