Class A and B described below by the Coast Guard. They are the same except in performance. Please note that class B is interoperable with
all other AIS stations
TYPES OF AUTOMATIC IDENTIFICATION SYSTEMS (PER ITU-R M.1371 AND IEC STANDARDS)
Class A | IEC 61993-2
Shipborne mobile equipment intended to meet the performance standards and carriage requirements adopted by IMO. Class A stations report their position (
message 1/2/3) autonomously every 2-10 seconds dependent on the vessel’s speed and/or course changes (every three minutes or less when at anchor or moored); and, the vessel’s static and voyage related information (
message 5) every 6 minutes. Class A stations are also capable of text messaging safety related information (
message 6/8) and AIS Application Specific Messages (
message 6,8,25,26), such as meteorological and hydrological data, electronic Broadcast Notice to Mariners, and other marine safety information (see
IMO Safety of Navigation Circular 289, GUIDANCE ON THE USE OF AIS APPLICATION-SPECIFIC MESSAGES (ASM) or the
IALA Application Specific Message Collection).
Class B | IEC 62287-1 and 62287-2
Shipborne mobile equipment which is interoperable with all other AIS stations, but, does not meet all the performance standards adopted by IMO. Similar to Class A stations, they report every three minutes or less when at anchor or moored, but, their position (
message 6/8) is reported less often and at a lower power. Likewise, they report the vessel’s static data (
message 18/24) every 6 minutes, but, not any voyage related information. They can receive safety related text and application specific messages, but, cannot transmit them. There are two types of Class B AIS, those using carrier sense Time-Division Multiple Access (CS-TDMA) technology and those like the Class A using Self-Organizing Time-Division Multiple Access Technology (SO-TDMA). Class B/SO is generally more capable; Class B/CS is generally less expensive. See this broader
comparison of Class A and Class B AIS.