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When the robot starts, it assumes it's at (0, 0) and with a heading of 0 (i.e., facing straight down the X-axis). When you start to move the robot, it has sensors, which are known as wheel encoders, that measure the number of rotations its wheels have made, and it uses that information to estimate both how fast it is going and how much it has moved. It can also estimate how fast it is turning and how much it turned using the same information. If you have a P3AT that came with a gyroscope, it uses that information to refine its heading and heading velocity.