Research And Implementation Of Empirical TEC Models

Total electron content(TEC)is a significant descriptive quantity for radio propagation correction and ionospheric theory research.TEC can characterize the ionization of ionosphere,which is widely used in calculating ionospheric delay for GNSS satellite signals and research on temporal-spatial variation of ionosphere.It provides reference for the study of space physics and radio science.In practice,the ionospheric empirical model is one of the important ways to acquire TEC.The ionospheric empirical model is a combination of empirical equations,based on the analysis of the temporal and spatial variation of the ionosphere and the observed data over a long period.Ionospheric empirical models based on GNSS TEC data can reflect the temporal and spatial variation of TEC in general.However,the accuracy of some TEC empirical models is not good,and they usually cannot accurately describe the temporal and spatial variation of TEC in some regions.The reason for this limitation can be summarized in two points.First,the ionospheric anomalies are not properly modeled,and the modeling function cannot accurately describe the characteristics of the ionosphere.Second,the accuracy of GIMs on a global scale is inconsistent due to the uneven distribution of tracking stations.It is not reasonable to build models using unequal precision data sets by equal weighting.Aiming at the factors that restrict the accuracy of ionospheric TEC empirical model,researches on improving method of empirical TEC models are studied.Several new empirical TEC models are proposed from the viewpoint of single-station,regional and global.Based on the analysis above,the main contents of this paper are as follows:(1)First,this study introduces the source of modeling data and data preprocessing methods for single-station/regional/global empirical TEC models.In particular,GIMs released by IGS are mainly used to establish regional and global empirical TEC models.GPS-TEC data is used to build single-station empirical TEC models.The IGS organization and its Associate Analysis Centers,GIMs product,and the IONEX format are introduced in detail.The overall accuracy of GIM-TEC and its relationship with the geographical position and solar activity are analyzed using the internal and external accuracy.Thereafter,we summarize the TEC algorithms,which are based on the GPS dual-frequency observation,as well as introduce the Reg-Est algorithm that is used in this research.Based on the analysis of the different solar activity parameters,this paper selects F10.7p as the index of the solar activity that is used as an input parameter for empirical TEC models.Lastly,the data preprocessing strategy of this model is introduced,that is,the TEC data for which F10.7>200sfu and Ap>30.0nT are excluded in the modeling data set.Accordingly,the data rejection rate is approximately 9%.(2)A new empirical TEC model for single-station is proposed based on the GPS-TEC data and nonlinear least squares fitting method,called SSM-T1 model.The SSM-T1 mode is tested at opmt station in Pairs,iisc station in Indian,cedu station in Australia,and the ohi3 station over Antarctic Peninsula.The result shows that the SSM-T1 model performed well at opmt,iisc,and cedu stations.However,this model cannot effectively describe the TEC variation on ohi3 station over Antarctic Peninsula.Ohi3 station is located in the region where mid-latitude summer nighttime anomaly(MSNA)happens.A seasonal difference exists in the diurnal variation of TEC in this area,and the correction of MSNA is lacking in SSM-T1 model.Therefore,the model is not suitable for station in MSNA area.(3)Two other new single-station empirical TEC models are proposed to address the needs of the MSNA regional stations.In particular,SSM-T2 is built on the basis of SSM-T1,adding MSNA correction items to the diurnal variation component.The model test result shows that SSM-T2 can effectively describe the MSNA phenomenon in obi3 station.In addition,the SSM-month model is a set of models that contains 12 sub-models to describe the 12-month TEC variation.SSM-month can also effectively describe MSNA phenomenon in ohi3 station.Lastly,this study uses the CODE GIMs and IRI2016 models to evaluate the prediction capabilities of SSM-T2 and SSM-month.The result shows that SSM-month and SSM-T2-ohi3 are in good agreement with CODE GIMs in the selected test days,better than the IRI2016 model.(4)Based on CODE GIMs from January 1,1999 to June 30,2015,this study uses the nonlinear least squares fitting method to establish a regional empirical TEC model(TECM-JJT)for Beijing-Tianjin-Tangshan area and another new regional empirical TEC model(TECM-NEC)for northeast China.Thereafter,these two models are evaluated using the residual distribution and statistical parameters.The fitting ability of the models to the modeling data set is tested from the diurnal and seasonal variations.Lastly,the two models are evaluated using the IRI2016,NTCM-GL model and GPS-TEC data under different solar activities.The results show that the TECM-JJT model is in good agreement with the GPS-TEC model,which is superior to the NTCM-GL model and the IRI2016 model.The TECM-NEC model can effectively describe the MSNA and winter nighttime TEC enhancement phenomena.(5)Baesd on analyzing the limitations on the global empirical TEC models,the idea of grid point modeling pattern is presented,and a new global empirical TEC model(i.e.,TECM-GRID)is built.This model contains 5183 sub-models that correspond to 5183 grid points worldwide(longitude 5°× latitude 2.5°).The modeling data set for each sub-model is the CODE GIMs grid point from January 1,1999 to June 30,2015.In which,SSM-T2 model is selected(based on the position)as the modeling method of the sub-model in the MSNA region,whereas SSM-T1 model is used as the modeling method of the sub-model in other regions.Lastly,the undetermined coefficients of each sub-model are determined in the 95%confidence interval using the nonlinear least squares method.Thereafter,the model residual distribution,the RMS and relative RMS values of the model residuals are analyzed.And then,the fitting ability of the TECM-GRID model to the modeling data is tested.Finally,the TECM-GRID model was validated using IRI2016,NTCM-GL model,and CODE GIMs at the testing time other than the modeling period.The results show that TECM-GRID can describe Equatorial Ionization Anomaly(EIA)and MSNA,which is consistent with CODE GIMs,and better than IRI2016 and NTCM-GL model.