mately 36 operating thrusters are required. Sixteen thrusters are located on the lower portion of each corner of the collector. Each thruster is gimbal - led individually to facilitate thruster servicing using a servicing cab, to permit operation of adjacent thrusters during servicing, and to provide redundancy. The system is nominally designed for X-POP operation (long axis per- pendicular-to-orbit-plane). The pertinent features and locations of the Attitude Reference Determination System (ARDS) are also given in figure 13. The average power required for the system is 34 megawatts. H. Microwave Power Transmission System The reference microwave power transmission system was developed considering environmental effects, antenna size tradeoffs, antenna thermal heating limitations, and ionospheric heating effects. The present microwave system has DC-RF power converters feeding a 1 km diameter phased array antenna with a 10-decibel (dB) Gaussian taper illumination across the array surface. This antenna shown in figure 14, is composed of 7220 subarrays, approximately 10 meters X 10 meters on a side, having slotted waveguides as the radiating surface with DC-RF power tubes mounted upon the backside of the subarrays. The antenna structure is a graphite composite material while the slotted waveguides are aluminum. Each subarray has its own RF receiver and phasing electronics to process a pilot beam phasing signal from the ground. The subarrays are phased together to form a single coherent beam focused at the center of the ground antenna/rectifying system (rectenna). This power beam has approximately 88% of its energy within a 5 km radius perpendicular to boresight, with a resultant beam width of 1.2 arc-minutes. Microwave System Parameters and Sizing Considerations - Some of the key parameters of the SPS microwave system are presented in figure 15. The power capability of the SPS system was sized by: (1) thermal limitations of o 22 kW/m in the transmit array due to waste heat from the DC to RF power con- 2 verter tubes; (2) a peak power density limitation in the ionosphere of 23<mW/cm
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