## 3 Proposed Method
### Stability Analysis of Satellite Ground Repeat Period and Original CNR  
In order to demonstrate that the characteristic of the CNR of GPS satellites performs almost consistently on adjacent days for static observation stations, real data sets collected from 32 globally distributed MGEX stations were used. The sample interval of data sets was 30 s, and the elevation cut-off sets were 5 deg. There were nine types of geodetic receivers, which include the Trimble, Leica, etc. It should be pointed out that the stations used were randomly determined and we tried to cover as many latitudes and longitudes as possible. The used MGEX stations are shown in Figure 2.  
Figure 2: Distribution of 32 MGEX stations (red dots) used in this experiment. Six satellites, which were randomly selected from thirty-two MGEX stations, were used to analyze the orbital repetition period for GPS satellites, and the results of the skyplot are presented in Figure 3. The blue line means the results of GPS satellites on DOY 200, 2021. The pink line denotes the results of GPS satellites on DOY 201, 2021. It is clear that the elevation (denoted vertically) and the azimuth (denoted by the circle) of these six GPS satellites remain the same as each other between these two adjacent days. This phenomenon can not only be observed in satellites with continuous observation arcs, such as G04 and G06 satellites, but can also be observed in discontinuous observation arcs, such as those of the G10 and G30 satellites. The above results further show that the orbital repetition period of the GPS satellite is nearly a sidereal day. Thus, the CNR of GPS satellites should remain consistent between two adjacent days for static observation stations if there is no change in the surrounding environment. The CNR on two adjacent days for some GPS satellites and stations is presented in Figure 4. The blue dots mean the CNR of DOY 200, 2021. The red dots mean the CNR of DOY 201, 2021. The CNR of these satellites on DOY 200 and DOY 201 are almost identical to each other since the blue dots significantly overlap the red dots. Even though there will be obvious fluctuations at the low elevation angle (both ends of the sequence), the trend of these two CNR series means they perform consistently with each other as a whole. In  
Figure 3: Results of ground track of 6 GPS satellites randomly selected from 32 MGEX observation stations. The blue and pink lines denote the DOY 200 and DOY 201, 2021, respectively. addition, to further demonstrate this phenomenon, the difference between these two CNR series is presented by the pink line. It is obvious that most of the differences are lower than 1 dB-Hz, and this decreases as the elevation angle or amplitude of CNR increases. Furthermore, the RMS of the difference is presented in each subplot. The maximum RMS is only approximately 0.56, which appeared on G06 at abmf station. This indicates that the difference between them is very small. In Figure 5, the RMS of the CNR difference between these two days on other satellites and stations is presented. It is obvious that almost all RMS values are rather small. Concretely, the average RMS is approximately 0.62. Among them, the minimum RMS is from the G25 satellite, which is merely approximately 0.32. Even for the G27 satellite, which has the highest RMS, it is still only approximately 0.95. Thus, on the basis of the above results, we found that the CNR remains almost consistent on two adjacent days at the same elevation and azimuth levels when the observation station is static. Considering that the flood usually lasts several days, the CNR on several adjacent days was compared by using the data set collected from jplm station from DOY 196 to DOY 200, 2021. The comparison results are demonstrated in Figure 6. From Figure 6, it can be  
Figure 4: CNR of GPS satellites on two consecutive adjacent days at L1 frequency. The pink line is the difference in CNRs between these two days. The blue dots mean the CNR of DOY 200, 2021. The red dots mean the CNR of DOY 201, 2021.