Now consider the amount of energy that the SPS could introduce into the atmosphere. The SPS converts sunlight into a 2.45-GHz beam that can pass virtually unattenuated through the atmosphere to Earth. If the power density of the beam is 2 limited to, say, 25 mW/cm , the power dissipation at a rectenna site would be comparable with that of a dormitory suburb of equal area: the weather/climate 2 effect of an operating SPS rectenna is small. As presently designed, the SPS could do very little to affect the atmosphere directly and thus to modify the weather. If the SPS transmitter were retuned from 2.45 GHz to the water vapor absorption line of 22.2 GHz, or if a separate transmission module operating at 22.2 GHz were added, however, the SPS's beam would then be able to transfer to the atmosphere a substantial part (roughly 15%) of the energy being transmitted by the SPS microwave beam. 3 3 This power absorption is only enough to increase the temperature of a 10 m volume of thermally and radiatively isolated surface-type air by about one Centigrade degree in an hour. Furthermore, if the remaining energy reaching the ground in the SPS beam (about 85% of the total beam energy) is intentionally misdirected away from the receiving platform, it can be absorbed by the ground and increase the near surface air temperature by a few Centigrade degrees. In comparison with the energies involved in atmospheric processes, the amount available from the SPS for altering the air is meager. It is therefore evident that, in order to modify the weather substantially, the operational SPS must exploit suitable inherent instabilities in the real atmosphere which could be triggered into development. To exploit the small amount of SPS energy transferable to the atmosphere in order to modify the weather to any useful extent (for whatever purposes), the power transmission beam must be physically linked with some process of atmospheric turbulence, a process comparable not only in scale with the cross- sectional size of the SPS beam in the atmosphere but also in power density. The higher beam frequency of 22.2 GHz required to enable weather modification and control, however, would increase sensitivity of the beam to the intense fluctuations of refractivity caused by clear-air turbulence around the high-level jetstreams and in the vicinity of the tropopause or caused by inversions and stable layers in the lower troposphere. The shift to a higher beam frequency would thus (perhaps seriously) degrade the capability for aiming the microwave beam accurately onto a small weather target as it migrates across the terrain. Disadvantages
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