Class B "CS". Base station Shore-based AIS transceiver (transmit and receive) which operates using SOTDMA. Base stations have a complex set of features and functions which in the AIS standard are able to control the AIS system and all devices operating therein. Ability to interrogate individual transceivers for status reports and or transmit frequency changes. Aids to navigation (AtoN) Shore- or buoy-based transceiver (transmit and receive) which operates using fixed-access time-division multiple-access (FATDMA). Designed to collect and transmit data related to sea and weather conditions as well as relay AIS messages to extend network coverage. Search and rescue transceiver (SART) Specialist AIS device created as an emergency distress beacon which operates using pre-announce time-division multiple-access (PATDMA), or sometimes called a "modified SOTDMA". The device randomly selects a slot to transmit and will transmit a burst of eight messages per minute to maximize the probability of successful transmission. A SART is required to transmit up to a maximum of five miles and transmits a special message format recognised by other AIS devices. The device is designed for periodic use and only in emergencies due to its PATDMA-type operation which places stress on the slot map. Specialist AIS transceivers Despite there being IMO/IEC published AIS specifications, a number of authorities have permitted and encouraged the development of hybrid AIS devices. These devices seek to maintain the integrity of the core AIS transmission structure and design to ensure operational reliability, but to add a range of additional features and functions to suit their specific requirements. The "Identifier" AIS transceiver is one such product where the core Class B CSTDMA technology is designed to ensure that the device transmits in complete compliance with the IMO specifications, but a number of changes have been made to enable it to be battery powered, low cost and more easy to install and deploy in large numbers. Such devices will not have international certification against an IMO specification since they will comply with a proportion of the relevant specification. Typically authorities will make their own detailed technical evaluation and test to ensure that the core operation of the device does not harm the international AIS system. AIS receivers are not specified in the AIS standards, because they do not transmit. The main threat to the integrity of any AIS system are non-compliant AIS transmissions, hence careful specifications of all transmitting AIS devices. However, it is well to note