for transmission lines could be reduced if receiving antennas were located in the vicinity of major users. • Resource Consumption. Materials requirements will be limited largely to those which are in plentiful supply, such as silicon and aluminum. Each SSPS would require less than 2% of the yearly supply of critical materials, such as platinum, available to the United States. • Energy Consumption. The energy required to produce the materials required during SSPS construction as well as the propellants to place it into orbit would be repaid — i.e., regenerated — in about two years of SSPS operation. • Atmospheric Pollution. According to present estimates of space vehicle emissions, the multiple launches required to orbit the SSPS are not expected to add any significant amount of pollution to the atmosphere, and will be within allowable goals. • Ionosphere Interactions. The microwave power density within the beam center is below the level where interactions with the ionosphere are projected to occur (50 milli-watts/cm2). • Microwave Exposure. Exposure to the microwave beam can be controlled by providing suitable enclosures for the maintenance crew to work on the receiving antenna. Within 10 kilometers from the beam center the microwave power density would meet the lowest international standards for continued exposure to microwaves (Figure 8). The microwave beam directional system and the phase control achieved by means of a pilot signal beamed from the center of the receiving antenna preclude the deviation of the microwave beam beyond allowable limits. In case of failure of the microwave beam pointing system, the coherence of the microwave beam would be lost, the energy dissipated, and the beam spread out so that the microwave beam density would approximate communication signal levels on Earth. The effects on birds exposed to microwave power flux densities within the beam at the receiving antenna and the effects on aircraft accidently flying through the beam are projected to be negligible, but they should be determined experimentally.
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