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Old 30-06-2015, 13:49   #61
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Re: AIS- A Love Story

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Originally Posted by Jon Eisberg View Post
AIS from a cell phone ???

Are you referring to accessing a web-based source of AIS info such as MarineTraffic.com? If so, that's not a very good idea... ;-)

From their FAQ page, emphasis mine:

Comparing the display from marinetraffic with an onboard AIS can often reveal some startling discrepancies. While some of the information MIGHT be better than nothing, it could just as easily be WORSE than nothing ;-) No way should such web-based reporting be relied upon for collision avoidance in limited visibility...
Point well taken!

I have not compared my MarineTraffic data head to head with an AIS receiver. My mistake. I can only say that ships I saw with my eyes near LA harbor a month ago did match what I saw on MarineTraffic on my phone, but I can certainly concede this is not the best way to see the traffic! Still, I would like to know how we can verify the info on MarineTraffic (aside from looking.) Anyone out there know the reliability of the MarineTraffic display? Each ship's position is displayed with how many minutes old the info is. I know that the land based antenna must be able to receive the signal to relay it, but if it IS receiving it, is the data presented on MarineTraffic still unreliable? After reading through the FAQ it seems that as long as you are broadcasting with enough power and there is a receiver nearby, you should show up on MarineTraffic fairly reliably. I am pretty close to the Southern Cal coast so I am ASSUMING (and I know how dangerous that can be) that there are enough receivers and ships close enough and broadcasting loud enough to make the info somewhat reliable. And I am not sure what "somewhat" is exactly... I will keep checking and looking around to see if what I see around me matches what is on MarineTraffic. I am sure MarineTraffic would not like to be considered a navigational safety tool and risk the lawsuits.
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Old 30-06-2015, 14:36   #62
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Re: AIS- A Love Story

Yes of course the position reports are accurate at the time of transmission.
Our position..
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Old 30-06-2015, 21:13   #63
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Re: AIS- A Love Story

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Originally Posted by monte View Post
Yep gotta love it. By the way Mark it's interesting to note on your screen shot just how many ships have their AIS bearing misaligned. You can see a big difference on the heading to cog on the ship closest to you. We have noticed that 8regularly as well. Fortunately the AIS calculate the cog correctly when showing vectors, but it can be confusing when the triangle ship icon is pointing up to 30 degrees from the actual cog. We have seen this occasionally and it's definitely not a variation caused by leeway or running one engine. I guess it's only a class A issue as they probably transmit heading as well as cog, whereas class B doesn't transmit heading (I don't think so anyway)
We are just finished 3 days sail in open waters and noticed several misaligned AIS triangles. It seems to me that whenever the triangle does not point exactly in the right direction it is skewed by either 90 or 180 degrees exactly.
As Monte mentioned, the relative motion vector was always correct.
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Old 01-07-2015, 03:45   #64
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Re: AIS- A Love Story

I picked up my yacht from Portugal and sailed it back to the UK. It was very basically fitted out (no AIS or Plotter), we crossed the Bay of Biscay in a force 9, lost the engine due to fuel contamination and were making 2 knots on a storm jib with clearly next to no manoeuvrability! The only thing that scared me was being hit by a ship at night, several seemed to be on a collision course with us and none responded to calls on the vhf but did seem to change course following transmission! Needless to say the first modification I did on reaching the UK was to install AIS, though only class B.
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Old 01-07-2015, 04:53   #65
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Re: AIS- A Love Story

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Originally Posted by meirriba View Post
We are just finished 3 days sail in open waters and noticed several misaligned AIS triangles. It seems to me that whenever the triangle does not point exactly in the right direction it is skewed by either 90 or 180 degrees exactly.
As Monte mentioned, the relative motion vector was always correct.
This is the fault of programming it. When you initially install the unit, you have to tell it where it is place on the boat (length to bow/stern/either side). On my unit I also had to tell it which direction was forward.

If you don't do this - the triangles will not be as they should be
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Old 01-07-2015, 05:16   #66
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Re: AIS- A Love Story

Where we sail AIS is a nice adjunct to radar but not to be depended upon in fog as your sole source of information.

A few year ago Tim R. and I (Tim is another forum member here) were in the thick Maine pea-soup (maybe 100 feet of vis) when I saw an AIS target on the plotter. I had already also noted my radar giving a return that looked almost as echos being repeated after the main target. They were good strong targets just oddly spread......

I knew my radar was well tuned, and not lying, so I picked up the VHF and hailed the AIS target, this is a beautiful benefit of AIS. Sure enough it was a tug towing two barges with 900 feet of cable between each barge. We were in a crossing situation!

If going by only AIS we could have been really dumb & split between the tug and barges in tow not having a clue they were even there.. The barges in tow were not displaying AIS signals, nor was there any data to tell us the target was towing a barge let alone two at 1800'. The AIS only identified the tug by name, speed etc....... Using AIS only could have been a death sentence or dismasting in that situation....

It was radar that alerted us to the actual positions of all three targets. AIS allowed us to hail the tug easily and confirm what the radar was showing, but alone AIS could have been very, very dangerous in that foggy crossing situation.

Both tools work really well together but if I must have only one device I will take my radar every day of the week for sailing in Maine and the Maritimes. Blind trust of AIS, in the fog, is just not a very good idea. I won't even go into slow updates & fast moving vessels, in the fog, on AIS.... For me AIS is a great complimentary tool to radar not a primary tool...
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Old 01-07-2015, 06:06   #67
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Re: AIS- A Love Story

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Originally Posted by Maine Sail View Post
Where we sail AIS is a nice adjunct to radar but not to be depended upon in fog as your sole source of information.

...

Both tools work really well together but if I must have only one device I will take my radar every day of the week for sailing in Maine and the Maritimes. Blind trust of AIS, in the fog, is just not a very good idea. I won't even go into slow updates & fast moving vessels, in the fog, on AIS.... For me AIS is a great complimentary tool to radar not a primary tool...
I couldn't agree more...

Other vessels may not be the only things you need to avoid running into out there, after all...

;-)


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Old 01-07-2015, 06:23   #68
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Re: AIS- A Love Story

some ducting in the med today...
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Old 01-07-2015, 07:07   #69
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Re: AIS- A Love Story

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Where we sail AIS is a nice adjunct to radar but not to be depended upon in fog as your sole source of information.
.....
Agree completely.
Was is really useful though, is when the tug broadcasts that it's towing on AIS, so you can do something 5 miles before you get to it. They don't all do that, but very useful when they do.
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Old 01-07-2015, 07:15   #70
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Re: AIS- A Love Story

Good story Mainesail, and a good recommendation for AIS targets displayed on the main plotter along with radar. We often use this to confirm what we are seeing as well as align the radar image directly over he target. Occasionally our radar needs a 5 degree bearing adjustment.
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Old 01-07-2015, 07:17   #71
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Re: AIS- A Love Story

Bcn, there are AIS repeaters in that area, hence the super range! A pretty good idea that hopefully we will see more of in the future.
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Old 01-07-2015, 07:18   #72
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Re: AIS- A Love Story

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Occasionally our radar needs a 5 degree bearing adjustment.
I can understand needing to adjust this once as part of a general calibration, but if it is happening more frequently, something is wrong - either with your heading sensor or with the radome.

Mark
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