Precise localisation of the Yutu-2 moon rover plays an important role in path planning, obstacle avoidance and navigating to target features. To provide highprecision localisation information, a stereo bundle adjustment method using the theory of the unit quaternions is presented for the first time. To improve the precision and robustness of the proposed method, the rover's pose, from a visual odometry technique assisted by an inertial measurement unit and the rotation angles of the mast mechanism, is viewed as a pseudo-observation. A reasonable weighting strategy and a rational geometric constraint condition of the stereo cameras is also invoked. Experimental results demonstrate that the proposed method provides more accurate localisation results than either a bundle adjustment alone or a weighted total least-squares method. The proposed method has been successfully used in Chang'e-4 mission operations. (a) Chang'e-4 lander (b) Yutu-2 rover FIG. 1. The Chang'e-4 probe. (a) An image taken by the Yutu-2 rover's Pancam camera of the lander. (b) An image taken by the lander's topographic camera of the rover.The Photogrammetric Record

The stereo vision system is the special engineering measurement instrument of the Chang'e-4 lunar rover. It is composed of the Navigation Camera (NavCam) and the Mast Mechanism (MasMec). An improved self-calibration method for the stereo vision system of the Chang'e-4 lunar rover is proposed. The method consists of two parts: the NavCam's self-calibration and the MasMec's self-calibration. A combined adjustment based on the points and lines is proposed. The baseline constraint of the NavCam is considered. The self-calibration model of the MasMec is established based on the product-of-exponentials formula. Finally, the premission laboratory calibration and the on-site calibration are carried out. The laboratory calibration shows that the proposed approach has high accuracy. The checkpoint with a distance of about 2.7 m to the left NavCam has a point error of about 4 mm. Finally, the proposed approach is applied in the on-site calibration.

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