## 7 Conclusions  
The CNR is an index of the strength of the GNSS signal, which is related to the direct and indirect signals. The multipath effect caused by floods will result in an attenuation of received signals by the GNSS receiver. Thus, the MP and CNR will be affected during a flood. The relationship between the CNR and GNSS signal is introduced, and the reason why the flood will lead to the decline of the CNR is analyzed theoretically. The MP combination model is presented. In addition, the ground track repetition period of the GPS, GLONASS, and BDS satellites for a static observation stations with the same elevation and azimuth were investigated. The skyplot was used to verify the validity of the repeat period. Real data set results showed that the ground track repetition period of the GLONASS satellite is eight days, and the BDS MEO satellite is seven days. Thus, to analyze the influence of floods on GLONASS and BDS MEO satellites, the data set collected on eight and seven days prior to the flood(s) should be used. Two real flood cases studies occurred in Zhengzhou and Xinxiang cities, China, 2021, and were used to analyze the influence of floods on the GNSS signal. The pseudo-range multipath of the GPS satellite during floods was analyzed, and results showed that the RMS of the MP during floods performed more seriously than on the days without a flood. The maximum RMS increase rate is approximately 17.85%, and the average of all other satellite is approximately 6. 55%. In addition, experimental results demonstrated that floods can decrease the CNR of GPS and GLONASS satellites, especially when the elevation is high. The reason for this phenomenon is that the multipath effect is serious at low elevations, and the multipath effect caused by floods can hardly influence the CNR. By contrast, when the elevation is high, the multipath without floods is much smaller than that of with floods. Thus, the multipath effect caused by floods has more influence. In terms of the BDS system satellite, although the influence of floods on three orbital-type satellites is slightly different from one another, the CNR of all them declined during the flood as a whole. In conclusion, based on the analysis of GPS, GLONASS, and BDS system satellites, it can be concluded that floods can lead to an increase of pseudo-range multipaths and  
Figure 16: CNR of BDS GEO B01 satellite B1 frequency. The blue and red dots denote the CNR collected on DOY:200 (no flood) and DOY:203 (with flood), 2021, respectively. induce a decrease of the CNR on the GNSS signal to a certain degree. Considering that the accuracy of positioning is related to the quality of the signal, whether the accuracy of GNSS positioning will be affected during flood, whether the elevation angle weight strategy can still be used during a flood, and whether the CNR can be used to detect the entire flood process should be answered in future research. Conceptualization and writing, M.S. ([PERSON]); data curation, X.C. and F.Z. ; methodology, J.S. ; software, L.Q. ; validation, X.T. and M.S. ([PERSON]); All authors have read and agreed to the published version of the manuscript. This research was funded by Zhejiang Provincial Natural Science Foundation of China grant number No. LQ22D040001 and GK218815299009. The data set used in this study was collected from the Infrastructure of National Earthquake Data Center, China and Changxin teams. Readers in China can apply for the data set from the agency 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). In addition, the data set can also be made available by contacting the corresponding author. We appreciate the authors [PERSON] and [PERSON] for providing the data set. We also appreciate the reviewers and editors' valuable comments to improve the paper's quality. In addition, compared with the people which affected by the flood, the impact of GNSS is negligible.