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