| With the development of global multi-GNSS system and real-time(RT)reference station network,the number of navigation satellites is increasing and the quality of observation data are improving.How to improve GNSS real-time precise point positioning(PPP)has become a hot issue.Real-time high-precision satellite orbit and clock products are the basic product of wide-area(or global)RT PPP service system,and its accuracy and quality are the key to guarantee positioning accuracy.At present,international GNSS service(IGS)organization is one of the most important organizations to promote real-time positioning service capability,and each IGS realtime analysis center(AC)provides different navigation systems and different kinds of real-time satellite orbit,clock and other products.Meanwhile,IGS provides combined products IGC01,IGS01,IGS02,IGS03,etc.However,the IGS RT combined products have problems with data interruption and jump,and there is room to improve the quality and stability of RT products.Therefore,it is of great research significance and application value to carry out the method and software of combination of real-time orbital and clock products for multi-GNSS system and realize the stable and reliable RT high-precision combined orbit and clock products.This paper addresses the method and key issues of BDS/GNSS real-time precise satellite orbit and clock products combination,focusing on the combination of multiGNSS products from each IGS real-time analysis center(RTAC).In this paper,the systematic deviation problem caused by the inconsistency of data flow,processing model and coordinate reference used by each AC is solved,and the orbit combination products are estimated in real time by using the robust least squares method with additional quasi stable datum.A RT clock product anomaly detection and processing method based on virtual reference satellite is proposed,and a robust Kalman filter clock combination processing model based on multiple constraints is constructed.On this basis,a real-time precise satellite orbit combination and clock combination processing software for multi-GNSS system is developed.The correctness and effectiveness of the algorithm model and software in this paper are verified by the measured data.The main research work and contributions of this paper are as follows.(1)The basic theoretical methods related to GNSS real-time products combination are systematically described from three aspects: leveling model,datum constraint and parameters robust estimation.For the problem of equation rank deficiency in GNSS RT orbit and clock combinations,the model of adjustment of free networks with additional quasi-stable datum and the indirect adjustment model with restrictive equation are given;considering the quality and anomaly of product,the robust least squares and robust Kalman filter estimation methods with equivalence weight function are studied.(2)Aiming at the inconsistency of coordinate datum and systematic error of RT orbit products from different IGS ACs,a seven-parameter method of similarity transformation is proposed to estimate the systematic deviation between RT orbit products.The effects of coordinate datum disparity and systematic errors of different orbital products are weakened.It is verified that the similar transformation method can effectively weaken the effects of coordinate datum disparity and systematic errors of different orbit products on the accuracy of combined orbit products based on the measured data.(3)Aiming at the problem of equation rank deficiency caused by simultaneous estimation of combined orbit parameters and seven-parameter of similarity transformation in GNSS RT orbit products combination,an additional quasi-stable datum constraint of seven-parameter is proposed,and the robust least squares is used to estimate the RT combined orbit product.This paper compares and analyzes three weighting strategies: equal weight,mean square deviation factor weighting and prior weight of early product evaluation accuracy.The measured results show that the the accuracy of combined orbit by multi-GNSS estimation at the same time is better than that by multi-GNSS estimation separately.The accuracy of the GPS/GLO/GAL/BDS RT orbit combination products is 26.5% higher than that of all ACs,20.3% higher than that of all IGS orbit combination products.(4)This paper proposes a RT clock anomaly detection method by using variation of clock relative to virtual reference satellite.The public satellites before and after the epoch are selected,and the average clock value is taken as the clock value of the virtual reference satellite.The method can avoid the influence of clock difference continuity and data anomaly when one or several satellites are selected as reference satellite.The measured results show that the method can effectively detect the occurrence of RT clock difference in each AC with the average accuracy improvement is 11.8% compared with the multi-GNSS combined clock products without anomaly detection processing.(5)At present,when IGS adopts Kalman filter to process the combined clock products,the separation and convergence of the parameters are achieved by a priori constraint of the given parameters and the variance of the process noise,but the inaccurate priori information of the parameters and the process noise will lead to the problems of slow convergence and low accuracy of the parameter estimation in the filter.The IGS single epoch weighted average estimation method suffers from the problem of instability of the combined clock products caused by selected reference AC and reference satellite.To address the above problems,this paper constructs a multiconstrained conditions model satisfying the minimum constraint solution,develops a RT clock products combination method based on robust Kalman filter with multiconstrained conditions.The parameters of combined clock,initial satellite clock bias and time reference deviation are effectively separated and accurately solved.The parameters of combined clock,initial satellite clock bias and time reference deviation are effectively separated and accurately solved.The measured results show that there is no obvious convergence process in the processing of RT clock products combination by this method,and the accuracy of ASC product is better than that of IGS clock combination products.The accuracy of ASC GPS/GLO/GAL clock products is more than 30.8% higher than that of all ACs clock products,13.9% higher than that of all IGS clock combination products.The accuracy of ASC BDS GEO/IGSO/MEO clock products is improved by more than 23.9% compared with that of all ACs clock products.(6)Based on the high-precision satellite navigation data processing and analysis software(PANDA)platform of Wuhan University,the real-time precise orbit and clock products combination software was developed.The data integrity,orbit and clock accuracy,spatial signal distance error(SISRE)accuracy and PPP performance of the combined product estimated by the software were also verified.The results show that:for the GPS/GLO/GAL system,the data integrity rate of all AC products is increased by more than 4.6% compared with the combined product ASC based on the method and software.The data integrity rate of all IGS combined products is increased by more than 3.0% compared with ASC product.Compared with all AC products and all IGS combination products,the SISRE accuracy of ASC products is increased by more than31.9% and 49.4% respectively.For BDS GEO/IGSO/MEO satellites,the data integrity rate and SISRE of ASC products compared to all AC products is increased by more than6.5% and 27.5% respectively.Through 20 MGEX reference stations distributed around the world,the performance of simulated RT PPP based on different navigation system combination schemes is verified,and the convergence time,simulated dynamic PPP accuracy and single-day static PPP accuracy of the combination products processed by the method and software of this paper are better than those of all AC products and IGS combination products in general. |
