Huff-Duff
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High Frequency Direction Finder is usually known by its acronym HF/DF, pronounced Huff-Duff. This has become the common name for this type of radio direction finder, and was coined during World War II.
Finding the location of radio and radar transmitters is one of the fundamental disciplines of Signal Intelligence SIGINT. In the WWII context, Huff Duff 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 WWII 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.
[edit] 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 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 for years. The Royal Navy was the first to design an apparatus that could take bearings on the high frequency radio transmitters employed by the German Kriegsmarine in World War II.
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.
The key feature of "Huff-Duff" was the use of an oscilloscope 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 that this would avoid detection of 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 ASDIC if it had dived.
[edit] Battle of Britain
In the Battle of Britain, "Huff-Duff" was the mechanism used by the RAF to track the location of their fighter squadrons in the air. RAF fighters had a HF channel of their radios set to broadcast a signal for fourteen seconds of every minute. A clockwork mechanism regulated this broadcast, which was codenamed "Pipsqueak".
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.
[edit] References
- "Secret Weapon: U.S. High-Frequency Direction Finding in the Battle of the Atlantic" by Kathleen Broome Williams, Naval Inst Pr (October 1, 1996), ISBN 1-55750-935-2
