twenty four ampere hour nickel-cadmium batteries. The battery charge is controlled by regulating the solar array power in sunlight, so that it initially provides a charge rate of C/2 (or C/3). The current is tapered off when the battery potential reaches a pre-selected temperature compensated voltage level. The unregulated bus voltage ranges from 24 to 37 volts. Power is distributed from the bus to various payload elements. The predominant demands are from the Earth viewing radar systems capable of surface imaging and precise altitude determination. The most demanding payload is the imaging radar system. When active, it consumes more than 1 kilowatt of bus power in order to radiate a peak RF power of 5 kilowatts with a duty cycle of about 6%. 3.1.13 COLUMBUS The power requirements for the COLUMBUS elements of the Space Station Freedom program represent the greatest power system engineeringchallenges to date for Europe's space industry. The COLUMBUS laboratory module attached to Space Station Freedom will be designed to accept and distribute up to 20 kilowatts of power via two independent 120 volt DC busses. Fifteen kilowatts of this will be available for payload functions. The requisite safety and reliability issues for manned space systems present a new and extremely demanding task for power system definition. This particular challenge must be addressed both for COLUMBUS, and for HERMES (discussed in the next section), in addition to the other technical challenges presented by them. The overall power distribution capability for the harness and associated switches greatly exceeds the capacity of the interface feeders, since there is a systems requirement that any one of the twenty two installed power distribution boxes be able to supply up to six kilowatts of power. Although the baseline switch design for COLUMBUS power distribution network incorporates a fault current limiting feature, the complexity of the overall distribution system makes selective zone protection rather difficult to achieve. The power distribution system for the COLUMBUS free flying laboratory module will handle 9.5 kilowatts in a smaller pressurized module. In this case ESA is responsible for generating the necessary 15 kilowatts of electrical power and its subsequent storage, management and control instead of simply distributing it as in the attached module. In order to meet the mission power requirements of the COLUMBUS free flying laboratory module, an additional resource module is included as an element of the overall spacecraft. Current design assumptions are that this resource module will support 19 kilowatts of solar arrays, power regulators for the two independent 120 volt busses, and six fifty ampere hour sixty cell nickel-hydrogen battery packs. A 120 volt, +1% -3.5% fully regulated DC bus is the baseline standard for all primary power supplies on COLUMBUS. This bus is routed as appropriate to the
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