Loran


	LORAN APN/4 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


		BACK to Home page

	* Check out our YouTube page for a short video showing how to take a simple Loran fix.


	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.

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.

In the image to the right we can see our Loran APN-4 indicator paired to a very nice exampleof its matching radio receiver unit. This item was recently donated to the Radar Room by the Defence Electronics History Society. Many thanks!

Note that with both this variant of Loran and the GEE set in our display, a separate matching receiver box is required. The later APN / 9 Loran receiver combined both of the units seen here into one cabinet.



		Loran AN-APN / 4 indicator seen alongside a matching R-9B APN / 4 receiver


	LORAN OPERATION


	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.


	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.


	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


	No signal see on this receiver frequency

No signal see on this one either


	FUNCTION 2 Expand and line up the signals



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


	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


	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


	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


	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.


	BACK to Home page