# Earth-to-Earth Microwave Rain Attenuation Measurements: A Survey On the Recent Literature
## 6 Research Challenges  
Challenging and limitation issues concerning both CML and customized experimental measurements including but not limited to are:  
* Short sampling times even with 24-h rates
* Limited access to received signal strengths
* Limited power resolution
* Fluctuations in the transmitted and received signal due to electronics' noise
* Signal's effect by various phenomena due to the path between the transmitter and receiver, such as fading due to reflections from nearby objects and the ground, but also by atmospheric parameters. * Wet antenna effects
* Jitter due to wind on antennas
* Heterogeneity in methodologies
* Signal processing and machine learning algorithms
* Lack of experimental measurements and different frequency bands and mainly in the lower microwave region of the spectrum, which is ruled by abundant remote sensing devices operating under specific wireless communications standards. These devices could potentially adopt scientific and technical aspects in the future, in terms of precision and accuracy to measure rainfall. The aforementioned issues have been addressed in detail by [PERSON] et al. [53], [PERSON] et al. [26], [PERSON] [87], [PERSON] and [PERSON] [88]. Microwave links can be used efficiently to estimate path-averaged rainfall employing either the path-integrated attenuation or the attenuation difference of two signals having different frequencies or polarizations. In [89], the authors have assessed the errors and uncertainties in microwave link estimation of path-averaged rainfall intensities due to rainfall variability. To this end, the simulated link signals using measured time series of raindrop size distributions, thus obtaining a dataset of almost 2 yr in size relevant to drop size distributions. This dataset has been used in combination with wind measurements to simulate link signals and compute the appropriate statistics. Microwave links have been also widely implemented for wireless data exchange, primarily between mobile phone networks or base stations. However, the ability to distinguish the attenuation during dry periods (i.e., the so-called attenuation baseline) from the one due to rainfall is a critical issue. Specifically, such a procedure requires identification and separation of dry from rainy periods. In [64], a method of implementing this procedure was proposed and was implemented with the use of path-integrated attenuation measurements which had been obtained from operational microwave links. Dry and rainy periods were separated by analyzing the local alteration of the link signal. The attenuation baseline was real-time estimated by employing the classification procedure into dry and rainy periods. Cellular networks which are constituted of commercial microwave links provide the benefit to measure near-surface rainfall. Nevertheless, the operated communication links introduce uncertainty relative to the dedicated installations because the properties of the working frequencies, and thus the geometry is oriented towards high communication performance instead of rainfall observation. Consequently, when the measurements are non-optimal it is obvious that quantification of the uncertainties is needed in order for the data to be usable. In [90], various instrumental impairments were studied and included signal variations originated from antenna wetting, uncertainty of baseline attenuation and digital quantization, and thus environmental ones, i.e., the observed variation of drop size distribution along a link's territory whereas is affected the accuracy of the path-averaged rainfall measurement and the spatial variability of rainfall which in turn affects the rainfall estimation accuracy outside the link's range. Accurate measurements of rainfall are significant in the scientific areas of meteorology, hydrology, agriculture, environmental policy, and weather forecasting. In order to obtain such measurements, one can use a number of microwave links to constitute a wireless communication network (WCN).