Control System Requirements The basic philosophy of the stationkeeping control system is to actively control semimajor axis variations caused by eclipsing, inclination changes caused by gravity perturbations, and east-west drift caused by gravity perturbations. The effects of microwave recoil are controlled by biasing orbital altitude to maintain the desired orbital period. Solar pressure induced changes to orbital eccentricity are not counteracted. In some other SPS studies, solar pressure induced eccentricity has not been controlled as long as fewer than 15 satellites were in the constellation. After the 15th satellite was installed, active control of eccentricity was started at the expense of large quantities of propellant consumed. Such control is not needed to prevent intersatellite collisions even in very tightly packed constellations. Solar pressure acts on each of the satellites in fundamentally the same way so that each member of the constellation moves from the original circular orbit to the same elliptic orbit then back to the circular orbit. During periods when the eccentricity is near its maximum, the deviation of each satellite from its nominal position exceeds the intersatellite spacing. However, no collision hazards are present since the deviations for all the satellites are of the same magnitude and in the same direction. In other words, the whole constellation moves together with little relative motion between the satellites. This is demonstrated in the following analysis. Consider a satellite moving in a nominal unperturbed circular orbit. The motions of two closely spaced satellites relative to the nominal position can be determined from a first order, linear expansion around the nominal where displacements and disturbances are small. Only planar motion is considered. The situation is illustrated in Figure 2.4-5. Figure 2.4-5. Satellite Motion
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