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Old 17-06-2010, 22:05   #31
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The failure of her Class B AIS transponder to show up on the ships in Sydney Harbor is disturbing. I know that I will see the ships before they see my lower power Class B transponder, but I have assumed that they will see me when we get a little closer.

A week or so ago I was entering the Chesapeake Bay early in the morning, when I got a call on Ch 16-"Sailboat at Lat-Long, speed, course, this is US warship 3, 6 miles on your starboard beam." Out of the haze was coming an aircraft carrier, and when I responded to his call he said he was going to follow me toward the bay. I asked him if he saw me on his AIS, and he said that he had my MMSI, but not my name. This is similar to the findings of the Australian safety review board, and indicates some shortcomings of the Class B system.

I have had pretty good luck with the Class B transponder in less crowded waters, like watching the Trinidad-Tobago ferry charge towards me at 37 knots, and make a course change to miss me at about 7-8 miles. I assumed he got the AIS information first, and confirmed it on radar, but now I'm not so sure. I have picked up other Class B units at about 8-10 miles with a pushpit antenna, and 12-15 with the masthead antenna. I have been using the masthead antenna offshore, to increase the AIS detection range, but I was overwhelmed with 81 targets off Miami, and switched back to the lower antenna to reduce the system to the closest 50 targets.


I have called a few ships when the AIS predicts a close CPA, and they have all reported seeing me on the AIS. After reading the report, I think that I will be calling more ships to see if they are picking up the Class B transponder and getting my name and vessel type.

I am using the Vesper Watchmate for collision avoidance, and once you set up its rules on CPA and TCPA for alarms, you have to manually mute each vessel that meets these rules. It is interesting that Jessica had taken a radar course (but still couldn't see a ship target 1 mile away), but there are no courses on AIS use. I'm sure she is now an expert on both systems.
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Old 18-06-2010, 20:57   #32
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Don,

Quote:
Originally Posted by donradcliffe View Post
The failure of her Class B AIS transponder to show up on the ships in Sydney Harbor is disturbing. I know that I will see the ships before they see my lower power Class B transponder, but I have assumed that they will see me when we get a little closer.
We should all be learning from these reports since that is what they are for and I agree with you. The official line is that Class B shows up on all displays and can't be turned off but the evidence seems to contradict the official statements.

I have often wondered why more "yachts" don't show up on the web displays (or Google) of AIS signals and am getting really worried about the utility of paying for (money and battery power) a class B transponder.



-Sven
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Old 18-06-2010, 21:28   #33
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There is a discussion on the Class A/ Class B situation here.

Panbo: The Marine Electronics Weblog: The Class B AIS filtering myth revisited, arrrrrgh!
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