STRV 1A & 1B: Power Systems

       Both STRV 1A and 1B use photovoltaic solar cells to convert solar radiation directly into electricity. The satellites run continuously from their batteries, which are charged periodically during each orbit.

       The power generating arrays on both satellites are composed of gallium arsenide cells, supplied by a variety of sponsors and manufacturers. Silicon cells would have degraded rapidly in the radiation environment and were therefore not suitable. Each panel generates a beginning of life power of between 20 W and 28 W (under normal illumination, at one solar constant). The variation is due to the surface areas of each panel not being equal and also because different sizes of cells and cell technology are used on each panel. Averaged over the spin period of the spacecraft, the power available from the arrays is 31 W for STRV 1A and 33 W for STRV 1B.

       The power generated by the arrays passes through protection diodes and dual-redundant separation microswitches to the Power Distribution Unit (PDU). A 28 V supply (± 2%) is achieved using shunt regulators and battery charge and discharge regulators.

The complete system is controlled by a majority-voting triplicated unit, connected to the spacecraft's control and status buses. The PDU controls all power switching via commands received from the Command Distribution Unit (CDU) in the On-Board Data Handling Subsystems (OBDH). Fifteen current-limited, output lines are available for switching in this way.

       The batteries are sized to accomodate the eclipses the can be encountered in GTO. The eclipse duration can very from zero to over 2 hours at apogee. The battery consists of 16 military specification Nickle Cadmium cells with maximum storage capacity (at 60% depth of discharge) of approximately 46 Whr.

       The average power consumption of the platform subsystems on each orbit is 13 W and therefore approximately 19 W (average) is available to the payloads. Supplementary power from the battery allows for short duration loads of up to 120 W.

       The performance of the STRV-1B power system has been to specification and no problems have been encountered. The STRV-1A power system suffered an anomaly early on in the mission, however the effect of this problem on mission operations has been minimised by appropriate battery management procedures.