## 5 Conclusions  
Taking advantage of the influence of flooding on GPS CNR, a new method to retrieve the whole flood process is proposed. The theory of the multipath effect and satellite ground track repeat period is presented, and the relationship between the multipath error and the CNR is introduced. In order to evaluate the feasibility and rationality of the developed method, real data sets collected from 32 MGEX stations distributed around the world are used. The orbital repetition period of the GPS on ground satellites is analyzed by skylplot. The stability of original GPS and fitted signal CNR is demonstrated. The results show that the average RMS of the original CNR difference between two adjacent days is approximately 0.62. In terms of fitted CNR, the coefficients between two adjacent days can reach approximately 0.99. Thus, the flood can be retrieved by using the fluctuation in CNR between two adjacent days.  
Real data sets collected during the flood in Zhengzhou City, China, were adopted to evaluate the performance of the developed method. Experimental results indicate that the rough flood process can be effectively retrieved by using all original and fitted CNRs. The flood appeared on DOY 200 and reach a peak on DOY 202. However, the accuracy of retrieval is influenced by CNR at a low elevation angle since the random noise is more serious at a low satellite elevation angle. Thus, to retrieve the accurate process of the flood, only CNRs larger than 40 dB-HZ or elevation angles larger than 45 deg were used. By comparing the fitted CNRs of two adjacent days, it can be found that the flood started at 15:30 pm on DOY 202 and reached a crest at approximately 8:30 am on DOY 202. After that, the flood disappeared at 10:00 am on DOY 202. However, how to find the critical point at which the CNR will be influenced by flooding, and improve the time resolution of the retrieval, should be further studied.  
[PERSON] worked out technical details. [PERSON] proposed the idea and wrote the main manuscript. [PERSON] designed the experiments. [PERSON] and [PERSON] drew the figure. [PERSON] and [PERSON] provided the data set. All authors have read and agreed to the published version of the manuscript.  
This research was supported by Zhejiang Provincial Natural Science Foundation of China. The founder is [PERSON]. The funding number is No. LQ22D040001.  
Figure 19: Results of consistency analysis on original and fitted by selected CNRs on DOY 202 and DOY 203, 2021. The subplot denotes the enlarged view of the critical point.  
Data Availability Statement: The data set used in this study is collected from the Infrastructure of National Earthquake Data Center, China. Readers in China can apply for the data set from the agency of Crustal Movement Observation Network of China on their own, and the website is: [[http://data.earthquake.cn](http://data.earthquake.cn)]([http://data.earthquake.cn](http://data.earthquake.cn)), (accessed on 20 July 2021).  
I ([PERSON]) appreciated the author [PERSON] for providing the data sets, and hope he can get married and settle down in the near future. Thanks to the author [PERSON], [PERSON], and [PERSON] for working out technical details and drawing the figure and hope they can pass the exam of postgraduate. I also want to appreciate my wife [PERSON]. Our wedding was postponed due to this flood, and thanks for her understanding and support. Finally, thanks to everyone who fights for the flood, and may there is no more floods in the world.  
The authors declare no conflict of interest.