Quality Assessment Method And Comparative Analysis Of BDS-3/galileo Observations

BDS regional system(BDS-2)has been officially operational by the end of 2012 and began to provide positioning,navigation and timing services to China and its surrounding areas.Currently,BDS is gradually expanding from regional to the whole world.Since 2015,five new-generation BDS(BDS-3)experimental satellites have been successfully launched.In 2016,on-orbit verification mainly focusing on the new navigation signal,spaceborne hydrogen atomic clock and inter-satellite links of BDS-3 experimental satellites has been basically accomplished,and the state of the BDS-3 working satellite has been basically cured.In the summer of 2017,BDS-3 working satellites will be launched successively.The European Galileo has also been officially operational on on December 15,2016,and currently the number of satellites in orbit has reached 18.According to the plan,BDS and Galileo will complete construction of the whole constellation in 2020.The positioning and navigation performance in terms of accuracy,reliability,and availability largely depends on the observation quality of the transmitted satellite navigation signals.This makes observational quality assessment a critical issue for global navigation satellite systems.Currently,although many researchers have carried out research on observational quality assessment of satellite navigation systems such as GPS,Galileo and BDS-2,there are still problems that need to be solved.On the one hand,research on observational quality assessment of navigation signals transmitted by BDS-3 experimental satellites is still not mature.On the other hand,observational quality assessment for Galileo is mostly based on GIOVE(Galileo In-Orbit Validation Element)and IOV(In-Orbit-Validation)satellites,and the data quality assessment of FOC(Full Operational Capability)satellites is basically not carried out.Consequently,the previous researches can not reflect the current Galileo data quality.Take this into account;this thesis aims to carry out research on the evaluation index and approach of GNSS observational quality assessment.A comprehensive and detailed assessment and analysis for BDS-3 experimental and Galileo satellites will also be carried out.This research will provide reference for the construction and application of BDS-3 and Galileo.The main work and results of this thesis are as follows:(1)The current development of BDS and Galileo is summaried.The characteristics of navigation signal transmitted by BDS-3 experimental and Galileo satellites are demonstrated with focus in terms of frequency and modulation mode.Finally,the time system and coordinate system of BDS and Galileo are introduced.(2)In-depth study of GNSS observational quality evaluation indicators and methods is carried out and a comprehensive and reliable evaluation and analysis program for observational quality assessment of GNSS navigation signals is formed.And the corresponding GNSS observational quality analysis and evaluation software,which can provide observational quality assessment in terms of SNR(signal-to-noise ratio),pseudorange and phase multipath,double-differenced residuals and single-differenced residuals,precision of zero-differenced phase and code observations,and performance assessment of SPP(single point pisitioning)and relative positioning,is developed.The research will provide reference for GNSS observational quality analysis and evaluation.(3)The characteristics of BDS-3 experimental and Galileo satellites are evaluated by using raw GNSS data in terms of SNR,pseudorange multipath,and phase multipath.It's shown that with regard to B1I,B3I,B1C,B2a,B2b signals that trsmaitted by BDS-3 experimental satellites,B3I has the highest SNR whereas B1C has the lowest.And the difference between B1I,B2a,and B2b is insignificant.For Galileo,E5 has the highest SNR,and followed by E1,E5a and E5b signals.The difference between E1,E5a and E5b signals is also insignificant.For the overlapping frequencies between BDS-3 and Galileo,the SNR of E1,E5a,and E5b signals are higher than their corresponding B1C,B2a,and B2b signals.With regard to pseudorange multipath,the B1C signal of BDS-3 experimental satellites has the largest pseudorange multipath and B1I,B2a,B2b and B3I is much smaller.For Galileo,the pseudorange multipath is from large to small,respectively,E1,E5b,E5a,and E5.For the overlapping frequencies between BDS-3 and Galileo,the pseudorange multipath of B1C,B2a,and B2b signals is smaller than their corresponding E1,E5a,and E5b signals.For the overlapping frequencies between BDS-3and BDS-2,BDS-3 have pseudorange multipath smaller than BDS-2.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 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.(4)The precision of BDS-3 and Galileo phase and pseudorange observations is evaluated and analyzed using single-differenced(SD)and double-differenced(DD)residuals.And performance assessment of single point positioning and relative positioning is carried out as well.It's shown that for BDS-3 experimental satellites,B1C has the largest DD residuals and followed by B1I.The difference between B3I,B2a and B2b signals are insignificant.For the overlapping frequencies between BDS-3 and Galileo,the DD residuals of E1,E5a,and E5b signals are smaller than their corresponding B1C,B2a and B2b signals.Overall,the SD residuals of Galileo are smaller than BDS-2 and GPS,and the precision of Galileo is better than BDS-2 and GPS.For the overlapping frequencies between BDS-3 and BDS-2,BDS-3 have DD residuals smaller than BDS-2.With regard to single point positioning and relative positioning,the addition of BDS-3 experimental satellites can improve the positioning accuracy,with respect to BDS-2 only or BDS-2/Galileo positioning.The positioning accuracy of Galileo is lower than BDS-2 in terms of single point positioning and relative positioning for short baselines.This is mainly due to the limited number of observed satellites and poor geometry for Galileo.When Galileo and BDS-2 are combined,the positioning accuracy can be significantly improved with respect to Galileo-only positioning.