AIS Class B – Whats the difference

AIS Class B was specified as a much less expensive, limited range and limited feature sub-set of the original Class A.

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)

So Class B:-

  • 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

Andy Norris of the University of Nottingham, writes in the Journal Of Navigation…

“The CSTDMA Class B system has been designed to prevent overloading of the AIS VHF data link. This is briefly reviewed but there are a number of other aspects that need to be considered. These include: the increased garbling of Class B messages compared to those of Class A; the problems accruing from the low update rate of Class B information; the increase in display information that will need to be managed; and the possible increase in inappropriate manoeuvres of leisure craft caused by misplaced reliance on AIS. As a result of the investigation the paper highlights the fact that Class B users must not assume that their own presence, in the form of Class B transmissions, will be particularly visible on the bridge of many SOLAS vessels. This will continue to be the case for many years into the future, until such vessels are mandated to carry radar with AIS target overlay capability….”


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