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ELT Homing
Air and Ground Search
Prepared By
Terry Cooper
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Visual Search 1• 500 AGL: Where’s the crash?
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Visual Search 2• How about now? (200 AGL)
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What is an ELT?
• Low power radio transmitter (~50-
75mw, 5W on 406)
• 121.5Mhz, 243Mhz (military) or 406Mhz
• Triggered by impact/shock
• 48 Hour battery life
• External and Built-in antennas
• Typically installed in the tail of an
aircraft (fixed wing).
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Typical ELT Location
ELT
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SARSAT/COSPAS
• Satellites on Polar orbits (LEOS)
• Satellite orbits take 100 minutes
• Monitoring 121.5*, 243*, and 406 MHz
• Primarily designed for 406MHz
• Relay ELT signals to local user terminals (ground stations)
• Doppler shift used to find possible location of the beacon
• 121.5 and 243 monitoring will cease by Feb. 2009
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Signal Confidence
• SARSAT hits are given a confidence
value.
Confidence Accuracy
1 50+ Miles
2 20-50 Miles
3 5-20 Miles
4 0-5 Miles
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Comparing ELT Types
TYPE 121.5 406
Location
Accuracy
12nm 2nm
Coverage Local Global
Signal Power .1W 5W
Alert time 1.5-2.0 Hours Instantaneous
Doppler
Location
2 Passes 1 Pass
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Radio Basics
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Radio Direction Finding
• Two methods of RDF
– Triangulation (signal strength, bearing)
– Homing (signal strength and direction)
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Tools of the Trade
Little L-Per
Handheld H Antenna
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Vehicle Mount Quad
Magnetic Vehicle mount antennas
Fore/Aft
Left/Right
Right
Left
Fore
Aft
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A/C External Antennas
Temporary Strut mount installation
Permanent Belly mount installation
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Little L-Per
How it works• REC Mode
– only used for Handheld
– Antenna forms directional beam (YAGI)
– Left side points to target at highest signal
– Needle indicates signal strength
– Useful for confirming multiple DF centres
• DF Mode– Antennas switched a high rate
– Needle points to side with strongest signal
– Needle centres with target in front or behind
– Functional with very weak signal (you may not hear it)
– Works well with moving vehicle or aircraft
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ELT search basics
• Look for obvious target areas near
SARSAT hit
– airports, heliports, Purolator depots
• Look for high ground
• Take numerous bearings
• Box the target or triangulate
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Handheld RDF
• Use L-Per on DF mode
• Find centres (there should be two)
– More than two centres use REC mode
• Take multiple bearings to the target
• Triangulate with bearings
• Watch for sources of error
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CENTRE
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CENTRE
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CENTRE
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CENTRE
CENTRE!
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CENTRE
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CENTRE
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CENTRE
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CENTRE
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CENTRE
CENTRE!
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CENTRE
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CENTRE
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CENTRE
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CENTRE
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CENTRE
CENTRE!
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Triangulation
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Using the Quad
• Little L-Per is always in DF mode
• While vehicle is moving, average the
indication (it will be swinging)
• Watch for centre or flip
• Keep needle at about half scale by
adjusting sensitivity control
• Box the target
• Always look for sources of error
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Using the Quad
Right
LeftFore
Aft
Target in Front
Right
LeftFore
Aft
Target Beside
Right
LeftFore
Aft
Target in Back
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Using the Quad
Right
LeftFore
Aft
Target Left
Right
LeftFore
Aft
Target Front or back
Right
LeftFore
Aft
Target Right
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Boxing the target (Quad)
Switch in Fore/Aft: needle right
Switch in Right/Left: Needle Left
Leave switch in Fore/Aft
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Boxing the target (Quad)
Switch in Fore/Aft: needle centres
Switch in Right/Left: Needle Left
Mark map
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Boxing the target (Quad)
Switch in Fore/Aft: needle centres
Switch in Right/Left: Needle Left
Find a road to left
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Boxing the target (Quad)
Switch in Fore/Aft: needle centres
Switch in Right/Left: Needle Left
Mark map
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Boxing the target (Quad)
Switch in Fore/Aft: needle centres
Switch in Right/Left: Needle Left
Additional sides reduces error
Get as close as possible with
Vehicle, then proceed on foot with
Handheld antenna
Entire track should be done
Without stopping vehicle for
Heading checks, errors are
Reduced by vehicle motion.
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Closing in
• Reduce L-Per sensitivity
• Fold H antenna
• Use Handheld Radio– Tune off frequency
– Remove antenna
• Use signal strength meter
• Use these methods for airports or Purolator depots where multiple choices of target exist
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Air Search
• RDF Homing
– Use Little L-Per and Strut or belly mount or
Spotter held antennae
– Similar to ground vehicle RDF with Quads
– Turn to centre, hold to track to target
– Fly perpendicular tracks to narrow target
area.
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Boxing the target (air)
Signal strength increasing,
Needle moderately sensitive
Fly tracks with needle centred at all times
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Boxing the target (air)
Signal strength very high,
Needle very sensitive
Brief turns can be used
to confirm station passage.
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Boxing the target (air)
Signal strength decreasing,
Needle less sensitive
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Boxing the target (air)
Probable target area
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Boxing the target (air)
Fly second track perpendicular to first starting
roughly in the centre of probable target area.
Keep the needle centred
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Boxing the target (air)
Intersection of tracks is target location
Keep the needle centred
Call in the ground team.
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Boxing the target (air)Additional tracks will refine target location,
reducing search altitude will also narrow the area.
Some error will still exist due to search altitude,
local terrain, and ELT position.
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Aural Null
Procedure A Procedure B
•Using aircraft communications radio
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Aural Null
• Procedure B is quicker, less flying
• Accurate navigation is required for
either procedure
• Signal propagation errors will affect
results
• Aircraft radio/antennas have a
directional bias.
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Wing Blanking
• Using aircraft comms radio
• Once a signal is picked up:
– Turn 360 degrees
– Note audible drop in signal
• The signal drop indicates that the belly of the
a/c points at the target.
• Repeat the process to triangulate the target.
• Doesn’t work with all antenna installations.
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RDF in the real world(stuff that will fool you)
• RDF in open areas
• RDF in Hills/Mountains/Wooded areas
• RDF in urban areas
• Effects of wreckage
• Effects of hangars
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Reflection Errors
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Hangar/Crash Effects
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Hawksbury East(Actual Search)
Runway, Grass
Trees
Paved apron
Metal open front hangars
Large Mtce hangar
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Canyon Effect
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Forest Effects
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Hills/Mountains
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Signal Polarization
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Airborne False Targets
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Airborne Reflections
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Information Sources
• www.ltronics.com (Maker of Little L-Per)
• www.CAP-ES.net (Scott E Lanis)
• Practice and Experience
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