There's been a myth circulating for some time that ships "turn off" Class B targets. I see it pop up now and then and occasionally a customer asks me about it. Recently someone even posted that ships have a big red IGNORE CLASS B button to declutter their displays. That's nonsense.
Ship's captains don't want to hit small vessels! The COLREGS are clear that all means to avoid collision must be employed. I mean, imagine a ship's captain in court after running down a small boat and saying he "turned them off".
Even industry people are promulgating this myth. As reported here, a tech support person at Simrad said "It is normal for Class A units not to be able to communicate with Class B units". This has since been clarified by Simrad of course.
This misinformation is very frustrating because the AIS system was designed for interoperability. Class A and Class B were carefully designed to work together. It's true that older Class A equipment, produced before the Class B standards were created, may not be able to display the name and size of a Class B target, but they still "see" the target and it's position reports. They will still have CPA and TCPA.
There's an excellent discussion about all of this here and it also talks about whether the 30 second reporting rate of Class B poses a significant problem. I'm sure there will be debate about it, but in our local waters, the Auckland Harbourmaster did a study of this last year. They tested a Class B unit on a reasonably fast ferry (26 knots) and determined that even on a vessel of that speed it did not pose a significant safety or reporting issue. He since went on to change the requirement to allow either Class A or Class B on the ferries. In my own experience navigating among, those now Class B equipped, ferries I do not see much "zig zagging". The ferries here must change course frequently to avoid recreational boats, yet their Class B data appears quite steady to me. Of course at that speed it isn't a continuous track but it isn't erratic and is easy to interpret. More importantly, CPA alarms work fine.
In the same Panbo article there is a fair bit of discussion about filtering of targets. This is something we have focussed a lot of attention in our products. An alarm that rings all the time is just going to be turned off and be of little use when it's really needed.
In the comments to the blog entry above, Ben Ellison from Panbo writes "I thoroughly agree with you about AIS alarms. I think every manufacturer has a ways to go to get them right, often a long ways. I hope every developer takes a good look at what Vesper Marine is doing. They are a tiny company, but almost entirely focused on AIS plotting and alarming.
For instance, the Vesper WatchMate not only has an alarm filter based on target speed but also one based on your own vessel speed. So if, say, you slow way down and maneuver around while landing a fish, you won't get bothered by CPA alarms that aren't actually meaningful. Vesper also supports profiles so you can have different alarm set ups for fishing, offshore, harbor, etc"
But people do need to understand the implications of target filters. In our products we have two schemes. One is used to suppress alarms and the other to filter (or declutter) the display. Naturally, if you disable the alarm for a particular scenario then you won't get a warning if that situation unfolds. We've tried to make it very easy for people to understand and give them the controls needed as well as simple ways (we call them profiles) for people to switch them on and off.