Modified SOTDMA (or Pre-Announced TDMA) is a simple TDMA access scheme defined for use in transmit only devices. It has specific application in emergency beacons such as AIS Search and Rescue transceivers (SART).

Whilst sharing the ‘SOTDMA’ name with the SOTDMA access scheme this technique has little else in common with SOTDMA. Modified SOTDMA is defined in ITU-R M.1371-4 Annex 9 and described for use in “devices that have limited range and operate in a low volume”. Further definition of the access scheme is given in the AIS SART equipment standard IEC61097-14.

The key elements of modified SOTDMA 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.
• A station randomly selects a slot for transmission. In the first transmission it announces its intention to use this slot for the following 8 minute period. At the start of the next period a new slot is randomly selected.
• Transmissions are made in ‘bursts’ of 8 messages once per minute. This is intended to ensure successful transmission when the device is operating near the surface of the sea and may be blocked from reception by periodic swell.
• The system is referred to as ‘modified’ SOTDMA as it uses the same pre-announcement system for future transmissions as the complete SOTDMA scheme.
• As modified SOTDMA randomly selects transmission slots without knowledge of their use by other stations it is likely to create data collisions. This has been deemed acceptable for use in emergency beacons where the cost benefits of the modified SOTDMA system outweigh the impact of data collisions (which are small as emergency beacons do not operate regularly or in high densities).
• Due to the likelihood of random transmissions colliding with transmissions from other AIS transceivers the modified SOTDMA technique is not suited to use in systems installed in large volume, or where large numbers of systems are used in a small area.

The essential hardware requirements to support modified SOTDMA 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 modified SOTDMA access scheme.

PATDMA is designed for Search and Rescue Transponders (SARTs) and transmits data as follows:

• SART transmits data eight times in one minute
• SART transmits safety message every four minutes