A few days ago we were hit by a thunderstorm while at anchor in Richmond
Island harbor, south of Portland Me. The wind held at about 40 knots
for 10 minutes or so and gradually diminished, and we were surrounded by
lightening, though there were no hits within a half mile. I had started
the engines in case we dragged, and was monitoring the position on GPS,
a year old Garmin 545. At the height of the storm, the GPS lost its
connection, and would only come back after a a full reset (faster than
the initial cold start, but slower than a normal power up). This was
repeated several times during the storm.

I assumed that the problem was the lightening, but I noticed also the
the little rubber cap on the unused connector was off, and it would have
been pelted with heavy rain, so it also could have been the problem.
The unit has worked perfectly since then.

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!)

BTW, my 35# Delta with 50' chain held fine in the hard mud (or is it
soft clay?). A neighbor with a CQR was not so lucky - he had set with
lots of rode, and deployed a kellet as the storm approached, but it let
go completely on the first 35 kt gust. He tried to reset a few times
but ended up circling for 45 mintues until the storm died down and then
he finally reset. As it turns out, we had switched to Delta anchors
because of problems trying to set a CQR in the very same anchorage 15
years ago.

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.....

"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.

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.

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?

On 2008-07-30 08:45:15 -0400, jeff said:

At the height of the storm, the GPS lost its connection, and would only
come back after a a full reset (faster than the initial cold start, but
slower than a normal power up). This was repeated several times during
the storm.

I assumed that the problem was the lightening, but I noticed also the
the little rubber cap on the unused connector was off, and it would
have been pelted with heavy rain, so it also could have been the
problem. The unit has worked perfectly since then.

Lightning can of course interfere. I find that we sometimes lose the
GPS under heavy cloud cover.

Our antenna connector sometimes needs a bit of a 'tweak' to improve
reception due to age & corrosion, as well, but you shouldn't have that
problem with a nearly new unit.

You *might* get a bit of problem if you get an unused connector wet.
Wouldn't hurt to seal it up.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/