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

The main purpose of the Geoscience Laser Altimeter System (GLAS) is to determine the mass balance of the polar ice-sheets and their contributions to global sea level change. For the mission, the required accuracy for the laser altimeter height measurements is 10 cm. In this case, the direction in which the altimeter beam is pointing relative to the Terrestrial Reference Frame must be known to an accuracy of 1.5 arcseconds assuming the average slope of the ice-sheet surface is one degree. The laser pointing direction will be determined relative to the star field measured by a star tracker in the GLAS spacecraft (ICESAT). Thus, the specification of one arcsecond pointing accuracy requires that the spacecraft attitude determination has comparable accuracy. A Charge Coupled Device (CCD) star tracker and gyros will be installed in an optical bench of ICESAT to determine the spacecraft attitude. Each star position measurement from the CCD star tracker contains approximately five arcseconds position uncertainty depending on the magnitude of the observed stars. Furthermore, gyro output accuracy is corrupted by measurement noise and bias.; The main purpose of this dissertation was to investigate the ability to determine the attitude to better than one arcsecond (1σ) using developed estimation algorithms. Extended Kalman Filters and a Batch method were developed and used to estimate the simulated GLAS attitude. The determined attitude showed that the root sum square of roll and pitch errors, which directly affect the laser beam pointing error, reduced to about 0.5 arcsecond (1σ), far better than one arcsecond. In order to support the study result, actual attitude data obtained from the X-ray Timing Explorer spacecraft, were processed with some of algorithms developed for this research. As a part of the generation of the measurement data, a star identification algorithm was developed.