Editing Direction finding (section)

==Applications==
===Radio navigation===
[[File:Gemtronics GT302 DF.JPG|thumb|A portable, battery operated GT-302 Accumatic automatic direction finder for marine use]]
''Radio direction finding'', ''[[radio direction finder]]'', or ''RDF'', was once the primary aviation navigational aid. (''Range and Direction Finding'' was the abbreviation used to describe the predecessor to [[radar]].<ref name="battleofbritain1940.net"/>) [[Beacons]] were used to mark "airways" intersections and to define departure and approach procedures. Since the signal transmitted contains no information about bearing or distance, these beacons are referred to as ''[[non-directional beacon]]s'', or ''NDB'' in the [[aviation]] world. Starting in the 1950s, these beacons were generally replaced by the [[VHF omnidirectional range|VOR]] system, in which the bearing to the navigational aid is measured from the signal itself; therefore no specialized antenna with moving parts is required. Due to relatively low purchase, maintenance and calibration cost, NDBs are still used to mark locations of smaller aerodromes and important helicopter landing sites. {{further | Non-directional beacon}}

Similar beacons located in coastal areas are also used for maritime radio navigation, as almost every ship was equipped with a direction finder (Appleyard 1988). Very few maritime radio navigation beacons remain active today (2008) as ships have abandoned navigation via RDF in favor of GPS navigation.

In the United Kingdom a radio direction finding service is available on 121.5&nbsp;MHz and 243.0&nbsp;MHz to aircraft pilots who are in distress or are experiencing difficulties. The service is based on a number of radio DF units located at civil and military airports and certain HM Coastguard stations.<ref>{{cite book | author=Smith, D.J.| title=Air Band Radio Handbook |year=2005 | publisher=Sutton Publishing | pages=104–105 | isbn=0-7509-3783-1| edition=8th }}</ref> These stations can obtain a "fix" of the aircraft and transmit it by radio to the pilot.
[[File:Whiteman localizer.jpg|thumb|right|[[Instrument landing system localizer|ILS Localizer]]]]

====Maritime and aircraft navigation====
[[File:Kolster radio compass.jpg|thumb|upright|Historic advertisement for Kolster radio compass]]
{{multiple image
|image1 = Automatic Radio Direction Finder R-5 ARN7 radio control box.jpg
|image2 = Automatic Radio Direction Finder R-5 ARN7 indicator.jpg
|image3 = Automatic Radio Direction Finder R-5 ARN7 radio compass unit.jpg
|footer = R-5/ARN7 radio compass components, with the radio control box (left), indicator (center), and radio compass unit (right)
|total_width = 880
}}
Radio transmitters for air and sea navigation are known as ''beacons'' and are the radio equivalent to a [[lighthouse]]. The transmitter sends a [[Morse Code]] transmission on a [[Long wave]] (150 – 400&nbsp;kHz) or [[Medium Wave|Medium wave]] (520 – 1720&nbsp;kHz) frequency incorporating the station's identifier that is used to confirm the station and its operational status.  Since these radio signals are broadcast in all directions (omnidirectional) during the day, the signal itself does not include direction information, and these beacons are therefore referred to as [[non-directional beacon]]s, or '''NDBs'''.

As the commercial medium wave broadcast band lies within the frequency capability of most RDF units, these stations and their transmitters can also be used for navigational fixes.  While these commercial radio stations can be useful due to their high power and location near major cities, there may be several miles between the location of the station and its transmitter, which can reduce the accuracy of the 'fix' when approaching the broadcast city.  A second factor is that some AM radio stations are omnidirectional during the day, and switch to a reduced power, directional signal at night.

RDF was once the primary form of aircraft and marine navigation. Strings of beacons formed "airways" from airport to airport, while marine NDBs and commercial AM broadcast stations provided navigational assistance to small watercraft approaching a landfall.  In the United States, commercial AM radio stations were required to broadcast their station identifier once per hour for use by pilots and mariners as an aid to navigation. In the 1950s, aviation NDBs were augmented by the [[VHF omnidirectional range|VOR]] system, in which the direction to the beacon can be extracted from the signal itself, hence the distinction with non-directional beacons.  Use of marine NDBs was largely supplanted in North America by the development of [[LORAN]] in the 1970s.

Today many NDBs have been decommissioned in favor of faster and far more accurate [[GPS]] navigational systems. However the low cost of ADF and RDF systems, and the continued existence of AM broadcast stations (as well as navigational beacons in countries outside North America) has allowed these devices to continue to function, primarily for use in small boats, as an adjunct or backup to GPS.

===Location of illegal, secret or hostile transmitters – SIGINT===
{{See also|High frequency direction finding|SIGINT}}
[[Image:British Post Office interference finding truck 1927.jpg|thumb|British Post Office RDF lorry from 1927 for finding unlicensed [[amateur radio]] transmitters.  It was also used to find [[regenerative receiver]]s  which radiated interfering signals due to feedback, a big problem at the time.]]

In World War II considerable effort was expended on identifying secret transmitters in the United Kingdom (UK) by direction finding. The work was undertaken by the [[Radio Security Service]] (RSS also MI8). Initially three U Adcock HF DF stations were set up in 1939 by the General Post Office. With the declaration of war, [[MI5]] and [[MI8|RSS]] developed this into a larger network. One of the problems with providing coverage of an area the size of the UK was installing sufficient DF stations to cover the entire area to receive [[skywave]] signals reflected back from the ionised layers in the upper atmosphere. Even with the expanded network, some areas were not adequately covered and for this reason up to 1700 voluntary interceptors (radio amateurs) were recruited to detect illicit transmissions by [[ground wave]]. In addition to the fixed stations, RSS ran a fleet of mobile DF vehicles around the UK. If a transmitter was identified by the fixed DF stations or voluntary interceptors, the mobile units were sent to the area to home in on the source. The mobile units were HF Adcock systems.

By 1941 only a couple of illicit transmitters had been identified in the UK; these were German agents that had been "turned" and were transmitting under MI5 control. Many illicit transmissions had been logged emanating from German agents in occupied and neutral countries in Europe. The traffic became a valuable source of intelligence, so the control of RSS was subsequently passed to MI6 who were responsible for secret intelligence originating from outside the UK. The direction finding and interception operation increased in volume and importance until 1945.

The HF Adcock stations consisted of four 10{{nbsp}}m vertical antennas surrounding a small wooden operators hut containing a receiver and a radio-[[goniometer]] which was adjusted to obtain the bearing. MF stations were also used which used four guyed 30{{nbsp}}m lattice tower antennas. In 1941, RSS began experimenting with spaced loop direction finders, developed by the Marconi company and the UK [[United Kingdom National Physical Laboratory|National Physical Laboratories]]. These consisted of two parallel loops 1 to 2{{nbsp}}m square on the ends of a rotatable 3 to 8{{nbsp}}m beam. The angle of the beam was combined with results from a radiogoniometer to provide a bearing. The bearing obtained was considerably sharper than that obtained with the U Adcock system, but there were ambiguities which prevented the installation of 7 proposed S.L DF systems. The operator of an SL system was in a metal underground tank below the antennas. Seven underground tanks were installed, but only two SL systems were installed at Wymondham, Norfolk and Weaverthorp in Yorkshire. Problems were encountered resulting in the remaining five underground tanks being fitted with Adcock systems. The rotating SL antenna was turned by hand which meant successive measurements were a lot slower than turning the dial of a goniometer.

Another experimental spaced loop station was built near Aberdeen in 1942 for the Air Ministry with a semi-underground concrete bunker. This, too, was abandoned because of operating difficulties. By 1944, a mobile version of the spaced loop had been developed and was used by RSS in France following the D-Day invasion of Normandy.

The US military used a shore based version of the spaced loop DF in World War II called "DAB". The loops were placed at the ends of a beam, all of which was located inside a wooden hut with the electronics in a large cabinet with [[cathode-ray-tube]] display at the centre of the beam and everything being supported on a central axis. The beam was rotated manually by the operator.

The [[Royal Navy]] introduced a variation on the shore based HF DF stations in 1944 to track U-boats in the North Atlantic. They built groups of  five DF stations, so that bearings from individual stations in the group could be combined and a mean taken. Four such groups were built in Britain at [[Ford End]], Essex, Goonhavern, Cornwall, Anstruther and Bowermadden in the Scottish Highlands. Groups were also built in Iceland, Nova Scotia and Jamaica. The anticipated improvements were not realised but later statistical work improved the system and the Goonhavern and Ford End groups continued to be used during the Cold War.  The Royal Navy also deployed direction finding equipment on ships tasked to [[anti-submarine warfare]] in order to try to locate German submarines, e.g. [[Captain class frigate]]s were fitted with a  medium frequency direction finding antenna (MF/DF) (the antenna was fitted in front of the bridge) and high frequency direction finding (HF/DF, "Huffduff") Type FH 4 antenna (the antenna was fitted on top of the mainmast).<ref name="WSInt III">Elliott (1972), p. 264</ref>

A comprehensive reference on World War II wireless direction finding was written by Roland Keen, who was head of the engineering department of RSS at Hanslope Park. The DF systems mentioned here are described in detail in his 1947 book ''Wireless Direction Finding''.<ref>
{{cite book
 | last=Keen | first=R 
 | title=Wireless Direction Finding 
 | edition=4th
 | year=1947
 | publisher=Iliffe
 | location=London, UK
}}</ref>

At the end of World War II a number of RSS DF stations continued to operate into the Cold War under the control of GCHQ the British SIGINT organisation.

Most direction finding effort within the UK now (2009) is directed towards locating unauthorised "[[pirate radio|pirate]]" FM broadcast radio transmissions. A network of remotely operated VHF direction finders are used mainly located around the major cities. The transmissions from mobile telephone handsets are also located by a form of direction finding using the comparative signal strength at the surrounding local "cell" receivers. This technique is often offered as evidence in UK criminal prosecutions and, almost certainly, for SIGINT purposes.<ref>
{{cite book
 | title=Electronic Countermeasures
 | editor1=J.A. Biyd |editor2=D.B. Harris |editor3=D.D. King |editor4=H.W. Welch Jr.
 | author=deRosa, L.A.
 | publisher=Peninsula Publishing
 | isbn=0-932146-00-7
 | chapter=Direction Finding
 | location=Los Altos, CA
 | year=1979
}}</ref>

===Emergency aid===

[[Emergency position-indicating rescue beacon]]s are widely deployed on civil [[aircraft]] and ships. Historically emergency location transmitters only sent a tone signal and relied on direction finding by search aircraft to locate the beacon. Modern emergency beacons transmit a unique identification signal that can include [[GPS]] location data that can aid in finding the exact location of the transmitter.

[[Avalanche transceiver]]s operate on a standard 457&nbsp;kHz, and are designed to help locate people and equipment buried by avalanches. Since the power of the beacon is so low the directionality of the radio signal is dominated by small scale field effects<ref>*{{cite journal
 |author1       = J. Hereford
 |author2       = B. Edgerly
 |name-list-style = amp
 |year          = 2000
 |title         = 457 kHz Electromagnetism and the Future of Avalanche Transceivers
 |journal       = International Snow Science Workshop (ISSW 2000)
 |url           = http://www.backcountryaccess.com/images/documents/457andFuture_000.pdf
 
 |url-status       = dead
 |archive-url    = https://web.archive.org/web/20110722082049/http://www.backcountryaccess.com/images/documents/457andFuture_000.pdf
 |archive-date   = July 22, 2011
}}</ref> and can be quite complicated to locate.

===Wildlife tracking===
Location of radio-tagged animals by [[triangulation]] is a widely applied research technique for studying the movement of animals. The technique was first used in the early 1960s, when [[radio transmitter]]s and [[battery (electricity)|batteries]] became small enough to attach to wildlife, and is now widely deployed for a variety of wildlife studies. Most tracking of wild animals that have been affixed with radio transmitter equipment is done by a field researcher using a handheld radio direction finding device. When the researcher wants to locate a particular animal, the location of the animal can be triangulated by determining the direction to the transmitter from several locations.

===Reconnaissance===
[[Phased array]]s and other advanced [[Antenna (radio)|antenna]] techniques are utilized to track launches of [[rocket]] systems and their resulting trajectories. These systems can be used for defensive purposes and also to gain intelligence on operation of [[missile]]s belonging to other nations. These same techniques are used for detection and tracking of conventional [[aircraft]].