# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 6 Research Challenges
The exploitation of measurements by wireless communication and navigation systems for environmental sensing has shown great potential for monitoring several atmospheric phenomena. On the other hand, data collection and storage are an open issue in terms of real-time measurements and measurements from different mobile phone companies stored under different protocols and with varying accuracy. There are open issues regarding the harmonization of heterogeneous measuring sensors in an area of its spectrum that is full of telecommunication protocols [102]. In [103] the authors performed a measurement campaign to test a straightforward rainfall retrieval algorithm whereas the latter was applied to the microwave link measurements as a function of a power-law relationship. Their results were compared to the results of five disdrometers positioned along the path. This methodology enabled the assessment of the quality of retrieval without taking into consideration the impact of other attenuation sources. In [88] authors assessed the current status and future challenges of using commercial microwave link networks for observing rainfall. In [104], the authors proposed a method to identify wet antenna attenuation (WAA) distribution and upper limits in accordance with rainfall climatology without the need for adjacent rainfall observations. The quantified contributions of antenna wetting owing to total loss had been obtained from 2 years of data, which had been acquired from eight short (48-497 m) CMLs, which worked at frequencies of 37.3 to 39.2 GHz within the cellular backhaul. In [105] a model, explaining and the same time correlating the WAA to the rain rate was proposed. In that model, excess attenuation reached approx. 3 dB per antenna in the condition of rain rate equal to 100 mm/h. The statistics of theoretically predicted rain attenuation, containing the WAA term, were compared to those of measured attenuation on a set of three other links which worked at the same frequency. Much work has also been done on the new prediction methods for rain attenuation. Since ITU-R rain attenuation mode does not perform accurately on a global scale, [PERSON] and [PERSON] [106] proposed a new prediction model without complex mathematical expressions that shew a very high accuracy level. This model uncovered the latent dependence of rain attenuation on operation frequency, path length, polarization, DSD, etc. The authors in [107] proposed a new model for estimating the statistics of rain fade slope for terrestrial microwave links inside tropical regions. The proposed model was validated with the use of the Chi-squared goodness-of-fit test. In [108], an analysis was presented relevant to the wet antenna attenuation from an electrodynamic perspective. That model has been predicated on the three antenna parameters such as reflectivity, efficiency, and directivity. Furthermore, the model makes possible the variation of those three parameters which depend on the degree of wetness. It has been shown that WAA is constituted of variations relevant to antenna impedance, efficiency, and directivity. Of particular interest are the rain attenuation and the ability to measure the path average raindrop size distribution with microwave link instruments. This feature is presented in a recently published work [109]. Two different methodologies were used where simulated data on radar and disrometer measurements from France were applied. The metadata was also applied to variables derived from 5 laser DSDms along the path and also to data originated from microwave link instruments in the Wageningen area, Netherlands. Preliminary results from the application of the method to an experimental setup of 26 and 38 GHz microwave links along a 2.2 km path link was also additionally presented.