1, 3. 8 MICROWAVE POINTING AND CONTROL 7. 3. 8.1 GROUND RULES/GUIDELINES The ground rules and guidelines given for the photovoltaic conversion system apply to the microwave power system pointing and control subsystem. One possible exception could be the use of control moment gyros (CMG) as stabilizing and control actuators for the antenna structure. For this exception to be feasible, however, an improvement in the present momentum to weight ratio CMG technology must be realized. 7. 3. 8. 2 POINTING AND CONTROL CONCEPT Pointing the transmitting antenna array to the ground rectenna with the desired accuracy requires the following approach: • Orientation of the SPS main body to the reference attitude will be to an accuracy of ±1° in three axes. • Acquisition of the ground rectenna by the microwave beam, and maintenance of this pointing direction, to an accuracy of ±1 arc min in two axes with an error of less than 8° about the line of sight will be accomplished by a gimbaled antenna pointing control system. • Fine pointing of the microwave beam to an accuracy of ±3 arc s will be accomplished by the antenna phase control subsystem. Initial acquisition of the ground rectenna will be accomplished by processing orbital parameters, the known attitude of the SPS with respect to Sun and Earth coordinates, and the rectenna location with respect to the SPS. After coarse pointing of the antenna, a scan sequence may be required for final acquisition. If a scan sequence is required, attitude errors will be obtained by processing a portion of the pointing information obtained by the phase control subsystem. For a given orbit and SPS attitude, the predominant antenna motions required to maintain pointing are well defined. For example, a rotation of 360° per day about the X axis is required as the Earth and SPS rotate B synchronously and the Z axis is oriented in a sunwardly direction. The antenna tilt motion and roll about the line of sight are also well defined once the SPS reference attitude has been acquired. Therefore, these predominant motions can be preprogrammed to take place in a well defined time sequence.
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