The Boundary Layer. As a result of heat, moisture and momentum exchanges at the ground, the lowest regions of the atmosphere can display a relatively high degree of refractivity turbulence. This activity is particularly the case during days when the air is humid and unstable or when the wind is strong. It appears, however, that over a 100 km^ rectenna, the low-level refractivity variations will not have serious deleterious effects on the efficiency of the system. Cloud Systems. Cloud systems represent regions where refractive indexes can be systematically different from those in the environment. This phenomenon is particularly true in the case of thunderstorms and squall lines. Such storms, especially when they produce heavy rain and hail, can cause very substantial scattering and attenuation of microwaves. It is essential that statistics be developed, at various representative sites, giving storm frequencies and dimensions, as well as rainfall and hail intensities and their special extents. Jet Stream Turbulence. It has been found that clear air turbulent conditions, associated with the high level jet streams, can lead to significant scattering of S-band (wavelength around 10 cm) radiation. This is particularly likely when the humidity varies substantially with height through the turbulent region. It is recommended that available data be used to estimate the effects of jet turbulence on the propagation of microwave radiation from the SPS satellite. Statistics on the frequency, extent and intensity of high level refractivity turbulence should be assembled. Stable Layers in the Atmosphere. Turbulence in the vicinity of the tropopause and other stable atmospheric layers can cause sufficiently intense fluctuations of refractivity to cause the scattering of S-band radiation. The effects are particularly important when the humidity varies substantially through the stable layer. The frequency of such conditions needs to be estimated in various geographical areas, including coastal and continental areas. Beam control is impacted to a major extent by the interaction of both the power beam and the pilot beam with refractive-index anomalies in the atmosphere. Slowly varying and drifting anomalies, large-area events, or numerous smaller-area anomalies in the power beam (main beam and principal sidelobes) and the pilot beam cause spatial and temporal modulations that vary
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