Class B AIS, part 2

... written for Panbo by Ben Ellison and posted on Oct 27, 2005

Today, I’m afraid, we must speak of “frequency agility.” Part of the Class A standard is that transponders must be able to change channels if ordered to by the presiding local coastal authority. The digital command is received on the VHF DSC calling channel, 70. As mentioned in the past only one port is currently using this function, Tokyo, and I’ve heard that even it may turn back to regular channels 87B and 88B.  Nonetheless, in September the U.S. apparently surprised international AIS committees by demanding frequency agility in the Class B standard. (This may have something to do with the still unsettled dispute with Maritel over use of 87B in the States, but, again, another story). At any rate, this change will mean that Class B transponders are more complicated and more expensive than originally conceived, and slower to arrive. Actual transponders available by the end of 2006 for around $2,000 is now sounding realistic. It’s also possible that there will be non-agile Class B transponders that are not approved for use in the U.S.

Now, as promised, let’s noodle a bit about what the Class B standard will mean. My thoughts:

 • Some bigger yachts are going to opt for Class A—perfectly legal for anyone to carry—because they’ll decide that the additional power and priority, plus things like ship-to-ship messaging (not in Class B), are worth the added expense and install hassle.

• I’m wondering if a single frequency receiver will be effective, especially in close quarters, when it’s only getting dynamic data every minute at best. Real time tracking starts to get unreal, but maybe not enough to make a difference.

 • Overall, Class B is going to greatly enhance AIS. Isn’t there a famous computer postulate that puts the value of a network equal to the square of its users? Isn’t AIS a more-the-merrier—and safer—situation, even for the lurkers (receive only).

PS. For more details of Class B, check out this page at Y-Tronic, noting that the Class B standard is supposedly going to use “Carrier Sense”, not “SOTDMA”. 

Posted by Ben E on October 27, 2005 2:43 PM

Comments

The command to change frequencies can also be received over the AIS channels. Message 22 handles this.

Posted by: Anonymous at October 27, 2005 10:52 PM | Reply

You're right, and thank you, but my understanding is that the USCG still wants DSC reception in Class B.

Posted by: Ben at October 27, 2005 11:08 PM | Reply

That is my understanding as well.

Posted by: Anonymous at October 27, 2005 11:51 PM | Reply

the thing i don't understand about the "class B" version is that the boats which will carry it are likely to be significantly *faster* and much more agile than the big boats with class-A hardware. this means the reduced reporting rates for class-B transponders is exactly the *wrong* thing to do if one cares about the fidelity of situational awareness. i realize that the big boats don't really care whether they see the little boats but there's a big chance for a success disaster here if AIS becomes significantly deployed outside the "must-carry" universe.

Posted by: Mike O'Dell at October 28, 2005 5:35 AM | Reply

In response to Mike O'Dell's point:

The international community is afraid that a higher reporting rate for faster boats carrying a Class "B" device will contribute to overloading the available bandwidth.

"Competent Authorities" like USCG are much more interested in tracking boats than in allowing boats to use AIS for collision avoidance purposes.

Personally I don't disgree with your point. Class "A" devices on ships faster than 14 knots while making a turn report their dynamic information every 2 seconds. Class "A" devices on ships faster than 23 knots report their dynamic information every 2 seconds even if they are not making a turn.

Class "B" devices were handicapped right from the start of the international negotiations that led to the new Class "B" performance standard.

Posted by: Fred Pot at October 28, 2005 4:38 PM | Reply

Is there any AIS equipment that support a NMEA2000 interface and has anyone tried them out? I would certainly like to pop a T into my 2000 backbone and attach an AIS receiver (or class B, when available...or class A, when I win the lottery :) Seems to be plenty of 0183 outputs.

I've seen some advertisements, but no reviews and sometimes the manufacturers claim compliance before they actually are (e.g. "compliance" sometimes means partial implementation).

Posted by: Robert at October 31, 2005 4:22 PM | Reply

Robert, I don't know of any NMEA 2000 receivers or transponders. I too would like to see them developed. Where did you see the ads?

Posted by: Ben at October 31, 2005 4:26 PM | Reply

Does the Y-tronic splitter thing comply with any ISO / IEC / ETSI standards? I'm thinking of transients when it switches from VHF to AIS modes in particular....

Posted by: del at December 13, 2005 12:06 AM | Reply

SRT has just launched a very low cost AIS Class transmit and receive technology solution. This will enable manufacturers to produce a professional Class B this year and retail it for $500. As standard DSC is implemented as is chanel agility. Thought you guys might want to know that we have been beavering away at this for 2 years and its now available.

Posted by: Simon at January 7, 2006 6:22 PM | Reply

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