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Old 15-07-2015, 04:55   #91
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Re: Live AIS Via Internet

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Originally Posted by conachair View Post
I've tried real AIS along side the google earth plug in with marine traffic on opencpn. Some ships had a mile between the real position and the internet position.
I prefer not to bother with old data. Maybe in areas with next to no traffic, but after playing with both systems side by side in a high traffic area, there is a strong argument to say that internet ais is actually worse than none. We are so used to computers spitting out exact info, knowing the data might be bad isn't enough, there's a very strong instinct to believe it's all real. We ain't as clever as we like to think we are all the time.
Bingo!!!!!!!!!!!!!!! Done the same and it is a veritable joke only safe for when at home sitting at your computer..

Even with actual AIS (not internet "AIS") there can be lots of errors between updates... Just last weekend a Saberline Express went blowing past us on full plane.. AIS showed it 1/4 mile behind us when she was 50 yards abeam. By the time AIS updated it was at least 3/4 mile ahead of us. That distance in the fog, at those speeds, is a death sentence. Interestingly enough the radar always showed the Saberline where it actually was...

In the fog I simply do not trust AIS positions, at least class B, but it is a good supplement to RADAR....
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Old 15-07-2015, 08:24   #92
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Re: Live AIS Via Internet

Internet AIS is not a replacement for 'real' AIS in any way. If you are not carrying a class B you are only getting about 25% of the benefit of AIS. The major benefit is to be SEEN rather than see others.

I know, I know, "I can alter course, blah blah blah...."

Bull sheeeet, that 1000ft container ship or tanker, can run you down so fast it will blow your mind. You really want him to see you loud and clear! Why make it 100% your responsibility to avoid them, why not work together!

There is a problem with land based AIS receivers that people don't talk about much and I will try to explain best I can. First thing to remember is that AIS was designed to work SHIP to SHIP.

So I have a customer, located in a southern US port, LOTS of traffic, with an AIS receiver located way up a tower, probably 150ft off the water. Problem is, there were boats out there that they knew were there and they didn't see them and they didn't know why. They saw boats much farther out and didn't understand why.

Remembering that AIS is designed to work SHIP to SHIP, when a class A unit wants to transmit it does whats called a "frame survey" looking for open slots in a frame. Might check a few frames (one second each) and if it sees slots that it thinks are not being used it schedules a transmit. Now you have two vessels that CANNOT SEE EACH OTHER due to range, say they are 60 miles apart straight line. They both do a frame survey and pic the same frame and slot.

Now the land based tower, with an antenna way the hell up there, can easily 'see' both vessels, but since they both transmit on the same frame and slot they receive neither! Two down for the price of one!

You might be thinking that this is a rare occurrence, you would be incorrect.

So you can bank on shore based internet AIS if you want to, but I personally believe that would be a big mistake.

If this has been previously covered please disregard.
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Old 16-07-2015, 05:38   #93
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Re: Live AIS Via Internet

what about using lots of ais receivers each with directional antennas? This would help to avoid the problem of two distant ships talking over each other.
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Old 16-07-2015, 07:23   #94
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Re: Live AIS Via Internet

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Originally Posted by Maine Sail View Post
Bingo!!!!!!!!!!!!!!! Done the same and it is a veritable joke only safe for when at home sitting at your computer..

Even with actual AIS (not internet "AIS") there can be lots of errors between updates... Just last weekend a Saberline Express went blowing past us on full plane.. AIS showed it 1/4 mile behind us when she was 50 yards abeam. By the time AIS updated it was at least 3/4 mile ahead of us. That distance in the fog, at those speeds, is a death sentence. Interestingly enough the radar always showed the Saberline where it actually was...

In the fog I simply do not trust AIS positions, at least class B, but it is a good supplement to RADAR....
With everything working as designed (AIS time slots, no missed transmissions, etc.), a vessel moving at 30kts using a Class B xcvr, there will be 1/4nm between updates.

In fog, if you have to venture out, radar is the best choice collision avoidance tool.
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Old 16-07-2015, 09:05   #95
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Re: Live AIS Via Internet

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With everything working as designed (AIS time slots, no missed transmissions, etc.), a vessel moving at 30kts using a Class B xcvr, there will be 1/4nm between updates.

In fog, if you have to venture out, radar is the best choice collision avoidance tool.
Exactly, AIS was always only a rough guess at who's out there not an acurate position tracking system.

The idea you can plot your course based on it and reasonably assume you will miss all other vessels is flat out wrong (with full blown AIS).

People are trying to use it for something that it is not. Internet based, provides information that is pretty much just as good as a dedicated reciever. In fact the known latency is probably helpful as you are less likely set the autopilot based on false information because you never trusted the exact position in the first place and you shouldn't be trusting it with a dedicated reciever either. The system let's you know someone is out there and to look for them.

To do real closest point calculations, we need something more like Connected Vehicles where you are getting second by second position and course information.
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Old 16-07-2015, 09:07   #96
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Re: Live AIS Via Internet

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...Internet based, provides information that is pretty much just as good as a dedicated reciever. In fact the known latency is probably helpful...
I don't think so.
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Old 16-07-2015, 09:42   #97
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Re: Live AIS Via Internet

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I don't think so.
Then you would think wrong. When humans see data that is typically correct, they tend to trust it. The problem is how much false data is acceptable. If you are talking about plotting closest point of approach in a congested area, the accuracy of full blown AIS is not sufficent.

I would much rather have data where the position part of the data is wrong, 1 out of 10 times vs one that is wrong 1 out of 100 times.

Users are far less likely to falsely trust position information that they regularly see correct but older data.

Unless you can reliably get current data (within a few seconds) on all boats, suggesting that you can implicitily trust the positioning of live AIS data is poor seamanship.
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Old 16-07-2015, 09:57   #98
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Re: Live AIS Via Internet

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Originally Posted by valhalla360 View Post
People are trying to use it for something that it is not. Internet based, provides information that is pretty much just as good as a dedicated reciever. In fact the known latency is probably helpful as you are less likely set the autopilot based on false information because you never trusted the exact position in the first place and you shouldn't be trusting it with a dedicated reciever either. The system let's you know someone is out there and to look for them.
You can't rely on the Internet-based system to do this! With the Internet based system, there is too much varibility in where the receivers are located in comparison to where you are located. Plus the fact that you really need a xcvr to play the collision avoidance game properly. AIS is very much dependent on your location, hence a xcvr/receiver must be located where you are, not on some rooftop 15 miles away. In addition to placement of the Internet-based receiver, you now are relying on home-based gateways riding on home-based broadband sending data at it's leisure to the other side of the world to unknown quality servers that may or may not process the data in a timely manner, if ever.

The on-board xcvr removes all this variability.

Hence, you get what you pay for!

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Originally Posted by valhalla360 View Post
To do real closest point calculations, we need something more like Connected Vehicles where you are getting second by second position and course information.
Given all the tools available and a competent operator, second by second updates are not required. You simply need the right tools, not some admittedly unreliable half-baked solution.
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Old 16-07-2015, 10:02   #99
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Re: Live AIS Via Internet

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Originally Posted by DotDun View Post
With everything working as designed (AIS time slots, no missed transmissions, etc.), a vessel moving at 30kts using a Class B xcvr, there will be 1/4nm between updates.

In fog, if you have to venture out, radar is the best choice collision avoidance tool.
Why not use both? AIS receiver is cheap and send you informations like MMSI number you don't see with radar.

Gerhard
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Old 16-07-2015, 10:02   #100
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Re: Live AIS Via Internet

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You can't rely on the Internet-based system to do this! With the Internet based system, there is too much varibility in where the receivers are located in comparison to where you are located. Plus the fact that you really need a xcvr to play the collision avoidance game properly. AIS is very much dependent on your location, hence a xcvr/receiver must be located where you are, not on some rooftop 15 miles away. In addition to placement of the Internet-based receiver, you now are relying on home-based gateways riding on home-based broadband sending data at it's leisure to the other side of the world to unknown quality servers that may or may not process the data in a timely manner, if ever.

The on-board xcvr removes all this variability.

Hence, you get what you pay for!



Given all the tools available and a competent operator, second by second updates are not required. You simply need the right tools, not some admittedly unreliable half-baked solution.
The point is even the full blown tool with reciever on the boat fails and in that case an obvious failure is less like to result in a mistake than one that is kind-of good.If you want true collision avoidance, you need second by second data.
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Old 16-07-2015, 10:32   #101
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Re: Live AIS Via Internet

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Originally Posted by valhalla360 View Post
Then you would think wrong. When humans see data that is typically correct, they tend to trust it. The problem is how much false data is acceptable. If you are talking about plotting closest point of approach in a congested area, the accuracy of full blown AIS is not sufficent.

I would much rather have data where the position part of the data is wrong, 1 out of 10 times vs one that is wrong 1 out of 100 times.

Users are far less likely to falsely trust position information that they regularly see correct but older data.

Unless you can reliably get current data (within a few seconds) on all boats, suggesting that you can implicitily trust the positioning of live AIS data is poor seamanship.
Your desire to have less accurate data is illogical. Perhaps you will also feel better off navigating with a chart that has sea monsters on it.

You make it sound like people will be down at their Nav table exclusively relying on their AIS.

With the exception on high speed motor boats using Class B covering long distances between transmissions, the information on the screen is accurate enough for most of us. After all, collision avoidance is not about playing chicken and making your avoidance manoeuvre at the last possible moment.

If a class b sailing boat or displacement cruiser has an uncomfortably close CPA, the 30 second gap between transmissions is not a problem and it is enough of a frequency to allow you to tell if they are making a course change. If the course change is only minor buy my sufficient to avoid issues, it can take longer to figure out using your eyes.

Using old unreliable data from the internet that has both delays and poor coverage is just plain dumb and pointless and has no place being included on serious navigational tools


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Old 16-07-2015, 10:35   #102
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Re: Live AIS Via Internet

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Originally Posted by valhalla360 View Post
The point is even the full blown tool with reciever on the boat fails
Not sure where you are getting this information. Any failures with on-board dedicated xcvrs discussed so far is seeing vessels at great distances, none of which cause collisions.


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and in that case an obvious failure is less like to result in a mistake than one that is kind-of good.If you want true collision avoidance, you need second by second data.
How much navigation experience do you have?

AIS is simply ONE tool in the collision avoidance bag of tricks! And, it's not the most important one!

You seem to be caught up in the idea that navigators use AIS exclusively and that's simply couldn't be farther from the truth.

Collision avoidance tools ranking of usefulness (IMO, of course):

1 - Human eyeballs
2 - Radar/ARPA
3 - AIS
4 - VHF Comms
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Old 16-07-2015, 10:43   #103
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Re: Live AIS Via Internet

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The point is even the full blown tool with reciever on the boat fails and in that case an obvious failure is less like to result in a mistake than one that is kind-of good.If you want true collision avoidance, you need second by second data.
I just unplugged my AIS power cable. Within 20second my MFD was warning a problem.

The failure rate with AIS is so small it's laughable that you bring it up. On the other hand, there are so many failure points for the internet it's also laughable that you can even consider it for anything more than finding vessels out of curiosity, stalking friends and for downloading porn.


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Old 16-07-2015, 10:53   #104
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Re: Live AIS Via Internet

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Originally Posted by hoppy View Post
I just unplugged my AIS power cable. Within 20second my MFD was warning a problem.

The failure rate with AIS is so small it's laughable that you bring it up. On the other hand, there are so many failure points for the internet it's also laughable that you can even consider it for anything more than finding vessels out of curiosity, stalking friends and for downloading porn.


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Old 16-07-2015, 10:57   #105
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Re: Live AIS Via Internet

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You make it sound like people will be down at their Nav table exclusively relying on their AIS.

With the exception on high speed motor boats using Class B covering long distances between transmissions, the information on the screen is accurate enough for most of us. After all, collision avoidance is not about playing chicken and making your avoidance manoeuvre at the last possible moment.


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That's pretty much what some people are suggesting is an option with full AIS. Once you accept that it's not, having data that's a couple minutes old, really loses almost none of the value.

Of course if congested harbors, high speed motor boats are typically very common. On open uncongested waters, AIS is a nice but really not of much use for collission avoidance. Trusting AIS to tell you where boats will be in a congested harbor setting, is foolish.
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