Pointing control is an important system technology for the success of a mission. The terms pointing accuracyEand attitude accuracyEare vague. We have to define pointing control accuracy, pointing stability, pointing determination accuracy, etc., in order to express them numerically. For example, SPICAs strictest requirement is as follows.
First, in order to align the telescope to a target star, the absolute pointing control accuracy of the detection system must be within 1 arc sec. Next, to photograph a planet after masking the star to locate planets around it, the directional stability for one minute must be within 0.03 arc sec. in half amplitude. This value is necessary to satisfy the condition that the image of the planet remains still and does not go out of focus during the telescopes exposure time. As 1 deg is 60 arc min, 1 arc sec becomes 1/3,600 deg. This is an angle to cover a one-yen coin (2cm in apparent diameter) 4km away. Although it is a simple question of how to measure this small angle, the first problem is how to realize these pointing accuracies in a vibrating environment where several mechanical refrigerators are operating.
On the other hand, regarding ASTRO-G, despite the fact that it is in unavoidably turbulent environment, the satellite actively moves itself as well. The satellite frequently repeats a motion called the fast-switching maneuver necessary for its observation mission. The satellite rotates its large antenna to 3 deg in 15 sec together with the satellite body, then comes to rest for the purpose of measurement for 30 sec, and rotates 3 deg in 15 sec again to return to its original position. As the large antenna is not a rigid body, there is a possibility that it vibrates slightly. The satellite motion must be controlled within an attitude-angle accuracy of around 0.005 deg each time. Imagine riding a bicycle (satellite) with a watermelon (antenna) hanging from the handle and trying to go around an S-shape bend without swinging the watermelon.
Fig. 4 illustrates these dynamic issues. In the figure, the intermediate vibration-frequency region is the interference region of the satellites flexible attachment and attitude control system as discussed with ASTRO-G. The high vibration-frequency region is the disturbance problem region as discussed with SPICA.
Along with advances in the satellite mission (the shift to high-tech), the typically low-tech satellite structure also has to become a precision instrument. Due to space limitations (a convenient excuse), I cannot describe how to solve these problems here. Anyway, many difficult issues remain and, of course, satellites can only be launched after solving the problems.
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