(see REFERENCES under ERDA) that each rectenna might require an "exclusion area" twice tne radius of the rectenna itsell or more, if for no other reason than public opinion. Should this higher estimate prove true, land investments might be extremely high ror a commercial SPS system, since each SPS requires at least one rectenna. A third possible environmental problem is damage to the atmosphere from exhaust fumes expelled by launch vehicles transporting personnel into space, especially considering the large number of launches required per SPS. Charles Bloomquist (see REFERENCES) has stated that with 360 shuttle launches per year, "up to a 2% increment in [water] may accrue in the 25-30 mile (40-50 km) altitude range," as well as possible difficulties with hydrogen chloride and aluminum oxide from tne solid rocket boosters. The HLLV planned for carrying SPS construction material would use liquid fuel so is not of concern in this regard. Radio Frequency Interference The DOE/NASA reference design for SPS selects 2.45 gigahertz as the frequency ror beaming the electricity produced by an SPS down to Earth from geosynchronous orbit. At the present time, there are a large number of operating satellites in space at various frequencies, and the trend appears to suggest that even more will be operating by the time an SPS system could become commercially available. Proposals call for a large number of SPS stations, which together with their high power levels, could cause a severe radio frequency interference pattern with other satellites and/or ground-based systems in the vicinity of the rectenna. This is another subject requiring further study. Resource utilization Some concern has been expressed by Summers, et al. (see REFERENCES) over the amount or aluminum required to construct an orbital photovoltaic SPS and its associated rectenna. Those authors reported that for each 10,000 megawatt SSPS, over one million metric tons of aluminum would ne needed, or 13% of U.S. reserves (0.8% of the world's reserves). Glaser has commented that "materials necessary for construction are largely those in plentiful supply, such as silicon and aluminum." Although his case may be true on a world-wide scale, it seems conceivable that imports might be required for some of tne construction materials. This, too, is speculative, however, for some new type of material might be developed prior to initiation or an SPS program. as mentioned earlier, if gallium arsenide is chosen for a photovoltaic SPS, there may ne a problem with gallium availability. DOE identifies four materials that could present problems in availability: mercury, tungsten, silver, and gallium. funding Prior to FI 77, NASA was responsible for conducting studies of SPS designs and aid so under general categories such as "advanced studies." Exact numbers are therefore not available for years prior to FY77. For FY77, $2.5 million was appropriated to NASA for SPS, and ERDA added another $.6 million. For FY78, NASA was appropriated $4 million, although $2.3 million of that amount was transferred to ERDA, which added another $.5 million to the effort. For FY79, NASA requested no funding for SPS, although Congress
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