SOTDMA and its system description.
Self-organising time division multiple access uses ASC II code and transmits on 162 MHz similar to channel 70. There are two channels with 256 timeslots in each giving 512 transmission points. A vessel may need up to 3 transmission points to send his full data when manoeuvring.
SOTDMA will use an algorithm to hunt the time transmission points to randomly transmit the vessels status. A vessel at anchor will transfer it every three minutes. A vessel doing 23 kn and altering course will transfer it every two seconds.
The vessel transmission can also be triggered by VTS using ASSIGNED mode. This insures that each vessel has its own time slot within the VTS reception range.
With POLLING mode the transmission is prompted by signal from a passing aircraft or ship these are usually military or government craft.
AIS transmits static data, dynamic data, voyage related data and safety messages.
Static data – Ships particulars and dimensions.
Dynamic data – Course and speed of vessel. Navigational status or position fixing system.
Voyage related data – Destination and ETA, draft.
Safety messages – text messages to specific ships or to all ships.