ELECTROMAGNETIC COMPATIBILITY The SPS must be designed and operated to satisfy established national and international regulations for the use of the electromagnetic spectrum. The SPS has the potential to produce interference because the amount of microwave power transmitted from an SPS to Earth is unprecedented and the size of the microwave beam would be very large at the Earth's surface. The SPS could interfere with military systems, public communications, radar, aircraft communications, public utility, transportation systems communications, other satellites; and radio astronomy. For example, the SPS field intensity would be 1 volt per meter at a distance of 30 km from the center of a receiving antenna site. Communication systems that generally operate with a received signal strength of several microvolts per meter would receive sizable signals from the SPS, even at distances of about 100 km. The interference potential of the SPS microwave beam would not be especially unusual except in the extent of the geographic area affected. High-power radar systems produce interference of similar electomagnetic intensities, but over limited areas. Prevention of SPS interference by direct energy coupling to any class of equipment would be part of the engineering deisgn of the transmitting and receiving antenna. This will reduce undesirable emissions at frequencies other than the operating frequency by constraining the size and shape of the transmitted microwave beam and its side lobes. Appropriate siting of receiving antenna will require a trade-off between the desire to locate antennas near energy load centers and the need to avoid interference with the large number of other users of the radio spectrum. For example, a minor change in location could substantially reduce the impact on national defense facilities without increasing interference with civil systems. Also to be considered are interferences with other satellites. Satellites in orbits lower than GEO, including those in transit to GEO, may pass through the microwave beam, causing interferences with satellite systems or potential damage to sensors. The duration of the encounter of satellites with the beam can be up to 2 seconds in LEO and up to 4 seconds during transport to GEO. It may be possible to choose a satellite orbit altitude and phase so that encounters with a known microwave beam location occur very rarely — if at all. RESOURCE REQUIREMENTS Indirect environmental impact of the SPS could result from physical resource requirements including land use, materials availability, and energy utilization. Studies showed that there are many suitable locations for receiving antenna sites in the United States. Site selection will be influenced by the desire to avoid migratory bird fly-ways, undesirable topography, proximity to defense installations, and population centers. The size and intensity of use of the contiguous land area necessary for a receiving antenna site and site construction will require that environmental impact be established for each specific site. Secondary uses of such sites, for example, for agricultural purposes or for terrestrial solar energy conversion systems, will need to be assessed. Locating receiving antennas off-shore may be attractive for major population centers near the seacoast, not only because of the possible proximity, but also because floating off-shore structures may be competitive with land-based structures. Floating structures would also provide opportunities for extensive mariculture; for example, one offthore antenna would produce from 5 to 10% of the fish products used by the United States. An analysis of the material requirements for the construction of the SPS indicated that no insurmountable material supply problems are evident in terms of world and domestic supply and potential manufacturing capacity. Over one half of the materials for the SPS reference system are readily available, but there are potential supply constraints on tungsten, silver, and gallium. An industrial infrastructure to fabricate SPS components (e.g., ion thrusters, dipole rectifiers, microwave generators, and graphite composites) will be adequate; however, facilities for the mass production of solar cell arrays will be required. Net energy analysis comparing alternative energy technologies in terms of energy produced by each system per unit of energy required indicated that when fuel is excluded, the energy ratio for the SPS reference system is marginally favorable with respect to other energy production methods. When fuel is included, the SPS energy ratio is very favorable, with the energy payback period projected to be about one year. LEGAL ISSUES The 1967 Space Treaty, Article VII, stipulates that each state is "internationally liable for damages" to others caused by activities in space. The 1973 “Convention on International Liabilities for Damages Caused by Space Objects" amplifies these responsibilities. The existing space law implies that if the global or local environment is damaged through SPS system operation, the SPS owners might face law suits or other forms of grievance procedures. Even if operation of an SPS system had no other effect than that caused by a nation making use of the power supplied to it, the design of a globaly marketable SPS system to meet widely varying national standards could add significantly to its cost. Furthermore, the possiblity of law suits could make insurance expensive or impossible to procure, unless the development, construction.
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