Before radio, a ship closing a coast in fog had a lead line, dead reckoning, and whatever bells, whistles or horns it could hear, and sound in fog is a poor guide to direction and an even worse guide to distance. In the 1920s the US Lighthouse Service began putting low-power transmitters on lighthouses and lightships so that any ship with a direction finder could take its own bearing on a charted point. This piece covers what the fog signal could not do, how a radiobeacon identified itself by its characteristic and its schedule, what early shipboard direction finders got wrong, and why the first-beacon date is less tidy than it looks.

Bells, sirens and diaphones: what sound could and could not do in fog

Through the nineteenth century, US light stations added sound signals for the days when the light itself could not be seen. Stations used fog bells struck by clockwork, steam whistles, trumpets and sirens driven by compressed air or steam, and, by the early twentieth century, the diaphone, whose low groaning note many coastal residents still associate with fog. Each station sounded a published characteristic, a fixed pattern of blasts and silences repeated on a regular cycle and listed in the Light List, so that a navigator with a watch could count the pattern and tell one station from its neighbors. That idea of identifying a fixed aid by its timing carried straight over into radio a few decades later.

The trouble was the air between the horn and the ship. Sound bends as it passes through layers of air at different temperatures and through wind that changes speed with height, and fog often comes with exactly those conditions. Joseph Henry, the physicist who served on the US Lighthouse Board and later chaired it, ran field tests on fog signals during his years on the Board, and John Tyndall carried out comparable trials in Britain. The trials recorded signals that faded or vanished at modest distances from the station and then reappeared farther out, along with large differences in audible range from one day to the next and with the direction of the wind.

For the navigator, the practical result was that loudness said very little about distance, and the apparent direction of a sound in fog could mislead a lookout as well. A faint horn might be close, and a horn that could not be heard at all might be closer still. The Light List has long carried cautions to this effect, warning mariners not to judge their distance from a station by the strength of its sound signal and not to assume a signal is operating because it cannot be heard. A ship that heard a fog signal learned that it was somewhere in the general neighborhood of a station, which was useful, although that fell well short of a position anyone could plot.

Lightships and the submarine bell: marking hazards a tower could not reach

Many of the worst hazards on the American coast lay offshore, on shoals where nobody could build a tower at reasonable cost. The Lighthouse Service and its predecessors answered with lightships, vessels anchored on station that carried a light, a fog signal and a resident crew. Most lighthouse histories date the first US lightship to 1820 in the lower Chesapeake Bay. Later stations included Nantucket Shoals, Diamond Shoal off Cape Hatteras, and the approaches to New York harbor, and the crews lived aboard for long stretches in all weather on a vessel that was not allowed to leave in a storm, because a lightship that left its station left the hazard unmarked.

Before radio there was an intermediate step that used water rather than air. From the early 1900s, the Submarine Signal Company of Boston supplied bells that hung beneath lightships and some other stations and were struck mechanically in a coded pattern. Ships fitted with the company’s receivers carried hydrophone tanks on each side of the hull, and an operator listening through earphones compared the sound from the two sides and turned the ship until the two were equal, which put the bell ahead. Sound in water does not suffer from the layered-air problems of the fog horn to the same degree, and accounts of the system describe it as giving a more dependable direction than listening on deck, although it required special equipment on the receiving ship, its range was limited, and the ship’s own machinery noise competed with the bell.

The lightship mattered for the radio story because it sat where traffic converged. A liner approaching New York or Boston from the open Atlantic aimed its landfall at a lightship, because the lightship marked the hazard and the entrance to the channel at once. That made the lightship an obvious place to put a radio transmitter, and, as a later section explains, it also made the lightship the vessel most exposed to a ship steering on that transmitter in fog.

Shore bearings or ship bearings: the choice behind the radiobeacon

Radio direction finding was established well before the First World War. Rotating loop antennas, and the fixed crossed-loop system with a rotating goniometer developed by Ettore Bellini and Alessandro Tosi, both let an operator find the direction of a transmitter by turning for the point where the signal faded to its minimum. The technique could be set up in either of two ways. A shore station could take a bearing on a ship’s transmission and tell the ship where it was, or a ship could carry the direction finder and take bearings on transmitters ashore.

The US Navy took the first approach after the war with shore radio compass stations along the coasts. A ship that wanted a position called the stations, transmitted so they could take bearings on it, and received back a bearing or a fix by radio. The arrangement had real advantages, because the shore stations had trained operators and carefully calibrated equipment and the ship needed only the transmitter it already carried for ordinary traffic. Its disadvantages showed up in exactly the weather where it was needed most. The stations could serve one ship at a time, so a fog bank off a busy port produced a queue, every ship asking for a bearing added transmissions to crowded frequencies, and the ship had to wait for the answer to come back before it could plot.

The radiobeacon reversed the arrangement. A transmitter on a charted lighthouse or lightship sent its signal, and any number of ships could take bearings on it at the same moment, each with its own direction finder and operator, without transmitting anything and without waiting on anyone ashore. The cost moved to the ship, which now needed a direction finder, a trained operator and a calibrated installation. The Bureau of Standards carried out much of the US engineering work on the shipboard radio compass in cooperation with the Lighthouse Service, and histories of the period describe the shore-station and beacon systems running side by side for some years before the beacon approach became the standard for coastal navigation.

Ambrose, Sea Girt and Navesink: dating the first US radiobeacons

The account most often repeated in Lighthouse Service and Coast Guard histories puts the first US marine radiobeacons at three stations on the approaches to New York: the Ambrose Channel lightship and the light stations at Sea Girt and Navesink in New Jersey. The year usually given for regular service is 1921. The choice of sites made navigational sense, because three transmitters spread around the harbor entrance gave a ship coming in from sea bearings that crossed at useful angles rather than lines running nearly parallel.

The date is where care is needed. Experimental work with the Bureau of Standards came before regular service, equipment was installed and tested before the Lighthouse Service announced the signals to mariners, and different accounts count from different events. Some date the first beacon to the first test transmission, some to installation, and some to the published notice that the signal was available for navigation. Accounts also differ on which of the three stations should be called the first. I give 1921 as the year commonly cited for regular public service on the New York approaches and I do not give a single day or name a single first station, because I have not seen the sources agree on either.

For anyone who needs a firmer date, the places to look are the Coast Guard Historian’s Office, the contemporary Notices to Mariners and the Lighthouse Service’s own published bulletins, and the writings of George Putnam, who was Commissioner of Lighthouses through this period and promoted the radiobeacon program. Whatever date a source uses, check which event it is dating. Through the 1920s and 1930s the network spread along the Atlantic, Gulf and Pacific coasts and onto the Great Lakes, and in 1939 the Lighthouse Service was merged into the Coast Guard, which ran the radiobeacons from then on.

Characteristic and minute: how the timed identification signal worked

The radiobeacon borrowed the fog signal’s method of identification and applied it to radio. US marine radiobeacons transmitted on low frequencies in the neighborhood of 300 kHz, in a band that later came to be associated with marine radiobeacons in the range of roughly 285 to 325 kHz. Each beacon sent its own characteristic, a distinctive group of dots and dashes, and the operator checked what he heard against the published list of radiobeacons before trusting any bearing. A bearing taken on the wrong station is worse than no bearing at all, because it plots as a clean line in the wrong place.

Time was the second half of the identification. Beacons did not transmit continuously in all conditions, and published schedules set when each one operated in clear weather, with more frequent or continuous operation in fog. As the network grew, beacons in the same area were grouped onto a shared frequency and assigned their own minutes in a repeating cycle of a few minutes, so that each one transmitted in turn and the others stayed silent. A navigator could stay tuned to one frequency and take bearings on several beacons one after another without retuning. The clock told the operator which beacon ought to be on the air, and the characteristic confirmed it, which meant that a bridge clock running a minute off, or a characteristic copied wrongly, could put a bearing on the wrong station.

The length of each transmission suited the equipment. A hand-turned loop needs a steady signal while the operator swings the antenna back and forth across the minimum to find it precisely, and a beacon that sent for a full assigned minute gave him that time. This is the opposite of the military problem covered in the HF/DF article elsewhere in this section, where the transmitter was trying to be brief, because the beacon wanted to be found and was built to stay on long enough for the slowest set to take a good bearing.

Some stations added a way to estimate distance by synchronizing a radio signal with a sound signal. The radio signal arrived effectively at once, while the sound took about five and a half seconds to cover each nautical mile, so a navigator who counted the seconds between the two and divided by that figure had an approximate range. The figure varies with air temperature, and the method still depended on hearing the horn, which brought back every acoustic problem the radiobeacon was meant to get around, although a range from timing was far better than a guess from loudness.

Sense, deviation and the coastline: limits of the shipboard direction finder

A loop antenna produces two minimums, half a circle apart, so a bare loop bearing could point either toward the beacon or directly away from it. Early operators often settled the question with dead reckoning, since a ship usually knew which side of the beacon it was on, and sets later added a sense antenna whose signal, combined with the loop’s, showed which end of the line was correct. Near a beacon, or after a long run without a fix, the ambiguity mattered, and an operator who resolved it from an assumption rather than from the equipment could plot the ship on the wrong side of a hazard.

The ship itself bent the bearings. Masts, stays, funnels, cargo booms and the steel hull re-radiate the incoming signal, and the resulting error, called radio deviation, changes with the direction the signal comes from and is often largest on bearings midway between the bow and the beam. Each installation had to be calibrated by swinging the ship through a full circle in sight of a beacon, comparing the radio bearing with the visual bearing at each heading, and drawing up a correction curve. The curve held only as long as the ship’s metal stayed where it had been during the swing, so rerigging, moving booms or adding fittings near the loop could make it wrong without anyone noticing.

Propagation added errors that calibration could not remove. A signal crossing a coastline at a shallow angle is bent at the boundary between land and sea, so bearings on a beacon whose signal ran along or across the shore were less trustworthy than those over open water. Around dusk and dawn, and at night at greater ranges, the skywave from a low-frequency beacon could mix with the ground wave and blur or shift the minimum, and navigation guidance treated night bearings on distant beacons with suspicion. At long range a radio bearing follows a great circle, which does not plot as a straight line on a Mercator chart, so navigators applied a conversion correction before laying it down.

The working rule followed from all of this. A single radio bearing was a line of position with an error the navigator could only estimate, and a usable fix needed bearings on two or three beacons crossing at good angles, preferably over water and preferably in daylight, checked against soundings and dead reckoning.

Steering on the beacon: the Nantucket lightship and a known hazard

A direction finder allowed a ship to do something simpler than plotting a fix, which was to keep the beacon dead ahead and steer toward it. The method was easy, because it needed no chart work and brought the ship to the transmitter. When the transmitter stood on a lighthouse a prudent navigator would turn off well before arriving, but when it was on a lightship anchored in the approach lane, a ship homing on it in fog was steering straight at a vessel that could not move out of the way.

The best-known case is the loss of the Nantucket lightship LV-117. On 15 May 1934, in fog, the White Star liner Olympic struck and sank the lightship on its station off Nantucket Shoals, and accounts give seven of the lightship’s crew as killed. Most accounts describe the liner as steering on the lightship’s radiobeacon at the time. I describe what is recorded and leave questions of speed, lookout and responsibility to the contemporary inquiry and court records, which are where anyone writing about the collision should go for the detail. The general point is supported independently of this one case, because a radio bearing gives direction only, and a ship steering down it has no measure from the radio of how close it is getting.

Navigation texts in later years warned mariners against running directly down a radiobeacon bearing toward a lightship and advised setting a course to pass the station at a safe distance. Over the following decades the Coast Guard replaced lightships with offshore light towers and large navigational buoys, which removed crews from the most exposed stations, and the last US lightship left service in the early 1980s.

From the beacon band to satellites: what replaced the radiobeacons

Marine radiobeacons served for most of the twentieth century alongside the systems that would eventually replace them. LORAN came out of the Second World War and gave positions over far greater distances, and satellite navigation followed. In the 1990s the Coast Guard used the radiobeacon band to broadcast differential GPS corrections from many former beacon sites, which gave the old band a second career, and conventional marine radiobeacons were phased out as GPS took over. The Coast Guard has since discontinued its differential GPS service as well, and the Coast Guard Navigation Center publishes the notices and dates for anyone who needs them. Aeronautical non-directional beacons are a separate system under the FAA and should not be confused with the marine network.

The radiobeacon was useful because its location was charted, its identity was published and its schedule was known in advance, so a receiver that heard it could say with confidence what it was and where it stood. A conventional repeater’s automatic station identification serves a smaller version of the same purpose, since it tells anyone hunting interference or checking coverage which transmitter is on the air. A fixed transmitter at a surveyed location also makes the best calibration target for direction-finding equipment, for the same reason the ships swung against beacons they could see.

The transition from fog signal to radiobeacon was gradual, and stations kept their sound signals for decades after the beacons went on the air, so a ship that lost its radio or its direction finder still had something to fall back on. Public safety now depends on GPS for vehicle location, for timing in simulcast radio systems, and often for the clocks that stamp CAD and recorder records, and the fallback for each of those dependencies is something a working agency should be able to name. The homing problem also has a modern form, in that a single strong reference, whether a beacon bearing or a GPS route, gives direction without telling the user what lies between him and the destination.

What to do at your agency

• The radio system administrator should listen to each conventional repeater this month, confirm that its automatic station identification is transmitting the correct callsign at the interval the license requires, and record the result in the maintenance log for that site.
• The radio shop supervisor should take bearings with the agency’s direction-finding equipment on one of the agency’s own repeaters from a surveyed point, compare them with the bearing calculated from the map, and write the error and the date into the equipment record.
• The radio system administrator should ask the system vendor for the written holdover specification for the simulcast or site timing reference, meaning how long the system keeps correct timing after GPS is lost, and add that figure to the existing communications continuity plan.
• The communications center manager should confirm in writing which time source the CAD system and the radio recorder use and what happens to their timestamps if that source fails, and add the answer to the existing continuity plan.
• The marine or water rescue unit supervisor should review the unit’s boat operator manual and remove or rewrite any procedure that relies on marine radiobeacons, replacing it with the current Light List as the reference for aids to navigation.
• The training officer for any marine unit should add one paragraph to the existing low-visibility operations procedure stating that crews pass fixed marks such as buoys and platforms at a set distance rather than steering directly onto them by GPS or radar.

Takeaways

• Fog signals identified stations by a published timed characteristic, but sound in fog bends and fades so unpredictably that loudness was no guide to distance and audibility was no guarantee of range.
• Lightships marked offshore hazards where towers were impractical, and the submarine bell let suitably equipped ships take a rough direction through the water before radio was available.
• The radiobeacon moved direction finding onto the ship, so that any number of vessels could take bearings at once without transmitting, which the Navy’s shore radio compass stations could not offer.
• The first US marine radiobeacons are commonly placed on the New York approaches at Ambrose Channel, Sea Girt and Navesink with regular service around 1921, although sources date installation, testing and public service differently.
• Radiobeacons identified themselves by a characteristic group of dots and dashes and by their assigned minutes in a shared-frequency cycle, so an accurate bridge clock was part of identifying the station.
• Shipboard direction finders suffered from sense ambiguity, radio deviation from the ship’s structure, coastal refraction and night effect, so one bearing was only a line and a fix needed several crossing at good angles.
• Homing on a lightship’s radiobeacon in fog put ships on a course toward the lightship itself, and the 1934 sinking of the Nantucket lightship by the Olympic is the case most accounts cite.
• Agencies that now depend on GPS for location, timing and timestamps should know the fallback for each dependency, much as light stations kept their fog signals long after the beacons went on the air.