Spacecraft attitude & rate estimation by an adaptive unscented Kalman filter

This paper presents a scheme for spacecraft attitude and rate estimation based on an Adaptive Unscented Kalman Filter (AUKF). The integrated attitude determination system here consists of rate gyros and attitude sensors as the measurement units. The process equations are formed combining the information from the rotational dynamics and kinematics of the spacecraft. As the measurement noise statistics of the rate gyros and the attitude sensors are often unknown, so an adaptive filter is employed here, which determines the measurement noise covariance (R) adaptively. Further, the choice of UKF facilitates with its high accuracy and derivative-free implementation. Two versions of the AUKF algorithm are shown here depending on the residual sequences and the innovation sequences. The residual based proposed AUKF comprises of two phase of measurement update. Further, a provision to regulate the speed of adaptation through a modulating factor, is also incorporated in the presented adaptive schemes. Positive definiteness and the diagonal structure of the adapted measurement noise covariance (R) is also ensured here. Simulation results show the superiority of the AUKF based attitude estimation scheme compared to the nonadaptive filter based schemes in terms of the estimation errors.