ENVIRONMENTAL IMPACT OF MICROWAVE POWER TRANSMISSION
The proposed system to transmit solar power to the Earth's surface involves microwaves as the conduit of, energy beamed from Earth-orbiting solar power stations through the atmosphere. Primary concerns about the impact upon the environment of such a system are:
The microwave beam from the satellite solar power station (SSPS) is triggered by a pilot signal beamed from the center of the receiving antenna to provide the necessary phase control to produce a coherent beam. Otherwise, if the beam were to drift, its coherence would be lost, the energy dissipated, and the resulting power density would approximate normal communication signal levels on Earth (ref. 10).
Radiation effects depend on the power density of the transmitted beam which in the present system is designed for a peak of 10-100 mW/cm^2. In the United States and other nations of the Western world, 10 mW/cm^2 is an accepted standard for radiation exposure, while the Eastern European nations have placed acceptable exposure limits as low as 10 pW/cm^2 (ref. 4). It is noted (ref. 11) that the U.S. Department of Health, Education and Welfare has set a limit for new microwave ovens of 1 mW/cm2 at a distance of 5 cm. The major biological effect of continuous microwave irradiation at levels between 10 and 100 mW/cm^2 is believed to be heating. Human exposure can be minimized by providing shielding for the personnel stationed in the rectenna area and by limiting public access to regions in which the Gaussian power distribution is below acceptable radiation levels. The system could be designed so that the microwave power density at 10-15 km from the center of the beam would be at most 10 HW/cm^2, meeting the lowest international standards for continued exposure to microwaves. Passengers in aircraft flying through the beam should be more than adequately protected by the metallic skin as well as the short transit times involved. By fences and a metallic screen under the rectenna, plant and animal life can be protected. Birds flying through the beam would experience elevation of body temperature (ref. 4).
Radiation effects do not appear to present substantial problems to transmission of power from space, but more research is required.
System efficiency and lack of atmospheric attenuation suggest 10 cm as the wavelength for transmission. The corresponding frequency, 3 GHz, can be controlled to within a few kHz (ref. 11). Due to the high power involved, electromagnetic noise could potentially interfere with radar, microwave and radio frequency communications, and possibly with radio astronomy. This will necessitate further restrictions near the rectenna.