Bill,
A directional signal to the satellites is not needed. The
omnidirectional signal put out by the ELT is sufficient as it is direct
line-of-site to all satellites above the horizon (unless you are below
the cliff face or down in the deep valley or already down under the
forest trees).
A 406 MHz ELT is pre-registered as being installed in a specific
airplane. When the ELT is triggered by the cockpit panel switch it
immediately (no wait) starts putting out an emergency signal with a
unique ID code. That signal and unique ID is picked up by all the
SARSAT satellites above the horizon. With only the basic squawk, it may
take up to 1-3 hours to determine the fix to locate you within a quarter
to 1-3 miles or so but they will immediately know who you are and what
type of aircraft right down to the paint (as recorded on your
registration form).
If the ELT contains a GPS chip AND keeps working long enough for the GPS
chip to receive signals, initialize and determine its location, then the
outgoing signal begins to contain a report of the exact GPS location
accurate to within a hundred yards or so - this may take from a minute
or two up to 10 or so minutes before the fix is determined and starts to
be included in the report.
(Note: if you have your ELT connected to an always-on GPS, the ELT can
transmit the GPS signal with it's first squirt. One I'm looking at on
my other screen even accepts the NEMA 0183 data format that my old
GARMIN GPSMAP60 (and most older GPS units) can output. My old GPS is
WAAS enabled and has accuracy down to 8-20 feet!
As it is an ELT registered to an aircraft, the action starts at once.
They can check the FAA database to see if you are on a filed flight plan
- if so, no more delay is needed. If there's no flight plan filed, they
may call your home, airport and (maybe) cell phone to see if they can
contact you. But, if you have the hookup to a GPS, they have an instant
pretty good fix and an emergency call. It's not a many hour delay like
there was for the old-style ELTs. For all I know, they may be sending
police (and an ambulance?) to your location within five minutes.
The current switch in the cockpit does activate the ELT and starts
sending out the help call.
I'd like a very big red button I could slap while anything is going on.
The SPOT system mentioned by Perry also has value (even though it does
not satisfy the ELT requirement for airplanes). It can be set to auto
report its location every so many minutes (I think 5) and that location
is recorded with the SPOT company. If you don't close your flight plan,
they have a trace to the last 7-10 miles of your location. If you
didn't file a flight plan and your friends or relatives note that you
didn't come home or arrive at your destination, that record can show
approximately where you went down. If you manage to activate the SPOT
emergency call, their system can immediately start the emergency
process, sending out help to your location.
I'd kind of like to have a SPOT along with a 406 MHz ELT. Or, being
poor, I would like to have at least an old-style ELT (the minimum
required by law) and a PLB (Personal Locater Beacon) (even though the
PLB must be hand triggered and has no impact activator). The PLB could
have most of the immediate location finding effectiveness of the 406 MHz
ELT - especially if you list it on your flight plan and especially if
you have a GPS enabled PLB.
http://en.wikipedia.org/wiki/Distress_radiobeacon#How_they_work
Ed
