results, however, indicate that orbit maintenance can be combined, at least partially, with attitude control to reduce total propellant consumption. In an eccentric orbit (uncorrected for radiation pressure), the large size of the SPS results in an unacceptable probability of collision with other synchronous satellites. Holding the eccentricity to zero will greatly reduce this probability. In summary, the preferred orbit for the SPS is geosynchronous with zero inclination and eccentricity; i.e., geostationary. Orientation and Attitude Control - The orientation of the SPS must satisfy two conditions: the solar energy collection system (SECS) must point at the sun and the microwave power transmission system (MPTS) must point at the ground station. The first consequence of this is that the MPTS must rotate continuously at one revolution per day relative to the SECS. This rotation is about an axis perpendicular to the orbit plane (POP), requiring that the MPTS be located at the north or south end of the SECS, mounted centrally on a rotary joint of sufficient size to span the SECS carry-through structure, or mounted centrally on a small rotary joint within dielectric structure for minimum interference with the microwave beam. Early SPS concepts used the last of these. Subsequent studies have avoided this approach in order to eliminate an unnecessary source of interference, the nature of which is not completely understood. The principal disturbance acting on the SPS is gravity gradient torque. Gravity gradient torque exists whenever the spacecraft principal axes are not orthogonal with the orbit plane and local vertical and the principal moments of inertia are not equal. An asymmetrical SPS, with the center of pressure offset from the mass center, will also experience torque from solar radiation pressure. Gravity gradient affects the SPS in two ways. The first occurs when the sun is not in the orbit plane (that is, at all times except at the equinoxes) if the SECS is pointed at the sun (figure A-2). This torque cycles with a period of one year, making momentum storage unattractive. Since the maximum sun angle is 23.4°, however, the SECS can be held in a POP attitude at all times. This eliminates the torque and causes a maximum power loss of only about eight percent.
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