excessive. Therefore, mounting of the thrusters on the back of the antenna (assuming the antenna is located in the center of the SPS) is being considered for use in conjunction with the end mounted modules. An undesirable plume impingement problem results, however, as the antenna rotates during thrusting. Another propulsion system mounting concept involves locating most of the thrusters on the back of the antenna and using smaller thruster modules around the periphery of the SPS for attitude control. Still another concept involves mounting the thrusters on each curved side of the SPS. This lattef mounting concept is advantageous in that the thrust is uniformly distributed across the surface of the SPS and bending is minimized. However, this mounting concept incurs steering losses and requires undesirable on-off operation of the thrusters. The thrusters on one side operate for approximately one-half orbit and are turned off; then, the thrusters on the other side are turned on for the other one-half orbit. This process would continue until the SPS attains geosynchronous orbit. In summary, an O2/H2 chemical propulsion system is proposed to maneuver the SPS from its low assembly orbit to a continuous sunlit orbit for electric thruster operation. Chemical propulsion must also provide attitude control during occultation. Based on cost, an MPD electric propulsion system is proposed to perform the SPS LEO to GEO transfer maneuver. To provide tangential thrust during transfer and to provide attitude control using transfer thrusters, end mounted SPS thruster modules, mounted on gimbals, appear necessary. 7.1. 6 ATTITUDE CONTROL SUBSYSTEM 7.1. 6.1 GROUND RULES/GUIDELINES The following ground rules and guidelines are applicable to the attitude control subsystem: • Accuracies — Attitude Control and Pointing Control — Main Body: ±1° in three axes — Microwave Antenna Pointing Coarse: ±1 arc min in two axes; less than 8° about line of sight Fine: ±3 arc s in two axes with respect to the ground rectenna center
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