Three-axis Attitude Control with Two Reaction Wheels and Magnetic Torquer Bars

The Far Ultraviolet Spectroscopic Explorer satellite was launched in 1999 and began a three-year prime mission to collect high-resolution spectra in the far ultraviolet bandpass. Two and a half years after launch, mechanical failure of two out of four reaction wheels reduced the satellite to two-axis control, halting science observations. This failure prompted modification of the FUSE attitude control system software to restore three-axis control using a hybrid of existing magnetic and reaction wheel actuators. Pointing accuracy and stability are once again at the sub-arcsecond level, close to the pre-wheel failure performance. The range of stable attitudes is limited, but ground-based modeling software now directs the planning process so that observations and maneuvers stay within the limits of the actuators. Despite these constraints, efficient science operations are ongoing, and over the course of a year, the entire sky is available for observation. Nomenclature ggN r = gravity gradient torque