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Old 26-06-2010, 22:07   #1
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Class B AIS

Here is a safety notice that the Australian Transport Safety Bureau has put out re Jessica Watson's "Ella Pink Lady collision with "Silver Yang Bulk Carrier.

Safety Advisory Notice issued to: Owners, operators and skippers of small vessels
Output No: MO-2009-008-SAN-014
Date Issued: 15 June 2010
Safety Action Status: Released
Background: Why this Safety Advisory Notice was developed
Output Text:
Minor safety issue

The evidence suggests that Class B AIS transmissions may not be reliably detected by watch keepers on board all ships. Therefore, operators of small vessels fitted with Class B AIS units should be aware that they cannot rely on the AIS unit alone to warn ships of their presence.

Safety advisory notice

The Australian Transport Safety Bureau advises that all owners, operators and skippers of small vessels should consider the safety implications of this safety issue and take action where considered appropriate.

Makes one think

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Old 26-06-2010, 22:41   #2
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I read the report when it came out and was surprised at some of the conclusions. But it seems that part of the AIS problem was Jessica Watson's unit and the fact that she had not entered the boat information correctly. But it does seem that some of the older Class-A AIS systems that predate Class-B might have trouble reading the messages.

-Zanshin (SV Zanshin)
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Old 27-06-2010, 01:06   #3
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Many people it seems are concerned of the value of Class B AIS particularly in a crowded environment of class A equipped vessels. This article explains very well how the two systems work alongside each other in such situations.

Class A vessels use SOTDMA (Self Organising Time Division Multiple Access) to send variable length telegrams, short ones on a regular basis, longer ones (static information like cargo, destination, etc.,) less often. Class B vessels using CSTDMA (Carrier Sense Time Division Multiple Access) mainly send fixed length telegrams using time slots not used by Class A vessels.
Class A vessels reserve a particular time slot and negotiate the use of that time slot with other Class A vessels within radio range. Class B vessels use any time slot as and when it is available. If no slot is available and a Class B vessel wants to transmit it simply misses out that 'go' and tries again after a pre-set time.
Its ability to provide a service therefore degrades gracefully and the user is informed if three potential transmission slots are missed by a warning signal. As soon as time slots become available the Class B unit picks up the reporting pace and goes back into the standard reporting timing.
So the environment is dynamic and depends on such issues as:
- How many Class A and Class B vessels are within radio range of each other
- What the Class A vessels are doing (moored, under way, etc)
- What the mix is of Class A and Class B vessels at that location

Now the maximum number of AIS time slots is 2250. However all this tells you is that, starting at slot 1, in theory 2250 vessels can send messages in sequence before slot 1 comes round again. A Class B vessel can 'borrow' a slot when not being used. In addition individual vessel antenna mounting will determine that vessels radio range, which throws another variable into the pot!
However, in general:
- 100 vessels within radio range of each other will be fine.
- 500 vessels within radio range of each other will be fine.
-1000 vessels in radio range of each other may show the occasional degradation to individual Class B unit performance.
- 2000 vessels in radio range will show a greater degradation to individual Class B unit performance.
In all cases Class A performance is protected as far as is practicable and the AIS system is designed to degrade gracefully, not fail catastrophically.
But to re-iterate: the actual AIS performance in a real-life scenario at a particular location all depends on the variables listed above, and it will be dynamic from minute to minute.
So we can assume that generally we will be seen by vessels equipped with modern class A transponders. It has been stated that its not possible with such equipment to gate out class B signals. However I view that statement with some doubt. I know my PC based chartplotter has a "do not display Class B vessel" option.

Furthermore there is nothing to stop commercial ships turning off their AIS completely. An acquaintance of mine was alarmed & angered when he nearly hit such a vessel whilst single handing mid Atlantic. His dialogue with the Captain revealed they had turned it off because the alarms were annoying them! Clearly he was putting an unhealthy faith in his AIS system for watch keeping, but it was working correctly.

IME the most relevant factor limiting class B AIS performance is the strength of the transmitted signal, which gives an average range around 8nm compared to 25nm or more for class A signals received by the same rail mounted antenna.

If in doubt RTFM
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Old 27-06-2010, 05:04   #4
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In a busy harbor, you will be at full alert, and whether your AIS B is getting out or not, it gives you the location of all the big ships. I had over 130 targets listed yesterday coming across New York Harbor, and I just figured it was up to me to get out of their way. In fog, however, it would be nice to show up on the big ships AIS...

Some of the Class A units can't decode the static info from Class B units, so you only show up with your MMSI number, position, SOG, and COG. This happened in the Pink Lady tests, and happened to me with an aircraft carrier outside Norfolk. Its better than nothing, and get their attention. On the Pink Lady tests where the Class B didn't show up at all, I suspect they were run at the dock, where the Class B only transmits every 3 minutes, vs every 30 seconds underway.

The Class B units aren't perfect, (in addition to the problems of being seen, there is no way to change your status, so you remain a 'sailing vessel' even when you are under power), but for $500 and a low current drain, they are worth it to me, especially at sea, where I have had several ships alter course to miss me at about 5 miles.
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Old 27-06-2010, 08:40   #5
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For those interested AIS data is transmitted at following intervals:

Class A:
Dynamic - Position, Time, SOG, COG, Heading, Navigational Status, ROT
At anchor/moored - 3 minutes
<14 knots - 10 seconds
<14 knots and changing course - 3.3 seconds
14-23 knots - 6 seconds
14-23 knots and changing course - 2 seconds
>23 knots - 2 seconds
>23 knots and changing course - 2 seconds
Static - 6 minutes

Class B:
Dynamic - Position, Time, SOG, COG
<2 knots - 3 minutes
>2 knots - 30 seconds
Static - 6 minutes

Class A Tx power 12 watts
Class B Tx power 2 watts

If in doubt RTFM
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