## 4 Ground Track Repeat Period of GPS, GLONASS, and BDS Satellites
### Results Analysis of Ground Track Repetition Period  
Considering that the BDS GEO satellite is a geosynchronous earth orbit satellite, the ground track of the GEO satellite remains almost stationary; therefore, only GPS, GLONASS, BDS IGSO, and MEO satellites are analyzed in this section. In addition, since the ground track repetition period of GPS and BDS IGSO is a sidereal day, both of them are analyzed together for simplicity. In order to be consistent with the subsequent analysis of the influence of floods, the data set collected during the flood(s) is used in this section. The ground track repeat period of the GPS G32 satellite is presented in Figure 4. The blue and pink lines denote the DOY:200 and DOY:201, 2021, respectively. The vertical denotes the elevation angle, and the circle represents the azimuth angle. The width of the pink line is half as wide as the blue line to make it clear that these two lines are almost exactly overlapped. This convention applies to all of the skyplots. From Figure 4, it can be seen that the elevation and azimuth of the G32 satellite remain identical to each other for two adjacent days. This result indicates that the ground track repetition period of the GPS satellite is a sidereal day, which is consistent with the theoretical results. Similar to the GPS G32 satellite, the ground track repeat period of the BDS IGSO satellite is also a sidereal day. Figure 5 demonstrates the ground track repeat period of the BDS IGSO B16 satellite. It can be found that the elevation and azimuth of the B16 satellite perform identically to each other for two adjacent days. Thus, in order to analyze the influence of floods on GPS and BDS IGSO satellites, two adjacent days before and after the flood should be used. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Constellation** & **i** & **\(T\)** \\ \hline GPS & \(55^{\circ}\) & 11 h 58 m \\ GLONASS & \(65^{\circ}\) & 11 h 16 m \\ Galileo & \(56^{\circ}\) & 14 h 05 m \\ BDS MEO & \(55^{\circ}\) & 12 h 53 m \\ BDS IGSO & \(55^{\circ}\) & 23 h 56 m \\ BDS GEO & \(0^{\circ}\) & 23 h 56 m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: General characteristic of GNSS orbits modes (i: inclination, \(T\): orbital period). Different from GPS and BDS IGSO satellites, the ground track repeat period of the GLONASS satellite is eight sidereal days. The satellite ground track repeat period of the R14 satellite from the perspective of the skyplot is demonstrated in Figure 6. The brown and blue lines denote the R14 satellite on DOY:200 and DOY:201, 2021, respectively. It can be seen that the elevation and azimuth of the R14 satellite on DOY:200 is totally different from DOY:201. Thus, the multipath of these two days will be different for the R14 satellite. This is completely different from the GPS satellite. For the GPS satellite, the elevation and azimuth will be repeated in two consecutive days if there is no orbital maneuvering [11]. The pink line denotes the elevation and azimuth of the R14 satellite on DOY:193. Thus, to investigate the influence of floods on the GLONASS satellite, the data set collected eight days prior to the flood should be used. Figure 4: Skyplot of the GPS G32 satellite on DOY:200 (denoted by blue line) and DOY:201 (denoted by pink line), 2021. The vertical means the elevation angle, and the circle represents the azimuth angle. Figure 5: Skyplot of the BDS IGSO B16 satellite on DOY:202 (denoted by blue line) and DOY:203 (denoted by pink line), 2021. The vertical means the elevation angle, and the circle represents the azimuth angle. To illustrate the repeat period of the BDS MEO satellite, the skyplot of the B20 satellite on DOY:196, DOY:202, and DOY:203, 2021 is presented in Figure 7. The brown and pink lines denote the trajectory of the B20 satellite on DOY:202 and DOY:203, respectively. The elevation and azimuth of the B20 satellite on DOY:202 is totally different from DOY:203, even though they are adjacent for two days.