DEAD RECKONING ON THE RED PLANET
Martian turf isn’t the easiest to navigate. When the Mars Exploration Rover is given a command to travel to an interesting target, its path will be littered with rocks and boulders. There’s no one on Mars to safely guide the vehicle from point A to point B and no one here on Earth can see the rover in real time to help it maneuver around various hazards. What is a rover to do? Just like humans, it uses its “eyes” and its "brain."
The "eyes" of the rover are two pairs of cameras located below the solar panel deck. They are called Hazard Avoidance Cameras or Hazcams for short. These stereo cameras provide depth perception at the front and rear of the rover. Front Hazcams scan for hazards as the rover creeps forward. Should it need to back up, rear Hazcams provide the necessary images.
The "brain" of the rover is the onboard computer software that tells the rover how to navigate based on what the Hazcams see. It is programmed with a given set of responses to a given set of circumstances. This is called "autonomy and hazard avoidance." The rover will be on its own (autonomous) for much of its journey across the Martian landscape. Communication with Earth is only twice per sol (Martian day). Scientists send commands to the rover in a morning "uplink" and gather data in an afternoon "downlink." During an uplink, the rover is told where to go, but not exactly how to get there. Instead, the command contains the coordinates of waypoints toward a desired destination. The rover must navigate from waypoint to waypoint without human help.
The process begins while the rover is stopped. Stereo images from the Hazcams are converted into an elevation map of the Martian terrain. Think of this map as a large checkerboard. When you play a game of checkers or chess you contemplate your move before you make it. You determine which squares are dangerous, which are safe. You use your eyes and your brain to plan your strategy. The rover evaluates each square of the grid on an elevation map for such features as slope and roughness. It assigns a number to each square. The higher the number, the safer the square. The rover selects the path to the next waypoint that has the highest value. It moves a short distance (approximately 35 cm or a little more than one foot) and the process is repeated. Information from each of these "steps" is organized into a larger model of the Martian terrain. A comprehensive view of the landscape begins to take shape and offers more options for the rover to choose as it slowly moves toward the commanded location.
As the rover travels over the surface of Mars, it may come across an obstacle that was previously undetected. In such a situation it uses its "eyes" and its "brain" to determine the height and distance of the obstacle. For example, if the rover determines that a rock 30 cm high is in its path, it will maneuver around the rock instead of attempting to drive over it. The rover has been programmed to avoid climbing over any feature with a height greater than the diameter of its wheels (25 cm). As the rover travels around the rock, it deviates from its straight-line path from point A to point B. Sensors will correct for this deviation so that the rover still understands how to find its way to point B.
The rover is programmed to handle many situations during its travels, even the ones that can be dangerous. What if the rover tilts too far? What if the arm of the rover is not properly stowed? What if there is unexpected contact with a rock? What if the rover comes upon a cliff? In each instance, onboard programming tells the rover to stop dead in its tracks and wait for instructions from Earth. But the rover is resourceful. Even at a standstill, it has the potential to get something done. Contingency plans programmed into the software for such occurrences may call for a Pancam panorama of the surrounding landscape or a diagnostic check so engineers can determine the rover's health. This information, as well as data that reveal the rover's sticky predicament, will be sent to Earth at the next opportunity to "phone home."
Web content editor/writer: Pamela R. Smith
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