Reseach On The Unified Modeling For Multi-GNSS Precise Point Postioning

GNSS is now becoming the main approach to obtain Geo-information,the demand for real-time and precise applications has been increasing dramatically.This theis aimed to establish an unified model for Multi-GNSS precise positioning,including PPP,PPP-AR and regional augmented PPP-AR,so that there would be no concern about the models used at user side.With this goal,several research jobs were carried out:(1)The difference and relation between different existing precise point positioning modelsThe existing PPP models,including the standard PPP model,UofC model and uncombined PPP model,were summarized and analyzed.Through theoretical argumentation,the following conclusion can be drawn: all three methods are equivalent when just using observations to estimate unkowns,regardless of the dynamic stochastic models of ambiguity and ionosphere;If the dynamic stochastic models of ambiguity and ionosphere are taken into account properly,the uncombined PPP convergences fastest with the most applicable information,then comes the UofC and the standard PPP is slowest in this case.Based on this conclusion,the uncombined PPP was slected out as the basis of unified model.Additionally,this paper also extended the dual-frequency uncombined PPP model for single constellation to Multi-GNSS and triple frequency uncombined PPP model.(2)New method for precise point positioning with ambiguity resolutionBased on the comparison and analysis of existing dual-frequency PPP-AR methods,a new method of dual-frequency PPP-AR was proposed.In our new method,unstable receiver widelane(WL)UPBs are removed by using an inter-satellite differencing,all un-diferenced WL ambiguities and un-diferenced satellites-side WL UPBs can be estimated by introducing a WL ambiguity datum for each station and a satellite-side WL UPB datum.In the new method,ionosphere-free(IF)UPBs are directly estimated,instead of estimating the integer clocks(‘Integer Clock' model)or phase clock(‘Phase Clock' model).In the estimation process,a satellite-side UPB or the average of all satellite-side UPBs is selected as the IF UPB datum,instead of choosing a receiver-side UPB in the ‘Integer Clock' model or the ‘Phase Clock' model.What is more,the satellite-side UPBs of L1& L2 observations,not WL UPB and IF UPB,are broadcasted in the new method.The triple-frequency PPP-AR,including UPB estimation solution in the server and positioning model in the user,was also discussed.(3)Regional augmented PPP-AR method and its implementionAn regional augmented PPP-AR method based on state-space corrections was developed,including the calculation and broadcasting of regional corrections,correction interpolation for the user,and positioning model in the user.In detail,zenith tropospheric wet delays and slant ionospheric delays(having absorbed receiver UPB)are calculated with precision ephemeris & clocks,UPBs products and observations(collected from regional CORS network);A technique,which is similar to MAC,was proposed to broadcast the un-diferenced corrections of zenith tropospheric dry delay,zenith tropospheric wet delay and slant ionospheric delay;The modified FKP model is used to interpolate correction for users,inter-satellite differencing is carried out before slant ionospheric correction interpolation in order to eliminate the influence of receiver UPB,what is more,height-related correction is recommended to apply to both zenith tropospheric dry delay,zenith tropospheric wet delay before interpolating;The users' positioning is implemented using the uncombined PPP-AR model,as the interpolated intersatellite single-difference slant ionospheric delay and the interpolated zenith tropospheric wet delay are treated as pseudo-observations.(4)Unified precise positioning and the corresponding data processing schemeBased on the above research on uncombined PPP,uncombined PPP-AR and regional augmented PPP-AR,a unified precise point positioning model was proposed.And the data preprocess was aslo presented,consisting of the preprocess of ephemeris,recognition and deletion of the wrong pseudo-range,the detection and mitigation of receiver clock jumps.An clock prediction method was proposed during IGS real-time stream outage.At the same time,new methods for detection and repairing of the cycle slips for dual and triple frequency observations were given.Taking into account that the strength of positioning model in the userside is not strong enough,it is difficult to fix all ambiguities and pass validation test in a short period of time,an ambiguity resolution algorithm,where all ambiguities are divided into different leve and then partial ambiguitiy resolution is used for ambiguities in the same level,was proposed.Based on the unified precise positioning model,the corresponding positioning scheme was constructed,including both server and user parts.The server part includes acquiring or computing precise ephemeris & satellite clock corrections(first-class product),UPB parameters(second-class product)and regional augmentation information(third-class product).Corresponding to first-,second-and third-class products,the user can carry out PPP,PPP-AR and regional augmented PPP-AR,based on the unified precise positioning model.In addition,a software system was also developed.Based on the software and real observations,the performance of PPP,PPP-AR and regional augmented PPP-AR were tested and analyzed,the results demonstrated the validity of the model,the scheme and the software.