Tradeoffs between directed and autonomous driving on the mars exploration rovers

NASA's Mars Exploration Rovers (MER) have collected a great diversity of geological science results, 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 attitude 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. 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. The MER rovers have driven more than a combined 10 kilometers over Martian terrain during their first 21 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 Autonomous Navigation and Visual Odometry drives.