page_title: Radio navigation
section_title: Reverse RDF
hierarchical_section_title: Radio navigation / Bearing-measurement systems / Reverse RDF
caption_attribution_description: The 'Black Beacon', Orford Ness
context_page_description: Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth. Like radiolocation, it is a type of radiodetermination.
  The basic principles are measurements from/to electric beacons, especially
  Angular directions, e.g. by bearing, radio phases or interferometry,
  Distances, e.g. ranging by measurement of time of flight between one transmitter and multiple receivers or vice versa,
  Distance differences by measurement of times of arrival of signals from one transmitter to multiple receivers or vice versa
  Partly also velocity, e.g. by means of radio Doppler shift.
  Combinations of these measurement principles also are important—e.g., many radars measure range and azimuth of a target.
context_section_description: The main problem with RDF is that it required a special antenna on the vehicle, which may not be easy to mount on smaller vehicles or single-crew aircraft. A smaller problem is that the accuracy of the system is based to a degree on the size of the antenna, but larger antennas would likewise make the installation more difficult.
  During the era between World War I and World War II, a number of systems were introduced that placed the rotating antenna on the ground. As the antenna rotated through a fixed position, typically due north, the antenna was keyed with the morse code signal of the station's identification letters so the receiver could ensure they were listening to the right station. Then they waited for the signal to either peak or disappear as the antenna briefly pointed in their direction. By timing the delay between the morse signal and the peak/null, then dividing by the known rotational rate of the station, the bearing of the station could be calculated.
  The first such system was the German Telefunken Kompass Sender, which began operations in 1907 and was used operationally by the Zeppelin fleet until 1918.  An improved version was introduced by the UK as the Orfordness Beacon in 1929 and used until the mid-1930s. A number of improved versions followed, replacing the mechanical motion of the antennas with phasing techniques that produced the same output pattern with no moving parts. One of the longest lasting examples was Sonne, which went into operation just before World War II and was used operationally under the name Consol until 1991. The modern VOR system is based on the same principles (see below).
caption: The Orfordness Beacon as it appears today.
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