s one terminal, several problems arise due to refraction, absorption and scattering Distance (km) effects, especially at microwave frequencies. For low angles of elevation of the Earth station beam, it is often necessary to evaluate the refraction produced by the troposphere, i.e. to determine the error in observed location of a satellite. The major part of the bending occurs in the first two kilometres above ground and some Figure 9.8 Median transmission loss, L, between statistical correlation exists between the magnitude of the effect and isotropic antennas in a temperate climate and over an the refractive index at the surface. For high-precision navigation average rolling terrain. The height of the transmitting systems and very narrow beams it is often necessary to evaluate the antenna is 40m, and the height of the receiving antenna is variability of refraction effects from measured values of the refrac- tive index as a function of height. A related phenomenon important in tracking systems is the phase distortion in the wave-front due to refractive index fluctuations, a feature closely linked with gain degradation. This phase distortion also affects the stability of fre- 9.8.3 Physical basis of tropospheric scatter quencies transmitted through the troposphere. propagation Absorption in clear air may affect the choice of frequencies, above about 40GHz, to minimise co-channel interference. Figure 9.9 Much effort has been devoted to explaining the fluctuating trans- horizon field in terms of scattering theory based on statistical models of turbulent motion. The essential physical feature of this approach is an atmosphere consisting of irregular blobs in random motion which in, turn produce fluctuations of refractivity about a stationary mean value. Using this concept, some success has been achieved in explaining the approximate magnitude of the scattered field but several points of difficulty remain. There is now increasing evidence, from refractometer and radar probing of the troposphere, that some stratificati