Non-Directional Beacon (NDB) System

Non-Directional Beacon (NDB)

• NDB is a simple radio transmitter that produces a stable radio wave that follows the curvature of the earth's surface, known as a ground wave.
• The wave is transmitted with equal strength in all directions, making it a non-directional signal.
• NDBs operate in the low to medium frequency bands at around 200 to 400 kHz.

Characteristics of NDB

• The range of NDB is not limited to line of sight, but varies with the power of the transmitter and the nature of the surface.
• Ground wave range is best over water and least over dry sandy country.
• Rated coverages of NDBs vary from 30 nm for low power transmitters over land to 500 nm for high power transmitters over water.

ADF Equipment and Commercial AM Broadcast Stations

• ADF (Automatic Direction Finding) equipment is used to receive the ground wave transmitted by NDBs.
• Commercial AM broadcast stations can also be used to obtain ADF bearings, but care should be exercised in identification since some broadcast stations are relay stations.
• Details on broadcasting stations are found in ERSA and are marked on the ERC charts.

NDB Reception and Interference

• The NDB transmits its signal in the low to medium frequency bands, resulting in a strong ground wave and a significant sky wave.
• The associated sky wave can sometimes interfere with the proper functioning of the ADF.

Range of NDBs

• The range of NDBs depends on the power of the transmitter, the nature of the country, and whether it is day or night.
• A single figure for the range of any particular NDB cannot be published due to these variables.
• A published range for each NDB is provided in ERSA, known as the rated coverage of the NDB.

Night Effect

• The ionosphere, a layer of electrically charged air, interacts with the sun's radiation, affecting medium frequency sky waves by absorbing or reflecting them based on the angle of arrival.
• This causes interference with the ground wave, especially at night, reducing the rated coverage of some NDBs.
• The ionosphere's reflection of the sky wave causes it to arrive at the aircraft later, resulting in out-of-phase signals that interfere with each other.

Co-Channel Interference

• Caused by nearby transmitters broadcasting on the same or adjacent frequencies, which is mitigated by careful frequency allocation.

Thunderstorm Effect

• Thunderstorms generate radio signals that affect the net signal received by the ADF receiver.

Mountain Effect

• NDB signals are reflected by mountains, causing errors that decrease with increased aircraft height.

Coastal Refraction

• Ground waves travel at different speeds through varying surfaces, causing the signal to bend when crossing from land to water at a coast line.
• This bending, or refraction, occurs when the ground wave crosses the coast at an angle, resulting in incorrect instrument indications.
• No refraction occurs when the ground wave crosses the coast at right angles.

Quadrantal Error

• Interference caused by the aircraft's structure interacting with the incoming signal.

Effective Use of ADF

• Requires interpretation of the displayed information, orientation with the aircraft's position and the station, and application of that information to the in-flight situation.