| On July 31,2020,China officially announced the completion and opening of the Bei Dou-3 Navigation Satellite System(BDS-3),marking the successful completion of the three-step development strategy of Bei Dou.For BDS,the combination of BDS-2 and BDS-3(BDS-2/3)can make full use of satellite resources and enhance the location service capability.Precise point positioning(PPP)can obtain high-precision 3D coordinate by using single station,with the advantages of simple operation,low cost and no regional restrictions.At present,the influence of receiver hardware factors on the consistency of BDS-2 and BDS-3 is not clear when using overlapping frequencies.Thus,some problems still need to be solved for the BDS-2/3 PPP: whether an additional inter-system bias(ISB)parameter should be introduced between BDS-2 and BDS-3;whether BDS-2 and BDS-3 can be treated as one system to estimate the fractional cycle bias(FCB)for the BDS-2/3 PPP ambiguity resolution.In view of the above problems,this thesis makes a study of BDS-2/3 PPP from the following aspects: with the observations collected from multi-GNSS experiment(MGEX)tracking stations,the characteristics of ISB between BDS-2 and BDS-3 with overlapping frequencies and its influence on BDS-2/3 PPP are analyzed in detail,and then it is confirmed whether an additional ISB parameter should be introduced;The FCB difference between BDS-2 and BDS-3 using overlapping frequencies at receiver side and its influence on the estimation of BDS-2/3 FCB are both detailly studied to confirm whether BDS-2 and BDS-3 can be treated as one system for FCB estimation;a method suitable for BDS-2 and BDS-3 FCB estimation is proposed to provide reliable FCB products for combined BDS-2 and BDS-3 PPP ambiguity resolution.Main research contents and contributions of this thesis are as follows:(1)The availability of current combined BDS-2 and BDS-3 constellation is detailly analyzed in global,and the performance of BDS-2/3 PPP float solution in different geographical regions is evaluated.The results show that the number of visible satellites and the position dilution of precision(PDOP)value of the BDS-2/3 constellation in the Asia-Pacific region are better than those of the GPS constellation,and are equivalent to those of the GPS constellation in other regions.In the Asia-Pacific region,the average convergence time of static and kinematic BDS-2/3 PPP in east,north and up components is(30.4,15.6,23.1)minutes,(27.1,11.4,17.3)minutes respectively;compared with BDS-2,the improvement percentage is(42.4%,57.4%,48.2%),(47.7%,59.3%,52.2%)respectively;However,in the American region,the performance of BDS-2/3 PPP is obviously weaker than that of other regions,especially for the kinematic PPP.This is mainly due to the limited BDS-3 satellites can be supported by the current receiver types,resulting in a small number of visible satellites and bad geometry in this region.(2)It is confirmed that there is an ISB parameter between BDS-2 and BDS-3 when using overlapping frequency,and the characteristics,optimal estimation strategy and existing reasons of the ISB parameter are studied in detail.Results demonstrate that,the ISB between BDS-2 and BDS-3 is very stable during a day and can be regarded as a constant to be estimated,which is caused by the facts that: the current IGS precise clock datums for BDS-2 and BDS-3 are different;the receiver hardware leads to the inconsistent pseudo-range hardware delays for BDS-2 and BDS-3.(3)The influence of ISB on BDS-2/3 PPP is quantitatively evaluated,and the influence mechanism is also explored.When an ISB parameters is introduced between BDS-2 and BDS-3,the performance of BDS-2/3 PPP could be improved,especially for kinematic PPP;the ISB parameter could be completely absorbed by the pseudo-range observation residuals of BDS-2 and BDS-3 if ignored,but it would not affect the accuracy of the phase observation.(4)By the comparative experiments,it is verified that the receiver hardware could lead to the inconsistent FCBs for BDS-2 and BDS-3 when using overlapping frequency,and the influence of the FCB difference between BDS-2 and BDS-3 on FCB estimation is analyzed in detail;a FCB estimation method considering the FCB difference between BDS-2 and BDS-3 is proposed,and the effectiveness and potential advantages of this method are evaluated by experiments.The results show that BDS-2 and BDS-3 cannot be treated as one system to estimate FCB;the FCB estimation method proposed in this thesis can take into account the influence of the FCB difference between BDS-2 and BDS-3 to realize the combined BDS-2 and BDS-3 FCB estimation;its products can be applied to achieve PPP ambiguity resolution between BDS-2 and BDS-3 satellites,which could improve the performance of BDS-2/3 PPP in the case of fewer visible satellites.Further,the quality of FCB product obtained in this paper is tested by using the observation data of five stations from day of year 100 to 120 in 2020.Results demonstrate that,compared with the BDS-2/3 PPP float solutions,the convergence time of BDS-2/3 static PPP fixed solution could be shortened by 38.9%,15.5% and 19.0% for the east,north and up components,and the coordinate accuracy of 1-hour solution is improved from(2.6,1.1,3.4)cm to(1.2,0.6,2.5)cm;For BDS-2/3 kinematic PPP,the convergence time in east,north and up components can also be shortened by 37.8%,2.8% and 10.3%.(5)The relationship between the FCB difference and the receiver types is detailly analyzed for BDS-2 and BDS-3 when using overlapping frequencies.The results show that the FCB difference between BDS-2 and BDS-3 is closely related to the receiver type.The FCB difference values of the stations with the same receiver type are similar with each other,which can be accurately calibrated;but for the stations with different receiver types,the FCB difference values of them may be quite different.(6)The optimal estimation strategies of ISB parameters between BDS-3 and GPS/GALILEO are studied,and the improvements of PPP fixed solutions for GPS/GALILEO,GPS+GALILEO with the combination of BDS-2/3 observations are detailly evaluated in terms of the PPP convergence time,time to first fix,coordinate accuracy and the fixing rate.Results demonstrate that,adding BDS-2/3 observations could obviously improve the PPP performance of single-or dual-system.Taking the GPS+GALILEO dual-system as an example,after the introduction of BDS-2/3 observations,the convergence time could be shortened from about 12 minutes,11 minutes to about 9min and 6 minutes for the static and kinematic PPP fixed solutions respectively,with improvements of more than 20% and 40%;similarly,the time to first fix could be shortened by 23.9% and 35.3% respectively;the RMS values of coordinate deviation for 10-minutes static PPP fixed solution could be reduced from(4.5,1.7,5.6)cm to(3.0,1.1,4.6)cm in the east,north and up components. |
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