Tradeoffs between directed and autonomous driving on the Mars Exploration Rovers

This is an extended abstract submission, not the final paper. NASA’s Mars Exploration Rovers (MER) have collected a great geological diversity of science re-sults, thanks in large part to their surface mobility capabilities. The six wheel rocker/bogie suspension provides driving capabilities in many distinct terrain types, the onboard IMU measures actual rover at-titude changes (roll, pitch and yaw, but not position) quickly and accurately, and stereo camera pairs provide accurate position knowledge and/or terrain assessment. Solar panels generally provide enough power to drive the vehicle for at most four hours each day, but drive time is often restricted by other planned activities. And driving along slopes in nonhomogeneous terrain injects unpredictable amounts of slip into each drive. These restrictions led us to create driving strategies that maximize drive speed and distance, at the cost of increased complexity in the sequences of commands built by human Rover Planners each day. Commands to the MER vehicles are typically transmitted at most once per day, so mobility oper-ations are encoded as event-driven sequences of individual motion commands. Motions may be com-manded using quickly-executing directed commands which perform only reactive motion safety checks (e.g., real-time current limits, maximum instantaneous vehicle tilt), slowly-executing position-measuring Visual Odometry (VisOdom) commands which use images to accurately update the onboard position es-timate, or slow-to-medium speed Autonomous Navigation (AutoNav) commands which use onboard image processing to perform predictive terrain safety checks and optional autonomous path changes. The MER rovers have driven over more than a combined 9 kilometers of Martian terrain during their first 16 months of operation using these basic modes. In this paper we describe the strategies adopted for selecting between human-planned directed drives versus rover-adaptive AutoNav and VisOdom drives