FATDMA is a manually managed TDMA access scheme where AIS devices are pre-configured to use specific TDMA slots for all transmissions. FATDMA is used only for AIS base stations and AIS AtoN stations. FATDMA is defined in ITU-R M.1371-4 Annex 2, §18.104.22.168.
The key elements of FATDMA operation are as follows:
- All stations share a common time reference (derived from GPS time) ensuring they can all accurately determine the start time of each TDMA slot.
- Stations are configured at installation to transmit in a specific TDMA slot or slots.
- Stations configured for FATDMA operation transmit a Data Link Management message which advises other stations of the FATDMA slot allocations. This blocks these slots from use by any other station in range. For this reason the use of FATDMA is minimised in order to minimise impact on the dynamic behaviour of the AIS network.
- Stations configured for FATDMA operation transmit only into the pre-defined slots.
The essential hardware requirements to support FATDMA are:
- VHF transmitter capable of operating on any AIS channel in the marine VHF band
- GPS receiver to provide a time reference for TDMA timing
Note that no receiver capability is required to support the FATDMA access scheme.
FATDMA is used by AIS Base Stations and AIS Aids to Navigation to transmit their data. It is a managed system and the slots are configured manually when the equipment is installed. The slots allocated to FATDMA transmissions are blocked for use by other AIS equipment.
- An AIS base station broadcasts messates to reserve slots in the AIS slot map for its own transmissions or for a nearby AIS Aid to Navigation
- The Base Station or AIS Aid to Navigation transmits data into the reserved slots periodically