A non-directional beacon, or NDB, is a radio broadcast station from an antenna in a known location used as air navigation aids. By broadcasting on the relatively long-range LF or low AM band, the signals being picked up by directional antennas in the aircraft to display the direction to the station. These beacons generally transmit a Morse code identifier of the beacon's name (eg "GLF" for Sydney Airport). NDB systems were originally refered to simply as "radio beacons", but the introduction of the directional VOR system led to a change in name.
Early NDB systems used a loop antenna attached to a radio receivers, rotated by hand (typically through a mechanical linkage to the navigator's station in the aircraft) until the minimum signal was located. The radio station then laid on the line measuring the radius of the loop. Such systems can often be seen on World War II-era aircraft, typically as a bare loop under the aircraft, or as a streamlined pod looking like a teardrop.
More modern systems used a highly directional solenoid that is spun at high speed with a motor. Electronics timed to the rotation of the antenna spin a pointer on a compass rose to indicate the direction of the station relative to the nose of the aircraft. These automated systems are known as automatic direction finders, or ADF. Even more modern systems use a phased array antenna to "rotate" electronically. However these systems are rare, and expensive. The expense is generally not considered worthwhile, notably given the low cost of competing navigation systems such as GPS.
NDB instruments point directly to the station, and therefore do not indicate the actual location of the aircraft. To determine location to NDB signals need to be tuned in order to take a fix. NDB stations are indicated on aerial navigation maps (known as charts) and are the easiest to use, but commercial AM radio stations can also be used with the receivers.
In general NDB navigation is simpler than with the more advanced VOR system. Fixes can be taken quickly by tuning in two stations in quick succession and then plotting the results. However one task is considerable more difficult with an NDB, navigating to enter the pattern to land. In this case the aircraft must locate itself both in place and direction to a very high degree of accuracy, and doing so with an NDB with any wind aloft is notoriously difficult.
