In-orbit identification of unmodelled disturbance torques acting on a spacecraft body

Extended Kalman filters (EKFs) are often used on low earth orbit spacecraft to estimate the attitude from directional vector measurements. These EKFs require a good mathematical model of the attitude dynamics to propagate the attitude estimate. If the model of the attitude dynamics is inaccurate, the EKF may take longer to converge or may not converge at all. Unmodelled disturbance torques need to be identified in order to improve the accuracy of the attitude dynamics model. This paper presents two methods of in-orbit identification of unmodelled disturbance torques acting on a spacecraft body. The first method uses rate sensor measurements combined with measurements from a single directional vector sensor, such as a magnetometer or sun sensor. The second method uses a combination of two non-collinear directional vector sensors, such as a magnetometer combined with a sun sensor