The Radar Room

THE LORAN NAVIGATION SYSTEM

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 operating in both Europe the Pacific Ocean, they 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 sometimes 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 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.

Restored Loran APN-4 indicator

Loran stands for LOng RAnge Navigation.

As the two separate systems worked well within their own maximum range limitations, there were occasions where 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 available range of the Loran system.

The biggest difference between Loran and GEE was that GEE only used ground waves and a VHF (very high frequency) signal. This essentially meant 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 complicated 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.

Ground and skywaves
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Loran ‘A’ APN-4  indicator alongside matching Loran signal receiver unit. (right)

 This is a small segment of Loran North Atlantic chart with sky waves correction table underneath. Note the hyperbolic curves around the Hebrides in NW Scotland (below)

APN-4 Loran receiver and indicator
Loran chart extract example
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