Relay Satellite (PRS) concept as described in the preceding subsection. The other three systems have received scant attention but the following observations have been made. The second system, an active power relay satellite (PRS), would require so much power for its own operation that it would present most of the problems associated with both generating solar power and relaying it, while having fewer of the advantages associated with either a pure generating or relaying system. The third system, i.e., the sun synchronous generation of solar electric power combined with a geosynchronous relay to earth, is a relatively recent concept that takes advantage of the sun synchronous orbit (best for power generation) and the continuous receipt of power at the same terrestrial location. The economics and competitiveness of this system are largely unknown but it does suffer from the requirements for active transponders and added power generation. The fourth system, i.e., a large mirror in geosynchronous orbit, has been proposed to direct solar radiation to a ground-based solar power plant for night operation. Because the sun is not a point source, the minimum size of the reflected image from geosynchronous orbit is 52,000 2 2 mi (134,000 km ). To provide a reflected solar image of one sun intensity, a mirror having the same area is required which would have a system mass of 45,000,000 tons (4 x lO^ kg). If targeted anywhere in the United States, the reflected solar radiation would involve at least 50,000 inhabitants, and the maximum daytime temperature in the affected region could reach 150°F. This could have severe environmental effects (Ref. A17). These considerations (the mass alone is over two orders of magnitude greater than the most massive orbital generation system) would seem to remove this alternative from further consideration. The safety aspects of all reflecting systems are very important. The nominal peak microwave power density level at the ground transmitting 2 2 . . antenna is 320 mW/in (50 mW/cm ), for example, and at the receiving an2 2 tenna is 130 mW/in (20 mW/cm ). This is to be compared to the U.S. safety 2 2 standard of 65 mW/in (10 mW/cm ) of continuous exposure. Other nations have set considerably lower continuous exposure levels. The comparative cost aspects for the baseline, passive PRS are given in the preceding subsection. No cost comparisons have been made for the other reflecting systems.
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