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Boat Forum / Cruising / July 2008Garmin losing satellites
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 are: 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. > So my questions are: 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..... >> So my questions are: does lightening often cause this problem? Is it >> more likely that a bit of water on the unused external antenna connector [quoted text clipped - 36 lines] > 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. >>> So my questions are: 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. [quoted text clipped - 8 lines] > 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. >>>> So my questions are: does lightening often cause this problem? Is it >>>> more likely that a bit of water on the unused external antenna connector [quoted text clipped - 22 lines] >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? > 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 [quoted text clipped - 5 lines] > 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.  SignatureJere 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/ |
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