For power levels in the kilowatt-range new sequential switching shunt-regulator concepts employing a combination of analogue and digital control of solar array sections have been developed in the U.K. and flown in space. This technique can be used in large multi-kW solar arrays too, since it is modularized. II.5.3 Microwave Power Transmission and Beam Steering and Phase Control A great amount of research has been invested in the area of antenna arrays with high directivity. Much of the work is directly applicable to SPS. Applications are: Multi-beam array modul, air traffic control, synthetic aperture radar. However, phase control and beam steering seem to be critical points in the SPS concept, which require investigations with priority. Amplitrons have been developed in Germany in the 196O’s. Progress in the development of solid state devices, which could replace amplitrons or klystrons has been observed. Field-effect transistors converting power up to 100 Watt at low voltages have been reported. II.5.4 Attitude/Orbit Control System and Propulsion Electric propulsion systems are developed in U.K. and Germany. The initially wide-spread development has progressively been narrowed with a view to achieve the first realistic application of North-South-Station-Keeping and of the four technology approaches that were subsequently brought to an advanced state of development: caesium ion contact and bombardment in France, mercury ion bombardment in the U.K. and mercury radio-frequency ionisation in Germany. Only the German program is now being pursued intensively to qualify for an early test flight. A new approach (Field emission electric propulsion, FEEP) seems promissing for AOCS application of SPS resulting from its claimed simplicity and modularisation capability. However, its application as well as the application of the other thruster systems (using ionized metal) for primary propulsion seems rather doubtful in the SPS case.
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