ELT Radio Basics
Module 2
Of
Casara ELT Training Plan
Radio Basics
• Radio waves travel
somewhat like light.
• Direct, Reflected,
Absorbed, Refracted.
• Slowed and attenuated
by ground effect.
• Horizontal or Vertical
polarization.
ELT Signal Strength
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•
•
•
•
•
•
•
•
1………….WEAK
•Signal strength decreases with
2
distance from the transmitter.
3
4
5
6
7
8
9…………STRONG
Radio Basics
• Ground effect causes signal to
slow and decrease closer to the
ground (blue area).
• Dead spot presents an aural cone
of silence directly above the end
of the ELT antenna.
• Polarization along axis of
antenna.
• The ELT signal fades away at the
end of the ELT signal range.
• Aviation frequencies are line of
sight and are affected greatly by
environmental factors.
Direction Finding in the real world
(stuff that can fool you)
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•
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DF in open areas
DF in Hills/Mountains/Wooded areas
DF in urban areas
Effects of wreckage
Effects of hangars
Reflection Errors
Radio signals travel and are reflected in the same way as light.
This means that if there is something blocking your view of the
target, but there is something to reflect the signal the reflector will
appear as the source of the signal.
(like the mirror in the top diagram).
Reflection Errors
If the reflector is near the transmitter, the error will not be as great as
compared to when the reflector is nearer to you (receiver).
You may in fact have the source of the signal indicating entirely the
wrong direction, i.e. (you may see the target behind you when in fact
it’s in front of you).
Hangar/Crash Effects
Hangars will block the signal or at least degrade it on any side with a solid wall.
On the other hand, an open hangar door will cause the hangar to act as an amplifier and
will strengthen the signal in the direction of the door.
A similar effect comes from the wreck itself if the ELT antenna is on one side of the
wreck . The signal will be much stronger in that direction than behind the wreck where
the signal is partially blocked.
Canyon Effect
Terrain causing directional effects, much like the hangar in the previous example.
Forest Effects
Trees, and / or heavy brush will tend to block the signal in the line of
sight.
Most of the signal received will be from overhead and will not be
directional.
Signals will line up with a road or power line cut giving very poor
Direction Finding readings, not at all related to the location of the ELT.
Find a larger clearing, more circular if possible.
Hills/Mountains
No direct line of sight.
You may get stronger reflections than the primary signal.
Get to higher location.
Hills / Mountains
No
Signal
Hill Shadows
Weak
Signal
Stronger
Signal
Signal Polarization
Horizontal polarization of the ELT signal will cause the
reflection to be seen as stronger than the real signal.
Some of the reflection will be vertical.
Airborne False Targets
Focusing by terrain can cause false targets, which disappear
when the search aircraft approaches them.
Airborne Reflections
Are there any questions?
CASARA
“THAT OTHERS MAY LIVE”
THANK YOU