Automatic Direction Finder (ADF)

Handout

Link to Handout

Aims and Objectives

Aims

• understand the principles of how an ADF works
• understand the principles of ADF operation
• understand the principles of ADF orientation
• understand the principles of track intercepts using the ADF

Aims and Objectives

Objectives

• explain the workflow associated with setting up the ADF
• recall three errors experienced by the ADF
• explain the five ADF navigation aid interception questions

ADF Introduction

ADF Introduction

Radio Waves

• Radio waves are transmitted from an antenna in all directions at the speed of light
• Frequency is a measure of the number of crests per second
• 1 radio wave crest per second is 1 Hertz
  • One thousand Hertz = 1 Kilohertz = 1 kHz
  • One million Hertz = 1 Megahertz = 1 MHz

ADF Introduction

Radio Wave Propagation

• Ground waves are in the frequency range 50-250 kHz
• Ground waves travel through the Earth’s surface
• Sky waves are in the frequency range 3-30 MHz
• Sky waves reflect or refract from the ionosphere and return to the ground
• Space waves are in the frequency range 30-300 MHz
• Space waves propagate through the atmosphere to outer space

ADF Introduction

Equipment

• NDB: Non-Directional Beacon, located on the ground
• Operates in the frequency range 200-500 kHz
• May be pilot-monitored, requiring a pilot report of faults
  • excessive hum
  • low power
  • loss of ident

ADF Introduction

Equipment

• ADF: Automatic Direction Finder, onboard the aircraft
  • Fixed Card ADF: A display of 360° compass rose, unable to adjust
  • Rotating Card ADF: A display of 360° compass rose, able to be adjusted by the pilot (most common)

ADF Introduction

Equipment

• Made up of two antennae
  1. Loop: determines the axis of the radio signal
  2. Sense: determines the direction of the radio signal along that axis
• Located under the fuselage of the aircraft

ADF Introduction

Equipment in the cockpit

• Fixed or Rotating Card ADF
• Tunable ADF Radio Receiver

ADF Introduction

Equipment

• This system is very similar to the AM radio in a car
  • A station transmits AM radio waves (NDB)
  • A AM receiver in the car interprets those radio waves (ADF)
  • Difference:
    • the ADF points to the direction of the source of the radio waves
    • the car radio converts the radio signals to sound waves

ADF Errors

ADF Errors

Summary

• Night effect
• Terrain effect
• Mountain effect
• Thunderstorm activity
• Co-channel Interference
• Coastal refraction
• Quadrantal error

ADF Errors

Night Effect

• The structure of the ionosphere changes at night
• Reflected sky waves arrive later to the receiver than the ground wave
• The result is a reduction in useful range of the NDB at night

ADF Errors

Terrain Effect

• The ground wave travels more efficiently through water
• Over land, the ground wave attenuates
• The result is a greater range of the NDB over water than land

ADF Errors

Mountain Effect

• The sky wave may be reflected by a nearby mountain
• The result is similar to night effect
• Mountain effect can be managed by climbing to a higher altitude

ADF Errors

Thunderstorm Activity

• Lightning transmits radio waves at a similar frequency to the NDB
• The ADF indication swings between the last lightning location and the NDB
• The ADF becomes unusable

ADF Errors

Co-channel Interference

• Interference caused by a station transmitting on the same or similar frequency to the NDB
• The ADF indication will be random

ADF Errors

Coastal Refraction

• When a ground wave crosses the coast at 90°, it continues in the same direction
• When a ground wave crosses the coast otherwise, it bends (refracts) towards the coast line

ADF Errors

Quadrantal Error

• The radio signal is distorted when it encounters the aircraft structure
• The ADF indicates the direction of the signal as it arrives at the antenna
• This effect is most common on bearings which are multiples of 45°

ADF Coverage

ADF Coverage

ADF Rated Coverage

• The rated coverage of each NDB depends on:
  • terrain between aircraft and NDB station
  • day or night
  • transmitter power

ADF Coverage

ADF Broadcast Stations

• Until 2020, broadcasting radio stations and their frequency were published in the ERSA
• The location, identifier, power output and frequency of the station were provided
• A pilot was able to listen to the cricket on ABC by tuning the ADF to the correct frequency
• AirServices no longer publishes information on these stations
• Today they can be found on the ACMA website

Orientation and Tracking

Orientation and Tracking

ADF Setup

T.I.T.

• Tune: set the correct NDB frequency on the ADF
• Identify: listen for the NDB morse identifier
• Test: test the ADF is functioning correctly

Orientation and Tracking

Fixed Card ADF

• Relative Bearing: the direction towards the NDB, relative to your heading
• 0: the nose of the aircraft
• 9: the right wing of the aircraft
• 18: the tail of the aircraft
• 27: the left wing of the aircraft

Orientation and Tracking

Orientation

• Transpose the orientation of the ADF needle on to the Heading Indicator (HI)
• Imagine:
  • centre of HI is the NDB station
  • aircraft is at the base of HI
  • track to NDB is at the head of HI

Orientation and Tracking

Orientation

• To begin orientating ourselves, we can ask five questions
  1. Where am I?
  2. Where do I want to be?
  3. Which way do I turn?
  4. On to which heading?
  5. How do I know I am there?

Orientation and Tracking

Orientation — Where am I?

• on heading 300°
• on 360° relative bearing to station
• track 360° to station

Orientation and Tracking

Orientation — Where am I?

• on heading 135°
• on 225° relative bearing to NDB station
• track 300° to station

Orientation and Tracking

Orientation — Where am I?

• on heading 270°
• on 045° relative bearing to NDB station
• track 315° to station

Orientation and Tracking

Intercept — Where do I want to be?

• Intercept: the heading used to cross the required track to be intercepted
• Intercept angle: the angle between the intercept heading and required track
• For intercept angles, use 30°, 45° or 60° for easy calculation

Orientation and Tracking

Intercept heading — Where do I want to be?

Once on intercept track, adjust for crosswind using the angle/percent rule of thumb

Orientation and Tracking

Intercept heading — crosswind correction

• For example
  • Track 360°
  • Wind: 04015KT
  • Heading: 005°

Orientation and Tracking

ADF Intercept — Inbound to Inbound

Orientation and Tracking

ADF Intercept — Outbound to Outbound

Orientation and Tracking

ADF Intercept — Inbound to Outbound

Orientation and Tracking

ADF Intercept — Outbound to Inbound

Airmanship/TEM/HF

Quiz on Objectives

• What are the three steps to setting up an ADF before using?
• What are three of the possible errors experienced by an ADF?
• What are the five questions to ask for navigation aid interception?