SPOT messenger reliability

[Hellman]

Last month, I posted here about a huge gap on my SPOT messenger track,
with an hour and a half between two fixes. My previous "record" was
about 40 minutes, which is why this seemed so strange, especially
since I wasn't banked steeply almost that whole time. (I was flying
from the Bay Area to Lake Tahoe in a motor glider. The soaring started
just as the track picked up again.) Since no one else seemed to have
had such a long drop out, I contacted SPOT and just now got their
response. They sent me a sample 6 hour track which they said was
typical, and it showed numerous 20 minute gaps (consistent with what
I've seen), a few 30 minute gaps (again consistent), but also one 40
minute gaps, and one 50 minute gaps. While still not as long as my 90
minute gap (8 missed transmissions), their sample was worse than I'd
ever seen prior to last month.

The bottom line is that SPOT is not claiming (at least when pressed
this way) high reliability. Even 10 minutes between fixes is a bit
long for aviation use, and these gaps make SPOT even worse. I'm not
suggesting getting rid of SPOT, but I'd guess it's about 50% effective
in helping SAR find a downed plane. Better than nothing, but far from
what I'd like. It makes me feel a bit less compulsive about also
carrying a PLB strapped to my parachute. (The SPOT is mounted in an
indent in my glare shield.)

Martin

[Darryl Ramm]

On Aug 5, 4:53*pm, Hellman > wrote:
> Last month, I posted here about a huge gap on my SPOT messenger track,
> with an hour and a half between two fixes. My previous "record" was
> about 40 minutes, which is why this seemed so strange, especially
> since I wasn't banked steeply almost that whole time. (I was flying
> from the Bay Area to Lake Tahoe in a motor glider. The soaring started
> just as the track picked up again.) Since no one else seemed to have
> had such a long drop out, I contacted SPOT and just now got their
> response. They sent me a sample 6 hour track which they said was
> typical, and it showed numerous 20 minute gaps (consistent with what
> I've seen), a few 30 minute gaps (again consistent), but also one 40
> minute gaps, and one 50 minute gaps. While still not as long as my 90
> minute gap (8 missed transmissions), their sample was worse than I'd
> ever seen prior to last month.
>
> The bottom line is that SPOT is not claiming (at least when pressed
> this way) high reliability. Even 10 minutes between fixes is a bit
> long for aviation use, and these gaps make SPOT even worse. I'm not
> suggesting getting rid of SPOT, but I'd guess it's about 50% effective
> in helping SAR find a downed plane. Better than nothing, but far from
> what I'd like. It makes me feel a bit less compulsive about also
> carrying a PLB strapped to my parachute. (The SPOT is mounted in an
> indent in my glare shield.)
>
> Martin

Martin

What type of aircraft (e.g. high wing?) was there track from? and
where was there SPOT messenger mounted? Could SPOT confirm there were
no outages or satelite gaps during your gap?

I normally see less than 10 percent loss almost all single points.
Obviously this
inceases with the percent of thermalling. I have compared missing
points to flight recorder traces and can often see suspect thermalling
that would explain missing gaps (you have to bracket for the ~1
minute? message send dither time).

The new messenger resends the last two fixes with each new one. So
will show movement between fixes so helps a bit but can also be looked
at as largely a cosmetic fix.

I like SPOT but I carry my PLB on my parachute harness.

Darryl

[Dean[_2_]]

Martin,
I have had the same experience with SPOT. Flew 3 hours yesterday
with about half the fixes 10 minutes apart and the other half 20
minutes. On other flights, and while hiking, I have had 30+ minute
time gaps. I wonder if the service is just oversubscribed and the
reliability has nothing to do with antenna placement. Thanks for
contacting SPOT and sharing their response with us.
Just one more piece of technology that we should not rely too
heavily upon. Better than nothing I suppose, but not quite what is
advertised.

Dean

[Hellman]

On Aug 5, 5:48*pm, Darryl Ramm > wrote:
> What type of aircraft (e.g. high wing?) was there track from? and
> where was there SPOT messenger mounted? Could SPOT confirm there were
> no outages or satelite gaps during your gap?

Darryl,

I don't think their track was from an aircraft and suspect that it was
not a real track, but a demo to show what you could expect. But I
have no idea how they came up with it.

Martin

[ContestID67[_2_]]

While I am a PLB proponent*, the SPOT is interesting. In an emergency
there are two different scenarios;

1) An event in which I can push the "HELP" button. The PLB and the
SPOT are basically equal* as they both transmit the exact coordinates
of where you are.

2) An event in which I can't push the "HELP" button. The PLB is
useless in this case. The SPOT at least gives someone some kind of
fix to rely upon, even if it is old. But let's calculate the search
area. If we assume that there is a fix every 10 minutes (an average
of 5 mins from the last fix) and going say 50mph average then I have
traveled (50mph / 60min/hr X 5 mins) 4 miles which gives a search area
of 54 sq miles (360 degree track possibility). Darn big search area.
We can narrow that big number down a bit if there are two SPOT points
giving an intended track so the search area if more like a cone.
Still a big area to search but at least there *IS* an area to be
searched.

People like the SPOT because of the price point (free in some cases),
that they get a "continous" I AM HERE! tracking signal and, of course,
the emergency alert function. Maybe the SPOT gives a false sense of
security with the large search area. Maybe the SPOT has an edge over
the PLB in scenario #2 even if it is tenuous.

What might be the best "get me out of a jam" option is a 406Mhz with
GPS ELT and 121.5 homing signal as it will fix problem #1 and #2 (if
it trips). Unfortunately they are just too expensive as yet. What
drives the cheapness and innovation of the SPOT (over the PLB and ELT)
is the money coming in from subscriptions - which also makes it
expansive in the long run. If the FAA mandates 406Mhz ELTs this might
drive the price of them down also as more players enter the market.

See http://aviation.derosaweb.com/presentations for a PLB/ELT/SPOT
presentation I gave to the ChicagoLand Glider Council in 2008. Needs
some updating.

My $0.02.

- John DeRosa

* I err on the side of relying on the government to bail me out versus
a commercial operation...who then contacts the government.

[ContestID67[_2_]]

Oops. Bad link.

http://aviation.derosaweb.net/presentations

[Eric Greenwell]

ContestID67 wrote:
> While I am a PLB proponent*, the SPOT is interesting. In an emergency
> there are two different scenarios;
>
> 1) An event in which I can push the "HELP" button. The PLB and the
> SPOT are basically equal* as they both transmit the exact coordinates
> of where you are.
>
> 2) An event in which I can't push the "HELP" button. The PLB is
> useless in this case. The SPOT at least gives someone some kind of
> fix to rely upon, even if it is old. But let's calculate the search
> area. If we assume that there is a fix every 10 minutes (an average
> of 5 mins from the last fix) and going say 50mph average then I have
> traveled (50mph / 60min/hr X 5 mins) 4 miles which gives a search area
> of 54 sq miles (360 degree track possibility). Darn big search area.

Can some experienced search and rescue people tell us if 54 square miles
is actually a large area to search? That's not even 8 x 8 miles. My
feeling is it's very small compared to the alternative: an approximately
120 by 130 mile rectangle equal to 15,600 square miles.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org