Non-Directional Beacon

A Non-Directional Beacon (NDB) is a ground-based radio transmitter that sends a signal in all directions (omnidirectional) on frequencies between 190 and 535 kHz (LF/MF band). Aircraft equipped with an Automatic Direction Finder (ADF) receive the NDB signal and display the direction to or from the beacon.

NDBs were among the earliest radio navigation aids used in aviation and provided the foundation for instrument flight long before VOR and GPS. They are used for:

• En-route navigation (as airway fixes)
• Non-precision instrument approaches (NDB approaches)
• Locator beacons (low-power NDBs) collocated with ILS outer markers

Limitations of NDB navigation include susceptibility to atmospheric interference (thunderstorms cause ADF needle deflection), terrain effects, shoreline refraction, and the lack of inherent distance information. Due to these limitations and the proliferation of GPS, NDBs are being decommissioned worldwide. However, they remain in use in many developing regions and as backup navigation aids.