Class A AIS VS. Class B AIS

Class A AIS transponders (SOLAS Compliant) operate using Self-Organizing TDMA (SOTDMA) broadcast mode and transmit at a power level of 12.5 watts. Dynamic information is transmitted every 2 to 10 seconds while underway and every 3 minutes while at anchor. Static and voyage related information, such as the vessel’s name and cargo, are transmitted every 6 minutes. Class A AIS transponders are required to have a DSC (156.525 MHz) receiver, external GPS, heading, and rate of turn indicator, and can also transmit and receive safety-related text messages.

Class B AIS transponders operate using Carrier-Sense TDMA (CSTDMA) broadcast mode and transmit at a power level of 2 watts. Dynamic data is transmitted every 30 to 180 seconds, while static data is transmitted every 6 minutes. A DSC receiver and heading are optional. Transmitting safety-related text messages is also optional, and only available if pre-configured into the Class B AIS transponder.

Class A AIS and Class B AIS Characteristics