## 1 Introduction  
Regarding global warming, extreme weather has been occurring more frequently in recent years, especially floods and urban water logging caused by heavy rainfall. Floods or urban water-logging disasters can result in many serious disturbances to communities and the environment [1, 2]. Thus, many researchers have been dedicating their efforts to detect and forecast floods. However, the flood having an effect on the GNSS (Global Navigation Satellite System) signal and the mechanism of influence have not been researched in depth. The surrounding environment of the observation station is changed during a flood, and this change leads to a fluctuation of multipath effects in theory. [PERSON] et al. investigated the influence of different observation environments on the multipath effect [3]. The results show that the pseudo-range multipath (MP) with water around the observation station is increased for the BDS (BeiDou System) and GPS (Global Positioning System) satellites compared with the open sky. However, the water environment of this experiment only exists on one side of the observation station. Thus, it can be predicted that the multipath error will become more severe when a flood occurs, since the observation station will be surrounded by water. In addition, [PERSON] indicated that the reflection coefficient of fresh water is nearly three times larger than that of dry soil [4]. However, how much the change of environment affects the MP has not been investigated in depth, and neither has the under-flood environment. A multipath error is largely related to a CNR of GNSS signal. [PERSON] et al. developed an experiment using the CNR to detect a GPS multipath error first [5]. The relationship between the multipath error, double-difference residual of carrier phase observation, and CNR is analyzed [6]. Experimental results indicated that the CNR decreases as the multipath increases. Moreover, [PERSON] et al. demonstrated that the multipath residuals of the carrier phase maintains a strong relationship with the CNR, and the multipath residual is inversely proportional to CNR [7]. [PERSON] et al. analyzed the multipath error of the BDS marine measurement, and the results indicated that the CNR received in marine environments performs less effectively than that of on-land environments for most BDS satellites because of the multipath effect [8]. In addition, some of the previous literature has directly or indirectly verified the correlation between the multipath and CNR, such as [9]. However, the reason why the CNR will be affected by the change of environment has not been analyzed in theory. In order to ensure the accuracy of a performance comparison analysis between the same satellites, the orbital repeat period of a satellite with respect to the static observation station at the same elevation and azimuth should be investigated. [PERSON] et al. analyzed the ground track repeat period of the GPS satellite. The results show that the period is nearly a sidereal day [10]. Thus, to research the variety of characteristics of the GPS satellite, a data set collected on two consecutive adjacent days can be used. Although [PERSON] et al. pointed out that the satellite maneuver can lead to a fluctuation of the repeat period, the influence is on the order of minutes or hours, and it hardly affects the analysis with units in days [11]. [PERSON] et al. demonstrated that the GLONASS satellite runs around the earth seventeen times about every eight solar days [12]. However, how to estimate the satellite repeat period of GLONASS has not been introduced in theory. In terms of the Galileo IOV (In-Orbit Verification) and FOC (Full Operational Capability) satellites, both of them have a repeat period of about 10 days [13]. In addition, [PERSON] et al. indicated that Galileo E18 and E14 satellites were launched into incorrect orbits [14; 15]; thus, these two satellites repeat their ground tracks about every 20 days instead of every 10 days [16]. The ground track repeat period of the BDS system was researched by [PERSON] et al.