ELT Homing - Tbaytelmy.tbaytel.net/noasara/docs/elthoming_ver2.pdf · What is an ELT? •Low power...

22/06/2010 CASARA Ottawa

ELT Homing

Air and Ground Search

Prepared By

Terry Cooper

Visual Search 1• 500 AGL: Where’s the crash?

Visual Search 2• How about now? (200 AGL)

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).

Typical ELT Location

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

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

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

Radio Basics

Radio Direction Finding

• Two methods of RDF

– Triangulation (signal strength, bearing)

– Homing (signal strength and direction)

Tools of the Trade

Little L-Per

Handheld H Antenna

Vehicle Mount Quad

Magnetic Vehicle mount antennas

Fore/Aft

Left/Right

A/C External Antennas

Temporary Strut mount installation

Permanent Belly mount installation

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

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

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

CENTRE

CENTRE!

CENTRE

CENTRE!

CENTRE

CENTRE!

Triangulation

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

Using the Quad

LeftFore

Target in Front

LeftFore

Target Beside

LeftFore

Target in Back

Using the Quad

LeftFore

Target Left

LeftFore

Target Front or back

LeftFore

Target Right

Boxing the target (Quad)

Switch in Fore/Aft: needle right

Switch in Right/Left: Needle Left

Leave switch in Fore/Aft

Switch in Fore/Aft: needle centres

Mark map

Find a road to left

Mark map

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.

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

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

Boxing the target (air)

Signal strength increasing,

Needle moderately sensitive

Fly tracks with needle centred at all times

Signal strength very high,

Needle very sensitive

Brief turns can be used

to confirm station passage.

Signal strength decreasing,

Needle less sensitive

Probable target area

Fly second track perpendicular to first starting

roughly in the centre of probable target area.

Keep the needle centred

Intersection of tracks is target location

Keep the needle centred

Call in the ground team.

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.

Aural Null

Procedure A Procedure B

•Using aircraft communications radio

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.

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.

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

Reflection Errors

Hangar/Crash Effects

Hawksbury East(Actual Search)

Runway, Grass

Paved apron

Metal open front hangars

Large Mtce hangar

Canyon Effect

Forest Effects

Hills/Mountains

Signal Polarization

Airborne False Targets

Airborne Reflections

Information Sources

• www.ltronics.com (Maker of Little L-Per)

• www.CAP-ES.net (Scott E Lanis)

• Practice and Experience

ELT Homing - Tbaytelmy.tbaytel.net/noasara/docs/elthoming_ver2.pdf · What is an ELT? •Low power...

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