An AIS transponder is a shipborne unit that automatically transmits and receives data over VHF links.
The International Maritime Organization lists main components as antennas, a VHF transmitter, multiple VHF receivers, a processor, GNSS timing and redundancy, sensor interfaces, display facilities, and a built-in integrity test. AIS uses two dedicated VHF channels with time-division access for stations to share the radio link; technical characteristics are specified in ITU-R M.1371-6.
Class A and Class B equipment have different capabilities and reporting behaviour. Class B does not necessarily provide all functions required of Class A equipment, meaning they are not interchangeable for all regulatory purposes.
