8.2.5. Noise impacts (28). The primary noise impacts would be at the launch complexes during the frequent launches of the launch vehicles while the SPSs are being constructed. Noise will also be generated during launches of various vehicles to supply the SPS with expendables and for their maintenance, but these launches would be less frequent. Noise impacts during the construction of the receiving antennas would be minor, because the sites would be remote from populated areas. The launch complex sites would most likely be located in the less populated Southwestern region of the United States. An international cooperative effort, however, could lead to the selection of launch complexes nearer the equator where there are large uninhabited land areas. 8.2.6. Microwave beam effects 8.2.6.1. Atmospheric attenuation and scattering (29). Attenuation of the microwave beam by rain, cloud droplets, snow, and hail will depend on their size, shape, and statistical distribution and composition. Rain, wet snow, melting precipitation and water-coated ice attenuation is low at frequencies below 3 GHz. The most severe condition is expected in rain clouds, where attenuation may reach 4% at 3 GHz. The attenuation produced by a 1-km path through wet hail could reach 13% at 3 GHz. Forward scattering by rain and hail will increase the field intensity outside the main microwave beam. For example, a 5-GW SPS operating at 3 GHz would scatter 3 mW nearly isotropically if the storm cell height were 1 km. At a range of 10 km, the scattered microwave beam power density would be about 2xl0-4 mW/cm2. Therefore, scattering by rain or hail is not expected to significantly increase sidelobe levels or broaden the main microwave beam. 8.2.6.2. Ionospheric propagation. Among the several possible interactions of the microwave beam with the ionosphere are the following: • Ambient refraction of the microwave beam by the ionosphere — This effect leads to a negligible displacement. If horizontal gradients are present in the ionosphere, they could result in displacements (less than 100 m) of the microwave beam (29). • Ionospheric electron density irregularities — These self-induced or ambient irregularities will cause phase fluctuations (less than 10 degrees) across the wave front of the reference beam propagated from the center of the receiving antenna to the transmitting antenna face. Random phase variations will subside within a few hundred meters and within tens of seconds (29). Power beam dispersion due to ionospheric density fluctuations will increase the field intensity at the beam edges by up to 30%. At low power densities, these fluctuations at the edges of the beam will not cause any significant power loss. Increased electron heating of the ionosphere at the microwave beam densities associated with the SPS microwave power transmission system is of concern because it may affect telecommunication system performance. Electron heating would be increased if the predicted electron temperatures in the lower ionosphere within the beam increase by a factor of 3 to 4. In contrast, ohmic heating would increase the rate of electron recombination, leading to an increase in electron density. However, in either case, the changes in ionospheric density would be small and any telecommunications impacts as a result of these changes would be insignificant.
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