"percentage of occultation" refers to the total amount of sunlight falling on the array. As discussed below, this is not necessarily a measure of useful power output. It is assumed for these calculations that all satellites are in precisely the same orbit, presenting an appearance similar to figure IV-A-3-15 if viewed from the center of the eclipsed satellite. In practice, such precision may not be achieved. An inclination difference of .033°, for example, would displace a satellite 24 km perpendicular to the orbit plane, shifting by about 12 days the eclipses of a satellite at 0.5° longitude separation. The overall result of such staggered eclipses would be less power loss on any given day, more days with some loss, and slightly more total loss per year. In the worst case, however, total power loss is less than 0.1% of the annual output. Going one step farther, an inclination difference of 0.22° with the line of nodes toward the sun would elevate the satellite 23.5° above the equator as seen from the next satellite in line, eliminating all eclipses by that satellite. The impact of eclipses by other satellites could be greatly reduced in this way, since the nearest satellite causes most of the shadowing (figure IV-A-3-16). However, the orbit maintenance problem would become more complex, particularly since both lunar/solar perturbations and nodal regression due to the earth's oblateness would adversely affect the desired orbits. A more serious question is the relative sharpness of the shadow. The width of the penumbra is proportional to the spacing between satellites; at 0.5° spacing, it is only about 3.4 km wide. Since the .array is much larger than this, the illumination intensity can vary over the array from full sun to total darkness (figure IV-A-3-17). If cooling rates are comparable to the maximum duration of the eclipse (15 to 20 minutes), differential contraction could produce thermal stresses and distortion. The severity of this problem is a function of configuration and materials; the column/cable configuration, for example, can be expected to experience greater stresses than a non-redundant truss structure (which could distort without developing internal stresses). However, the need for some tension in the solar cell blankets and concentrators, together with the limited choice of materials for these items, may make some thermal stresses unavoidable in any structural concept. Variable SPS Orbits for Eclipse Avoidance It is conceivable that the power interruptions caused by eclipses could be eliminated by changing the orbit of the satellite from time to time so as to maintain continuous illumination. The following discussion explores this possibility in detail.
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