Lighthouse Radio Beacons and Racons
From Light to Radio Signal
The lighthouse's primary purpose has always been to give a ship's officer a fixed, identifiable reference point — something unambiguous in the visual noise of a coastline at night or in failing visibility. For most of lighthouse history that reference was entirely optical: the intensity of the light, its characteristic flash pattern, its colour sectors. Radio technology offered a fundamentally different approach. Beginning in the 1920s, transmitters installed at lighthouse stations could be detected by shipboard receivers at distances far beyond the visual range of any light, and in total darkness, heavy rain, or dense sea fog that would have rendered even a first-order Fresnel lens invisible.
How the Marine Radio Beacon Worked
A marine radiobeacon transmitted a continuous or interrupted medium-frequency signal in the 285–325 kHz band, typically at a power of between 50 and 2,000 watts. The ship carried a radio direction finder — an antenna that could be rotated to identify the bearing from which the signal was strongest. By taking bearings on two or three beacons whose positions were precisely known and charted, a navigator could fix the ship's position using the same triangulation principle as visual cross-bearings. The system was relatively cheap, worked in all weathers, and required no modification to the lighthouse structure itself beyond a small transmitter building and an antenna mast.
Radiobeacons were assigned individual identification codes — typically two or three letters in Morse — so a navigator could confirm which station was being received. Many lighthouses transmitted on grouped schedules, with several stations in the same sea area sharing a frequency and transmitting in a pre-arranged sequence so that their signals did not overlap. The Admiralty List of Radio Signals, published annually, catalogued every operational beacon with its frequency, characteristic, range, and the times it broadcast.
Major Beacon Stations and Their Role
Some of the world's most strategically important radiobeacons were collocated with major lighthouse stations. Bishop Rock, at the western tip of the Scilly Isles, carried a beacon that was among the first references a transatlantic vessel encountered when approaching the English Channel from the west. Its light — a first-order optic showing a flashing white with a range of 24 nautical miles — was already well known; the beacon extended its usefulness to vessels still hundreds of miles offshore. Similarly, the lighthouses on Ushant (Île d'Ouessant) off the Breton coast of France and on Cape Race in Newfoundland operated radiobeacons that became standard references on the North Atlantic route.
In American waters, the US Lighthouse Service began installing radiobeacons at major stations from 1921 onward, with Cape Henry, Virginia and Cape Cod, Massachusetts among the early sites. By the mid-1930s there were more than 70 marine radiobeacons operating along the US coastline and the Great Lakes. The US Coast Guard, which absorbed the Lighthouse Service in 1939, continued the programme and eventually operated more than 200 stations. Similar systems expanded throughout British, Norwegian, Australian, and Japanese territorial waters during the same period.
The Differential GPS Transition
Marine radiobeacons survived largely intact until the 1990s, when the Global Positioning System became widely available and genuinely reliable. A ship with a GPS receiver no longer needed shore-based references at all — position could be calculated from satellite geometry to within a few tens of metres. Most general lighthouse radiobeacons were decommissioned by the late 1990s and early 2000s. However, many beacon transmitters at lighthouse stations were converted to a different and more precise function: differential GPS reference stations, abbreviated DGPS. These transmitted correction data that allowed a ship's GPS receiver to improve its accuracy to around one to three metres, by comparing the GPS-derived position at a precisely surveyed fixed point with the position the satellites were indicating at that instant. The medium-frequency infrastructure — the transmitter buildings, the antenna masts, the precisely surveyed coordinates — proved directly reusable.
Racons: The Radar Transponder
Alongside the radiobeacon, a separate technology emerged that worked with shipboard radar rather than with dedicated radio receivers. The racon — a contraction of radar beacon — is a passive transponder that detects the sweep of a ship's radar beam and responds by transmitting a coded signal back along the same frequency. This signal appears on the ship's radar screen as a distinctive mark: typically a line extending from the racon's position in the direction away from the vessel, sometimes preceded by a coded Morse identifier. The navigator can read the code, look it up in the chart or pilot, and confirm which mark is being observed — useful in pilotage waters where several buoys or beacons might appear as undifferentiated dots on the radar display.
Racons began appearing on lighthouses and major buoys in the 1960s and became standard equipment at critical pilotage marks from the 1980s onward. They are particularly valuable at rock hazards and offshore shoals where a vessel navigating in restricted visibility needs unambiguous confirmation of a danger's precise position. The Bishop Rock lighthouse carries a racon; so do the Lizard in Cornwall, Fastnet Rock off County Cork, and many North American offshore lights including Nantucket South Shoal and the lighthouses guarding the approaches to the Chesapeake Bay.
Racons operate across the X-band (3 cm, 9.3 GHz) and S-band (10 cm, 3 GHz) radar frequencies used by most commercial vessels. The transponder needs only a small amount of power — a modern solid-state racon can run continuously on the same solar-panel and battery system that powers the lighthouse's LED lantern. This efficiency has allowed racons to be fitted to remote offshore stations that were previously inaccessible for conventional radio infrastructure.
AIS and the Current Picture
The Automatic Identification System, mandatory on most commercial vessels since 2004, added another layer to the shore-based aid network. AIS base stations are now collocated with many lighthouse stations, receiving and rebroadcasting the position, identity, heading, and speed of vessels in the surrounding sea area, and transmitting that data to vessel traffic services and coast guard operations centres. Some lighthouses also transmit their own AIS position as a virtual or real AIS aid-to-navigation mark, allowing a vessel's chart plotter to display the lighthouse as an identified object even if it is outside visual or radar range.
The practical result is that a modern lighthouse station can simultaneously carry a LED lantern with a range of 20 nautical miles, a racon readable at 20 miles on radar, a DGPS correction transmitter, and an AIS base station — each technology addressing a different failure mode, each exploiting the lighthouse's unique attribute of a known, precisely surveyed position on a prominent piece of coastline. The optical light remains fundamental; radio, radar, and satellite technologies have accumulated around it.
Open the map to see which lighthouse stations in any sea area you are researching currently carry racons or AIS marks — the data drawn from OpenStreetMap includes operational details for many stations worldwide.