# Analysis of the Influence of Flood on the L4 Combination Observation of GPS and GLONASS Satellites
## 4 Experiment Analysis and Results
### Influence of Flood on L4 for GPS Satellites
#### 4.1.2 Influence of Flood on GPS L4 Observation  
The comparison results of L4 residuals on G10, G18, G23, and G32 satellites between DOY 199 and DOY 202, 2021 are presented in Figure 6. The red line denotes the residuals of L4 on DOY 202 (with the flood), and the blue line denotes the residuals of L4 on DOY 199 (without flood). It can be seen that the fluctuation of L4 residuals on DOY 202 performs obviously higher than that of DOY 199 for these four satellites. To better demonstrate this phenomenon, the RMS of L4 residuals on DOY 199 and DOY 202 are given in each subplot. For example, the RMS of L4 residuals on the G10 satellite increases from 0.14 cm to 0.41 cm during the flood on DOY 202, and the increase rate of RMS reached approximately 192.86% compared with DOY 199. In addition, the 90% confidence intervals (CI) of the L4 residuals are also calculated. By comparing the CI for the same satellite on DOY 199 and DOY 202, it can be concluded that the range of fluctuation of the L4 residuals is significantly larger on DOY 202 than on DOY 199. It should be noted that, in order to eliminate the influence of random noise, only data sets which elevation between 30\({}^{\circ}\) and 90\({}^{\circ}\) are used for calculations and comparisons. Based on the above analysis, it can be drawn that the presence of floods increases the fluctuations of the L4 residuals for these satellites. To better demonstrate the difference in L4 residuals between DOY 199 and DOY 202, 2021, the histogram of the amplitude of L4 residuals and relative frequency are shown in Figure 7. The residuals of L4 combination observation can be divided into four categories. From Figure 7, it can be observed that the distribution of L4 residuals on DOY 199 and DOY 202, 2021 is significantly different from each other. The residuals of L4 which are larger than 0.25 cm on DOY 202 are more than that of DOY 199. The maximum difference is approximately 0.35. Considering that the total number of G10 satellite epochs is 754, thus, the difference between them can reach approximately 261 epochs, which accounts for 34.60% of the total epochs. Thus, it can be concluded that the residuals of L4 on DOY 202 fluctuate more significantly than the residuals of DOY 199. In addition, it should be noted that the same phenomenon can also be found in other satellites. Figure 6: Residuals of L4 on G10, G18, G23, and G32 satellites. The blue line denotes the L4 residuals collected on DOY 199 (without flood), and the red line denotes the L4 residuals collected on DOY 202 (with flood). In order to further analyze the influence of flood on L4 of GPS satellites, the RMS increase rate of other GPS satellites is presented in Figure 8. The RMS increase rate can be expressed by the following equation:  
\[\text{RMS increase rate}=\frac{\text{RMS}_{202}-\text{RMS}_{199}}{\text{RMS}_{199}} \tag{7}\]  
where RMS\({}_{202}\) refers to the RMS of the L4 residuals of a certain satellite on DOY 202; RMS\({}_{199}\) refers to the RMS of the L4 residuals of a certain satellite on DOY 199. Figure 8: RMS increase rate of L4 residuals on DOY 202 (with the flood) of GPS satellites compared with DOY 199 (without flood), 2021. Figure 7: Histogram of L4 combination observation residuals and relative frequency. The blue and red bars denote the L4 collected on DOY 199 and DOY 202, 2021. \(\mid-\),25) denotes that the amplitude of L4 residuals is lower than 0.25 cm. 10.75,+) means that the amplitude of L4 residuals is larger than 0.75 cm. Symbol 1 and) denote the inclusion and non-inclusion, respectively. It should be noted that to ensure the accuracy and convenience of data processing, the satellites selected here are all satellites with high elevation angles and continuous observations. It can be seen that the residuals of L4 of all these satellites on DOY 202 increased to a certain degree compared with DOY 199. The average RMS increase rate is approximately 109.53% for all GPS satellites.