AIS Class B, messages & data fields
Geez, this morning I found myself confused about Class B AIS details touched on yesterday, and realized that my sources are somewhat sketchy. The USCG AIS site is pretty informative generally, but its Class B particulars seem suspect, the statement “IEC has begun work on a Class B certification standard, which should be completed by 2003” suggesting that it has not been updated in a long time. I am grateful for the Documentation section of AISParser.com, which is where I found the following message info, much edited and with a few notes added:
Message 18 - Standard Class B CS Position Report: UserID/MMSI, Time, SOG, COG, Longitude, Latitude, True Heading. {CS, or Carrier Sense, integrates “politely” with the Class A protocol known as Self-Organizing Time Division Multiple Access or SOTDMA, illustrated above}.
Message 19 - Extended Class B Equipment Position Report: Same as 18, plus Ship Type and Dimensions {measured bow/stern and port/starboard relative to GPS, which may be why they are often screwed up}.
Message 24 - Class B CS Static Data Report: UserID/MMSI, Boat Name, Ship Type, Call Sign, Dimensions, and Vendor ID {i.e. transponder manufacturer, to help track down problems}.
While I don’t understand the purpose of #19, which seems redundant, I think that the list includes every significant data field that a Class B can transmit. But I could be wrong!
Hmmm ... yes it can get confusing...
msg19 is a hang-over from the days when Class B was expected to use the SOTDMA protocol, however it is two slots long, which means that when the CSTDMA protocol was dreamed up, it wouldn't work (CSTDMA can ONLY be one slot long), so they invented msg24.
However the problem of how to get two slots worth of info into a single CSTDMA slot still remained, so msg24 was split into two, 24a and 24b. The 24b is sent within a minute of the 24a so when both have been received, all the data is then available.
...oh and I believe that there is a Class B SOTDMA standard somewhere, so msg19 may come back at sometime in the future.....