Figure 6-41. Waveguide Weight and Packaging Density 6.8 ATTITUDE CONTROL AND ALIGNMENT The subarrays must be aligned to a nominal, ideal surface within about one arc minute to maintain efficiency. It is proposed that the reference alignment plane be established at the center subarray by means of interferometers that determine the angle of arrival of the pilot beam. This primary sensor with triple redundancy would be backed up by other instruments: star sensors, earth sensors, inertial sensors. A best estimate of line-of-sight to the ground would be determined by redundant processors in the central subarray, which becomes the command-control center not only for MPTS, but for an entire SPS as well. Alignment of each subarray at installation and at intervals throughout the system lifetime (perhaps following eclipse periods) requires a similar reference. It is recommended this also be based on interferometers and that the necessary adjustments be made using the screwjacks noted above and described in detail in Section 8. Alternate sensing approaches would be (1) to employ the
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