aluminum pipe with an outer diameter of 12 mm and 0.5 mm wall thickness. The total mass of the structure is 4 tons. Power conversion and distribution The solar array consists of about 1500 rolls of 1 m x 100 m strips of solar panel, which will be stretched between the top longitudinal beam to the lower beams of the main structure. Conversion efficiency of the panel is assumed to be 14 %. Then each strip of solar panel generates 1 kV x 20 A of DC electrical power at maximum. The panels are electrically connected to each other in parallel. The cable network of the system is used not only to conduct the electrical current to the transmitting antenna but also to serve as magnetic torque for attitude control of the overall system. The power conducting cable is one of the heaviest elements of the system, especially in the case of the high power density antenna. Transmitting antenna Transmission is possible only when a rectenna is in the field of view of the controllable microwave beam, which is assumed to be movable as much as 30 degrees in any direction from the center position. Therefore, a rectenna located on the equator can receive power from a single satellite in a 1000 km equatorial orbit for 200 seconds in one orbit, and about 1600 seconds in a day. The relation of diameters of the transmitting antenna, Dt and rectenna, Dr and distance between them, d is given for transmitting frequency f (MHz) as; f x Dt x Dr/ d= 0.68 (length in km), In this case where d is approximately 1100 km, typical values of Dt and Dr are 100 m and 3 km respectively. If Dr is as large as that of the Reference System, Dt is only 30 m. One of the concepts of the antenna based on a current design [5] is shown by Figure 2. Guidance and control Reduction of the operating cost has been selected as the primary requirement of the guidance and control system design. From the standpoint of spacecraft design, easy orbit management and simple attitude and thermal control systems are desired to satisfy this requirement. To make this approach possible, the electrical power system should be operable over a wide range of conditions of the related factors. The standard orbit altitude is 1000 km for this study, although it may be subject to change to 800 or 1200 km to avoid the most crowded altitude of 1000 km. The annual loss of altitude for these altitudes is several km, which will be acceptable for a decade or more period of operation without orbit maintenance. The gravity gradient force will be used to keep the system vertically in the orbital plane. Attitude keeping in the north-south direction can be achieved by the restoring magnetic force of the electric current loop in the geomagnetic field. Active control may be necessary if periodical forces caused by nonuniformity of the geomagnetic field could act resonantly on the structure.
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