Class B AIS here?, but worthless?


Today two reliable sources told me that all five FCC commissioners have now signed off on Class B AIS for US waters, though neither knows when the Order will become effective. The final step should be an announcement in the FCC Daily Digest. Having been wrong about the timing so many times in the past, I certainly won’t venture a guess. But it does seem like a good time to discuss an e-mail I recently received from a marine electronics industry veteran and Class B AIS skeptic who would like to remain anonymous. Based on his own “on-the-water analysis” he thinks it’s “completely worthless”! His points, in italics, along with my comments:

1)  The practical range is only 5-7 miles line of sight, which is simply too short. My own offshore testing showed a 6–10 mile range, and that was with the low-mounted antenna seen above.

2)  At a speed of 30 Kts a large commercial vessel transits 6 miles in 12 minutes.  If it takes 4-5 minutes (not a lot of time) for the observer on the commercial vessel to notice a potential collision, 4-5  minutes to decide what to do, they only have 2-4 minutes to effect a maneuver to avoids a collision.  Not realistic in my estimation. I don’t think many ships are doing 30 knots, but even so 8–10 minutes is time enough in most cases.

3)  If your vessel is stationary it will only transmit the vessel position every 3 minutes, when you are moving the position is transmitted every 30 seconds. Both conditions increase latency. Think about a small vessel colliding with a large commercial vessel. If the position is only updated every 30 seconds the commercial vessel is going to assume that during the 30 second between fixes that the recreational boater has altered course. If your vessel is stationary in front of a large commercial vessel they are going to get very few position reports, and again I think the commercial vessel will assume that you will move out of their way. There are issues with the slow Class B data rates, as I’ve often discussed, but I’m convinced that when Class B proliferates, developers will come up with better ways to plot them.

We’ve discussed Class B skepticism here before, but now there are a number of boaters outside the U.S. who’ve actually been using it for a while. I’d particularly like to hear from them in regard to the issues above. If finally us Yanks can actually buy a Class B transponder, should we?

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.

36 Responses

  1. Phil Koken says:

    Sounds like a bunch of whiners to me. I will take ANY tool available to reduce the threat of collision at sea, and we keep a full time watch 100% of the time underway aboard Samadhi! Complaining that the system is imperfect, and therefore shouldn’t be implemented is stupid and shortsighted. Samadhi is (well) outside of the USA now, and a Class B AIS may just be on my Panama City order list…

  2. Ron Rogers says:

    In the current context, Mr. Koken’s selection of the word “whiners” says it all. How silly! Safety systems that don’t contribute to safety are to be avoided. I’d save my pennies for a class “A” system. Meanwhile, a dual channel receive-only system is the best interim solution as the Class “B” system could contribute to a false impression of your vessel’s intentions. It is the ships which will require improved software to better interpret the movements of a Class “B” equipped vessel.

  3. Roger says:

    ” . . . . I think the commercial vessel will assume that you will move out of their way.”
    Yes, I probably would. I have no wish to take on a large commercial vessel at sea any more than I wish to take on an 18 wheeler on the road.

  4. Ian Zane says:

    Based on my professional experiences in the Chesapeake, Miami area, Los Angeles Harbor, and Houston Ship Channel, commercial vessels must continue to assume small craft will do the WRONG thing (think: three year old in a Mall parking lot).
    In the last few months I have seen three pleasure boats try to cross car carriers steaming at 21-23 knots only to veer away at the last chance. Five whistles are a common sound where commercial and pleasure meet.
    I will install one of these when the infant mortality issues are worked out, but I am sad to believe they will only better equip those already inclined to safe operations. The rest of the yahoos out there will continue to be a danger to themselves and others. And government still hasn’t fore-sworn using these to keep tabs on us all.

  5. John says:

    Has anyone addressed the ability of Class A receivers being able to see the new B’s?? I couldn’t wait and ended up getting a “A”. When I called one of the big ships and verified my AIS info. He later asked me if I was a class B transmitter. I said no, and he then told me of other ships who had problems seeing the class B units.
    As far as range, it would be interesting to see how he came up with 5-7 miles line of site. I think what I’m looking at is a low gain antenna mounted near the deck. What would the range have been mounted 10, 15, 20 ft higher?? Also many of the big ships have their antennas significantly higher then that.
    I also have another question Ben. My understanding is the B-class standard for the minimum of what they have to do. So can they for instance choose to transmit more often?? Broadcast at a higher power like the A’s? Use both channels, etc.. It would be nice to keep a running tab of the class A and B units where the columns show the features each of them employ.
    By the way I have not regrets having the A unit. It’s kind of nice to keep it running at anchor thinking that at least someone see’s me.

  6. marinate says:

    The issue regarding Class A transponders not being able to ‘see’ Class B transponders is a common concern. The facts are:
    1. Class B transponders transmit position reports (your lat, long, COG, SOG and MMSI) using a message that all Class A transponders can understand. This is AIS message 18.
    2. Class B transponders transmit static data reports (vessel name, call sign etc.) using a new AIS message introduced with the class B standard (message 24). Some older Class A units don’t decode this message, however all Class A manufactures have resolved this in their latest software.
    The worst case is therefore that a Class A unit will only see a Class B units position, COG, SOG and MMSI (i.e., all the essential information from a safety point of view) and not see the vessel name.
    The Class B AIS standard is a minimum in some areas, however reporting rate is not permitted to be increased beyond the spec.
    Lets hope the FCC finally make a move..!

  7. Ben Ellison Ben Ellison says:

    Thanks for Class B message explanation, Marinate. I’ve never heard, though, that Class B wattage and data rates are just minimums. I believe the 2 watts and 30 seconds will be maximum in most places. But also think those standards work better than the skeptic does.

  8. ibsailn says:

    We are planning an AIS install over the winter and have not decided on how to proceed yet. Options I have come up with are Furuno Class A, Shine Micro Class B (firmware dissabled transmitter), or another class B unit (ACR unit we tested with Ben was nice) if the FCC does in fact approve these things by March or so. From the range we saw with a low mounted antennae, I am not worried about range of class B, but the update frequency and potential ability for ships to filter class B units out of sight is a concern. The big downside of class A is cost and power requirements (these aren’t a small drain for a sailboat that tries to avoid running engines).
    John, what class A unit did you buy? How have you found the real life power draw? Also, do all the units have a “silent” mode or just some of the class B units? I have heard that in certain areas you get harassed by harbor control as they think you are a large ship and get upset when you aren’t in a shipping lane. It would be nice to turn off your transmitter, but keep receiving in those circumstances.
    Also, to further correct the skeptic’s calculations, 30kts is a rarity for a ship (only a few container ships on specific routes have this sort of speed and they aren’t going to get out of your way in either case). 20 kts is still fast for the worldwide fleet (16 to 18 seems to be normal). Even assuming a closing speed of 30kts (20+10), and a 10nm class B range (very achievable with any sort of normal antenna location, would be better easily with masthead), that still gives 20 minutes to see the target, make contact, and take action. This is longer than I currently have in most circumstances as it takes a lot longer to get a Marpa fix to determine if it is really an issue or not. I don’t really expect the ship to alter course to avoid me (although I have been surprised how often they offer to do so when you do manage to get them on the radio). I just want them to be aware of me, call on the radio (or answer my call), and confirm how I should avoid them.

  9. Dan (b393capt) says:

    I don’t follow the concern about reporting position only once every 3 minutes if your boat is not moving. The larger boats AIS still shows you in the same spot with no velocity no matter your boat reports once or six times in 3 minutes.

  10. Dan (b393capt) says:

    I don’t follow the whining about range. Would there be a substantial safety benefit to smaller boats choosing Class-B if the range of Class B could be another 5, 10, or 15 miles ? Wouldn’t that extra time be squandered anyhow ? Except where avoiding action obviously needs to be taken well in advance (like between two large boats, congestion, etc.) .. it would seem to me most boats will do what they do today .. observe to see if the collision risk continues and take avoiding action only a minute in advance, or even less if they believe they are the right of way boat.
    Am I correct? Am I naïve? My plan is to place the antenna somewhere convenient, rather than at the top of my mast. I don’t view the extra miles of range I could get at the top of the mast worth while.

  11. Ben Ellison Ben Ellison says:

    From Steve Dashew:
    We are in the UK, and have now put about 20,000 miles under our keel with AIS A-type. Few comments from the real world:
    1-Distance is line of sight, and a function of antenna height at both ends. We regularly see ships at 20+ miles – longer with ducting. The ships with their high receiving antenna are going to see yacht height AIS signals at 10 miles or more.
    2-We have yet to see any ship moving at 30 knots. 18-22 seems to be the norm. The time to CPA is based on whether you are on reciprocal courses or the ship is overtaking (or coming from abeam).
    3-There are going to be issues with the ship interpreting CPA based on the yacht’s output of SOG and COG. However, they are used to this as many smaller ship COGs wonder back and forth in a seaway (we have seen a lot of this).
    4-Any ship which sees a CPA of under two or three miles, and cannot get a visual on the target is going to be doubly watchful.
    In general, I would put an AIS B way ahead of most other “necessities” for a cruising yacht, especially when cruising in areas with lots of rain.
    Regards – Steve Dashew

  12. Sandy says:

    If big brother is watching me he’s bored to tears.
    Now I’ve got some shopping to do and I don’t think I’m going to find it on Ebay.

  13. del says:

    I find the comments in the original article absolutely amazing – I guess this guy objects to seat belts in his car as well?
    I see that several posters have already dispelled some of the myths that continue to circulate about Class B – hopefully they will die out sometime soon? Please?
    Several of the moans in the original article are the result of specific design decisions – Output Power v. range being just one of them. This choice allied with the 30s report rate allows for many more Class B’s to co-exist in the same geographical area without disrupting the Class A operation. This was a prime requirement for Class B.
    The at anchor update rate is the same for Class A and Class B, so I don’t understand that complaint at all.
    As the owner of a 7m sailing cat, I heartily endorse Phil Kokens comment – “I will take ANY tool available to reduce the threat of collision at sea” – and that DEFINITELY includes a Class B.

  14. John says:

    2) Some older Class A units don’t decode this message, however all Class A manufactures have resolved this in their latest software.
    GREAT!! I hope that’s resolve on my Furuno FA150 that I just had installed.
    The worst case is therefore that a Class A unit will only see a Class B units position, COG, SOG and MMSI (i.e., all the essential information from a safety point of view) and not see the vessel name.
    Well thats great, I’d rather see a target in front of me then nothing at all. In fact thats not much better then many boats with Class A anyway with there static information being wrong.
    As far as power, I’ll be on the boat this weekend will provide a real world power draw.
    About being able to shutdown transmitting I seem to remember reading somewhere that was a undocumented possibility. But others told me as soon as they populated the correct information about there boat, size, draft, type, etc that they no longer were harassed in NY and other places.
    Just this past weekend I looked at my AIS info on a few boats at Annapolis Yacht Club and one of the boats had his info completely wrong. He had himself as 60m wide and 20m long. He actually is 60ft long and 20ft wide. Kind of funny seeing the image navnet3d superimposes on the docks. He covered up 1/2 the marina. Then there are two other boats, with no home port, no width, height, or depth, and type of boat set at Fishing. This mirrors also what I see on some of the big boats static information that are running up and down the bay. Another common problem I see is the boat pointing north no matter what. I found out that I was doing the same thing, and after doing some research I had to send the True North heading to the AIS unit. Once this occured, my rate of turn and other things started to work. So my guess is that harbor control has just learned not to trust the information unless told otherwise or the information looks valid.
    As far as speed, I see the occasional container ship running down the bay at nearly 30knots but like what others have said, 99% of them are running well under 20knots.
    Anway, I can’t wait till class B does come out. I just was impatient, but I have no regrets other then my wallet being thinner.

  15. I, too, am interested in the Furuno FA-150 (Class A) unit and was hoping to reduce the apparently considerable power draw by switching off the transmitter and running in receive only mode.
    When I contacted Furuno tech support, they told me that it was possible to do this on the unit but they would share how to do so only with the military or coast guard.
    Looking forward to hearing some real world power consumption figures for the FA-150.
    BTW, recently installed an AIS WatchMate from Vesper Marine. Great device that works as advertised with very little power drain.

  16. Drew Clark says:

    Re all of the posters talking about going with Class A for their 25-50 ton yachts, what are the regs regarding who can deploy Class A AIS? I seem to recall that the service was intended for all ocean-going ships over 300 gross tons, as required under the international SOLAS standard. Is there any guideline or policy that would prevent a smaller vessel from acquiring and switching on a Class A device? I realize cost and power draw are issues for some, but what about increased clutter and noise for the “legitimate” Class A users? Isn’t that why Class B was put forward in the first place?

  17. Russ says:

    With regard to the FA-150, the specifications and operator’s manual show that it can be be run at transmit power levels of 1w, 2w and 12.5w. It’s not clear to me why you would want to pay $4,000 for a Class A device and then turn off the transmitter. The transponder nominally draws 7w at 12v, this presumably is at 12.5w of transmit power and in my experience Furuno’s actual power consumption numbers are well under what is shown in the specs.
    With an FA-30 receiver I routinely see ships which are not in line of sight. Strictly by the numbers, with the antenna on my spreaders at 24′ and a commercial vessel’s antenna at 100′, the nominal range is 18 miles. At a worst case closing speed of 30 knots (ship 22 knots, me 8 knots) that’s still 36 minutes.
    The skeptic is entitled to his view, but a Class B is probably going to draw no more than 1a which is not much of a price to pay for some chance at being seen first.
    A major flaw in the skeptic’s thinking is that in reality you’re not relying on some AB on watch seeing your return on his radar. His AIS receiver will have TCPA and CPA alarms set and AIS gives you a chance to trigger those alarms and attract his attention.
    And the same goes for my watch standing where his signal can trigger my alarm. This happened just last week when I didn’t see the ship (actually a large MY) because I was distracted with piloting, but the alarm did see it and brought it to my attention.
    Class B is no panacea, but it’s one more arrow in the quiver.

  18. Piotr says:

    Additional benefits of AIS:
    “ORBCOMM has successfully launched six AIS-equipped satellites(…)The satellites are equipped with Automatic Identification System (AIS) technology to receive and report transmissions from AIS-equipped maritime vessels from anywhere within the satellite coverage area(…)”
    I hope Panbo can dig into it and get some more info, especially about possibility to receive by satellite weaker signals from AIS B transponders.
    I do have AIS Class B transmiter (CBS200) on my sailboat. Antenna is installed ~3 meters above water. Receive range is more then a line of sight, I got signal from AIS objects (not only vessels, lighthouses also transmit it) of more then 50NM away, sometimes even 100MN! I’m not sure about my transmit range, will have to ask ships nearby and VTS operators.
    Baltic Sea

  19. Dan (b393capt) says:

    More info? I am reading many comments above about how important AIS-B is to boaters outside the US, like Steve Dashew. My question to all of you:
    1. Has AIS become important to you, just because you can see AIS-A users ?
    2. Has the adoption of AIS-B by other working boats (fishing, etc. etc.) significantly increased the important of AIS to you ?
    3. Has the adoption of AIS-B by recreational power boats significantly increased the important of AIS to you ? What do you think the tipping point is, 10% adoption, 50% adoption ?
    4. Has the adoption of AIS-B by recreational sailboats significantly increased the importance of AIS to you ? What do you think the tipping point is, 10% adoption, 50% adoption ?
    A combined answer for 3&4 is fine.
    Thanks !!
    Dan (East Coast, USA)

  20. Sandy says:

    about where any such services are in effect. I am just assuming that such a product would retransmit AIS messages to a broader area. I suspect they are in use based on the reports of exceptionally long range acquisition of information, particularly around the Mediterranean. Am I mistaken? [no worry, I’m used to it]

  21. Sandy says:

    Sorry about the clipping. I meant to say
    Google “AIS repeater” and you will find a list of manufacturers with product to deliver, but nothing about where any such services are in effect. I am just assuming that such a product would retransmit AIS messages to a broader area. I suspect they are in use based on the reports of exceptionally long range acquisition of information, particularly around the Mediterranean. Am I mistaken? [no worry, I’m used to it]

  22. Larry Brandt says:

    Two clarifications: 1) AIS is NOT a transponder. 2) AIS is NOT radar.
    I suggest we all avoid using these two terms in reference to AIS function, whether Class A, Class B or receive-only. Further, an AIS manufacturer who purposefully uses the term “radar” in its description of AIS is misleading the consumer.
    More accurately, AIS is an autonomous VHF broadcast transceiver; or if receive-only, then merely a receiver…though none the less valuable for that, let me add.
    Pilot-types out there will find the aviation system most analogous to AIS to be ADS, Automatic Dependent Surveillance. Again, neither the aviation transpoder nor aviation radar are accurate analogies.

  23. Richard says:

    On the range of 2 Watt Class B AIS units.
    Two Watts is more than adequate to deliver a reliable signal at the longest line-of-sight range likely encountered.
    If you assume a sailboat antenna at the top of a mast 65′ above water and a freighter with its antenna at 120′ the combined line-of-sight range is about 23 NM.
    Assuming no antenna gain on either end and 6 dB coax loss on each end; 2 Watts delivers a signal 15 dB above the threshold of the SmartRadio SR-161 specification of -112 dBM. That is a very solid signal.
    Antennas mounted lower, on either end, will reduce the combined line-of-sight range so the case above is approximately the practical “worst” case from a transmit power standpoint for recreational boating on open waters.
    I assume that the 12 Watt Class A transmit power specification is intended to provide additional margin for diffracted radio paths (e.g., ships around a bend of a river or a point and not visible to each other).

  24. Ben Ellison Ben Ellison says:

    Larry, I thoroughly agree that AIS is not radar, and have tried to swat that analogy down.
    But I think “AIS transponder” is close enough to being a correct expression, or at least it’s in wide use. If you google “AIS transponder”, you’ll see that the term is used by the US Coast Guard, several AIS manufacturers, and Wikipedia, among others.
    I don’t think you’ll have much luck getting people to change to “AIS autonomous transceiver.” Language evolves.

  25. Dr. X says:

    My experience with AIS in the med is that you can see target up to 100NM away (my antenna is 20M above water). But I am not sure how useful it is to see boats that far!
    In terms of speed, we routinely see boats going in excess of 30KTS. Fast ferries (catamaran or Hydrofoil type) hit the 40-45KTS range.

  26. Russ says:

    Dr. X – That’s good to know. Since you’re seeing ships traveling at 40 knots, the 100nm visibility is actually useful. With a closing velocity of 50 knots, that’s two hours warning, though I suspect they don’t hold that course for 2 hours.
    If however, we assume a 50 knot closing speed, then even the 18-23 range that has been estimated for Class B offers 20 minutes of TCPA to trigger the alarms on either vessel.
    To rebut the skeptic, this is a much more time than would be available with the naked eye on watch and comparable to what we’d expect from ARPA.
    Class B isn’t Class A, but why turn down 10-15 minutes of warning on a possible collision?

  27. We’ve used Class B AIS for the past few months in Asia. Ship positions are received at more than 120Nm (!) in mid-ocean areas of the Sth China Sea (good S/N ratio ?) As we approach Singapore the effective maximum range decreases (quite logically) but it is at all times very useful. We logged over 1000 targets in less than 48 hours, using a Comar CSB.200. It is a very effective primary safety system. Even amongst several hundred class A transmitters in the Singapore Straits I could watch a class B vessel crossing the shipping lanes ahead of me. I don’t understand the negativity of some contributors – this system WORKS. Check our reveiew at

  28. Ben Ellison Ben Ellison says:

    Thanks, Crystal Blues! Good to hear that Class B is working in the busiest Class A environment in the world. Can you recall how far away you’ve seen Class B targets, or been seen yourself? Also neat to see that you have underway 3G cellular out there. Thanks for an informative blog.

  29. bobetter says:

    “Two clarifications: 1) AIS is NOT a transponder. 2) AIS is NOT radar.”
    Actually, it IS a transponder. It can and does respond to queries from Base Stations. No, it is not “Radar”.

  30. Jim Hebert Jim Hebert says:

    Regarding the term TRANSPONDER, I am all for precision in language and accurate use of words, and thus I have to note that a random dictionary definition says:
    TRANSPONDER: a radio… set that upon receiving a designated signal emits a radio signal of its own and that is used especially for the detection, identification, and location of objects….
    This is really quite an accurate description of the AIS system. An AIS unit first listens to detect the presence of other vessels, and upon receiving their transmissions, learns which time slot is available for use. It then responds to those “designated signals” and “emits a radio signal of its own” which fits in an open time slot.
    This definition seems quite descriptive with the way AIS operates. On that basis I don’t see a problem using “transponder” as a description of an AIS unit. In fact, it seems preferable to “autonomous transceiver” because that term implies nothing about the use for identification and the way the device responds to signals it receives.

  31. Crystal Blues says:

    Ben, we’ll be sure to take note of remote Class B targets and let you know distances etc., but it will be a while as we’re now refitting (on the hard) in Phuket. Meanwhile, our AIS Class B installation experiences are now on the blog at

  32. Mark says:

    There are some bargains out there. Just got an ACR/Nauticast 2007 Class A set for US $1300. It was a return but unused. Only disadvantage is that it runs on 24 VDC and my boat has 12 VDC, but I have a DC-DC converter that can take care of that. I think class B sets will come waaay down in price in 2009-2010, but not class A. It was tempting to save $500 and buy a class B set, but I like the extra power, more frequent reporting and self contained (keybd and screen included) feature of the one I bought.

  33. High Concept says:

    I installed an ACR Nauticast-B Class B AIS transceiver on my 38 ton motor yacht. VHF antenna is mounted on the radar arch 28 feet (9 meters) above the water. The yacht is located in Marina del Rey, Los Angeles. Topographically, the marina is more or less in the center of a semicircular basin, ringed with mountains and punctuated with a few hills.
    I built a mobile test jig with an SR-162 AIS receiver, a laptop, and an AIS VHF antenna, mounted on the roof of my car. I then drove to various hilltops and mountain ridges around Los Angeles where I could obtain a line of sight view of the marina. Nearby to my yacht was a commercial vessel with a Class A beacon that is always on. Line of sight from a mountain top is useful because there are no horizon issues.
    I was able to reliably receive my own Class B ACR beacons at 1.5 miles, 200 feet MSL, and 4.8 miles, 200′ MSL.
    I was NOT able to receive my beacon at 9.8 miles, 1500 feet MSL, nor at 15 miles, 1500 feet MSL. In both of these failed tests, I was able to consistently receive the beacon from the commercial vessel near mine. I was also, by the way, able to receive transmissions from 10-20 ships at sea — some 40 miles away, and another 40-50 ships moored in and around Los Angeles and Long Beach Harbors, about 30 miles away.
    I’m happy that the ACR meets its 5-mile spec, but I’d be happier with more range.

  34. Beacon says:

    Since no one has mentioned it, I thought I would point out that AIS Class B is restricted to 2W transmitter power per the spec, while AIS Class A is permitted to transmit at up to 12W. Which translates to big difference in effective range, especially near the coast where there is a lot of interference. You can optimize your transmitter range by using the ACR supplied, frequency specific antenna.

  35. Ben Ellison Ben Ellison says:

    That’s the ACR antenna duct taped to the GPS in the photo above. You can see that it’s base is only about 8′ above sea level, at best, but in my offshore tests in June ’07, the attached ACR Nauticast B was seen by five ships at 6 to 11 mile ranges and not seen by two at 10–12 mile ranges. I think High Concept’s rig might do better outside Marina del Rey, but it’s great that boaters are testing AIS B like this.

  1. May 16, 2019

    […] USCG safety regulation changes move along a much slower timeline — heck, I got close to whiny about Class B AIS — and also knew that they are very careful about causing boaters unnecessary expense (by […]

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