Autonomous Avoidance of Structural Resonances on the STEREO Mission

In October of 2006, the STEREO satellites w ere launched. The STEREO mission consists of two spacecraft in heliocentric orbits, one leading and one lagging the Earth, that carry a number of different instruments to study solar coronal mass ejections in three dimensions and how they interact with th e Earth’s magnetosphere to create “space weather.” While not a dynamic spacecraft, the STEREO mission had tight attitude control and jitter suppression requirements. Jitter is required to be held below a few arcseconds, and there are additional stability requirements from exposure to exposure. Analysis indicated that the performance was largely driven by the interaction of the wheel imbalances and the structural modes of the platform the instruments are mounted. The mission goal of maximizing time availa ble for science made operationally intensive solutions less desirable. This paper describes the algorithm developed to adjust the wheel speeds through the null vector to avoid the structural resonances and describes its demonstrated performance in flight, as well as the analysis which motivated it.