Volume integral equation solution of microwave absorption and scattering by raindrops

Frequencies above 5 GHz, especially in the microwave or millimeter wave range, used for future terrestrial repeater system or Earth-satellite communication systems may cause various degrading effects on the performance of communication lines due to the attenuation by hydrometers (e.g., rain, hail, fog, snow, and ice crystals). We consider only attenuation (absorption and scattering) from rainfall, since hail, fog, snow, and ice crystals are considered as secondary affects in the studies of RF signal propagation. The problem of attenuation by rain has been studied for more than four decades. In most theoretical studies, the raindrop is considered as a sphere or an oblate spheroid for the computation of rain attenuation. Indeed, raindrops are nonspherical in shape and have preferred axial orientation angles different from vertical direction. A typical and well-accepted model for describing the realistic raindrop shapes was developed by Pruppacher and Pitter (1971) who solved a pressure balance equation at the surface of falling raindrops by numerical techniques and determined the shapes of raindrops of various sizes theoretically. Li et al. (1995) used a different functional expression to further simplify the P-P model for the calculation of rain attenuation. This new modified P-P model is adopted for the studies of microwave.