NDB & ADF

Nondirectional Radio Beacon (NDB) Intro

Ground-based radio transmitter

• NDB transmits radio energy in all directions.

Automatic direction finder (ADF) determines the bearing.

– From the aircraft to the transmitting NDB station.

– The needle points to the NDB ground station.

Nondirectional Radio Beacon (NDB) Summary

• Capable of voice transmission.

• Often used for transmitting the Automated Weather Observing System (AWOS).

The aircraft must be in operational range of the station.

Nondirectional Radio Beacon (NDB) - How to identify the station

– Identify the station by listening to Morse code before relying on ADF.

– Usually two letters or an alpha-numeric combination

Automatic Direction Finder (ADF)

The airborne equipment includes two antennas

– Receiver

– Indicator instrument

Automatic Direction Finder (ADF) - Two antenna types

– “Sense” antenna (non-directional)

• Receives signals with nearly equal efficiency from all directions.

– “Loop” antenna (bidirectional)

• Receives signals better from two directions.

ADF Types

• Fixed card

• Rotatable compass-card

• Radio magnetic indicator (RMI)

– One needle

– Dual needle

Magnetic Bearing

– The direction TO or FROM a radio transmitting station measured relative to magnetic north.

Magnetic Heading

– The direction the aircraft is pointed with respect to magnetic north.

Relative Bearing

– The number of degrees measured clockwise, required to point directly at the station

ADF Types - Fixed card

– Also known as the relative bearing indicator (RBI).

– Always indicates zero at the top of the instrument.

– The needle indicates the relative bearing (RB) to the station.

• MH + RB = MB

ADF Types - Rotatable compass-card

– Allows the pilot to rotate the aircraft’s present heading.

– The head of the needle indicates MB to the station.

– The tail indicates MB from the station.

• MH + RB = MB

ADF Types - • Radio magnetic indicator (RMI)

– Automatically rotates the azimuth card to represent aircraft heading

• Remotely controlled by a gyrocompass

– The RMI has two needles

• Can be used to indicate information from either the ADF or the VOR receiver.

• Tail indicates the radial.

When a needle is being driven by the ADF

• The head of the needle indicates the MB TO the station tuned on the ADF receiver.

• The tail of the needle is the bearing FROM the station

When a needle is being driven by the VOR

• The head of the needle points the bearing TO the station.

• The tail of the needle points to the radial of the VOR the aircraft is currently on or crossing.

Functions of ADF

– Plot your position,

– Track inbound and outbound,

– Intercept a bearing, and

– Execute holding patterns, non-precision instrument approaches.

ADF Homing

Flying the aircraft on any heading required to keep the needle pointing directly to the 0° RB position.

ADF Homing Procedure

– Tune the station,

– Identify the Morse code signal, &

– Turn the aircraft to bring the ADF azimuth needle to the 0° RB position.

– Turns should be made using the heading indicator.

– Make small corrections as necessary by checking the ADF needle.

ADF Tracking

– Flying the aircraft to maintain a constant MB to or from the station.

– Tracking uses a heading that maintains the desired track to or from the station regardless of crosswind conditions.

ADF Tracking Procedure

– Turn to the heading that produces a zero RB,

– Maintain this heading until off-course drift is indicated by the needle,

– When there is a definite (2° to 5°) change in needle reading, turn in the direction of needle deflection to intercept the initial MB, &

– Fly the heading that maintains the desired track to or from the station

ADF Errors and Limitations

• Susceptible to ground-wave propagation

– May reflect off terrain, such as mountains.

• Will give erroneous indications near electrical disturbances – Lightning

– Precipitation static (P-static)

• Must listen to audio morse code ID when using for navigation

– No “OFF” flag