nondirectional beacon

A radio navigational aid that enables the pilot to obtain his bearing from a transmitting station. A transmitter operating in long- or medium-wave frequencies, or even in UHFs (ultrahigh frequencies), with the code of the station superimposed on its transmitted signal. The signal indicates a bearing or homing and is picked up by the radio compass or ADF (automatic direction finder) in the aircraft. NDB facilities normally operate in the frequency band of 190 to 535 kHz and usually transmit a continuous carrier with either 400 or 1020 Hz modulation. When used in conjunction with ILS (instrument landing system) markers, they are called compass locators. The aircraft equipment can also lock onto other transmitters like radio broadcasting stations for this purpose. All radio beacons except compass locators transmit repetitive continuous three-letter identification signals except during voice transmissions. The availability of this facility is indicated on aeronautical charts by symbols .