Dynamic Positioning is a method used by ships and other vessels to control their position, using thrusters to counteract the effects of wind, current and any other external forces.
A free floating ship has 6 axis of motion, it can move vertically up and down over a wave (heave) it can tilt longitudinally (pitch) and horizontally (roll), it can move forwards or backwards (surge), from side to side (sway) and rotate (yaw).
The idea of dynamic positioning is to control three of these movements, surge, sway and yaw.
The above demonstrates the three axis that can be controlled by dynamic positioning, namely surge, sway and yaw
The above shows the three axis that cannot be controlled by dynamic positioning these are pitch, roll and heave
By measuring the vessels actual position and comparing it to a set position the deviation from the setpoint can be found, when this is measured again, both the new deviation and the rate of change of that deviation can be found.
If you are walking away from your house, after 1 minute you are 10 metres away, the deviation is 10 meters and that deviation is changing at 10 meters per minute providing you started at your house.
If you needed to be back at your house (say the phone rang) in a hurry, you can turn around and run back, if you did not slow down before you get there (control your rate of change of position) you would run into the door and knock yourself out.
However if you were only 2 meters away from the house you would not need to run back as it would be time to slow down before you started running, so it is important to know how far away you are from where you want to be, as well as how fast you are moving.
This motion perception and control is relatively easy for a human, but things are a little more complicated when machinery is involved. (see DP more detail)
With me so far?
So we need know how far we are from where we want to be and we need to know how fast we are moving, we also need to know directions, both what direction we are moving in and what direction it is back to where we want to be.
Going back to house and telephone analogy, if your phone starts ringing and you start running away from the house you are not going to answer it any time soon, in this example it would appear that you do not know where your house is, or what direction you are moving in (of course you may not want to answer the phone)
In reality you know where the house is because you can see it and you know that you are moving towards it for the same reason. Being able to see the house gives you a way of fixing your position in relation to an object that you know the position of.
What about if you were blindfolded, yes you could tell where the house was by working out the direction of the telephone ringer, and as you moved closer you could hear the ringer getting louder. In this case you are using the sound of the telephone ringer to work out your position and what direction you are moving in.
For a ship it is not quite so easy, if you can see a house from a ship, the chances are you have done something pretty wrong already and you are unlikely to be able to move even if you wanted to.
It is not impossible though, and there are many methods of fixing a ships position very accurately. I will mention a few but there are many more.
The use of a reference to a known location (house) and sound (telephone ringer) in the above examples is deliberate, because that is what we use, although in a different form