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Radiometric measurements could provide continuous information about atmospheric conditions. In this paper, a sky status indicator (SSI) is proposed as a real-time recognition criterion for the detection, in particular, of the presence of rain events along the propagation path. The computation of the SSI is based on ground-based brightness temperature measurements, at 23.8 and 31.4 GHz, collected in Cabauw, Netherlands, in 2009 by the ESA Atmospheric Propagation and Profiling System (ATPROP) multichannel radiometer. A validation analysis is carried out between simulated data, which are computed by applying the radiative transfer equation to a database of radiosonde profiles collected in De Bilt, Netherlands, by the Royal Netherlands Meteorological Institute, and two data sets of radiometric observations at two elevation angles (\theta equal to 90^\circ and \theta equal to 69.6^\circ). The analysis based on SSI probability distribution functions has allowed for calculation of the boundary threshold values that are able to discriminate the status of the sky. Furthermore, performances of the SSI were validated against rainfall measurements collected at the ground by a rain gauge located near the ESA ATPROP multichannel radiometer. [ABSTRACT FROM AUTHOR]

A large database of radiosoundings, brightness temperatures, and satellite beacon data was used to verify the ability of radiometers in estimating the attenuation in nonprecipitating conditions at millimeter wavelengths. The mass absorption models proposed by Liebe and Rosenkranz have been used coupled with the Helsinki University of Technology (TKK) cloud-detection model to derive coefficients for the calculation of the attenuation due to integrated water vapor and liquid water contents. Comparison of predicted and measured attenuation cumulative distribution functions in the 20-50-GHz band confirms radiometers as a unique tool for retrieving attenuation in nonrainy condition and shows the very good performance of the TKK model for propagation applications. [ABSTRACT FROM AUTHOR]