China's Chang'e-4 (CE-4) probe will explore the South Pole-Aitken basin in late 2018. Its preselected landing area is located on the southeastern floor of the Von Kármán crater. Landing experience of China's Chang'e-3 probe may not be applied to CE‐4 mission directly because the topography of lunar farside is more rugged. Moreover, due to scale dependence and smooth effects, the previous topographic studies derived from digital elevation model data with lower resolution cannot represent the meter-scale topographic features. Because of low time and space complexities in lunar images, image segmentation algorithm is especially suitable for recognizing lunar features. We divided the narrow angle camera (NAC) mosaic image into three parts: dark area, bright area, and flat area, based on a double-threshold Otsu method. The first two parts corresponded to the undulating areas (positive and negative terrains). And then we calculated the flat area percentage (Fap) of the previous successful lunar landing missions (including Chang'e-3, Apollo, Surveyor, and Luna series) and obtained the Fap threshold (>0.6) for lunar safe landing. According to Fap map (~1.5 m per pixel) of CE-4 preselected landing area, the divided square grids with a size of 0.01° can be classified into safe grids (Fap > 0.6) and unsafe grids (Fap ≤ 0.6). The CE-4 preselected landing areas can be greatly reduced to five potential landing areas. In the last, we proposed a route planning method, which took both the distance and security into account, for CE-4 rover to dynamically generate a safe and short route between current grid and target grid.

Figure 1 Lunar Reconnaissance Orbiter wide angle camera mosaic of Von Kármán crater. The white box shows the Chang'e-4 (CE-4) preselected landing area.

Figure 2 (a) Lunar Reconnaissance Orbiter narrow‐angle camera mosaic of CE-4 preselected landing area marked with a

white box in Figure1.The Lambert Conformal Conic Projection was used and some small gaps (white areas pointed out by

yellow arrows) remained.(b) The image segmentation results of CE-4 preselected landing area through a double-threshold Otsu method. CE-4=Chang'e-4.

Figure 3 The Fap of each divided square grid with a size of 0.01°in Chang'e-4 preselected landing area, superposing

on the narrow-angle camera mosaic image. The five potential landing areas (yellow polygons) are marked with numbers

1-5, while the three least safe landing areas (red polygons) are marked with A-C.

Figure 4 (a) The Fap map of the study area, marked in the white rectangle in Figure4, superposing on the narrow‐angle camera mosaic image. Grid A in PLA 1 and grid B in PLA 2 are current grid and target grid. Grid C is an intermediate grid. (b) Route planning results.The black arrows show the shortest path, while the red and blue arrows show the safest path and the best path from grid A to grid B. The green circles show the best path from grid A to grid B going through grid C. Number i and j are the row and column numbers of the grid in rectangle AB. PLA = potential landing area; ND=normalized distance.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018EA000507