Definition
A ground-based, low or medium frequency radio transmitter that broadcasts a signal equally in all directions, allowing a suitably equipped aircraft to determine its bearing to or from the station using an Automatic Direction Finder (ADF) receiver. NDBs are used as enroute navigation aids, as approach aids for non-precision instrument approaches, and as compass locators associated with ILS approaches.
Plain English
A radio beacon on the ground that sends out a signal in every direction. An aircraft instrument can sense which direction the signal is coming from, so the pilot knows which way the station is.
Context Anchor
Seen in instrument procedures, approach planning, and IFR alternate requirements when a procedure or airport depends on an NDB for navigation.
Derivation
Called 'non-directional' because the signal is broadcast equally in all directions, unlike a VOR which transmits directional information. The aircraft's receiver does the work of figuring out the bearing, not the beacon.
Why Pilots Care
NDBs remain a required navigation option for many IFR alternate airports and serve as a backup when GPS or VOR coverage is unavailable.
Intuition Check
“Non-directional” does not mean the pilot gets no direction. It means the ground station broadcasts in all directions; the aircraft equipment determines the direction to it.
Example Sentence 1
The pilot tuned the NDB frequency and watched the ADF needle swing to point at the station.
Example Sentence 2
Because the destination lacked a suitable ILS, the alternate airport was selected based on its published NDB approach.