Kalman filter based attitude estimation for CubeSat missions in free-space optical communications

Kurzfassung

In the last few years, optical satellite communications have recently gained increasing attention due to the necessity of transmitting high data rates on smaller satellites like CubeSats. Thus, the German Aerospace Center DLR’s OSIRIS program (Optical Space Infrared Downlink System), developed a highly compact and power-efficient laser communication payload for small satellites, OSIRIS4CubeSat, to test laser communication for CubeSats. This payload will be exploited in the CubeISL mission to establish an Inter-Satellite Link (ISL) between two CubeSat terminals in LEO. To establish this link, it is essential to have precise information on the current attitude of the communication terminal in space. This attitude information is now available only if a link has already been created. Nevertheless, it is likewise important to accurately know the attitude during the link acquisition phase to decrease the miss-pointing error and shorten the search time until the opposing target is found. Indeed, in this thesis, the CubeISL communication terminal’s attitude estimation concept is presented, aiming to estimate the OSIRIS terminal’s attitude and find its miss-pointing error with respect to the pointing target. The estimation process relies on the attitude information given by the CubeISL satellite and on the measurements coming from the OSIRIS terminal’s sensors, a magnetometer and a gyroscope. Hence, the estimation algorithm consists of a USQUE UnScented QUaternion Estimator Kalman Filter. Two versions of this filter have been developed: one version includes the angular velocity into the state vector while the other is dynamics-free with a state constituted by the local error quaternion and the gyroscope bias according to the USQUE model of Crassidis and Markley. Both versions demonstrated a good convergence of the estimated quantities with respect to the actual attitude within reasonable times. Nevertheless, being the dynamicfree USQUE computationally lighter, it is preferred in order to shorten the search time for the ISL link establishment.