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Old 03-13-2016, 04:07 PM   #1
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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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Old 03-13-2016, 04:10 PM   #2
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Tied to a dock, mooring or at anchor?
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Old 03-13-2016, 04:12 PM   #3
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Tied to a dock, mooring or at anchor?
All tied up at a dock. Engines down. No other stationary vessel I can find on AIS shows that.
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Old 03-13-2016, 04:50 PM   #4
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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.

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Old 03-13-2016, 05:10 PM   #5
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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.
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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Old 03-13-2016, 05:31 PM   #6
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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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Old 03-13-2016, 08:13 PM   #7
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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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Old 03-13-2016, 08:19 PM   #8
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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.
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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Old 03-13-2016, 08:23 PM   #9
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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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Old 03-13-2016, 08:26 PM   #10
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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.
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Old 03-13-2016, 08:36 PM   #11
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Quote:
Originally Posted by psneeld
some GPS inputs do jump around a lot while sitting still...
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.
There we go. 'Splains a lot.
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Old 03-13-2016, 08:45 PM   #12
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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.
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Old 03-14-2016, 12:02 AM   #13
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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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Old 03-14-2016, 12:10 AM   #14
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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.
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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Old 03-14-2016, 03:15 AM   #15
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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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Old 03-14-2016, 08:58 AM   #16
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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?
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.
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Old 03-14-2016, 09:05 AM   #17
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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.
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Old 03-14-2016, 09:24 AM   #18
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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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Old 03-14-2016, 09:47 AM   #19
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Most likely a rift in the space/time continuum. It's a quantum physics thing Haaaahaha!!
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Old 03-14-2016, 10:00 AM   #20
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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."
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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