How does an NDB tell a pilot which direction the station is in?
A ground station sends a radio signal equally in every direction, the aircraft direction finder shows where it comes from, and the mast, earth wires and open ground around the station are shared with the airfield lighting.
An NDB (non-directional beacon) is a ground station that sends a radio signal equally in every direction, with no direction coded in it. The aircraft's ADF (automatic direction finder) works out which way the signal is coming from, and its pointer shows the direction of the station from the aircraft. The pilot can fly towards the station or keep to a chosen bearing to or from it. Each NDB sends a Morse code identification so the pilot can confirm the right station.
An NDB gives direction only, not distance. It works at low and medium frequency, so the signal can be received a long way off, but it is easily disturbed by thunderstorms, night-time sky-wave signals, hills and coastlines, which makes it less precise than a VOR. At an airport an NDB is often used as a locator for an ILS or as a simple approach aid, and many are being withdrawn as satellite navigation takes over.
An NDB is not part of the AGL system, but when a station stands on or near an airfield it meets the lighting on the ground. The mast is usually marked and lit as an obstacle, and a network of buried copper earth wires spreads out around it, so tall structures, metal fences, overhead lines and new cable routes close by can bend the bearing. Works near the station are agreed with the navaid owner first, so that the signal and the lighting both stay right.
Frequently Asked Questions
Q1What does an NDB do?
An NDB, or non-directional beacon, is a ground station that sends a radio signal in every direction. The aircraft's automatic direction finder (ADF) works out which way the signal is coming from and its pointer shows the direction of the station. The pilot can then fly towards the station or keep to a chosen bearing to or from it.
Q2Does an NDB give distance, and how does the pilot know which NDB it is?
An NDB gives direction only, not distance. For distance the pilot uses another aid, such as a DME, or crosses bearings from two stations. Each NDB sends a short Morse code identification, which the pilot listens for to confirm the right station is being used.
Q3Why is an NDB less precise than a VOR?
An NDB works at low and medium frequency, so its signal can follow the ground for a long way, but it is easily disturbed. Thunderstorms, night-time sky-wave signals, hills and coastlines can all make the pointer wander or point the wrong way. A VOR is much less affected, which is why NDB bearings are treated with more care.
Q4Why must the area around an NDB be kept clear?
Metal fences, overhead lines, large buildings and other tall structures close to the mast can re-radiate the signal and bend the bearing. An NDB also has an earth system of buried copper wires around the mast, and digging or cable work through it can upset the signal. The site is therefore kept clear, and new structures close to it are reviewed with the navaid owner.
Q5Is an NDB part of airfield ground lighting?
No. It is a radio navigation aid owned and maintained by the air navigation service provider or the airport's navaid team. It meets the AGL system on the ground when a station stands on or near the airfield, because its mast is usually marked and lit as an obstacle and its clear site is shared with lighting masts, cable routes, earthing and works.
Q6What should be done before working near an NDB?
Tell the navaid owner before the work and agree the dates and the method. Keep tall equipment, large metal objects and new cable routes away from the mast and do not cut or dig through the buried earth wires. Work on the mast or its lights only when the navaid owner has made the transmitter safe, because the mast carries radio power. Ask whether the NDB needs a check afterwards.
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