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firepiper
RO# 22740
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 Posted - Jul 25 2010 : 20:40:49
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Weird question....can dense fog affect GPS reception? I understand the principles of it, but, after a super dense fog yesterday a.m..... i was mid channel, per gps, but, found myself in very skinny h2o on edge of channel. Thanks
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Fire Escape
1987 35' Luhrs Alura
1986 Twin Crusader 454,FWC,MPI,VD 1.91
1993 Whaler Rage Jet Dink |
Homeport: Plymouth, MA
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adru
RO# 19517
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Posted - Jul 25 2010 : 22:29:49
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Fog should have no affect. I lost my GPS single for about 10 to 15 seconds directly after a close lightning strike last week in rain so heavy I could barely see the anchor on the bow of my boat, a big relief when it was reacquired.
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Homeport: Branford, CT
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JLC
RO# 18971
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Posted - Jul 25 2010 : 22:38:49
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You should keep an eye on your accuracy meter that usually reads in feet.
My Garmin will tell you depending on how many sat's your acquiring your accuracy ie 5 10 15 feet etc.
Radar sure helps fill the gaps and especially helps you not hit bouys in the fog.
If your down to 3 acquired sat's and you lose one for a few seconds its easy to be off course by 20 ft.
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Holes that I currently throw money into; ..and have problems with :)
34' Sea Ray Sundancer SOLD!!! Bub Bye.... Bub Bye now
32' Galaxy Sloop ~For Sale~
21' Chris Craft Bow Rider
15' 1970 Sidewinder
13' Mad River Canoe
12' 1966 Gruman 9.9hp
11'4 Zodiac FR340 9.9
9'2 Zodiac 2hp Honda
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Homeport:
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FlyWright
RO# 3887
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Posted - Jul 25 2010 : 22:39:24
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Fog shouldn't affect the GPS accuracy but many other factors can, with or without the fog. Often it's the map database that is in error, not the GPS accuracy. Radar and good pilotage techniques are required to supplement GPS to stay safe.
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Al
1977 34 Californian LRC Trawler
2006 Achilles LEX-96 w/ Honda 2HP O/B
"Give a man a fish and he will eat for a day.
Teach him how to fish, and he will sit in a boat and drink beer all day." Anonymous
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Homeport: CA Delta, Riverboat Marina
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November Charlie
RO# 824
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Posted - Jul 25 2010 : 23:15:18
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quote:
Originally posted by FlyWright
Fog shouldn't affect the GPS accuracy but many other factors can, with or without the fog. Often it's the map database that is in error, not the GPS accuracy. Radar and good pilotage techniques are required to supplement GPS to stay safe.
Indeed. Piloting is, the way I see it, a synergistic effect achieved from properly using all available tools to the appropriate extent of their capabilities, and evaluating the information and perception from each against the others, to the end of maintaining optimal situational and spatial awareness. These tools can be electronic, they can be human. They can be radar, GPS, depth sounders, magnetic compasses, charts, chartplotters, eyes, ears, Loran-C, smells, feelings, perceptions, warm and fuzzy feelings or that 'something's not right here' feeling that should never be ignored. My philosophy on it, after running boats professionally for almost 11 years, is that piloting is not a procedure, it's not a protocol. It's not a standard method, although many formal methods do exist. It's an effect fueled by and resultant from situational awareness.
And checking everything against everything, even (especially) without conscious intent is the key. I've seen a few jetties ornaments where the owner was adamant it shouldn't have happened - "the GPS showed me in the middle of the channel!" Well, no it didn't. The GPS was likely receiving an accurate lat and long, but the chart it was representing that position on may have been 'off', the chart datum settings may have been wrong, the icons may have been wrong - who knows for sure. What IS for sure, is that "driving the chartplotter" - that is, forgetting to pilot the boat because you're playing a video game trying to keep the blinking icon between imaginary features on a variable electronic background instead. Now, what if two waypoints, that had been verified by any of the easy means to be midchannel, were entered into that GPS, and the course between those lat and longs was run, instead of playing the video game? A quick glance at where the chartplotter showed the channel to be on that pretty picture behind those verified waypoints would have showed a problem. What if the radar was tuned up? A quick check of the chart with a pair of dividers could show the distance from midchannel to the jetty running parallel to it - hell, even seeing which finger is closest, then putting it down on the latitude scale - then running out a VRM or eyeballing a range ring would have guided you right down midchannel. Add in keeping the SHM (or C/S vector if the radar is so equipped) between the gated buoys on the screen, and Saint Raytheon would not only guide you in but might alert you to a misperception or miscalculation in the use of the other tools.
It can go the other way, too. People that haven't been 'enlightened' to it (usually in a difficult to forget way) are very prone to what could, I suppose, be considered a kind of spatial disorientation. The 'objects on radar have a much closer CPA than they appear' effect of flat, two dimensional relative motion displayed on a, how do I put this, 'center-of-a-circle-centric' display. (Bonus points: Where did the term 'Radar assisted collision' originate?)
Anyone ever get 'spun up' in the fog? Going nice and slow, checking every tool at your disposal and somehow still spun off where you should (or think you should) be heading?
What about eyeballs and spotlights? What about range towers? Range boards/lights are amazingly accurate. What about ears? The bells and those low moaning whistles on buoys mean they're nearby. That creaking rusty metal noise from the hinges on the bell strikers means they're very near. That horrible crashing, grating, crunching noise means they're in your forward stateroom. (NB: Not a recommended location or method for installing aftermarket through hull fittings.)
If you don't feel like you're missing something and/or using every advantage you can, you're not paying enough attention.
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-Mike
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Homeport: Northeast
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vic33004
RO# 27361
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Posted - Jul 26 2010 : 08:57:24
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Fog is not the error, GPS is not always as accurate as its proponents advertise. yes GPS can be incredibly accurate, however that type of accuracy is not 100% of the time. GPS is like all other types of navigation products, is an aid to navigation. there are many other aids to navigation that a seasoned mariner will use to help, particularly in low light. ie. depth finder, radar, bow watch to listen for traffic or waves on the shore/jetty. all of these things in their totality gives the skipper the info he/she needs to safely operate a vessel and none of them are fool proof or capable of giving the skipper an absolute.
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Vic33004
02 Regal 4260 |
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Homeport: Fort Lauderdale, FL
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Robski97
RO# 7334
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Posted - Jul 28 2010 : 08:03:58
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One fact we are omitting is the slower the boat moves the less accurate the gps. In fog with the vessel barely making way there will be fluctuation. Also if boat is not equipped with a heading sensor I think it compounds the issue. So in a narrow channel I could see it happening . ( also I know with raymarine equipment in fog they want u to switch to north up if u dont have a heading sensor . This will stop the screen from flipping around at idle speeds . Just my 2 cents.
Rob
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| 2006 44 Sundancer |
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Homeport: Merrick, NY
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PascalG
RO# 12212
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Posted - Jul 28 2010 : 09:48:40
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good points above except that i probably disagree with Rob about accuracy impacted by speed. heading and course, maybe but not position.
hard to tell the reason without knowing how wide the channel was. if it's very narrow, that may be enough to get you on the sides. also, is it a channel were you've been thru before? as other mentioned, GPS position is usually VERY accurate but the charts may not be. it's usually the chart that's off a little.
in dense fog I rely mostly on radar when in a narrow channel, then plotter;and cross check both (well all 3 actually since i also use a laptop) with the sounder.
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Pascal
1970 Hatteras 53 MY
12' Westphal Catboat
16' Hobie Cat
2007 Sandbarhopper 13
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Homeport: Miami, FL
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Capt. Bill1
RO# 2017
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Posted - Jul 28 2010 : 11:02:14
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"One fact we are omitting is the slower the boat moves the less accurate the gps."
It shouldn't be any less accurate. If that were true than GPS would be useless for sailboats and hikers.
This is most likely a simple case of the charts not matching the reality of the current geography of the area.
"I understand the principles of it,"
Don't mean to be a dick, but by asking this question you've shown that to not be completely true. A little more review on how GPS works and why it doesn't always match up to your charts might be in order.
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The two most common elements on Earth are oxygen and stupidity.
Creationism is to science what storks are to obstetrics.
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Homeport: Sarasota/Ft. Lauderdale/ Palmas del Mar PR
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Robski97
RO# 7334
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Posted - Jul 28 2010 : 13:08:58
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Pascal,
My bad... Thats what I meant. ( coarse / Heading )
Rob
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| 2006 44 Sundancer |
Edited by - Robski97 on Jul 28 2010 13:09:49 |
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Homeport: Merrick, NY
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Billylll
RO# 24494
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Posted - Jul 28 2010 : 13:13:56
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I agree with Rob a position/heading sensor hooked to your Chartplotter is a must have.
Bill
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WirelessOne,
40 Mainship
Sedan Bridge
Little Egg, N.J. |
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Homeport: Tuckerton, N.J.
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Kerry
RO# 2599
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Posted - Jul 28 2010 : 16:40:43
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No it won't be the fog. The difference between accuracy and operation is quite different due to the dynamics of the system, accuracy is only defined for 95% of the time.
This is one reason why nobody can ever say a certain GPS setup (especially internal antenna models) is predictable just because the unit was sat in the intended mounting position for 5 minutes and it worked perfectly, this does not mean it will always be perfect, far from it, so many different unpredictable variables.
As for charts, charts are charts and unless the chart has been upgraded then charts don't simply don't move around by themselves. If you know your local area/chart then these surprises should be known.
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Homeport: CQ Coast
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gps/fog question
