On the Accuracy of Simplified Models for Water Vapor Attenuation Prediction at Ka band and Q band

The accuracy of the approximate prediction models included in recommendation ITU-R P.676 aimed at estimating the attenuation due to gases (water vapor and oxygen) from ground meteorological inputs is assessed. To this aim, the gaseous attenuation derived from a multichannel radiometer installed at the experimental station of Spino d’Adda is used as reference. Two versions of the same recommendation are considered, defining different simplified expressions for the gaseous specific attenuation, but both taking advantage of the same equivalent height concept. Results, evaluated over a full year of data, indicate that the most recent model (version 11 of P.676) significantly underestimate the attenuation due to gases; on the contrary, the previous version of the recommendation turns out to be accurate enough to be used reliably in supporting the derivation of total tropospheric attenuation from the received beacon signal in electromagnetic wave propagation experiments, especially when no radiometric data are available at the site.