disadvantage in physical and control tolerances must be recognized. Some advantages accrue from the reduced potential for EM interference, negligible ionosphere effects, and smaller physical system. Because we do not yet know the importance of the perturbations of the rectenna (in terms of heating and turbulence in the boundary layer), we cannot yet address the ways in which such perturbations will feed back to effect the system performance. Problem 1: The effects of the atmosphere on beam wandering, beam spreading, and gain reduction. Problem 2: The effects of the atmosphere on Doppler shift and Doppler spread of signals and the resulting impact on the ability of the pilot-control signal to minimize beam wandering, etc., and on the frequency control system. Problem 3: The identification of the various atmospheric phenomena which may affect the factors listed in Problems 1 and 2. Some of the most important ones appear to be the following: (a) Storms and clouds passing through the propagation path. It is expected that the important ones will be mainly of a convective type. (b) Clear-air refractive index perturbations in stratified layers throughout the troposphere. Some initial indications suggest that the higher levels (i.e., near tropopause) may be most important, but the strong refractivity effects near lower level inversions should also be considered. (c) Clear-air refractive-index perturbations associated with convective plumes and bubbles. (d) Sharp horizontal discontinuities in refractive index associated with either dry (i.e., sea breeze fronts, dry lines) or moist fronts. Problem 4: Possible heating of the ground and evaporation of soil moisture where the beam wanders off the rectenna, especially under conditions of failure of the control loop. Problem 5: Atmospheric heating by absorption of the microwave energy by clouds, rain and hail and the probability of occurrence of effects of various magnitudes. Problem 6: Possible effects on the signals at the rectenna of depolarization by nonspherical raindrops, hail, and snow crystals.
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