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A Non-directional Beacon, or NDB, is a radio broadcast station in a known location, used as a navigational aid by aircraft pilots. NDB usage is standardized by the ICAO. NDBs are assigned ICAO-standard three-letter identifications, which are broadcast in Morse code to allow the pilot to identify the station. Most NDBs also transmit voice identification. With the advent of VOR systems and GPS navigation, NDBs are decreasing in use; however, they are still the most widely-used navigational aid in use today.
The NDBs have one major advantage over the more-sophisticated VOR. The signals follow the curvature of the earth so NDB signals can be received at much greater distances at lower altitudes. However, the NDB signal is affected more by atmospheric conditions, mountainous terrain and electrical storms.
Automatic Direction Finders
NDB navigation actually consists of two parts – the Automatic Direction Finder (or ADF), equipment on the aircraft that detects an NDB's signal, and the NDB's transmitting unit itself. The ADF can also locate transmitters in the standard AM broadcast band (535kHz to 1615kHz).
ADF equipment as implemented today uses a rotating solenoid to determine the direction to a broadcast signal. Equipment then plots the direction to the station on a compass found on the instrument panel of the aircraft. The pilot follows the needle to fly toward the station. In more complex aircraft, such as modern jetliners, ADF equipment may plot the bearing to a station on a so-called horizontal situation indicator.
When flying in crosswinds and navigating by ADF the pilot has to compensate for crosswinds. For example, with the VOR, if the pilot keeps the needle centered he will follow a straight line to the VOR transmitter. With the ADF, if the pilot keeps the nose of his aircraft pointed at the radio transmitter, the aircraft will drift left or right in any crosswind. As the pilot compensates by repointing the nose of the aircraft at the transmitter he will follow a curving path, first drifting to one side of the NDB then making an increasingly tight turn before overflying the it. Therefore, the pilot must compensate for crosswinds and point his aircraft to the left or right of the NDB to follow a straight track to it.
The principles of ADFs are not strictly limited to NDB usage; such systems are also used to detect the location of a broadcast signal for many other purposes, such as the location of emergency beacons.
Usage of NDBs NDBs provide rudimentary navigation – essentially, the ability to fly a line through the sky. However, with the advent of VOR navigation, NDBs have found their niche in several applications.
Radials and Airways First, using the compass equipment on his aircraft, a pilot can track a specific radial over the station. A radial is a line passing through the station that points in a specific direction, such as 270 degrees (due West). NDB radials provide a charted, consistent method for defining paths aircraft can fly.
In this fashion, NDBs (and VORs as well) create 'airways' in the sky. Aircraft, jets in particular, follow these pre-programmed routes to complete a flight plan. Airways, or vectors, are numbered and standardized on charts; for example, J24 (jet) is a high-altitude airway, and V119 (victor) is a low-altitude airway. Pilots follow these routes by tracking radials across various navigation stations, and turning at some. While most airways in the United States are based on VORs, NDB airways are common elsewhere, especially in the developing world and in lightly-populated areas of developed countries, like the Canadian Arctic, since they can have a long range and are much less expensive to operate than VORs.
All standard airways are plotted on aeronautical charts, such as U.S. sectional charts.
Fixes The ability to intercept fixes is a long-used application of NDBs. A fix is, literally, a point in the sky. These fixes are computed by drawing lines through navigation stations until they intercept, creating a triangle with the fix as one vertex: Plotting fixes in this manner allows a pilot to determine his rough horizontal location. This usage is important in situations where other navigational equipment, such as VORs with distance measuring equipment (DME), have failed.
Instrument Landing Systems NDBs are most commonly used as markers for an instrument landing system approach and standard approaches. NDBs may designate the starting area for an ILS approach or a path to follow for a standard terminal arrival procedure, or STAR. In the United States, an NDB is often combined with the outer marker beacon in the ILS approach (called a Locator Outer Marker, or LOM); in Canada, low-powered NDBs have replaced marker beacons entirely.
Technical NDBs typically operate in the frequency range from 190 kHz (kHz) to 535 kHz (although they are allocated frequencies from 190 to 1750 kHz) and transmit a constant carrier at modulations of either 400 or 1020 Hz. NDBs have a variety of owners, mostly governmental agencies and airport authorities.
Further Reading - International Civil Aviation Organization (2000). Annex 10 — Aeronautical Telecommunications, Vol. I (Radio Navigation Aids) (5th ed.).
- U.S. Federal Aviation Administration (2004). Aeronautical Information Manual, § 1-1-2. Available online at http://www.faa.gov/ATpubs/AIM/
- Southern Avionics Company, Non-Directional Radiobeacons (NDB) and their Place in a Worldwide Navaid System. Available online at http://www.southernavionics.com/sac1g.htm
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