Editing Direction finding (section)

===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.