Class B AIS Transponders

Class B AIS Transponders are intended for use on crafts not covered by a mandatory carriage requirement under SOLAS Chapter V Regulation 19.

There was concern that large numbers of Class B AIS equipped vessels could have a detrimental effect on the safe operation of the AIS network by Class A vessels, Base Stations and Aids to Navigation (AtoN AIS). As a result, new network access technology was developed, which allows large numbers of Class B fitted vessels to coexist with Class A with a negligible detrimental effect on AIS network.

The new technology, “Carrier-Sense TDMA (CSTDMA)”, requires that the Class B ”CS” AIS listens to the AIS network to determine if the network is free of activity and, only if the network is free, can transmit its information. The Class B AIS is also required to listen for reservation messages and comply with these reservations. This polite operation ensures that a Class B AIS will not interfere with Class A, Base Station or Aids to Navigation AIS operations.

Class B messages

Class B transponder transmits the following messages:

• message 14 -- Safety Related Broadcast Message (optional, transmit with predefined text only)
• message 18 -- Standard Class B Equipment Position Report
• message 19 -- Extended Class B Equipment Position Report (transmit ONLY as a response on Base Station interrogation)
• message 24 -- Class B “CS” Static data (Part A and Part B)

Contents of message 14:

• MMSI number
• Safety related text
• Some special parameters

Contents of message 18:

• MMSI number
• Speed over ground (SOG)
• Course over ground (COG)
• True heading
• Position accuracy
• Longitude
• Latitude
• Some special parameters

Contents of message 19:

• MMSI number
• Name of ship
• Speed over ground (SOG)
• Course over ground (COG)
• True heading
• Position accuracy
• Longitude
• Latitude
• Type of ship and cargo type
• Dimension of ship/reference for position
• Type of electronic position fixing device

The message 24 (Class B ”CS” Static data report) consists of two parts: Part A and Part B.

Contents of message 24 Part A:

• MMSI number
• Name of ship
• Some special parameters
• Contents of message 24 Part B:
• MMSI number
• Type of ship and cargo type
• Vendor ID
• Call Sign
• Dimension of ship/reference for position
• Some special parameters

Reporting intervals

The Class B transmits position report (message 18) in reporting intervals of:

• 30 sec if SOG > 2 knots
• 3 min if SOG ≤ 2 knots

Static data submessages 24A and 24B are transmitted every 6 min in addition to and independent of the position report. Message 24B is transmitted within 1 minute following message 24A.

Extended Class B equipment position report (message 19) is transmitted once every 6 min ONLY as response on Base Station interrogation.

The Class B ”CS” AIS transmits a message 14 once a minute by a user manual input. Automatic repetition is not allowed.