Research of sun vector in the attitude estimation of interplanetary spacecraft

In the interplanetary spacecraft attitude determination based on vector observation, the geometric relation between the reference vectors is an important factor which influences the accuracy of attitude estimation. This paper proposes an optimal algorithm using the Sun line-of-sight vector to improve the attitude estimation accuracy. With the introduction of the Sun vector, the attitude quaternion obtained from the star-tracker is converted into a pair of mutually perpendicular reference vectors perpendicular to the Sun vector. The normalized weights are calculated according to the accuracy of the sensors. Furthermore, the optimal attitude estimation in the least square sense is achieved with the quaternion estimation method. Finally, the practical data of the Deep Impact mission is used to demonstrate the presented algorithm. The estimation error of attitude angles are within 150μrad, which meets the requirements of the Deep Impact mission.