## 3 Proposed Method
### Influence Analysis of Flooding on CNR  
In order to demonstrate the effect of flooding on the CNR of GPS satellites, the data sets collected on DOY 199 and DOY 201, 2021 from ZHNZ station were used. ZHNZ station is located in Zhengzhou City, China. It was influenced by a flood from DOY 200 to DOY 202, 2021. The sample interval was 30 s. The elevation cut-off angle was 5 deg. Concrete information about the flood is available in [PERSON] et al. [20].  
The CNR of the GPS G27 satellite at L1 and L2 frequency is presented in Figures 8 and 9, respectively. The blue line means the CNR of DOY 199, 2021, and there is no flood on this day. The red line denotes the CNR of DOY 201, 2021, and there is a flood on this day. The green line denotes the elevation angle. From Figures 8 and 9, it is obvious that the CNR of DOY 201 performs significantly worse than the CNR of DOY 199, 2021, and the decrease increases as the satellite elevation angle increases. Thus, the CNR of the G27 satellite at the low satellite elevation angle is hardly affected. The reason for this phenomenon is that the multipath error at the low satellite elevation angle is larger than that at the high satellite elevation angle in the normal environment; thus, it is relatively less affected during the flood.  
Figure 8: Results of the CNR of G27 satellite at L1 frequency on DOY 199 (blue line, no flood) and DOY 201 (red line, with the flood), 2021. The green line denotes the satellite elevation angle.  
Figure 7: Results of consistency analysis of fitted CNR of different stations and satellites on DOY 200 (blue dots) and DOY 201 (red dots), 2021. R is the correlation coefficient of CNRs between these two days.  
Moreover, the histogram of CNR and relative frequency of G27 satellite on L1 frequency is demonstrated in Figure 10. Based on the range of CNR values, three categories are used to divide the CNR. From Figure 10, it can be found that the CNR lower than 43 dB-Hz on DOY 199 is more less compared to DOY 201, 2021. On the contrary, the CNR larger than 43 dB-Hz on DOY 199 is more than that of DOY 201. This phenomenon indicates that the CNR of DOY 201 performs decrease during the flood. Thus, based on the results of Figure 10, it can be concluded that the CNR of GPS satellite will decrease under the influence of flood.  
In addition, it should be pointed out that the above phenomenon not only appeared for the GPS G27 satellite at L1 frequency but can also be observed in other satellites and frequencies. In addition, the CNR of other GNSS satellites, such as GLONASS satellites, is also seriously affected during flooding [27]. Moreover, the decrease in the CNR during the flood is not caused by other aspects, such as rain, receiver hardware, satellite malfunction, etc. The above conclusions have been investigated in depth by [PERSON] et al. [20]. In conclusion,  
Figure 10: Histogram of the CNR and relative frequency for G27 satellite L1 frequency.  
Figure 9: Results of the CNR of G27 satellite at L2 frequency on DOY 199 (blue line, no flood) and DOY 201 (red line, with the flood), 2021. The green line denotes the satellite elevation angle.  
the effect of flooding on the CNR of the GPS signal is significant. Thus, it is feasible to retrieve the whole flood process by using the fluctuation in CNR.