performance electric thrusters, and use of the Y-POP, X-IOP orientation and inertia balancing to minimize attitude control propellants. Figure 9 shows such a system employing eight RCS quads, two on each comer of the spacecraft. The total RCS propellant requirements (see table) are low, due primarily to the high specific impulse (13,000 s) which is believed to be feasible with the argon ion bombardment thrusters. The dominant stationkeeping propellant requirement is the complete correction of the solar pressure perturbation. This requirement can be eliminated if a ±3.1-degree longitude stationkeeping accuracy is acceptable. The present stationkeeping accuracy goal is 0.5 degree. An effort is currently underway to define a solar pressure correction policy which meets the 0.5-degree accuracy requirement, but will reduce the propellant requirement. The ACSS attitude reference determination system features charge-coupled device (CCD) star and sun sensors as well as electrostatic or laser gyros and dedicated microprocessors. Five attitude reference determination units are at various locations on the spacecraft to sense thermal and dynamic body bending and to desensitize the system to these disturbances. The control algorithms will feature statistical estimators for determining principal axis orientation, body-bending state observers or estimators, and a quasi-linear RCS thrust command policy to provide precise control and minimize structural bending excitation. The ACSS hardware mass is very small relative to the 30-year propellant requirement. Satellite antenna The basic satellite antenna configuration is shown in Fig. 10. Three main components comprise the structure—a tension web made from composite wires or tapes, a catenary cable that transfers the web tension to the vertices of the third component which is a hexagonal compression frame. Original analyses of this structure assumed an allowable midspan web deflection of only 0.75 cm. As a result, the structure weight was comparable with other concepts such as the rigid matrix system. Recent Fig. 10. Satellite antenna.
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