HIGH-POWER MICROWAVE OPTICS FOR FLEXIBLE POWER TRANSMISSION SYSTEMS K. Eric Drexler - M.I.T. Space Systems Laboratory, Cambridge, Massachusetts B. Ray Sperber - Boeing Aerospace Company, Seattle, Washington In the conventional SPS concept, a one kilometer diameter phased array broadcasts directly to a ten kilometer wide rectenna. Diffraction optics, economics, and microwave power density limitations at the transmitter and in the ionosphere set the power of this system at 5 GW, and have restricted consideration of alternative systems to powers within a factor of two of this level. While such a system might prove attractive, a system with far greater flexibility appears feasible. A non-optimized concept is presented below. A ‘large concave microwave mirror near the transmitter can magnify the apparent size of the Earth as seen from a phased array, and vice versa, permitting a small phased array to be coupled to a small rectenna while preserving the transmission efficiency (the reflection loss is slight) and peak power densities characteristic of the reference system. This augmentation of the phased array aperture with a large mirror gives the system greater resolution (in the optical sense), and opens new degrees of freedom in SPS design. The consequences of such an approach for a prototype satellite have been explored (1,2). The following will discuss its consequences for a mature SPS system. Using this approach, the mature SPS will have many phased array feeds utilizing a common mirror to couple to many rectennas. Total satellite power might be some 20 to 50 GW (reducing the number of orbital slots needed), with a mirror perhaps 5 kilometers in diameter, and of 100 kilometer focal length. Such a mirror must be actively configured and could be quite light (3 A» 5). Figure 1 illustrates a gravity gradient stabilized configuration. Since a mature SPS system will surely involve active structural control, no attempt has been made to make the structure rigid (permissable deflections in the microwave optical path are minute). System mass is discussed in Table 1. As Figure 2 indicates, the phased array feeds are located in front of the mirror's focal plane, at a point where a power density equal to that of the reference system's transmitter will produce the reference system's power density at the ground. At this point aberration from the mirror produces only minor variations in phase and power density relative to a perfect optical system. The array is large enough and close enough to the mirror to have independent control over the phase and power density at some 100 resolution elements on the mirror, justifying the assumption made regarding control of the outgoing beam. Calculations assuming a spherical mirror indicate adequate performance, which can surely be improved on. This augmented-aperture system behaves like a retrodirective array five kilometers across and able to form many beams. Since it is five times the diameter of the reference system antenna, it can efficiently serve a 2 kilometer, 200 MW rectenna. Busbar power cost will be slightly higher than for the reference system, because of the added system element, but busbar cost is only part of the system cost. Power transmission on the ground adds sub-
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