NDB, ADF and bearing references

What does the ADF needle actually point to?

Start with the reference frame, then add the heading.

Relative and magnetic bearings

An NDB radiates a nondirectional signal; the airborne ADF determines a direction to its source. On a fixed-card indicator, the needle head reads relative bearing clockwise from the aircraft nose. Magnetic bearing to the station, often called QDM, is magnetic heading plus relative bearing, reduced into 000–359°. The reciprocal, QDR, is the magnetic bearing from the station.

Heading 350° and relative bearing 030° give QDM 020° after wrapping through north. QDR is 200°. The aircraft is south-south-west of the station, although the needle is only 30° right of the nose. A bearing is a direction; it does not reveal distance.

Card and pointer

A correctly aligned rotating magnetic card can show magnetic bearing directly. A manually set card must be kept aligned; an RMI uses its heading reference. Understand which heading source drives the display. An error in that reference changes the number read from the card even if the antenna-derived relative direction is unchanged.

Read the same station after a heading change

At one instant, magnetic heading is 020° and relative bearing is 340°. The station is due magnetic north: QDM = 020 + 340 = 360°, normalised to 000°. The reciprocal position line from the station is QDR 180°. If heading changes to 050° with negligible position change, the station’s relative bearing becomes 310° while QDM stays 000°.

This is why adding a relative bearing to track rather than heading introduces an error when drift exists. If the aircraft’s ground track is 030° but heading is 020°, using the track gives a false 010° bearing. The antenna measurement is referenced to the airframe nose. Identify the correct reference before carrying out an otherwise correct addition. Magnetic-reference differences between sources also need attention in precise comparisons.

Why can a stable needle still be wrong?

The absence of an alarm is not proof of a usable NDB bearing.

Antenna system and ambiguity

A directional loop responds differently to signal direction but has a 180° ambiguity. A sense antenna allows the system to distinguish the two possible directions. Installation and operating mode matter: a receiver in a non-directional antenna mode is not providing the same bearing function as ADF mode.

ADF indications can be affected by thunderstorms and electrical disturbance, sky wave, terrain and coastal refraction. The pointer may seek an electrical source rather than the intended beacon. Many traditional ADF installations have no navigation-warning flag that reliably identifies all erroneous indications.

Use corroboration

Monitor the correct identification using the installed procedure and compare the bearing trend with heading, expected geometry and independent navigation. A smooth false indication can be more persuasive than an erratic one. In this laboratory, switching the signal to disturbed labels the bearing unreliable instead of presenting a computed steer command.

Compare airborne and ground direction finding

ADF places the directional receiving equipment in the aircraft and receives an NDB signal. Ground direction finding reverses that arrangement: a ground station determines a direction from the aircraft’s transmission and may provide a bearing according to the service and reference used. These are different measurement chains. Neither should be confused with a range measurement merely because a bearing number is supplied.

Imagine a ground direction-finding bearing broadly agreeing with a suspect ADF needle. That can be useful corroboration only if the sources and references are understood and their limitations considered. A common transmitter-location assumption or poor reception can still affect interpretation. Ask who measured the direction, which end of the line it points from, whether it is true, magnetic or relative, and what accuracy/service applies before plotting it.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.