Constraints-Based Optical Hybrid Navigation for Small-Body Close Flybys

Abstract

This paper proposes a new optical hybrid navigation method for use in close flybys of small celestial bodies. Two steps are taken to estimate the inertial position and attitude of the spacecraft: 1) The relative motion between the spacecraft and the center of mass of the small body is determined by processing the images taken from the onboard monocular camera using the photogrammetric technique. 2) The absolute state is determined by establishing an accurate displacement increment equation, which is based on various constraints and integrates both optical and radiometric observations. No a priori precise alignment information of onboard cameras is needed for the whole process. The flyby of asteroid Toutatis by Chang’e-2 in 2012 is selected as a case study. The position accuracy of Chang’e-2 in the inertial frame is improved significantly from 15 to 5 km. The inertial pointing of the camera is obtained with an uncertainty of 0.003 deg, which is more precise than the given initial value of 0.5 deg. The proposed method is also beneficial to the camera alignment calibration, with the error reduced to 0.05 deg from the nominal value of 0.5 deg.