Automatic Direction Finder (ADF-NDB)

Wing Commander Rajagopal

Non-Directional Beacon (NDB) & Automatic Direction Finder (ADF)

NDB (Non-Directional Beacon) is a ground-based radio transmitter used for aircraft navigation. It provides a reference signal that helps pilots determine the direction of the station using an Automatic Direction Finder (ADF). NDB is used for en-route navigation and instrument approach procedures.

Introduction to NDB

  • A Non-Directional Beacon (NDB) is a ground-based radio transmitter operating in the LF and MF frequency bands.
  • NDB transmits a vertically polarised interrupted carrier-wave signal.
  • The transmission is radiated uniformly through 360°.
  • Operating frequencies range from 190 kHz to 1750 kHz.
  • NDB antennas are generally large T-shaped aerials because of the long wavelengths involved.
  • Typical emission designators are NON-A1A and NON-A2A.
  • The operational range varies from approximately 25 NM to 500 NM, depending on the purpose of the beacon.
  • Each NDB continuously transmits a unique three-letter Morse code identification.

Principle of ADF

  • The Automatic Direction Finder (ADF) uses NDB transmissions for navigation.
  • ADF operates within the frequency range of 190–1750 kHz.
  • It determines the direction of an NDB using the loop antenna principle.
  • The loop antenna consists of two vertical elements that receive the radio signal.
  • Signals arriving simultaneously produce no phase difference.
  • Equal phase results in zero current flowing through the loop circuit.
  • Zero current indicates that the beacon lies perpendicular to the loop antenna.
  • The measured current is processed to determine the bearing of the station.

Null Position in ADF

  • The polar diagram of a loop antenna is shaped like a figure of eight.
  • The null position occurs when the NDB is either directly ahead of or behind the aircraft.
  • A sense antenna resolves the front/back ambiguity.
  • Combining the loop and sense antennas produces a cardioid (heart-shaped) radiation pattern.
  • This cardioid pattern has only one null position.
  • The front/back ambiguity of the loop antenna is therefore eliminated.
  • The polarity of the sense antenna may be switched periodically to improve accuracy.

Components of an ADF

  • The loop and sense antennas are mounted together inside a teardrop-shaped housing.
  • The antenna assembly is usually installed on the underside of the aircraft fuselage.
  • Modern ADF systems use two fixed loop antennas mounted at right angles.
  • The antennas are connected to a goniometer that measures the bearing.
  • A search coil detects the null position.
  • A motor rotates the search coil until the null position is reached.
  • The position of the search coil determines the relative bearing displayed.

ADF Control Panel

  • The ADF is controlled from a cockpit control panel.
  • Digital control panels normally provide active and standby frequency selection.
  • The frequency selector is used to tune the desired NDB.
  • The ANT (Antenna) mode is used to listen to the Morse code identification.
  • Bearings should be ignored while operating in ANT mode because the loop antenna is disconnected.
  • The Beat Frequency Oscillator (BFO) enables reception of NON-A1A transmissions.
  • Bearings should also be ignored while BFO mode is selected.
  • The TEST button checks the serviceability of the indicator.
  • A serviceable indicator needle moves approximately 90° during the test.

Beat Frequency Oscillator (BFO)

  • Every NDB continuously transmits a unique three-letter Morse identification.
  • NON-A1A transmissions require the BFO to be selected ON for audio reception.
  • The heterodyne circuit inside the BFO generates an audible beat frequency.
  • The beat frequency is produced by mixing the received carrier with an internally generated frequency.
  • BFO should be selected during tuning, identification, and signal monitoring.
  • NON-A2A transmissions can be heard without using the BFO.
  • Amplitude modulation used by A2A transmissions reduces their effective range.
  • The BFO switch may also be labelled TONE or VOICE.

ADF indications inside the Cockpit using RBI and RMI

Relative Bearing Indicator (RBI)

  • The Relative Bearing Indicator (RBI) displays the position of the NDB relative to the aircraft’s longitudinal axis.
  • QDM is the magnetic bearing to the station.
  • QDM = Aircraft Heading + Relative Bearing.
  • If the total exceeds 360°, subtract 360°.
  • QDR is the magnetic bearing from the station.
  • If QDM is less than 180°, add 180°.
  • If QDM is greater than 180°, subtract 180°.
  • A movable compass card allows the pilot to manually set aircraft heading.
  • The head of the needle indicates QDM, while the tail indicates QDR.

Radio Magnetic Indicator (RMI)

  • The Radio Magnetic Indicator (RMI) displays magnetic bearings directly on a rotating compass card.
  • The rotating compass card continuously shows the aircraft heading.
  • The head of the needle indicates QDM.
  • The tail of the needle indicates QDR.
  • Many RMIs contain two independent needles for two NDBs, two VORs, or one of each.
  • True bearings can also be obtained after applying variation and deviation corrections.

Inherent Errors of ADF

Terrain Effect

  • Terrain effect is caused by reflected radio waves from hills and mountains.
  • Flying at higher altitudes in mountainous terrain reduces terrain effect.

Static Interference

  • Static interference is caused by electrical discharges from thunderstorms and charged clouds.
  • ADF indications should be treated with caution during thunderstorm activity.

Quadrantal Error

  • Quadrantal error is caused by distortion of radio waves by the aircraft structure.
  • The greatest error occurs on quadrantal headings.
  • Modern electronic compensation systems significantly reduce quadrantal error.

Cone of Silence

  • The cone of silence is located directly above the NDB where the ADF needle fluctuates rapidly and becomes unreliable.

Dip Error

  • Dip error causes the ADF needle to indicate toward the side to which the loop antenna is tilted.

Coastal Refraction

  • Coastal refraction occurs when radio waves cross the boundary between land and sea.
  • Radio waves travel faster over seawater than over land.
  • The waves bend toward the slower medium (land).
  • The effect decreases as frequency increases.
  • NDBs located near coastlines generally operate on higher frequencies to minimize this error.
  • The angle at which the coastline is crossed also affects the magnitude of the error.
  • Minimum coastal refraction occurs when crossing the coastline at right angles.

Night Effect

  • Night effect reduces the accuracy of ADF bearings after sunset.
  • During daylight, the D-layer absorbs most sky waves.
  • After sunset, the D-layer disappears, allowing sky waves to return to Earth.
  • The returned sky waves interfere with the surface waves from the same NDB.
  • Sky waves usually arrive out of phase and with different polarization.
  • Horizontally polarised sky waves induce unwanted currents in the loop antenna.
  • ADF indications often wander during dawn and dusk.

Station Interference

  • Station interference occurs when two or more NDBs operate on similar frequencies.
  • Designated Operational Coverage (DOC) ensures accurate bearings during daytime operations.
  • DOC is achieved through careful frequency allocation.
  • DOC limitations do not apply at night because of night effect.
  • ADF systems provide no automatic failure warning.
  • Positive identification of the NDB using its Morse code is essential before navigation.

Range and Accuracy of ADF

  • ADF range depends on both transmitter characteristics and environmental conditions.
  • Transmitter power, operating frequency, and emission type affect the usable range.
  • Mountainous terrain and precipitation reduce the effective operating range.
  • High-quality receivers improve reception and usable range.
  • A minimum signal-to-noise ratio of 3:1 is required for reliable ADF operation.
  • Within the Designated Operational Coverage (DOC), daytime bearing accuracy is typically about ±5°.