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4.1.2 Rain AttenuationRain attenuation is, perhaps, the most important single phenomenon thatimpairs transmission of a satellite signal. How the loss due to rain is calculatedis the subject of this section. Rain attenuation is modelled asA r ¼ aLr b rð4:24Þwhere1. a and b are coefficients that are calculable theoretically fromconsiderations of electromagnetic propagation in spherical raindrops. These coefficients are polarization- and frequency-dependentbased on raindrop characteristics and can be approximated to thefollowing analytical expressions [2]:(a ¼ 4:21 10 5 f 2:42 ; f 54 GHz4:09ð4:25aÞ4:09 10 2 f 0:699 ; 54 < f 180 GHz(b ¼ 1:41 f 0:0779 ; f 25 GHzð4:25bÞ2:63 f 0:272 ; 25 < f 164 GHzOutside these frequency ranges, the coefficients are equated as zero.If the coefficients are linearly polarized vertically or horizontally,the coefficients for a circularly polarized wave can be calculatedusing [3]a c ¼ 0:5ða h þ a v Þð4:26aÞb c ¼ a hb h þ a v b v2a cð4:26bÞThe subscripts of the constants indicate their polarization. Forexample, subscripts c, h, and v denote circular, horizontal, andvertical polarization, respectively.2. r r ¼ rain rate (mm=hr). Average values of r r can be obtained fromyour country’s Department of Meteorology (or its equivalent).3. L ¼ rain geometric path length (km). A simple geometric expressionfor calculating the pathlength has been given by Ippolito et al. [4]Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.