AIS Class B

AIS Class B is a lower-cost, lower-power automatic identification system transponder intended for smaller vessels not subject to the SOLAS Class A carriage requirement, including fishing vessels, recreational craft, and small domestic passenger ferries. It operates on the same VHF frequencies as Class A — channels 87B and 88B at 161.975 MHz and 162.025 MHz — and transmits positional data at lower reporting rates and with a maximum transmit power of 2 watts compared with the 12.5 watts of Class A. Class B does not transmit rate of turn information and does not receive all message types. The Class B standard is defined in ITU-R M.1371 and ETSI EN 301 843-4. Class B transmissions are visible on Class A transponders and on AIS displays, but Class A units are given network access priority and Class B transmissions may be suppressed in congested traffic areas. Radio operators and watchkeeping officers must be aware that a vessel fitted only with Class B may not appear on AIS displays in dense traffic because of this suppression. Several maritime administrations now require Class B fitment on domestic passenger vessels and fishing boats above a defined size as a means of improving small vessel collision avoidance and search and rescue response.