NDB and ADF in Aviation

The NDB is a ground-based radio aid that transmits an omnidirectional signal. The ADF is the airborne instrument that receives this signal and determines the direction to the station.

Operating Principles

• Frequencies: Operates in the Low Frequency (LF) and Medium Frequency (MF) bands, typically between 190 kHz and 1750 kHz.
• Modulation:
  • N0N: Unmodulated carrier (basic signal).
  • A1A: Morse code identification by carrier interruption (requires activating the BFO - Beat Frequency Oscillator on the ADF to be audible).
  • A2A: Amplitude modulated identification (directly audible, does not require BFO).
• ADF Antennas: Uses two antennas:
  • Loop Antenna: Determines direction but with 180° ambiguity.
  • Sense Antenna: Resolves the ambiguity to indicate if the station is in front or behind.

Instruments and Indications

• RBI (Relative Bearing Indicator): Displays the Relative Bearing (RB), which is the angle between the aircraft's nose and the station. The card is fixed (North is always up).
  • Formula: $QDM = Magnetic Heading (MH) + Relative Bearing (RB)$.
• RMI (Radio Magnetic Indicator): Combines a directional gyro with the ADF needle. The card rotates automatically to show the aircraft's magnetic heading at the top.
  • Needle Head: Indicates QDM (Magnetic Heading TO the station).
  • Needle Tail: Indicates QDR (Magnetic Bearing FROM the station).
• Accuracy: Typical ADF accuracy during the day is $\pm 5^\circ$.

ADF System Errors

The ADF is susceptible to numerous errors due to LF/MF wave propagation:

1. Night Effect: At night, the ionosphere's D layer disappears, allowing sky waves to interfere with ground waves. This causes fading and needle oscillation. It is worse at sunrise and sunset.
2. Mountain Effect: Diffraction and reflection of waves in mountainous terrain cause erroneous readings.
3. Coastal Refraction: Waves speed up over water, bending towards the coast when crossing the coastline at an oblique angle.
4. Thunderstorm Effect: Lightning (static discharges) emits powerful signals in LF/MF. The ADF needle may momentarily point towards the center of the storm (Cumulonimbus).
5. Quadrantal Error: Deviation caused by signal refraction on the aircraft's metal fuselage.
6. Dip Error: When turning, the loop antenna tilts, causing reading errors.

Operational Use

• Homing: Flying keeping the ADF needle on the nose (RB 0°). If there is a crosswind, the aircraft will fly a curved path.
• Tracking: Flying a heading corrected for wind to maintain a straight path to or from the station (maintaining a constant QDM or QDR).
• NDB Approaches: Considered non-precision approaches. The pilot must remain within $\pm 5^\circ$ of the published approach course to be considered "established".
• Failure: Unlike VOR or ILS, the NDB has no failure flag. The pilot must continuously monitor the Morse code identification; if it ceases, the station has failed.

Locator Beacons

These are low power NDBs (10-25 NM range) used in terminal approaches, often located at the ILS Outer Marker (forming a LOM - Locator Outer Marker).