Direction finding

Direction finding (DF), or radio direction finding (RDF), is the use of <a href="/facts/Radio_wave/XmB6WPpF">radio waves</a> to determine the direction to a radio source. The source may be a cooperating <a href="/facts/Radio_transmitter/nFYghtKv">radio transmitter</a> or may be an inadvertant source, a naturally-occurring radio source, or an illicit or enemy system. Radio direction finding differs from <a href="/facts/Radar/jk9BTbFA">radar</a> in that only the direction is determined by any one receiver; a radar system usually also gives a distance to the object of interest, as well as direction. By <a href="/facts/Triangulation/lpmlmhGd">triangulation</a>, the location of a radio source can be determined by measuring its direction from two or more locations. Radio direction finding is used in <a href="/facts/Radio_navigation/sRkrEaG9">radio navigation</a> for ships and aircraft, to locate emergency transmitters for <a href="/facts/Search_and_rescue/pdLTUPgl">search and rescue</a>, for tracking wildlife, and to locate illegal or interfering transmitters. During the Second World War, radio direction finding was used by both sides to locate and direct aircraft, surface ships, and submarines.
RDF systems can be used with any radio source, although very long <a href="/facts/Wavelength/kcJU0ymz">wavelengths</a> (low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless used for marine <a href="/facts/Radio_navigation/sRkrEaG9">radio navigation</a> as they can travel very long distances "over the horizon", which is valuable for ships when the <a href="/facts/Line-of-sight_propagation/w4xfQd2o">line-of-sight</a> may be only a few tens of kilometres. For aerial use, where the horizon may extend to hundreds of kilometres, higher frequencies can be used, allowing the use of much smaller antennas. An <a href="/facts/Automatic_direction_finder/nNbfWLDW">automatic direction finder</a>, which could be tuned to radio beacons called <a href="/facts/Non-directional_beacon/Jnx4rAKX">non-directional beacons</a> or commercial <a href="/facts/AM_radio/oS0rn1lg">AM radio</a> broadcasters, was in the 20th century a feature of most aircraft, but is being phased out.
For the military, RDF is a key tool of <a href="/facts/Signals_intelligence/Iwzvw6AW">signals intelligence</a>. The ability to locate the position of an enemy transmitter has been invaluable since World War I, and played a key role in World War II's <a href="/facts/Battle_of_the_Atlantic/PV9uF2Jw">Battle of the Atlantic</a>. It is estimated that the UK's advanced "<a href="/facts/High-frequency_direction_finding/dlrTsofM">huff-duff</a>" systems were directly or indirectly responsible for 24% of all <a href="/facts/U-boat/GgjbMzMT">U-boats</a> sunk during the war. Modern systems often used <a href="/facts/Phased_array/5wvA9WLV">phased array</a> antennas to allow rapid <a href="/facts/Beamforming/4VpeHoGc">beamforming</a> for highly accurate results, and are part of a larger <a href="/facts/Electronic_warfare/c6XGz9y0">electronic warfare</a> suite.
Early radio direction finders used mechanically rotated antennas that compared signal strengths, and several electronic versions of the same concept followed. Modern systems use the comparison of <a href="/facts/Phase_(waves)/cpGtuVll">phase</a> or <a href="/facts/Doppler_effect/bzrK1BKf">doppler techniques</a> which are generally simpler to automate. Early British <a href="/facts/Radar/jk9BTbFA">radar</a> sets were referred to as RDF, which is often stated was a deception. In fact, the <a href="/facts/Chain_Home/XPwgbaQr">Chain Home</a> systems used large RDF receivers to determine directions. Later radar systems generally used a single antenna for broadcast and reception, and determined direction from the direction the antenna was facing.

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Direction finding