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Radio Magnetic Indicator (RMI)
- is a gyro-stabilised ADF which automatically aligns the card, and indicates
magnetic bearings.
RMIT Flight Training ADF Operations
Brief 18
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 3
Considerations:
Limitations
- various limitations effect the readings of the ADF:
a) Coastal Refraction
- when passing from land to sea at an oblique angle, NDB radio waves may
be refracted (bent) due to different conducting and reflecting properties of
land and water. Always refracts towards the land (away from the normal).
b) Co-Channel Interference
- signals received from other NDBs operating on the same frequency, may
give false bearing information. (eg: SHT and BNS) Generally NDBs of
similar frequencies are spaced well apart geographically.
c) Thunderstorm Effect
- thunderstorms generate a lot of radio energy, and the ADF needle may
point towards it; a poor mans weather radar.
d) Terrain Effect
- radio signals have greater range over water than over sandy or
mountainous country where the range is considerably reduced.
Brief 18
RMIT Flight Training ADF Operations
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 4
e) Mountain Effect
- fluctuating bearings may result from the mixing of radio waves reflected
from the surfaces of mountains.
- higher frequencies minimise this effect.
f) Night Effect
- radio waves take two paths; ground waves (along the Earth’s surface) and
sky waves.
- during the day the sky waves are absorbed by the lower ionosphere. At
night the ionosphere reflects the sky wave back, mixing with the ground
wave, causing errors.
g) Quadrantal Error
- results from the metal construction of the aircraft. Needle of ADF will align
itself to the cardinal points of aircraft. (ie: N, E, S, W) when station in
between these quadrants.
Brief 18
RMIT Flight Training ADF Operations
RMIT Instructor Rating Mass Briefs Issue 1 /2004 Page 5
Air Exercise:
T - Tune to NDB using receiver
I - Identify using Morse code
T - Test by depressing TEST switch
Airmanship:
- Always use NDB within it’s rated coverage
- TIT checks before use
- AM stations not to be used for navigation
- Instrument taxi checks to be completed
Brief 18
RMIT Flight Training ADF Operations
ADF Operations
Aim
• To learn the principles and considerations
behind the operation of the ADF
navigation system.
Application
• Supplementary navigation information
• Orientation information (where am I?)
• NVFR rating
• IFR rating
Overview
• Definitions
• Principles
• Considerations
• Air Exercise
• Airmanship
Definitions
• NDB
– Non-Directional Beacon, is the ground radio
transmitter that radiates energy in all directions.
• ADF
– Automatic Direction Finder, is the airborne
receiver which when tuned, indicates the
direction TO the NDB.
• Locator
– Low powered NDB used for approaches in
conjunction with an ILS.
Principles
• NDB Ground Stations
Principles
• NDB
– Operates on frequencies between 200Hz and 500
KHz (LF to MF radio range).
– Primarily short distance navigation aids of usually <
200Nm.
– Transmitter power is between 100 and 3000W with
majority in the 100 to 500W range.
– Greater the transmitter power, the greater the
range.
– Details found in ERSA and ERC charts; VTC, TAC
and VNC.
Extract from ERSA (POINT COOK)
FAC P – 292 Not for operational use
Principles
– The emitted wave has a Morse code identifier
superimposed upon it, of 2 to 3 letters.
– The carrier wave may also contain voice/
ATIS information.
Extract from ERSA (POINT
COOK) FAC P – 292 Not
for operational use
Extract from TAC-2 Not for
operational use
Principles
• ADF
– Airborne equipment consists of three
main components:
1. Antennae
2. Receiver
3. Cockpit display
Principles
• ADF
– Airborne equipment consists of three main
components:
1. Antennae
• loop antenna aligns itself perpendicular to the
direction of the received signal. As it is a loop,
two nulls are received - one removed 180 from
the other.
• To resolve the ambiguity the signal from the
sense antenna is combined, to drive the loop
(and hence the pointer) in the correct direction.
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