Research On High-precision Global Ionospheric Modeling Based On GNSS

Ionospheric delay is one of the error sources that restrict the accuracy of navigation and positioning.Practice has proved that the high-precision ionospheric model is of great significance for the correction of ionospheric delay errors.With the completion of the BeiDou-3 global network and the increasing improvement of the Galileo system,the number of satellites reached or exceeded GPS,and there were more than 200 ground tracking stations of Galileo and BeiDou,it is possible to use Galileo and BeiDou to build a global ionospheric model.In addition,there are differences in the number of satellites,constellation types,and signal frequencies of different systems.It is necessary to verify the consistency of the global ionospheric models established by different systems.In view of this,this paper studies the feasibility of the Galileo and BeiDou systems to establish a global ionosphere model separately,and builds a multi-system GNSS high-precision global ionosphere model on this basis.In addition,the new model is applied to PPP for analysis and evaluation.The specific work and results are as follows:(1)The global ionospheric model of GPS,GLONASS,Galileo and BeiDou each single system has been established,and it is found that the VTEC of different systems has high consistency,and they can better reflect the temporal and spatial distribution of global VTEC.Among them,Galileo and GPS systems have the highest consistency.The average Bias of Galileo relative to GPS VTEC is 0.02 TECU,and the STD from 2019 to 2020 is less than 1.5 TECU.Due to the small number of tracking stations in the Western Hemisphere,average deviation between GPS VTEC and BeiDou VTEC is-0.11 TECU,and the STD is between 1?2.5 TECU,which is equivalent to GLONASS.(2)A global ionospheric model combining the four systems of GPS,GLONASS,Galileo and BeiDou has been established.The results show that the VTEC difference between multi-systems and single systems mainly exists in areas where tracking stations are scarce,such as oceans and poles.The RMS of multi-systems is reduced compared with each single system,which is 0.3 TECU lower than GPS,1.6 TECU lower than GLONASS,0.6 TECU lower than Galileo,2.5 TECU lower than BeiDou,and the decline is particularly obvious in areas where single-system observations are missing,such as oceans.(3)The accuracy of the new model was verified from multiple aspects using post-event GIMs products,GNSS dSTEC observations and CAS DCB.The results showed that the single-system model has high accuracy.The accuracy of BeiDou system in the eastern hemisphere,especially in the Asia-Pacific region,is comparable to other systems.Multi-system fusion can significantly increase the reliability of the model while improving the accuracy of the model.The comparison between the modeling products in this thesis and the products of IGS analysis centers shows that the accuracy of the modeling products is consistent with the products of various IAACs,the estimated DCB of each system satellite is very close to the CAS DCB product,and the consistency of the multi-system DCB is better,which verifies the correctness of the modeling methods and data processing strategies in this article.(4)Taking the standard single-frequency PPP as a reference,the PPP positioning effect of different ionospheric modeling products in this dissertation is evaluated.In terms of positioning accuracy,the average RMS value of the positioning errors of the five ionospheric products obtained in this paper is close,which is equivalent to the results of CODG,ESAG,IGSG and WHUG.In terms of convergence time,the average convergence time of multi-system products(GREC)and Galileo is 47.5%and 46.7%shorter than standard single-frequency PPP,slightly worse than CASG(57.1%)and WHUG(48.6%),but better than CODG(46.2%),ESAG(44.7%),IGSG(39.3%),JPLG(21%)and UPCG(37.2%).BeiDou's convergence time improvement effect is slightly worse(35.3%),but it is still better than JPLG(21%).In general,the performance of the modeled products in this thesis is comparable to that of ESAG?IGSG?JPLG and UPCG.