# Rain Monitoring with Polarimetric GNSS Signals: Ground-Based Experimental Research
## 3 Results and Discussion
### Statistical Results of G27 Satellite in 2015  
#### 3.1.1 Phase Shift in No-Rain and Rainy Days  
No-rain days are judged by the weather radar echo and the weather record. It refers to the day when the nearby stations have no record of rain during the observing period and the weather radar reflectivity of GNSS ray crossed under 1 km is 0 dBZ. Considering the availability and integrality, data from G27 is processed based on the above algorithms (Equations (4)-(8)). For our observation site, this satellite is on a descending trajectory, which makes the receiver less lock lose at the beginning of being tracked. After converting time series into elevation series, it is found that some days' data suffers from lock lose at lower elevations. In fact, lower elevation can induce more serious phase shift [26]. Thus, in order to do statistical analysis, data with elevations ranging from 1.5\({}^{\circ}\) to 21.5\({}^{\circ}\) is selected. It is shown that usable data comes from 72 days, of which four are rainy days. In addition, daily data lasts 69 min in this elevation range. By computing the data from no-rain days, the average \(\overline{\Delta m}\) and standard deviation \(\sigma_{\Delta m}\) can be obtained, which can be seen in Figure 4.  
From Figure 4, it can be seen that the average \(\overline{\Delta m}\) (\"black+\" line) varies with the elevation. The 1 and 2\(\sigma_{\Delta m}\) thresholds are marked in dark grey and light grey, respectively. Since the carrier phase is recorded by units of length, the phase delay is obtained in mm, which is more convenient to analyze the relationship between phase delay and rain rate. The standard deviation is less than 4 mm, ranging from 1.42 mm to 3.84 mm, which indicates that the data on no-rain days has good consistency. This also implies that the multipath effect on G27 is basically stable. Furthermore, the mean of standard deviations is 2.32 mm, which is consistent with the experimental results of [PERSON] et al. [24].  
Figure 5 presents the variations of corrected phase shift with elevation (\"black+\" line), whose value can be seen according to the left vertical axis. Additionally, the \(\Delta\phi_{+}(\beta)\) is shown by red line and its value is based on the right vertical axis. It is obvious that their \(\Delta\phi_{C}(\beta)\) are both within \(\pm 2\sigma\), even their majority are in the range of \(\pm 1\sigma\). Therefore, the \(\Delta\phi_{+}(\beta)\) are all zero, which is well matched with the fact that there is no rain during the observation period. Due to space limitation, we only take these two cases for example. But the other no-rain days' data has the same feature.  
Figure 4: Statistical characteristics of G27 data in all no-rain days.  
Phase shift of two rainy days is reported in Figure 6. Since some corrected phase shifts are beyond 2\(\sigma\), phase shift \(\Delta\phi_{+}(\beta)\) are not all zero, which can be seen in red line. In addition, its maximum is more than 5 mm. By verification, it is found that there are precipitation clouds on the GPS signals' passing area during their receiving periods (13:46-14:55 on 6 July 11:24-12:33 on 10 August). This reveals preliminarily that signals of G27 is probably affected by raindrops, resulting in creating phase shift, which still requires further research.