# Rain Monitoring with Polarimetric GNSS Signals: Ground-Based Experimental Research
## 4 Conclusions  
This paper continues to study the feasibility of rain monitoring from polarimetric GNSS signals by performing a ground-based experiment. Firstly, based on previous theoretical research [12; 25], the experiment was designed, including experimental equipment, conditions, site, and time. Especially, the key instrument, a conical horn dual-polarized GNSS antenna, was developed and used for the first time. Then, to solve some problems such as quality control, unlocking, hardware effect, phase ambiguity, multipath effect, some methods were provided successively mainly following [PERSON] et al. method [24], which has been applied to this ground-based experimental data. Since the multipath effect was inevitable for ground-based experiment, the method to eliminate its effect and extract phase shift proposed by [PERSON] et al. [24] was used. According to this data processing algorithm, experimental data in 2015 and 2016 was processed to obtain phase shift in rainy days. Finally, the phase shift was validated by weather radar and weather station data. Figure 15: Cases of accumulated phase shift from different GPS satellites compared with accumulated rainfall observed from four nearby weather stations: (**a**) G1 in Oct. 2; (**b**) G3 in Sep. 15; (**c**) G3 in Nov. 7; (**d**) G9 in Nov. 7; (**e**) G11 in Oct. 2; (**f**) G13 in Oct. 21; (**g**) G22 in Nov. 7; (**h**) G23 in Nov. 7; (**i**) G24 in Oct. 21. By analyzing data from 2 GPS satellites in 2015 and 12 GPS satellites in 2016, the results show that phase shift doesn't exist on no-rain days. Furthermore, the standard deviations of the term \(\Delta m\) induced by multipath effect are small, which is well matched with the results of [PERSON] et al. [24]. Fortunately, the experimental system catches some rainy cases and the rain-induced phase shift is obtained. These results verifies the feasibility of this experimental system and the data processing algorithm. They also reveal that phase shift can be observed in rainy days. Compared with radar data, it is found that there are radar echoes along the GPS ray when phase shift occurs. In addition, by examining individual cases, the trends of accumulated phase shift and accumulated rainfall from one or more rain gauges are very similar, with some correlation coefficients up to 0.99. However, accumulated rainfall retrieved from radar reflectivity shows a more consistent trend with accumulated phase shift. This is due to the fact that rain gauge is just a point while radar can match with GPS rays in space. These comparison further demonstrates the reliability of the obtained phase shift. Since this rain monitoring method is still in an initial phase, more research can be carried out. In this ground-based experiment, due to the effect of phase ambiguity and multipath effect, the phase shift is just relative, not absolute. Therefore, a spaceborne experiment is necessary, in which the phase ambiguity and multipath effect are easy to be solved. Our research here can provide experiment system and data processing methods for the future spaceborne experiment. Conceptualization, H.A. and W.Y. ; Data curation, S.B. ; Formal analysis, H.A. and S.B. ; Investigation, W.Y. ; Methodology, H.A. and S.M. ; Validation, S.B. and S.M. ; Writing--original draft, H.A. ; Writing--review and editing, W.Y. This research was supported by the National Natural Science Foundation of China (Grant Nos. 41805026 and 41505016) and the State Key Laboratory of Geo-Information Engineering (Grant No. SKLGIE2019-ZZ-2). The authors are thankful to [PERSON] and her team for their advice on the processing of experimental data. The authors would like to express their thanks to National Space Science Center, Chinese Academy of Sciences (NSSC/CAS) for technical support on the experiments. Many thanks to the National Weather Information Center for providing the meteorological data.