Fig. 6. Effects of phase control on microwave power distribution at receiving antenna. Source: Reference 9. damage. Experimental results on animals exposed to microwave radiation indicated that irreversible tissue damage occurred at power densities of about 100 mW/cm2. Based on the premise that thermal effects predominate at levels higher than 10 mW/cm2 and that below this power density level, nonthermal effects predominate, the American National Standards Institute recommended in 1966 that 10 mW/cm2 be used as an acceptable standard. The USSR Microwave Exposure Standard of 0.01 mW/cm2 is based on the empirical findings by Soviet scientists that microwave exposure could affect the nervous systems of animals and humans. Their studies indicated that the central nervous system is particularly sensitive to microwave radiation, and that chronic occupational exposure of humans to very low power microwave radiation leads to a variety of psychological and physiological effects. In view of the current interest in low-level effects of microwave radiation, it is very likely that international standards will be adopted in time to be incorporated in the SPS design. The basic premise is that the SPS microwave transmission system must be designed so that the microwave power flux densities to which the public would be exposed outside the receiving antenna site will meet international standards. The SPS microwave transmission system design incorporates the principle of retro-directive control of the microwave beam to make it impossible for the beam to be pointed to any location but that of a receiving antenna, instantaneous shut-off of power fed to the microwave generators and graceful system failure modes (see Fig. 6). An SPS system consisting of a large number of satellites will create background microwave radiation levels of less than 10 4 mW/cm2. There is no evidence to suggest that there are any biological effects associated with continuous exposure to this extremely low-level micro wave radiation.
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