c. Yaw Mode Yaw mode dynamics has been defined to be rotation about the spacecraft's Z axis in the X-Y plane as shown in Figure 42. FIGURE 42 YAW AXIS The baseline rigid body mass properties of the SSPS about the Z axis are listed in Table 12. TABLE 12 YAW AXIS MASS PROPERTIES An analysis of the structural dynamics about this axis identified four lateral bending modes for the baseline structure. The mode shapes for each of these modes are shown in Figures 43 through 46. As indicated, Figures 43 and 45 represent symmetric bending modes, while Figures 44 and 46 represent anti-symmetric modes. The modal data at the extremities of the structure in this yaw mode are given in Table 13. Data are presented for only one side, since the structure is assumed to be symmetrical. A detailed discussion of these mode shapes is given in Reference 36. in its nominal orientation the spacecraft's axis is perpendicular to the orbit plane and the Z-axis oscillates in the X-Z plane about the Y-axis at 23-1 /2 deg/year. In this orientation the SSPS only experiences a solar pressure torque which is proportional to the angular deviations of the spacecraft about the Z-axis. Numerical values for these torques are given in Table 14. For the baseline spacecraft it was found that in order for the yaw-mode dynamics to be characterized by a 0.5 damping ratio and an undamped natural frequency of 0.0025 rad/sec 1/10 lowest anti-symmetric yaw bending mode, it is necessary that the attitude control have the following control gains (from Equations (1) and (2).
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