Orbital Profile and Environment The spacecraft orbiting the earth encounters eclipse (i.e., no-sun) periods at some points in time due to the spacecraft traveling through the earth's shadow. The eclipse duration is a function of the orbit inclination angle (with respect to the equator), orbit altitude, and launch date and time. It is easily and accurately predictable, especially for circular orbits and negligible orbit decay. Figs. 8 and 9 show typical variation of eclipse duration and Beta angle with time for LEO and GEO spacecraft, respectively. The significant characteristics of the eclipse profile are the following: (1) the eclipse profile for one year repeats yearly, (2) for the LEO, over 5% of the orbits in a year contains no eclipse durations ( They are in continuous sunlight); over 40% of the orbits have eclipse durations of at least 25 minutes (see Table 4), and (3) for the GEO, about 24% of the orbits in a year are in the eclipse season and 76% in continuous sunlight. The principal solar array degradation sources are the radiation from the Van Allen belt, solar flares, UV radiation, plasma interactions, and thermal cycling. Among them, the largest uncertainty is the effects of plasma interaction. This phenomenon may result in some loss of array power due to high voltage insulation failures. In array sizing, a large safety margin is often used to allow for both known and unknown causes or uncertainties in degradation, including thermal cycling and random damages. The net result is that the arrays are significantly oversized to meet the beginning of life (BOL) power needs. Thus a large surplus in power exists at BOL, theoretically diminishing to zero at end of life (EOL). The array degradation factor, therefore, is a large contributor to the substantial power margin in the early phases of the mission. Spacecraft Operational Modes The Space Station is expected to see many of the operational modes encountered by the SKYLAB-- solar inertial and gravity gradient orientation, rendezvous and docking, various experiment operation, full-power and powered-down modes, etc. Dramatic differences from any of the past spacecraft are the result of an indefinite lifetime criteria and the concept of building up the
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