The ground irradiance produced by specularly reflected light from Space Power System spacecraft is given by4 where K is the degradation due to atmosphere and/or instruments, N0 = 2.0 x 107 watts/ster-nr is the average visual disk radiance of the sun, r$ is the specular reflectance of surfaces, a is the area of the surface in m^, a is the angle between the incident and reflected rays, R is the distance from the SPS subsystem to the earth in meters, a is the angle at the SPS subtended by the solar disk, p is the diffraction limit for coherent reflection from an element of SPS area 6a m2, and t is the angular divergence of the solar image due to the fact that the reflectors are not optically flat mirrors. The ground illumination from sunlight reflections off the Space Power System spacecraft have been evaluated for a variety of configurations, orientations, and operational conditions, that are thought to produce the brightest irradiances. A summary of ground irradiance levels that have been calculated is presented in the accompanying table. The diffuse cases are all relatively bright in comparison with stellar sources. For example, the SPS in GEO casts an order of magnitude more light than Venus at its brightest. The OTV/SB combination in LEO is visible during daylight hours but, of course, is at too low an altitude to be illuminated at night. The specular cases cited in the table produce much brighter ground illumination. However, this irradiance is restricted .to small, fast moving spots. The actual duration of these "glints" of specular reflections varies from about one second for the OTV/SB in LEO to two minutes for the SPS antenna. An important consideration is the sudden onset of the specular irradiance compared to the much dimmer diffuse irradiance. Enhancements of 10$ are common. An exceptionally bright specular reflection is produced by the backside of the OTV solar panels during LEO construction. Although perfectly flat solar panel surfaces are assumed as worst cases for the OTV and SPS, more realistic situations are represented by the curved or misaligned surfaces that are also analyzed. These worst case conditions in the table 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. 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 panels
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