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.