Non-directional beacon
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A non-directional beacon (NDB) is a radio broadcast station in a known location, used as an aviation or marine navigational aid. NDB usage for aviation is standardized by ICAO Annex 10 which specifies that NDBs be operated on a frequency between 190 kHz and 1750 kHz. Each NDB is to be identified by a two or three-letter Morse code group. 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.
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, coastal refraction and electrical storms, particularly at long range.
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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 (535 kHz to 1615 kHz).
ADF equipment determines the direction to the NDB station relative to the airframe of the aircraft. This may be displayed on a relative bearing indicator, RBI. This display looks like a compass card with a needle superimposed, except that the card is fixed with the 0 degree position corresponding to the centreline of the aircraft. In order to track toward an NDB with no wind the aircraft is flown so that the needle points to the 0 position, the aircraft will then fly directly to the NDB. Similarly, the aircraft will track directly away from the NDB if the needle is maintained on the 180 degree mark. With a crosswind, the needle must be maintained to the left or right of the 0 or 180 position by an amount corresponding to the drift due to the crosswind.
When tracking to or from an NDB it is also usual that the aircraft track on a specific bearing. To do this it is necessary to correlate the RBI reading with the compass heading. Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 plus or minus drift as required. An NDB may also be used to locate a position along the aircraft track. When the needle reaches a RBI reading corresponding to the required bearing then the aircraft is at the position. However, using a separate RBI and compass this requires considerable mental calculation to determine the appropriate relative bearing.
To simplify this task a compass card is added to the RBI to form a 'Radio Magnetic Indicator', RMI. The ADF needle is then referenced immediately to the aircraft heading which reduces the necessity for mental calculation.
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
Airways
As described above, a pilot can track a specific bearing to or from an NDB. A bearing is a line passing through the station that points in a specific direction, such as 270 degrees (due West). NDB bearings provide a charted, consistent method for defining paths aircraft can fly. In this fashion, NDBs can, like VORs, define 'airways' in the sky. Aircraft follow these pre-defined routes to complete a flight plan. Airways 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:
Image:NDB Article Airspace Fix Diagram.png
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
Image:NDB ZBAA 36L MM.JPG 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.
Common errors
Navigation using an ADF to track NDBs is subject to several common errors:
- twilight error
- radio waves can be reflected back by the ionosphere can cause fluctuations 30 to 60 nautical miles (approx. 54 to 108 km) from the transmitter, especially just before sunrise and just after sunset (more common on frequencies above 350 kHz)
- terrain error
- high terrain like mountains and cliffs can reflect radio waves, giving erroneous readings; magnetic deposits can also cause erroneous readings
- electrical error
- electrical storms, and sometimes also electrical interference (from a ground-based source or from a source within the aircraft) can cause the ADF needle to deflect towards the electrical source
- shoreline error
- low-frequency radio waves will refract or bend near a shoreline, especially if they are close to parallel to it
- bank error
- when the aircraft is banked, the needle reading will be offset
While pilots study these errors during initial training, trying to compensate for them in flight is difficult; instead, pilots generally simply choose a heading that seems to average out any fluctuations.
Monitoring NDBs
Besides their use in aircraft navigation, NDBs are also popular with long-distance radio enthusiasts (DXers). Because NDBs are generally low-power, typically between 25 and 100 watts, they normally cannot be heard over long distances, but favorable atmospheric conditions can allow NDB signals to travel much further than normal. Because of this, radio monitors interested in picking up distant signals can gain considerable enjoyment in listening for and logging faraway NDBs. Also, because the band allocated to NDBs is free of broadcast stations and their associated interference, and because NDBs do little more than transmit their own Morse Code callsign, they are easy to listen to and identify, making NDB monitoring a very accessible and fun niche within the radio hobby.
The NDB band runs approximately 200-530 kHz, ending at the lower end of the AM radio dial in the US. A few NDBs can therefore be heard on older radios that can tune slightly below the official 530 kHz (such as the "HEH" beacon in Newark, Ohio at 524 kHz which is within the bandwidth of most AM radios at the bottom of the dial, and the "OH" beacon in Columbus, Ohio at 515 kHz), but for the most part the NDB band requires a general communications receiver or other radio that will tune within that band.
It's also worth noting that most AM radios, while close (within a few miles) to an NDB, can tune to the harmonics of that NDB frequency on a standard AM dial. (In fact, the occurrence of hearing repeated morse code over the standard AM band has been a mysterious phenomenon for years, with many possible sources. However, it is usually NDBs.
Radios which can receive the longwave broadcasting band are especially suitable for reception NDBs. Especially some older radios can also receive the range between 281 kHz and 353 kHz, which is not used for broadcasting purposes any more.
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
See also
- Instrument Flight Rules (IFR)
- VHF Omni-directional Range (VOR)
- Distance Measuring Equipment (DME)
- Global Positioning System (GPS)
- Instrument Landing System (ILS)
- List of NDBs