ndB

NDB, or non-directional beacon, is a ground-based radio transmitter used in aviation navigation. It operates in the low-frequency or medium-frequency bands, typically around 190 to 535 kHz, and broadcasts a continuous carrier with an identifying Morse code signal. Aircraft equipped with an Automatic Direction Finder (ADF) receiver can determine the bearing to or from the beacon by interpreting the direction of the received signal.

Because the beacon itself radiates a non-directional signal, the information provided to the pilot is a relative bearing rather than a precise azimuth. The ADF display helps the pilot steer toward the beacon, away from it, or establish a line of position when used with other navigational aids. The practical range of an NDB varies with transmitter power, antenna design, terrain, and atmospheric conditions; low-power beacons may be tens of miles away, while high-power stations can be hundreds of miles distant, especially at night due to ionospheric propagation.

NDBs have been a mainstay of en-route navigation and instrument approach procedures since the mid-20th century. They are commonly used to mark airways, fixes, and approach paths. A subset known as compass locators is associated with certain instrument approaches and can be co-located with ILS facilities. NDBs are valued for their simplicity and low cost, but their signals are susceptible to static, electrical interference, and reduced bearing accuracy compared with more modern systems.

In many regions, the aviation network is shifting toward VOR, DME, and GNSS-based navigation, leading to decommissioning or retirement of some NDBs. Nevertheless, remaining NDBs continue to provide coverage in remote or sparsely served areas where alternative facilities are limited. Operators and pilots rely on aeronautical information publications to track current NDB availability and procedures.