providing mutual rescue capabilities between the various systems. Routine operational power transfer between HERMES and COLUMBUS has been addressed by industry under contract. This has proven to be a valuable concept, with the advantages of extending the HERMES mission duration and allowing greater flexibility in docking operations. 4.1.2 Power System Development for GEO Missions A number of different power system architectures have been used for GEO missions in recent years. This is particularly true of communications satellites. The basic system concepts employed for telecommunication satellites range from fully regulated power busses through solar array regulated power busses and all the way to unregulated configurations. Such trends have resulted in a lack of standardization. Power supply performance and quality have been quite variable. In the latter two approaches, there is little system modularity and hence little standardization or growth is possible. The present situation has led ESA to initiate a contract to study advanced power systems for use on telecommunications satellites in the 1990's. One of the fundamental goals of this contract is to assess the optimization of power system architectures stemming from a consideration of the complete power conditioning requirements - all the way from the power source to the end user's hardware. The use of HEXFET and Thick Film Hybrid devices in various space power systems has certain attractive features. These are being studied thoroughly at the power system level, and also with respect to their effect on TWT electronic power conditioner topologies. Power conditioning for communication payload elements using solid state RF components is also being studied under contract. 4.1.3 Additional Developments The power systems and power conditioning laboratories at ESTEC have developed several novel circuit concepts over the last ten years. These include the Sequential Switching Shunt Regulator and the Conductance Control technique. The SSSR is now used extensively for solar array power control on ESA spacecraft, and the CC is being applied to battery power regulators in regulated bus systems. Power switching stages controlled by a pulse drive circuit using N-channel FETS have been developed through recent laboratory work. A unique high bandwidth (600 kHz) Hall effect current sensor has also been developed recently. It independently processes and combines the DC and AC components of the measured signal.
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