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Old July 6th 08, 07:59 AM posted to rec.aviation.soaring
Darryl Ramm
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Posts: 2,403
Default SPOT Versus ELT Analysis (Long)


The problem I have with this is it is a false paradox, you don't need
to choose between a PLB/ELT or a SPOT. Especially since a SPOT is
cheap enough (think the cost of a few tows, a long aeroretrieve, a lot
cheaper than annual glider insurance... and so on) let alone the worry
of friends and relatives or the hassle of people trying to find you if
you land out etc.

I've used a spot for about 9 months now, from when they first became
available. There are quite a few people I fly with now who also use a
SPOT, universally the interest in them is for tracking (aka
SPOTcasting). This allows just tracking people out of interest,
facilitating easier aero and ground retrieves and hopefully being able
to find somebody in a crash or similar should the SPOT still be
transmitting after the crash or at least being able to have a good
last know location (and likely some direction intelligence) if the
SPOT is not transmitting after a crash.

I've also flown with a PLB on my parachute harness for several years.
My sentiment is that if I'm in serious trouble then I do want a full
PLB with 121.5 MHz homing beacon etc. for local SAR resources to find
me with. Also since for best GPS reception and GlobalStar transmission
the SPOT unit is sitting on the canopy rail on my glider and I have
the PLB on my parachute harness so it goes with me in case of a bail
out.

But anyhow a couple of corrections to the original post -

ELTs and PLBs transmit a homing beacon on 121.5 MHz (not 121.6 which I
assume was a typo - although you'll see 121.6 MHz mentioned as a test
frequency by ELT/PLB manufactures their distress beacon is always on
121.5Mhz).

Comparing internal antennas on SPOT vs. external anennas on PLBs or
ELTs is nonsensical. The beacon anennas on PLBs and ELTs have to be
large becasue of the 121.5MHz beacon frequency. All current PLBs with
GPS use a planar GPS antenna (GPS is ~1.5 GHz), SPOT uses a similar
GPS antenna. The GloabalStar L-band antenna in SPOT operates on a
similar frequency to GPS and uses the same planar antenna technology.
The antenna are different because they operate on significantly
different frequencies. PLBs and SPOT use planar and not helical coil
antennas (which would also work at these frequencies) for various
technical reasons and practical ones including compactness and
robustness.

The 1 year life for SPOT batteries vs. a shelf life for PLB batteries
also is not a valid comparison. SPOT claims a ”Powered On unused”
battery life of one year. That means the unit is turned on by the user
but not sending any messages. SPOT claims that batteries installed but
with the device turned off the batteries will “last several
years” (see the SPOT User's Manual) . The Energizer e2 Lithium (non-
rechargeable Li-FeS2) batteries SPOT uses have shelf lives of more
than a decade and are the same basic chemistry as the batteries used
in the best PLBs and ELTs. PLB battery packs however are larger than
SPOT because a PLB draws more power than SPOT does in any of it's
operational modes. The sealing and construction of battery packs for
PLBs is more impressive than for a SPOT messenger, but the battery
compartment seals and construction on SPOT appear more than adequate
for our uses. SPOT messengers seem to give ample warning of battery
exhaustion, and it is easy to carry a spare set of batteries and
replace them if needed (tool less access to a well built, sealed
battery compartment). I'd actually be more worried about PLB
batteries because people don't test their PLB, so how confident should
they be that the actual device really will work if needed and the
batteries really are OK?). Bottom line is that batteries and SPOT just
do not seem to be an issue.

Not all 406MHz ELTs or PLBs transmit a GPS location. If I was buying a
PLB today I'd only look for one with a GPS and I'd look for one with
the longest battery life (I care about the built in 121.5 MHz homing
beacon lasting as long as possible so the local SAR organization can
home my location if needed). Effectively all current 406 MHz ELTs that
you would install in a glider do not encode a GPS location.

---------------------

While the SPOT service may be a black box in terms of how things
operate, ELTs and PLBs are not themselves magical devices and knowing
how they work in some detail might help in emergencies, so at the risk
of drifting off topic and getting boringly long...

A common misconception about 406MHz devices is they just transmit on
406Mhz - all of these also transmit a 121.5MHz homing beacon. PLBs
typically put out a less powerful 121.5Mhz beacon than an ELT and (in
the USA) also encode a morse code "P" (dit dah dah dit) about every 50
seconds on the beacon to signify it is a PLB. So if you suspect
somebody is missing and may have activated a PLB listen for their
121.5Mhz beacon just like you would with an ELT. This is also the
reason that 406 MHz ELTs and PLBs have long antennas - they are sized
for the 121.5 Mhz signal (some high end ELTs likely intended more for
corporate/jet aircraft do have separate 406 MHz and 121.5 MHz
antennas).

121.5 MHz ELT beacons can be Doppler located by low earth orbiting
(LEOSAR) satellites within the COSPAS-SARSAT network. It takes several
satellite passes to determine the approximate position and due to how
this is done at 121.5Mhz and the poor frequency stability specs or
these ELTS you end up with something like typically 4 hours needed for
a final position estimate and a position uncertainty of something like
up to 15 nautical miles (over 100k acres to search). It is this COSPAS-
SARSAT monitoring of 121.5MHz beacons that is going away in Febuary
2009. But airlines will still monitor 121.5 MHz, SAR organizatons like
CAP and the Coast Guard will still be able to DF/home on them etc.

The 406 MHz ELT/PLB beacons transmit a much more stable signal, and
the signals are handled by the LEOSAR satellites differently and a fix
can be determined within about an hour to within a few miles
uncertainty, say typically less than a 1,000 acres to search. The 406
Mhz signal is transmitted in pulses roughly every 100 seconds, the
higher power during the pulse, the improved frequency stability specs
and different signal handling in the satellites allow this faster and
more precise Doppler location.

Now if the 406 MHz ELT (most glider ones won't today) or PLB (try to
buy one with GPS) also optionally encode GPS coordinates in the 406
MHz signal then the geostationary (GEOSAR) COSPAS-SARSAT satellites
can relay the location very accurately within a few minutes of
activating the ELT/PLB - it may take several minutes (up to 10 minutes
on older units) for the GPS unit in the PLB/ELT to obtain a cold GPS
fix.

The other thing the GEOSAR satellites always do is pick up a 406 MHz
ELT/PLB signal almost instantly and that allows the coordination/SAR
organizations to identity through the digital serial number who the
beacon is registered to. The SAR coordinators can attempt to contact
the owner or emergency contacts and quickly try to determine if this
is a real emergency or not. That's a big problem with 121.5 MHz ELTs,
there are just a huge number of false alarms. BTW there is a
significant fraction fo false alarms with 406MHz beacons, I don't have
the numbers handy, but the ability to identify the owner an contact
them helps quickly resolve those. Many of those false alarms have
historically come from 406 MHz marine beacons accidentally triggered
during testing/inspection (I believe due to having to remove units
from their shipboard mounts to read labels and thereby accidentally
activating it - a pretty amusing design mistake).

If the GPS enabled ELTs or PLBs can't get a GPS fix then they revert
and behave like a non-GPS unit. Just let it sit there and be Doppler
located in case there is a problem with the GPS (know what any GPS
status LEDs on the device mean), also you can verify the 121.5Mhz
beacon is working by listening with the glider VHF radio (or the
backup handheld you carry in your survival kit). The GPS enabled ELTs
and PLBs are not designed to provide continuous position tracking, in
fact they specifically avoid this for signal qualiy reasons so thigns
like obtain a location fix and continue to transmit that location for
a while. The exact behavior will depend on what spec the device is
complaint with but it may fix obtain a GPS fix and transmit that
location in every 406 Mhz pulse for 20 minutes or so or longer. So you
could imagine corner cases where this could create some problems (like
ideas people might have for trying to trip a PLB in flight if they
think they are in trouble then managing to flying some way before
crashing...) - but that hopefully should be rare wacky stuff and
ultimately that last location stuff can often be helped by SAR using
the 121.5Mhz homing beacon.

BTW a SPOT messenger that can't receive a GPS signal will transmit the
unit ID information and "911" or "Help" message type (but apparently
not an OK or SPOTcast message?) without the GPS coordinates and so the
same thing can apply there with people at least able to try to contact
the messenger owner or emergency contacts. Like many PLBs the SPOT
messenger also shows via LEDs wether it has a GPS fix or not.

----------------

SPOT is a tremendous tool for tracking gliders, it is the only
practical technology that I can think of that can do this. Like all
technology it is not goign to wakr always, but my experience with it
has been very impressive. It provides "I'm OK" or "I've landed out and
need a retrieve" type messages. The only other technology that has
anything as broad coverage and reliable as this would be an Iridium
satelite phone where you can call your crew once on the ground
(practically impossible in flight becasue of the large fold out
antenna--unless you have an external Iridium antenna installed on the
glider). There is a big piece of mind as well in flying with a SPOT
with tracking and knowing people have been able to watch your tracks –
which is my experience so far. Yes there are times when people have
seen problems with this - it would be useful to know if this was
really just the shared pages, whether it affects the private account
pages as well or whether position reports were really never received.

SPOT is so convenient for tracking for lots of reasons and is such low
cost that people should just get one, if you also fly in desolate
areas (most pilots I know do), fly by yourself a lot, etc. want a back-
up for real emergencies then I would also carry a PLB on your
parachute harness as well. If I have no interest in tracking or non-
emergency retrieve messages etc. and only wanted a distress beacon
then I'd buy a PLB - but I don't know of any glider pilot who want a
SPOT to use just as a distress beacon (i.e. in "911" mode).

Darryl