Non-Directional Radio Beacon


  • The Non-Directional Radio Beacon (NDB) is a low or medium frequency radio beacon transmits non-directional signals whereby the pilot of an aircraft properly equipped can determine bearings and "home" to the station
  • The pilot, through the use of an Automatic Direction Finder, uses these signals in order to determine relative/magnetic bearing and therefore position
  • The entire system consists of:
    • Ground station
    • ADF receiver
    • Antenna:
      • Loop Antenna (Magnetic Bearing from the airplane to the station)
      • Sense Antenna (Directional Information)
      • Bearing Indicator
  • Ultimately, the Standard Service Volume dictates the reception limits of the NAVAID
NAVAIDS Depicted On Legend
Figure 1: NAVAIDS Depicted On Legend


  • These facilities normally operate in a frequency band of 190 to 535 kilohertz (kHz)
    • According to International Civil Aviation Organization (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
    • The NDB frequency can sometimes bleed over to the AM Radio frequency band and likewise radios can bleed over onto the NDB frequency


  • All radio beacons except the compass locators transmit a continuous three-letter identification in code except during voice transmissions
  • NDB frequency and identification information is found on aeronautical charts and in the Chart Supplement U.S.
  • The Morse codes are used to identify the NDB stations while the commercial broadcast stations are identified at random times by the station's announcer
  • These signals can be used to either home or intercept and track a course for navigation


  • Relatively simple and low cost
  • Accuracy is suitable for navigation but subject to numerous limitations
  • Not limited by line of sight which permits reception at low altitudes over great distances due to ground waves

Automatic Direction Finder Receivers:

  • Fixed Compass Card
  • Movable (Rotating) Compass Card
  • Single-needled Radio Magnetic Indicator (RMI)
  • Dual-needle Radio Magnetic Indicator (RMI)
  • Fixed Compass Card:

    • A fixed compass card simply means the face of the instrument cannot rotate, leaving only the needles to move
    • Always represent the nose of the aircraft at 0° and the tail as 180°
    • Visualizing the situation with this type of indicator can be daunting
    • RB + MH = MB
      • (relative bearing) + (magnetic heading) = (magnetic bearing)
      • Relative Bearing: Degrees flown to station (clockwise)
      • Magnetic Heading: MH
      • Magnetic Bearing: Distance from magnetic north
  • Movable Compass Card:

    • Pilot can rotate the face of the card
    • The ADF needle will directly indicate the magnetic bearing to the NDB when the aircraft heading is shown at the top
  • Single-Needle Radio Magnetic Indicator:

    • Combines radio and magnetic information to provide continuous heading, bearing and radial information
  • Dual-Needle Radio Magnetic Indicator:

    • Has a second needle
    • The second needle typically points to a VOR station


  • Navigation
  • "Colored" Airways (limited)
  • Intercepting and tracking
  • Holding
  • Instrument Approaches
  • Homing
  • Tracking

Non-Directional Beacon Limitations:

  • Radio beacons are subject to disturbances that may result in erroneous bearing information
  • Such disturbances result from factors such as lightning, precipitation static, etc.
  • 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
  • Twilight Error (Night Effect):

    • Radio waves can be reflected back by the ionosphere and can cause fluctuations 30 to 60 NM (approx. 54 to 108 KM) from the transmitter, especially just before sunrise and just after sunset
  • Terrain Error:

    • High terrain like hills and mountains can reflect radio waves, giving erroneous readings especially if they contain magnetic deposits
  • Electrical Error:

    • Electrical storms, and sometimes also electrical interference can cause the ADF needle to deflect toward the electrical source
  • Shoreline Error:

    • Low-frequency radio waves will refract or bend near a shoreline, especially if they are close to parallel to the shore
  • Bank Error:

    • When the aircraft is banked, the needle reading will be offset

Standard Service Volume (SSV):

  • The SSV defines the reception limits of unrestricted NAVAIDs which are usable for random/unpublished route navigation [Figure 1]
  • Each NAVAID operates at the power necessary to provide coverage within their assigned operational service volume
  • A NAVAID will be classified as restricted if it does not conform to flight inspection signal strength and course quality standards throughout the published SSV
    • However, the NAVAID should not be considered usable at altitudes below that which could be flown while operating under random route IFR conditions (14 CFR Section 91.177), even though these altitudes may lie within the designated SSV
  • Service volume restrictions are first published in Notices to Airmen (NOTAMs) and then with the alphabetical listing of the NAVAIDs in the Chart Supplement U.S.
  • The SSV of an NDB is indicated by using the class designator
    • Example: Compass Locator, MH, H, HH
  • NDBs are classified according to their intended use
  • The distances (radius) are the same at all altitudes
Non-Directional Beacon Service Volumes
Figure 2: Non-Directional Beacon Service Volumes
Non-Directional Beacon Service Volumes
Figure 2: Non-Directional Beacon Service Volumes

ADF Navigation:

  • Many airplanes are equipped with ADF radios which operate in the low and medium frequency bands [Figure 2]
    • By tuning to low frequency (LF) radio stations such as NDBs, or to commercial broadcast (AM) stations, a pilot may use ADF for navigation in cross-country flying
    • Some major commercial broadcast station locations and frequencies are shown on sectional aeronautical charts
  • Most ADF radio receivers signals are in the frequency spectrum of 190 kHz to 1750 kHz, which includes LF and MF navigation facilities, and the AM commercial broadcast stations
    • Primarily for air navigation, the LF/MF stations are FAA and privately operated non-directional radio beacons
    • Some broadcast stations operate only during daylight hours, and many of the low powered stations transmit on identical frequencies and may cause erratic ADF indications
  • The ADF has automatic direction seeking qualities which result in the bearing indicator always pointing to the station to which it is tuned
    • That is, when the bearing pointer is on the nose position, the station is directly ahead of the airplane; when the pointer is on the tail position, the station is directly behind the airplane; and when the pointer is 90° to either side (wingtip position), the station is directly off the respective wingtip
  • The more commonly used ADF instrument has a stationary azimuth dial graduated from 0° up to 360° (with 0° at the top of the instrument to represent the airplane's nose)
    • In this type, the bearing pointer shows only the station's relative bearing, i.e., the angle from the nose of the airplane to the station [Figure 3]
    • A more sophisticated instrument called a Radio Magnetic Indicator (RMI) uses a 360° azimuth dial which, being slaved to a gyro compass, rotates as the airplane turns and continually shows the Magnetic Heading at the top of the instrument
    • Thus, with this rotating azimuth referenced to a magnetic direction, the bearing pointer superimposed on the azimuth indicates the Magnetic Bearing to the station
  • The easiest, and perhaps the most common method of using ADF, is to "home" to the station
    • Since the ADF pointer always points to the station, the pilot can simply head the airplane so that the pointer is on the 0° or nose position. The station, then will be directly ahead of the airplane. With a crosswind, however, the airplane would continually drift to the side and, unless a corrective change in heading is made, would no longer be flying straight to the station. This would be indicated by the pointer moving to the upwind side of the nose position on the dial. By periodically turning the airplane into the wind (toward the head of the pointer) so as to continually return the pointer to the 0° position, the airplane can be flown to the station, although in a curving flightpath as a result of wind drift. The lighter the crosswind and the shorter the distance from the station, the less the flightpath curves. Upon arrival at and passing the station, the pointer will swing 180° from a nose position to a tail position
  • ADF should be considered as a moving, "fluid" thing
    • The number to which the bearing indicator points on the fixed azimuth dial has no directional meaning to the pilot until it is related to the airplane's heading. To apply this relationship, the magnetic heading must be observed carefully when reading the Relative Bearing to the station. Any time the airplane's heading is changed, the Relative Bearing will be changed an equal number of degrees
  • To determine the Magnetic Bearing to a station on a fixed ADF azimuth dial, the pilot may imagine the airplane as being in the center of the fixed azimuth, with the nose of the airplane at the 0° position, the tail at the 180° position, and the left and right wingtips at the 270° and 090° positions, respectively
    • When the pointer is on the nose position, the airplane is heading straight to the station and the Magnetic Bearing can be read directly from the magnetic compass. If the pointer is left or right of the nose, the pilot should note the direction and number of degrees of turn that would (if the airplane were to be headed to that station) move the pointer to the nose position, and mentally apply this to the airplane's heading. For example, in Fig. 12-11, when the airplane is headed 090°, the pointer is 60° to the left of the nose position. A turn 60° to the left would place the pointer on the nose position. If the airplane were to be turned 60° to the left, the heading would be 030°. The bearing from the station is the reciprocal - or 210°
  • One of the several valuable uses of ADF is the determination of the airplane's position along the course being flown. Even though the airplane is following a course along a VOR radial, obtaining an ADF bearing that crosses the course will establish a "fix" or position along that course. This is particularly advantageous when an off course VOR is not available for a cross bearing or when the only VOR receiver must be used as the primary tracking system
ADF Receiver
Figure 3: ADF Receiver
ADF Receiver
Figure 3: ADF Receiver
Figure 4: ADF and RMI
Figure 4: ADF and RMI


  • Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the primary signal generator for a ground-based navigational transmitter is inoperative
    • Pilots should disregard any navigation indication, regardless of its apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative
  • 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)
  • Do not include a flag to warn of inoperative conditions so signal must constantly be monitored
  • Additionally tools are available to better increase your knowledge of navigation including VOR/NDB Simulators