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.