The STEREO Spacecraft's Power System Extended Mission Flight Performance

Abstract

The Solar TErrestrial RElations Observatory (STEREO) observatories are two NASA spacecraft orbiting the sun close to earth’s orbit with one spacecraft “ahead” of the earth and the second “behind” the earth drifting away from the earth at a rate of 22 degrees per year. The two spacecraft were launched on October 25, 2006 for a two-year mission. Stereo is a very successful scientific mission and was awarded a mission extension through the end of FY2012. STEREO is the third mission in NASA’s Solar Terrestrial Probes Program. The mission is sponsored by NASA’s Science Mission Directorate; APL designed, built and operating the twin observatories for NASA. Each spacecraft is designed for approximately 500 watts and has an unregulated nominal 28 volt bus. A single 23 AH NiH2 battery with two cells per common pressure vessel (CPV) is used to support the spacecraft loads during launch and eclipse periods. The battery charge control is performed using ampere-hour integration charge-to-discharge (C/D) ratio control and maximum pressure limit. The spacecraft computer via a Mil-STD-1553 interface bus performs the battery charge control and the peak power tracking. A hardware backup V/T control that limits the battery voltage to preset safe levels is incorporated into the design. Each battery pressure vessel is bypassed by a switch that is automatically activated to remove the vessel from the battery in case of open circuit failure of that vessel. The two STEREO spacecraft’s have been performing as designed. This paper will describe the power system performance of the two spacecraft’s to date.