These categories represent different beam control response problems. Effect magnitudes and the statistical precessing of pilot beam parameters to extract atmosphere "signatures" must be carefully addressed in the pilot beam-array, control-loop design to positively minimize ambiguities and instability probabilities. These areas have important implications for future modular and subsystem laboratory testing. Attenuation and scattering due to precipitation also reduces the amount of total power at the rectenna. This mechanism is generally proportional to rain rate and the problem varies with geographical location. Blowing dust or sand also contributes to beam power attenuation and scattering of energy away from the rectenna. Although each mechanism alone would have seemingly small effects stated as a percentage of main beam power or in dB power loss, because of the large power levels involved with SPS, the magnitude of power fluctuations due to these mechanisms may be unacceptable to the distribution system. The pilot beam required for frequency/phase reference and orbital antenna control is also affected by these same atmospheric mechanisms. Under circumstances such as very heavy rain, severe sand storms, etc., any of these mechanisms could readily cause control-loop instabilities resulting in power beam wander and frequency fluctuations, which in turn adversely affect power transfer efficiency and greatly exaggerate the interference problems. The above discussion addresses those mechanisms which cause degradation of received power at the rectenna and pilot carier modulation. Another major area of concern is the EMC problem. If no ionospheric or atmospheric effects were present, there would still be impact outside the rectenna area due to the power beam sidelobes, emission of harmonics of the primary frequency and spurious components, noise sidebands, and terrain reflections. The SPS emissions from these sources will cover the hemisphere (Ref. 4.1.6); however, EMC problems will most likely occur within 100 km radius of the rectenna where the power densities are highest. Energy coupling to "victim" systems will involve in-channel and nonlinear responses by out-of-band components, relative to the primary receiver pass band, and coupling through cabling and circuit element apertures to other electronic circuiting. Different scoring procedures are required because of the wide variations in the latter in the characteristics of energy coupling
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