Study On The Application Of The Ionoshperic TEC Models Over The Polar Regions Based On GNSS

The polar regions are the location of the earth's geographical pole and geomagnetic pole.The magnetic lines penetrate into the ground or open outward to connect with the interplanetary magnetic field in the regions,so that the polar ionosphere is closely coupled with the magnetic layer,the solar wind,and the interplanetary magnetic field.Complex polar ionospheric variations have caused certain harm to radio systems such as satellite navigation.Meanwhile,based on the dual-frequency or multi-frequency GNSS observation data,the ionospheric total electron content(TEC)information between the satellite and the receiver can be retrieved,which provides a data basis for continuous monitoring of the ionospheric variation in the polar regions over a long period of time.Due to the special ionospheric variation characteristics in the polar regions,GNSS-based ionospheric researches have also spawned new challenges.Due to the influence of various factors,the polar ionosphere is different from that in the low and middle latitudes.The applicability and consistency of the commonly used ionospheric models have uncertainty under temporal and spatial characteristics of the polar regions,such as polar nights and nights,Weddell Sea Anomalies(WSA),Antarctic ice sheets(AIS)and the Arctic Ocean(AO)as well as ionospheric storms.In addition,the commonly used ionospheric models are mostly based on the assumption of thin layer model,ignoring the existence of ionospheric spatial gradients.Therefore,the strong ionospheric spatial gradients caused by the ionospheric heterogeneity and patches reduce the reliability of thin the ionospheric layer model in the polar regions.Although many scholars have used multi-source ionospheric data to analyze the characteristics of the polar ionosphere during some special events in recent years,most of the current studies are based on short-term observation data or focus on one of the Arctic and Antarctic.Few studies have used the observed data from a solar cycle to analyze the variation characteristics of the ionosphere in the Arctic and Antarctic and their coupling relationship.In view of the application of ionospheric model and the ionospheric TEC variation characteristics in the Arctic and Antarctic,the main contents of this thesis are:1.Applicability and consistency analysis of multiple ionospheric models over the polar regionsDue to the complex variations of polar ionosphere,the applicability of the ionospheric models is greatly affected over the polar regions.The ionospheric model can not only provide ionospheric correction for navigation users to accelerate the convergence of ambiguity,but also can be easily and accurately used to analyze the changes of the ionosphere itself.At present,the research on the ionospheric model mostly focuses on the middle and low latitudes.With the increasing attention of the polar regions,it is of great significance to further improve the performance of the ionospheric models based on analyzing the applicability and consistency of ionospheric models by means of muti-source observation data over the polar regions.A comprehensive evaluation of Global Positioning System(GPS)Klobuchar,Galileo broadcast Ne Quick2,international reference ionosphere(IRI)and global ionospheric map(GIM)models in estimating ionospheric total electron content(TEC)is performed using GPS-derived,constellation observing system for meteorology,ionosphere,and climate and JASON-2 TECs under various solar conditions in the Arctic and Antarctic.The performances of the four ionospheric models are first analyzed for the overall polar regions.In addition,according to the temporal and spatial characteristics of the polar regions,the accuracies of the four models are evaluated for the polar days and nights,the AIS and the AO,the WSA as well as for ionospheric storm.The results show that the deviation of the four models has different seasonal variation characteristics in the Arctic and Antarctic.In the Antarctic,the real TEC is underestimated in the summer and the real TEC is overestimated in winter,while the seasonal variation in the Arctic is the opposite of that in the Antarctic.Since the Klobuchar model uses a constant(5 ns)to fit the global ionospheric changes at night,the relative correction accuracy of the model is negative during the polar nights.Galileo officially claims that the Ne Quick G model can correct about 70% of the ionospheric delay globally,but due to complex ionospheric variations in the polar regions,the model can eliminate only about 50% of the ionospheric errors in the polar regions.The newly published IRI-2016 model has a large systematic deviation when used in the polar regions.During the WSA period,the deviation of the model can reach 13.09 TECU,which seriously underestimated the actual TEC.In the AIS and AO regions,due to the lack of GNSS ground station observation,the relative correction accuracy of the GIMs model is only about 50%,which is about 30% lower than the global average accuracy.Currently,IGS has seven ionospheric analysis centers offering GIMs products.In this paper,the d STEC method is used to analyze the consistency of the seven GIM models of IGSG,CODG,JPLG,UPCG,ESAG,CASG and WHUG under different temporal and spatial characteristics in the Arctic and Antarctic,which provides reliable reference information for the researches of the ionosphere in the polar regions with different requirements.The results show that the consistency of each GIM model in the Arctic is obviously better than that in the Antarctic,and the RMS error values have the characteristics of annual and seasonal periodic variations.2.Influence of satellite azimuth on thin layer ionospheric modelsThin layer ionospheric model is widely used for GNSS navigation and ionospheric TEC modeling based on GNSS observation data.This model shares a common assumption that all free electrons of the ionosphere are constrained in a layer of infinitesimal thickness at a given reference altitude,ignoring the existence of ionospheric spatial gradients.It is considered that the STEC of different directions is the same with the same satellite elevation and ionospheric piercing point(IPP).However,due to the influence of various factors,the ionospheric heterogeneity and patches often appear in the polar regions,forming a strong ionospheric horizontal gradient,resulting in a large mapping error of the thin layer model.The Ne Quick model is a 3D ionospheric electron density model that can directly attain STEC and VTEC at arbitrary satellite azimuthal angles for a given IPP,which avoids the projection function error caused by the ionospheric gradient,and provides a new idea for studying the influence of the ionospheric horizontal gradient on the thin layer model.However,according to the existing research,since Galileo only uses a small amount of polar observation data in the calculation of the broadcast ionospheric model coefficients,the accuracy of the Ne Quick G model is low in the polar regions.Aiming at this problem,a calculation method of Ne Quick model coefficients based on regional GPS tracking network is designed in this paper,and calculates the model coefficients applicable to the Arctic and Antarctic.A comprehensive simulation analysis based on Ne Quick model was conducted for the relationship between the mapping function errors caused by the satellite azimuth and the satellite elevation,time as well as season in the polar regions.In addition,according to the "real" mapping values calculated by the Ne Quick model,a new method to evaluate the performance of the mapping functions is designed.and the common projection function which is closest to the "real value" is explored by this method.According to this method,the commonly used mapping function which is closest to the “real value” is explored.It was found that there is a significant difference in the STEC from different directions at the same IPP and satellite elevation,the maximum mapping function error can reach 9 TECU in the polar regions.However,when the elevation angle is increased to approximately 40°,the diferences of mapping values among the diferent azimuth angles become small.Compared to the diferent spatial direction “real” mapping values,the Ou and MSLM mapping functions outperformed other commonly used mapping functions,the F-K mapping function is of the lowest accuracy and is not suitable for use at low elevation angles.3.Determination of the optimal ionospheric thin layer height over the polar regionsThe 2D ionospheric TEC models fix the thin-layer to a specific height in process of modelling and using,without considering the variation of the thin layer height caused by the ionospheric vertical variation with seasonal factors and time.In this paper,we first analyze the long-term variation characteristics of peak electron density height(hm F2)and the relationship between the hm F2 and the solar activity index F10.7 using COSMIC data in the Arctic.Then,a new method detecting optimal thin layer height is proposed based on UQRG GIM model and d STEC method.This method overcomes the limitations of traditional thin layer height detection methods that require dense GNSS tracking network and unreliable results during geomagnetic storm.According to the traditional CPP method and the new method proposed in this paper,the variation characteristics of the optimal thin layer height are analyzed during a solar cycle,which provides an important information for the modeling of ionospheric TEC in the Arctic and Antarctic.4.Study on the variation characteristics of ionospheric TEC in the Arctic and AntarcticIt is of great significance to fully understand the morphological characteristics and physical mechanism of the polar ionosphere by detecting the variation characteristics of the ionosphere during a solar cycle and the response of the ionosphere during the polar days and nights,the Weddell Sea anomaly as well as the geomagnetic storm.In this study,we use polynomial function model,generalized trigonometric series function model,spherical harmonic function model and spherical cap harmonic function model to model the ionospheric TEC during a solar cycle over the Arctic and Antarctic.According to the comparative analysis of the four models,the optimal spherical cap harmonic function model is selected to study the temporal and spatial distribution characteristics of ionospheric TEC in the polar regions.Based on the ionospheric TEC model established by the spherical harmonic function,the variation trends of the Arctic and Antarctic ionospheric TEC are analyzed during a solar cycle.Besides,we analyzed the distribution characteristics of ionospheric TEC in polar day and night,Weddell Sea anomaly as well as different seasons.The formation of ionospheric patch is associated with the occurrence of geomagnetic storms.The generation and evolution of the ionospheric patch were traced and analyzed in the Antarctic during the geomagnetic storm on DOY 76,2015.In addition,by analyzing the spectrum of the time series of spherical cap harmonic function model coefficients,the periodic term of each coefficient is determined,and then it is modeled using Fourier series.The ionospheric TEC can be predicted according to the given time and coefficient model.