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October 25th 04, 10:49 PM

(Mark Zenier) wrote in message ...
In article ,
Tony Calguire wrote:
postal97321 wrote:
...
Ryan Gardner/Gazette-Times
Chris van Rossman's television sent out a distress signal that was
picked up by an orbiting satellite.
...

What exactly are these search and rescue people listening for? Wouldn't
an emergency beacon be sending some kind of intelligent signal, like SOS
in morse code, or some kind of RTTY? At the very least, a certain kind
of modulated tone. It seems to me that if they're going to chase after
every spurious signal that pops up on 121.5, no matter what it sounds
like, that's a recipe for failure. No wonder 90% of their hits are
false positives.

It's some sort of a warble tone on a carrier.

They use a transponder on polar orbiting satellites (weather and earth
sensing ones, usually) that allows the ground stations to do a doppler
measurement. Sort of the reverse of the Transit satellite navigation
system, where the timing and frequency shift of the signal allowed the
ground station to determine its position to a fraction of the meter.
The Transit system used atomic clocks, so the SARSAT accuracy would be
lower. But accurate enough to get the search and rescue close enough to
use portable direction finding gear.

Mark Zenier Washington State resident

I bought an older (circa 1970) 121.5 ELT at a auction several years
ago.
And this unit had no modulation. Simple oscillator, followed by a
trippler,
a driver and a power amp. It had several shock switches in parallel.
This unit was designed to be clamped to the inside frame.
I always wondered just how well it radiate from the inside.
I have heard three ELTs, two hard landings, I live about 6 miles from
Lexington BlueGrass airport in Lexington Kentucky, and one was a
accidental trip in a local neighborhood. All three had a distinctive
"Wail" that is hard to discribe. I keep one scanner tuned to 121.5,
243 and some other hot freqs, and one Saturday morning I heard the
wailing of an ELT, and after 30 minutes, decided to go find it. With
my wife drivign and me RDFing it took us about 15 minutes.
I used a Pr02004 with an adjustable RF antenuator to find the street.
Then used my Pro34, wrapped in aluminium foil to narrow it down. We
listened to the CAP for the next 4 hours until they found it. I did
call the tower and tried to expalain what I had found, but they
thought me a kook.
Maybe the SAR satellite has to accept any signal on 125.5 as valid. I
do know that older ELT did not have moudlation. But I would have
expected the FAA/FCC to require replacement by of all of the older
units by now.
Terry

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