 is given by Equation 6.86 where T, is the extraterrestrial fog and cloud) and absorption by atmospheric gases. Curves show- noise temperature (usually taken as 2.7°K for the frequency range 2 ing the variation of specific attenuation with frequency are publish- to 50 GHz, unless the receiving antenna is pointing near the sun). ed by the CCIR (1990f). For most practical purposes, this source of attenuation may be (6.86) ignored for frequencies below 1GHz. Waves passing through rain precipitation are subject to attenua- tion due to absorption and scattering, the attenuation being a func- Figure 6.24 shows the variation of effective sky noise temperature tion of frequency, polarisation, the microstructure of the for clear air with 7.5g/m3 water vapour density for various elevation precipitation and the distance the wave travels through the precipi- angles, for the frequency range 1 to 60GHz. Noise contribution due tation cell. to absorption by rain or cloud may also be calculated from Equation In practice, due to air resistance, falling raindrops, other than the 6.87, provided that the temperature and specific attenuation vari- very small droplets, assume an approximately oblate spheroidal ations along the radio path are known. Figure 6.25 shows an shape and this causes the specific attenuation coefficient to be example of the theoretical variation in sky noise temperature due to higher for horizontal polarisation than for vertical polarisation. The attenuation from clouds on a radio path. calculation of the total rain attenuation along a radio path requires a knowledge of the rainfall rate distribution along the path and an 6.4.15 Depolarisation due to hydrometeors allowance for inclined paths. The total attenuation is obtained by integrating the specific attenuation coefficient over the path length. The use of orthogonal polarisation in communications systems is an In most practical cases, predictions are required for the attenuation important technique by which economy of frequency spectrum The space wave 6/19 0