Modeling antenna noise temperature due to rain clouds at microwave and millimeter-wave frequencies

A characterization of the antenna noise temperature due to precipitating clouds at Ku band and above is described by deriving a closed-form solution of the scalar radiative transfer equation. Following the so called Eddington approximation, the analytical model is based on the truncated expansion of unpolarized brightness temperature angular spectrum in terms of Legendre polynomials. The accuracy of the sky-noise Eddington model (SNEM) is evaluated by comparing it with an accurate numerical solution, taking into consideration a wide variability of medium optical parameters as well as a typical rain slab model. The effect of the antenna pattern for ground-based antennas is also quantified. Physically-based radiative cloud models, characterized by a vertically-inhomogeneous geometry, are also introduced. Hydrometeor optical parameters are calculated and modeled for a large set of beacon channel frequencies. Nimbostratus and cumulonimbus models are finally applied to SNEM for simulating slant-path attenuation and antenna noise temperatures for ground-based antennas. Results are compared with ITALSAT satellite receiver measurements and co-located radiometric data between 13.0 and 49.5 GHz for various rain events during 1998.