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Old 03-14-2016, 11:51 AM   #21
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If you are seeing the status as "moored" then it is a class A ais signal and position data is uploaded every 180 secs. Twenty snapshots an hour may look like movement in some areas so the "moored" status line is helpful.


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Old 03-14-2016, 02:01 PM   #22
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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..
Are you seeing these aberrations on a Class A or Class B AIS receiver? If so, what model?
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Old 03-14-2016, 02:58 PM   #23
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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."
I can see why marinetraffic.com has delays. But I don't see why it would have inaccuracies. It relies on data sent by reporting stations that receive AIS transmissions. Do those somehow get corrupted before being displayed on marinetraffic?

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Old 03-14-2016, 03:19 PM   #24
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Richard, I run marinetraffic and boatbeacon on my tablet while underway. Both have inaccuracies with boats in the wrong position (delay?), boats shown present which are not (ghosting?) present commercial vessels not displayed and docked vessels showing speed which might be from historical data. I've never used a proper AIS.
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Old 03-14-2016, 03:35 PM   #25
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it's an app for crying out loud, that relies on 2nd hand data. Is anybody really surprised it's not dead nuts accurate all the time!?
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Old 03-14-2016, 04:03 PM   #26
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it's an app for crying out loud, that relies on 2nd hand data. Is anybody really surprised it's not dead nuts accurate all the time!?
Aha! Sorta like reading TF, huh?
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Old 03-14-2016, 06:51 PM   #27
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Richard, I run marinetraffic and boatbeacon on my tablet while underway. Both have inaccuracies with boats in the wrong position (delay?), boats shown present which are not (ghosting?) present commercial vessels not displayed and docked vessels showing speed which might be from historical data. I've never used a proper AIS.
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it's an app for crying out loud, that relies on 2nd hand data. Is anybody really surprised it's not dead nuts accurate all the time!?
Here's my thinking. It's one thing for it to be out of date, it's another for it to publish inaccurate data. After all, these are AIS data packets that are being reported by volunteer stations. They may be delayed, they may be intermittent, but they would need to be corrupted for them to be inaccurate.

The example given here shows that the last update was 41 mins ago. So in all likelihood that vessel was not there when I looked at marinetraffic.com. That doesn't mean it was wrong, just out of date.

I agree that it's not a replacement for an AIS - because it's not real time. I'm just curious if anyone has found it to be inaccurate - like it shows an update from 30 seconds ago and clearly the vessel is not there.

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Old 03-14-2016, 06:51 PM   #28
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it's an app for crying out loud, that relies on 2nd hand data. Is anybody really surprised it's not dead nuts accurate all the time!?
You've got that right, Bill!

But it's better than nothing for cheap SOBs like me.
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Old 03-14-2016, 08:08 PM   #29
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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.
Ahh, that explains it. Interesting that will all the trouble I had, they never suggested one of those guys drive the 30 minutes to my boat to take a look.
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Old 03-14-2016, 08:22 PM   #30
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Just to be clear, we are talking about a vessel that reports a non-zero SOG via AIS when the boat is moored. Right?

On both Class A and Class B AIS the SOG comes from the attached GPS. With Class B it's part of the AIS, and with Class A it's external, with internal as a backup. So the SOG is computed by the GPS and reported by the AIS.

I agree that sites/apps like marinetraffic are unlikely to present anything other than actual AIS data. There is no reason for them to make it up, and internet protocols ensure it's reliable and intact delivery. Delayed, yes, but corrupt, no.

I know if I look at any of my GPSs when standing still, they all report random small SOG values. It's just the nature of the beast. The slower you move, the less accurate the SOG. And when you are standing still, it's utter garbage. Save with COG. This is why some GPSs average/filter to present more "sensible" data when going very slow or at rest.
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Old 03-14-2016, 08:23 PM   #31
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Oh, and Ronald Reagan...he was an actor and a politician, neither of which present things accurately.

A twisted tree is more believable.
Hummm, low bar.....
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Old 03-14-2016, 08:50 PM   #32
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Aha! Sorta like reading TF, huh?
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Old 03-14-2016, 11:30 PM   #33
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Speed input can be via GPS or transducer. If transducer is selected it will read current but show no movement.
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Old 03-15-2016, 06:53 AM   #34
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HMMMM, Constant errors ,

And some adventurous folks allow their GPS to steer in a channel?
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Old 03-15-2016, 06:52 PM   #35
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I suspect the problem is a poor GPS antenna or poor connection. I had the same problem--the AIS target of my boat jumped all over the place before I attached a GPS antenna. Attached the antenna and voila! Much better. The GPS antenna is optional on my class B unit but on some vessels, it's essential if there is too much equipment or other shielding surrounding the AIS box.


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Old 03-15-2016, 11:59 PM   #36
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Speed input can be via GPS or transducer. If transducer is selected it will read current but show no movement.
AIS gets its data from a GPS so I'm not sure that is possible. I don't believe you can select what type of speed input the AIS sees. It may not even be able to read a speed transducer signal.
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Old 03-16-2016, 12:11 AM   #37
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You've got that right, Bill!

But it's better than nothing for cheap SOBs like me.
That's fine for you. You seem to understand its limitations.

But some don't seem to get it.
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Old 03-16-2016, 12:32 AM   #38
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My 740s has the ability to select speed input from multiple sources. The data provided from the 740s to the AIS includes that information.

Is the target in question actually moving on the screen or are you just seeing the its information?
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Old 03-16-2016, 05:30 AM   #39
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My 740s has the ability to select speed input from multiple sources. The data provided from the 740s to the AIS includes that information.
Yes but are you sure the AIS can see/read transducer speed? Can you switch between the two on your AIS? Just because the data is present doesn't mean the AIS unit can use it.

If an AIS did read transducer speed the information it would be outputting to other vessels would not reflex the vessels true SOG and could be dangerously misleading.
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Old 03-16-2016, 03:34 PM   #40
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Well, this has been an interesting discussion and it is again obvious that when it comes to regulations, more time can be spent reading all the information than actually operating the equipment. As a result there is a wide range of understanding among users.

It appears that all AIS devices must be in operation at all times except when moored and when safety is compromised. This appears to apply to Class A & B both in mandatory and voluntary use.

To me, compulsory use is counterproductive to a degree and therefore I would look long and hard at my personal choice to have it.

The print is original, the bold is mine:

6. When must AIS be in operation? Vessels equipped with AIS (either by mandatory carriage or voluntarily) must abide by the requirements set forth in 33CFR 164.46(d) and should especially ensure their AIS is in properly installed, using an assigned MMSI, and, that its data is accessible from the primary conning position of the vessel. Also, that it be in 'effective operating condition', which entails the continuous operation of AIS and the accurate input and upkeep of AIS data fields during all times that the vessel is navigating (underway or at anchor), and, at least 15 minutes prior to unmooring. Should continual operation of AIS compromise the safety or security of the vessel or where a security incident is imminent, the AIS may be switched off.This action and the reason for taking it must be reported to the nearest U.S.Captain of the Port or VesselTraffic Center and recorded in the ship's logbook. The AIS should return to continuous operation as soon as the source of danger has been mitigated.
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