Table 2♦4-2. Reference Attitude Control System Mass Summary 2.4.2 Stationkeeping The purpose of the stationkeeping system is to maintain the orbit in the presence of various perturbations. Each satellite must be kept near its nominal position to maintain good efficiency in the microwave power link. Additionally, each satellite must have its position with respect to the remainder of the SPS constellation tightly controlled because of the close spacing of the satellites. The most important perturbations affecting the SPS orbits are solar pressure, gravity perturbations, microwave recoil, and insertion velocity errors. Microwave recoil is a perturbation that is unique to SPS. No previous spacecraft has transmitted sufficient power to experience a significant recoil effect. Subsequent sections discuss each of these perturbations and their effects on the SPS orbit. The last section describes the requirements of the stationkeeping system to counteract these effects. Solar Pressure Effects During periods when the satellite is not being eclipsed, the only long term affect of solar pressure is to cause periodic variations in the orbital eccentricity. Starting from a circular orbit, eccentricity increases to a maximum value, then decreases back to zero in a period of one year. Maximum eccentricity is given by^ ^Kampos, B., Guidance, Flight Mechanics, and Trajectory Optimization, Vol. IX-General Perturbation Theory. NASACR-1008 (April 1968).