The small body explorations have received attention around the world in recent years. In these missions, high-accuracy optical navigation is important for the landing or rendezvous. Therefore, Terrain Relative Navigation (TRN) to estimate deviations by comparing nominal terrain information with actual terrain information is often used. Enough observation of a target body to make a shape model is possible after arrival, especially in the small body explorations. Accordingly, the shape model of the target body is utilized for the generation of the nominal terrain information.
Recently, we are studying the autonomous optical navigation method based on TRN. Firstly, the reference image from a nominal position is generated by rendering in the case of the proposed method. Three-dimensional positions on the shape model relative to each pixel of the reference image are also memorized in addition to luminance. Secondly, multiple small images extracted from the reference image are compared with a captured image by template matching. Therefore, the relationships between the three-dimensional positions on the shape model and the multiple small images in the captured image can be determined. Finally, the actual position of the spacecraft can be determined by estimation of perspective projection, that projects a three-dimensional shape onto a two-dimensional plane. For these reasons, three-dimensional positions of the spacecraft can be estimated directly in high accuracy by utilizing the shape model. The estimation accuracy and computational time are evaluated by comparing the proposed method with other methods. As a result, the high estimation accuracy of several image resolution in real-time is achieved. We believe that the proposed method will be a key technology for landing on small bodies with higher accuracy in the future.
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