7 Electromagnetic Waves O = 0° O = 5° O = 10° O = 20° O = 30 O = 60° O = 90° Fig. 7.24 Sky noise for clear air and infinitesimal antenna beamwidth. (Water vapor concentration = 7.5 g/m³ 3. surface temperature = 15°C; surface pressure = 1023 mb). Frequency (GHz) Path Diagram for This Example Cloud Layer B Cloud Layer A Clear Air and Cloud Layer A Clear Air and Cloud Layers A Clear Sky and B Fig. 7.25 Example of the effect of clouds on sky noise temperature. (Clear air: water vapor = 7.5 g/m 3 at surface; cloud layer: 0.5 g/m³ liquid water). Frequency (GHz) technique relies on the Cross-Polarization Discrimination (XPD) achieved by the careful design and alignment of the transmitting and receiving antennas involved. A significant factor affecting cross-polarization discrimination achieved in practice at SHF is the depolarization caused by hydrometeors, in particular rain and ice crystals. Depolarization due to Rain For rain-induced depolarization, there is a strong statistical correlation between the degrada- tion in XPD and the attenuation of the wanted polarization, usually called Copolarized Path Attenuation (CPA). The Sky Wave The relationship between XPD (XdB) and CPA (CdB) is, however, complex and depends on frequency, polarization, angle of inclination of the path and raindrop size, shape, and canting angle distributions along the path. Data is continually being amassed from propagation experiments, in many cases using communications satellites, and semiempirical relationships based on this data are in common use. One such semiempirical relationship is given in CCIR (1990b), with further parameters in CCIR (1990c). Depolarization due to Ice Crystals Depolarization due to clouds of ice crystals can be considerable and can occur in the absence of significant attenuation and depolarization due to rain. Because the major axes of the crystals are normally close to the horizontal, depolarization tends to a minimum for hori- zontal or vertical polarization. Abrupt changes in XPD have been observed to coincide with 