Taut Wire

The taut wire reference system is probably the easiest to explain and visualise.

It basically consists of a weight and a piece of wire, the weight is placed on the seabed with the wire led back to the ship. When the wire reaches the ship it is fed through an inclinometer (measures angles) and on to a constant tension winch.

As the ship moves the angle of the wire changes, this angle is measured by the inclinometer, and by applying the depth of water we can obtain the position of the vessel, relative to the weight on the seabed.

The winch maintains a constant pull on the weight so that the wire is taut at all times (hence the name "taut wire") the pull is in the region of 40 to 60 % of the mass of the weight. As the tension increases the winch pays out a little, as it reduces the winch heaves in a little

From the diagram, any direction that the ship moves  can be resolved into its X and Y components relative to the taut wire weight, thus the ship motion can be measured and therefore controlled. Length D is measured when the weight is deployed (there is a counter on the winch).

Taut wires are an excellent position reference, they are simple, easy to operate and it is very easy to observe, and therefore react to, problems with the data.

Unfortunately they are limited to shallow and moderate water depths, due to actual size of the weight, and consequently the wire needed for deep water.