4.2 RECOMMENDATIONS The worst case conditions in Table 5 have ground irradiance levels that may exceed acceptable limits. Evaluation of the ocular irradiance levels that correspond to these ground irradiance levels is required to completely assess the reflection limitations that will be imposed on the Space Power System. Nevertheless, it is prudent to consider options for reduction of reflected sunlight from these vehicles. Possible methods for reducing reflections fall into three major categories. Vehicle Orientation. Since the major ground illumination is produced by large flat surfaces on the OTV and SPS, it is appropriate to inquire about reorienting the vehicles to direct specular reflections away from earth. Since solar power collection falls with the cosine of the tilt angle, for example, an 8° tilt of the solar panels causes a 1% power loss, but specular reflections are shifted 16° off the sun-earth direction. In the case of the SPS, if modest power losses can be tolerated on its solar panel, specular reflections can be eliminated. Specifically, it requires a 5° tilt at geosynchronous altitude for specular reflections to pass over the north or south pole. If the solar panels are misaligned +3° from a perfectly plane surface, an 8° tilt would assure no specular light is returned to earth from the SPS solar panels. The SPS antenna reflections will necessarily strike 2 earth due to ground receiver pointing requirements, but other modifications are suggested below to alleviate this ground irradiance. Similar power-reflection tradeoffs are workable for the OTV during its transit. Near LEO, the tilt constraint is quite small (a few degrees) since the vehicle is eclipsed. Orbit inclination considerations are required to evaluate the tilt profile during the full LEO to GEO transit. The construction phase requires OTV solar panel orientation to be in the orbit plane to reduce atmospheric drag, and therefore no reorientation seems feasible. Surface Curvature. Most of the large surfaces that produce strong reflections are nominally flat in the Baseline Design. In practice, however, the vehicles are expected to flex under thermal and propulsion loads causing some misalignment of flat elements. Intentional misalignment of large solar
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