Research On Ionospheric Tomography And Ionospheric Disturbances

The ionosphere is one of the important components in space,and it is also a research hotspot in the field of satellite navigation.Studying the spatial and temporal variations of ionosphere and monitoring the abnormal characteristics not only helps to grasp the spatial and temporal variations of ionosphere and improve the precision of navigation and positioning,but also finds out the mechanism of ionospheric disturbance to explain or even forecast the extreme events(such as tsunamis,earthquakes,etc.).With the development of satellite navigation,the ionospheric research is leaps and bounds.The researchers can obtain high-precision ionospheric TEC by using dual frequency observation data,and then high precision 3D electron density can be obtained by ionospheric tomography technology,which lays a solid foundation for studies of ionospheric spatial structure.In the long-term earthquake monitoring and forecasting,ionosphere is proved to be affected by earthquakes,and abnormal changes of the ionosphere can be detected which has obvious regularity of time.However,the ionospheric anomalies are affected by a variety of factors such as magnetic storms.Therefore,studying the ionospheric anomalies during magnetic storms helps to detect ionospheric abnormalities and to analyze the abnormal causes better.With the development of the ionospheric tomography technology,studies of ionospheric anomaly extend from time domain to space domain.Ionospheric CIT technology has been developing rapidly in the past twenty years.However,due to the complexity of the electron density inversion,there are still a series of problems to be solved.Ionospheric tomography algorithm is studied in this paper,and an adaptive multiplicative algebraic reconstruction algorithm is proposed to improve the iterative accuracy of the electron density.Then,the VTEC projection function is improved by using tomography technique,and a VTEC projection function model based on ionospheric tomography is proposed.Finally,the variation of the ionospheric disturbance before the earthquake and during the geomagnetic storm is studied.The main achevements of this paper are summarized as follows:1.A new adaptive ionospheric tomography method(AMART)is proposed for a series of problems of the multiplication algebraic reconstruction algorithm(MART)inversion of electron density.On one hand,basing on the MART algorithm,an adaptive relaxation factor related to the electron density is proposed to overcome the influence of propagation noise on electron density inversion.On the other hand,the relaxation factor in the iterative process is improved to improve its adaptability and accuracy.The experimental results show that the new method is superior to the MART algorithm in terms of the accuracy of the electron density.2.The VTEC projection function used in the ionospheric single-layer model only considers the height factor of the satellite-to-station connection.Without considering the azimuth factor,the projection results deviate from the true values,especially when the height angle is low.To solve this problem,a new VTEC projection model is proposed by using the tomography technique.The new model adopts the method of subsection projection,taking full account of the ray orientation factor,and the results show that the projection accuracy of the new model is better than that of the traditional projection model.3.The ionospheric disturbances during the geomagnetic storm and before the earthquake are studied in this paper.By analyzing the correlation between the mean values of the ionospheric anomalies and geomagnetic indices during the geomagnetic storm,it is found that the the average value of ionospheric anomaly is positively correlated with the intensity of geomagnetic storms,and there is a certain lag.By analyzing the ionospheric anomalies before earthquakes,it is found that the ionospheric anomalies occur in a long time before the earthquake,and it can be regarded as earthquake precursor.