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Old 22-01-2017, 08:54   #1
bcn
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Why AIS makes sense

especially when the AIS objects are well presented in your plotter.

After three days of hard weather the buoy which guards the main entrance to the Barcelona commercial port drifted away and is now 2.2nm southwest.

Compare the place on the chart with the actual one represented by the red AtoN.
The navigational warnings by radio do mention the incident every 4-6 hours....
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Old 22-01-2017, 10:18   #2
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Re: Why AIS makes sense

That's handy! I didn't know they do that.
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Old 22-01-2017, 10:47   #3
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Re: Why AIS makes sense

Quote:
Originally Posted by bcn View Post
especially when the AIS objects are well presented in your plotter.

After three days of hard weather the buoy which guards the main entrance to the Barcelona commercial port drifted away and is now 2.2nm southwest.

Compare the place on the chart with the actual one represented by the red AtoN.
The navigational warnings by radio do mention the incident every 4-6 hours....
Of course, if you didn't have AIS you'd head for where the buoy should be, instead of where AIS shows it, and you'd be good.
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Old 22-01-2017, 10:51   #4
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Re: Why AIS makes sense

It drifted due to heavy weather - 8m waves last night. Which is quite extreme in this part of the Med.
But AIS allows to signal this situation - making it "visible".
If your plotter knows about the difference.

This buoy is a very important reference in this zone.
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Old 22-01-2017, 11:22   #5
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Re: Why AIS makes sense

Interesting.....

To the best of my knowledge AtoNs are 'synthetic' ... the buoy or whatever isn't doing the transmitting... that comes from a central TX in the Harbourmaster's office or wherever.

So that 'off position' buoy needed human intervention from ashore to adjust its position *after* it was observed and reported to be out of position. During the gale and when it was dragging it would have still shown as being at the start of the fairway.

They always used to teach us not to rely on buoys as they may be out of position........... nothing changes... this would be a case where - during the gale and before adjustment - I would rely on AIS rather than eyeball.....
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Old 22-01-2017, 11:30   #6
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Re: Why AIS makes sense

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To the best of my knowledge AtoNs are 'synthetic' ... the buoy or whatever isn't doing the transmitting... that comes from a central TX in the Harbourmasters office or wherever.
Nope, look at the screenshot: Real not Virtual
This one has a transponder - one will have to trigger the "Off Position". Not being a manufacturer of this devices there might be even automatic triggers - what I doubt.

The Barcelona Harbour Authority should perhaps have placed a Virtual AtoN in the chart or intended "Sierra buoy" position instead of telling vessels that they should ignore "Sierra buoy".
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Old 22-01-2017, 12:04   #7
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Re: Why AIS makes sense

An interesting situation: You don't want to run into it in your sailboat, so it's nice to know it's off position. However, if the ships are using it to square their entrance to the harbor, they have both to avoid it and align themselves without it.

Life's tough, sometimes.

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Old 22-01-2017, 18:30   #8
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Re: Why AIS makes sense

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Nope, look at the screenshot: Real not Virtual
How do you know it's Real? A shore based transmitter designed for the purpose and located nearby could generate a sentence that had been manually programmed.
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Old 22-01-2017, 18:55   #9
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Re: Why AIS makes sense

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How do you know it's Real? A shore based transmitter designed for the purpose and located nearby could generate a sentence that had been manually programmed.
While a shore based transmitter "could" generate it by sending out false data, why on earth would they?

The display clearly shows Real(Off Position) which is derived from two message 21 parameters:

Virtual AtoN flag
0 = default = real AtoN at indicated position; 1 = virtual AtoN, does not physically exist.

Off Position Indicator
For floating AtoN, only: 0 = on position; 1 = off position.
NOTE 1 This flag should only be considered valid by receiving station, if the AtoN is a floating aid, and if time stamp is equal to or below 59. For floating AtoN the guard zone parameters should be set on installation
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Old 22-01-2017, 19:06   #10
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Re: Why AIS makes sense

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Originally Posted by bcn View Post
Nope, look at the screenshot: Real not Virtual
This one has a transponder - one will have to trigger the "Off Position". Not being a manufacturer of this devices there might be even automatic triggers - what I doubt.
They frequently have a set "guard zone" which automagically triggers the Off Position flag if they drift outside it.
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Old 22-01-2017, 19:26   #11
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Re: Why AIS makes sense

Most of the ones I have so far come across have been synthetic - simply marking submerged rocks and the like - and I made the bold assumption ( memo to self...beware the bold assumption!) that all were.

An interesting read here..
https://www.nauticalcharts.noaa.gov/.../ais_aton.html

EDIT... May be Real ( self explanatory ), Synthetic ( a physical navaid exists but AIS is generated elsewhere...and Virtual ( no navaid exists eg a shoal or submerged rock).
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Old 23-01-2017, 04:48   #12
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Re: Why AIS makes sense

A prudent navigator should not rely on only one system, but take into considerations others, such as radar screen look compare to the charts.
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Old 23-01-2017, 05:33   #13
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Re: Why AIS makes sense

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While a shore based transmitter "could" generate it by sending out false data, why on earth would they?
Power, reliability, cost, coverage.

Put a transmitter on the buoy, and then it has to have power, and an antenna, and electronics -- all in the buoy, in the salt spray.

A transmitter on shore will have easy access to power, will be more or less out of the salt spray, has ready access for maintenance, and an antenna up higher where it can be "seen" by receivers from further away. One transmitter can synthesize any number of navaids, which means that all the buoys in the harbor can be synthesized without the bean counters picking and choosing which ones are worth it.
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Old 23-01-2017, 06:10   #14
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Re: Why AIS makes sense

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A prudent navigator should not rely on only one system, but take into considerations others, such as radar screen look compare to the charts. Amen to that.
Regards.
This is one scenario of many where radar is more than a little handy. As it shows you in real time what's truly out there, not what should be, or is charted. And it's not dependent on external sources of information such as beacons. Not to mention that it shows things that aren't charted, or broadcasting; other boats, land masses, fishing floats..
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Old 25-01-2017, 20:43   #15
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Re: Why AIS makes sense

Quote:
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Power, reliability, cost, coverage.

Put a transmitter on the buoy, and then it has to have power, and an antenna, and electronics -- all in the buoy, in the salt spray.
Easily done with a battery, solar panel, and AIS transmitter.

Quote:
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A transmitter on shore will have easy access to power, will be more or less out of the salt spray, has ready access for maintenance, and an antenna up higher where it can be "seen" by receivers from further away. One transmitter can synthesize any number of navaids, which means that all the buoys in the harbor can be synthesized without the bean counters picking and choosing which ones are worth it.
If they want to do that all they need to do is code it as virtual, not real.

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