High-frequency Direction Finding (electronics) - Battle of The Atlantic

Battle of The Atlantic

Along with ASDIC (sonar), Ultra code breaking (COMINT) and radar, "Huff-Duff" was a valuable part of the Allies' armoury in detecting German U-boats and commerce raiders during the Battle of the Atlantic.

The origins of the technique lay in the work of the National Physical Laboratory, Slough, in the late 1920s. A team of scientists had been investigating the Ionosphere and part of the work involved tracking thunderstorms. Due to the very short duration of the lightning flashes the scientists found the normal loop aerial was not capable of being rotated quickly enough to obtain a bearing pinpointing the electromagnetic pulse of the lightning. The laboratory, however, contained one of the then-new oscilloscopes, and in conjunction with a new aerial design called the Adcock aerial, consisting of two large loop aerials at right-angles aligned north-south, and east-west, which didn't have to rotate, these two innovations allowed an instantaneous indication of the direction of any signal's source. This equipment formed the basis for "Huff-Duff"..

Another source was the work of the French engineers Maurice Deloraine and Henri Busignies which succeeded in leaving France just in time in 1940 to continue the development in the U.S.

The Royal Navy designed a particularly sophisticated apparatus that could take bearings on the high frequency radio transmitters employed by the German Kriegsmarine in World War II. There were severe technical problems of engineering effective high frequency direction finding systems for operation on ships, mainly due to the effects of the superstructure on the wavefront of arriving radio signals. However, these problems were overcome under the technical leadership of the Polish engineer Wacław Struszyński, working at the Admiralty Signal Establishment.

Many shore-based installations were constructed around the North Atlantic and whenever a U-boat transmitted a message, "Huff-Duff" could get bearings on the approximate position of the boat. Because it worked on the electronic emission and not the content of the message, it did not matter that the content was encrypted using an Enigma machine. This information was then transmitted to convoys at sea, and a complex chess game developed as Royal Navy controllers tried to manoeuvre wide convoys past strings of U-Boats set up by the Kriegsmarine controllers.

A key feature of the British "Huff-Duff" was the use of an oscilloscope display and fixed aerial which could instantaneously reveal the direction of the transmission, without the time taken in conventional direction finding to rotate the aerial; U-boat transmissions were deliberately kept short, and it was wrongly assumed by the U-boat captains that this would avoid detection of the sender's direction.

Another feature was the use of continuously motor-driven tuning, to scan the likely frequencies to pick up and sound an automatic alarm when any transmissions were detected.

In 1942 the allies began to install Huff-Duff on convoy escort ships, enabling them to get much more accurate triangulation fixes on U-boats transmitting from over the horizon, beyond the range of radar. This allowed hunter-killer ships and aircraft to be dispatched at high speed in the direction of the U-boat, which could be illuminated by radar if still on the surface and by ASDIC if it had dived. It was the operation of these technologies in combination which eventually turned the tide against the U-boats.

The distances involved in locating U-boats in the Atlantic were so great that d/f accuracy was relatively inefficient and in 1944 a new strategy was developed by Naval Intelligence where localised groups of 5 shore-based d/f stations were built so that the bearings from each of the 5 stations could be averaged to gain a more reliable bearing. Four such groups were set up in Britain at Ford End in Essex, Anstruther in Fife, Bower in the Scottish Highlands and Goonhavern in Cornwall. Traces of some of the last two groups can still be seen in Google satellite images. It was intended that other groups would be set up in Iceland, Nova Scotia and Jamaica but confirmation is needed. Simple averaging was found to be ineffective and later statistical methods were used. Operators were also asked to grade the reliability of their readings so that poor and variable ones were given less importance than those that appeared stable and well defined. Several of these df groups continued into the 1970s as part of Composite Signals.