ADF (Automatic Direction Finder) Basics

The ADF, or Automatic Direction Finder, is a short-range navigational aid used in aviation. It provides aircraft bearing information relative to a ground station called an NDB (Non-Directional Beacon).

The bearing is measured clockwise from the aircraft’s longitudinal axis to the NDB. The reading displayed in the cockpit indicator is called the ADF bearing, which is a relative bearing.

To determine the NDB’s magnetic bearing, we use the following calculation:

NDB Magnetic Bearing (with respect to Aircraft) = Aircraft Magnetic Heading + ADF Relative Bearing

Applications of ADF

ADF has several key applications in flight:

• Track Intercept: Used to get back on track when encountering crosswinds.

• Station Homing: Navigating directly toward or away from an NDB. Be cautious: improper station homing, where the ADF bearing aligns with the aircraft’s longitudinal axis, can result in a curved path due to crosswinds.

  • Correct Station Homing:
    • Estimate the wind drift angle (approximately 5 degrees) to return to the original track.
    • Add an extra angle (around 3 degrees) to compensate for future crosswinds once the track is regained.
    • Correct the aircraft heading to fly at 90 degrees + 5 degrees + 3 degrees = 98 degrees.
    • Path correction is achieved when both the ADF and the heading needles remain constant.

• Triangulation Position Fix: Determining the aircraft’s location using multiple NDBs.

  • Tune the ADF receiver to an NDB.
  • Obtain the ADF bearing.
  • Calculate the NDB magnetic bearing.
  • Calculate the NDB true bearing.
  • Draw a corresponding line on a map.
  • Repeat these steps using another NDB station.
  • The intersection of the lines indicates the aircraft’s 2D position.

ADF Specifications

Ground Station (NDB) Specifications:

• Frequency of operation: 220 to 550 Hz
• NDBs transmit a Morse code identifier for aircraft ADF receivers.

Aircraft ADF Specifications:

• Frequency of operation: MF from 550 to 1750 KHz, covering the commercial AM broadcast band.
• The ADF bearing needle points only to the tuned NDB.
• When flying directly towards an NDB, the ADF bearing pointer indicates 000 degrees.
• When flying directly over an NDB, the ADF bearing pointer indicates 180 degrees.

Advantages

• ADF is not subject to line-of-sight limitations because it operates in the MF band.

Disadvantages

• Ground wave error can be approximately +/- 5 degrees, while sky wave error can be +/- 30 degrees.
• Quadrantal error, caused by radio wave bending due to the aircraft’s metal structure, can reach a maximum of +/- 10 degrees.
• During nighttime, lower ionospheric layers can create strong sky waves, leading to significant errors. It’s best to limit the use of ground stations during these times.
• Terrain, such as mountains and coastal regions, can affect ADF accuracy due to signal reflections.
• Rain, snow, and thunderstorms can interfere with antenna operation and overall ADF system performance.