During assembly in low Earth orbit, the major perturbation that must be corrected is that resulting from aerodynamic forces acting on the system. The aerodynamic drag force acting on the system is where the projected area (A) is measured in square meters. The area is continually changing during the SPS assembly. Assuming the solar array is 25 per cent deployed at the end of the assembly and the start of the transfer to geosynchronous orbit, the SPS will experience an aerodynamic deceleration of 2 x 10“5 g, which will require a propulsive force of 26 000 N to maintain altitude. Station keeping the SPS at its operational geosynchronous orbit is complex in that stringent position control must be maintained in the presence oi a number of perturbing forces that tend to cause the spacecraft to drift from its position. The perturbations that are encountered include those due to Earth oblateness, solar and lunar gravitational attractions, solar radiation pressure, and micro wave pressure. The gravitational anomalies due to Sun, Moon, and Earth oblateness produce short period oscillations in all the orbital elements, but the major perturbation of concern is the nodal precession in the plane of the perturbing body. This nodal precession causes a long-period oscillation of the inclination of the spacecraft orbit to the Earth1 s equatorial plane. By remainin unchecked, these perturbations would cause the inclination of the SPS orbit to increase to a maximum of 15° in approximately 26. 5 years. Table 7-6 shows typical motions that might be imparted to the operational SPS by gravitational anomalies. TABLE 7-6. SYNCHRONOUS ORBIT PERTURBATIONS a. Typical values expected; actual values dependent on launch date and time. Because of the large area of the SPS, the dominant perturbing force in the operational orbit is due to solar radiation pressure. Assuming a solar radiation constant of 4. 5 x 10“6 N/m2 (no reflection) and an area of 242 x 106 m2, the solar pressure exerts a force of approximately 1090 N on the SPS. This
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