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The ionosphere total electron content (TEC) has always been one of the important error sources in satellite-based navigation and positioning applications. Besides TEC, the rate of TEC index (ROTI) which is derived from TEC, is also served as a significant parameter in monitoring the status of the ionosphere. Based on more than 2,800 GNSS stations in China, as part of the Beidou Satellite Ground Augmentation System (BDSGAS), the ROTI maps were constructed from 2019 to 2022, with the temporal resolution of 300 seconds and the spatial resolution of 0.5°and 0.25° along longitude and latitude, respectively. In order to analyze the seasonal characteristics of ROTI maps during those years, the geomagnetic quiet days in each season are chosen based on hourly geomagnetic Dst index. Among those ROTI maps, the seasonal characteristics are obvious and differ along latitude, in the low latitude the strongest ROTI occurring and related to the scintillations in both spring and autumn, in the middle latitude ROTI active only in summer, and in the high latitude ROTI displaying no activity during each winter.
Keywords
Total Electron Content (TEC)
Rate of TEC Index (ROTI)
scintillation
Beidou Satellite Ground Augmentation System (BDSGAS)
Qianxun Spatial Intelligence (Zhejiang) Inc., Deqing, Zhejiang, China
*Address all correspondence to: snake553@163.com
1. Introduction
In recent years, the pattern of global satellite navigation systems is undergoing great changes. China’s self-developed Beidou Satellite Navigation and Positioning System (BDS), officially opened on July 3, 2020, becomes the third mature satellite navigation system after GPS and GLONASS. The comprehensive completion of BDS indicates that China has an independent voice and provides an opportunity in the field of satellite navigation. At the same time, relevant researches and applications could also be accelerated and upgraded.
At present, people’s needs for navigation and positioning are becoming more and more diversified, generating various application scenarios such as automatic driving, UAV operation, safety monitoring, etc. Among those applications, GNSS satellite signal is an indispensable signal source. However, the ionosphere, as the medium for the radio-wave transmitting from GNSS satellite to ground equipment, can never be ignored due to its refraction, scattering, and reflection upon radio signals. Therefore, to investigate the ionospheric influences based on the GNSS signals becomes an important problem to be solved. Traditionally, the ionospheric total electron content (TEC) can be inversed from double-frequency GNSS signals by integrating the ionospheric electron density along the propagation path, which is not only an important parameter in ionospheric physics research, but also an important error source to be eliminated in navigation and positioning applications.
Besides TEC, the TEC Rate of Change Index (ROTI) derived from TEC, is also frequently adopted to monitor the ionospheric state. Meanwhile, TEC and ROTI maps are favored by researchers and engineers in order to display the difference of overall regions. TEC maps are much more popular and widely-used than ROTI maps. So far, there are several research institutions regularly releasing their global ionospheric maps (GIM), such as Center for Orbit Determination in Europe (CODE, Switzerland), Jet Propulsion Laboratory (JPL, USA), European Space Agency (ESA, Germany), and Universitat Politècnica de Catalunya (UPC, Spain), Chinese Academy of Sciences (CAS, China) [1, 2, 3, 4]. Those GIMs are submitted to International GNSS Service (IGS) to generate the combined version [5].
Particularly, in the Arctic and Antarctic, low latitudes and equatorial regions, where the ionosphere tends to change intensely, ROTI map is often used to locate the position of the active area of the ionosphere. As early as 1997, Pi et al. [6] obtained the global ROTI map in the form of scattered points with the help of global GPS data, and used it to analyze the global distribution of ionospheric irregularities. In Europe, due to the influence of aurora, Cherniak et al. [7] used more than 700 GNSS stations in the middle and high latitudes of the Northern Hemisphere to develop the Northern Hemisphere high latitude ROTI map in the geomagnetic coordinate system (the geomagnetic latitude is above 50°), whose resolutions along the magnetic latitude and magnetic local time are 2° and 8 minutes, respectively, which can easily indicate the regions with active TEC changes under the influence of the aurora. In India, Harsha et al. [8] utilized 26 Indian GNSS stations to build TEC and ROTI maps to monitor the status of the ionosphere in the equator and low latitude regions.
In China, especially before the Beidou system was fully built, TEC estimations generally rely on GPS and GLONASS signals, and the number of GNSS stations used for ionospheric monitoring is not as dense as it is now. In low latitude of southern China, the lack of observation often leads to inaccurate estimation of the ionospheric TEC, then resulting inaccurate TEC maps. Meanwhile, the researches about ROTI are mainly focus on two aspects, one is the conventional observations of ROTI in the low latitude area of southern China, and the other is the case studies on the ionospheric ROTI response under the condition of geomagnetic activities. The ROTI maps have not been regularly released and studied, whose climatological characteristics are still not clear on the overall scope of China. It is urgent to fill the gap.
In this chapter, based on the GNSS observations from over 2800 Beidou Satellite Ground Augmentation System (BDSGAS) stations in China, a set of ionospheric ROTI maps will be constructed from 2019 to 2022, and only the ones under geomagnetic quiet condition will be adopted to statistically analyze its characteristics, including the features along each season and the low, middle, and high latitudes.
The GNSS observations used in this chapter are from the reference stations of BDSGAS [9]. The system adopts advanced system architecture, data processing system, software and multiple broadcasting means to enhance the positioning accuracy by multi-GNSS and multi-modes. The enhanced data products can be broadcasted by multiple means with various precisions such as meter, decimeter, and centimeter level for real-time service and even millimeter level for the post-processing positioning service. The acceptance of the system was completed in 2019. Currently, this system is operated by Qianxun Spatial Intelligence Co., Ltd., established jointly by China North Industries Group Corporation Limited and Alibaba Group in August 2015. As successful business cases, a group of customized products and services for specific application scenarios has been launched, such as dangerous building detection, precision agriculture, and autonomous vehicles.
Up to now, the system has established more than 3000 GNSS reference stations in China, covering all provinces, cities, and major counties. Each station is equipped with multi-GNSS receiver which fully supports BDS, GPS, Galileo, and GLONASS signals, with the sampling rate as high as 1Hz. Those stations can provide sufficient GNSS data for the construction of the ROTI map in this chapter.
In order to measure the level of the geomagnetic disturbance, the geomagnetic Dst index is chosen from the World Data Center (WDC) in Tokyo, Japan. According to the trend of the Dst index, the geomagnetic quiet days are selected from every month, acting as the reference for the geomagnetic disturbances. When analyzing the seasonal climatological characteristics of ROTI, only the geomagnetic quiet days during the equinox/solstice months are considered for simplicity, i.e. March, June, September, and December, respectively. The time span covers from spring 2019 to winter 2022, nearly four years long. Table 1 lists each selected geomagnetic quiet day and the daily minimum of hourly Dst index. The minimum hourly Dst of those quiet days is no less than −10 nT.
season
Date of the geomagnetic day
Minimum of hourly Dst (nT)
Spring
2019-03-22
−1
2020-03-11
4
2021-03-11
6
2022-03-22
−1
Summer
2019-06-12
−1
2020-06-15
−6
2021-06-14
−7
2022-06-13
−4
Autumn
2019-09-19
−8
2020-09-10
−3
2021-09-12
−7
2022-09-18
−7
Winter
2019-12-17
−8
2020-12-14
−6
2021-12-15
−10
2022-12-18
−1
Table 1.
Geomagnetic quiet days and daily minimum of hourly Dst indices during 2019 to 2022.
The process of ROTI map construction can be divided into three steps. At first, ionospheric slant TEC along the ray paths from each GNSS station to satellites is inversed. The algorithm about TEC inversion is very mature, and the method based on local spherical symmetry and thin layer model assumption in literature [10] is chosen. The ionospheric puncture point (IPP) is 450 km above the Earth surface. Secondly, Rate of TEC (ROT) and ROTI for each station is obtained with the common method [7]. It should be noted that, the cut-off limit of ground elevation is set as 30°, in order to avoid the impact of multipath effects caused by low elevation and obtain the most reliable ROT and ROTI information. At last, all the ROTI data are gridded into a two-dimensional map divided by longitude and latitude [11], with the longitude and latitude resolution of 0.5° and 0.25°, respectively, and the temporal resolution is 300 seconds. The value of ROTI in a grid is the averaged ROTI of all the valid ROTI data from the ground-satellite ray paths crossing through the grid, without any additional interpolation applied.
After the above three steps, the ROTI map is completed by the form of grid. Due to the uneven distribution of GNSS stations, there are some empty grids in the map. In fact, the ROTI map is only a partially filled grid map.
On the one hand, according to the geographic latitude, the whole China and nearby region can be divided into three parts: (a) the low latitude region, located below 25°N, mainly including Hainan, Guangdong, Guangxi, and Yunnan; (b) the middle latitude district, between 25°N and 45°N, such as Sichuan, Hubei, Henan, Shandong, and Hebei; (c) the high latitude area, above 45°N, mainly including Heilongjiang, Jilin, northern Xinjiang, and northern Inner Mongolia. On the other hand, due to the fact that ROTI is more active during night than that in white day, only the ROTI maps at nighttime (18:00–03:00 LT) are considered. Then, the nighttime ROTI maps are compared by each season and each region, in order to discuss the seasonal characteristics of ROTI maps from 2019 to 2022.
3.1 Spring
Ionospheric scintillation in low latitude areas is common phenomenon in spring. For example, Li Guozhu confirmed that ionospheric scintillation is more likely to occur in spring and autumn but less likely to happen in winter and summer by using the ionospheric GPS amplitude scintillation data of Sanya station in Hainan from 2004 to 2006 [12]. Liu K. utilized the GPS data from Sanya in China from 2004 to 2012 and found that the scintillations have a maximum occurrence during equinox of solar maximum [13]. Considering the close relationship between scintillation and ROTI, the value of ROTI in spring tends to be higher in low latitude than that in middle and high latitudes.
As a group of typical examples listed in Table 1, the geomagnetic quiet days in spring from 2019 to 2022 are March 22, 2019, March 11, 2020, March 11, 2021, and March 22, 2022, respectively, with the hourly Dst index no less than −1 nT. The corresponding nighttime ROTI maps for each day are shown in Figure 1. It can be seen that ROTI is less than 0.1 TECU/min in most of the area from 2019 to 2022, expect in the low latitude of Hainan and nearby region, where the geographic latitude is less than 20°N. Since the value of ROTI acts as the sign of scintillations to some extent, the higher the ROTI is, the stronger the scintillation is. The case of ROTI less than 0.1 TECU/min can be marked as “blank” without any scintillation. Therefore, there is nearly no scintillation in 2019 spring, the quietest spring among those years. In 2020 spring, scintillations were obvious only in Hainan and around during 22:00 LT to 02:00 LT, below 20°N. In 2021 spring, scintillations are observed only far south of Hainan around 23:00 LT, also below 20°N but with much smaller size than those in 2020 spring. In 2022 spring, the scintillations occurred below 20°N from 22:00 LT to 04:00 LT, lasting longer than other years.
Figure 1.
The nighttime ROTI maps in spring.
The strength of scintillation in each year is also different. To measure the strength, a group of ROTI thresholds are considered. The strength can be compared by counting the total number and calculating the average of ROTI among the grids exceeding the thresholds. For simplicity, the thresholds chosen in this chapter are 0.1 TECU/min and 0.5 TECU/min, referring to no scintillation and moderate scintillation, respectively. Three LTs are selected to evaluate the temporal difference, i.e.: 19:00 LT, 21:00 LT, and 23:00 LT. The results are shown in Table 2. It can be clearly seen that, 2022 spring is the strongest among those years, with the most number and the largest average of grid exceeds 0.5 TECU/min, while 2019 spring is the weakest. In 2020 and 2021 spring, scintillations are almost equivalent in strength. The difference of such strength from 2019 to 2022 may be related to the different level of solar activity, increasing from solar minimum to growing phase.
LT
Thresholds (TECU/min)
Number
Average (TECU/min)
2019
19:00
0.1
156
0.193
0.5
4
2.902
21:00
0.1
108
0.328
0.5
6
3.764
23:00
0.1
212
0.287
0.5
13
2.850
2020
19:00
0.1
94
0.142
0.5
2
1.177
21:00
0.1
137
0.140
0.5
2
1.117
23:00
0.1
364
0.230
0.5
22
0.909
2021
19:00
0.1
66
0.143
0.5
2
0.712
21:00
0.1
102
0.175
0.5
4
1.023
23:00
0.1
177
0.330
0.5
38
0.838
2022
19:00
0.1
91
0.323
0.5
10
1.823
21:00
0.1
81
0.342
0.5
8
2.064
23:00
0.1
225
0.757
0.5
70
2.01
Table 2.
Number and ROTI average of grids for ROTI exceeding the thresholds in spring.
3.2 Summer
Different from the ionospheric scintillation mainly in the low latitude region in spring, the ionospheric nighttime activities in summer often correspond to the ionospheric electron density depletions caused by the mid latitude ionospheric trough (MIT) phenomenon [14]. For example, in the middle latitude of East Asia, He Youwen and Long Qili analyzed the seasonal characteristics of the ionospheric scintillation from the scintillation observations of four stations, and found that the occurrence rate of scintillation is highest in summer, followed by autumn and weakest in winter [15]. However, due to the restriction of observation conditions, there are few reports using ROTI maps to study this phenomenon.
June 12, 2019, June 15, 2020, June 14, 2021, and June 13, 2022 are chosen as the geomagnetic quiet days in the summer of each year, as listed in Table 1 all the hourly Dst indices in summer greater than −9 nT. The nighttime ROTI maps are shown in Figure 2. It can be seen from the figure that, in the night of each summer, ROTI greater than 0.1 TECU/min is widespread in mid latitude regions during 21:00 LT to 02:00 LT, but rarely occurs in low and high latitudes, with the size of active ROTI much wider than that in spring. Meanwhile, as time goes on, the active region of ROTI larger than 0.1TECU/min in the map moves from east to west, which is mainly related to the Earth’s rotation resulting in the movement of direct solar radiation point from east to west.
Figure 2.
The nighttime ROTI maps in summer.
As the same ROTI thresholds used in spring, the total number and the average of ROTI among the grids exceeding the thresholds are listed in Table 3. Since the number of grids exceeding 0.5 TECU/min is too small, such grids can be considered as gross error to be ignored. The situation in summer is quite different from that in spring. In summer nighttime, the number of grids exceeding 0.1 TECU/min can reach 2023 around 23:00 LT in year 2022, but in spring only 364 grids are captured, much more than that in spring, spreading much more widely in summer. The averages of ROTI for grids over 0.1 TECU/min in summer are generally from 0.123 TECU/min to 0.216 TECU/min, but the same averages in spring are from 0.142 TECU/min to 0.757 TECU/min, less in summer than that in spring. From 2019 to 2022, the number of grids exceeding 0.1 TECU/min is smallest in 2020, and largest in 2022, also following different pattern of activity other than that in spring. Even though the solar activity reached its minimum during 2019–2020, much less active than in 2021, the nighttime ROTI map in summer is still as strong as in 2021, nearly uninfluenced by the level of solar activities.
LT
Thresholds (TECU/min)
Number
Average (TECU/min)
2019
19:00
0.1
479
0.189
0.5
10
3.269
21:00
0.1
1002
0.138
0.5
3
1.919
23:00
0.1
1575
0.142
0.5
9
2.768
2020
19:00
0.1
70
0.129
0.5
1
0.555
21:00
0.1
109
0.165
0.5
2
2.398
23:00
0.1
377
0.123
0.5
0
—
2021
19:00
0.1
109
0.216
0.5
3
3.267
21:00
0.1
581
0.127
0.5
2
0.952
23:00
0.1
1603
0.124
0.5
0
—
2022
19:00
0.1
144
0.183
0.5
6
1.214
21:00
0.1
743
0.134
0.5
4
0.989
23:00
0.1
2023
0.153
0.5
10
1.472
Table 3.
Number and ROTI average of grids for ROTI exceeding the thresholds in summer.
3.3 Autumn
Similar to spring, autumn is also another season with high occurrence of ionospheric scintillations in low latitudes. Zhang Hongbo et al. [10] used the ionospheric scintillation data of Haikou Station in China’s low latitude region from 2010 to 2017 to analyze the statistical occurrence of ionospheric scintillation in spring and autumn, found that both the occurrence and the intensity of ionospheric scintillation were generally higher in spring equinox than those in autumn equinox, and attributed the inconsistency between spring and autumn to the asymmetry of the equinox in the ionospheric background.
As listed in Table 1, the geomagnetic quiet days in each autumn from 2019 to 2022 are September 19, 2019, September 10, 2020, September 12, 2021, and September 18, 2022, respectively, with the hourly Dst index no less than −8 nT. The nighttime ROTI maps are shown in Figure 3. Seen from Figure 3, the scintillations were almost blank both in 2019 and 2020 autumn, occurred only in a tiny small area of the southeast region in 2021 autumn from 22:00 LT to 00:00 LT, and extended to larger area in 2022 autumn than those in 2021 from 21:00 LT to 03:00 LT. The intensity of scintillations in each autumn can also be clearly revealed, the strongest in 2022, followed by 2021, and the weakest in 2020.
Figure 3.
The nighttime ROTI maps in autumn.
The total number and the average of grids exceeding the ROTI thresholds of 0.1 TECU/min and 0.5 TECU/min are listed in Table 4. As the strongest autumn in 2022 among those years, around 23:00 LT in 2022, the number and the ROTI average of grids over 0.5 TECU/min is 140 and 1.398 TECU/min, respectively, while the number of grids over 0.5 TECU/min in other years is no more than 11. The ROTI average of grids over 0.1 TECU/min is a little larger in 2021 than those in both 2019 and 2020. When comparing spring and autumn of the same year, the number of grids over the thresholds is usually larger in spring than that in autumn except in 2022 and 19:00–21:00LT in 2019, while the ROTI average is also larger in spring than that in autumn except the gross error with only several scattering grids over 0.5 TECU/min. Generally, both the range and amplitude of scintillations are weaker in autumn than those in spring. Therefore, the difference of scintillations in spring and autumn in each year is consistent with the performance of ionospheric scintillation stronger in spring than autumn as mentioned in the literature [16].
LT
Thresholds (TECU/min)
Number
Average (TECU/min)
2019
19:00
0.1
175
0.155
0.5
4
1.527
21:00
0.1
153
0.216
0.5
11
1.396
23:00
0.1
155
0.140
0.5
3
1.185
2020
19:00
0.1
159
0.120
0.5
0
—
21:00
0.1
89
0.137
0.5
1
1.887
23:00
0.1
33
0.174
0.5
1
1.757
2021
19:00
0.1
87
0.167
0.5
1
3.992
21:00
0.1
109
0.240
0.5
5
2.208
23:00
0.1
170
0.166
0.5
2
0.689
2022
19:00
0.1
94
0.183
0.5
6
0.765
21:00
0.1
248
0.494
0.5
48
1.909
23:00
0.1
346
0.686
0.5
140
1.398
Table 4.
Number and ROTI average of grids for ROTI exceeding the thresholds in autumn.
3.4 Winter
In winter, the direct sunlight point moves to the southern hemisphere, thus the ionosphere changes in the northern hemisphere are relatively stable and without scintillation at night. The geomagnetic quiet days for each winter from 2019 to 2022 are December 17, 2019, December 14, 2020, December 15, 2021, and December 18, 2022, respectively, with the minimal value of hourly Dst index no less than −10 nT.
The corresponding nighttime ROTI maps are shown in Figure 4. Different from all the ROTI maps in other seasons, the nighttime ROTI maps always keep in very low level in each winter, without any active area. Therefore, there is no scintillation in winter. Then, the analysis about the ROTI thresholds is also omitted in this subsection.
Under geomagnetic quiet conditions, the nighttime ROTI maps for each season from 2019 to 2022 in China is constructed, and its seasonal characteristics are analyzed in this chapter. The following conclusions can be drawn: (1) In spring and autumn, ROTI is mainly active only in the low latitude areas in southern China. The value of ROTI in active areas is larger than that in other seasons, which is also related to ionospheric scintillations; (2) In summer, ROTI is active only in the middle latitude of central China, with the amplitude of ROTI less than that in Equinox. The size of active ROTI area is much wider than that in spring and autumn. It is generally related to the ionospheric nighttime trough in summer; (3) In winter, the ionosphere is generally quiet, without any active ROTI area.
In addition to the ROTI map mentioned in this chapter, the ionospheric scintillation S4 index is also often used for ionospheric state monitoring. In the future, more comparative studies about various parameters can be carried out to provide comprehensive ionospheric monitoring services.
This work was supported by the National Natural Science Foundation of China (41704158) and the National Key Research and Development Program of China (2021YFA0717300). Thanks to the World Data Center (WDC) in Tokyo, Japan for providing geomagnetic Dst data.
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Written By
Chengli She
Submitted: 20 January 2023Reviewed: 27 January 2023Published: 27 February 2023
Open access peer-reviewed chapter
