Class B AIS filtering, the word from Dr. Norris

Ben Ellison

Ben Ellison

Panbo editor, publisher & chief bottlewasher from 4/2005 until 8/2018, and now pleased to have Ben Stein as a very able publisher, webmaster, and editing colleague. Please don't regard him as an "expert"; he's getting quite old and thinks that "fadiddling fumble-putz" is a more accurate description.

91 Responses

  1. John - gCaptain says:

    I think it’s important that we put up a wall of distinction between identification and avoidance. The filtering of Class B signals is suppose to assist in allowing ships to avoid other ships in busy waterways… at the time watchkeepers are at their most heightened state of awareness. At these times there are plenty of eyes on the water and, often, there is not much a ship can do to get out of the way of small boat traffic (with or without AIS).
    Now identification and improved awareness in light to moderate traffic areas is where AIS shines. Let’s face it, a boat’s navigation light is never mounted high enough or is bright enough to be seen clearly by ships. So, unless the watch-keeper forgot to disable the filtering, having a working class-b system is going to help prevent a collision.
    And, while Andy’s comments are dead on target, I would like to make a point about the following statement;
    ” 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.”
    CPA is nearly useless in ship-boat encounters and, yes, CPA alarms are often of great nuisance BUT modern ecdis & radar systems with AIS overlay also show bow and stern crossing time and range. I rarely worry if a boat is set to pass a mile astern of my ship (unless I think he might be asleep) but if he is crossing ahead of me by mile I’ll be ready to act quick.
    The point is that few in the boating community understand how we, aboard ships, use AIS and the full spectrum of advantages that come with installing a class B transceiver.

  2. Cattledog says:

    So… I think I should install Class A and program it to indicate I am an Arleigh Burke Class (Aegis), Guided Missile Destroyers?
    That’ll keep em away!

  3. Furuno Tech says:

    Andy provided an excellent summary of CLASS B Filtering concerns and capabilities with respect to IMO(Big Ship)Radar systems.
    Many of the issues that Andy mentioned have already been explained to me by several WA State Ferry Captains. They are simply; the Class B AIS Update Rate (30 sec/update)is simply too slow and smaller vessels navigate too closely for Class B information to be useful to larger vessels in confined waters, especially alarm parameters.
    The WA State Ferry (WSF) Fleet is the largest passenger ferry system in the western hemisphere. WSF has been a valued Furuno Customer for the past two decades and their opinions/demands carry a lot of weight with our company. The fact that Andy reaffirmed their same concerns in his explanation justifies the reasons why these settings exist on our products even though many posters in this forum might not agree.
    Furthermore, as Andy points out, because of feedback from the WA State Ferry Captains and other Furuno customers, we have already implemented certain types of AIS Alarm Filtering Criteria to enhance the usefullness of the AIS Target Overlay capability on many of our products, including Navnet 3D and other Furuno radar/plotter systems. Please trust that these new features have not been implemented to mask certain classes of AIS Targets but instead, we are trying to make AIS monitoring for ALL CLASSES more useful and less distracting to our customers, thus improving navigation safety and the usefulness of our products.
    Of course, Furuno encourages feedback from any customers through this forum or directly to our company. We are reading and listening.
    A Happy and Prosperous New Year to All,
    Furuno Tech

  4. norse says:

    It’s unfortunate that the boats (with Class B with its slow update rate) are the ones which need a higher update rate most, as their course and speed are less predictable (or even erratic) than a ship’s.
    For some AIS targets, there is certainly zero collision danger. Those should be safe to hide.
    The rest have a wide range of collision risk, ranging from something to be aware of to something to be warned about. The need-to-monitor ones must not be hidden. Only the high-risk cases should cause an alarm.
    It must be possible to reduce clutter and false alarms without subverting the safety feature of AIS.

  5. Cattledog says:

    Oh this is going to be costly to Furuno someday.
    I sometimes wonder if have the legal dept and their insurance carrier be part of the product development team might be a good thing? I know it’s really not a good thing because it will stifle development / improvement.
    Someday, a big black ship is going to run over a family of 6 in someplace that it just shouldn’t happen. This “accident” will be witnessed and possibly recorded by nearby ships, or thru USCG recordings of AIS/Radar history. Even worse, the local witness might even have video of the “accident” as this is becoming quite common on small vessels.
    Now the legal team for the deceased will be digging thru all records, documents, emails and blog posts plus the USCG recordings. They will paint a picture of corporate malfeasance, apathy and outright subservience to “large customer interests” in lieu of solid engineering and safety protocols.
    It will be argued in front of a jury. Ignoring all rational reasons for these engineering choices, the jury will see photos of mangled bodies of small children and vote with the manufacturer’s dollars to hurt the manufacturer as much as they can. It will be an emotional decision, not one based upon solid customer input or great engineering.
    Somewhere, a lawyer has read Furuno Tech’s blogging and is now watching all marine collisions and actually hoping for it to happen. After all, he only has an 80 footer and really wants that new 164 from Westport Yachts.
    Sometimes, you can be too close to the subject to see how the outside world views you.

  6. Kurt Schwehr says:

    All the more reason that all of these specification documents should be open and not hidden behind pay walls. In my opinion, not knowing what exactly is in these documents is detrimental to mariner safety. While I think that Andy is a pretty smart guy, it would be a different type of discussion if everyone reading this article could see the actual text.

  7. CAW says:

    Well, given what I have just read, re-read, and re-re-read, Class B suppliers had better start working on regulators if they want people to consider the delta cost between Class A and Class B to be worth it. Unfortunately for Vesper, I was about to buy their new B unit, now it seems the Standard radio with embedded Class A AIS + a better GPIRB + a SART are a better way to allocate my funds.

  8. Ben Ellison Ben Ellison says:

    Good grief, CAW, part of what you just read and reread is an expert telling you that Class B AIS is worth carrying, and an experienced ship captain (John) agreeing with him. If you were told that some ships turn up their radar filters so they only see big targets in some situations, would you then think it pointless to carry a good radar reflector?
    Also, what is a “Standard radio with embedded Class A AIS”? The Matrix AIS VHFs contain a dual channel receiver that gets all Class A and B messages, just like every Class A and B transponder does.

  9. Anonymous says:

    In the more crowded Chesapeake and Delaware Bays I can duck into shallow water where the ships can’t go so their turning off B doesn’t matter as much there. Even if they do not see me they cannot hit me. Where I really really want the ship users having B enabled is offshore where they may be coming up on me at much higher speeds and no matter where I go they can follow because there is sufficient depth and I may not have the speed (6kt) to get out of their way if they make maneuvers not seeing me at all.
    Another comment on the reporting rate – the 30 seconds works great for most class B scale displacement boats but someone should increase the reporting frequency with speed – thus if a class B boat is doing 30 kn it reports position a lot more frequently, if not mandate A for them. With this design issue addressed on the transmitting end, the people doing the display logic have much more ability to flag the boats at risk and ignore the ones that are not a problem.

  10. Capt.Kenn E. Beck says:

    Good post, Dr. Norris has given a good explanation, Furuno Tech remarks were interesting as well. The fact that ships will have the option to not display Class B is, practicably speaking, a non-issue. It’s like saying no point in having a light on your bicycle at night because some drivers might not be looking out the window. Watch standers on ships have always had the option of suppressing small targets on radar, it’s called sea return. I would think the the fact that WSF system wants the option to not display Class B as would put an end to the discussion. Can anyone reasonably argue the the the the Washington State Ferries don’t care if they can see recreation craft?

  11. nick says:

    What worries me is that there will be countless cases when someone on the big ship switches the filter ON and it will remain so for ages.
    For me this cancels any potential benefits of a (class B) transponder. Having a receiver is a great idea either way.
    Wondering why IMO (or whoever rules here) will not step in and say that filtering out class B signal is ILLEGAL. I am sure manufacturers would follow with adequate software updates.
    Why wait for an accident? A ‘class B’ citizen less important than a ‘class A’ citizen? Sure, we do not have the cash a cargo ship owner has … we do not count. Rules are made by the rich.

  12. Evan says:

    Ben, I do not believe for a minute, based on other articles I have read about ship operations, that the crew of these ships are only going to switch on the filter in certain situations, and then switch the filter back off (unless every time you turn the unit on, the filter defaults to off). For the most part, they are going to switch the filter on when they are in a crowded waterway and they are going to leave it on.
    I was going to upgrade my AIS receiver to a transponder, and now I am thinking, what is the point, especially as I sail in the Puget Sound where all those Washington State Ferries are.

  13. Bremer Speck says:

    What this entire discussion boils down to is, that it is best to have all the safety tools you can afford. That it is better to have a Class B transponder, than not. That is also better to have an AIS receiver, than not. And, if you can afford it, is it best to have a Class A transponder. More powerful and much more information and nobody is going to filter you out. Bottom line is, get all the safety tools that you can afford. Do not worry or be concerned about who filters what. To have any kind of AIS on board will add to your personal safety because YOU CAN REACT to what you see on the display. For the time being, the Vespermarine Watchmate transponder is the best insurance your money can buy! (This comes from a guy with a Class A transponder). Cheers, Ronald

  14. Dave says:

    So it would seem that while we all accept that Class B transponders are useful devices. The myth has proven to be correct. Large ship radars/ECDIS can and do filter out class B as an option

  15. Anonymous says:

    I am not sure I understand what the fuss is about Washington State ferries. The first principle of driving a boat in the PNW is to stay the hell out of their way. I have zero expectations that they will ever alter their course because I am there.
    Fortunately, they are extremely fine radar targets and an AIS receiver will see them around corners.
    Big ships and ferries with expensive electronics that are capable of selective filtering are precisely the sorts of boats that cannot (and will not) alter course for small boats. It really does not make much difference whether they see you or not, it is your business to stay out of their way.

  16. Ben Ellison Ben Ellison says:

    No, Dave; if anything, this thread is showing how alive and disturbing the myth is. It’s a complicated myth; we talk mostly about the tools — like the imaginary “ignore all Class B” button (it’s not nearly that easy) — but really it’s about the perceived attitudes of the people using the tools.
    CAW, Nick, and Evan are demonstrating how facts and rationality are irrelevant if you want to jump to conclusions based on a prejudice about how ship drivers don’t care about small vessels. Sorry, fellas, but I find your comments both ignorant about real world collision avoidance and offensive to professional mariners, and I’m depressed about them. I’ll try to respond with a kinder attitude tomorrow.

  17. Anonymous says:

    But the fundamental myth that was denied and then reintroduced by Steve dashew is that there exists settings on ” big” ship radar that turn off ( or filter that’s really semantics) class B targets and the IMO position is vague , essentially leaving open the issue to the radar companies how they choose to do it.
    I fully accept the issue is complex ie handling multiple dense class B traffic., I can understand allowing say filtering of ” sleeping ” targets of CPA filtering but overall blocking of B traders seems unfortunate

  18. RED says:

    Aside from the debate about whether or not class B filtering occurs, I for one found Dr. Norris’ reasoning for *why* filtering would be desirable to be compelling. Often decluttering has been suggested as the reason. Dr. Norris points to a much more valid purpose for filtering class B targets, that of alarm saturation in congested waters. Alarm, or “task” saturation is a serious issue because it leads to ignoring critical alarms that may be embedded in a flurry of alarms that aren’t an actual threat. This is a classic “wheat from chaff” or “fog of war” problem which places a heavy burden on the defensive actor to remain alert even when there are mostly nuisance distractions. Dr. Norris points to innovation in alarm algorithms as a path to improving the art, that certainly seems the most productive path forward.
    Richard D. s/v RED

  19. John - gCaptain says:

    ” I do not believe for a minute, based on other articles I have read about ship operations, that the crew of these ships are only going to switch on the filter in certain situations, and then switch the filter back off ”
    Let me tell you the secret to operating a large ship… checklists.
    The type of problem you described was prevalent in our industry for many years but we learned from mistakes not only in our industry but others. Today every maritime officer in the world, regardless of citizenship, is required by the IMO to take Bridge Resource Management Training. This training was derived directly from the aviation community after the NTSB realized that new electronics where creating more information than one man (the pilot) could process alone. This training teaches you specifically to how to work as a team to prevent sensory overload.
    Now it’s true that not all vessels have multiple ECDIS and Radar stations with AIS capabilities just like not all cars have snow tires. It’s also true that some vessels, including many ferry’s, are undermanned and do not have a “team” on the bridge to help the captain process the information. And even the most experienced of us make mistakes ( ). But that doesn’t mean it’s wise to run YOUR car in the snow without the added, passive, protection of good all-weather radials.
    Now back to the checklists. Publications like the ICS’s “Bridge Procedures Guide” have departure and arrival checklists and it is required that all potential dangers, including electronic ones like filtering, are included in a ship’s voyage plan. Just like airline pilots, we simply can’t take a ship out to sea without a well though out plan that includes checklists to make sure we don’t forget things like AIS filters.
    And I can assure you that those of us with a voice in the industry are doing our best to make sure professional mariners are aware of, and incorporate into their ship specific procedures, the potential for this specific problem:
    But I can assure you, those mariners lazy to fill out federally mandated departure voyage planning paperwork and are blaise about the possibility of going to jail are the same people who have no idea how to use the filters in the first place.

  20. Evan says:

    Ben, I am sorry you are disappointed, but i believe you are the one who posted a story here not long ago about the professionally crewed ship that just about ran down that single handed sailor last year. I am also sure that you are aware that this is not the only instance of large ships, that were not in a constrained waterway hitting or almost hitting small vessels. A lot of times this has more to do with poor visibility and the poor radar targets small vessels make, but it also has to do with short crewed vessels, this by the way was in an article written by an experienced large ship captain a couple of years ago.
    The whole point of spending money on a transponder is to make my vessel more visible to these large vessels, as i am not as worried about the small ones. If they are going to turn the capability to see me off, then what is the use. If there are no reminders from the equipment, or no standing orders at the begining of the watch to check the setting, you can bet that human nature is that the professional crew will forget that the previous watch turned the filter on.
    Also, someone mentioned that it is not as easy as throwing a switch to filter out the class B’s, in which case i will lay you long odds that it isnt that easy to turn it off either and beeping alarms are much more incentive to work your way through those menus than a quiet empty screen.

  21. Preston Calvert says:

    I’m sorry it is frustrating to see these conclusions being drawn, but the fact is that you seem invested in the idea that class B filtering by large vessel radars is a ‘myth’. Clearly that is not true. The only thing worth discussing now is how well the ship bridge crews manage their radar displays, and how reliable their protocols for removing class B filtering are as they enter less crowded waters, where they might encounter a small craft with only class B transmissions. I don’t know of any way of determining how well this management is done by ship crews, so small boat operators should never rely on class B AIS as a key component of their visibility to large ships. Vigilant watch-standing, with the proper use of small vessel radar and appropriate guard zones, and piloting with great respect for large vessels are the best measures. AIS class B transmission is just an additional aid with an unknown probability of helping our visibility at any given moment of time.
    The myth is dead; long live the myth.

  22. Evan says:

    Steve, actually other than when they arrive or depart from dock, I see WS Ferries manuveur to avoid vessels that they see all the time. At a max speed of 7 knots, and often under sail at 1 to 3, I certainly have limited ability to avoid a ferry travelling at 20 knots. My concern is whether they do “see me” in bad visibility.
    And while I take rule 11 very seriously and will take very visible action to give way even when I am the stand on vessel (there is nothing to get the heart going like a large ship bearing down at you in the middle of the night at sea), when not in a traffic channel or limited manuveurability area, it is precisely those vessels that are the give way vessel and MUST alter course to avoid collision with my sailboat, because I am officially the least manuveurable vessel.

  23. Bill Bishop says:

    Ben, I think Dr Norris’s closing comment, in response to your question re: “Should recreational vessels have a class B unit” sums this discussion up.
    “Safety at sea, just as when driving, is governed by probabilities, not certainties. We need to be always decreasing the probability that an accident will occur to us, as individuals.”
    The more tools you have to be seen (whether perfect or not, the better your survival probability, statistically speaking at any rate. You are always responsible for your own rear end, so keep a good lookout, and turn your radar on.
    One final thought, being right, in the official accident report, will not offer much consolation. If it is a dangerous area for a small boat with limited maneuverability to be in, don’t be there, statistically speaking again.

  24. Allan Seymour says:

    Ben I had two experiences last year of big vessels seeing the SallyW’S AIS. Going into Newport RI in the dark a large cruise ship was coming out through the narrows. He did his security call and I replied could he see me on AIS coming in? He replied shortly he did and I was no problem. Last Summer crossing Penobscot Bay to Camden a large tanker was coming down the ship channel as I was about to cross it. I called him and he said he saw me on AIS and to keep coming as I would cross ahead of him. I don’t count on them seeing me but I take the initiative to call them so they can look for me. Being in a small slow boat means you have to be more aware and AIS helps me see them. To bad the Maine State ferries don’t have AIS.

  25. Capt.Kenn E. Beck says:

    Allan Seymour describes the method used by professional mariners. I would be very reluctant to place my vessel in the path of a commercial vessel with out some kind of agreement before hand. AIS greatly facilitates this type of communication.
    Bill Bishops comments are correct as well. Every vessel has a duty to maintain a lookout. To depend upon any one method alone is poor seamanship.

  26. BrianM says:

    With the talk of slow AIS B updates, does anyone make a class b ais that implements class b using SOTDMA (per ITU 1371-4). While the update rate is still slower then class A, the update rate depends on speed, unlike the (over 2knts)fixed 30s rate of CSTDMA.
    I also wonder if anyone will ever make a “black box” class a, with just antenna, 0183 & n2k connections.

  27. Ben Ellison Ben Ellison says:

    There are numerous definitions of myth, but this one will get me started: “An unproved or false collective belief that is used to justify a social institution.”
    Yes, I misunderstood how strict the IEC/IMO is about AIS alarm filtering on the relatively new AIS-integrated radars. But I very much stand by the idea that there is a “false collective belief” around filtering, and I think some of the comments here demonstrate that false belief loud and clear.
    The core of the filtering myth is that ship drivers don’t give a damn about small vessels. Nick expressed it most vividly here — “we do not have the cash…… we do not count…etc.” — but I’ve been hearing similar stuff since before Class B AIS even existed as a real technology.
    I reject the idea that most marine professionals don’t care, or are grossly careless. Of course there are examples of very poor ship driving, and I’ve written about some here, but why do so many sailors presume that those watchkeepers are the norm? (And why is it so hard to differentiate between places like much of Puget Sound, where ships are confined to channels and largely preoccupied with avoiding each other, and the open seas, which is quite a different situation?)
    Incidentally, I also find it offensive when professional mariners speak of amateur boaters like they’re all idiots. But as far as I can tell they are not making decisions about safety gear based on that false belief. Check out the comments to this 2006 Panbo entry:
    Class B filtering was presumed before the B transponders were available, and well before the IEC wrote guidelines for AIS tracking on IMO mandated radars. And that myth took hold pretty solidly despite the fact that no filtering is allowed on the displays that come with mandated Class A transponders. (Yes, Nick, the IMO made it “ILLEGAL” to filter out Class B on the primary AIS target display from the very beginning of AIS.)
    The presumption that ships don’t care is on this thread. How else can we explain Nick’s certainty that “…there will be countless cases when someone on the big ship switches the filter ON and it will remain so for ages.” Or Evan’s “I do not believe for a minute…that the crew of these ships are only going to switch on the filter in certain situations, and then switch the filter back off.” Did they actually read Furuno’s manual for the FAR-2117 radar and figure out how all the filtering options and activation algorithms work together? (In my view, it’s a darn challenging manual, and it’s hard to truly disappear a dangerous Class B target.) For instance, do they (and CAW, Dave, etc.) understand that the display filtering shown on Steve Dashew’s radar doesn’t stop tracking, doesn’t apply to active targets, and is over ridden if the filtered target enters the vessel’s ARPA target acquisition zones? Or did they jump to conclusions based on a false premise?
    What Andy Norris explained well, and I should have gathered in the first place, is that IEC/IMO is giving manufacturers and watchkeepers some latitude about how AIS targeting is integrated into the radars being slowly mandated on SOLAS ships. After all, AIS is already helping quite a bit with collision avoidance (including with Class B vessels, as you can find dozens of user testimonies about, like Allan’s, if you scan comments to Panbo’s AIS entries). And in retrospect it seems obvious (as it is to Richard D of s/v Red above) that alarm filtering can be a safety improvement in ports crowded with numerous Class B and/or small vessels that ships are obliged to “stand on” amongst anyway. (Nick might call that being a “second class citizen” but it’s written right into the rules of the road, for good reasons).
    Of course the IMO and IEC are not presuming that many professional mariners will use AIS filters to ignore vessels they should be paying attention to. And, incidentally, I didn’t make up the quotes and graphs I expurgated from IEC 62388, even if I misunderstood the leeway regarding AIS CPA alarms; the IMO really is trying to improve AIS tracking, but carefully. (Kurt has a good point, I think, that this document should be more easily available):
    It’s noteworthy that Dr. Norris describes the regulators as hoping “innovation by manufacturers would be the best way to evolve both filtering and acquisition strategies in these relatively early days of AIS/radar integration.” One suggestion I have for Furuno is to rewrite that FAR manual completely, or write a user guide that really lays out how the filtering and activation work, with examples. Another is to consider setup profiles for harbor and offshore uses, and perhaps automatic reminders activated when a ship’s speed indicates that it’s changing from one mode to another.
    I’d also like to point out that I have never, ever suggested that all ships will see you if you have Class B AIS. Quite the contrary, I’ve been noting the limited target tracking on many ships for years, like here: So the idea that this particular filtering wrinkle all of a sudden makes Class B much less useful is a straw man argument (hello, Preston ;-).
    But it doesn’t much matter what I and others say. If you’re motivated by a deep, dark belief that ships don’t give a damn about you — regardless of how unrealistic that is, and rude to the pros who are also commenting here — well then it’s easy to use this arcane filtering business to justify a dismissal of Class B technology. Mythology defined.

  28. Paul says:

    As for big ships leaving their “Ignore Class-B” filters switched on, I’ve had many mid-ocean ship encounters between California and Hawaii, and all the ones I spoke with did see my Class-B signal. Some couldn’t decode our boat’s name (common with older pre-Class-B AIS gear), but they could all see our position, course, and speed.
    Anecdotal evidence to be sure, but it makes me happy I got the Class-B transponder. One advantage of the Class-B is that it consumes less power than a Class-A — which could be a big deal on a sailboat.
    A receive-only unit would suit me almost as well as a transponder (since I always assume that I’m invisible, transponder or not), but I figure that if the other guy does want to watch for me, I should make it as easy for him as I can.

  29. Dan Corcoran (b393capt) says:

    1) Thank you Dr. Norris & Ben.
    2) I am really having a tough time comprehending the position some commenters are taking that the value of class b transponders are greatly diminished, when the contrary is clearly explained here. I would encourage anyone thinking of choosing between a receiver and a transponder, to choose the transponder. The collision avoidance feature of these products will be a lot more useful if everyone bought transponders. I for one will go out of my way to avoid your boat if I receive your AIS signal. Maybe I am not as threatening as a class-a ship, but it would sure ruin the day for both of us if we hit.
    3) My recent experience with a class b transponder, has me fully appreciating how much AIS alarms are distracting. With just a few transponders out on long island sound, its amazing how many alarms i get. Get just two of us in the same channel and its unbearable. Hopefully the talked about evolution of filtering capabilities will come to class-b users as well as class-a. If not, just maybe my next transponder will be a class-a for no other reason then having better filtering.
    4) Keep an open mind everyone. I wouldn’t be surprised if some future class-b transponders/receivers selectively filter out other class-b boats. Do I really need to see all the class-b boats ahead of me as I come into a harbor, cluttering my display and preventing me from seeing the charted depths of water ahead?
    5) It sure would be ideal, for both big ships and small, if the AIS filtering was self adjusting. So for example a bridge crew turning on a filter than involves filtering class-b boats wouldn’t be burdened with turning it off promptly when moving to a less densely populated area. As much as larger ships might use checklists to avoid such errors, us smaller boats do not.
    6) It would also be ideal, if even filtered ships appear at least as small dots on our chartplotters both in the position they are reported at (one color) and the position they are projected to be at (in between 30 second transmissions). Such an approach might make it more comfortable for manufacturers to adopt complex approaches to filtering without fear users will misuse filtering and cause an accident.

  30. Kurt Schwehr says:

    > I also wonder if anyone will ever make a “black box”
    > class a, with just antenna, 0183 & n2k connections.
    In reply to BrianM, it is not currently legal to create a Class A device that doesn’t exactly have the RS422 (or is it RS488?) and the Minimum Keyboard Display (MKD) in a more black box style. IMO would have to change the rules to allow Class A units that look more like a Class B. Having setup both, the Class B devices are *much* better put together. All the Class A’s I’ve worked with are pretty fragile physically. They might look tough on your console, but be careful working in them or around their wiring. I would be very much in favor of ditching the crappy MKDs and requiring a “real” display of some sort.
    I’ve also struggled trying to get the USCG to allow AIS on AUVs to reduce the risk of a vessel hitting one (as has happened at least 2x). Us sending out freewave, wifi, and iridium position messages doesn’t help the other ships in the area.

  31. Ben Ellison Ben Ellison says:

    Thank you, Dan, but you’ve gotten a bit confused. Class A transponders do not do any target filtering. In fact, I don’t think there’s any appreciable difference between the output of a Class A and Class B transponder.
    Filtering happens at the target display level, in this case on IMO radars. And let’s distinguish between ignoring targets and filtering them off the screen while continuing to track them. (The skeptics often don’t understand the difference.)

  32. BrianM says:

    So building something that complies with ITU 1371-4 Class A and the appropriate NMEA standards – but not IMO Class A would be illegal?
    In any case, it would be interesting to see a class b device built using SOTDMA – at least in the USA, the patent seems to be held invalid. Or I misunderstand what the reexamination certificate says. (
    Another item found a bit amusing is the statement about filtering the MKD is illegal. Really, does it matter? Alot of car stereos have larger displays then a MKD.

  33. CAW says:

    I’ve been involved in and dogged by risk benefit calculus my entire professional life, especially when it involved missiles and explosives.
    We talk in terms of the “9s.” If something is good to “one 9” that means there is a 10% chance of a bad outcome. “Six 9s” means there a 1:10000 chance of a bad outcome. Some industries such as space, air travel, energy and communications have “9” metrics they apply and track. I’ve found nothing similar in ocean commerce/safety other than passive safety equipment reliability specs.
    At sea, I have encountered ships from whose decks it looked like I might safely eat. I have also encountered those that looked like SAR cases waiting to happen. There is a statistical distribution of professionalism at sea just as there are varying cultural attitudes toward risk and responsibility. I impugn no one other than to call all humans human.
    I cannot see any statistically sound or financially substantive reason to have a Class B AIS aboard my vessel other than faith and a warm fuzzy. The opportunity cost between an A receiver I monitor and B transceiver I equally monitor and others may or may not track is, perhaps, $700.
    That is too much to spend for a technology that does not appear to do anything to increase the “nines,” and which if it does, does so randomly according to the operating orders and cultural disposition of the ship involved, and with those variables, introduces rather than reduces risk.

  34. Dave says:

    Kurt, its only “illegal ” where the class A device is to be fitted to a IMO compusory fit ship. We can fit any type of A or B device, with or without Keyboards displays etc.
    Ben, I think you are confusing some of the skeptics, I for one fully support class B transponders and I was sailing very recently off the coast of italy where the unit was a godsend.
    The main condern expressed is the radar filtering, its not at all clear that class B alarms on the FAR are activated is the “ignore Class B ” option in on. Thats the kernal of this debate isnt it.

  35. Ben Ellison Ben Ellison says:

    Please, CAW, there is no such thing as “an A receiver”…there are just AIS receivers, single or dual channel. They are not regulated or classed. You need to check the marketplace too. With a couple of Class B transponders (which include dual channel receivers) in the $500 range, and dual channel receivers at about $300, your $700 “opportunity cost” is rather flawed.
    And never mind for now your preference for unavailable statistics over the multiple positive reports here about real world Class B use. But I certainly am interested in how you figure a Class B “introduces rather than reduces risk”?

  36. Anonymous says:

    Ben: The next question is can you actually install a Class B Transponder correctly and effectively on a sailboat, given the need for separation between VHF and AIS antennas, the special GPS receiver, etc. I’d say getting clear AIS warnings at the helm might be worth more?

  37. Ben Ellison Ben Ellison says:

    Well, there certainly are a lot of sailboats around with successful Class B installs. You can get an antenna splitter and use the masthead VHF stick (though they’re rather pricey and there is usually some signal loss), or mount the AIS antenna on a spreader of rail. My first real world test of Class B was a very temporary install on a sailboat, and I was still seen by ships at 6 to 10 miles:

  38. Evan says:

    There are a number of ways to “correctly and effectively” install a Class B Transponder on a sailboat.
    Several antenna splitters are specifically designed to allow the sharing of a single VHF antenna between the VHF and a Class B transponder as one option (I seem to recall that there is one Class B transponder with the splitter built in).
    The other option is to place the VHF antenna on the top of your mast and mount the AIS antenna on the stern of your vessel (this does reduce the range somewhat). I have the second antenna mounted at the top of the mizzen mast on my ketch.
    The “special” GPS receiver is no more than an ordinary dedicated GPS receiver and some Class B units have one built in. There is no issue with mounting multiple GPS receivers on a boat (I have three), and the Class B units all provide the capability (requirement) to program exactly where the gps receiver dedicated to your transponder is located.

  39. norse says:

    CAW mentioned he was thinking of buying the Vesper WatchMate which has a Class B transponder. The extra cost of that over the Vesper WatchMate which has an AIS receiver is $400. That’s an apples-apples comparison. His question of how much extra safety you get for that money is a good one, or at least one that many others will also be asking. You could ask the same question about money spent on life-rafts, SARTs, EPIRBs, radar reflectors, insurance, etc. Those things are also in the “warm fuzzy” category.
    Or sometimes even “priceless”.
    The AIS transponder has the advantage over the SART option (for example) in that it works on the prevention side instead of the rescue side.

  40. jeff says:

    Thanks to all for an excellent discussion.
    Brian, the default reporting rate of a Class B is dictated by the standard, not the technology. In fact, all certified Class B’s must be capable of at least a 5 sec reporting rate. It is also already speed sensitive, just not the same as Class A. Class B reports positions at 3 min intervals below 2k and 30 sec above 2k. The default 30 sec reporting interval was chosen based on computer simulations to ensure that a large population of Class B units won’t overload the system.
    So from a technical point of view there is nothing preventing a faster reporting interval. It’s just that in order to certify it, and hence sell it, the end user cannot alter any of this. But if a “local” authority (USCG or maybe a harbormaster?) decided it was in the interest of safety appropriate to increase the reporting interval, they can do it and you don’t have to buy or do anything. Your existing Class B will do as it is instructed via telecommand.
    Evan, there is a difference in the required built-in GPS receiver for a Class B transponder. It must meet certain accuracy and interference standards in order for the Class B to be certified. I don’t believe that general-purpose GPS receivers must meet these standards, since I don’t think there are certifications required in order to sell recreational GPS receivers.
    Jeff Robbins
    Vesper Marine

  41. Anonymous says:

    I keep reading this thread and thinking COLREGS, and why is there is air of wanting a large vessel or ferry to use electronics to figure out we are there and then avoid us. A couple of years ago a sailboat named Ouzo was lost in the Solent near a ferry, the Pride of Bilabo. It was assumed that it was run down by the ferry. In the following inquiry, the ferry crew was not found at fault, and the everyone involved said, small boats should stay out of the way of larger vessels to the point of not having to invoke the COLREGS and should keep a continuous watch, and that they likely were never visible on Radar (I know an AIS thread).
    So, what does one conclude? An AIS transponder would have been better than nothing, or would an Active radar reflector have been more useful (Ben a new area of discussion?). I will admit that in a very crowded harbour I will switch the CPA alarm off, and keep my wits about me (and the crew’s). I suspect a commercial vessel will do the same. I do see an AIS transponder being very useful at sea or in fog, but so would an active radar reflector.
    Now, my earlier comment on good class B AIS installations came from reading the Comar manual (, it states the GPS should be “dedicated”, and an LNA type, which should not be installed within 5 Metres (16’3″) of a VHF antenna, and that the AIS antenna should be dedicated and not installed within 2 Metres (6’6″) of a VHF or DSC antenna. If you want the best range out of your AIS, a stern rail mount likely won’t cut it… spreader, yes, but that mast would be in the way, and 2m on a masthead… not on my 30ft sailboat.
    So, with that said, I would love to see the product specifications or details on the splitter that works with those requirement?

  42. Quitsa says:

    I want to join the others in thanking Ben for stimulating an interesting debate and causing more information to be available to all of us who try to operate vessels with the greatest degree of safety. After four years experience with an AIS receive-only system on a relatively fast (28 kt) powerboat, my sense is that the greatest benefit of the AIS system is not on the bridge of the big ships, but rather in giving the small boat operator a much more accurate picture of his “threat environment.” Even a cruising sail boat (I owned one for 20 years so I do have experience) operating at 4-8 kts has a reasonable amount of ability to manuever.
    Does anyone really think in this day and age of dodgy foreign flag ships undermanned with minimally qualified watchkeepers it is ever prudent to rely on the equipment and talent of the big vessel to stay safe based on your status under COLREGs? Sure, I know that there are many, many highly qualified foreign-operated merchant seamen out there and some incompetent US crews. It’s the 1 in 200 that is going to run you down. I cruised up the east coast last spring past Norfolk and did not spend a lot of time worrying about whether one of the many Navy ships with an Aegis system on board I saw would collide with me. But going past the New York harbor shipping lanes was a lot less reassuring. The AIS target display allowed me to identify and call ships in marginal crossing situations to advise them of my intentions. With just my very good open array radar, I would have been far more cautious in altering my course to take no chances and probably would not have succeeded in contacting the relevant ships with a more general hail on 16. I have similar experiences with large vessels all the time on my offshore fishing trips crossing the Boston and New York harbor approaches.
    This is another example of the “great being the enemy of the good.” Yes, cheap rapid update Class B and more sophisticated and consistent target display on the big ships would be great. That does not mean that an AIS receiver or Class B transceiver isn’t a significant contributor to safety for a small boat, with or without radar on board.

  43. Capt.Kenn E. Beck says:

    The adaption of AIS by commercial vessels has made radar reflectors less and AIS more valuable as watch standers now expend less effort to ensure radars are properly tuned.
    Broadcasting an AIS signal alone does not relive a vessel of the duty to maintain a lookout. In Granholm v. TFL Express a single handed sailor sleeping was run down by an overtaking vessel in board daylight. Both vessels were found at fault.
    In most cases a recreational sailors with sufficient skills to play COLREG games with deep sea vessels likely has enough experience to know not to, and will stay well clear.

  44. BrianM says:

    Jeff, all I can go by is ITU 1371-4 – since it can actually be downloaded for free. And I look at “Table 2” and
    I am assuming that all of the Class B AIS transceivers out there implements CSTDMA (1371-4 Annex 7).
    I’m wondering if anyone is working on/making a Class B AIS transceiver that implements SOTDMA (1371-4 Annex 2). This is allowed, according to
    Looking at Table 2 (the update rates for AIS B transmitters), this would end most of the update rate comments – by design.
    If this really is not allowed, please tell me where that information is, or just quote it – since I’m not going to spend $$$$s for safety standards that should be free.
    If you have links for other AIS-related standards that are available for free, please link them.

  45. Ben Ellison Ben Ellison says:

    SRT’s new miniature Class B AIS board, called Cobalt, “conforms to both CSTMA & SOTDMA IEC AIS standards”…but I don’t know when and where SOTDMA might go into use. You can read a bit about Cobalt and several other SRT innovations if you download their latest Pulse newsletter here:

  46. Evan says:

    *Capt Kenn, I agree in that the long standing application of COLREGS is that all vessels including the Stand-On vessel are responsible for avoiding collisions, and are held accountable even if the Give-Way vessel did not take the mandated action to avoid collision. Besides which, only a crazy person would prefer being run down to enforce a point of right a way with a large vessel.
    Having said that, I have been in a number of situations, including “offshore” (once was in the very busy Mediterranean) in gales twice in a 13m sailboat, where I do not have the maneuverability to “stay clear” when a large vessel approaches. (I.e. I am in danger of pitch poling or capsizing if I change course, and there is basically only one direction that I can go anywhere “fast”).
    It is in these situations where I am most anxious for technologies such as AIS to assist in ensuring that the large vessel knows where I am (even with active radar reflectors, when your boat is dipping beneath the wave tops, you make a very poor radar target).
    AIS receiver’s certainly help, in that I at least I know the name of the vessel approaching in order to hail it, rather than saying “Vessel approaching position xx”. But I am hoping that an investment in an AIS transmitter could also be counted on to always improve my visibility to these large, not particularly maneuverable, vessels so they know exactly where I am and give them a lot of time to plan a course change.

  47. jeff says:

    Brian, To my knowledge, the only standard the USCG and FCC currently will certify against is IEC 62287-1. This is for Class B using CSTDMA. That standard is generally in line with ITU-R M.1371 but it is the one that provides the certification requirements and testing methodology. Unfortunately, it is not available for free.
    My understanding is the forthcoming IEC 62287-2 is for Class B SOTDMA and it is targeted for committee draft w/vote in 2012.
    Regarding the comment about mounting antennas and splitters. A masthead VHF antenna gives the most range possible. But… in order to use it most people choose to share it with their VHF radio via a splitter. If you get a splitter, check that it is designed specifically for AIS transponders and has a fail-safe mode or you run the risk of damaging your equipment. There are of course trade-off’s when using a splitter. You aren’t able to transmit and receive at the same time. You’ll get the most range on transmit but you may have some loss on receive (which might be made up somewhat by antenna height). Our splitter has been designed to solve that problem by including a low-noise amplifier on the AIS RX path.
    However, keep in mind that Class B is designed around an approx. 7 mile range (not receiving of AIS targets, rather them receiving you). Increasing the height of the antenna will have the most effect and will certainly increase that range, but I’ve personally found that a high quality VHF antenna/cable on a stern arch, pole or even the rail works well and gives adequate range for the intended purpose (collision avoidance). Also like any radio on-board, installation and equipment quality makes a big difference.

  48. Russ says:

    A liveley discussion which from my perspective is focused on these questions:
    Q1: Do ships give the same attention or care to avoiding small boats as they do other ships?
    A: The prudent small boat operator should assume “No”. The filtering discussion is a red herring. In the absence of a conversation with the ship’s watch, I assume a ship at sea will hold course and a ship in or entering a harbor will proceed directly to its destination, usually via a known channel or route. It is foolhardy to assume the watch will see your small boat with any visual or electronic system and avoid you. The small boat must always keep clear.
    Q2: Is an AIS receiver a worthwhile investment?
    A: Absolutely. You can’t avoid the ship you can’t see and AIS extends your vision. However, I have seen ships on radar that didn’t appear on my AIS, and vice versa. Eyeballs, radar and AIS are all complimentary, and in that order.
    Q3: Is an AIS transponder a worthwhile investment?
    A: Yes, it will enhance the communication with the ship’s watch if they can see you on a screen and an AIS target can be directly associated with your vessel whereas radar returns do not include any information which uniquely identify your small boat. That said, with only an AIS receiver, you can still provide your range and bearing to the watch who may be able to then associate you with a radar target.
    Q4: Is an AIS class B transponder a worthwhile investment versus AIS class A?
    A: The cost trade off on these choices is not as simple as it appears. Unless your installer works for free, or you have unlimited free time, installation on your boat and integration with your existing electronics is a significant part of the cost. This closes the gap in total cost. Additionally, class A units have a control head or MKD which must be fit into the equation and significantly complicates the problem on a small boat where panel space is at a premium; this effectively increases the total cost of class A.
    A black box class A without IMO approval would significantly reduce both product cost and installation cost.
    For the record I have only a receiver and believe Class B is a waste of money due to it’s lower power and less frequent updates. If I’m going to invest in a transponder, I want to have the best possible chance of being seen. There aren’t a lot of ships between Panama and French Polynesia so I’ll revisit this question in 2012.
    Russ Irwin
    s/v New Morning

  49. Ben Ellison Ben Ellison says:

    Good points, Russ, but I’m a little taken aback by your last paragraph. Aren’t you using a Furuno FA-30, which (while no doubt an excellent AIS receiver) costs more than many perfectly good black box Class B transponders?
    Honestly, I don’t see the sense of just getting an AIS receiver these days, unless one is willing to go with one of the single channel models, which are cheap but also not as effective. If you want a decent receiver, the installed cost of a Class B that can also transmit if you want (and contains a GPS that can serve as backup navigation on some systems) is just a few hundred dollars more. Even if you’re skeptical about Class B, your Q3 point above — which has been confirmed by so many people actually cruising with Class B — seems worth the money.
    By the way, I don’t know if a black box Class A transponder is possible, but ones with a screen are getting less expensive, and less bulky and hard to install.

  50. BrianM says:

    My question about the CSA-300 (and any of the Class A units, really) – Is IEC60945 Edn 4.0 equivalent to at least IPX4 water protection? If not – is there a Class A unit that one wouldn’t be scared to mount at the helm of an express or center console powerboat?

  51. Russ says:

    Ben – Yes I have an FA30 which is an excellent receiver. Unfortunately Furuno has limited it’s range when used with NN3D to 24nm. But when I use MaxSea to view the feed I can see ships on the Pacific side of Panama (I’m on the Caribbean side) at a range of 80nm.
    In any case, I think there were few, if any, Class B transponders available in early 2008 when I installed the FA30.
    It’s the installation cost/hassle, not the equipment cost, that prevents me from moving to Class A. I would buy an FA-150 tomorrow if I could drop it into the same position as the FA30. But I need to carve out a home for the control head and run wires to it, then run new wires to somewhere for the GPS, etc., etc. The benefit does not exceed the cost for me.
    It’s an issue that the marine industry ignores. Other industries moved to standard form factors and interconnects and saw their businesses grow. The marine industry is still plagued with proprietary form factors, connectors and protocols and they suffer because of it.

  52. Russ says:

    Here are the specs for some entrepreneur for a black box class A for vessels not required to have IMO approved devices.
    – Class A power and update protocol.
    – Accept external GPS feed. (I already have two, I don’t need a third)
    – N2K and 183 data feed
    Recreational vessels can install class A today, why burden them with a bunch of requirements that are not relevant to their vessels?
    I suspect the build cost premium over class B would be less than $200 (more power and the license for a protocol stack) and the street price premium probably less than $400 on the street. Such a device would make the Class B filtering discussion irrelevant but of course filtering based on speed, CPA, etc. would continue to be relevant.
    Why would any government agency, international body or professional mariner not want to see such devices on the market?

  53. Sandy Daugherty says:

    As a student of human behavior, I’m intrigued that so many important decisions are made based on emotional rather than objective information. Allegiance to one side or the other of the Have’s vs Have-not’s schism seems to have more influence on the decision to install Class B AIS than the frank and balanced suggestion of one of the system’s most knowledgeable developers.
    Why get lost in the minusculae of range and antenna placement, costs and data rates, or certifying authorities when one who knows says its worth the time, cost and effort?
    Or is it more meaningful to ask if a “faceless, foreign, money-driven big thing” will kill you unless you wave a big sword back at him? If that is the question that matters the most to you, AIS isn’t going to matter at all. If you haven’t already, you might find a more sympathetic audience in one of the Gun Threads on an internet forum.
    When deciding whether or not to install AIS B, you should limit your consideration to these few aspects: It works. It increases your safety margins significantly. It is not perfect, There will be conditions where it will not work, whether as a reslt of human interaction, electronic failure or an Act of God. It _supplements_ traditional collision avoidance practices. It will not, nor has it ever been suggested to be a replacement for Radar, Radar management, and eyeball vigilence. So the decision is in fact limited to
    1. Can I afford it.
    2. Can I afford not to have it.

  54. Ben Ellison Ben Ellison says:

    Russ, I doubt there will ever, ever be an AIS Transponder without its own built-in GPS. They’re critical to the message timing and its trivial to stick another GPS antenna somewhere, even on the smallest boat (on many boats, the antenna doesn’t even need to be on deck).
    I’m also dubious about your imagined price difference of $400. I think there’s more to Class A than many realize. Besides I don’t think a black box version can be approved at any level until the IMO/IEC comes up with some rules about how to enter user data from a third party display and keyboard. Not too hard, maybe, but I don’t think the regulators have shown any interest.
    So I have question for you. Let’s say you had no AIS gear at all right now, and you realized that the under-$1,000 Class A transponder is many years away (and possibly a fantasy). What would you buy now…receiver or black box Class B transponder?

  55. Kees says:

    Sandy, well said.

  56. Kurt Schwehr says:

    First, there really is not much difference between a class A or class B other than the RF power and some minor settings. The cost difference is mainly from economies of scale and the complexity of the junction box and mkd. The USCG and FCC can both veto an AIS device being approved for use in the US. The USCG is not going go against the IMO specs. The discussion of a black box here is great, but I doubt that it will get anywhere without engaging the IMO and IEC members…
    Class B’s are around to reduce the load on the VHF channels, so there are a lot of people who do not want the black box version of A to happen from my observations. But don’t let that stop the discussion here!
    As for not having a class B on board, that’s like not having a life jacket or lights on your vessel. It is not a guarantee of anything, but it definitely reduces your risk. As someone who analyzes that risk, I just can’t follow the people who go for the recv only over the B. Even with a sub optimal antenna and mounting position, many vessels will see you via AIS.
    From a court of law standpoint, it will not be long before they say that a prudent mariner caries an AIS transmitter just like happened with VHF voice radio.
    BTW, look up the definition of transponder… only 2 messages are transponder messages and they are not currently in use in the US. Hence my referring to AIS transceivers.

  57. BrianM says:

    It appears that Furuno will be happy to sell you the “interesting” part of the FA-150 kit without the MKD…
    Or is there some newer requirement that everything is made in 1 box now?

  58. Ben Ellison Ben Ellison says:

    Thanks, Kurt. And I know you’re right about the strict definition of transponder, but once I saw that even the USCG is calling them AIS transponders, I figure that that’s the least confusing term for them.
    Brian, I doubt that box will work without the MKD, which is probably why the manual is with the full FA150 setup.
    Happy New Year to all!

  59. Larry Brandt says:

    Thanks, Ben, for the great discussion. I have learned a lot. And I am more convinced than ever that there is a new AIS-B in S/V High Flight’s upgrade plan in 2011.
    But Kurt, good for you! I’m with you. AIS is undeniably NOT a transponder. It is incontestably a transceiver. (And for aviation buffs, the aviation analogue of AIS would not be TCAS, but rather ADS; that is, Automatic Dependent Surveillance.)

  60. robert says:

    Thank you for this information. i am a professional mariner in the us navy and have a 42ft sailboat, so i can relate to both sides of this. some of the comments im reading here are very disturbing. i do try to believe that most professional mariners will do the right thing, but these worries about big ships running you down seem to point more to small boats not wanting to do whats needed or required. i mean simply COLREGS. yes the give way vessel must give way, but you must give way and take emergency action if he doesn’t. any device that may help any boat or ship see me be it radar or ais, the better for me and my family. why aren’t people keeping visual lookouts? why aren’t people paying attention to thier surroundings? why are we waiting to the last moment to call a ship and let them know what we are about to do? the person on the helm is accountable for all of this. right now it looks like excuses to pass the blame to big ships or Furuno and make it thier responsibility to not run into us. buts thats not the case. its MY responsibility to ensure I do everything in my power to not get hit, to ensure I follow the COLREGS, and by doing so keep myself, my family, and any passengers safe. AIS is just a tool to assist in this. when i leave my dock its not big ships i am worried about. its people in boats that dont know the COLREGS, and big powerboats that are partying and don have anyone sober to lookout or even anyone on the wheel

  61. Ron Rogers says:

    First of all, it depends on the situation. Is the mix of ships and small craft occurring in the English Channel,Puget Sound, the inland rivers, the ICW, Chesapeake Bay, or on the high seas? Is the small craft a tug with tow, a sailboat, a commercial fishing vessel, or a powerboat? What are the comparative chances the operators know the COLREGS and will they reply on VHF with correct radio procedure in English (the IMO standard)? BTW, on the Chesapeake, USN “warships” will not reply at all and in Norfolk, they are basically abusive. I think the latter behavior is because they are young OOD performing prescribed maneuvers restricted by draft.
    With this as backdrop, it should be apparent that AIS “A” or “B” is not a protective shield which will cause the big boys to avoid you. It’s no better than pleasure boat navigation lights which often cannot be seen by ships. I take my cue from tug boat captains – they run outside the shipping channels and use their VHF in restricted waters.
    When a ship filters AIS by length alone, it is likely that many Class B emitters will be blocked. Unless the software is written so that any AIS target on a potential collision course will override filter settings, both AIS A and B offer little to small craft. I *think* that the Furuno rep said that is how their radar/AIS software is written.
    For me, AIS is a tool which provides me with information which facilitates contacting other vessels and reduces the frequency of my queries as to a large vessel’s intentions. My personal opinion is that unless there is an American pilot on a foreign merchant ship, there is no reason to place any faith in the competence of that ship’s officers who often are independent contractors who barely know one another. I have no faith that they are resetting their AIS filters as they approach or depart coastal waters. These are not airline crews with check-rides, myriad mandated checklists and external course monitoring and control. Well, maybe small commuter airline crews. There are a startling number of marine casualties each year.
    Ron Rogers

  62. del says:

    The IEC standard for Class B using SOTDMA (as shown in ITU-1371-4) is still in the committee stages, so no product will be avialable to conform to it for a while (I’m guessing 12-18 months, but don’t quote me….). By design this unit WILL be more power hungry than the existing Class B-CSTDMA variant, so would not be appropriate for some installations.
    I’ve read the SRT docs on their forthcoming products several times, and will believe them when I see ’em.
    In answer to someone else, installing a Class B on a 7m sailing catamaran is easy – I put the combined GPS&AIS antenna on a bar running between my backstays, so giving it a 3m ASL height. It worked just fine, thanks. Granted, on a mono with single backstays, you couldn’t do this…..

  63. Russ says:

    Ben: You can always construct a hypothetical to get the answer you want.
    If I was intalling the first device on New Morning today it would be a Furuno FA-150.
    PS: I spent 30 years in high tech product development, I have a pretty good sense of BOM cost and total product cost.

  64. BrianM says:

    Del, if you read the various revisions of ITU 1371, you would see that when class B was added, it was SOTDMA. From -1 (2001) until -2 (2006), the only way to do class B was with SOTDMA. You mean to tell me the IEC is just acting on something that has been in the standard for 10 years… now?
    What do they need to draft? It has been there all along…
    For those reading that have access to IEC specs: The AIS testing protocols circa 2005 (pre-CSTDMA) – was it impossible to market a Class B AIS unit?

  65. Ben Ellison Ben Ellison says:

    Thanks, Russ. I didn’t realize that anyone was so into the AIS class differences that B could be considered a “waste of money” at about $500 and A the way to go at about $4,000.
    Incidentally, do you think there’s something inferior about the new Comar/Comnav Class A units at about $2,500? They certainly look easier to install on a yacht.
    Also, would you venture any guesses about the world wide market for Class A transponders? Looking at the SRT investor pages, they’re darn excited about some 32,000 EU fisheries and inland waters mandates yet to be filled. I’m pretty sure the USCG will mandate more Class A here eventually (though maybe just a few thousand vessels), and I think there are similar moves afoot in the far east. But aren’t these sorts of numbers trivial compared to consumer electronics?

  66. Russ says:

    Ben: It’s your hypothetical. You’re question was in the context of new install, with no prior AIS. Street price for an FA-150 is about $3,500. If I were outfitting New Morning today I would spend the incremental $3,000 for class A, keeping in mind the total cost with installation (new power from the electrical panel, data cabling, antenna mounts on the spreader, antenna wires up the mast, cutting a hole in the nav station panel, possibly reconfiguring the nav panel to make room for the control unit, etc.) would probably end up costing another couple thousand. I would spend the incremental $3,000 for class A rather than waste all my installation hassle and expense on class B. I certainly realize that other boat owners have different budget, would not be willing to spend the incremental $3,000 and I respect their choice.
    I would install Furuno because I like to limit the number of vendors with whom I have to work and because I have a NavNet 3D system. Furuno answers my calls and emails and I know they’re going to take responsibility for any issues interfacing with NavNet3D or MaxSea. If I were to choose another vendor’s system then I set myself up for the two support organizations pointing fingers at each other. Keep in mind the support you get as a journalist is different than the support we get as consumers. The knee jerk reaction from most vendors on interface issues is that it’s “the other system”.
    Let me be clear I have no reason to believe the Comar/Comnav unit, or any other vendor’s AIS unit, is anything less than perfect. It’s just that I think purchase price is only a small part of the vendor selection decision.
    Regarding market size, I would guess that the TAM for AIS units is about 50-75% of the size of the installed base of fixed mount VHF units. The market for class A vs B is a function of price. If class A cost the same as class B, I believe 95% of the units sold would be class A. Since they’re not the same price the market is much smaller; but I firmly believe there is a lot of elasticity in the demand for class A. I don’t know how many units that actually is, but I completely agree it’s not millions of units.
    I’m not talking about consumer electronics build cost. If a class B transceiver is $500 on the street then it’s finished goods cost is probably about $250 so I’m saying a class A black box could be built for $450 and on the street for $900. That’s twice the build cost of a 16GB WiFi iPad so we’re not talking consumer electronics build cost. But the black box has no display, no UI software, no battery, battery charging circuit or battery mangement, no storage and no minituarization. The easy solution to RF problems (the black art of the electronics world) is shielding but shielding is tough in a minituarized product like an iPhone or iPad; not an issue when the black box product can be the size of 5 iPads stacked on top of each other with essentially an unlimited power budget (i.e., no concern for battery life). The balck box also has no expense for industrial design or custom tooling, the chassis is an off the shelf item. So the I’m suggesting twice the build cost for a lot less product.
    There are a lot of smart people in this forum, some of whom actually build AIS units. Could one of them help us understand why a black box class A would be more than a $200 increase in build cost and $400 increase in street cost over class B?

  67. Anonymous says:

    no you cant blackbox the fa-150 it does not tx unless the display is on …

  68. Ben Ellison Ben Ellison says:

    Thanks, Russ. Probably the more important question is whether the regulators will ever permit a black box Class A AIS. I think that at least the perceived market for Class A is pretty small, and that the regulators figure they’ve already created a less expensive black box version (Class B) for small commercial and recreational vessels. And while I’d not heard it before, Kurt suggested that there “are a lot of people who do not want the black box version of A to happen.”

  69. Dan Corcoran (b393capt) says:

    It dosn’t need to take thousands of dollars to install a class b transceiver, and a stern rail installation is really quite good enough.
    I am no master installer, but it took all of three hours to physically install on my 39 ft sailboat, including a cable gland to route cables to the stern rail combination vhf/ gps antenna, and rerouting 0183 cable and power.

  70. Russ says:

    Dan – Would it be reasonable to assume that you have a 25w fixed mount VHF with its antenna on the masthead?
    Why is the masthead important for your 25w VHF, but not your 2w AIS?

  71. Dan Corcoran (b393capt) says:

    Yes, in fact I do Russ (25w fixed mount, masthead)
    My reasoning, beyond that I have the AIS on loan setup this way:
    (1) VHF Antenna installation costs / installation difficulty are much lower with a stern mount, vs. adding another VHF or sharing existing VHF antenna
    (2) Having both the helm and the autopilot in the stern, I have multiple points where I can easily pickup NMEA-0183 wiring and instrument power.
    (3) I don’t want to degrade the VHF performance with a splitter, not even a little bit, nor have the AIS disabled when I use the VHF.
    (4) In practice the range with the stern mount is already 7+ miles for small vessels, 10+ miles for large vessels. If (but I don’t) cross oceans single handed, I would desire longer ranges, but for the sailing I do (we keep watch) I don’t see a significant advantage in having more range as neither I nor another vessel are going to make decisions on an AIS contant until they are a bit closer than those ranges.
    Note the stern mount antenna is only about 12″ high. If I wanted more range I could mount it higher or use a longer antenna, but it just dosn’t interest me in the slightest to get more range on top of what I enjoy now.

  72. Ben Ellison Ben Ellison says:

    I’m not at all sure that antenna height makes a big difference when your AIS signal is only 2 Watts. The best ranges Class-B-to-a-ship I’ve heard of are about 20 miles, and to get that line-of-sight it only takes antennas that are 10 and 125 feet high according to this calculator:
    I’d still like to have my AIS antenna high, but don’t think it’s as big a deal as it is when you’re pushing 25 watts of signal and may want to get the attention of far off vessels or shore stations if you’re in trouble. Also, in my limited experience offshore with Class B, the range I did get seen at — 6 to 12 miles, one passage with an antenna 8 feet off the water, another with the antenna at about 16 feet — felt sufficient. Though more is better, especially further offshore.

  73. Ben Ellison Ben Ellison says:

    Steve Dashew has a follow-up on this entry. The money quote (as they say): “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.”

  74. Ron Rogers says:

    This is not about range, per se; it is about time. Ships that one encounters on Chesapeake Bay or Puget Sound are moving at 15 to 20 knots. At 15 knots, they are covering .25 nautical miles a minute. Your visual horizon from your flybridge (10 feet above the waterline?)is 3.7 nautical miles. So the ship is “on you” in 14.8 minutes. Assuming that the ship’s AIS antenna is 50 feet in the air and your antenna is 10 feet in the air, you have increased acquisition range to 14 miles and have added 40 minutes until the ship is on you. BIG difference. However, as you point out, were you to raise your AIS antenna to 20 feet, you would only add 2 miles to your acquisition range. The point of raising an antenna on a small boat is to get it clear of interfering objects.

  75. Ron Rogers says:

    Can someone explain why a Class A AIS costs so much more than a Class B unit? Personally, I doubt that the difference can be justified by component or assembly costs. I *assume* that the difference is a marketing decision. Were they closer in price, there would be little reason to buy a Class B unit.

  76. Ben Ellison Ben Ellison says:

    Ron, Class A’s have more sensor inputs, a required display and keyboard controls, and of course more wattage. The Comar CSA-300 at $2,500 doesn’t seem wildly off given that the three Class B transponders with displays — from Vesper, Icom, and Simrad — are in the $1,200 range. If you check out the Comar manual, you’ll see some complexity you might not be aware of:
    Also, I’m not quite getting your horizon math. If my height of eye is 10′, and conditions clear, I should be seeing the waterline of a ship at about 3.7 miles (why it’s handy to have an HE table). If its deck is 50′ off the water — which equates to an 8.3 mile horizon — that deck should have come over my horizon when we were 12 miles apart.
    When I looked for a Height of Eye table, I came across a familiar chapter of Bowditch about lighthouse ranges, both luminous and geographic. It’s somewhat analogous to my point about Class B AIS; your height of eye (or antenna) doesn’t matter if the light isn’t bright enough.
    PS A quick look on Marine Traffic at Puget Sound, Chesapeake Bay, and even the bigger waters nearby suggests that only about one third of the ships are going over 15 knots.

  77. Ron Rogers says:

    I was trained to plan for the worst case. Container ships carry their speed most of the way in and usually don’t have to wait for a berth. Colliers for Baltimore have long waits and do not waste fuel bucking the tide. Tankers book just like box ships. Cruise ships are on unique schedules and rarely hit box ship speeds. On the Chesapeake between Norfolk and the Bay bridge, I assume that they are going at least 15 knots. Of course, half the time I think that the ships in the Annapolis Anchorage are underway! That’s why I want AIS. I wonder if VTS zones have speed limits?

  78. Dan Corcoran (b393capt) says:

    Filtering – On topic, different tack:
    Andy Norris wrote “It may have been at the back of the mind of some legislators that innovation by manufacturers would be the best way to evolve both filtering and acquisition strategies in these relatively early days of AIS/radar integration … manufacturers will be implementing their own best ideas in these areas.”
    I ask, what kind of sophistication is being contemplated for future AIS alarms? I can think of a lot of complex rules based on CPA, TCPA, vessel length, steady COG of my vessel or approaching vessel, and if the TCPA of all potential collisions is overall increasing or decreasing substantially.
    For example:
    1. If a vessel under 40 feet has a CPA of 100 feet to me, a TCPA of 5 minutes (e.g. distance 1.25 nm, moving at 15 knots), I am not interested in an alarm until such vessel is much closer.
    2. If a vessel over 80 feet has a CPA of 500 feet to me, a TCPA of 5 minutes, I could be very interested.
    3. In the case (2) above if the 80 foot vessel is not steady, e.g. COG is changing over the last minute (or ROT is substantially above 0), my interest is greatly diminished until the TCPA is under 2 minutes, as the CPA calculation is transitory.
    4. Even then, might not be interested in an (audio) alarm if I already have received and acknowledged an alarm for other vessels of similar size and TCPA recently.
    Maybe to support this, a filter (of audio alarms) would prevent me from getting subsequent alarms unless threats are getting substantially closer in a specific quadrant of my compass rose since the last recent alarm I acknowledged. (note, this would eventually have the effect of automatically silencing the alarms while entering and staying in a busy channel, unless say a vessel approached from a new direction, e.g. from behind or crossing ahead, or a much larger vessel (say a ferry) entered the channel vs. those already there.
    To restate my question, what kind of sophistication is being contemplated for future AIS alarms in manufacturer products?

  79. Evan says:

    Ben, I am curious, are there AIS coastal repeaters?
    I ask because I have a screen shot I took a year and a half ago, when I had my AIS antenna temporarily mounted on my stern about 5-10 feet above the water, and I had two AIS contacts showing on my chart plotter at over 12NM, one of which was around the tip of Texada Island (mind you I do not know height of the island at that point).

  80. Paul says:

    Evan, I don’t believe that there are coastal repeaters in operation. The range you report isn’t too surprising though — there can be some amazing propagation effects. With my old stern-rail AIS antenna I often detected ships at 60+ miles. I get at least that range (on occasion) with the current spreader-mount antenna. From my house at 1000ft elevation I’ve received ships from 2000+ miles. It’s called tropospheric ducting.
    Also, while a land mass often blocks the signals, sometimes it will refract the signal, bending it back towards your receiver. It’s not exactly magic, but it’s pretty hard to predict.
    Regardless, I work on the assumption that my Class-B transponder will pick up the other guys at six miles worst-case. Well, worst case is I never pick them up.
    As for the “Transponder” definition, IMHO that battle is lost. The governing specs for AIS describe the units as transponders. In the interest of speaking a common language, I suggest we all adopt that term.

  81. DaveV says:

    Regarding cost of Class A AIS – the issue is likely also related to GPS INTEGRITY. The comment from Vesper earlier hinted at this originally. There is NO certification of GPS data provided by recreational GPS receivers. These receivers can produce FALSE locations, speeds and altitudes (other than sea level) that may not be recognized by the operator. I know that seems impossible but it is true and I have observed it. I have seen my boat placed on a mud flat for a short period of time (When I was safely at sea) and moments later had my vessel moved to the correct location by the chartplotter.
    Electron storms from sunspot activity, jamming from nearby sources etc can cause false position and velocity reports. On Reccreational GPS receivers there are no tests by the receiver to exclude erroneous GPS satellite data that can also be transmitted by a failing satellite.
    Certified GPS receivers in the Aviation world must provide “Fault Detection and Exclusion” with separate receiver channels and special software to eliminate false / corrupted data from the satellites. Aviation receivers must also provide multipath rejection ( false positions computed based on GPS signals reflected from nearby land, buildings, other ships etc) and use special antenna designs that specifically exclude telecom or VHF harmonics from the GPS receiver.
    There is a LOT that goes into a GPS receiver that is designed to meet “several 9’s” of reliability and demonstrated integrity and resistance to jamming etc. That certification process costs money and that investment must be recovered in a few short years to make the business investment pay off. That has a lot to do with prices that does not include materials and assembly costs.
    So a CLASS A transponder must provide its own CERTIFIED GPS receiver in addition to the TX / RX functions of the AIS itself. The data it transmits MUST be accurate and reliable.

  82. Kees says:

    Dave, a Class B transponder must also include its own GPS receiver, for the same reasons.
    What you are talking about has a name as well: Receive Autonomous Integrity Monitoring (RAIM)
    In fact the AIS messages that Class B transponders pump out include a RAIM flag in the NMEA0183HS and NMEA 2000 messages.
    Heck, the NMEA 2000 messages even include a flag whether the Class B is SOTDMA or CSTDMA. Not sure about ‘183HS.

  83. Cattledog says:

    I knew it wouldn’t take too long to find a current example of “professional mariners” not looking out the window…
    These guys ran over an island, a rather large island in good weather, under power about 1500 miles in the middle of nowhere at 4:30 in the morning.
    AIS receiver – nice – AIS transponder – nice – will the transponder save you out in the open ocean if you fall asleep – nope… Because the other guy is asleep too.
    And I hope this dampens the enthusiasm for the myth of professional mariners.

  84. Ben Ellison Ben Ellison says:

    Cattledog, I don’t know what your problem is with professional mariners, but all you’re proving is how irrational you are about it. Of course there are going to be mistakes; the question is how many mistakes relative to successful voyages.
    Have there been any other ship wrecks like the MS Olivia in 2011? Compared to how many ship transits? I think you’ll find the percentage is exceedingly small.
    So it’s ridiculous to conclude: “AIS receiver – nice – AIS transponder – nice – will the transponder save you out in the open ocean if you fall asleep – nope… Because the other guy is asleep too.” Of course the other guy MIGHT be asleep, but probably not.
    There is no “myth of professional mariners.” It’s like most every other profession. Most are true professionals; some are incompetent.

  85. Peter says:

    when did islands start installing AIS’s?:)

  86. Kurt Schwehr says:

    Anyone who knows patents, please help!
    It looks like this link is gone…
    Does anyone have a link to documentation that shows the SOTDMA patent is no longer valid in the US? Wikipedia has this, but no link to the actual information:

    USPTO ex-parte reexamination certificate (7428th),
    issued on March 30, 2010

    And for the person looking for more open documentation on AIS… the nuts and bolts of what comes out the serial port on an AIS unit is discussed here:
    I like the document, but then again, I talked with Eric Raymond a good number of times while he was writing the first draft.
    And there is always my giant pile-o-references… the majority are publicly available.

  87. DaveV says:

    You can find the patent on Google Patents here:

  88. Kurt Schwehr says:

    Yes, that’s definitely a good URL for the patent, but I’m not seeing the 2010 USPTO document that revokes the patent. That’s the part that I’m missing.

  89. DaveV says:

    Kurt- As of March 30th the patent had a legal status code of “FPB1” I suspect that this may simply mean fees due to maintain the patent in the US specifically have been paid and the patent is in force. I found that the patent is filed internationally and has paid fees across all of Europe for protection in all of those countries in June / July of last year. Hakan Lans is protecting that invention actively and is spending considerable fees on it.

  90. BrianM says:
    Seems to have the reexamination as part of the entry, but I was unable to view it in either firefox or IE.
    The official gazette – which was linked to before – only has the last year online.

  91. roger hudson says:

    Remember that AIS is for SOLAS compliant vessels, Navy ships won’t emit AIS, though their lookouts are usually very good.
    My basic rule ( at 27 feet long sailing), small gets out of the way of big.

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