Abstract: A linear algorithm is proposed to determine the intrinsic parameters of zoomed monocular camera in the vision based landing of an UAV (Unmanned Air Vehicle). With the knowledge of the camera's principal point, the images of the two circular points are derived only from a group of squares without any metric measurement in one planar landing target image, then two linear constraints on the intrinsic parameters are established via the absolute-conic. Experiments with simulated data as well as with real images show that: the linear algorithm is accurate, real-time and efficient, and can be used either in the vision based landing of an UAV,3-D motion estimation, 3-D reconstruction or visual surveillance where the camera intrinsic parameters such as focal length are need to be changed.

Key words: camera calibration of intrinsic parameters, landing plane target, circular point, absolute conic, square