or an Arizona-Spain transmission link represents near maximum transmission distance when the effect of elevation angles are accounted for. (4) The peak transmitter power density parameter is inordinately 2 2 driven by capital cost. The 320 mW/in (50 mW/cm ) point is at the "knee” of the cost curve; lower densities imply great risk of cost escalation, higher densities increase the biological/environmental risk. (5) For efficient microwave performance, the surface roughness must be less than 0.2 inches (5 mm) across the 0.6 mile (1 km) reflector (Ref. A4 indicates surface flatness must be held to about 50 mils (1 mm)) . In addition, the relative vertical displacement of all reflector subarrays must be maintained to within a fraction of a wavelength. (6) In terms of comparative costs, the baseline PRS would provide less costly energy transmission than present 765 kV ac overhead lines only for distances greater than 3500 miles (5600 km). It is also higher in cost than most other projected systems with the exception of underground cables. This relatively higher cost is the result of both high capital costs and unavoidably high transmission losses. 5. Reflecting Systems Four reflecting systems have been suggested, none of which would generate power but which could be useful in transmitting it. The four systems can be identified as follows: • Ground to ground microwave transmission via a passive reflector at geosynchronous orbit. • Ground to ground microwave transmission via an active transponder at geosynchronous orbit. • Sun synchronous satellite to geosynchronous active transponder relay to ground. • Large orbiting mirror at geosynchronous orbit to reflect sunlight to the ground for night operation of terrestrial solar power plants. The first listed system has received the most attention and appears to be the most nearly feasible system. It is essentially the baseline Power
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