AIM Text
- A low or medium frequency radio beacon transmits nondirectional signals whereby the pilot of an aircraft properly equipped can determine bearings and “home” on the station. These facilities normally operate in a frequency band of 190 to 535 kilohertz (kHz), according to ICAO Annex 10 the frequency range for NDBs is between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation. All radio beacons except the compass locators transmit a continuous three-letter identification in code except during voice transmissions.
- When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a Compass Locator.
- Voice transmissions are made on radio beacons unless the letter “W” (without voice) is included in the class designator (HW).
- Radio beacons are subject to disturbances that may result in erroneous bearing information. Such disturbances result from such factors as lightning, precipitation static, etc. At night, radio beacons are vulnerable to interference from distant stations. Nearly all disturbances which affect the Automatic Direction Finder (ADF) bearing also affect the facility's identification. Noisy identification usually occurs when the ADF needle is erratic. Voice, music or erroneous identification may be heard when a steady false bearing is being displayed. Since ADF receivers do not have a “flag” to warn the pilot when erroneous bearing information is being displayed, the pilot should continuously monitor the NDB's identification.
Source: FAA Aeronautical Information Manual · current edition · paragraph 1-1-2.
The short answer
This AIM section is FAA advisory guidance on the Nondirectional Radio Beacon (NDB), a low- or medium-frequency beacon transmitting nondirectional signals so a properly equipped pilot can determine bearings and home on the station. It describes the operating band, three-letter coded identification, voice capability, and disturbances affecting ADF bearings, supporting NDB-based navigation under 14 CFR Part 91.
Research Notes
Common Questions
What frequency band do NDBs operate in?
What is a compass locator?
Per the AIM, when a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a Compass Locator. All radio beacons except compass locators transmit a continuous three-letter identification in code except during voice transmissions.
Why should I keep monitoring an NDB's identification?
The AIM warns that radio beacons are subject to disturbances (lightning, precipitation static, and at night interference from distant stations) that produce erroneous bearings. Since ADF receivers have no flag to warn of bad bearings, pilots should continuously monitor the NDB's identification; noisy ID usually accompanies an erratic ADF needle.