## 6 Influence of Floods on GPS, GLONASS, and BDS CNR
### Influence of floods on BDS with 1 s Data Set
#### 6.4.2 BDS IGSO and GEO Satellite  
For BDS IGSO and GEO satellites, their ground track repeat periods are sidereal days. Thus, to analyze the influence of floods on the CNR of BDS IGSO and GEO satellites, the data set collected one day prior to the flood should be used. However, there was localized flooding on DOY:201 and DOY:202, which could have affected the accuracy of the comparison results. Thus, the data set collected on DOY:200 is used in this experiment. Figure 15 demonstrates the comparison results of the CNR of the BDS IGSO B13 satellite on DOY:200 and DOY:203, 2021. The green and yellow lines denote the fitted results of the original CNR on DOY:200 and DOY:203. It can be seen that the difference of CNR between DOY:200 and DOY:203 is mainly concentrated on low and high elevations. For example, the CNR slightly decreases when the elevation is lower than 20 deg and larger than 65 deg. In addition, it can be found that the fluctuation of the B13 IGSO satellite performs more severely than the B20 MEO satellite. This phenomenon may be related to the characteristics of the satellite's track. The orbit height of the BDS IGSO satellite is much higher than that of the MEO satellite, which is approximately 3.6 \(\times\) 10\({}^{4}\) km away from the surface of the earth. The relationship between signal attenuation and propagation distance is positively correlated. Thus, the BDS IGSO satellite performs more severe fluctuations than the BDS MEO satellite. Figure 14: CNR of BDS MEO B20 satellite B1 frequency comparison results with elevation as the horizontal axis. The blue and red dots denote the CNR collected on DOY:196 (no flood) and DOY:203 (with flood), 2021, respectively. The green and yellow lines are the fitted results of CNR on DOY:196 and DOY:203. The comparison result of the BDS GEO B01 satellite B1 frequency is presented in Figure 16. It should be noted that since B01 is a geostationary satellite, the elevation angle of the satellite remains almost constant. Thus, the time series of the CNR is used for comparative analysis. Theoretically, the B01 satellite can be observed all day. However, there is no observation data in many epochs because of the observation environment and low-cost receiver, especially during the flood on DOY:203. Thus, in order to ensure the reliability of the analysis' result, only the same epochs on DOY:200 and DOY:203 are selected for comparison. From Figure 16, it can be seen that the CNR on DOY:203 is lower than that of on DOY:200 in nearly two thirds of the epochs. The amplitude of decrease is approximately 1 dB-Hz. In some of the epochs, the CNR on DOY:200 is lower than that of on DOY:203. For example, the CNR on DOY:200 is lower than DOY:203 by around \(2.0\,\times\,10^{4}\) and \(3.8\,\times\,10^{4}\). This phenomenon is caused by environmental changes and is seen as more of a random fluctuation. Overall, the influence of floods on the BDS GEO satellite can be observed, and the CNR is decreased because of the influence of floods as a whole. In addition, when comparing Figures 15 and 16 with Figure 14, it can be found that the CNR of the B13 and B01 satellites is lower than that of the B20 satellite as a whole. The reason for this phenomenon is that the B13 and B01 belong to IGSO and GEO satellites, respectively, which are approximately \(3.6\,\times\,10^{4}\) km away from the earth's surface. However, for the BDS MEO satellite, the height of the orbital is only \(2.2\,\times\,10^{4}\) km. Thus, the signal attenuation of the MEO satellite is smaller than that of the IGSO and GEO satellites under the same signal power. In addition, the CNR of the B01 satellite fluctuates in a range from 32 dB-Hz to 38 dB-Hz, but is concentrated on 35 dB-Hz. This is because the B01 satellite is stationary, and the strength of the signal is relatively stable compared with IGSO and MEO satellites. Moreover, from the relationship between the CNR and the elevation of the B13 and B20 satellites, it can be found that the CNR has a positive correlation with the elevation angle.