Class B rollout, looking better, but mysterious

... written for Panbo by Ben Ellison and posted on May 21, 2009

Ever since I realized that the free AIS pro web viewer can count and/or filter AIS types, I've been using it occasionally in an attempt to gauge the spread of Class B in the USA. You may recall the day last October when my Li'l Gizmo was the only Class B to show up on the whole East Coast (or at least within range of a certain mix of listening towers). And, frankly, I haven't seen many B transponders in US waters since then...until today when there were six in the general New York area, almost all underway. And, surprisingly, some were being seen well offshore, in fact way offshore... 

This was no fluke either; I just checked the viewer again, seven hours later, and three of those offshore Class B's are still out there, including the sailboat furthest out.  This is mysterious.  I was just developing the opinion that the various online AIS viewers would naturally tend to under count 2 watt Class B transponders because their shore antennas were generally arrayed for 12 watt Class A ranges (and also because Class B type boats are much more likely to shut down their transponders at the end of a voyage).  And I've seen this locally; a Class A transponder outside Camden can be seen by the Pilots' distant antennas but so far Class B doesn't show up.  So now I'm wondering just what receivers (and how high) are providing these feeds to Siitech, or is a satellite involved somehow?  Below is a zoomed out view with Class A turned back on (you can still see the B's by their icon shape).  Note how far away the Jacques Jacob is.  What's going on here?



Cool. I'm looking at the Siitech viewer again and the Class B "William I" that was off Atlantic City yesterday afternoon has almost made Montauk, and a Class B is headed West off New Haven. I don't understand how Siitech is getting such consistent wide and long range coverage but it will be a boon for Class B vessels in this area who'd like friends and family to keep track of them.

Posted by: Ben at May 22, 2009 7:21 AM | Reply

Ben, I dont think it is un-reasonable that those signals are being received by a well-placed land based antenna - although it is clearly unusual.

I once rigged up my AIS receiver in a hotel in Barcelona, and one evening my screen managed to pick up some ships about 310nm to the south, enroute to Gibraltar. I was on the 15th floor, so that helped, though the antenna was pretty crummy, chosen more for its ability to fit in my backpack than anythign else! Interestingly, the daytime range was noticeably worse than at night.

The Jacques Jacob appears to be about 195nm from Halifax, where I imagine there's a receiver because of that cluster of ships.

Alternatively, could a ship be sending their AIS targets to Siitech? If one were equiped with VSAT on an 'unlimited traffic' basis, I dont see why not.

Posted by: Gustav at May 22, 2009 7:41 AM | Reply

On the Columbia River there is an AIS repeater network set up so that a reciever in Portland can see traffic offshore of Astoria. That's around 80nm straight line distance through a coastal mountain range. There are repeater networks elsewhere so consider that when you see your receiver picking up ships at crazy distances for VHF.

I haven't been up there lately to see if any class B units have shown up.

Posted by: John at May 22, 2009 10:21 AM | Reply

Im the provider of the AIS info from Halifax. I can tell you that i cant see more than about 5nm during the day (I too have noticed reception is better at night) - My receiver is 7 stories up, but is only a 6 inch antenna. and is partially obstructed by landforms and the downtown. I can clear cover the inner harbour though.

I wonder what kind of equipment they are using to see 195 miles?

Posted by: Peter at May 22, 2009 10:33 AM | Reply

It would be interesting to see if the phenomenon persists, or was a fluke. My first thought is Sporadic E propagation (see general intro at Wikipedia. I have seen it, and anomalous tunneling, in the VHF range.


Posted by: Steve Roberts at May 22, 2009 11:16 AM | Reply

Ranges of up to 200nm are unusual, but not impossible with the right atmospheric conditions and a decent high gain directional antenna. Shine Micro have some super sensitive base station receivers for this type of application also.

Ionospheric ducting can get you a looooong way!
See the bit on propogation here:

Alternatively there is an AIS repeater, or AtoN AIS set up to repeat this information somewhere in range. These are often used to get AIS data from shipping lanes far off shore back to land.

Posted by: marinate at May 22, 2009 11:32 AM | Reply

How consistent is the reporting? If you are getting regular updates then the receiver is probably relatively near the transmitter. If the updates are erratic then it may be ducting as marinate suggested.

Posted by: bcl at May 22, 2009 11:51 AM | Reply

From my location on the coast about 40 miles north of San Francisco, elevation 1000 ft, I regularly receive AIS targets out 100 NM, and often out several hundred NM. When the tropospheric ducting is happening signals as far as 2000 NM have come through. My antenna is a standard marine whip on the roof.

There are quite a few AIS multi-receiver networks up and running. I contribute my AIS feed to a couple: - This site aggregates the data from several northern California hilltop locations, and provides vary good coverage. See the bottom of his "About" page for access to the raw TCP feed. - a very international AIS sharing site, combining signals from all over the world. You need to contribute a feed in order to access the raw TCP data stream.

My sailboat VALIS (on San Francisco Bay) has an ACR Class-B transponder, but I usually do shut it off when not sailing. There are several other Class-B boats on the Bay.

Here is a link to my blog entry that shows a typical track-history of the AIS targets received from my location:

And if you enjoy playing with AIS and navigation software that is still a work-in-progress, here's a link to something I am giving away:

Posted by: Paul at May 22, 2009 1:22 PM | Reply

Thanks a lot Paul. This looks like a very useful utility, and I'm relly looking forward to using it.

Posted by: Sandy Daugherty at May 22, 2009 7:01 PM | Reply

The ionosphere lowers in altitude at night - at least it did in Vietnam. "Tunneling" is the phenomenon used to explain Bermuda Radio's reaching out 350 miles (VHF) from time to time. An engineer there told me that sometimes they can copy vessels close-in to the South Carolina coast.

Therefore, picking up the AIS VHF signal at unusual ranges is not unheard of, but in no way can it be depended upon. In the Puget Sound area, there are AIS repeaters which facilitate vessel operators seeing behind physical obstacles like a high peninsula.

Posted by: Ron Rogers at May 22, 2009 7:33 PM | Reply

Certainly ducting can give VHF ranges of over 400 miles and I have experienced that on a few occasions. But looking at the standard line of sight VHF calculators, a ship with an antenna at 100 feet, and the shore antenna at 1000 feet gives a range of 44 miles. 2000 foot shore antenna gives a range of 77 miles and 5000 feet gives a range of 114 miles.

Certainly the military, homeland security and CG are monitering AIS from aircraft--I wonder if any of this information is being relayed back to some coastal station?

Posted by: Bob Austin at May 23, 2009 9:59 PM | Reply

When I first fired up the AIS dual-channel receiver on my boat as a "smoke check" in a marina in Portland, Oregon, I hung the VHF antenna over a lifeline. I received targets all up and down the Columbia River, including one ship inbound just inside the Bar at Astoria. I am sure I was not seeing ALL of the traffic at farther ranges downstream towards Astoria.

In my opinion it was likely a fluke, but not in propagation characteristics sense; rather that a particular AIS installation on that ship might have been optimum: a hot transmitter, installed high in an optimum location, with the very best antenna grounding, low loss coax, etc.

Longer range with AIS can also be attributed to its digital nature. A VHF transmission at that range when you're trying to pick out voice modulation from ambient noise would be impossible, but a digital burst can get through. Old time hams know that CW can make it through when voice can't.

Posted by: Larry Brandt at May 25, 2009 9:23 PM | Reply

By the way, I have some Class B questions.

Is the sailing vessel Class B user able to easily reset his vessel type when he ceases to sail and goes under power, and vice versa? And what are the right of way ramifications in restricted longer confined to radar alone - and thus to Rule 19D - are we back to the standard R-O-W rules as per unrestricted visibility???

Posted by: Larry Brandt at May 25, 2009 9:28 PM | Reply

Larry, A Class B vessel can not change its vessel Type, but in the AIS system Type is just that, not its Navigational Status. A sailboat is still a sailboat even if it's motoring. Class A vessels can easily change their Navigational Status, but so many are poor at it -- hence so many "moored" ships underway and "underway with engine" ships tied up -- that experienced users are leery of it.

I don't know how AIS is being integrated into the Rules of the Road, but do know that it's not uncommon these days for an accident to be recorded by someone's AIS. Like this one:

Posted by: Ben at May 26, 2009 8:07 AM | Reply

The way I read it, Larry, Rule 19 - Conduct of vessels in restricted visibility - removes the concept of privileged or stand-on vessels.

In other words there are no rights of way and therefore a motor vessel, sailing vessel, super-tanker, ALL vessels are equal and have an equal obligation to avoid a collision.

Posted by: Roger B at May 26, 2009 9:10 AM | Reply

Rule 5 - Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.

The key is "all available means" If a vessel is equipped with AIS then it is a tool which can and should be used to maintain a proper lookout.

Posted by: Kennebec Captain at May 26, 2009 9:56 AM | Reply

Ben, I think your statement has an error in it, or maybe I am misreading it. When a sailing vessel is under power, it is a powerboat for R-O-W rules. When under sail alone, it is a sailing vessel. Is the change from motoring to sailing treated in the Class B units as a Navigational Status change? Or is it treated in the Class B units as a Type change? This change has significant right-of-way implications in good visibility.

In restricted visibility, it is unclear to me how AIS will be treated. As Roger B points out Rule 19 essentially makes all vessels in restricted visibility give-way vessels. But Rule 19d goes further and mandates (vessel "shall take") specific avoiding actions when collision avoidance is done "by radar alone". Does the use of AIS - in addition to radar - mean that Rule 19d would not apply? I would think that this is the correct interpretation.

Posted by: Larry Brandt at May 26, 2009 1:29 PM | Reply

What I'm trying to say, Larry! In AIS "Type" terms, a sailing vessel is sailing vessel no matter what it's doing (I'm pretty sure). The "Navigation Status" field does seem to have much more to do with Rules of Road terms, but Class B AIS transponders do not broadcast Navigation Status.

Posted by: Ben at May 26, 2009 2:05 PM | Reply

Larry ... I think 19d is very clear. E.g. AIS + Radar does not equal "by radar alone", just as VHF bridge-bridge + Radar does not equal "by radar alone"

The question to ask ... is should 19d be modified "by radar and/or AIS alone"? I would love to here how that discussion went and why it favored 19d being apparently unmodified to date.

Larry ... what do you think? Should 19d(i) and 19d(ii) apply if you additionally have AIS information ?

Would you argue that 19d exists to avoid "radar assisted" collisions, and AIS eliminates that risk ?

Would you argue that 19d more broadly exists to cover the situation where one vessel detects another without the certainty of recripical detection and that (i) and (ii) is necessary even with AIS?


Posted by: Dan Corcoran (b393capt) at May 26, 2009 4:04 PM | Reply

According to Farwell's the term "RADAR alone" means not sighted visually or not hearing a fog signal. Rule 19 applies regardless if a vessel is on AIS.

Posted by: kennebec Captain at May 26, 2009 9:05 PM | Reply

Dan, I agree that Rule 19d's "radar alone" means exactly that, and your elaboration was eloquent. I would argue at this point, though, that 19d(i) and (ii) need not be changed. Frankly, I haven't thought through the full implications of AIS-B yet, and so may eventually be convinced otherwise by a strong advocate.

Thinking aloud, say we have a collision risk between a ship with radar and AIS-A, and perhaps a sailboat under sail with radar and AIS-receive only. Certainly both vessels are better off than if the smaller vessel had radar alone. The best use of AIS in this situation, I believe, would be to facilitate bridge-to-bridge contact. And so Rule 19a,b,c and e, and Section I to which 19c refers, would properly cover the situation.

My concern here is that AIS-B transmissions apparently do not permit a receiver to discriminate between a vessel under sail and the same vessel under power. I think that is a missed opportunity on the part of the Class-B design/manufacturing community because the rights of the vessel in these two situations are significantly different. (Note: in restricted visibility, they're not different...but in good vis, they are different.)

Posted by: Larry Brandt at May 26, 2009 9:14 PM | Reply

Larry, You may be right about a "missed opportunity" but it's not the "Class-B design/manufacturing community" to blame. The designers and manufacturers had to work with very strict specs established by the IMO.

Posted by: Ben at May 26, 2009 9:29 PM | Reply

Ben, you said it in an earlier post and it just didn't sink into my brain: Class B does not broadcast Navigation Status. (Too much sun on the boat yesterday, I suppose.)

I was just now looking at various locations using the excellent Siitech viewer. I can guess one reason why the IMO Class B spec didn't include broadcast of Navigation Status...of the existing Class A users, a surprisingly large percentage have entered an erroneous Nav Status.

Posted by: Larry Brandt at May 26, 2009 10:03 PM | Reply

"Thinking aloud, say we have a collision risk between a ship with radar and AIS-A, and perhaps a sailboat under sail with radar and AIS-receive only......"
"My concern here is that AIS-B transmissions apparently do not permit a receiver to discriminate between a vessel under sail and the same vessel under power."

In the ship/sailboat case, other rules may apply because of draft or ability to maneuver. A sailboat under sail that does not recognize that tonnage rules will not be helped by AIS. More importantly, will the captain of the sailboat under power be one that knows that when under power, a sailboat is a vessel under power and not a sailboat.

With ships colliding in broad daylight with pilots aboard and all the electronics functioning, AIS could be another tool that tells the respective Captains they are colliding after the fact.

I wonder if the bridge allision in CA and subsequent release of oil might have been prevented if someome watching the AIS position had called the ship and told them they were not in the channel. -Doug

Posted by: MaineFog at May 27, 2009 10:58 AM | Reply

"I wonder if the bridge allision in CA and subsequent release of oil might have been prevented if someome watching the AIS position had called the ship and told them they were not in the channel. -Doug"

They were called by the VTS watchstanders....obviously it didn't help

Posted by: Anonymous at May 27, 2009 11:29 AM | Reply

With reference to long distance AIS reception, you may find this interesting:

Satellite AIS from USCG

Posted by: JonnyBoats at May 27, 2009 10:03 PM | Reply

Hey all, just a FYI.. I was hailed yesterday coming into Annapolis Harbour by a sailboat called something lady.. Anyway the point is the captain asked if I could see his class b AIS. I have the Furuno Class A unit and could see all of his info quite clearly. So I believe Class A units not seeing Class B's must be tied possibly to the older units or maybe it's not a issue all together.

Just a FYI.

Posted by: Anonymous at June 8, 2009 11:59 PM | Reply

The Washington State Ferries see my Class B target, but it lacks a vessel name, so from their perspective I'm sort of unidentified. Again, it's probably older equipment.

Meanwhile, the Vessel Traffic Service at USCG's state-of-the-art Sector Seattle JHOC doesn't see me at all! Or at least they didn't last fall. They might have fixed this, since it's a simple software problem.

Posted by: Tim Flanagan at June 9, 2009 2:03 PM | Reply

Tim, Are you confident that you've gotten your Class B within range of the VTS's receivers? Remember, your antenna is low and broadcasting at 2 watts, while the VTS antennas were designed for receiving 12 watt transmissions coming from high sticks. It would seem odd that they couldn't even display the Class B dynamic data message, which was built into the original AIS specs.

Posted by: Ben at June 9, 2009 2:29 PM | Reply

Trying to trouble shoot why USCG and Seaway Welland and just 5 minutes ago no one can see my vessel name on my Furuno FA-50 a " B " unit. It was installed a few weeks ago in Florida and was very usefull on the trip up to the Great Lakes. In one severe squall line I managed to screw up the radar tuning and had no targets to get back in tune. The AIS showed all the lakers and the projected courses made staying out of their way a lot easier. Plus now I had a target to tune to amid the sea clutter and rain.I am aboard a 29m yacht. We heard that some harbors are going to mandate AIS on ships over 65 feet but we added the B class for safety. Other stations can see our MMSi but they do not see our name. This Mad Mariner site
had this info:(from this site above)* Has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to every 10 sec. for Class A):
I believe this is a problem with bandwidth. The maritime industry pressed for quick implementation after 9/11 which meant they had to use existing technology. They chose to send the data transmissions via VHF, which results in ships competing for air time. I would really like to see future models have a WIMAX/cell/other based primary data output with the VHF channels providing redundancy.

* Does not transmit the vessel’s IMO number or call sign.
Both are useless to boaters... the big question now is: does it display an MMSI #??

* Does not transmit ETA or destination:
These are rarely updated by mates aboard ship anyway.

* Does not transmit navigational status:
Not terribly important. I also never trust this data.. or any AIS data that has to be manually changed regularly. Hopefully once Integrated Bridge Systems develop AIS messages will be automatically updated by the system when nav lights are changed.

* Is only required to receive, not transmit, text safety messages:
I don't see this as a major limitation.

* Is only required to receive, not transmit, application identifiers (binary messages):
Not sure what this means.

* Does not transmit rate of turn information:
This is an important point! Also, will these receivers be tied to any heading indicators? What percentage of boats have anything but magnetic compasses?

* Does not transmit maximum present static draught:
Not important.

So perhaps your name does not get picked up by A stations???

Still a very useful tool for staying out of the way of the big guys.


Capt Tedd
Go Fourth

Posted by: Capt Tedd Greenwald at June 11, 2009 6:07 PM | Reply

Some basic AIS info is here:

Bandwidth is not really an issue, the system is designed to handle thousands of vessels at a time, there is no "competition". VHF is a much more robust transmission means than WiMax or cellular will ever be.

Posted by: Anonymous at June 14, 2009 11:38 AM | Reply

Pat Harman emailed in some positive Class B experiences:

I share your enthusiasm for AIS. I would like to share with you my experience with the AIS class B transceiver I installed on my new boat. Two incidences occurred which proved the value of little guys like me letting the big guys know that I am there.

The first incident was with the Washington State Ferry Chelan. The Chelan was checking into the Victoria traffic system about 6 miles from my location. The Chelan was concerned about a vessel "Sun Dancer" on his intended track. Victoria Traffic advised the Chelan that I was not checked into the system, but they were aware of me. I broke into the conversation and advised the Chelan that I was aware of him and would stay out of his way. The Chelan passed me in a narrow location near Sidney Spit, but I was well off to the side. The Chelan slowed down so as not to wake me. A very professional experience

The second incident was with a British Columbia Ferry moored to her birth in Nanaimo, BC. I was making a run to make slake water at Dodd Narrows. I did not have visual contact with the ferry until I rounded Jack Point. The ferry did not appear to be departing immediately so I crossed her bow. Just as I crossed her bow the ferry advised Victoria Traffic that the were departing their birth. I was not as prudent as I should have been, but AIS made it virtually a non-incident. The ferry delayed departure a minute or two to avoid traffic.

This was my first experience with broadcasting my position. I now know that having a transceiver was a wise investment


Posted by: Ben at December 30, 2009 11:33 AM | Reply

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