requirements imply a transmitting antenna on the SPS about 1.0 km in diameter. The pointing loss becomes unacceptable if the miss radius exceeds 200m, which, in angular terms, corresponds to 5.6 x 10^-6 radians = 1.1 arc seconds. Because of the obvious difficulty in mechanically pointing a 1.0 km diameter antenna to this accuracy, the proponents of the SPS suggest using an ARA for the spacecraft antenna with the pilot source located at the center of the rectenna. An equally important reason for using an ARA for the SPS is safety, specifically, the need to protect the public from exposure to the high power beam. Although no beam pointing system is infallible, the ARA would seem to be the most inherently reliable system for this application since its retrodirectivity is inseparable from the beam forming process itself. Such pointing errors that are known to exist (discussed in Sect. II) produce only slight (compared to rectenna diameter) mispointing. Moreover, the response time of an ARA is determined by its own dimensions, not by the ground to spacecraft round trip delay as it would be for a conventional closed loop control system. It would, therefore, be of the order of microseconds, not 2 x 36 x 10^6 /(3 x 10^8 ) = 0.24 seconds, the round trip delay for an SPS in geosynchronous orbit. Communication Satellites The applicability of ARA’s to communication satellites was noted by Hansen [6] and others. Large antennas on communication satellites will be required not only to serve ground receivers with small apertures (as in direct TV transmissions) , but also to provide the directivity needed for spectrum conservation. A communication satellite ARA could use either frequency or time division multiplexing. In a frequency multiplexed version designed to communicate with each of n ground stations, each element of the array is equipped with n phase
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