Research On Methodology And Application Of GNSS Decoupled Clock Estimation And Precise Point Positioning Ambiguity Resolution

Precise point positioning(PPP)is a spatial positioning technology proposed in the 90s of the 20th century.With the support of external precision product information and comprehensive consideration of the accurate correction of various errors,it can accurately determine the absolute position of the target worldwide by using a single receiver.Compared with the customary differenced positioning technology,PPP does not rely on the observation information of the reference station,and the positioning method is more flexible.It has been widely used in scientific research fields such as space atmospheric sensing.In recent years,with the continuous improvement and development of global navigation satellite systems(GNSS)such as BDS in China,GPS in the United States,GLONASS in Russia and Galileo in the European Union,and the improvement of precise satellite orbit and clock product accuracy,PPP technology has gradually expanded from the field of scientific research to more popular applications such as unmanned aerial vehicle and even automatic driving.PPP constitutes an important part of integrated Positioning,Navigation,Timing(PNT)and has become a research hotspot of GNSS data processing.Nowadays,precise satellite clock estimation has formed a relatively complete system,but some problems still need to be further studied,such as the factors affecting the convergence speed and stability of satellite clock estimation.In addition,in satellite clock estimation,due to the absorption of ionospheric ambiguity to deviation parameters,it is impossible to obtain PPP ambiguity-fixed solution only by relying on precise satellite clock products,which limits the further improvement of positioning accuracy.Moreover,as an important ambiguity resolution model,the decoupled clock model is rarely deeply studied in the current literature,and there is no expansion of the decoupled clock model from a single system to multiple systems.Based on the above contents,it is also necessary to develop the corresponding software platform to ensure the feasibility and effectiveness of the algorithm model and lay the foundation for the follow-up application research work.This dissertation focuses on research topics such as "preicse satellite clock estimation","PPP","ambiguity resolution",and"software platform development"and carries out research on the multi-GNSS(including China’s third generation Beidou system)precise satellite clock estimation and PPP ambiguity resolution model,algorithm and software platform.The main work and contributions of this paper are as follows:1.A satellite clock estimation method extracting code bias is proposed,and the influence of time-varying satellite code bias on the convergence speed and stability of satellite clock estimation is studied.It usually takes a long convergence time to make the satellite clock reach a relatively stable state when estimating the satellite clock.In order to study the factors hindering the convergence of the satellite clock,after analyzing the processing methods of bias parameters in the traditional ionosphere-free model,this paper considers that the time-varying code bias parameters are the main factors affecting the convergence speed of the satellite clock,and deduces three clock estimation models extracting satellite code bias,receiver code bias and both satellite and receiver code bias at the same time.By analyzing the stability of the satellite clock after extracting various code biases,it is confirmed that time-varying satellite code bias is the main factor hindering the convergence of the satellite clock.2.The key problem of the decoupled clock model at the server end is studied,and the implementation scheme of the decoupled clock model is improved.The decoupled clock model separates the code clock and phase clock,takes into account the time-varying characteristics of the wide-lane ambiguity bias parameters,and estimates three kinds of clock(or clock like)products at the same time.Users can easily obtain the ambiguity-fixed solution by using the decoupled clock product.However,at the server end of the decoupled clock model,due to the complexity of datum parameter selection,there is little research on it at present.Starting from the basic observation equation,this paper deduces the specific form of the decoupled clock model,studies the selection strategy of ambiguity datum,generally analyzes the degree of freedom of the model,puts forward a specific scheme to adapt to the ambiguity resolution at the server end,and verifies the effectiveness of the product and algorithm through the positioning practice at the user end.3.Considering computational efficiency,a method of satellite clock estimation for multi-GNSS based on the decoupled clock model is proposed.In order to improve the calculation eficiency of clock estimation,the influence of ambiguity parameters is usually eliminated by epoch difference.However,for the decoupled clock model,the ambiguity parameters must be estimated and fixed in order to obtain the decoupled phase clock.For a single system,relying on an external computing software package can basically solve the problem of low clock calculation efficiency at the server end.However,when the decoupled clock of multiple systems is estimated at the same time,the ambiguity parameters increase sharply,resulting in the sharp increase of the dimension of the normal equation,which seriously reduces the computational efficiency.Therefore,this paper proposes an improved multi-GNSS decoupled clock estimation model.The model mainly adopts two strategies,one is to calculate the decoupled clock products of each system respectively,and the other is to solve the MW combined virtual observation equation alone to estimate the wide-lane ambiguity bias,so as to improve the calculation efficiency.4.Design a software platform that takes into account multi-GNSS satellite clock estimation and PPP,and open source server and user algorithms of GPS data processing.Based on the research of satellite clock estimation and PPP in this paper,a software platform Cube supporting multi-GNSS(BDS/GPS/Galileo)is studied.Cube data processing software is a secondary development based on RTKLIB,which is written in C language,easy to transplant,modular structure,easy to expand and integrate with new algorithms.The software provides important support for subsequent research work.The function of data processing software can be divided into two parts:satellite clock estimation at the server end and PPP at the user end.At the server end,it supports satellite clock estimation based on the traditional ionosphere-free model,satellite clock estimation extracting satellite code bias and decoupled clock estimation based on the decoupled clock model.At the user end,the supported positioning models include the traditional ionosphere-free model,the integer-recovered clock model and the decoupled clock model.Due to the lack of clock estimation and positioning software based on the decoupled model,the part of GPS data processing in the software is open-source.