GLAS spacecraft attitude determination using CCD star tracker and 3-axis Gyros

The Geoscience Laser Altimeter System (GLAS), which will be carried by the Ice, Cloud and land Elevation Satellite (ICESAT) in 2001, needs to determine spacecraft attitude with better than 1 arcsecond (1 sigma) accuracy in post-processing. For this attitude requirement, the GLAS attitude system includes a zenith-oriented CCD star tracker and 5-axis Hemispherical Resonator Gyros. For the pointing knowledge of the laser beam in the nadir direction, it is required that we know accurate rotations about the roll and pitch axes. By applying two well-known estimation algorithms, batch least squares and extended Kalman filter, to the simulation data, the attitude of ICE-SAT/GLAS could be determined with approximately 0.3 arcsecond (1 sigma) uncertainty in each of the roll and pitch axes, without significant difference between algorithms. The RSS of roll and pitch errors thus amounts to 0.43 arcsecond(1 sigma).