A seagoing vessel is subjected to forces from wind, waves and current as well as from forces generated by the propulsion system. The vessel's response to these forces, i.e., its changes in position, heading and speed, is measured by the position-reference systems, the gyrocompass and the vertical reference sensors. Reference systems readings are corrected for roll and pitch using readings from the vertical reference sensors. Wind speed and direction are measured by the wind sensors. The DP control system calculates the forces that the thrusters must produce in order to control the vessel's motion in three degrees of freedom – surge, sway and yaw – in the horizontal plane.