Signals and sensors used in ship navigation

Not all communication between ships is done by radio or satellite and this is unlikely to change so long as the redundancy that comes from older methods that add to safety and permits communication with leisure and non SOLAS vessels is considered desirable. Many of the older communication means are not located on the bridge itself but are controlled from there and it falls to the navigation team to interpret the signals given by others.

Conceivably the oldest and most important are navigation lights. The types of lights required and their use are specified in The International Regulation for Preventing Collisions at Sea (COLREGS 1972). The regulations are notable in themselves given that they have their roots in the 19th century and represent a much older international co-operation on maritime matters than SOLAS or the IMO although they now come under the umbrella of both.

Initially devised in the age of sail when collisions between vessels most often resulted from the variability of the wind and the inability of ships to change course rapidly, the ‘rules of the road’ have been revised over time taking account of technology advances and the advent of powered vessels. Knowledge of COLREGS is an aspect of basic navigator training but proper application of them relies upon the signals given by ships in different circumstances.

Requirements and rules

Navigation lights must be displayed from sunset to sunrise and during time of low or poor visibility so that nearby ships can navigate safely after seeing the navigation lights. Five separate lights are fitted at different positions on the ship according to the requirements of the rules and these allow easy identification of the displaying ship’s size, direction of travel or to indicate the ship is at anchor. Navigators learn the use of lights at an early stage in their education.

The lights are:

  • Foremast – Bright White with a horizontal arc range of 225 degrees.
  • Mainmast – Bright White is also known as all-round light has a horizontal arc visibility of 360 degrees.
  • Port side – Bright Red with a horizontal arc visibility of 112.5 degrees
  • Starboard side – Bright Green with a horizontal arc visibility of 112.5 degrees
  • Stern of the ship- Bright White with a horizontal arc visibility of 135 degrees.

In addition to the above, two anchor lights are fitted forward and aft and are bright white in colour. The power for the navigational lights must be supplied from a separate distribution board which has no other supplies attached to it. This is done so that they cannot be extinguished by inadvertent operation of a wrong switch. The mast head lights must be visible from at least six nautical miles away for vessels over 50m in length and from three nautical miles for smaller vessels.

Due to the critical nature and essential safety requirement of navigational lights, they are fitted in duplex manner at each position. Two separate lamps or a lamp holder with dual fitting can also be used. All lights are switched from the bridge and any fuses must also be able to be changed on the bridge. A control panel must have indicator lamps and an alarm in case of failure of any light.

Despite being a fixture of ships for a considerable length of time, navigation lights are kept under review. At NCSR 4 in March 2017, the Sub-Committee agreed an IACS draft Unified Interpretation on application of the COLREGs with respect to the placement of side lights as an interim measure, ahead of a future revision of the COLREGs.

Another visual aid that must be available and used from the bridge is the Daylight Signalling Lamp which is mandatory on all ships above 150gt and all passenger vessels. The lamp must have its own emergency power source and in most cases this will be a battery kept in a constant state of charge. It will be used to send Morse signals to other vessels when appropriate.
The use of the signalling lamp is now one of only two instances where Morse code remains in shipping (the other being the ship’s whistle) following its phase out for radio communications under GMDSS.

Another requirement of COLREGs is that during daylight hours, ships that are at anchor, being towed or not under control should display an appropriate shape from the fo’c’sle. These are known as the Black Ball or Black Diamond, the former being for ships at anchor and the latter for towed or Not Under Command (NUC) vessels.

As far as visual aids and signals go, the final requirement for ships that harks back to an earlier era but which is still relevant and mandatory is the carriage and occasional use of signal flags. The 26 alphabet flags and the substitute and numerical pennants can be used to signal a range of information with each of the letter flags having a unique meaning assigned in accordance with The International Code of Signals 2003. More complex messages can be sent using combinations of letters and numbers. Navigators are expected to know the meaning of the signal flags but a copy of the code is also required on the bridge. In cases of needs it is also possible to signal using semaphore flags but the use of VHF radio has made most such forms of signalling somewhat outmoded.

SOLAS requires all ships to have a means of signalling by sound and for merchant vessels this is covered by the requirement to be equipped with a whistle often erroneously referred to as a foghorn. Sound signals are made using Morse and the meanings attributed by the Signals Code.

Ships generally have two whistles, one electric and the other powered by compressed air. Sound signals are only of use if they can be heard, and in the fully enclosed bridge of modern ships that would not always be possible, therefore it is a requirement of SOLAS that such vessels should be equipped with a Sound Reception System that enables the navigating officer inside the cabin to listen to the sound signals and horns from other ships.