Martin:

I did a serious investigation on the problem involving some
experiments and analysis and I made a presentation to my local club.
My presentation is available for download from my Wing Rigger website:
www.wingrigger.com. (click Extra Soaring Content in lower right
corner).

Here is the bottom line: the radio link between Spot and the Global
Star network is a bit fussy. There are some 40 Global Star satellites
in low earth orbit. Statistically 2 or 3 satellites are likely to be
in view but sometimes just one. When you consider the geometry of
the low earth orbit and the statistical distribution of the satellites
it will almost always be the case that the satellites are very near to
the horizon. So what matters is that Spot has a line of site view to
the horizon. Since you don’t know exactly where on the horizon the
Global Star will be, the only sure thing is to have a clear view of
the entire horizon. This is often rather difficult. My experiments
proved that when you do have a 360 degree view of the horizon Spot
works perfectly. So part of the answer relates to the mounting
location in your glider. What I found works best is a location
behind my headrest high against the underside of the canopy. I have a
picture of this mounting in the mentioned presentation.

There is more to the story. Even with optimal mounting you will miss
some Spot transmissions because gliders spend a portion of their time
in banked turns. When the glider is banked, a certain percentage of
the horizon becomes blocked by the glider’s wings, fuselage etc. If
there aren’t any satellites in the unblocked portion of the horizon,
your message won’t get out. I studied this effect by carefully time
correlating the exact times at which Spot succeeded or failed in its
transmissions to the time stamped record created by an IGC file that I
examined in See You. I did this for two long flights. With my
“optimal mounting” I miss about 20% of transmissions while banked and
I miss only 2% when in substantially level flight. So my Spot
tracking performance is pretty good, but certainly not perfect.

One other consideration relates to the performance of the patch
antenna that is used in the Spot device. The antenna preferentially
transmits to one hemisphere. So the unit works best when the unit is
mounted flat – label side up. Any other mounting angle reduces your
chance of linking to a satellite on the horizon. I’m sure this also
comes into play when the glider is in a banked turn.

Steve Koerner (GW)