The Far Ultraviolet Spectroscopic Explorer is a NASA Origins mission
launched in June 1999 to obtain high-resolution spectra of astronomical sources at far-ultraviolet wavelengths. The science objectives require the satellite to provide inertial pointing at arbitrary positions on the sky with sub-arcsecond accuracy and stability. The requirements were met using a combination of ring-laser gyroscopes, three-axis magnetometers, and a fine error sensor for attitude knowledge, and reaction wheels for attitude control. Magnetic torquer bars are used for momentum management of the reaction wheels, and coarse sun sensors for safe mode pointing.
The gyroscopes are packaged as two coaligned inertial reference units of three orthogonal gyroscopes each. There are four reaction wheels: three oriented along orthogonal axes, the fourth skewed at equal angles (54.7°) with respect to the others. Early in the mission the gyroscopes began showing signs of aging more rapidly than expected, and one failed after two years of operation. In addition, two of the orthogonal wheels failed in late 2001. The flight software has been modified to employ the torquer bars in
conjunction with the two remaining wheels to provide fine pointing control. Additional new flight software is under development to provide attitude control if both gyroscopes fail on one or more axes.
Simulations indicate that the pointing requirements will still be met, though with some decrease in observing efficiency. We will describe the new attitude control system, compare performance characteristics before and after the reaction wheel failures, and
present predicted performance without gyroscopes.