Study On The Monitoring Of Global Ionospheric TEC Based On Multi-GNSS And Multi-Source Space Observation Techniques

Based on the tracking observations of dozens of GNSS satellites by hundreds of IGS stations with long term period and unprecedented high temporal and spatial sampling rate,the space-time distribution of global ionospheric total electron content was retrieved by groundbased GNSS techniques,which greatly advanced our knowledge on the ionosphere conditions.However,the global distribution of IGS stations was not even.Most of the stations were located in the continental areas of the Northern Hemisphere.There were few or no GNSS stations in the vast ocean area.In areas where ionospheric observations were lacking,the accuracy of global ionospheric models was significantly decreased.The main factors affecting the accuracy of global ionospheric models were model data sources and model algorithms.In recent years,with the construction and development of new navigation satellite systems(Galileo,BDS,QZSS,NAVIC),more navigation satellites could provide more ground-based GNSS ionospheric observation data.LEO satellites were also one of the important technologies for monitoring the ionosphere,such as DORIS,RA,IRO and so on.Unlike GNSS satellites,LEO satellites could obtain uniformly distributed global ionospheric observations.Therefore,making full use of multi-GNSS data and multi-source satellites observation data was one of the important ways to improve the accuracy of global ionospheric models.Although the establishment of global ionospheric TEC model based on GNSS data had been developed for nearly 20 years,the current model algorithms could be improved by decomposing the error items of the model,refining the model item by item,and constructing more rigorous models,so as to improve the accuracy of the global ionospheric TEC models.Focusing on the goal of building a high-precision global ionospheric TEC model,this thesis carried out research work from three aspects:model data,model algorithm and model application.The global ionospheric TEC model based on multi-GNSS and multi-source satellites observation data fusion was studied.The refinement algorithms of global ionospheric TEC models was investigated.The real-time global ionospheric TEC model algorithm and its application in positioning were also analyzed.The main research work and contributions of this thesis were as follows:(1)Two new algorithms(one-step method and two-step method)of GPS/GLONASS global ionospheric TEC modeling were proposed considering GLONASS receiver IFDCB.Based on the data from day of year 049 to 108 in 2014,results showed that the ionospheric VTEC increased by 0.15 TECU and the stabilities of GPS satellite DCBs increased by 5%after adding GLONASS data.The influence of GLONASS receiver IFDCB on global ionospheric VTEC was less than 2 TECU.The difference between GLONASS receiver IFDCB and DCB could reach 5 ns(15 TECU).Considering IFDCB,the systematic error of GLONASS residuals could be eliminated.(2)The influence of multi-GNSS(GPS/GLONASS/Galileo/BDS/QZSS/NAVIC)data fusion on global ionospheric TEC model was studied.Data analysis based on September 2018 showed that the global ionospheric model using only GLONASS or Galileo data could be established with the same accuracy as GPS.With the fusion of multi-GNSS data,the difference of VTEC was less than 1 TECU,and it could significantly improve the accuracy of TEC estimation in the Southern Hemisphere,especially in the Asia-Pacific region.The stabilities of GPS satellite DCBs were improved by 8%.(3)The influence of multi-source(GNSS/DORIS/RA/IRO)data fusion on the global ionospheric TEC model was studied.Data analysis based on October 2013 showed that the fusion of Multi-source satellites data had little effect on ionospheric VTEC in the areas with abundant ground-based GNSS data,but it could significantly decrease the ionospheric VTEC(4-8 TECU)in low latitudes and increase the ionospheric VTEC(3-6 TECU)in the middle and high latitudes of the Southern Hemisphere,and significantly improved the ionospheric VTEC estimation accuracy(about 3 TECU)in the marine areas.There was no significant difference between the global ionospheric TEC model of multi-source data fusion and the global ionospheric model of IGS.When compared with Jason-2,the accuracy of global ionospheric model in low latitudes and in the Southern Hemisphere had been improved by 30%.(4)A new global ionospheric TEC modeling algorithm based on spherical harmonic expansion and epoch-differenced carrier phase observation data was proposed.Based on the analysis of GPS/GLONASS data from day of year 032 to 061 in 2016,results showed that the ionospheric VTEC with new algorithm was about 0.9 TECU larger compared with the traditional algorithm.When compared with IGSG,the root mean square of the model was about 2.2 TECU.When compared with dSTEC,the accuracy of model in the Southern Hemisphere could be improved obviously by adding GLONASS data.The accuracy of model in the middle and high latitudes was improved by 8%,but it was slightly lower in low latitudes.When compared with Jason-2,the accuracy of model was about 4.3 TECU.(5)A new global ionospheric TEC modeling algorithm based on spherical harmonic expansion and multi-layer assumption model was proposed.Based on GPS data analysis from day of year 060 in 2014 and 212 in 2008,it was shown that the multi-layer model decreased the ionospheric VTEC in low latitudes by about 9 TECU(double-layer model)and 18 TECU(three-layer model).The multi-layer model significantly reduced the value of receiver DCBs.The multi-layer model could significantly improve the fitting accuracy of STEC for satellite elevation angle smaller than 30 degrees.When compare with the single-layer model,the accuracy of multi-layer model was improved by 4%(double-layer model)and 14%(three-layer model).(6)Anew real-time global ionospheric TEC modeling algorithm based on real-time observation data and predicted spherical harmonic coefficients was proposed.Whe compared with IGSG,dSTEC and Jason-2,the accuracy of the model was 1.2,1.9 and 2.5 TECU,respectively in low solar activity year,but was 4.9,7.5 and 7.1 TECU,respectively in high solar activity year.The ionospheric delay correction percentage with this new algorithm was about 83%.The influence with this new algorithm on positioning accuracy was better than 1.4 m.When using the new real-time global ionospheric model,the accuracy of standard point positioning in horizontal direction and vertical direction were better than 0.6 m and 1.4 m,respectively using only single frequency pseudorange.(7)A high performance and high precision GNSS Ionosphere Monitoring and Analysis Software named GIMAS was developed using C++language.The OpenMP parallel computing scheme was introduced into the software for computing intensive tasks,which improved the efficiency of global ionospheric modeling by about 6 times.The application of GIMAS software in the IGS Ionosphere Associate Analysis Center of Wuhan University was analyzed.The results showed that GIMAS could monitor the activity of global ionosphere steadily and effectively for a long time,reaching the international advanced level in the same field.