signals at sea can be broadly divided historically into day signals, night signals, wireless telegraphy, and modern radio communications, and a brief history of the development of each can be found below.

Day Signals.

The first record of a system of communication between ships with flags was the fighting instructions of the 16th century. Only five flags were used at first and though their significance depended on where and how they were hoisted communication was very restricted. However, by the end of the 17th century five more flags, and the Union flag, had been added. When these were supplemented by firing a particular number of guns, it was possible to make 22 manoeuvring signals for the purposes of battle tactics. In the 18th century the number of flags, and flag signals, steadily increased. A code of signals for the general chase was also introduced for warships, and these were the first signals to include compass directions. By the 1790s there were 57 signals in the fighting instructions alone and these were increased further by a numbered code with 28 designs of flags (numbered 1 to 28) that could be used singly or in combination and could be hoisted anywhere without affecting their meaning. After modifications, the Admiralty issued the Signal Book for the Ships of War in 1799 which remained in force with minor alterations for nearly 30 years. The numerical signals of the 1799 signal book were supplemented in 1803 by a vocabulary signal book, Telegraphic Signals or Marine Vocabulary. For this a different set of numeral flags was introduced, three or four flags denoting a word or phrase. Its use continued throughout the 19th century until the advent of the International Code of Signals which was also used by merchant ships to hoist their signal letters. By then there had been two important additions to signalling by day, semaphore and the Morse code.

Night Signals.

In addition to signalling by flags during the day, various night signals were developed. From the 18th century to the middle of the 19th, they were based on arrangements of four lanterns or fewer, disposed in horizontal or vertical lines or in squares or triangles, sometimes supplemented by a masthead light, either alone or with rockets, blue lights, or false fires. Then, in 1867 a British naval officer, Captain Philip Colomb (1831–99), invented a form of Morse code using short and long flashes from a lantern. Later the Morse code itself was adopted which, with the advent of electricity aboard ships, was transmitted by Aldis lamp.

Wireless Telegraphy.

A development of the electric telegraph, a 19th-century invention which used Morse code to send and receive messages. The first transmission took place in 1892, and by 1901 the Italian Guglielmo Marconi (1874–1937) was regularly transmitting Morse code signals to ships at sea. At first strong electrical power and a long wavelength were needed to send signals long distances. Initially, it was thought that short-wave sets on low power would be valuable in warfare at sea because, it was assumed, they could not be detected more than a few kilometres away. However, it was later found that the transmissions included some waves that inclined upwards which bounced off a layer of the atmosphere so that after about 160 kilometres (100 mls.) the signal could again be received. An apparatus to signal to submarines underwater was also invented early in the 20th century by the Canadian–American radio pioneer Reginald Fessenden (1866–1932), who was also one of the pioneers of radiotelephony. This directed sound waves over a distance of up to about 1.6 kilometres (1 mls.). After the Second World War (1939–45) sonar was used, in which a supersonic beam was transmitted underwater a considerably greater distance.

Modern Radio Communications.

Radiotelephony, voice radio, was developed for use at sea during the first decades of the 20th century. Nowadays VHF (very high frequency) radio, with the help of various satellite systems such as Inmarsat, is universally used for ship-to-ship and ship-to-shore communications. A more recent addition is the Automatic Identification System (AIS), a Swedish invention, which transmits high-speed information about the movements of ships in an area. This broadcasts data, in digital format, on a dedicated VHF channel at very high speeds lasting only seconds. Because it transmits in very small bursts at least 2,000 ships in any one area can use the system simultaneously without causing radio interference with each other. The information is received aboard a ship via its AIS transponder which has three main components: a GPS receiver, a VHF transceiver, and in between them a computerized data processor.

The minimum data a ship must broadcast using AIS is its nine-digit Maritime Mobile Service Identity number, what type of ship it is, and its position, course, speed, and rate of turn. AIS collects this information from all ships within VHF range and then either sends it to the recipients as text or marks it alongside the blips on the recipient's radar screen or electronic chart. Such information is extremely useful if a ship's master considers himself on a possible collision course with another ship, particularly as it is also possible to ask for additional information about the ship causing concern. AIS was developed for ships to inform VTS (vessel traffic systems) authorities of their hazardous cargoes. This is because, in certain congested shipping lanes such as the Straits of Dover, VHF voice traffic has just about reached saturation point. Lighthouse authorities, and some port authorities, have also started to use it to broadcast instant data about their aids to navigation. In the USA, the system, for security reasons, is now compulsory for vessels as small as 20 metres (65 ft) LOA. The European Union (EU) requires all its coastal states to have established the AIS infrastructure by 1 July 2007. See also digital selective calling; navtex.