Study On GNSS Precise Point Positioning And Ambiguity Resolution

Precise point positioning(PPP)technology has been rapidly developed in surveying and mapping industry with advantages of convenient operation,high precision and low cost.After the past decades studying and applying,abundant achievements have improved the basic theoretical issues of PPP technology.However,the drawback of long convergence time of parameters estimation,relatively low positioning accuracy and reliability with a single system limit the ability to meet the requirements of high positioning accuracy and high reliability in wide applications.With the multi-GNSS observables and ambiguity resolution(AR),the high precise positioning with high reliability is achieved in a short time,significantly.The estimation of fractional cycle bias(FCB)which destroys the integer nature of PPP undifferenced ambiguities is the critical prerequisite of achieving PPP ambiguity resolution.The observation information can be multiplied and the redundancy of parameter estimation can be greatly increased with multiconstellation global navigation satellite system(GNSS)combination and multi-frequency signals.Hence,the multi-frequency and multi-GNSS PPP with ambiguity resolution,which can achieve higher positioning accuracy and shorter convergence time,is a critical technology to be studied.Under the improving of PPP AR method,the ionosphere modeling and differential code bias(DCB)estimation should be further researched.This thesis is intended to:(1)Estimation of fractional cycle bias(FCB);(2)The study on the multifrequency and multi-GNSS PPP with ambiguity resolution(AR);(3)the study on the modified uncombined PPP method considering the dramatic variation of receiver code bias;(4)the ionosphere modeling and DCB estimation using the ionospheric delay measurement from the uncombined PPP AR.The main work and contributions are as follows:(1)The equivalent of FCB products in different PPP methods is confirmed from the conversion formulas driving and results comparison.Firstly,the theoretical equivalence of FCB estimation between different PPP methods is presented with the analysis of difference among ionospheric-free PPP model,unconstrained and uncombined PPP model and ionosphere constrained uncombined PPP model.Then,the different methods to recover the integer nature of float ambiguities are analyzed with three different PPP models,and the corresponding methods of FCB estimation are also introduced.One month data is collected from IGS network to estimate the FCBs using the IF-PPP,UU-PPP and IC-PPP methods,respectively.Finally,the accuracies of FCBs in three different PPP models are evaluated by the residuals of ambiguities and single-differenced FCBs.The equivalence of FCB estimations is verified with the theoretical and experimental comparison.(2)The comparable positioning performance between IF-PPP,UU-PPP and IC-PPP models is verified in theorization and experiments.With the augment of high accuracy of ionosphere corrections,the rapid PPP AR is processed in a short time achieving high precise positioning.The performances of ambiguity-float and ambiguity-fixed solutions with IF-PPP,UU-PPP and IC-PPP methods in terms of the positioning accuracy,the convergence time and the ambiguity success fixing rate are evaluated.The FCB products estimated from three different PPP models are used to achieve the PPP AR using the precise satellite orbit and clock products with IGS stations in static model.The comparison of ambiguity-float and ambiguity-fixed solutions confirms that the positioning performance of PPP AR between IF-PPP,UU-PPP and IC-PPP models are quite close to each other.(3)The triple-frequency uncombined PPP model is established with raw observables and the corresponding FCBs are estimated.The uncombined PPP model is flexible and efficient to deal with the multi-frequency observations.Extending the dual-frequency PPP model to triple-frequency PPP model,considering the inter-frequency clock bias(IFCB),the tripe-frequency FCB estimation model is introduced to create the triple-frequency FCB products with the dual-frequency precise satellite orbit and clock products.The accuracy performance of FCB estimation and triple-frequency PPP AR with BDS and Galileo observations are evaluated using the MGEX stations.(4)The multi-GNSS PPP model is established with GPS,BDS and Galileo observations considering the characters of the inter-system bias(ISB)between different GNSSs,and its performance is analyzed.Considering the ISB parameters caused by the time and coordinate differences between different GNSSs,the characters and stability of ISB parameters are discussed to improve their stochastic model in multi-GNSS PPP method.Firstly,the ISB parameters are estimated with white noise process to analyze the their stability over a short time and a relatively long time,respectively.The differences between different receiver types are also evaluated.Then,the performance of FCB estimation and PPP AR with GPS/BDS/Galileo observations are confirmed.(5)The modified uncombined PPP(M-UPPP)method is proposed to mitigate the effects of receiver code bias large variation.With the many experiences,it has been confirmed that the dramatic variations shown in receiver code biases strongly degrade the performance of positioning accuracy and PPP AR.Considering this variation,we proposed a new PPP model estimating the phase receiver clock offsets and a code bias in equations.The significant improvements are shown in positioning accuracy and shortening the convergence time.(6)The improved method for estimation of ionospheric delays and satellite DCBs is proposed using the uncombined PPP method with ambiguity resolution.The uncombined PPP method can directly estimate the ionospheric delays with high precision.After fixing the ambiguity,the estimation accuracy of ionospheric delays is also further improved as well for other parameters.The global ionospheric map(GIM)products and satellite DCBs are created with the ionospheric delay measurements from ambiguity-fixed solutions.The results show that accuracy of GIM products and satellite DCBs are significantly improved with ambiguity resolution.