=10° Frequency (GHz) Figure 6.24 Sky noise for clear air and infinitessimal antenna beamwidth. (Water vapour concentration = 7.5g/m³ 0 = path elevation angle; surface temperature = 15°C; surface pressure = 1023mb) usage may be improved by reducing mutual interference between 6.4.15.1 Depolarisation due to rain systems sharing the same, or adjacent, frequency channels. This technique relies on the cross polarisation discrimination (XPD) For rain induced depolarisation, there is a strong statistical correla- achieved by the careful design and alignment of the transmitting and tion between the degradation in XPD and the attenuation of the receiving antennas involved. wanted polarisation, usually called the co-polarised path attenuation A significant factor affecting the cross polarisation discrimination (CPA). achieved in practice at SHF is the depolarisation caused by hydro- The relationship between XPD (XdB) and CPA (CdB) is, how- meteors, in particular rain and ice crystals. ever, complex and depends on the frequency, polarisation, angle of PATH DIAGRAM FOR THIS EXAMPLE Clear air and Clear air and cloud layer A Cloud layer B cloud layers A & B Cloud layer A Clear sky Frequency (GHz) Figure 6.25 Example of the effect of cloud on sky noise temperature. (Clear air: water vapour 7.5g/m 3 at surface. Cloud layer: 0.5g/mi 3 liquid water.) 6/20 Electromagnetic waves inclination of the path and the raindrop shape and canting angle mechanism for the low frequencies where the refractive index distributions along the path. changes more rapidly with real height and, more specifically, height Data is continually being amassed from propagation experiments, measured in wavelengths. in many cases using communications satellites, and semi-empirical On the other hand, for the higher frequencies, refraction domi- relationships based on this data are in common use. One such nates over reflection, even though the amount of refraction de- semi-empirical relationship is given in CCIR (1990b) with further creases as the frequency increa