On Wed, 30 Jul 2008 09:59:05 -0400, jeff wrote:

Jonas Grumby wrote:
"Larry" wrote in message
...
jeff wrote in :

So my questions a does lightening often cause this problem? Is it
more likely that a bit of water on the unused external antenna connector
was the real problem? Is it likely my unit is defective? Should I just
assume that I'll always lose GPS at the worst moment? (Yes, I know the
answer to that one!)
...
The system link budget was designed to overcome rain fade. Next time try a
handheld GPS along with your boat mounted GPS and see if they both fade out.
Larry does not account for scattering by the rain droplets, 99% of the GPS
signal is reflected/scattered from rain drops. There is very little
absorption; it's only about 1.5 GHz and the raindrops are very small
compared to a wavelength.

Where is your antenna? Is it under something that can build up some sitting
water during a rain storm?

Is your antenna dc grounded at the center conductor? Charge buildup can
cause front end problems.



The unit has a built-in antenna. It sits under a hardtop with a plastic
window that could accumulate some standing water, but it was not a
torrential downpour (which we were in a few weeks ago without loss) and
the wind would have blown away any large puddles. However, this is
still a possibility. In normal use the GPS locks onto 7 or 8
satellites, so one puddle shouldn't consistently kill it, though there
could have have been enough moisture in the air to attenuate the signals.




It is better to have your GPS antenna mounted where it has an
unobstructed view of the sky. A popular place on sailboats is the top
rail of the stern pulpit where it is out of the way.

We have never lost GPS tracking due to heavy rain on any of our boats.

Heavy rain could push the total fade over the edge but there is probably
something else adding to the problem.

http://www.gigabeam.com/images/attenuationChart_02.gif

From that graph, the maximum rain fade you could hope for is less than 10
dB, still plenty of link margin.

Would the military guide atomic weapons with a system that fades out in
rain?