## 4 Experiment Results and Analysis
### Retrieve the Rough Process of the Flood by Using All Original CNRs  
In order to demonstrate that the original and fitted CNRs are consistent before and after the flood, the original and fitted CNRs on DOY 198 (blue), DOY 199 (red), and DOY 203 (green) are presented in Figure 12. The original and fitted CNRs are denoted by dots and lines, and this convention applies to all the following figures. Five satellites are selected to cover the whole day. It can be seen that the trends of the original and fitted CNRs on these three days are almost the same as each other. Although some epochs are different from each other at low elevation angles, this is mainly caused by random noise. The average correlation coefficient of fitted CNRs between these three days is also demonstrated, which is denoted by R\({}_{3}\). It can be found that the R\({}_{3}\) of all these satellites is nearly 0.99, which further verifies the consistency. In conclusion, the surrounding environment of the observation station is similar and stable for these three days. Thus, it can be used as a reference since all of these three days are not affected by the flood.  
In Figure 13, the results of the original and fitted CNRs on DOY 199 and DOY 200, 2021 are demonstrated. The red dots and blue dots mean the original signal CNR of DOY 199 and DOY 200, 2021. The red line and blue line denote the fitted CNR of DOY 199 and DOY 200, 2021. Five satellites cover the whole day. Based on the fitted CNR, it is obvious that the CNR decreases after 16:00 pm on DOY 200. Prior to this time, the original and fitted CNRs on the G21, G06, and G23 satellites are almost the same as each other. Thus, it can be retrieved that the flood first occurred in the afternoon of the DOY 200, 2021. This conclusion can be verified in Figure 14, which demonstrates the results of a consistency analysis of the CNRs of the original and direct signals on DOY 200 and DOY 201, 2021. In addition, it is obvious that the CNRs of G21, G06, and G23 on DOY 200 are larger than on DOY 201, 2021. However, the CNRs of G16 and G08 are almost the same as each other.  
Figure 12: Results of consistency analysis on original and fitted CNRs on DOY 198, DOY 199, and DOY 203, 2021. The R\({}_{3}\) denotes the average correlation coefficient of CNRs between these three days.  
Therefore, the above results demonstrate that the flood occurred on DOY 200 and extended to DOY 201.  
Figure 15 demonstrates the original and fitted CNRs of DOY 201 and DOY 202, 2021. Since it is confirmed that there exists a flood on DOY 201, the subsequent fluctuation in the flood can be directly obtained by comparing DOY 201 with DOY 202. From Figure 15, it is clear that the CNR of DOY 201 on the G06 satellite is larger compared to DOY 202, 2021. This indicates that the effect of flooding on the CNR for DOY 202 is larger than that for DOY 201, and this means that the flood on DOY 202 is more serious than on DOY 201 at this stage. In addition, it can also be found that the CNR of DOY 201 on the G23, G16, and G08 satellites is lower compared to on DOY 202, 2021. This phenomenon shows that the flood begins to subside compared with the morning on DOY 202. Thus, it can be concluded that the flood peak appeared on the morning of DOY 202.  
It is worth noting that the above analysis can only retrieve a rough trend of the flood since all of the CNRs are used in the process of retrieval. As is well known, the CNR at low elevation angles is largely affected by random noise, and this will reduce the sensitivity of CNR to the change in the surrounding environment.  
Figure 14: Results of consistency analysis of the CNRs of the original and direct signals on DOY 200 (red line and dots) and DOY 201 (blue line and dots), 2021.  
Figure 13: Results of consistency analysis on original and fitted CNRs on DOY 199 and DOY 200, 2021.