Study On The Multi-GNSS Uncombined Precise Point Positioning:the Construction Of Low-cost Single-frequency Model And The Refined Methodology Of Multi-frequency Model

Precise Point Positioning(PPP)is a kind of global absolute positioning technology with the support of external precise satellite orbit and clock products and after considering various errors based on a stand-alone receiver,which is proposed in the 90s of 20 century.PPP has the ability to provide absolute position information of any positions in the world area for the users compared with the customary differenced positioning technology such as relative positioning.Nowadays,the Global Navigation Satellite System(GNSS)such as American GPS,Chinese BDS,European Galileo and Russian GLONASS are developing rapidly.They provide abundant original observations for the GNSS research and applications.Benefit from the continuous improvement of the accuracy of satellite orbit and clock products,PPP technique has become one of the main methods in GNSS applications,which is widely applied in Positioning,Navigation,Timing(PNT),space atmosphere sensing and some other relevant geoscience applications.Ionosphere-free(IF)model is the main PPP model which is widely used in early years.However,with the development of uncombined PPP(UPPP)model,the IF PPP model is gradually replaced by UPPP model,because the UPPP model retains more original information and is more suitable for multi-frequency multi-GNSS data processing.But there are still many thorny problems need to be solved although more and more researchers have paid a lot of attentions on UPPP model.On the one hand,the existing UPPP models are mainly based on geodetic-grade receivers while the studies on uncombined single-frequency PPP model based on low-cost single-frequency receivers are limited.On the other hand,many scholars have found that the receiver code biases(RCBs)may be remarkably time-varying in some cases while the previous studies implicitly assumed that the RCBs stay constant over time in formulating the functional model of PPP,thus the current multi-GNSS UPPP model needs to be refined.In this regard,in order to enrich the UPPP models and expand the applications of UPPP technique,this paper focuses on solving the deficiency of the current UPPP model,such as the multi-GNSS uncombined single-frequency model and applications as well as the refined multi-GNSS UPPP model considering the time-varying RCBs.The main contributions and innovations of this paper are as follows:1.A multi-GNSS uncombined single-frequency PPP model based on low-cost single-frequency receivers is proposed,and is applied to lane-level navigation for massive users as well as space atmosphere sensing studies.The math model of multi-GNSS uncombined single-frequency PPP based on low-cost single-frequency receivers includes two main parts:functional model and stochastic model.First,the full rank multi-GNSS uncombined single-frequency PPP functional model is deduced by analyzing the type and number of rank deficiencies among the parameters in raw observation equations.Then this paper summarized and proposed the stochastic model of multi-GNSS single-frequency observations and a set of single-frequency GNSS data processing strategy after analyzing the quality and characteristics of the observations collected from real low-cost single-frequency receivers.Finally,the uncombined single-frequency PPP model proposed in this paper is evaluated by applying it to lane-level navigation and space atmosphere sensing(ionosphere and water vapor).The uncombined single-frequency PPP model enriches the current UPPP model in single-frequency area and greatly expands the applications of the UPPP technique.2.Considering the time-varying RCBs,a modified multi-GNSS multi-frequency PPP(MPPP)model is proposed,and is applied to precise ionosphere retrieval as well as precise timing.The characteristics of RCB variation and factors causing RCB variation are also analyzed.Based on the customary UPPP functional model,this paper proposed a modified PPP(MPPP)model by modifying the time-constant RCBs as time-varying parameters.The rank deficiencies among parameters like receiver clock,ionospheric delay and ambiguities are solved by S-basis theory.Thus the adverse effect to the observation residuals and the estimable parameters like receiver clock,ionospheric delay and ambiguities caused by the time-varying RCBs is solved in the parameter estimation substantially.The MPPP model ensures that the estimated parameters are accurate and optimal,and provides a new solution for precise ionospheric delay retrieval and timing.Furthermore,the variation characteristics and factors causing RCBs variation are studied by the extracted RCB drift series.The results show that the variation of RCBs is closely related to ambient temperature.3.Based on the realtime data stream,a set of realtime multi-GNSS uncombined PPP algorithm is implemented,and is applied to the lane-level navigation as well as the on-line autonomous calibration of RCBs.Generally,the vehicle navigation and disaster monitoring based on GNSS require high timeliness,thus realtime PPP is more and more significant in the modern GNSS applications.With the continues improvements of realtime satellite orbit and clock estimation methods and strategies,the accuracy and timeliness of the realtime products can completely satisfy the normal realtime positioning and monitoring needs for general users.First,this paper systematically studies the realtime product coding,broadcasting according to network and decoding process based on RTCM format while also following the NTRTP protocol.Then the recovering method of precise satellite orbit and clock SSR corrections is introduced,and the precise SSR corrections published by French CNES are evaluated and compared.Finally,a realtime multi-GNSS uncombined single-frequency PPP solution for lane-level navigation is developed,and another realtime multi-GNSS multi-frequency PPP is realized for on-line autonomous calibration of RCBs and high-precision navigation and positioning.The performance of the proposed realtime PPP method above is validated by realtime data streams in this paper.4.Designing and implementing a set of multi-GNSS uncombined PPP and application software platform and open-source a multi-GNSS Single-frequency Uncombined PREcise point positioning Multi-parameter Estimation(SUPREME)software.Multi-GNSS UPPP model has gradually become one of the main techniques in GNSS applications benefiting by its own advantages and potential,and has been widely used in space atmosphere sensing,especially ionospheric delay retrieval.Based on the multi-GNSS UPPP model proposed above,this paper designs and implements a Multi-GNSS Solutions(MGS)software platform,which can be used for Positioning,Navigation,Timing(PNT)and atmosphere sensing applications.MGS has powerful GNSS data processing capabilities,including supporting multiple constellation(BDS/GPS/Galileo/GLONASS),arbitrary frequency(single frequency,dual frequency and triple frequency),and multi-temporal(realtime and post)processing mode.The new models and algorithms are easily added to MGS because of its own abundant function library and modular structures.MGS is a powerful software platform for the future research work.Furthermore,the author opens source a Single-frequency PREcise point positioning Multi-parameter Estimation software(SUPREME),it provides a good software platform for low-cost GNSS data processing and applications.