## 6 Influence of Floods on GPS, GLONASS, and BDS CNR
### Influence of Floods on GPS with 30 s Data Set  
Similarly, since the ground track repeat period of the GPS satellite is a sidereal day, the data set collected on the previous day before the flood can be used to compare the characteristics of the CNR. Figure 10 presented the CNR of the G32 satellite on DOY:200 (blue dots) and DOY:201 (red dots), 2021. DOY:200 is used to denote the characteristics of the CNR for a normal day, since there was no flood on DOY:200. In order to make the comparison result more intuitive, the satellite's elevation is used as the horizontal axis. From Figure 10, it can be seen that the CNR decreases on DOY:201, which is represented by red dots. The decrease trend performs more seriously with an increasing elevation angle, especially when the elevation is larger than 50 deg. The maximum decrease in the CNR is approximately 5 dB-Hz at an elevation between \(5^{\circ}\) and \(90^{\circ}\). Compared with DOY:200, the percentage of decrease in CNR ranges from 9.0% (compared with 55 dB-Hz) to 16.67% (compared with 30 dB-Hz). However, this is the maximum decrease, but not the average. In terms of the average, the percentage would be much lower, which is only 1.36%. This phenomenon is mainly because the CNR mainly decreases when the elevation greater than \(50^{\circ}\), but the reduction is not significant at a low elevation. The reason is that the multipath effect is serious at low elevations both on the days with floods and days without floods. However, at the larger elevation, the multipath effect performs more seriously during a flood compared with a day without a flood. Thus, the difference of the CNR at larger elevations during a flood will more obvious it would be at lower elevations.  
Figure 9: RMS increase rate of all other GPS satellites’ L1 frequency MP compared with DOY:200.  
To further illustrate the difference in CNR between DOY:200 and DOY:201, the histogram of the CNR and relative frequency are presented in Figure 11. The CNR is divided into four categories. It can be found that the distribution of the CNR on DOY:200 and DOY:201 is different from each other. The CNR larger than 50 dB-Hz on DOY:200 is more than that of on DOY:201. The maximum difference is approximately 0.20. The total number of G32 satellite epochs is 878. Thus, the difference between them is about 176 epochs, which is approximately 20.05% of the total epochs. This is a remarkable phenomenon. Considering that the relationship between the CNR and elevation creates a positive correlation, this result is consistent with Figure 10. In addition, it should be noted that it is not only the G32 satellite that is affected during a flood, but also other satellites are influenced during a flood. More information can be found in [25].  
Figure 11: Histogram of the CNR and relative frequency. The blue and red bars denote the CNR collected on DOY:200 and DOY:201, 2021. \(|-\)30) means CNR lower than 30 dB-Hz. \(|\)50,+) means CNR larger than 50 dB-Hz. Symbol \(|\) and denote inclusion and non-inclusion, respectively.  
Figure 10: CNR of GPS G32 satellite L1 frequency comparison results with elevation as the horizontal axis. The blue and red dots denote the CNR collected on DOY:200 (no flood) and DOY:201 (with flood), 2021, respectively.