LandSAfe - The Landing Site Risk Analysis Software Framework (2014)

Background

The Aurora program of the European Space Agency (ESA) aims at the robotic and human exploration of the Solar System in the next decades. One key element of this program is the Lunar Lander mission planned for the year 2018 which targets the polar regions of the Moon. To ensure a safe and successful landing a careful risk analysis has to be carried out. This is comprised of identifying favorable target areas and evaluating the surface conditions in these areas. Concretely, features like craters and boulders of size bigger than 50 cm have to be identified which pose a potential hazard for the lander. Additionally, shadows and steep slopes have to be avoided in order to allow for a successful touchdown and subsequent surface operation of the lander.

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LandSAfe

The Landing Site Risk Analysis software framework (LandSAfe) has the objective to provide software modules for the automatic production of lunar digital surface models (DSM) or digital terrain models (DTM) respectively, derived products like hazard and illumination maps as well as tools for analysis and visualization of these data. The combination of image and height data allows for a quantitative evaluation of landing site hazards like craters and boulders with respect to detection, counting, sizing and distribution. The final hazard maps support decision making in visualizing potential unsuitable landing areas by incorporating vital factors like surface roughness, shadows, slope, boulders and craters. The results have relevance in two stages of the mission: First, the software helps in defining a suitable landing site in the preparatory stage of the mission. Second, during the descent stage the lander shall navigate automatically to the designated landing site while avoiding obstacles like craters and boulders in a self-acting way. In this stage live footage of the descending has to be compared and aligned with a landmark database stored on the lander in real-time. The landmark database has to be build up by LandSAfe with the help of suitable feature elements like points or edges.

Within the LandSAfe activity the Institute of Photogrammetry and GeoInformation (IPI) of Leibniz Universität Hannover (LUH) has the task to develop techniques, algorithms and software modules for the automatic derivation of lunar DSMs. In a narrow sense this incorporates software modules for digital image matching which automatically determines corresponding image points in stereo imagery. Auxiliary modules for rectification of the images for the generation of the final hazard maps and to support the matching are necessary. For the combination of overlapping single DSMs or ortho photos into a common image a geometric mosaicking module is needed. Geometric mosaicking refers to assembling the ortho photos cartographic correctly. Radiometry is accounted for by averaging gray values and not by complex radiometric adjustments like de-/reshading. In order to obtain high accuracy final products a bundle adjustment is mandatory which improves the relative accuracy of the orientation data of the camera/ spacecraft and the absolute accuracy with respect to the body-fixed coordinate system of the Moon. The sequence of tie point matching - bundle adjustment - DSM matching - ortho rectification - mosaicking constitutes a complete photogrammetric processing chain.