Editing Dynamic positioning (section)

==Control systems==
[[Image:Control-Kalman.svg|350px|thumb|Block diagram of control system]]
In the beginning [[PID controller]]s were used and today are still used in the simpler DP systems. But modern controllers use a [[mathematical model]] of the ship that is based on a [[Hydrodynamics|hydrodynamic]] and [[aerodynamic]] description concerning some of the ship's characteristics such as [[mass]] and [[Drag (physics)|drag]]. Of course, this model is not entirely correct. The ship's position and heading are fed into the system and compared with the prediction made by the model. This difference is used to update the model by using [[Kalman filter]]ing technique. For this reason, the model also has input from the wind sensors and feedback from the thrusters. This method even allows not having input from any PRS for some time, depending on the quality of the model and the weather. This process is known as [[dead reckoning]].

The [[accuracy and precision]] of the different PRSs is not the same. While a DGPS has a high accuracy and precision, a USBL can have a much lower precision. For this reason, the PRS's are weighted. Based on [[variance]] a PRS receives a weight between 0 and 1.