High-frequency direction finding
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The idea of using two or more radio receivers to find the bearings of a radio transmitter and with the use of simple triangulation find the approximate position of the transmitter had been known and used since the invention of wireless communication. The general principle is to rotate a directional aerial and note where the signal is strongest. With simple aerial design the signal will be strongest when pointing directly towards and directly away from the source, so two bearings from different positions are usually taken, and the intersection plotted. More modern aerials employ uni-directional techniques.
HF/DF was used by early aviators to obtain bearings of radio transmitters at airfields by rotatable aerials above the cockpit, and during World War I shore installations of all protagonists endeavoured to obtain information about ship movements in this way. The requirement both to tune a radio and rotate an aerial manually made this a cumbersome and slow business, and one which could be evaded if the radio transmission were short enough. Films depicting World War II spies transmitting covertly will sometimes show detection vans attached to patrols performing this activity.
Finding the location of radio and radar transmitters is one of the fundamental disciplines of Signal Intelligence SIGINT. In the World War II context, HF/DF applied to direction-finding of radio communications transmitters, typically operating at high frequency (HF). Modern direction finding of both communications and noncommunications signals covers a much wider range of frequencies.
Within communications intelligence (COMINT), direction finding is part of the armoury of the intelligence analyst. Sister disciplines within COMINT include cryptanalysis, the analysis of the content of encrypted messages, and traffic analysis, the analysis of the patterns of senders and addressees. While it was not a significant World War II tool, there are a variety of Measurement and Signal Intelligence (MASINT) techniques that extract information from unintentional signals from transmitters, such as the oscillator frequency of a superheterodyne radio receiver.
Battle of the Atlantic
Along with Sonar ("ASDIC"), intelligence from breaking German codes 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 HF/DF..
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, HF/DF 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 HF/DF 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 HF/DF 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 from shore-based d/f stations 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. It was intended that other groups would be set up in Iceland, Nova Scotia and JamaicaConfirmation 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 the Composite Signals Organisation.
Battle of Britain 
During the Battle of Britain the Royal Air Force (RAF) deployed an Identification Friend or Foe (IFF) system codenamed "pipsqueak", a term also used for a small calibre German shell during World War I. RAF fighters had a clockwork mechanism that regulated the broadcast of a signal over an HF channel for fourteen seconds of every minute. Each Fighter Command sector had HF/DF receiving stations that would monitor the "pipsqueak" broadcasts and telephone the bearings back to the sector control rooms where they could be triangulated and the squadron's location plotted.
- Elephant Cage
- Operation RAFTER – remotely confirming that a superhet radio receiver is listening to a certain frequency
- Kathleen Broome Williams (1996-10-01). Secret Weapon: U.S. High-Frequency Direction Finding in the Battle of the Atlantic. Naval Inst Pr. ISBN 1-55750-935-2.
- Arthur O. Bauer. "HF/DF An Allied Weapon against German U-Boats 1939–1945" (PDF). Retrieved 2008-01-26.: A paper on the technology and practice of the HF/DF systems used by the Royal Navy against U-Boats in World War II
- Patrick Beesly (1978). Very Special Intelligence: The story of the Admiralty's Operational Intelligence Center in World War II. Spere. ISBN 0-7221-1539-3.
- deRosa, L.A. "Direction Finding". In J.A. Biyd, D.B. Harris, D.D. King & H.W. Welch, Jr. Electronic Countermeasures. Los Altos, CA: Peninsula Publishing. ISBN 0-932146-00-7.