Model And Approach Of Multi-GNSS Tightly Combined Precise Relative Positioning

With the development and construction of global(GPS,GLONASS,BDS,and Galileo)and regional(QZSS,IRNSS etc.)satellite navigation systems,satellite navigation and positioning technology is entering the multi-GNSS era.It is expected that combining signals from different GNSS systems will increase the number of available satellites,provide stronger geometry,and thus improve the positioning and navigation accuracy,availability,and reliability.However,significant differences exsit beteween different GNSS sytems in terms of the constellation configuration,signal design,and time and coordinate reference systems,which also brings challenges to multi-GNSS integration.In this context,recently it has become a hotspot in GNSS positioning and navigation areas to make full use of multi-GNSS multi-frequency signals to further enhance the positioning performance and reliability,and truly realize the compatibility,interoperability,and interchangeability between GNSS systems.In precise relative positioning applications,an effective approach to improve the interoperability and interchangeability of GNSS systems is the tightly combined or inter-system double-differencing of observations from different constellations.In that case,a single-pivot satellite is chosen for all the observations from different constellations.Consequently,both inter-system and intra-system double-differenced(DD)observations can be created.Compared to the loosely combined model or classical DD model(a single pivot satellite is chosen for each system and only intra-system double-differenced(DD)observations are created),the tightly combined model can reduce the number of reference satellites,increase additional DD observations,improve geometry,and further improve the positioning accuracy,availability,and reliability.In the past few years,although some progresses have been achived in the field of mufti-GNSS tightly combined relative positioning,but there are still a series of problems that need to be resolved.For example,a comprehensive and detailed analysis of the between-receiver differential inter-system bias(DISB)is still needed in terms of the generation mechanism,the time-varying characteristics,and the influences,the.model of the tightly combined relative positioning for non-overlapping frequencies is still not very mature,and lack of research on China's BDS,especially BDS global system(BDS-3).This paper focuses on the model,approach,critical issues and performance analysis of tightly combined multi-GNSS precise relative positioning.The research will provide support for interoperability of GNSSThe main work and contributions of this dissertation are as follows:(1)For the first time this dissertation present an initial characterization and performance assessment for the new-generation BDS-3 experimental satellites and their signals.The characteristics of the B1C,B2a,and B2b signals are evaluated in terms of observed carrier-to-noise density ratio,pseudorange multipath and noise,triple-frequency carrier phase ionosphere-free and geometry-free combination,and double-differenced carrier phase and code residuals.The results demonstrate that the observational quality of the new-generation BDS-3 B1C,B2a,and B2b signals is comparable to that of GPS and Galileo.However,the analysis of code multipath series shows that the elevation-dependent code biases,which have been previously identified to exist in the code observations of the BDS regional(BDS-2)satellites,seem to be not obvious for B1C,B2a,and B2b signals of BDS-3 satellites.With regard to the triple-frequency carrier phase ionosphere-free and geometry-free combination,it is found that different from the BDS-2 satellites,no apparent bias variations(also called IFCB)could be observed in all the new-generation BDS-3 experimental satellites.(2)Mathematical model of tightly combined multi-GNSS precise relative positioning between overlapping frequencies is first derived from the basic observation equations.Taken into account that the ionospheric errors cannot be cancelled and ignored for long baselines,this dissertation analyzed the correlation and reparameterization approach between the DISB parameters and the DD ionospheric parameters,and introduced the mathematical model of tightly combined multi-GNSS precise relative positioning between overlapping frequencies for long baselines.(3)The post estimation of DISBs between overlapping frequencies is first described and the generation mechanism and the influences of DISBs are studied.Then a comprehensive and detailed analysis of the time-varying characteristics of the phase and code DISBs between overlapping frequencies(i.e.,GPS L1/L5-Galileo El/E5a,GPS L1/L2/L5-QZSS L1/L2/L5,and BDS B2-Galileo E5b)are made using raw GNSS data from Feb.1,2015 to May 30,2016.It is shown that for baselines with identical receivers(including the same firmware version),the DISB is close to zero;for baselines with different receiver types,the DISB is non-zero but with long stability and remains unchanged after receiver restart.Meanwhile,the update of receiver firmware,the state change of anti-multipath filter,and the type of the observations also have an impact on the estimation of DISBs.Finally,taken into account the time-varying characteristics of DISBs,the approach of real-time estimation and modelling for DISBs between overlapping frequencies is introduced,and the convergence time of DISBs is evaluated.It's shown that the DISBs can converge after only a few epochs under environments with good geometry and where DD ambiguities can be quickly and reliably fixed.This could meet the timeliness requirements in real-time tight combined relative positioning applications.(4)The performance of the tightly combined relative positioning was evaluated using GPS L1/L5,Galileo E1/E5a,and QZSS L1/L5 observations for short baselines.Meanwhile,taken China's BDS regional system(BDS-2)into account,for the first time this dissertation evaluated the performance of tightly combined relative positioning using BDS B2 and Galileo E5b observations and verified the feasibility of tightly combined relative positioning using observations from overlapping frequencies of BDS and Galileo.Results of short baselines consisted with identical and different types of receivers demonstrated that identic,al results are abtained for the loosely combined model and tightly combined model with unknown DISBs.On the other hand,the tightly combined model with corrected DISBs can provide us with a much higher empirical ambiguity resolution success rate and lower empirical failure rate with respect to the loosely combined model.The improvement can be especially significant(10%-25%)under environments where the navigation signals are serious blocked or the observed satellite for each system is limited.(5)For the first time this dissertation investigated the size and stability of phase and code DISBs of BDS-3 B1C/B2a/B2b signals with respect to GPS L1/L5 and Galileo E1/E5a/E5b signals using raw data.It is demonstrated that the DISBs are very stable in the time domain,which verified the feasibility of tightly combined GPS/Galileo/BDS-3 relative positioning.Taken this into account,for the first dissertation evaluated the performance of tightly combined GPS/Galileo/BDS-3 relative positioning.It's shown that the tightly combined model can provide us with a much higher empirical ambiguity resolution success rate with respect to the loosely combined model.The improvement can be especially significant(30%-50%)under environments where the observed satellite for each system is limited and only single frequency observations are available.(6)Study of the tightly combined relative positioning using observations from overlapping and non-overlapping frequencyies was carried out.The mathematical model of tightly combined multi-GNSS precise relative positioning between non-overlapping frequencies was first introduced and the impact of different frequencies on the DISB and ambiguity resolution was discussed.Then the time-varying characteristics of DISBs between GPS L1/L2-BDS B1/B2 are analyzed using raw GPS/BDS data.It is found that the phase DISB are very stable in a continuous observation session,but will change after the receiver is restarted.And taken this into account,strategies of real-time estimation and modeling for DISBs between non-overlapping frequencies is discussed and the tighly combined GPS/BDS relative positioning between non-overlapping frequencies is realized.Results show the tightly combined model can provide us with a much higher empirical ambiguity resolution success rate with respect to the loosely combined model.The improvement can be especially significant(30%-60%)under environments where the observed satellite is less than 10 and only single frequency observations are available.Finally,the performance of GPS L1/L2/L5 and Galileo E1/E5b/E5a relative positioning was also evaluated based on the mixed tightly combined model which can simultaneously process inter-system DD observations between overlapping and non-overlapping frequencies.It's shown that the mixed tightly combined model can further improve empirical ambiguity resolution success rate with respect to tightly combined model between overlapping frequencies.