| With the technological revolution and the development of global digitalization,the space-time location information service system has become a new infrastructure for human production and life.The gradual improvement of multi-frequency and multi-system GNSS provides a more convenient tool for space-time position information.As another high-precision positioning technology different from RTK,PPP can realize single-station high-precision positioning.However,due to too many parameters to be estimated,strong parameter correlation in the function model,and weak strength of the normal equation,PPP usually takes a long time to converge the position parameters to centimeter-level accuracy,and the re-initialization time after signal loss lock is almost as long as the first initialization time,which limits its popularity in real-time applications.Through the introduction of products such as observable-specific signal bias and inter-frequency clock deviation,the user end can achieve ambiguity fixation and thus improve the convergence speed.With the development of emerging systems such as BDS-3 and Galileo and the modernization of GPS,PPP ambiguity fixation has developed from dual-frequency PPP-AR to triple-frequency or even multi-frequency PPP-AR.Multi-frequency observations can narrow the search range of ambiguity and effectively improve Ambiguity fixed efficiency.At the same time,the number of ground-based reference stations is increasing,and the atmospheric enhancement information provided by them can reduce the correlation between ambiguity and other parameters to be estimated,and realize the rapid initialization of PPP ambiguity.This technology is called PPP-RTK.Fine atmospheric modeling is an important prerequisite for the rapid convergence of PPP-RTK.With the upgrading and transformation of China’s CORS stations,the enhanced information provided by domestic regional CORS stations is used as an external constraint to quickly realize PPP-RTK high-precision positioning.At present,navigation and positioning have developed from the era of reliable measurement and complete navigation to the era of credible PNT.Discussion on PPP-based data trustworthiness,product trustworthiness and algorithm trustworthiness determines the application value of GNSS in science and engineering.Therefore,in response to the rapid positioning requirements based on PPP technology,this paper conducts BDS-3/GNSS OSB,ionosphere,troposphere and other enhanced product estimation,and PPP-RTK rapid initialization key technology research,and provides theoretical support for GNSS trusted positioning.This enables the PPP technology to be practically applied in real-time positioning of users.The main work and contributions of the paper are as follows:(1)The function model and stochastic model of BDS-3/GNSS precise point positioning are systematically described.Starting from the original observation equation,two PPP positioning models,ionosphere-free combination and non-difference non-combination,are deduced in detail,and the commonly used multi-frequency linear combination is given.Two parameter estimation methods commonly used in non-difference PPP,least squares and Kalman filter,are introduced;the resistance performance of the robust Kalman filter method to outliers in observations is verified.Experiments show that compared with the standard Kalman filter,it is improved by 65%,49%,and 25%,respectively.The basic theories and methods of BDS-3/GNSS data preprocessing and quality control are sorted out.(2)Aiming at the low calculation efficiency of FCB,a fast FCB estimation method with epoch constraints is proposed.The conversion relationship between FCB and OSB is carefully deduced,and dual-frequency OSB generation is realized.The results show that the stability of the phase deviation products at each frequency of the three systems of BDS-3,GPS and Galileo is better than 0.05 ns.Extend the OSB estimation of dual-frequency BDS-3/GNSS to tri-frequency,and propose a tri-frequency OSB estimation considering IFCB correction.The absolute deviation OSB is corrected to the original observation value,which can be directly used for non-difference and non-combination multi-frequency PPP ambiguity fixed.The results show that the accuracy of the generated absolute deviation OSB products: GPS and Galileo stability is better than 0.03 ns and BDS-3 stability is better than 0.06 ns.(3)Aiming at different regions and areas with large elevation fluctuations,tropospheric modeling methods considering height correction and ionospheric modeling methods considering latitude regions are proposed.Considering the large height difference area,it is pointed out that the tropospheric normalization is carried out by using the exponential attenuation function,then the regional tropospheric model is built,and the accuracy of the tropospheric layer of the interpolated grid is analyzed.The specific process of regional ionospheric modeling by low-order spherical harmonic function is described in detail,and the accuracy of regional ionospheric modeling with different system combinations and different latitudes is analyzed.The user uses regional atmospheric products and deviation products to quickly achieve high-precision and rapid positioning of regional PPP-RTK.The research shows that for tropospheric modeling,in the areas with large elevation fluctuations,the wet delay RMS without additional height correction is 7.1mm,and the additional height correction RMS is 6.2mm.The wet delay accuracy after additional height correction is 12.7% higher than that without additional elevation correction.For ionospheric modeling,the ionospheric modeling accuracy of GCE multi-system is better,and the ionospheric modeling accuracy of BDS-3 is similar to that of GPS;the RMS of ionospheric modeling in low latitudes is obviously higher than that in middle and high latitudes.(4)Aiming at the satellite attitude anomaly in the earth shadow area,a BDS-3/GNSS ambiguity fixed step by step PPP-AR considering the satellite attitude anomaly is proposed.The application of satellite attitude quaternion products to PPP-AR can effectively improve the positioning accuracy of PPP-AR,shorten the convergence time,and improve the success rate of ambiguity fixing.The results show that,compared with the nominal attitude model,the convergence time of satellite attitude quaternion products used in PPP and PPP-AR can be shortened,and PPP is shortened from 95.5min to 70.2min,which is increased by 26.5%.PPP-AR has been shortened from 44.8min to 23.5min,an increase of 47.5%.PPP uses quaternion attitude model products to improve the percentage of U direction is relatively high,about 8% of the percentage of improvement of the E direction of PPP Muir AR is about 12%.(5)BDS-3/GNSS PPP-RTK credible positioning is achieved based on the additional atmospheric error stepwise-relaxed constraint.The GNSS PPP-RTK rapid precision positioning system with BDS-3 as the core is established through the non-difference and non-combination model,and the regional PPP-RTK experimental analysis of atmospheric products is carried out by using the step-by-step relaxation constraint method.The results show that according to the measured CORS station data,the convergence time of BDS-3/GNSS PPP-RTK is better than that of 8min,and different data sampling rates have no great influence on the convergence time;the level of positioning accuracy is better than 2cm,the elevation is better than 3cm,and the fixed rate of ambiguity is more than 93%.Aiming at the reliability differences caused by data quality,error products and location algorithm diversity between cloud and terminal,an evaluation method of GNSS data quality,product accuracy and algorithm credibility based on credibility is proposed,which provides theoretical basis and technical support for GNSS trusted location.The results show that the reliability of the data quality of a CORS station in southwest China is 0.91 and the product credibility is0.86.There are 97 pictures,41 tables and 174 references. |
