A sloped terrain makes control of vehicle dynamics challenging. Roll (tilt from side-to-side), pitch (move¬ment from front-to-rear), and yaw (rotation around the vertical axis) alter the GPS antenna location with the projected center of the vehicle (Figure 5). For example, when driving across a slope, the horizontal position of the GPS antenna is downhill with respect of the center of the vehicle, and the guidance will be in error down the slope (Figure 6). To compensate, some systems include gyroscopes, accelerometers, or additional GPS antennas. Less advanced terrain-compensation mod¬ules deal with only roll and pitch angles of the vehicle, while others can measure total dynamic attitude in six degrees of freedom and enable the system to compen-sate for variable terrain.