The Radar Room

LORAN NAVIGATION AID

(LOng RAnge Navigation)

GEE was a hyperbolic navigation system good for ranges up to a maximum of around 450 miles, at which distance the increasing inaccuracies made precise navigation extremely difficult. As the US forces operated in both Europe the Pacific Ocean, they

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Rebuilt Loran side view

required an alternative product that worked at many times the range of GEE. The final system was called Loran and was also based on using hyperbolic principles, though the early models had at a lower degree of accuracy when compared side by side with GEE. Unlike Gee, the new Loran sets had a usable range of  up to 1,500 miles and occasionally more. To give assistance with the Loran development, Robert Dippy, (the chief designer of GEE), went over to the ‘States to give valuable assistance. Hence some almost identical features are found with both systems. Noting also that the size and connection of the two different ‘boxes’ was made similar enough to allow either one of the two systems to be easily exchanged for the other.

Loran APN-4

As the two systems worked well within their own maximum range limitations, there were occasions when it was possible to see both Loran and GEE systems installed alongside each other in a single aircraft. The GEE system being primarily used up to the limit of its range, at which point the operator would move over to using the longer range available from the Loran system.

One of the most significant differences between Loran and GEE was that GEE only used ground waves and a VHF (very high frequency) signal, meaning that GEE was only good for line of sight distances. Loran on the other hand, used a very LOW frequency signal and therefore could be used with either ground waves or skywaves, or a combination of both. However, the calculation of the location using a Loran set was much more complex as correction charts had to be used, with a final figure being hand-calculated on a piece of paper from a number of readings.

The diagram to the right shows the different waves that could be received by the Loran set from the same transmitter source. By  looking at this, one can easily see why it was necessary for the operator to add correction figures in order to account for the longer time it took for the skywaves ro reach the receiver. These correction values were marked down onto special Loran charts.

Loran chart example
Ground and sky waves

To the left we see a small segment of a Loran North Atlantic chart. Note the easily recognisable hyperbolic curves around the Hebrides in NW Scotland. Below, you can see one of the charts detail boxes at the bottom of the sheet, explaining the corrections that will need to be made for sky waves.

Loran chart sky-wave corrections

To most visitors to this site, the above will give sufficient information to outline just how an early Loran position fix was achieved. However, there may be a few who are curious to know a little more about the actual calculations carried out from the waveforms seen on the screen. The following images were therefore taken from our rebuilt Loran indicator using simulated transmitted markers to show an example of how to calculate a fix from a single transmitted signal. Note that we’ll not be correcting the end result with the chart tables. Also note that at least one more fix will need to be made from another transmitter to give another coordinate unlike with GEE, where the two coordinates appear on the screen simultaneously.

FUNCTION 1       Locate the signal

Fuction 1 locate the signal

No signal see on this receiver frequency

Fuction 1 locate the signal

No signal see on this one either

FUNCTION 2       Expand and line up the signals

FUNCTION 2  Expand and line up the signals FUNCTION 2 Choose the signal you wish to use
Fuction 1 signal located

The lower line shows a set of three pulses we can use

FUNCTION 2  Match the pulse position and width

In the first screen on the left, we see the three pulses, although there is actually a fourth pulse visible now. From left to right, the pulse train is therefore: Ground wave, 1 hop E, 2 hops E and the small one is almost certainly a 1 hop from the ionosphere F layer. In the second screen we move the marker on the lower trace using the large ‘coarse’ control to pick the best pulse for us to use. In this case it is the reflected skywave, 1 hop E . (The second pulse in.) The third screenshot shows us lining the lower pulse up and expanding it, using the leading edge of the upper pulse as the marker

FUNCTION 3       Superimpose and align signals

FUNCTION 3       Superimpose and align signals FUNCTION 3       Superimpose and align signals

In the left hand screen we can see the top and bottom traces superimposed on each other. Again, by using the coarse and fine controls on the front panel, the operator will need to match the size and position of the pulses exactly. The right hand shot shows this now done.

FUNCTION 4       Take approximate reading

Function 4  Take rough reading

Using the 1,000 micro second markers, we count from right to left. The small markers at A each identifying 5,000 uS. The downward pointing pulse shows where our rough reading should be taken. From right to left, you can see that this is somewhere between 7,000 and 8,000 uS. The navigator would write this figure down.

FUNCTION 4       Take accurate reading

Function 5   Take accurate reading

In this final screen image, we calculate the accurate reading to be plotted on the chart. Using the 7,000 uS obtained from the rough reading, we automatically add another 100 uS delay which is due to the circuitry in the Loran unit. The operator will then add the 762 uS you can see obtained from the screen to the left. By adjusting the indicator controls it is possible to increase the size and sharpness of the tiny pulses for measuring purposes, but this would have made it difficult to see the overall picture viewed here. On paper, the navigator would therefore add 100 uS delay to the 7,000 uS, plus the 762 uS from the screen. The total is therefore =  7862 uS        

This will be the first of the two (or more) coordinates to be plotted on the chart.

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