essentially passive (microwave reflectors). Geosynchronous orbits for the relay points have most often been considered, although the use of transponders in low earth orbit has been proposed. The feasibility of a PRS significantly depends on large-scale, highly efficient transmission of energy by microwaves. In addition, the PRS shares many of the requirements for other SPS concepts, such as large construction in space, transportation, and supporting terrestrial systems. The PRS concept under study by A.D. Little et al (see, for instance, Ref. All) consists of a primary structure that is built up of 354 ft x 354 ft x 66 ft (108m x 108m x 20m) deep bays. A substructure provides support for the microwave reflector. The ground transmitting antenna is a phased array with waveguides and converters, similar to that of the orbital SPS except larger. The PRS receiving antenna is a rectenna similar to the ground-based SPS rectenna. If ground power densities are to be 2 2 held to about the 130 mW/in (20 mW/cm ) level, the ground transmitting array and rectenna would have to be 6 miles (10 km) in diameter, and the orbital reflector should be 0.6 miles (1 km) in diameter for 5 GW to 10 GW systems. Exhibit 15 depicts a PRS system and lists some of its characteristics. There are a number of unique PRS requirements that will impact on its design, development and operation. These are described in detail in Ref. All, and are summarized in the following six paragraphs. (1) Atmospheric effects and errors at the ground-based transmitting antenna require that it be sectored into subarrays which must be controlled. Control can be accomplished by adaptive control which requires a reference beam sent from the reflector. Alternatively, a sensor matrix at the reflector could provide command control. (2) Transmission losses increase at lower elevation angles, and at angles below 20° excessive land areas would be required to maintain the required transmission power levels. This places a limit of about 60° latitude on the ground rectenna location. (3) Maximum PRS transmission distance with one satellite would be 4,500 miles (7,200 km) due to the earth's curvature. An Arizona-Japan
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