A blind approach beacon system or beam approach beacon system (BABS) is an automatic

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

landing system developed in the early 1940s. It is a responder (or

transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...

) mounted in a

Hillman Hillman was a British automobile marque created by the Hillman-Coatalen Company, founded in 1907, renamed the Hillman Motor Car Company in 1910. The company was based in Ryton-on-Dunsmore, near Coventry, England. Before 1907 the company had ...

van A van is a type of road vehicle used for transporting goods or people. There is some variation in the scope of the word across the different English-speaking countries. The smallest vans, microvans, are used for transporting either goods or ...

and placed at the end of the

runway In aviation, a runway is an elongated, rectangular surface designed for the landing and takeoff of an aircraft. Runways may be a human-made surface (often asphalt concrete, asphalt, concrete, or a mixture of both) or a natural surface (sod, ...

. In some cases fixed sites were installed and by the mid-1950s Standard 9 vans were in use. It was interrogated by a ''

Rebecca Rebecca () appears in the Hebrew Bible as the wife of Isaac and the mother of Jacob and Esau. According to biblical tradition, Rebecca's father was Bethuel the Aramean from Paddan Aram, also called Aram-Naharaim. Rebecca's brother was Laban (Bi ...

'' unit mounted in an

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

. It responded by transmitting

dot A dot is usually a small, round spot. Dot, DoT or DOT may also refer to: Orthography * Full stop or "period", a sentence terminator * Dot (diacritic), a mark above or below a character (e.g. ȧ, ạ, İ, Ċ, ċ, etc.), usually to indicate sou ...

s on one side of the runway and

dash The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash , generally longer than the hyphen ...

es on the other. The strength of the reception in the aircraft depended on the aircraft's position relative to the centre line of the runway. The aircraft's equipment could then determine its position relative to these dots and dashes. This is an early version of the localizer from the more modern instrument landing system (ILS). The localizer uses two tones (90 and 150 Hz) modulated on a carrier (108.1-111.95 MHz) and transmitted on a wide aperture antenna array, this can be seen at almost all airports at the end of the runway. The strengths of which (or Difference in Depth of Modulation) are detected by electronics in the aircraft and present the pilot with an indication to fly left or right and indeed up or down with a co-located glidepath (328.6 to 335.4 MHz) equipment. See instrument landing system for further details.

References

See also