This hazard avoidance algorithm uses a single flash lidar range image to select a safe landing site for NASA missions. The advantage of flash lidar is that the entire range image is captured instantaneously, and therefore, is trajectory-independent. This is unlike a scanning lidar on a moving vehicle, where ranges have to be assembled in spacetime to form a complete range image.
A range image of a level plane has variable ranges that are a function of the field of view of the lidar and the attitude of the lidar relative to the plane. In the simplest case with the lidar pointed nadir at a plane, the ranges on the edges of the image appear to be farther than the center. Because this variation in range could be mistaken for a surface slope, it is removed before computing other metrics for safe site selection. This procedure is called range flattening.
After the flattening, each pixel in the image is processed through a 3×3 median filter. The current pixel plus the 8 pixels adjacent to it are collected, and the median range is computed. The current pixel is then assigned the median value. This “median filter” step removes noise and outliers up to 4 pixels in area from the image.
During site delta range (SDR) image construction, maximum slope constraints set by the rover (or lander for non-MSL-type landers like Mars InSight) are dealt with using the site delta range image. A window the size of the rover in pixels is passed over every pixel of the filtered range image. The difference between the maximum and the minimum range within the window is set as the SDR value for the pixel at the center of the window.
During rock delta range (RDR) image construction, rover constraints also provide an approximate size of rocks that can cause harm to the rover. A window the size of these rocks is passed through each pixel of the filtered range image. The center pixel of that window is set as the difference in minimum and maximum range. To obtain the site rock delta range (SRDR) image, a window the size of the rover is passed through the RDR image to get the effects of rocks on the rover. In a similar manner to the site delta range image, the center pixel of that window is set as the maximum RDR.
The cost of a particular pixel is computed as a combination of the SDR and SRDR images. The site with the lowest cost can be selected as the safe landing site.
This work was done by Andrew E. Johnson of Caltech and Amit B. Mandalia of Georgia Institute of Technology for NASA’s Jet Propulsion Laboratory.