# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 1 Introduction  
Rainfall is a meteorological parameter that presents high spatio-temporal variability [1], Rainfall can be measured through standard rain gauges, disdometers, weather radars, meteorological satellites, and microwave signal attenuation. Rain gauges are the typical instrument for measuring rainfall and they present accurate in-situ measurements. Disdometers also conduct in-situ measurements with further information regarding drops such as rainfall velocity, number of droplets, kinetic energy. Remote techniques include weather radars and meteorological satellites, with the main disadvantage being the inaccuracy of estimating rain amount at a ground level [2]. It is well known that the more rainfall data we have from several measurement sources, the better the spatio-temporal rainfall measurement is. In remote sensing techniques is also included microwave rain attenuation with roots in the 1940s. Specifically, the first pioneering experimental studies of the signal attenuation by rain was carried from 1942 to 1962 [3]. In their cutting edge work of 1947, [PERSON] et al. determined experimentally the attenuation of a 24 GHz microwave link over a distance of 2 km [4]. In order to correlate uniform rainfall over the link with signal attenuation, nine rain gaugeswere used over the link path, and the received power was measured every 30 s. The measurements took place in Hilo, Hawaii and the transmitter-receiver equipment was based on 24-inch parabolic antennas with 50 mW of maximum transmitted power. Six uniform rainfall events from 5 to 41 mm/h where selected and analyzed. At that time, radar technology was used to estimate the attenuation of the transmitted signal during rain in contrast to the dry season [5]. The distance between radar and reflector was 6.9 km, and along with the line of sight (LOS) path, eight typical rain gauges were used to give the rain rate. The radar wavelength was 6.8 mm (\(\sim\)34.8 GHz). The theoretical linear relationship between attenuation and rain rate has not been confirmed experimentally for 0.86 cm radar (34.8 GHz) for rain intensities lower than 2 mm/h. The calculated attenuations are not with good agreement for low rain intensities (\(<\)0.3 mm/h). Additionally, at higher wavelengths of about 3 cm, 5 cm, and 10 cm, the calculated attenuation is lower than the measured one compared with that of 0.86 cm, especially for low rain intensities. Prediction of rain rate via radars is possible via two methods, by attenuation or by reflectivity, with the first being more suitable for wavelengths less than 3.21 cm and the latter for wavelengths higher than 3.21 cm (9.3 GHz) [6]. [PERSON] and [PERSON] in 1977 [7] showed that one-way methods between the transmitter and receiver are better than radar methods for measuring path average rainfall. In all the papers of [PERSON] et al. and the rest of the works until then, there was a significant difference in the experimental results concerning the theory. The measured attenuation of the signal due to rainfall was much more significant than the attenuation resulting from the theory [2]. Generally, the rain attenuation is a complex function depending primarily on the signal wavelength and the drop size distribution. The latest statement has been also confirmed by measurements taken in three regions of India using a variety of techniques [8]. It has been shown that signal attenuation due to rain exhibits significant variations and deviations from the theoretical models. Drop size distribution (DSD) plays an essential role in calculating signal attenuation due to rainfall. Considerable efforts have been made at the beginning of the millennium to measure rainfall through microwave links with high operating frequencies. A research team from Japan used a microwave link of 50 GHz to measure the average rain path [9]. The measurements took place in 2003, and additional sensors were used for temperature corrections, water vapor, and wet antenna. The path was 820 m long, and the power data was recorded by sampling 50 Hz averaged one second.