The operation of AIS Aids to Navigation is defined the equipment and test standard IEC62320-2 Edition 1.
AIS Aids to Navigation use either the FATDMA or RATDMA access scheme depending on their hardware configuration.
- A Type 1 AIS AtoN with transmit only capability uses the FATDMA access scheme. This requires a nearby base station to reserve the slots used by the AIS AtoN via data link management messages.
- A Type 3 AIS AtoN with both receive and transmit capability can use either the FATDMA access scheme (with base station reservations) or the RATDMA access scheme. The latter allows the AIS AtoN to autonomously allocate slots for its own transmissions as required.
An AtoN AIS unit operating in RATDMA (Random Access Time Division Multiple Access) mode uses its receiver to listen to both AIS frequencies for about one minute, and makes and stores a map of all the AIS "slots" [or message spaces] on the VHF data link [VDL]. It then looks for two free adjacent slots in which to send its [2-slot long] AtoN message 21 or meteorological and hydrological message 8.
RATDMA is ideal for many applications because the AtoN or weather/tide AIS unit can be placed at any location, and requires no reservation of slots by a base station. It can be used whether base stations exist in the area or not. Its drawback is that in order to transmit to the AIS slot map the unit must turn its receiver on for at least one minute before transmitting, and this is the main power consuming factor with an RATDMA AtoN or weather AIS unit.
An AtoN AIS unit operating in FATDMA (Fixed Access Time Division Multiple Access) mode will transmit in a pair of slots which are reserved by an AIS base station. Ships receive a message from the base station, indicating that certain slots are reserved. The ship AIS transponders store this reserved slot information in their slot maps, and do not transmit in these slots. The FATDMA AtoN is programmed to transmit in two consecutive slots of those reserved by the base station. It is possible to "re-cycle" slots by having a number of AtoN units use the same pair of slots, but use them sequentially. FATDMA allows greatly reduced power drain for an AtoN AIS unit, because no receiving period to build a slot map is needed.