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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:

The essential hardware requirements to support modified SOTDMA are:

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: