Non-directional beacon

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.

Contents

1 Automatic Direction Finders

2 Usage of NDBs

2.1 Radials and Airways
2.2 Fixes
2.3 Instrument Landing Systems

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 better-equipped aircraft, such as complex singles, twins, and airliners, 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:

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Airspace Fix Diagram

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

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The NDB station co-located with Middle Marker of Beijing Capital International Airport ILS RWY36L

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 (60 to 110 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 "OS" and "HEH" NDBs in Columbus, Ohio, at 515 and 524 kHz respectively), but for the most part the NDB band requires a general communications receiver or other radio that will tune within that band.