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.

Simple metrics used to assess landing safety in the simple safe site selection (S4) algorithm.
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.

The software used in this innovation is available for commercial licensing. Please contact Dan Broderick at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to NPO-49440.

Topics:
Computer software and hardware Electronics & Computers Exteriors Imaging and visualization Information Technology Lidar Mathematical models Noise Optics Simulation and modeling Software Soils Windows and windshields
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Simple Safe Site Selection (S4) Algorithm

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NASA Tech Briefs Magazine

This article first appeared in the January, 2015 issue of NASA Tech Briefs Magazine (Vol. 39 No. 1).

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Overview

The document outlines the development and implementation of the Simple Safe Site Selection (S4) algorithm, a crucial component of NASA's Mars Technology Development efforts aimed at ensuring safe landings for rovers and landers on Mars. The S4 algorithm is designed to detect and avoid small-scale hazards that could pose threats during the terminal phase of flight, specifically when the spacecraft is a few hundred meters above the Martian surface.

The Lander Vision System (LVS) incorporates the S4 algorithm as part of its Hazard Detection and Avoidance (HDA) strategy. This system utilizes a single range image from a flash lidar to perform site selection in a computationally efficient manner. The algorithm's simplicity and speed are significant advantages, allowing it to run in less than two seconds on a flight processor, which is critical for real-time decision-making during descent.

The document highlights the performance of the S4 algorithm, noting that it can run on various processors, including standard processors and field-programmable gate arrays (FPGAs). The algorithm's run times are impressive, with the S4 algorithm achieving a runtime of 0.016 seconds on a 2.5 GHz Intel Core i7 processor and 1.3 seconds on a 60 MHz flight processor, demonstrating its efficiency and suitability for flight scenarios.

The research was conducted by Dr. Andrew E. Johnson from the Jet Propulsion Laboratory and Amit Mandalia from the Georgia Institute of Technology, under NASA's sponsorship. The findings emphasize the importance of the S4 algorithm in enhancing the safety and reliability of future Mars missions by enabling the selection of scientifically interesting sites while avoiding hazards.

Overall, the document serves as a technical support package that provides insights into the S4 algorithm's development, performance, and potential applications in Mars exploration. It underscores NASA's commitment to advancing aerospace technology and ensuring the success of its missions through innovative solutions that address the challenges of landing on the Martian surface.