Non-directional beacon
NDB signals follow the curvature of the Earth, so they can be received at much greater distances at lower altitudes, a major advantage over VOR.
However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range.
The system, developed by United States Army Air Corps (USAAC) Captain Albert Francis Hegenberger, was used to fly the world's first instrument approach on May 9, 1932.
The ADF needle is then referenced immediately to the aircraft's magnetic heading, which reduces the necessity for mental calculation.
The principles of ADFs are not limited to NDB usage; such systems are also used to detect the locations of broadcast signals for many other purposes, such as finding emergency beacons.
As of September 2022, only one colored airway is left in the continental United States, located off the coast of North Carolina and is called G13 or Green 13.
NDBs have long been used by aircraft navigators, and previously mariners, to help obtain a fix of their geographic location on the surface of the Earth.
NDBs may designate the starting area for an ILS approach or a path to follow for a standard terminal arrival route, or STAR.
[5] German Navy U-boats during World War II were equipped with a Telefunken Spez 2113S homing beacon.
Vertical NDB antennas may also have a T-antenna, nicknamed a top hat, which is an umbrella-like structure designed to add loading at the end and improve its radiating efficiency.
Apart from Morse code identity of either 400 Hz or 1020 Hz, the NDB may broadcast: Navigation using an ADF to track NDBs is subject to several common effects: While pilots study these effects during initial training, trying to compensate for them in flight is very difficult; instead, pilots generally simply choose a heading that seems to average out any fluctuations.
Radio-navigation aids must keep a certain degree of accuracy, given by international standards, Federal Aviation Administration (FAA), ICAO, etc.
; to assure this is the case, Flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify NDB precision.
Specialized techniques (receiver preselectors, noise limiters and filters) are required for the reception of very weak signals from remote beacons.
Reception of NDBs is also usually best during the fall and winter because during the spring and summer, there is more atmospheric noise on the LF and MF bands.
As the adoption of satellite navigation systems such as GPS progressed, several countries began to decommission beacon installations such as NDBs and VOR.
[10] Airservices Australia began shutting down a number of ground-based navigation aids in May 2016, including NDBs, VORs and DMEs.
