A non-directional beacon (NDB) is a radio transmitter at a known location, used as a navigational aid. NDB signals follow the curvature of the Earth, so they can be received at much greater distances at lower altitudes, a significant advantage over VOR. However, NDB signals are affected more by atmospheric conditions, terrain, coastal refraction and electrical storms, especially at long range.
There are four types of non-directional beacons in aviation, the last two used in conjugation with an ILS.
- En route NDBs, used to mark airways
NDB bearings provide a charted method for defining paths aircraft can fly. This way, NDBs can, like VORs, define “airways”. Pilots follow these routes by tracking radials across various navigation stations.
- Approach NDBs
A runway equipped with NDB or VOR (or both) as the only navigation aid is a non-precision approach runway.
- Localizer beacons
- Locator beacons
Most commonly used as markers for an ILS approach, NDBs may designate the starting area for an ILS approach or a path to follow for a STAR.
NDBs can also help plot fixes, by extending imaginary lines out from multiple navigation stations until they intersect.