Attitude Determination and Autonomous On-Orbit Calibration of Star Tracker For GIFTS Mission

A novel split field of view star tracker is being developed for the EO-3 GIFTS mission (2004). The camera is designed to be autonomously self-calibrating, and capable of a rapid/reliable solution of the lost-in-space problem as well as recursive attitude estimation. Two efficient Kalman filter algorithms for attitude, camera principal point offset, and focal length estimation are developed. These algorithms make use of three axis gyros for the rate data and star camera split field-of-view line-of-sight vector measurements. To model the optics of the camera the pinhole model is used, which is found to be sufficiently accurate for most of star cameras. The relative merits of the two algorithms are then studied for estimating the principal point offset, focal length and attitude of a simulated spacecraft motion. Simulation results indicate that both algorithms produce precise attitude estimates by determining the principal point offset, focal length and rate bias; however, reliability and robustness characteristics favor the second algorithm. Introduction Spacecraft attitude determination is the process of estimating the orientation of a spacecraft by utilizing on-board observations of celestial bodies or other reference points. Combinations of these observations are used to generate more accurate estimates of the spacecraft rotational attitude. If these observations are error free then the spacecraft attitude can be determined precisely. But, in practical problems these measurements are not error free, so some sensor noise is always present. Several attitude sensors exist, including: three axis magnetometers, sun sensors, Earth-horizon sensors, global positioning sensors, star cameras, and rate integrating sensors. The accuracy of the attitude estimation depends on the quality of the attitude sensor. For example, the attitude accuracies that can be achieved with a sun sensor are approximately 0.015 degrees for the best available instruments.