LORAN A and C - Back in the day

Loran-A used two frequency bands, at 1850 kHz and 1.950 kHz, smack in the middle of a frequency band exclusively allocated to radio amateurs.

The cathode ray tube had the advantage that the pulses were still recognizable on the screen even when the amplitude of the interference surpassed that of the pulse. It was the only steady signal amongst all kind of interference jumping around. It was even possible to use Loran under circumstances that would have shut down medium and short wave communication. Quite remarkable.

I only sailed once with a RCA Loran A which was attached to the bulkhead in the chart room. It was never used but as the youngest officer I was intrigued by it and started to read the manual. It was not to difficult too learn how to get a fix with the help of the special Loran charts, but only in the western part of the Atlantic. The captain shrugged and said his faith was in a good old fashioned star fix.

Only on one occasion, after five days in very bad weather and failing to see sun or stars, I was asked to get a fix. They did even wake me up for that, it was suddenly urgent. It appeared that we were blown a good distance in the direction of Greenland… That felt good! But after that it was back to normal again.


My first experience with navigation was in Alaska with a RCA LORAN A like you describe. Ours had a little wheel that you’d turn to get the two signals to line up on the cathod ray tube to get the time delay between the two stations.

This was very often the case in Alaska, the availability of celestial was not something that could be depended upon.

I do recall using the sky wave correction tables a few time mostly out of curiosity but I recall the fixes plotted close to our DR.

Loran A was used during the war extensively in the Pacific. For the fishery it was also a very important navigational equipment. Long after the US had shut down their chains it was still used by the Japanese fishermen using their own chain.

We were then en route from Europe to Three Rivers between Quebec and Montreal. Great circling brought us already somewhat close to Greenland.

My interest in the Loran A on board was because in school we had a class in hyperbolic navigational systems, which was brand new then, with a dictation which I still have, it was not yet in the books. We were one of the first, also getting our radar certification. The dictation described Loran A, Decca and Consol.


I remember the old DX Navigator receiver was standard on the vessels early '70s. The later models were Loran A and C. I don’t recall anyone using C until there was digital displays.

500 KHz has plenty of skywaves too as long as it late night or even better a winter night with the D layer gone. I never messed with a Loran-A, but I did plenty with Loran C. C was on 100 KHz and at the time I lived in sight of the radio station in Annapolis that talked to subs on something like 90 or 120 KHz with some gigawatts of power. You had to go into the menus and set up notch filters on the sub frequencies if you wanted the thing to work near home.

I don’t believe it’s possible to use Loran C without extensive computerization as all signals are on 100 KHz and distinguished only by delay.

The old DX Navigator had the CRT for A and C. I never saw anyone use C.

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I have only had experience in using Loran A and it must have been the Japan or Philippines chain. I don’t remember much reliance been placed on the position by my seniors. I had more experience with the Decca system in Europe and the Persian Gulf. There was a fourth lane accessible by the pilot in Rotterdam to mark the deep water channel.

What do you mean? We had a LORAN C on the CG cutter. It was jury-rigged into the nav equipment rack on the bridge. The story was it was surplus from an aircraft at the airbase (Kodiak).

It had a CRT display and IIRC the TDs were displayed on an LED display. The displayed TDs were plotted directly on the chart.

No manual or instructions. Never trusted it because the signal could be “forced” to display a different position if you didn’t like the one obtained.

Today, Digital Signal Processing (DSP) technologies are used to perform most of the complex calculating tasks. These technologies have reportedly improved the absolute accuracy of LORAN-C to approximately 50 feet. Earlier generations did not have access to such techniques. For example, in the 1970s, the first digital LORAN-C receivers measured TDs by using a local oscillator to sync with the input pulses. At the time, analog-to-digital conversion (ADC) was so primitive that engineers could only determine whether a point on the LORAN-C signal was positive or negative.

Despite these limitations, engineers built receivers that were accurate to 200 feet. Engineers in the 1950s had it even worse: digital technology wasn’t even invented, so signals were obtained and manipulated with analog circuits. Yet, these receivers were still accurate to 1500 feet. Clearly, today’s technology provides advantages and opportunities that LORAN’s architects never had.

And risks. The accuracy of the navigation equipment used today may exceed the accuracy of the survey that charted the hazards in the area you are operating in…


Zones of Confidence (ZOC) indicated on charts refer to the following:



That interesting info and maybe partly explains but “back in the day” we didn’t sail close to navigation hazards relying on electronics alone.

In the middle of the Gulf of Alaska for example having an error of 3 miles or even more would not be an issue. On the other hand on a transit through Whale Passage we’d use radar and visuals.

As the ship approached the coast with it’s navigation hazards the methods used become more accurate. This wasn’t considered esoteric knowledge, it just seemed like common sense.

That was for us also good enough, even 10 miles on the big ponds.

How did you choose what chain to use? Everything is on the same frequency.

It’s supposed to synchronize on the third pulse of a shaped pulse train at 100 kHz. It may accidentally get the wrong pulse, or you can manually cycle-skip to get a stronger signal. Either way each cycle away from the third one nets you a 10 uS difference in the relevant TD (or both if you cycle-skip on the master).

So? It was selected by a microprocessor? This was 1975 and it was surplus aircraft.

The hyperbolas could be seen on chart. The correct TDs were known approximately from the DR.

They existed. Intel 4004 came out in 1971. But I’m being stupid. You set the Group Repetition Interval into the receiver, and now only members of that chain are stationary on the CRT. And the secondaries (not truly slaves any more) come in a defined order within the group.

A less known feature of Loran C is that it has the capability of time synchronization of remotely separated clocks and where there was a need to measure a common event. Specifically, this timing capability was very important in the early space program, and in measuring the arrival time of pulses from nuclear detonations.

And then there was Enhanced Loran or eLoran with an accuracy of 8 meters that was designed to provide back up/redundancy to GPS/GNSS in safety critical applications but never got off the ground as Loran was terminated shortly after.

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That makes sense.

The Loran C receiver was mounted in rack about three times bigger then the unit so I wonder if the unit from an aircraft had replaced a tube set? Ship was built in 1966.

I was a lowly E-4 so someone else would have changed chains. We stayed in Alaska or sailed coast-wise so no need to change.


The signal from a single LORAN transmitter will be received several times from several directions. This image shows the weak groundwave arriving first, then signals after one and two hops off the ionosphere’s E layer, and finally one and two hops off the F layer. Operator skill was needed to tell these apart.

Like Dutchie said, it was nice to be able to be the only one on watch that could find the signal. But the ground wave could be easily seen during the day and as night fell the ground wave would get weaker and weaker while the sky wave would get stronger. But if you’d been watching all along you could still pick it out.

Not hijacking, but my first bridge watches were with Omega, and it wan’s that hard to be off one lane, about 8nm…

I could tell the story about an amphibious landing during an exercise in NC in the early '70s that missed the beach by 8nm, but…