## 1 Introduction  
Using the attenuation of electromagnetic signals to measure rainfall has received much attention recently. This approach is strongly supported by the theoretical basis for calculating attenuation due to the water content along a signal path, which has been established and validated by numerous propagation tests [1, 2]. In 1991, it was suggested that custom-designed microwave links could be used for the reconstruction of rainfall fields [3]. The usage of the backhaul links of commercial wireless communication networks (CWCNs) for rainfall monitoring was first proposed by [PERSON] et al. in 2006 [4] and then by [PERSON] et al. in 2007 [5]. Since then, this technique has been improved rapidly through extensive studies (e.g., [6, 7, 8, 9]). The Earth-space links of commercial geostationary Earth orbit (GEO) satellites have also been studied to serve the same purpose. It has been demonstrated that the GEO system is capable of path-integrated rain rate estimation [10, 11, 12], and that the estimation has a high accuracy for heavy rain events [13]. The recent development in large constellations of low Earth orbit (LEO) satellites for global broadband services hints at a new opportunity for rainfall measurements. Several companies are pursuing large constellations of LEO satellites to provide global broadband access, e.g., Starlink's system with more than 10,000 satellites and Telesat's system with 300 satellites [14]. Once the satellite broadband service is available to the public, it can be foreseen that millions of user terminals (ground receivers) will be deployed across the globe. A dynamic network of satellite-to-ground links will be formed on an unprecedented scale. It will become a very large sensor network that could provide off-the-shelf signal attenuation data for the observations of the atmosphere. Meanwhile, because of the increasing demand for multimedia applications and thus the requirement of high data rates, satellites broadband services need to use Ku (12-18 GHz), Ka (26.5-40 GHz), and/or Q/V (40-50 GHz) frequency bands [15]. Signals at these frequencies are severely attenuated by gas, cloud and rain in the atmosphere [16]. They are especially sensitive to liquid water, which indicates that measuring precipitation through them can be a valid approach. The opportunistic use of satellite communication links can provide global real-time observations at a very low cost. One of the key advantages of using a LEO satellite system for rainfall estimation is that it can offer three-dimensional (3-D) rain field tomography. First proposed by [PERSON] et al. in 2016 [17], this approach exploits the similarity in the revolutionary motion between the LEO satellites and the signal source in computed tomography (CT) [18] and performs tomographic reconstruction of rain attenuation fields. Various investigations have been carried out recently to further examine this approach. For example, it has been suggested that 3-D attenuation fields can be retrieved by using the estimated signal-to-noise ratio (SNR) at the ground receivers and an iterative method to estimate the sky noise [19]. Furthermore, some signal processing strategies have been explored to optimize the performance of the rain field retrieval [20; 21; 22]. The attenuation of microwave links also provides the opportunity for raindrop size distribution (DSD) measurements. Knowledge of the DSD is fundamental to quantitative precipitation estimation and realistic DSD representation is very important to atmospheric research. Studies have suggested that the DSD is best modeled by a gamma distribution [23], which is in the following form:  
\[N(D)=N_{0}D^{\mu}e^{-\Lambda D}, \tag{1}\]  
in which \(D\) is the equivolumetric raindrop diameter in mm, \(N_{0}\) is the intercept parameter in mm\({}^{-1}\cdot\)m\({}^{-3}\), \(\mu\) is a dimensionless shape parameter and \(\Lambda\) is the slope parameter in mm\({}^{-1}\). One straightforward approach to estimate the three parameters is through three different attenuation measurements.