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  #21  
Old 03-09-2011
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AIS "B" versus "A"

I just purchased and installed a AIS send and receive unit. This is a class "B" unit and it has to be programed with a MMSI code which is available for free from Boat US in example. Class "B" is relatively inexpensive and it is not mandatory yet. Commercial vessels over 65' must have a class "A" AIS unit which cost several thousand dollars.
I live in the San Juan Islands WA were boating is an all year affair.
After installation I was eager to try my new gadget and found that I could receive no problem but when I went on the bridge of vessels with an "A" class receiver they were not programed to receive transmissions from an "B" class like mine.
The likeliness that a tanker / ferry etc. in example will be looking for your "B" signal is slim if not non existent, you will however see an be seen by other class "B" vessels, and you will "see only" class "A" vessels.

Gunnar SJIwatertaxi.net
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Old 03-09-2011
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AIS is a great feature..however I have talked to a number of commercial vessels on the high seas via Ham radio especially near known pirate areas that they will in fact turn off AIS not to alert the bad guys of their location...
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  #23  
Old 03-09-2011
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Panbo: The Marine Electronics Weblog: Class B AIS filtering, the word from Dr. Norris
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  #24  
Old 03-10-2011
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Quote:
Originally Posted by wickman View Post
I just purchased and installed a AIS send and receive unit. This is a class "B" unit and it has to be programed with a MMSI code which is available for free from Boat US in example. Class "B" is relatively inexpensive and it is not mandatory yet. Commercial vessels over 65' must have a class "A" AIS unit which cost several thousand dollars.
I live in the San Juan Islands WA were boating is an all year affair.
After installation I was eager to try my new gadget and found that I could receive no problem but when I went on the bridge of vessels with an "A" class receiver they were not programed to receive transmissions from an "B" class like mine.
The likeliness that a tanker / ferry etc. in example will be looking for your "B" signal is slim if not non existent, you will however see an be seen by other class "B" vessels, and you will "see only" class "A" vessels.

Gunnar SJIwatertaxi.net
Hei Gunnar, can you elaborate more on that, I mean that are distressing news. For me one of the big problems with sailboats is that they are very difficult to spot on a radar especially if the sea has waves. A class B AIS (send and receive) makes all the sense to be seen by ships.

But they are expensive. You have said that you have found one relatively inexpensive. What are you talking about and what is the price?

And you have said this:

"The likeliness that a tanker / ferry etc. in example will be looking for your "B" signal is slim if not non existent, you will however see an be seen by other class "B" vessels, and you will "see only" class "A" vessels. "

And I hope you are wrong

For what I have read there is not a difference between A and B receivers except that the A are more powerful and have precedence over B signals. This means that B signals will only "arrive" in between A signals.

You seem to know of what you are talking about. Do you mean that in the area where you have tested it, trying to find B signals from a A receiver in a ship, the space was saturated, like in a Port, or that happens in normal traffic circumstances? I mean, the B signals not arriving to an A receiver making invisible for them (ships) small boats equipped with a B transmitter. And you say you can only see signals from Class A on a Class B receiver. You mean that your class B will not show the other boats equipped with class B transmitters? This makes not much sense.

If it is so something is very wrong about this technology. It was supposed to work!

Regards

Paulo

Last edited by PCP; 03-10-2011 at 09:00 AM.
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  #25  
Old 03-10-2011
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There is a good explanation of AIS Class B filtering here.

The salient points are;
Quote:
... Particularly in busy areas, small craft often pass closer to ships than is generally considered safe for ship-to-ship encounters, even though needing particular alertness by the small craft skipper.
For this reason, especially in areas that are crowded with small craft but that also have appreciable shipping movements ... it could well be the case that any activation of Class B targets will cause almost constant activation of the Closest Point of Approach (CPA) alarm on the ship - continually distracting the navigating officer/pilot. A 1.0 NM CPA may be appropriate for ship-to-ship encounters in such an area, but many small craft skippers will be quite happy approaching ships at very much closer distances. Therefore, filtering of all AIS Class B targets, together with preventing their activation, may be the appropriate strategy in such areas to avoid possibly dangerous alarm distraction of the bridge team.
Many such areas are found around the world, justifying the inclusion of such a mode in manufacturers' equipment. It should not be forgotten that the bridge windows form the most widely used navigational aid. When operating in busy areas in reasonable visibility they normally form the primary collision avoidance tool.
In inclement weather in such waters it would generally be the correct practice to switch off the AIS Class B filter, considerably improving the probability of identifying small craft in poor visibility and bad radar clutter conditions. In general, there would be fewer Class B targets in such conditions. These would naturally wish to keep a greater distance from ships and, in any case, any detrimental over-alarming of the ships system would, in these conditions, be compensated by the benefits of increased probability of target detection. Of course, in other than crowded waters in good visibility, the AIS Class B filter should generally be switched off...

...My view is that is preferable for recreational craft to carry and use AIS Class B transponders.
This makes craft highly visible to an ever increasing number of ships in normal circumstances. It does not guarantee that you will be observed on a particular ship's AIS system but it increases the chances way above 0%, which is what happens if you don't use an AIS transponder.
Remember that a ship's radar will not be guaranteed to see you either, neither are you guaranteed to be seen from the bridge windows. However, the chances of being seen visually, by radar or by AIS will be greatly increased compared with relying on just visual and radar visibility.
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  #26  
Old 03-12-2011
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Older Class A don't see class B

Unless, for all the reasons given previously, a Class A vessel decides to ignore Class B signals; your class B info will be seen on a Class A vessel.
It may be that the Class A vessel you tested your unit with was running early software on their device. I understand this may happen because early Class A was designed before the Class B protocols were envisioned or finalized. There is probably an update available for the suspect class A unit, but it has not been applied. I've also heard from people that the Class A vessel can see them but their vessel info is not showing up on the Class a vessel screen, just the blip is there. Again, a related software incompatibility issue. If other Class B vessels can see you then try contacting another Class A vessel sometime and they will probably respond that "I see you now".
Another pretty typical problem with new Class B installations is a bad antenna or shorted connector on the VHF side. Same as with your VHF radio; you may still get very strong signals, but you can't transmit.
Hope this helps.
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