ce of refractive-index variations at heights of the order of a kilometre began to be recognised and studies of the correlation between the height variation of refractive index and field strength began in several laboratories. Free-space curve With the development of more powerful transmitters and antennae Pr x 1/d2 of very high gain it proved possible to establish communication well beyond the horizon even in a 'well-mixed' atmosphere with no surface ducts or large irregularities in the height variation of refrac- tive index. To explain this result, the concept of tropospheric scatter was proposed. The trans-horizon field was assumed to be due to incoherent scattering from the random, irregular fluctuations in refractive index produced and maintained by turbulent motion. This procedure has dominated much of the experimental and theoretical work since 1950 and it certainly explains some characteristics of troposphere propagation. However, it is inadequate in several re- Free space Diffraction Scatter spects. It is now known that some degree of stratification in the range range range troposphere is more frequent than was hitherto assumed. The possi- Distance, d bility of reflection from a relatively small number of layers or sheets of large vertical gradient must be considered, especially at v.h.f. At Figure 9.5 Tropospheric attenuation as a function of u.h.f. and s.h.f. strong-scattering from a 'patchy' atmosphere, with distance in a homogeneous atmosphere. Direct and ground local regions of large variance in refractive index filling only a reflected rays interfere in the free space range; obstacle fraction of the common volume of the antenna beams, is probably diffraction effects perdominate in the diffraction range; and the mechanism which exists for much of the time. refractive-index variations are important in the scatter range The increasing emphasis on microwaves for terrestrial and space systems has recently focused attention on the effects of precipitation The range over which the space-wave mode is domina