# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 1 Introduction
In work ([PERSON] et al., 2007), three well-known prediction models for the specific rain attenuation were used in order to calculate the attenuation of microwave links at different frequencies and path lengths. The models are namely the ITU-R, the Crane Global and Moupfouma model. Generally, the ITU-R model gives the lowest prediction values at all frequencies between 10 and 40 GHz, whereas the Moupfouma gives the highest at 10 GHz and the Crane model gives high values for the greatest frequency of 40 GHz. From measurements at 19.5 GHz it is shown that all three models fall inside measurement bounds for rain rates of up to 50 mm/h, while above 50 mm/h, only the ITU-R stays within the bounds. The presented work ([PERSON] et al., 2007) takes into account 22 microwave links at a frequency of about 20 GHz along with rain gauges covering an area of about \(15\times 15\) km\({}^{2}\). In the proposed method, the microwave links are divided into several equal intervals, forming a regular grid, assuming constant rain rate inside each of the grid cells. A novel iterative algorithm is applied in order to estimate the rain rate at any given point inside the area of interest, therefore generating a two-dimensional precipitation map. The algorithm may be applied in a network with arbitrary geometry and with different frequencies. The frequency range of the used links was 17-24 GHz. Concerning the attenuation model, the ITU-R power law is utilized. Results showed good similarity compared to rain gauges. Subsequently, the study named \"Frontal Rainfall Observation by a Commercial Microwave Communication Network\" introduces a technique of reconstructing the rainfall spatial-temporal conditions with the use of RSLs into wireless networks and extended Kalman filtering (EKF) ([PERSON] et al., 2007). Specifically, the technique above was applied on a cellular system (network with star topology), which was constituted of 23 microwave links. Moreover, all the transmitter-receiver links (from 0.8 to 18 km) were vertically polarized with working frequency bands of 18, 22, and 23 GHz. Nevertheless, as gaps between links existed, a stochastic model (based on advection) was implemented in order to gather the data along the direction of rainfall motion; thus the theory of operation assumed that during short periods, the predominant rain cell's force was advection. The temporal resolution was set at 1 min. In regard to the need for comparison, instantaneous rainfall estimations were tested in contrast to measurements originated from five rain gauges (RG). Additionally, this technique could be applied to thunderstorms nowcasting as, in this case, high accuracy is needed during measuring precipitation and rainstorm conditions ([PERSON] et al., 2007). The estimation of rain rate through measurements of the signal attenuation which is caused by rainfall using microwave links is a research field of high interest which keeps growing during recent years. The remote techniques used so far (weather radars, meteorological satellites) do not provide accurate measurements for ground level rain rate. Besides, rain gauges can provide accurate in-situ measurements, and their high accuracy is limited in spatial points that are close to them due to the high spatio-temporal variation of rainfall. As a result, the idea of determining the rain rate through microwave attenuation gained much interest. Numerous papers which are focused in this direction have been published, presenting many different approaches and results. Some of the authors used backhaul cellular microwave links for their measurements, while others used customized experimental setups. This paper presents information gathered from works published during recent years. The literature is organized based on the technique used for conducting the measurements (backhaul cellular microwave links or customized experimental setups). The key points and results of each published work are outlined.