ELT Radio Basics

Module 2Of

Casara ELT Training Plan

Radio Basics

11/03/2008

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

• 1………….WEAK• 2• 3• 4• 5• 6• 7• 8• 9…………STRONG

•Signal strength decreases with distance from the transmitter.

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.

11/03/2008

Radio Basics

Direction Finding in the real world(stuff that can fool you)

• 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

Hill Shadows

No Signal

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?

““THAT OTHERS MAY LIVE”THAT OTHERS MAY LIVE”

THANK YOUTHANK YOU

CA

SA

RA

CA

SA

RA