High - frequency direction finding High - frequency direction finding , usually known by its abbreviation HF / DF ( nicknamed huff - duff ) is the common name for a type of radio direction finding employed especially during the two World War s . 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 - boat s 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 oscilloscope s , as well as a new aerial design called the Adcock aerial consisting of four equidistant vertical elements that 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 . Another source was the work of the French engineers Maurice Deloraine and Henri Busignies , who 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 , 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 Jamaica . 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 . RAF fighters had a clockwork mechanism that regulated the broadcast of a signal over an HF channel for fifteen seconds of every minute . Each Fighter Command sector had HF / DF receiving stations that would monitor Pipsqueak broadcasts and telephone the bearings back to the sector control rooms where they could be triangulated and the squadron ' s location plotted . It had the drawback of tying up the aircraft ' s radio while broadcasting its DF signal .