Stand-alone spacecraft attitude determination using real flight GPS data from UOSAT-12

This paper presents the results of an experimental attitude determination system employing GPS (Global Positioning System), using phase difference data logged in orbit on the UoSAT-12 minisatellite, which was launched in 1999. The basics of GPS attitude determination are described, and two attitude determination algorithms are described and compared: one based on least squares estimation, and the other based on Kalman Filtering. The algorithm developed for resolving integer ambiguities in the carrier phase differences is tested to achieve instantaneous attitude from a set of only four measurements collected from two antenna baselines. A quaternion estimator based on a standard extended Kalman filter (qEKF) uses all GPS measurements to enable continued attitude determination taking into account the spacecraft dynamics. The attitude information derived from on-board magnetometers and a horizon sensor is used to evaluate the results of GPS attitude determination. Most recently, the LSQ attitude algorithms have been demonstrated in orbit and compared with the qEKF method. (C) 2002 Published by Elsevier Science Ltd.