L. Leopold L. Livingston J. Seyl IV-C-5 ANTENNA POINTING CONTROL a. RATIONALE Antenna pointing is required to point the SPS antenna to the ground rectenna to effect the maximum transfer of microwave energy from the solar power station to earth. The first priority is to accomplish the initial acquisition of the beam of the spacecraft antenna. The second is that of maintaining the antenna beam so that the maximum power density remains at the center of the rectenna. b. DEFINITION The possible methoas employed to perform the above are: (1) By utilizing the known orbital parameters of the satellite. (Guidance and Control System). (2) Obtain attitude of the spacecraft from star trackers on board. (Guidance and Control System). (3) Obtain pilot beam from rectenna to remove ambiguities of antenna pointing away from rectenna. (4) By utilizing phase control to supplement inaccuracties in precise pointing of the antenna. c. OPERATING CHARACTERISTICS The antenna pointing system operates in either an acquisition mode or a tracking mode. The acquisition mode is primarily required during start-up, when the antenna may not be oriented accurately enough for the tracking mode to function. The tracking mode operates while power is being transmitted and also operates during eclipses in order to minimize restart time. In the acquisition mode, for initial start-up or after an extended period of down time, it must be assumed that the antenna is in an arbitrary attitude from which the phase control receivers cannot receive the pilot beam. However, the ephemeris of the orbit will be accurately known from ground tracking methods and SPS attitude can be determined from onboard star tracker and horizon scanner data. Thus, the required antenna gimbal angles can be computed for the known rectenna location on earth, and the antenna driven to the computed desired position. The accuracy of this position will probably be limited by structural alignment errors and flexibility. (No quantitative assessment of these errors has been made, but it should be possible to hold them well under one degree). If necessary, attitude sensors could be mounted directly on the antenna to reduce the errors, but this does not appear to be required. The tracking mode utilizes the retrodirective phase ("pilot") beam as received at seven subarrays (see Figure IV-C-5-1). Primary pointing information is obtained from the four receivers at A and B, whose signals are fed to two phase comparators in pairs to determine pointing errors in azimuth (A) and elevation (,B). To minimize distortion errors, the receivers are located symmetrically on the axes of the antenna. The separation distance "d" will be determined by the maximum height differential between subarrays (see Figure IV-C-5-2), since even the
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