Updated 11 Oct 2019
Important as it is for a ship to know its exact position at any given time, the purpose of a vessel is that it moves between ports. Therefore, it is vital that the ship can head in the correct direction and the chief tool for this is its compass.
Modern ships have access to many types of compasses. The GPS system can determine in which direction a ship is heading by comparing current position with a previous one. This is done continuously and, so long as the GPS signal is not being manipulated or faulty, should be sufficient.
Today all ships are required to have GPS installed but in addition they also require two other compasses: the traditional magnetic compass and a gyrocompass. A magnetic compass is of course only able to indicate magnetic North which is not a fixed point and local geo-magnetic conditions can cause the compass to be in error, as can the metallic structure of the ship itself (especially if there have been changes to the superstructure or after drydockings) or the cargo the ship is carrying.
The strength of the earth’s magnetic field has reduced noticeably in recent times and the movement of the magnetic north pole has accelerated. It is even considered possible that the earth’s magnetic core could flip, reversing its natural polarity and making magnetic compasses point South instead of North. Even without natural changes, over time the accuracy of a magnetic compass will deviate and it will be necessary to correct the compass and record the deviation.
This is done by a process known as swinging the ship which should be carried out in open water at regular intervals. The compass is checked using a reference point such as the sun or a visible landmark on a known bearing. Deviations will be checked with the ship on all eight of the main headings and corrections made by repositioning the corrective elements that are located around the binnacle.
The compass card on which the direction markings are shown is isolated from movement as much as possible by suspending the card on a jewelled mounting and in a liquid-filled housing. Both of these damping means can become defective; the mounting by wear and the fluid by leakage or appearance of air bubbles. Tests can be done using a magnet to deflect the card through 90º and then releasing it and timing how long before the card returns to showing North. If the time is excessive, the compass may need to be calibrated or repaired.
All vessels should have their compass swung/adjusted and a new deviation card issued at maximum two-yearly intervals. When a new vessel is commissioned, compass deviation on any heading should be no more than 3°. Thereafter, deviation on any heading should be 5° or less. Vessels transiting the Panama Canal are required by the canal authorities to have had a valid compass deviation card issued within the previous 12 months. Some flag states and many shipowners will stipulate that the magnetic compass is to be swung and adjusted annually.
The limitations of the magnetic compass were a driving factor for the development of the gyrocompass in the early years of the 20th Century. Invention of the device is usually credited to Raytheon Anschütz but there were earlier variants in its evolution.
A gyrocompass makes use of gyroscopic principles and the Earth’s rotation to give a bearing that remains aligned to true North once the initial heading is set and the gyro put in motion. Unlike a magnetic compass, the gyrocompass is not hampered by external magnetic fields but can be affected by rapid changes in the orientation and attitude of the ship.
Before the advent of GPS, a magnetic compass would be used to set the gyrocompass to the correct heading. On most modern ships, the GPS or other navigational aids feed data to the gyrocompass allowing a small computer to apply a correction. Alternatively, a design based on an orthogonal triad of fibre optic gyroscope or ring laser gyroscopes will eliminate these errors, as they do not depend upon mechanical parts.
The fibre optic gyrocompass is a complete unit that, unlike a conventional compass, has no rotating or other moving parts. It uses a series of fibre-optic gyroscope sensors and computers to locate North. It has very high reliability and requires little maintenance during its service life. The system usually includes a sensor unit, a control and display unit and an interface and power supply unit. It is often linked with the ship’s other navigational devices including GPS.
The gyrocompass does not need to display the heading mechanically on a single display because it uses a sensor and the information can be sent to repeater units. These would be located at the steering station, in the emergency steering room and on the bridge wings. The exact number and location of repeaters is governed by SOLAS and will depend upon the size of the vessel. Although a gyrocompass is unaffected by magnetic interference from the ship or surrounding equipment, it is reliant on a stabilised power source and in some instances on a GPS input to remain functioning.
Ships that are not obliged to carry a gyrocompass are required to have a transmitting heading device (THD) that shows the ship’s true heading. Most THDs in use today are commonly known as a GPS compass and make use of an antenna with two or three GPS sensors. Where there are two, they will be placed either side of the vessel’s centre line and at equidistance from it so as to be able to calculate the orientation of the vessel. The sensors can also be used to calculate pitch and roll or trim.
GPS positioning can feed into other systems on the ship beyond navigation. For example, it allows recording of where and when systems such as oil-water separators and ballast treatment systems have been employed. This can be used as evidence of compliance with environmental regulations in case of PSC investigations.