of 0.1 and a mast diameter of 50 meters, of torque is included by the slip rings. Figure 3.3-5 shows this torque variation with power level. Antenna gravity gradient torques on the rotary joints are calculated using the equations shown in Fig. 3. 3-6. The disturbance torque is approximately offset between the antenna boresight and the local vertical. The nominal antenna azimuth angle is offset 2.6° as a result of locating the spacecraft at the stable node point, 123° West longitude, and the rectenna at 104° West longitude. An additional 3.5° offset results from eccentricity drift caused by solar pressure on the solar arrays. The elevation angle offset is 6.5° caused by locating the spacecraft on the equator and the rectenna at 40° North latitude. The next highest disturbance on the antenna servo system results from electromagnetic radiation forces. An estimate of the force created by the electromagnetic energy radiation from the antenna has been computed (Ref 6) assuming a total power input of watts and a frequency of 3 GHz. The force is calculated by replacing the electromagnetic fields at the aperture of the slot array by equivalent current sources and computing the forces on the image currents which replace the aperture ground plane. The results predict an electro- magnetic force pressure of 2.3 x 10 N/m normal to the antenna and a corresponding total force of 18N. Although the electromagnetic forces do not place a significant design requirement on the antenna control system, the constant force in the radial direction does require the SSPS to continually perform orbital corrections. An acceleration along the radial direction does not significantly modify the energy of the orbit. The orbit will develop an eccentricity but the orbit period will remain almost constant. The force of 18 Newtons will cause a radial perturbation of by the 80th day. (This same acceleration along the velocity vector would change the semimajor axis by (120 n mi) in the same time.) The propellant requirement to make an altitude correction of 1. 0 n mi for an SSPS of slugs and an ISP = 8000 seconds is, This would be equivalent to a yearly propellant requirement of, 3. 3.1.2 Pointing and Control Qualitative and limited quantitative data has been generated for defining mechanical steering of the transmitting antenna. This data will be used in an overall assessment of
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