Fig. 7. Specular reflection from solar array with attitude error. the sun and the orbit plane, a reflected ray could strike the earth only when the earth is between the SPS and the sun. Hence, specular reflection from the array will not be a problem in the nominal orientation. However, as Ekstrom (4) has pointed out, the array will not in general be precisely in the nominal orientation, but somewhere within a small attitude deadband. This deadband has not yet been defined in detail, but can be assumed to be less than a degree. If the error is such as to tilt the normal to the array toward the sun when the sun is near the earth’s limb as seen from the SPS, then a reflection could strike the earth. This can occur only near midnight during six-week periods centered on the equinoxes. If the reflected beam is seen only near the horizon, atmospheric absorption will mitigate any adverse effects. The angle of incidence at the ground, 0, may be readily calculated from the attitude error, -y (see Figure 7): This function is plotted in Figure 8, from which it may be seen that a deadband of 0.1° will allow no reflection to be visible more than about 12° above the horizon. A significant decrease in this maximum elevation requires a major reduction in deadband which may not be practical. For y = 0.1°, the reflection will be visible no more than 26° from the terminator if atmospheric refraction is neglected. The maximum duration at a given location is about two minutes, and the times at which a reflection could occur can be calculated in advance. Consequently, the impact of such reflection on astronomical observations is not likely to be of major importance. If, because of possible retinal damage to observers (4) or for other reasons, all specular reflection from the array must be avoided, a non-POP attitude can be flown during the periods of possible reflection. If the commanded angle between the orbit plane and the normal to the array is and the maximum error is y, then the following
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