requiring further in-depth studies as types of terrain, soils mechanics, and environments are established. It is likely, for example, that the rectenna elements as currently envisioned cannot withstand hurricane force winds. The recommended rectenna elements are linearly polarized dipoles with GaAs Schottky barrier diodes. Development of these elements is also needed. e. Phase Front Control It was shown theoretically in 1961 that microwave power can be transferred at high efficiency when the transmitting antenna is illuminated with an amplitude distribution that is near Gaussian, and when the phase front of the beam is focused on the receiving antenna (see Exhibit 19) . To maintain high efficiency the front must be controlled with high precision, and this requires that the antenna be sectored into numerous subarrays to provide the necessary center to edge amplitude taper. Each subarray must have phase control electronics, and efficiency and safety considerations dictate that it be closed loop. Two control approaches are possible, adaptive and command. The command approach would utilize a matrix of sensors at the ground antenna to determine the received power beam center and shape. A processor would then develop commands for transmission back to the SPS via the telecommunications link. This approach has limited resolution but it is anticipated that it could be developed to counter SPS antenna thermal distortions. A potentially more accurate, adaptive scheme would utilize a reference beam aimed from the center of the ground antenna. This reference beam would be sensed at the SPS and interpreted through its phasing; corrections would then be made in the phase of the transmitted power beam. The electronic phase control subsystems for the SPS have been identified as a critical technology item requiring early development. These subsystems must perform with high accuracy to keep beam scattering losses at acceptable levels, and also to point the beam at the center of the rectenna for both high efficiency and safety. The tolerable errors are sufficiently small that early development and demonstration is mandatory.
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