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A non-directional (radio) beacon (NDB) is a radio transmitter at a known location, used as an aviation or marine navigational aid.

As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF omnidirectional range (VOR) and TACAN.

NDB signals follow the curvature of the earth, so they can be received at much greater distances at lower altitudes, a major advantage over VOR. However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range.

NDBs used for aviation are standardised by ICAO Annex 10 which specifies that NDBs be operated on a frequency between 190 kHz and 1750 kHz, although normally all NDBs in North America operate between 190 kHz and 535 kHz.

Each NDB is identified by a one, two, or three-letter Morse code callsign. In Canada, privately-owned NDB identifiers consist of one letter and one number.

North American NDBs are categorized by power output, with low power rated at less than 50 watts, medium from 50 W to 2,000 W and high being over 2,000 W

Use of non-directional beacons

NDB navigation consists of two parts — the automatic direction finder (or ADF) equipment on the aircraft that detects an NDB's signal, and the NDB transmitter. The ADF can also locate transmitters in the standard AM medium wave broadcast band (530 kHz to 1700 kHz at 10 kHz increments in the Americas, 531 kHz to 1602 kHz at 9 kHz increments in the rest of the world).

ADF equipment determines the direction to the NDB station relative to 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 degree 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. (Aircraft Heading +/- ADF needle degrees off nose or tail = Bearing to or from NDB station).

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 adjusted for drift. An NDB may also be used to locate a position along the aircraft track. When the needle reaches an 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 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.

More info on using a NDB can be found at the following location: http://www.navfltsm.addr.com/ndb-nav-adf-1.htm