Reading the Class B specifications (http://www.navcen.uscg.gov/?pageName=AISMessagesB) , there's a Message 18 transmission interval that's SOG dependent (https://www.panbo.com/archives/2015/06/sotdma_class_b_ais_the_new_middle_way.html). That makes sense as you need more frequent updates at higher speeds.
The question is about triggering a transmission on course changes?
Class A is apparently configured to do so, but have not found documentation for Class B.
We raced Wednesday Night and I retrieved this track from Marinetraffic.com https://www.facebook.com/photo.php?fbid=10154548505206664&set=pb.542601663.-2207520000.1464359034.&type=3&theater
Our speeds were 3-6 kts SOG except during tacks, where we might drop to 2. It pretty clearly shows all of the tacks, buoy roundings and most of the times I can recall bearing away on the course. There are intermediate positions which would be consistent withtime intervals, and I would expect that Marinetraffic downsamples to give a cleaner track.
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