A. Design Considerations This section summarizes several trade studies dealing with orbit selection, orientation, system sizing and constructability. These trades have a direct, major influence on the overall configuration of the satellite. Orbit Selection - Geosynchronous altitude (35800 km) has been used in all studies. Near uninterrupted transmission is possible, antenna steering accelerations are low, and the satellite is stationary with respect to a point on earth. Three inclinations have been considered (figure A-l). Zero inclination gives a stationary satellite, simplifying the rectenna design. A 7.3° inclination eliminates the lunar and solar perturbation which would otherwise cause the inclination to vary between zero and 15° over a period of years, if uncorrected. A 23.4° inclination places the orbit in the ecliptic plane, permitting the satellite to be oriented simultaneously toward the sun for maximum output and perpendicular to the orbit plane (POP) for minimum gravity gradient torque. Non-zero inclinations cause a daily variation in the angle of incidence on the rectenna of about twice the inclination, increasing the rectenna area required and imposing constraints on the rectenna design for which solutions have not yet been identified. The zero-inclination orbit can be maintained with a relatively small propellant budget, however. All studies have concluded that zero inclination is preferred. Solar radiation pressure acting on the large, low density solar array induces a small, variable eccentricity in the orbit. This causes a daily oscillation of several degrees of longitude. The design impact of this oscillation is relatively minor: the resulting angular acceleration of the transmitting antenna is manageable and the moderate east-west motion of the microwave beam can be easily accommodated by most proposed rectenna designs. Moreover, the propellant penalty for maintaining zero eccentricity is substantial, amounting to roughly half of the total attitude control and orbit maintenance requirement for typical configurations. Consequently, early studies tended to tolerate the eccentricity in order to reduce propellant resupply requirements. Subsequent
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