Configuration 3 provides a "dog-leg" structure mounted to the solar array at axis C, which positions the mass center of the antenna more nearly on the primary axis of the SPS and positions axis A parallel to the earth's polar axis. Configuration 4 combines axes "A" and "B" into a ball-joint, and rearranges the antenna configuration from a disc to a cone having its mass center at the ball-joint. Configuration 5 is same as configuration 4 except that the solar array is oriented to be perpendicular to the orbit plane (P.O.P.), eliminating the requirement for axis "C". (Axis C was necessary to allow the array to be faced toward the sun while the dog-leg was positioned so that axis A was parallel to the earth's polar cixis). Figure IV-C-8-3 indicates antenna configuration 3 of the prior figure installed on the SPS, and shows the overall geometry and required basic rotation about the three axes. Figure IV-C-8-4 shows the nominal forces acting on the antenna support structure. These forces, resulting from orbital mechanics, are necessary to keep the antenna attached to the other parts of the SPS. It should be noted that the gravity gradient torque on the disc configuration antenna is on the order of 1000 N-m, when the antenna is pointed toward a rectenna at an earth location as indicated on the drawing. There is also a dynamic unbalance due to the antenna's rotation about the axis normal to the orbit plane. This unbalance must be reacted by a torque applied to the antenna of about 200 N-m, as shown in the figure. Thus, about 1200 N-m of torque must be applied to the antenna to hold its attitude. The torques to correct antenna pointing errors would be applied in addition to the torque necessary to hold a fixed attitude. These forces and torques would appear to be relatively insignificant when considering the joint drive system. However, it is believed that a system to point the antenna by driving each joint mechanically, with torques acting on the antenna and reacting on the SPS structure, is not likely to provide the required pointing accuracy of about 1 arc minute. Figure IV-C-8-5 attempts to illustrate the problem and its possible solution. If the antenna structure and the SPS structure were very rigid, such that the sketch on the left of the figure were representative of the situation, a direct drive at each joint could probably be designed to point the antenna within one arc minute. However, the SPS is more nearly like the middle sketch. A servomechanism operating under these conditions is not likely to achieve precise pointing of the antenna. The solution is believed to be the concept on the right. Control moment gyros mounted on the antenna are used for attitude control torques input to the antenna, with the antenna being mounted through the "softest" possible mount to the SPS (low stiffness interconnection). The necessary rotary joint through which the electrical energy must pass would be driven by a servomechanism to track the antenna as its position and attitude
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