All NDBs are being decommissioned and will
be phased out.
Automatic Direction Finder
The Automatic Direction Finder (ADF) is an aircraft radio navigation system that senses
and indicates the direction to an NDB ground transmitter. Direction is indicated to the
pilot as a magnetic bearing or as a relative bearing to the longitudinal axis of the aircraft,
depending on the type of indicator installed in the aircraft.
Limitations of NDBs
Radio beacons and receiving equipment are subject to atmospheric disturbances, which
can make their use undesirable. The radio compass is subject to signal fade and static
during stormy weather, which can result in erratic indicator operation. This can make
NDBs unsuitable for homing approaches or for holding during thunderstorms. At night,
other distant stations interfere with signal reception in the same way as standard radio
receivers. Also, homing normally results in a curved course being flown rather than a
straight course because of crosswinds acting on the aircraft.
VHF/UHF Omnidirectional Ranges
Omni is from the Latin word omnis, which means "all." An omnifacility provides an
unlimited number of courses (called radials) in all directions. This is in contrast to the
first nationwide system of airway beacons (four-course ranges) which provided
guidance in only four directions.
Many different types of omnifacilities are in operation today: the VOR facility, the
VOR/DME facility, the TACAN facility, and the facility that uses both VOR and TACAN
called the VORTAC.
VHF Omnidirectional Range
The VHF Omnidirectional Range (VOR) is a radio facility that eliminates many of the
difficulties previously encountered in air navigation. VOR course information is not
affected by weather or other factors common to ADF. With a course indicator, it is
possible to select and precisely fly any one of 360 courses to or from a VOR.