| Global Navigation Satellite Systems(GNSS)include the American GPS,Russian GLONASS,Chinese Bei Dou Navigation Satellite System(BDS)and European Galileo systems.With the modernization of GPS and GLONASS plus the rapid developments of BDS and Galileo,the satellites of new generation can transmit at least triplefrequency signals.Multi-frequency and multi-constellation GNSS bring both opportunities and challenges for GNSS precise data processing.Research show that the precise ionosphere products and reliable GNSS orbits help to improve the performance of ambiguity resolution and shorten the convergence time for multi-frequency,multiconstellation GNSS positioning.Therefore,aim to serve the multi-frequency,multiconstellation GNSS navigation and positioning,this article focuses on improving the precision of multi-frequency,multi-constellation GNSS ionosphere modelling and orbit determination.Currently,the global ionosphere modelling is mainly based on GPS and GLONASS dual-frequency observations.The potential and advantages of multifrequency and multi-constellation GNSS in ionosphere modelling have not been fully explored.In addition,most of the GNSS stations are located on the land,which means the ionosphere derived by GNSS over the oceans has a relatively lower precision.Therefore,it is necessary to study and realize the ionosphere modelling based on multisource fusion.Meanwhile,the ambiguity resolution can obviously improve the precision of multi-constellation GNSS orbit determination,while there is no practical solution for the multi-constellation GNSS ambiguity resolution.The methods of GPS ambiguity resolution have been deeply studied,comprehensively analyzed and widely used.However,contaminated by the Inter-Frequency Bias(IFB),it is quite difficult to achieve GLONASS ambiguity resolution especially for long baselines,and the algorithm of BDS and Galileo ambiguity resolution is under study.Therefore,studying the effective methods of ambiguity resolution for GPS,GLONASS,BDS and Galileo help to further improve the precision of multi-constellation GNSS positioning and orbit determination.Viewing the shortcomings of the current GNSS ionosphere modeling and precise orbit determination,this study mainly focuses the methods of multi-frequency and multi-constellation GNSS ionosphere modelling and orbit determination.We make the most of advantages and potentials of multi-GNSS to improve the precision of ionosphere and orbit products.The major research work and contributions of this study are as follows:(1)The methods of multi-frequency,multi-constellation GNSS ionosphere modelling have been deeply and systematically studied.To maximum the advantage of multi-frequency and multi-constellation GNSS,we make full use of multi-frequency and multi-constellation GNSS observations to model the global ionosphere.The increasement of satellites and observations significantly increases the number of ionospheric pierce points,enhancing the strength of the ionosphere estimation and improving the precision of ionosphere model.Though comparing the ionospheric results with those of IGS,CODE,ESA,JPL and UPC,the averaged RMS are 1.5,1,1,1,8,2,6 and 1.9 TECU,respectively.Compared with the results of ionosonde,the RMS of the estimated ionospheric results in 2017 and 2018 are 2.5 and 2.4 TECU respectively,which improves 0.1~0.3 TECU with respect to the RMS of IGSG.It proves that the new method of ionosphere estimation can improve the precision of the ionosphere.Especially during the period of ionosphere scintillation,the ionosphere derived from multi-frequency and multiconstellation GNSS has a better precision and the advantages of the new method are more obvious.In addition,we realized the simultaneous estimation of the ionosphere and multi-frequency,multi-constellation GNSS Differential Code Biases(DCB),the results shows that the DCB for GNSS satellites in 2017 is better than 0.5 ns,and compared with the DCB of CAS,the mean bias is lower than 0.5 ns,the standard deviation is no more than 0.3 ns.(2)Based on multi-source data fusion from multi-GNSS and satellite altimetry,we construct a global ionosphere model with a high precision over the land and oceans.This model makes full use of the advantages of the multifrequency,multi-constellation GNSS,introduce the satellite altimetry data over the oceans,overcome the deficiency of GNSS ionosphere sensing,realized global ionosphere modelling based on multi-source data fusion and improved the ionosphere precision on the oceans.Compared with the weighted mean values from different ionospheric products,the new ionosphere based on multi-source data fusion has the highest precision,its RMS in 2014 and 2018 are 4.19 and 1.36 TECU,which is just half of the RMS of IGSG and CODG.In 2014,the percentages of differences within 5 TECU for GISG,IGSG,CODG,ESAG with respect to ITEC are 81.24%?79.52%?80.74%?77.69%,while in 2018,the percentages are reaching to 95.75%?94.28%?95.33%?94.98%.In addition,benefitting from the data of satellite altimetry,the improvements of the estimated ionosphere is more obvious for the regions near the ocean.The improvements mainly focus on the southern hemisphere,where the percentage of ocean area exceeds 80%.(3)Regarding to the difficulty of GLONASS ambiguity resolution especially for long baselines,we propose an effective method of GLONASS ambiguity resolution in orbit determination.With the help of GPS observations,we estimated and then calibrated the fractional parts bias(FCB)of GLONASS ambiguities.After FCB calibration,the success-rate of GLONASS AR can reach up to 90%,which is at the same level compared with that of GPS ambiguity resolution.Compared with the IGS final orbit products,the RMS of GLONASS orbits in along-track and cross-track components are improved by 12% and 18%.The orbit misclosures in along-track component improved by 25%,from 6.7 cm without ambiguity resolution to 5.0 cm with ambiguity resolution.The orbit misclosures in cross-track component improved by 18%,from 4.5 cm without ambiguity resolution to 3.7 cm with ambiguity resolution.The orbit precision in radial component and SLR residuals are improved only by several millimeters,this is because the radial orbit errors are mainly absorbed by satellite clock parameters,and ambiguity resolution has limited improvement to radial orbit.(4)Through estimating and calibrating GLONASS FCB,correcting BDS satellite induced code bias,we successfully realized the ambiguity resolution in precise orbit determination for GPS,GLONASS,BDS and Galileo.The fixing rates of ambiguity resolution for GPS,GLONASS,BDS and Galileo can reach up to 98.1%,96.4%,84.6% and 92.6%.Compared with IGS final orbit products,the 3D orbit RMS for GPS is improved from 3.3 cm without AR to 1.6 cm with AR,which has been reduced from 3.7 cm without AR to 3.2 cm with AR for GLONASS.Compared with the orbit products of CODE,the 3D orbit RMS of BDS IGSO is improved from 11.8 cm without AR to 7.3cm with AR,which is from 9.3 cm without AR to 7.3 cm with AR for BDS MEO and from 14.9 cm without AR to 10.1 cm with AR for Galileo.As for orbit misclosures,after realizing ambiguity resolution,the 3D orbit misclosures for GPS,GLONSS,BDS IGSO,BDS MEO and Galileo are 2.5 cm,4.6 cm,10.7 cm,10.4 cm and 8.9 cm,which are improved by 54%,23%,25%,13%,15%,respectively.The SLR residuals are reduced by 2~6 mm.By orbit comparison,orbit misclosures,and SLR residuals analysis,it proves the effectiveness of the strategies of ambiguity resolution.Therefore,the integrated precise orbit determination of GPS,GLONASS,BDS and Galileo with integer ambiguity resolution improves the precision of orbits,promoting the integrated data processing of multi-GNSS and their applications in the future. |
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