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Author Topic:   AIS Class A and Class B Confusion
jimh posted 02-21-2009 02:21 PM ET (US)   Profile for jimh   Send Email to jimh  
WEST Marine has some motion picture and audio presentations from the Miami International Boat Show (MIBS) available on-line. In one of these, Chuck Hawley from WEST Marine interviews an ICOM spokesman, identified as Rick. About the MXA-5000 AIS receiver, ICOM's Rick says:

"It's a class A and B simultaneous receive AIS box."

"It's two channels, class A and class B, and it automatically just receives both, and simultaneous, so you'll never miss a class A or a class B target."

I found this to be confusing. From what I can tell, the radio channels used to transmit AIS signals are not divided into class A and class B users. All AIS transponders use the same channels, whether they are Class-A transponders or Class-B transponders. From the statements made by Rick, one could make the inference that a particular radio channel was assigned to Class-A transponders and a separate channel was assigned to Class-B transponders. I can't find any reference to support that.

According to the USCG, an AIS Class B transponder differs from a AIS Class A transponder as follows:

Class B
Shipborne mobile equipment provides facilities not necessarily in full accord with IMO AIS carriage requirements. The Class B is nearly identical to the Class A, except the Class B:

--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)
--Does not transmit the vessel’s IMO number or call sign
--Does not transmit ETA or destination
--Does not transmit navigational status
--Is only required to receive, not transmit, text safety messages
--Is only required to receive, not transmit, application identifiers (binary messages)
--Does not transmit rate of turn information
--Does not transmit maximum present static draught


Class-B transponders also transmit at lower power level, 2-watts, compared to Class-A transponders, which transmit at 12.5-watts. There is no mention of distinctive radio channel assignments.

As far as the radio portion of an AIS receiver goes, it is going to hear both Class-A transponders and Class-B transponders. However, since they transmit slightly different data, the data demodulator of an AIS receiver may need to be able to respond to both data formats. Perhaps this was the point the ICOM spokesperson was making: the MXA-5000 will process and output data from both Class A and Class B transponders.

I don't know if there is such a device as a Class A only receiver. Class-B transponders have only been approved for use in the United States for a few months, but they have been used elsewhere for some time. I would expect that if there were some particular difficulty to be overcome by an AIS receiver in handling Class-B transponders, that problem would have already arisen in Europe or other areas where Class-B transponders have been in use.

It is possible to filter AIS data when it is being displayed on a chart plotter or other device, and in some cases to reduce clutter it may be desirable to discard some of the data that has been received. For example, you might want to filter out all vessels that are moving at less than 5-knots. However, this sort of data filtering does not generally take place in the AIS receiver. The AIS receiver receives radio signals, demodulates the information on them, and then outputs that data in serial data transmission for use by other devices. This is the realm of the ICOM MXA-5000. In this regard it seems like all other AIS receivers. If there is going to be filtering of the data, it will happen at a higher level in the processing of the data, not in the radio receiver.

From the WEST Marine video, it looked like ICOM had a very cool set up at MIBS with their MXA-5000 connected to a computer running a marine navigation program, ultimately displayed on a very large screen. It was probably a very impressive demonstration. I hope ICOM takes that display on the road and hauls it to some other boat shows.
Cf.: DisplayPageDynamicView?storeId=10001&langId=-1&catalogId=10001& page=Content&content=miami-landing.html&icid=HM2
The AIS receiver discussion is in the last half of the ICOM segment.

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