Research On The Ionospheric Characteristics In Low-Latitude Regions Of China Based On BeiDou Satellites' Observations

Navigation and positioning systems involve all aspects of human production and life,and the changes of the ionosphere are a major obstacle that affects the accuracy of navigation and positioning.The characteristics of ionospheric changes in low-latitude regions of China are quite different from those in other low-latitude regions(i.e.,South America and Africa).At present,the understanding of the characteristics of the ionosphere in low-latitude regions of China is insufficient.Therefore,it is of great significance to study the characteristics of the ionosphere in low-latitude regions of China.This dissertation used Beidou satellites' observation data to study the characteristics of the ionosphere in low-latitude regions of China.The main work and research results of this dissertation are summarized as follows.1.Using Beidou GEO(Geostationary Earth Orbit)satellites' observation data,the impact of the solar eclipse,a special space weather event for the ionosphere in low-latitude regions of China was studied.An annular solar eclipse with a maximum eclipse of 0.994 occurred on 21 June 2020.The central trajectory of this annular eclipse passed through most of the low-latitude regions of China.This dissertation analyzed Beidou GEO satellites' observation data provided by the four low-latitude GNSS(Global Navigation Satellite System)observation stations of Wuhan Jiufeng(JFNG),Guilin(GUILIN),Hong Kong Wong Shek(HKWS),and Taiwan Tainan(CKSV)from 19 to 23 June 2020.This dissertation developed firstly a program for processing and analyzing Beidou GEO satellites' observation data,and obtained the graphs of changes in ionospheric TEC(Total Electron Content)over time at each station on each day.Then,two days before and two days after this annular eclipse(i.e.,19,20,22,and 23 June 2020)were used as reference days to analyze the impacts of this annular eclipse on the ionospheric TEC observed by the above-mentioned four stations.The research in this dissertation showed that during the solar eclipse,compared with the reference days,the ionospheric TEC observed by the four stations all experienced the process of a first drop to a minimum value(this minimum value was 3.0?21.4 TECU lower than the reference days)and then gradually recovered.On the day of the solar eclipse,at the ionospheric piece point of each GEO satellite,the ionospheric TEC began to fall at the earliest 101 minutes before first contact,fell to the minimum value at the earliest 8 minutes before maximum obscuration,and returned to the value before the fall at the latest 51 minutes after forth contact,the declining speed of ionospheric TEC was slower than the recovery speed.The reason why the minimum ionospheric TEC at individual ionospheric piece point was ahead of maximum obscuration might be that the eclipse time at a location with a small geographic longitude on the path from the GEO satellite to the ground receiver was earlier than the eclipse time at the ionospheric piece point,which made the ionospheric TEC along the path from the GEO satellite to the ground receiver recover in advance.In addition,CKSV,HKWS,GUILIN,and JFNG stations observed the maximum ionospheric TEC declining quantity during the solar eclipse about 5.48,4.7,4.05,and 2.13 TECU,respectively.The study in this dissertation showed that the maximum ionospheric TEC declining quantity during the solar eclipse had a significant latitude effect,which might be related to the location of the Beidou GEO satellite ionospheric piece point corresponding to individual stations closer to the north peak of the equatorial anomaly structure.2.Ionospheric tomography combined with Beidou and GPS(Global Positioning System)observations were used to study the characteristics of the ionosphere in Chinese low-latitude regions(take Hong Kong as an example).This dissertation first developed an ionospheric tomography program that combined Beidou and GPS observation data,and used the program to conduct the simulation reconstruction for the ionospheric electron density distribution over Hong Kong at 20:30 UT(Universal Time)on 19 June 2020.The simulation reconstruction results verified the rationality of the ionospheric tomography algorithm used in this dissertation and the correctness of the program developed in this dissertation.Then,the processing method of the observation data of the Hong Kong ground-based GNSS network was introduced,and the correctness of the calculation method of ionospheric TEC in this dissertation was verified.Finally,the reconstruction results of the measured data from the Hong Kong ground-based GNSS network were used to analyze the ionospheric characteristics of the Hong Kong area during the geomagnetic storm on 26 August 2018.The analysis result showed that compared with the geomagnetic quiet condition(i.e.,24 August 2018),at 02:30 UT on 26 August,the peak electron density on the 113.5°E and 21.75°N profiles both increased by 11.3%.And at 08:30 UT on 26 August,the peak electron density on the 113.5°E and 21.75°N profiles increased by 16.9%and 16.0%,respectively.The above results showed that the ionosphere above Hong Kong was affected by this geomagnetic storm,and a positive storm occurred on 26 August.The work of this dissertation has deepened the understanding of the characteristics of ionospheric changes in low-latitude regions during special space weather events such as solar eclipses and geomagnetic storms,and will help to promote the study of ionospheric modeling in low-latitude regions of China,which is of great significance for improving the early warning capability of special space weather events in low-latitude regions of China.