How AIS works

The benefit of AIS over other technologies is that it transmits data in an organised, interoperable, way; fusing static and dynamic data together.

GPS data gives dynamic data including a vessel’s course, speed and position, which is fused with static data including a vessel’s MMSI number, its size and it’s destination to give a comprehensive navigation system.

This data is broadcast on a common international VHF frequency – AIS 1 and 2 (161.975MHz and 162.025MHz) – ensuring devices are able to see each other and interoperate within the AIS slot map.

The AIS slot map is the gateway to ensure that each AIS product interoperates within the AIS system. This interoperability ensures the integrity of the AIS system is maintained.
Different AIS products use the slot map in different ways, some can reserve their slot, some send without having a slot, whilst others scan for a free slot to transmit data into.

Self Organised Time Division Multiple Access (SOTDMA) is used by Class A transceivers.  This enables the Class A unit to reserve its time slot and any subsequent time slots.

Carrier Sense Time Division Multiple Access (CSTDMA) units scan for available time slots in the slot map and transmit when one is found. Unlike SOTDMA, CSTDMA units aren’t able to reserve a slot, but continue the scanning process at regular intervals to find the next available slot.

Fixed Access Time Division Multiple Access (FATDMA) is used by AtoNs instead of SOTDMA and CSTDMA. FATDMA is used by AtoNs when their time slots are controlled by an AIS base station, this system stop ships using those slots and ensures data is sent and received.

Random Access Time Division Multiple Access (RATDMA) is also used by AtoNs. It behaves like CSTDMA, where the slot map is not controlled by an AIS base station, so the AtoN must scan for an available space to transmit into.

Pre Announced Time Division Multiple Access (PATDMA) is used by SARTs to transmit data. The SART transmits it’s data regardless if the slot is reserved or not. This system is designed to be used in emergency situations.