An NDB or Non-Directional Beacon is a ground-based, low frequency radio transmitter used as an instrument approach for airports and offshore platforms. They were added yesterday in a new Infinite Flight hotfix, seen below.
NDBs are highly reliable, typically provide decades of uninterrupted service. Because of this, NDBs are the most widely used navaid in the world.
In addition to serving as stand-alone primary instrument approaches at airports, NDBs are also used as Locator Outer Markers (LOM) for Instrument landing Systems (ILS). Outer Markers designate the starting area of an ILS approach or flight path to follow for a standard terminal arrival or STAR procedure.