## 4 Results and Discussion
### Influence of Satellite Distribution on the Detection Rate  
In this section, the GPS L1 L2 data of four typical SRB events in Table 5 are used to analyze the significance of the satellite distribution on the detection rate. The satellites are divided with respect to the incident direction of the Sun into two categories: \"near-satellites\" and \"distant-satellites\". Figure 8 shows a sky image of the KUNM station at 03:30:00 (UTC) on 13 December 2006. The star-marked point represents the position of the Sun. The near-satellites G1 and G27 are consistent with the direction of the elevation of the Sun, while the distant-satellites G08 and G28 are in opposite directions. Figure 8: Sky image of ICS station KUNM at 03:30:00 (UTC) on 13 December 2006. Figure 7: Detection rate in the range of 0.1–1 kSFU at the GPS L2 frequency for 4 intense L-band SRB events. The histogram shows the number of samples in each flux range. Table 6 lists the azimuth (\(Az\)) and elevation (\(EI\)) of the Sun and the satellites at 03:30:00 (UTC). Two near-satellites and two distant-satellites are selected for each station. There are few satellites that meet the requirements of \"near-satellites\" or \"distant-satellites\", and thus, the values of \(N_{sat}\) = 3 and \(N_{station}\) = 2 from the optimization in Sections 4.1 and 4.2 cannot be used for this section. This part of the experiment includes the following two steps:  
* For a single station, the intersection of the valley periods of two near-satellites (distant-satellites) is taken. * For each SRB event, the valley period common to at least two stations is judged as the final detection result. The detection results of each flux range for four typical intense L-band SRB events in Table 5 are shown in Figure 9. For GPS L1, the detection results of the near-satellites and distant-satellites are different in several flux ranges. The results for the two categories are generally equivalent. Specifically, the detection rate of the distant-satellites is higher in the ranges of 1-2 KSFU, 3-4 kSFU, and 7-8 kSFU but lower in the ranges of 4-5 kSFU, 6-7 kSFU, and 8-9 kSFU. For GPS L2, the differences between the detection rate of the near-satellites and that of the distant-satellites are \(-\)3.70%, 5.00%, \(-\)5.56%, and \(-\)6.25%, respectively, which are smaller than those at the L1 frequency. Moreover, the detection rate at the L2 frequency is much smoother than that at the L1 frequency. Consequently, the distribution of the satellites relative to the Sun has no effect on the overall detection rate. Figure 10 presents the carrier-to-noise ratio observation data of GPS, GLONASS, and Galileo at the HARB station during 11:30:00-12:30:00 (UTC) on 6 September 2017, and the shaded area indicates the period when the SRB occurred. At the time of the peak flux density (12:02:30), the \(C/N_{0}\) values of GPS L2, GPS L5, GLONASS G2 and Galileo E5 drop sharply by approximately 5-7 dB-Hz. Additionally, there is a slight drop near 12:08:00 (UTC). In each subdiagram of Figure 10, the decline degree of \(C/N_{0}\) at each satellite appears to be similar at the same frequency. However, the \(C/N_{0}\) values at various frequencies are affected by the SRB to different extents. The \(C/N_{0}\) values of the high frequencies of GNSS (GPS L1, GLONASS G1, and Galileo E1) show an inconspicuous decrease, which is consistent with the conclusions of the study in [22]. They also pointed out that a possible reason for this is the different sensibilities of the GNSS frequencies and systems to SRB