What is AIS?

AIS stands for Automatic Identification System and is an automatic tracking and monitoring system that is used on-board vessels and aircraft in order to identify and locate vessels. Identification, position, course and speed are part of the information that is exchanged with other AIS equipped vessels or aircrafts and control centers on-shore. 

The International Maritime Organization (IMO) and the International Convention for the Safety of Life at Sea (SOLAS) requires AIS to be fitted aboard international voyaging ships with gross tonnage of 300, cargo ships of 500 gross tonnage and all passenger ships regardless of size.

What are the differences between a Class A and Class B transponder?

There are several important functional differences between AIS Class A and Class B transponders:  

• Class A meets the requirements of the IMO AIS carriage requirements and is standardized in SOLAS, while the Class B provides capabilities which are not fully compliant with IMO requirements.

• A main difference is the data communication technology; Class A units utilize Self-Organizing Time-Division Multiple Access (SOTDMA) technology.  This technology ensures that VHF transmissions of different transponders do not occur at the same time by synchronizing their data transmission to an exact timing standard, providing that no transmissions are missed.  Class B units utilize Carrier-Sense TDMA (CSTDMA), which listens to the AIS network to determine if a slot is being used. Once the unit determines that the slot is open, it powers up its transmitter and broadcasts a 1 slot position report.  The Class B is also required to listen for reservations from base stations and comply with these reservations. If the unit cannot find an open slot, it will not broadcast. 

• The reporting intervals are lower with a Class B transponder than a Class A transponder.

• The transmitted AIS data from a Class B transponder is limited; a Class A transponder transmits and receives all required AIS data.

What is SOTDMA?

Self-Organizing Time Division Multiple Access (SOTDMA) is a concept in which the time slots are synchronized to UTC, and stations advertise their intention to transmit in a specified time slot by means of a reservation protocol carried in a prior transmission.

For convenience, a group of contiguous time slots spanning a period of 60 seconds is termed a super-frame. Each time slot may be used by a radio transceiver (mounted on aircraft, ground vehicles, and vessels or at fixed ground stations) for transmission of data. The exact timing of the slots and planned use of them for transmissions are known to all users in range of each other, so that the data link can be used efficiently and users do not transmit simultaneously.