NASA Curiosity rover,
Curiosity, NASA’s Martian rover, periodically checks to see how much its drive wheels are slipping in loose gravel or sand by comparing the distance it actually travels to how far it has been told to move. To do this, the rover checks dozens of nearby features, such as nearby pebbles and shadows on rocks. NASA engineers have ensured Curiosity can carry out this visual odometry even if there aren’t any distinguishable landmarks.
NASA designers etched the Morse Code letters J-P-L (for Jet Propulsion Laboratory) in all six wheels so that they leave a distinctive pattern in the sand as the rover moves. Curiosity’s cameras can discern the pattern and use it for measurements. If Curiosity determines there’s a significant position error from wheel slip, it recalculates the path to its next target. This should let the rover get to targets more quickly and directly.