The main lower- and intermediatelevel modules and their functions are the following:

• A global path-planning module makes an initial choice of points that define an approximate obstacle-avoiding path from the starting point to the destination. This module also memorizes the previously acquired knowledge of the terrain to improve performance with experience.
• A trail-selection module uses visual cues to identify potential path segments in both the near and far fields and attempts to assemble the segments to define a tentative path for the robot to follow toward a location in the far field.
• A terrain-classifier module uses data from raw stereoscopic imagery to create a traversability map (see figure).
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  The Raw Image Was Processed, along with its stereoscopic counterpart, by a fuzzy-logic terrain classifier. The portion above the dark mask in the middle image shows obstacles identified by the classifier. The results of the classification were projected onto a two-dimensional Cartesian grid to obtain a traversability map, on which magenta indicates dangerous obstacles.
  There are several local path-planning modules: a trail-following module that seeks local openings and avoids local obstacles, a dead-reckoning module that strives to keep the robot close to the prescribed path, a hazard-avoidance module that utilizes sensory data and classical obstacle-avoidance techniques to detect dangerous slopes and obstacles, and a position-error-correction module that derives information on wheel slip from a combination of motor-current, opticalflow, and inertial-navigation data.
• An open-space module seeks to steer the robot toward more-open areas in order to center the robot on a trail, counteracting what might otherwise be a tendency to choose a path too close to obstacles when striving to minimize the total path length.
• A stabilizer module favors the previous heading decision in order to suppress steering oscillations that can otherwise occur when the obstacle-avoidance module cannot decide whether to steer to the right or the left around an obstacle.
  
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