International vessels over 300GT and Costal vessels over 500GT all carry Class A AIS Transceivers. These devices broadcast vessel information such as location, speed and course, along with other less critical data, all in real time. The transmitted data can be received by any other vessel, and this data can be used to display that vessel on your own chartplotting or Radar display.
More and more smaller commercial and leisure vessels are now carrying Class B Transceivers or Receivers
For the leisure sailor and small boat operator, these fall into two basic types.
These units receive the data transmitted by transmitter equiped vessels and either displays the vessel location on an integral screen, or outputs a data stream which can be fed into most chartplotting displays. The down side of these units is that you do not appear on the screens of other vessels.
These units both receive other vessels and transmit your location, speed and course data to other vessels. With a Transceiver, you appear on everyone elses screens! Class B Transceivers are aimed at vessels under the 300T threshold. They are not a staturtory requirement, and do have the ability to be silenced. This is a useful feature when racing so your opponents do not know your exact location and speed, and also rather handy when passing through areas where Pirates frequent!
Class A Transceivers
are primarily for larger commercial vessels, but certain commercial environments for example windfarm construction, the clients demand the higher specification Class A systems on all vessels.
allow AIS units to share an existing VHF antenna with your VHF transceiver, saving the need to install a second antenna.