Attitude and angular rate estimation using the sliding mode observer with additive quaternion corrections

The sliding mode observer is used to design attitude and angular rate determination routines for the University of New Hampshire Cooperative Astrophysics Technology Satellite (CATSAT). The key design goal includes reducing the computational load of traditional nonlinear filters and providing accurate and stable estimates in the presence of modeling uncertainties, disturbances, and measurement noise. The estimation routines are particularly suited to satellites (i.e. CATSAT) not equipped with gyroscopes which provide body rate measurements. (For gyro-equipped satellites, the proposed application can also provide angular rate data in the case of gyro failure, a common phenomenon.) Quaternions are used to parameterize spacecraft attitude. In particular, additive quaternion corrections are applied to the SMO. The proposed sliding mode observer uses continuous state propagation with discrete measurement corrections to estimate attitude quaternions and angular rates. This method is implemented on the CATSAT simulation model. The observer yields satisfactory attitude and rate estimates in the presence of sensor noise, unknown bounded disturbances/uncertainties, and uncertain system parameters.