The rendezvous and berthing of the reflector is more easily accomplished in GEO because gravity gradients are much reduced. Also, both elements are active and cooperative. A human operator in the control loop can be considered, if desired, because the delay time for direct round-trip communications to GEO is about 0.2 seconds. The advancing state-of-the-art in robot mechanisms, perceptions, planning, and task control, coupled with demonstrated capabilities in C3I, are applicable to all phases of reflector deployment, rendezvous and berthing. An alternate concept, the "maypole" or spoked wheel fully deployable concept, is shown in Figure 3. In this concept, the Kapton film reflector is stretched across the entire diameter of the structure. The only rigid members are the center mast and the rim truss. Stay tapes or guys extend from the ends of the center mast to the rim to stabilize and shape the rim. This system is somewhat like a bicycle wheel in that the overall shape and flatness are controlled by tension of the stays. Attitude control will be accomplished by electric thrusters. Attitude control can be accomplished by performing station keeping with a low level of continuous thrust and allocating the station keeping thrust to generate the necessary attitude control torques. Although the attitude control requirement is moderately stringent, the station keeping thrust level will induce maximum angular accelerations on the order of 0.01 arc second per sec^; these are small enough to maintain adequate attitude precision. Since the structural dynamic response of the reflector will be slow, the attitude control system will have to include dynamic compensation. At GEO, station keeping involves compensating for irregularities in the Earth's gravitational field, Sun-Moon effects, and solar and microwave radiation pressure. For a large low-density object such as the reflector, radiation pressure is a significant part of the requirement. The microwave pressure can be partially compensated for by operating at slightly less than GEO altitude. The integrated change in velocity (delta V) requirements for station keeping at GEO would require an annual delta V requirement for a 4000-t. reflector and a 1200-t. reflector of 47.6 m/sec per year for the more massive object and 54 m/sec for the less massive one. For this annual delta V, the propellant mass fraction for 30 years station keeping at 3000 seconds Isp is about 6%. The power requirement is less than 150 kW. Reflector Assembly in Space Gravity gradients, atmosphere drag, the debris environment, frequent sun occultation, transportation considerations, and communications all favor assembly in GEO orbit. The only factor that favors low-Earth orbit is human access. Therefore, all- robotic assembly is preferred. Human access would only be required in the event of a breakdown that could not be repaired robotically but this would be a very infrequent occurrence. In the time period of a microwave reflector project, an advanced crew transportation system is expected to be available to transport people to GEO if needed. Therefore, GEO assembly, has been chosen.
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