Class B AIS Filtering – The “Myth” Is Real

We recently created a hornets nest of comment here and elsewhere on the Internet when we mentioned what appeared to be a class B AIS filter on our IMO approved Furuno 2117 radar. Ben Ellison, who we consider the dean of marine tech writers, took us to task for spreading a false rumor. Ben has gotten so much conflicting comment that he went to one of the AIS tech mavens for the answer (you can read the final word on this subject here on Ben’s website). There are also a myriad of amateur and professional mariner comments, some of which are of interest.

There are many issues being addressed by commentators. We think most are missing the points which are critical to cruising yachts. To briefly summarize:

  • There is both a vessel size and AIS B filter, which when activated will filter out targets. This is confirmed by Furuno and Dr. Norris, one of the AIS fathers.
  • In crowded areas you had better assume that your AIS B or AIS A on a smaller size yacht is being filtered.
  • Note that we turn off our AIS receiver in almost all harbors and coastal environments because of the constant annoying CPA alarms and we assume the big guys do as well.
  • Offshore, properly trained ship crews will use a check list to turn theirAIS filters off. But we assume there are plenty of crews who will not take this step.
  • Over the past two years we have had six occasions in the open ocean when we were forced to alter course on passage for ships that did not respond to VHF calls and were the burdened vessel. Note that COLREGs require the stand on (right of way) vessel to maintain its course so the burdened vessel can maneuver around it.
  • Even though AIS B is not 100% foolproof, we suggest that it is a highly valuable safety feature, and would choose it and a good radar before a life raft, if there was a budgeting conflict.
  • Carrying an AIS, A or B, even if you fudge on the programmed vessel length, does not mean a lazy,  undermanned, or inebriated watch on a ship will see you.
  • The highest value for an AIS transponder is going to be in dirty weather, i.e. rain or a big sea, which masks radar returns and visual observation.
  • We always assume the other guys do not see us and plan accordingly.

You might think that with a careful visual watch and radar you would be OK. This is the case except for in heavy rain or fog. We have seen 20,000 ton and larger ships disappear from our radar screen in squalls. If they don’t show up, what do you think happens with a yacht?

With AIS B transponders now available for under $1000 they are worth serious consideration. Even if they are not a 100% guarantee of being seen you are a lot more likely to show up with than without.

Posted by Steve Dashew  (January 2, 2011)

19 Responses to “Class B AIS Filtering – The “Myth” Is Real”

  1. Phil Says:

    Would you consider an active radar reflector to be a better investment than an AIS with this new information?

  2. Steve Dashew Says:

    Hi Phil:
    Ideally you would have both active radar reflector and AIS B. But if you need to make a choice, I think I would go with AIS because of what it tells you about the other guys. But this is a tight call.

  3. Matt Marsh Says:

    I know that none of the boats I use will show up on radar in anything but perfect conditions. They’re just too small and don’t have enough metal. When ships are around, I assume they won’t see me, and I stay well clear of them.

    If the other vessel is thousands of times bigger than I am and moving twice as fast, I do not want to get close enough for COLREGS to matter. Being legally “in the right” doesn’t count for much when a two tonne boat at 10 knots collides with a 20,000 tonne boat at 20 knots. This is where AIS shines; it lets a small boat identify a big ship (and call it directly) before visual contact is made. With verbal confirmation that he sees your AIS transmission, you can ensure that both parties know how the crossing will occur, at a time when the ship still has options available to him.

    Assuming that ships will see a small boat’s AIS signal and adjust course accordingly, without actually talking to them, is not sensible. The AIS makes it easier to make contact and establish who’s doing what before it’s too late for the ship to react. It doesn’t replace mariner’s sense or the self-preservation instinct, and I think that’s an inherent flaw in the debate: the idea that AIS will make the ship avoid you. It won’t; it’ll just make sure he has a way to see your course and speed when you hail him.

    Filters are an essential tool in tight quarters, and a huge liability in open water. Since none of us small-boat folk know what a particular ship has done with them, and it’s easier for us to see the ship and react than it is for the ship to see us and react, it’s usually up to us to identify a ship, call him, and figure out each other’s intent.

  4. Cattledog Says:

    Steve, interesting thought process/choice on AIS and Radar over life raft. At first I was shocked, but after thinking it through, yes, not getting hit is better than sitting in the raft. And you are far more likely to get hit than sit in the raft.

    So in this way of thinking, SSB vs Sat Phone – which do you choose first? Assume both give me a weather solution.

  5. Steve Dashew Says:

    Hey Cattledog:
    We would pick a satphone over an SSB, or maybe two of them (one for the dink, one for the ship) with enough $ left for the AIS B

  6. Ben Ellison Says:

    Thanks for the compliment, Steve!

    However, I’d like to point out that it’s still not clear how all the Furuno FAR filters and automatic activation algorithms work together. I still believe that the Display Filter menu you posted earlier only applies to sleeping (non activated) targets, and that even if filtered off the screen they are still tracked and are automatically activated if the target enters either one of the automatic acquisition zones (which are the same for ARPA and AIS) that might be set up. This strategy seems to correspond to what I thought I read in the IEC specs last year:

    But what I missed in the Furuno manual (which is darn hard reading, I think) is section 4.12.3, which is about limiting the function of the collision alarm and which describes the Class B alarm filter thusly: “EXCEPT CLASS B: Select ON to prevent automatic activation of class B AIS targets.”

    So I think to actually ignore a Class B target completely, a FAR operator would have to apply two filters that are off by default and maybe also not use automatic acquisition zones. It would be good to get Furuno’s explanation of how this all works, but so far I haven’t gotten a specific response. Clearly, though, I overstated how strict the IMO is about filtering, as explained by Dr. Norris, and in retrospect I can see the need to filter, especially alarms, in crowded areas. But why skeptics then immediately presume that such filtering will often happen in open waters, I truly don’t understand.

  7. Cattledog Says:

    Ben, you will also notice that FURUNOTECH did not directly answer the questions about filtering etc on your blog. That is because they probably really do filter out us little guys. And as most of your readers are little guys, we wouldn’t be happy with his answer.

    I think the correct answer is filtering based upon radar distance range. Tigher pictures mean less data for the watch to assimilate. If the watch is running radar on a 5 mile range, then anything outside of this range can be filtered. Open up the range for work in open ocean and filters should not be allowed.

    Steve, have you played with the filters in real world? How does it appear they are working?

  8. Steve Dashew Says:

    We have not tested the filters, but will do so in April.

  9. Ben Ellison Says:

    Thanks, Cattledog; you voice another good illustration of a point I was trying to make in the comments section of the Panbo entry regarding Dr. Norris. The core myth about AIS filtering is not really about filtering specifics but rather about the idea that most ship personnel don’t give a crap about small vessels. If you’re deep into that mindset, actual filtering facts don’t matter, and the IMO, Furuno, and gosh knows who else are all in on the conspiracy to run you down. Pardon my bitterness, but I’ve been trying to demythologize this subject for years now, and frankly I’m kind of astounded by the level of prejudice I’ve seen.

    At any rate, if you accept the idea that most deck officers take their profession seriously, then a specific range on radar AIS filters might seem a bit arrogant. Consider shallow areas like Tampa Bay and the approaches to New Orleans, for instance; ships there run long channels where their draft makes them the stand on vessel in almost all crossing situations, and their main safety concern is just where and when they will meet ships coming the other way, ships that might be more than five miles away. Just saying. I don’t like filtering at all, because it feeds the prejudice about ships ignoring yachts, but I do understand how the IMO is being careful not to screw up professional level radar use. (Remember, too, that Class A transponder displays can not do any filtering, despite all the rumors otherwise.)

    Question for Steve: How many times in the last two years did ships respond to your VHF calls and/or gave way when they were the burdened vessel?

  10. Steve Dashew Says:

    Howdy Ben:
    Re give way vessels IN THE OPEN OCEAN not answering calls when CPA issues were obvious, probably six times in the last two or three years.
    Another point, Ben, which needs to be clear, is that the AIS issue is not a factor in our minds in high traffic areas. Everyone – yachties and big guys – are then on alert. It is OFFSHORE where things are more relaxed that we are concerned. From many years of voyaging I can tell you that we have had mostly good experiences with ships. But there are exceptions, and that is what you have to plan for. On countless occasions we have spoken with ships and asked if they see us followed by a pause on the other end. Then, after giving range and bearing data, “Oh, we see you now”.

    The key is to use defensive seamanship, assume the other guys are not going to see you, and use AIS B.

  11. Ben Ellison Says:

    Thanks, Steve. What I’m curious about is how many times you’ve had good experiences with ships at sea when “CPA issues were obvious”? I’m going to guess that it’s many more times than the six bad experiences. I thoroughly agree that nothing should be taken for granted, but what I’ve observed is that any hint of bad behavior on the part of ships leads some sailors to presume they’re nearly all bad, and hence there’s no value to things like Class B.

  12. Steve Dashew Says:

    You are correct, Ben, in saying we have had many more positive experiences than negative with ships at sea. However, in situations where these is a tight CPA the ratio has been recently surprising.

    Take 2008 between Greenland and Ireland, the North Atlantic. In EVERY close CPA – four times – the burdened ship did not answer VHF and we were forced to alter course. In all four cases the CPA was under a mile. Each situation had us on a ESE course and the ship heading SW, so there is a good chance these were Russian vessels. If they had been Norwegian, the close CPAs would not have occurred. I want to reiterate that AIS B is a great tool, one that we recommend highly. But it is not going to protect us from every situation, which is why we stand watch even in the middle of the South Atlantic or Indian Oceans where you can go weeks without seeing another vessel.

  13. Cattledog Says:

    “At any rate, if you accept the idea that most deck officers take their profession seriously”

    Dear Ben, they may take the paycheck seriously, but many are incompetent. Let me list a few to remind you:

    Exon Valdez
    Queen of the North –
    and you know I can go on and on…

    My personal experience – I like to wiggle my way on planes, trains and boats and drive them. Mainly in 3rd world countries. I am not licensed to operate any of these but most of the time someone competent is watching me and giving me instruction. HOWEVER, on several occasions, mainly boats, I have been left alone. Completely alone for hours. Two of those times going up the Pearl River in south China. The largest of these was a coastal container ship in the range of 300 feet. The stupid crew spent 15 mins watching me, explained where they wanted to go up the river and then went below to nap and play cards. The only time they checked on me was when they brought me a bowl of food after 3 hours. I won’t detail the second Pearl River experience as I know for a fact that one of those involved reads this blog once in a while… lol

    I have also commercial fished with my father. On several occasions while drifting overnight, we have had container ships with rather large brand names go by, no response on radio and came way too close. Close enough that I shot a flare onto the bridge deck and still no response.

    So your and my ideas on “professionalism” are different. These guys in the open ocean are NOT looking out the windows as much as you think they are. The Queen of the North is a prime example. I am sure the people involved feel like Xrap over it, but exboy/girl friend were arguing and lost situational awareness and flat hit the rocks. In this case, I will bet they had radar alarms suppressed. And in my mind, this should not be allowed.

    On your point about size and caring – you have this mindset already too but don’t realize it. Think about your travels over the last few years, at times you have seen debris in the water, on most occasions you see it it harmless and proceed thru it. On other occasions, you have changed course to avoid a percieved threat to your boat. If you think honestly about this, you will realize you have made these types of decisions for wildlife and the occasional dink at the harbor entrance. It is human nature. Now when you are in a bulk carrier or container ship, just about everthing other than another “super vessel” is not a worry. Certainly a 50′ fiberglass playboat is nothing to worry about 1000 miles from anywhere. Hell, you only see them once every few years out there. So the mind plays tricks on us and makes us complacent. We forget our checklist to reset the alarms opting for a hot meal when we are 50 miles offshore… It happens. It happens alot.

  14. wolff Says:

    “With AIS B transponders now available for under $1000 they are worth serious consideration”

    An option here is the Standard Horizon GX 2100 VHF 25w radio with class b AIS for around $300 USD – This radio received the 2010 NMMA Innovation Award

  15. Ben Ellison Says:

    Wolff, the GX 2100 Matrix AIS VHFs are great, but they only receive AIS. They are NOT Class B transponders. It may be impossible to combine a transponder with a VHF transmitter in one box due to regulatory restrictions and/or interference problems. At any rate no manufacturer has done it yet, though there are special antenna splitters for sharing a single antenna with a 25w VHF and Class B transponder.

    Cattledog, your last paragraph rendered me pretty much speechless. I feel sorry for you.

  16. Paul Shard Says:


    You mention you turn AIS off in harbour and even in coastal environments?

    # Note that we turn off our AIS receiver in almost all harbors and coastal environments because of the constant annoying CPA alarms and we assume the big guys do as well.

    Perhaps your unit needs upgrading. I find our unit (Raymarine AIS500) is very useful in harbour since it shows the other ships so clearly on the chart. The audible alarm can be turned off and then the ships in dangerous situations just blink and have a bigger target on screen. This summer we were in Solent/North Sea/Baltic and it was very useful all the time. I would hate to have it turned off.

    Best regards

    Paul Shard

    ps. We did a podcast on using AIS in New York Harbour… and included the segment in our television series “Distant Shores”.

  17. Steve Dashew Says:

    Hi Paul:
    In harbor the radar works fine and we find that having both AIS and radar targets showing up is too much clutter.

  18. Wolff Says:

    Ben – you are correct the Standard Horizon is a receiver only and not a transponder. Thanks for the clarification 🙂

  19. Paul Shard Says:

    Hi Steve,

    I know what you mean about clutter. We do leave the radar on as well but find the AIS well worth keeping on in port. Boats like ours that have just an 18 inch Radome which will be much less precise than your large (48″?) open array.

    ps. Thanks for doing such a detailed blog. Always good to read and so informative!