Before the electronic age
Until the early 20th century, a ship in fog, rain or darkness had few ways of knowing what lay ahead. The navigator used the compass, the lead line, the log and, when the weather allowed, observations of sun and stars. Near land, fog signals from bells, whistles and horns, along with lights, gave warning. Radio changed this first, in the early 1900s, by allowing ships to receive time signals and weather warnings and, with a directional aerial, to take bearings on shore transmitters. Radio direction finding became a common tool between the world wars. The gyrocompass and the echo sounder also arrived in the same period, so that an officer could steer by a true-north reference and read the depth of water without hauling a lead.
How radar works, in brief
Radar, the name derived from “radio detection and ranging”, sends out short pulses of radio energy and listens for echoes. The delay between sending and receiving gives the distance to an object, and the direction of the rotating aerial gives its bearing. The result is shown on a circular screen as a map-like picture of the surroundings, with coastlines, buoys, ships and rain squalls appearing as echoes.
Wartime development
The basic idea of detecting objects with radio waves was explored in several countries in the 1930s, mainly for detecting aircraft. During the Second World War, Britain and the United States, among others, developed it rapidly for use at sea and in the air. A key advance was the cavity magnetron, developed in Britain in 1940, which made it possible to produce powerful signals of short wavelength. Shorter wavelengths allowed smaller aerials and sharper pictures, and sets small enough to fit on escort vessels and aircraft.
Radar helped warships find enemy ships and aircraft in the dark, and it became an important tool in the Atlantic convoy battle, where escorts and patrol aircraft used it against surfaced submarines. Navies also used it for navigation in convoy and near coasts, but its main role in the war was in detection and gunnery.
Radar in merchant ships
After the war, surplus equipment and new designs made radar available to merchant ships and passenger liners. By the early 1950s, it was being fitted to ocean liners, ferries, tankers and fishing vessels, and in time it spread to large and small ships alike. Later rules governing larger ships made radar a required piece of equipment.
For a watchkeeper, it brought two benefits. He could see other vessels in poor visibility, and he could take ranges and bearings of coastline features to check the ship’s position. In ports and narrow waters, a harbour tugboat, a pilot cutter or a ferry could now work in conditions that earlier would have meant waiting at anchor.
Radar had a drawback that took time to understand. Reading it correctly needs skill, since a screen does not show everything, and a clear picture can encourage a watchkeeper to run at speed in fog. A frequently discussed example is the collision of the liners Andrea Doria and Stockholm off the American coast in 1956; both ships had radar, and the accident is often cited in studies of how radar was understood and used. Over the following decades, training, rules for avoiding collisions in restricted visibility and new equipment dealt with these problems. Collision-avoidance plotting aids, usually called ARPA, arrived in the 1970s and tracked other ships’ courses and speeds automatically.
Hyperbolic radio systems
Radar showed what was nearby, but it did not give position far from land. Hyperbolic systems filled this gap. Stations on shore sent synchronised radio signals, and a receiver on the ship measured the small differences in the time at which the signals arrived. Each difference defined a curved line of position, and two or more of them crossed to give a fix. The Decca Navigator System, developed in Britain in the 1940s, and Loran, developed in the United States during the war, were widely used for several decades. Loran-C, a later development, covered long ranges. These systems gave more accurate positions than celestial navigation in all weathers, though their coverage was limited to areas near the chains of stations.
Satellites
The first satellite navigation system, Transit, was developed by the United States Navy and entered use in the 1960s. It gave positions at intervals as satellites passed overhead, which was good enough for ocean navigation. It was replaced by the Global Positioning System, which began as a military programme in the 1970s and reached full operation in the mid-1990s, followed by other national and regional satellite systems. By the end of the century, a small receiver could give a continuous position in nearly all weather, and the older hyperbolic networks were gradually shut down.
The electronic bridge
Several newer technologies were built on the same foundation. The Automatic Identification System, which broadcasts a ship’s identity, position, course and speed to its neighbours and to shore stations, became required for larger vessels in the early 21st century. Electronic chart displays show the ship’s position on a digital chart, and in many countries they are accepted in place of paper charts, provided that certain standards are met.
These systems can fail or be disrupted, and seamanship still rests on cross-checking. Officers are generally trained to confirm electronic information by other means, including the compass, depth sounder, visual bearings and, where practical, observations with the marine sextant. The tradition of using more than one method at a time continues, even though the instruments have changed.