Orfordness Beacon

The Orfordness Rotating Wireless Beacon, known simply as the Orfordness Beacon or sometimes the Black Beacon, was an early radio navigation system introduced by the United Kingdom in July 1929. It allowed the angle to the station to be measured from any aircraft or ship with a conventional radio receiver, and was accurate to about a degree. A second station operating on the same principle was set up to provide wider area coverage and allow two-bearing fixes between Orford Ness and Farnborough Airport. The system was similar to the earlier German Telefunken Kompass Sender and the later Sonne system.

In navigation, the determination of a "fix" requires two measurements to be taken. Using classical triangulation techniques, this was normally the measurement of two angles, or bearings, along the line-of-sight to prominent landmarks, like a lighthouse. After taking the two measurements, lines of position are drawn radiating from the landmark along the reverse angle. They will cross at some point, and their intersection determines the location of the navigator.

The introduction of portable radio systems in the early 20th century gave rise to the possibility of using radio broadcasters (beacons) as a landmark that would be visible to a radio receiver at very great ranges, hundreds of miles or more. The angle between the navigator and the beacon can be taken by using a simple mechanism known as a loop antenna which can be rotated around the vertical axis. As the antenna is rotated, the strength of the received signal varies, and drops to zero (the null) when the loop is perpendicular to the line to the beacon.

This technique, known as radio direction finding (RDF), is useful but only moderately accurate. Measurements better than a few degrees are difficult with a small antenna, and because of the electrical characteristics, it is not always easy to make a larger version that might provide more accuracy. Moreover, the addition of a loop antenna may be difficult on smaller vehicles, or difficult to operate for those without a dedicated navigator.

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