NDBs

NDBs, or non-directional beacons, are radio navigation aids used in aviation to provide bearing information to pilots. They transmit a continuous radio signal on a single frequency in the low- to medium-frequency bands (roughly 190 to 1750 kHz). The transmitted tone includes a Morse code identifier, enabling aircraft receivers to identify the beacon.

Aircraft rely on ADF (automatic direction finder) equipment to determine the direction to a selected NDB. Since the beacon radiates equally in all directions, the radio bearing displayed by the ADF points toward the station but does not indicate distance. Pilots use these bearings, along with map data and other navigation sources, to determine their position and course, especially on en-route legs and on instrument approaches.

History: NDBs were among the first radio navigation aids to reach global coverage in the mid-20th century, replacing earlier optical methods and enabling more reliable navigation over oceans and remote areas. They remain in use in many regions but are gradually being phased out in favor of satellite-based and other precision approaches.

Operational considerations: NDB signals can be affected by atmospheric conditions, thunderstorms, day/night propagation effects, and coastal terrain, causing bearing drift or range variability. Interference from electrical systems and other radio transmissions can degrade readability. For this reason, NDB procedures require careful monitoring and are often coexisting with VOR/DME or GPS-based approaches.

Modern context: Many air navigation systems are transitioning toward GNSS-based solutions. Some countries are reducing network coverage of NDBs, particularly in busy airspaces, while others retain a minimal NDB network for backup navigation or operations in areas with limited satellite coverage.