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03-13-2016, 03:07 PM
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#1
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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How can a moored boat be moving?
There's a 112 foot yacht tied up on the river. AIS shows it moving at anywhere from 0.3 to 1.2kts. How can that be?
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03-13-2016, 03:10 PM
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#2
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Guru
City: Sarasota/Ft. Lauderdale
Join Date: Apr 2014
Posts: 5,438
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Tied to a dock, mooring or at anchor?
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03-13-2016, 03:12 PM
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#3
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by Capt.Bill11
Tied to a dock, mooring or at anchor?
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All tied up at a dock. Engines down. No other stationary vessel I can find on AIS shows that.
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03-13-2016, 03:50 PM
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#4
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Guru
City: Fort Myers, FL... Summers in the Great Lakes
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 12,834
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Simply, if the system isn't malfunctioning, either the boat is moving back and forth quickly (not tight to the dock) or the GPS antenna is moving around. If you watch the AIS display, the heading of the boat should be changing if it's moving back and forth.
Ted
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03-13-2016, 04:10 PM
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#5
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by O C Diver
Simply, if the system isn't malfunctioning, either the boat is moving back and forth quickly (not tight to the dock) or the GPS antenna is moving around. If you watch the AIS display, the heading of the boat should be changing if it's moving back and forth.
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That makes sense, Ted. Direction does not change because there is not enough heading change. It is very tidal with wide river outflow speed and current shifts. I guess it could be working on the lines like a bungee with the current. Interesting. Two other boats nearby show a constant 0 speed.
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03-13-2016, 04:31 PM
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#6
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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On the other hand, there could be a GPS glitch or local disturbance because the Midnight Sun, pic 1, which is downstream and very much in the water is shown on a nearby rooftop. So much for 100% reliability.
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03-13-2016, 07:13 PM
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#7
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Guru
City: Sarasota/Ft. Lauderdale
Join Date: Apr 2014
Posts: 5,438
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I thought you were looking at AIS on your plotter.
There is a difference between what you'd see on the AIS on your plotter in real time and what you're seeing on an AIS site like Marine Traffic I'll bet.
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03-13-2016, 07:19 PM
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#8
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by Capt.Bill11;
There is a difference between what you'd see on the AIS on your plotter in real time and what you're seeing on an AIS site like Marine Traffic I'll bet.
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Could be but either way you wouldn't think it would show movement when it has been fast to the dock for days.
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03-13-2016, 07:23 PM
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#9
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,151
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some GPS inputs do jump around a lot while sitting still...not sure why but some (like my Raymarine) is all over the place until moving...
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03-13-2016, 07:26 PM
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#10
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Guru
City: Vermont
Join Date: Jan 2013
Posts: 10,093
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Normal GPS error will result in more or less random COG and SOG when standing still. Some GPSs do averaging so they mostly report zero SOG when anchored or moored, but not all do.
__________________
MVTanglewood.com
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03-13-2016, 07:36 PM
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#11
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by psneeld
some GPS inputs do jump around a lot while sitting still...
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Quote:
Originally Posted by twistedtree;
Normal GPS error will result in more or less random COG and SOG when standing still. Some GPSs do averaging so they mostly report zero SOG when anchored or moored, but not all do.
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There we go. 'Splains a lot.
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03-13-2016, 07:45 PM
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#12
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Master and Commander
City: Vallejo CA
Vessel Name: Carquinez Coot
Vessel Model: penultimate Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 12,559
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Observed an AIS signal moving east at 60 mph. It was crossing the Richmond-to-Marin-County bridge (SW end of San Pablo Bay). Concluded the vessel was being towed by a land vehicle.
__________________
Kar-KEEN-ez Koot
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03-13-2016, 11:02 PM
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#13
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TF Site Team
City: Saltspring Island
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 5,663
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Hawg
I have been watching Midnight Sun move around on shore close to her shelter in the Fraser over the last few weeks. May be something that being covered by the shelter roof is doing to the signal? I'll let the owner know, as he is a friend of mine and it is time I called.
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03-13-2016, 11:10 PM
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#14
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by koliver
Hawg
I have been watching Midnight Sun move around on shore close to her shelter in the Fraser over the last few weeks. May be something that being covered by the shelter roof is doing to the signal? I'll let the owner know, as he is a friend of mine and it is time I called.
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Funny only those two showed movement.
I know you can't answer this but, why do so many folks leave the AIS on while it's in the nest? Do they set the alarm and wake up at 3am to see if it's still there?
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03-14-2016, 02:15 AM
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#15
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TF Site Team
City: California Delta
Vessel Name: FlyWright
Vessel Model: 1977 Marshall Californian 34 LRC
Join Date: Apr 2008
Posts: 13,728
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Hawgwash and markpierce, I think you're confusing AIS with an online lookalike such as marinetraffic or boatbeacon. Those online versions have serious delays and inaccuracies. I wouldn't trust them like one could trust a real AIS. But even with a real AIS, it's just smart to cross reference with other sources like radar, eyeballs, etc. In the words of Ronald Reagan, "Trust but verify."
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03-14-2016, 07:58 AM
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#16
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Guru
City: Vermont
Join Date: Jan 2013
Posts: 10,093
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Quote:
Originally Posted by Hawgwash
Funny only those two showed movement.
I know you can't answer this but, why do so many folks leave the AIS on while it's in the nest? Do they set the alarm and wake up at 3am to see if it's still there?
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Class A is meant to be left on all the time. They are not equipped with a switch, and the IMO definition of class A AIS says it must be on all the time. There is an exception that allows you to mute transmission for safety, i.e. to not tell the pirates where you are, but it's of marginal value since a radar can see you before AIS can pick you up. But the AIS tells you more about the boat than radar, so there is reason to mute it.
I'm not sure what the spec says for Class B, but thinking about it, the ones I've dealt with don't have on/off switches either. Of course you can cut power to any device, but the absence of a switch suggests how it's intended to be used.
__________________
MVTanglewood.com
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03-14-2016, 08:05 AM
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#17
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Guru
City: Vermont
Join Date: Jan 2013
Posts: 10,093
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I too have seen a couple of amusing AIS targets. One appeared to be a duplicate transmission from one of the Washington State Ferries, and I almost called the boat to let them know they seemed to have a problem.... until the ferry docked and the "duplicate" target drove away down the road.... clearly a boat on a trailer with the AIS still on.
Another was a friend who shipped their boat across the atlantic. The AIS was left on (by mistake) and they could track their boat the whole way via one of the sat services.
And last, somewhere in the Seattle area well inland up towards Edmunds someone has a boat parked in their driveway (or maybe it's a land receiver that is also transmitting) called "Simrad". I see it on my plotter anytime I run up/down the sound.
__________________
MVTanglewood.com
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03-14-2016, 08:24 AM
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#18
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Guru
City: Edmonds, WA
Vessel Name: WESTERLY
Vessel Model: 1974 Pacific Trawler 37
Join Date: Oct 2007
Posts: 502
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TwistedTree says: "And last, somewhere in the Seattle area well inland up towards Edmunds someone has a boat parked in their driveway (or maybe it's a land receiver that is also transmitting) called "Simrad". I see it on my plotter anytime I run up/down the sound."
That's Simrad's office in Lynnwood where they have a testing lab.
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03-14-2016, 08:47 AM
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#19
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Veteran Member
City: Marblehead,Oh
Vessel Name: Castoff
Vessel Model: 77 Heritage West Indian 36
Join Date: Feb 2016
Posts: 54
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Most likely a rift in the space/time continuum. It's a quantum physics thing Haaaahaha!!
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03-14-2016, 09:00 AM
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#20
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Guru
City: Sidney
Join Date: Feb 2015
Posts: 2,258
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Quote:
Originally Posted by FlyWright;
I think you're confusing AIS with an online lookalike such as marinetraffic or boatbeacon. In the words of Ronald Reagan, "Trust but verify."
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No confusion here. I am fully aware of the limitations of the mobile app. due to web technology, older browsers, range, terrestrial vs satellite bla, bla, bla...
This is borne out every time I use it in "cross reference with the naked eye" on land and at sea but none of that explains why 2 vessels out of dozens are tossing garbage.
Oh, and Ronald Reagan...he was an actor and a politician, neither of which present things accurately.
A twisted tree is more believable.
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