# Rain Monitoring with Polarimetric GNSS Signals: Ground-Based Experimental Research
## 1 Introduction  
Rain, a common weather phenomena, reflects the physical and chemical processes of atmospheric dynamics, thermodynamics and water vapor cycle [1]. It also plays an important role in transportation of atmospheric energy and global hydrologic cycle. However, its changing spatial and temporal distribution often leads to natural disasters such as floods and droughts [2]. Therefore, its forecasting, monitoring, and estimation is crucial for the research of scientific issues such as atmospheric dynamic analysis, numerical weather forecasting, weather and climate change, global water balance, hydrological model construction, meteorological disaster forecasting, flood disaster monitoring, and agricultural flood control et al. [3; 4; 5]. Traditionally, rain monitoring mostly relies on rain gauge, weather radar and satellite remote sensing [6]. Obviously, rain gauge, which is widely utilized, can provide accurate \"point\" measurement. The network of many rain gauge stations can be used to get regional rain rate by interpolation [7]. Regional rain information can also be obtained from weather radar based on its parameters such as radar reflectivity (\(Z\)), differential reflectivity (\(Z_{DR}\)) and specific differential phase shift (\(K_{DP}\)) [8; 9; 10]. Especially, these radar polarization parameters, \(Z_{DR}\) and \(K_{DP}\) offer more microscopic information about rain [11]. Additionally, remote sensing data from spaceborne payloads such as infrared/visible sensors on Geosynchronous Earth Orbit satellites, passive microwave sensors and precipitation radar on Low Earth Orbit satellites has capable of yielding rain information [5; 12]. Then, some spaceborne rain products including the Climate Prediction Center Morphing technique product (CMORPH) [13], the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis [14], Global Satellite Mapping of Precipitation (GSMaP) [15] are produced. Recently, several novel methods based on microwave link signals are being developed to achieve rain information. When propagating though rain area, microwave link signals, usually come across attenuation and depolarization, which is a problem for communication. However, it is beneficial for meteorology. Some researchers have found it is possible to estimate rain rate using rain-induced attenuation [16; 17]. Path-averaged rain rate was studied in theory and experiments according to the relationship between microwave attenuation and rain rate [16]. The feasibility of monitoring rain using wireless communication networks was investigated by [PERSON] et al. [18]. Based on the cellular wireless network covering all parts of the Netherlands, more than 2400 links of rain attenuation were obtained through experiments, and country-wide rainfall maps were retrieved [19]. A microwave link experiment was also set up to assess several error sources associated with rain rate estimation from cellular communication networks [20]. Besides, in 2018, the feasibility of rain detection over oceans by Global Navigation Satellite System (GNSS) reflectometry observations was firstly investigated at weak winds [21]. Rain-induced depolarization can also be used to obtain rain information. A polarimetric concept that uses the depolarization of GNSS signals to detect heavy rain was first proposed by [PERSON] et al. [22]. Then, they did some theoretical research to assess the sensitivity of GNSS signals for remote sensing of heavy rain [23]. A field campaign was also carried out to investigate the detectability of polarimetric phase shift and the feasibility of estimating rain information [24]. These research intended to solve some problems for the the Radio-Occultation and Heavy Precipitation (ROHP) experiment aboard the Spanish Earth Observation satellite PAZ. [PERSON] and [PERSON] et al. [12; 25] also conducted some research on the physical process of GNSS signals propagating through rain-filled medium and the relationships between cross-polarization discrimination, polarimetric phase shift and rain rate, respectively.