Black Beacon - Wireless Navigation

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​Background

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The 1920’s witnessed a huge growth in experimentation and development of radio technology. There was a great interest in broadcast radio, where a large audience could be reached by transmitting signals on the long, medium and short wave frequency bands, with the BBC being created in 1922. Maritime radio systems slowly developed supporting telegraph and then voice services. By the late 1920’s, valve technology had improved to a point where reasonably high power transmitters were economically possible. Receiver designs, however, were often quite basic due to power supply and affordability limitations. There was also an increasing need for better navigation systems for ships and aircraft. Government scientists recognised that radio systems had potential in this area and funded research in this area.

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​The Rotating Beacon concept

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In 1928, the Royal Aircraft Establishment (RAE) at Farnborough was working on a system that would use two rotating beacon transmitters at known locations. Ships and aircraft would establish their bearing to each station and then could plot their position by triangulation on a chart. The design objectives were that the system should have a useful range, be easy to use and not require specialist receiver equipment.

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Construction

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In early 1929, building work commenced on identical beacons at RAE Farnborough and Orfordness in order to provide navigation coverage of the English Channel area. The system used a sturdy 3m square wooden aerial frame structure, vertically mounted on an elevated motorised rotating platform. A copper conductor loop and associated large air spaced tuning capacitor were attached to the rotating frame and connected to the transmitter located below by means of electrical slip rings. The aerial rotated clockwise at a rate of 1 revolution per minute. Every second of rotation of the aerial corresponded to 6 degrees of bearing change. The transmitter was built on a brick ground floor base with a two-storey octagonal wooden windmill-like structure above to house the aerial and associated equipment. The system was initially supplied with DC power from the site central 30kW generator located near the MT workshop. Cables were laid from the generator house, over Stoney Ditch then over the shingle to the beacon, a distance of about 0.5 mile.
 

Design

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The system transmitted a 288 kHz (1080m) long wave continuous signal from the rotating square aerial. Aerials of this type transmit a maximum signal strength when a receiver is in line with the aerial frame or plane. It subsequently transmits a near zero ‘null’ signal when a receiver is positioned at 90 deg to the aerial plane. When the aerial plane is facing north, an amplitude modulated morse signal V is sent (…_). Then it rotates 90 deg to face east and a morse signal B is sent (_..) . Orfordness would transmit for 5 min, and then Farnborough would transmit for 5 min at the same frequency and so on. The system could be run 24/7 when sufficient funding was available.

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How the system worked

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A ship out at sea would tune their ordinary receiver to 288 kHz and their signal strength meter would rise and fall as the beacon rotated. A stopwatch is required. When the ‘V’ code is heard the stopwatch is started. The operator observes the signal strength meter and, when the signal falls to its lowest point, the watch is stopped and elapsed time in seconds is noted. The number of seconds is multiplied by 6 to calculate the bearing in degrees. A customised stopwatch with an integral compass rose was produced based on a ‘sports’ 30 secs per revolution product. As before, when hearing the ‘V’ code the user starts the watch running. When the ‘null’ is observed, the watch is stopped. The bearing can be read directly from the watch compass rose, thus avoiding the need to calculate the bearing.

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The operator keeps monitoring and recording with similar results. The operator will then notice a change in the null timing (the transmitters have changed over) both bearings are then plotted on a chart from both beacons. There will usually only be one intersection which is your position. The easterly ‘B’ signal is there to provide a second sanity check reference for ambiguous situations.

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How well did the system perform ?

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It was simple to use and did not require specialist receiver equipment or training, only a stopwatch. It could be accurate to 2 degrees with a practical range of up to 100 miles and under certain conditions much further. Performance at night was degraded over longer distances. It was however unsophisticated and cumbersome to use and the rapid pace of radio development internationally made the system look increasingly obsolete. In early 1933, a dedicated power house containing a diesel generator was built nearby, which was shared with the newly constructed Bomb Ballistics Building. 24/7 operation was costly and funding was shared between the Air Ministry and Trinity House. Use declined steadily and funding was finally withdrawn in October 1934, when the service closed.

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Historical reflection
 

The rotating beacon was in fact the ‘first of the beams’.  Orfordness was used for the initial development of Chain Home Radar and subsequently major over the horizon experimental radar installations during the Cold War period. The rotating beacon building and nearby powerhouse have been restored by the National Trust and are now used as an exhibition space and visual observatory. The building is Grade II listed by English Nature.

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