Creating maps to help guide the rover

While the rover is stopped, a pair of images from the front (and occasionally also the rear) Hazcams is captured and processed, into a set of (x,y,z) coordinate points in front of the rover. This resulting map of points is processed into a set of terrain features (steps, slopes, roughness) which serve as a three-dimensional model of the actual terrain in front of the vehicle. The Sojourner Rover on the Pathfinder mission in 1997 took 20 separate measurements for every step. The Mars Exploration Rovers will take between 6,000 to 10,000 points of measurements per step.

This model is used to determine if the terrain features represent obstacles for the rover (for instance, a feature with height of 30 centimeters (12 inches) or greater will be considered an obstacle). A small number of short potential paths in the direction to the destination are checked within this model and a safe path avoiding obstacles is chosen.

The rover moves a short distance (about 30 cm/1 foot) along this path and the process is repeated. As new terrain models are acquired, they are organized into a "world" model with the rover at the center of approximately a 10m-by-10m area. Due to the possibility of wheel slippage, the rover is programmed to only remember a small map around it in order to remain accurate about the surrounding obstacles. Information about the terrain surrounding the rover beyond the 10m-by-10m area therefore must be deleted every additional 5 meters driven. So, as the rover goes forward (roughly six times the length of its own body), it forgets where it has been.

Some of this map data is stored elsewhere onboard the rover and some will be sent back to Earth to help build a master map; however, there is a limit to how much data can be sent back to Earth. The amount that the rover can store is more than the amount it can uplink to Earth so the flight team must choose which data is sent ­ science or rover-related. (For further information about data flow to and from the rover, please see Rover Communications

Once constructed, the map helps the rover select safe movements to the destination and prevents the rover from encountering obstacles already avoided during prior segments of the drive. The rover proceeds to move to the destination until the accepted distance near the destination is achieved, or (in failure cases) either the rover has driven a distance farther than the target or for a time which exceeds that accepted for the path.

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