On May 14, 11:34*pm, Darryl Ramm wrote:
On May 14, 4:37 pm, "Matt Herron Jr." wrote:



On May 14, 9:13 am, wrote:
*snip

I also suggested to the company that they offer an aviation tracking
option that reports every minute instead of every 10 minutes, but was
told the unit takes 4 minutes to get each GPS fix. I guess that's how
it last so long on a set of batteries. I then suggested they offer an
every 4 minute tracking option and was told they will think about it.

Makes me wonder what is going on during that 4 minutes. are there
inaccuracies introduced by the fact that we are moving faster than a
hiker? *how about the fact that we change altitude over that 4
minutes? *Is it averaging a bunch of signals? *Is it gathering
components of a single fix? *Maybe it gets an instant fix, and then
waits for an opening to upload to the satellite? *Has anyone ever
compared the spot time stamped location to an IGC trace temporally? In
other words, even though there is a lag in getting the fix from spot,
I wonder if the time it puts on the fix is accurate. *If you flew over
some point of interest and pushed the OK button to mark it would you
get the right position 20 minutes later, or some random in between
position n minutes later?

Matt

You are making assumptions from what Martin said. In track mode I
suspect it is sleeping, wakes up the GPS and then sends the location/
time (and it tries to resend it later as well, since it is simplex it
never knows it the message goes out). But it knows exactly the time
when it was exactly at some coordinate and tries to send that exact
info, possibly delayed up to ~10 mins. Who know what exactly it does
when you press OK, Help or 911. if I built it, it would wake up the
GPS and grab a new fix and send that location - why would you build it
any different? The very modern GPS chipset used *definitly should not
take 4 mins to acquire a (warm) fix and I'd eat my hat if they have to
do a cold fix each time.

The SPOT does not "wait for an opening" to talk to Globalstar, it has
no idea where the Globalstar satelites are, it just sends blind, then
retransmits the same message later. It's pure simplex, there is no
"carrier detect", no handshake, no ACK. Nothing. *The Globalstar
satellites are just dumb one-way bent pipe repeaters. Sometime being
simple is a beautiful thing.

And to Martin's earlier comment, OK etc. does not always take 20
minutes to send, it might take up to that (or may never successfully
send) but you can see that messages often go out quicker than this.
I'm missing the point of the argument used, I use tracking becasue it
is convenient and I don't want to press buttons, not because I think a
track position report is more likely to get out than an OK report.

BTW kind of a throw away comment but the circle of uncertainity for a
10 minute fix spot at reasonable fast cruise speed is about the same
order as for a SARSAT doppler located plain old 121.5 MHz ELT. (which
SARSAT service goes away next year) Of course if SAR is activated and
they can home your ELT then they can find you (or your corpse). If
SPOT survives they also find you or the corpse. The big issue with an
ELT is they just don't activate on impact frequently enough and nobody
know what the stats are for gliders ( ~12 percent for GA aircraft). At
least with a SPOT in track mode you are not relying on impact
activation and not at the mercy of the unit not being damaged and the
antennas able to see the sky after impact. If SPOT simply halved the
10 minute location transmit times (which I've also heard from other
sources they are looking at doing) the are of uncertainity drops a
lot. The other thing is I suspect a lot more crashes/landouts happen
when gliders are not cruising at high speed but scratching around low
in weak lift or close in on ridges etc. In those cases the area of
uncrtainty will be smaller. Previous flight trace fixes will often
allow some guessing by SAR groups on the indended flight path, speed,
etc. and that will often reduce the area of uncertainity as well.

Executive summary: go buy yourself a SPOT messenger!

Cheers

Darryl

Good summary Darryl, thanks. I agree with your search area opinions
as well. I assume it makes more sense to have the spot attached to
the chute rather than the plane for pretty much all situations.

Matt