Radio navigation beacons

A most basic aviation radio navigational aid is the NDB or Non-directional Beacon. These are simple low frequency and medium frequency transmitters and they are used to locate airways intersections, airports and to conduct instrument approaches, with the use of a radio direction finder located on the aircraft. The aviation NDBs, especially the ones marking airways intersections, are gradually decommissioned, as they are replaced with other navigational aids based on newer technologies. Due to relatively low purchase, maintenance and calibration cost, they are still used to mark locations of smaller aerodromes and important helicopter landing sites.

There were[1] also marine beacons, based on the same technology and installed at coastal areas, for use by ships at sea. Most of them, especially in the western world, are no longer in service, while some have been converted to telemetry transmitters for differential GPS

In addition to dedicated radio beacons, any AM, VHF, or UHF radio station at a known location can also be used as a beacon with direction finding equipment.