"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!)



No. The overhead cloud full of water is a giant attenuator of RF signals,
especially at the upper UHF and microwave frequencies GPS uses.

The GPS satellite cluster is around 8000-9000 miles away with some
seriously powered, but very distant, transmitters. The signal available
on
a clear day is just a whisper of RF to begin with by the time it makes the
long trip to your position. Your other problem is the size of your
receiving antenna. Everyone wants tiny....tiny sellphones, tiny radios,
tiny GPS units. Manufacturers give them what they want....at the expense
of a good helical rotating polarity GPS antenna...what used to be in those
long tubes the old GPS receivers had plugged into them that folded up for
storage. The new antennas are just little pieces of PC board under the
plastic. So you have a very far away transmitter and a poorly antenna'd
receiver, exascerbating the attenuation problem. You don't have an
antenna
big enough to catch much signal (See that big dish antenna on that tower?
It's there for a reason.)

Now....Take your GPS receiver out on deck on a clear day. Let it lock
onto
the birds for a few minutes. (GPS is a very slow responding system.)
Switch the display to the satellite signal and position page and notice
the
signal levels it's receiving. While watching these levels, cover the
antenna panel, usually on the face of it at the top, with your hand. See
all the signal levels dropping? That's what happens when an inch of human
flesh and bones is in the way. Imagine how much the signals drop when
FIVE
MILES of rainwater are between the antenna and the birds! That's bouat
how
thick a good thunderstorm is. It eats satellite TV, my bank's computer
linking system (It was dead during a storm yesterday when I was there.),
anything that uses satellite microwave signals.....


http://gpsinformation.net/gpsclouds.htm

http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf

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.