RATDMA is a simple TDMA access scheme available for certain types of data transmission and AIS device types. RATDMA is defined in ITU-R M.1371-4 Annex 2, §184.108.40.206.
RATDMA is used when a station needs to allocate a slot, which has not been pre-announced. This is generally done for the first transmission slot during data link network entry, or for messages of a non-repeatable character (such as transmission of text messages).
The key elements of RATDMA 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.
- The station transmitting using RATDMA uses its internal ‘slot map’ to randomly select a slot that is not currently in use by another station. It does not announce use of this slot, or subsequent slots used for non-periodic transmissions.
- RATDMA is not suitable for periodic transmissions as slots allocated using this technique can not be known by other AIS devices. Use of RATDMA for periodic transmissions by many devices would result in significant data collisions and compromise the integrity of the system.
- RATDMA is used by Class A AIS stations for ‘network entry’. This occurs when a Class A device is first switched on and has not previously announced its own slot allocation using SOTDMA. An initial RATDMA transmission is used to solve this problem.
The essential hardware requirements to support RATDMA are:
- VHF transmitter capable of operating on any AIS channel in the marine VHF band
- Two VHF receivers capable of operating on any AIS channel in the marine VHF band
- Full time decoding of all received messages in order to populate an internal slot map
- GPS receiver to provide a time reference for TDMA timing
- Sufficient memory (RAM) to store a slot map for at least five minutes of TMDA slot allocations (the allocation status for 22500 TDMA slots)
AtoNs using RATDMA behave like Class B transceivers, and transmit and receive data as follows:
- AtoN scans for available slots in the AIS slot map
- Data transmited into available slot
- Receives data from the AIS slot map
- Depending on configration, scans for available slot in the map
- Transmits data into the AIS slot map