3.3 CONFIGURATION ANALYSIS 3.3.1 Control Analysis The characteristics of a phased array Microwave Power Transmission System eliminates the need for mechanical fine pointing of the antenna. Signal phasing compensates for misalignment and distortions in antenna surface up to 1/16 of a wavelength with minimal loss in transmission efficiency. An overall antenna misalignment error of ± 1 arc-min can be tolerated by the subarrays. The purpose of this subtask is to define the environment and load requirements for the design of the antenna pointing system servomechanisms, define the best accuracy that can be achieved with this system and identify the likely design approach. 3.3.1.1 Spacecraft Torque Environment Figure 3.3-1 summarizes the torque environment for the baseline SSPS system (Ref. 2). Torque calculations are based on the configuration data presented in Fig. 3.1-2 and the following additional groundrules: • Baseline orbit - equatorial geosynchronous • Baseline attitude - long axis (X-axis) perpendicular to orbit plane, the solar array normal (Z-axis) parallel to the projection of the sun vector onto the orbit plane. The external disturbance torques are induced by aerodynamic, solar pressure, magnetic and gravity gradients. Gravity gradient torque predominates the induced torque environment by several orders of magnitude and will be the only source of external torque used to define mechanical system requirements. Control system torque levels are limited by SSPS structural bending. A force level of 2980N (667 lb) used for orbit keeping and applied at the comers of the solar array, was found in Ref. 3 to be the maximum force at which structural deflections can be limited to ± 1 deg. This force, however, induces symmetric bending and does not affect antenna motion. A 44.5N (10 lb) coupled jet firing is used for attitude control and induces antisymmetric bending modes which do impact control system design. 3. 3.1.1.1 Antenna Motion Relative To Spacecraft - Figure 3. 3-2 shows a typical system that provides rotation in azimuth and elevation. The azimuth rotary joint is located at the mast interface between the antenna assembly and the solar arrays. Azimuth motion is provided by variable speed motor drives located at this interface. An actuator for elevation control could utilize proportional linear control (worm gears and linkage) and would be located at an offset distance from the main antenna-to-mast rotary joint.
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