The operation of Class A AIS transceivers is defined by the equipment and test standard IEC61993-2 Edition 1.

Class A transceivers use the following TDMA access schemes:

  • SOTDMA is used for the majority of transmissions including all periodic transmissions (position reports).
  • RATDMA and ITDMA are used during network entry (when the transceiver is first switched on) to enable initial transmissions and slot allocation
  • RATDMA is used for non-periodic transmissions which can not be pre-announced. This would include transmission of text messages or safety related messages
  • ITDMA is used to allocate slots for periodic transmissions when the reporting interval of a periodic message is changed (for example when the position reporting interval changes due a vessel speed change)

Using SOTDMA, units transmit to locate a vacant time slot reserve it, transmit the data and reserve the next time slot automatically. Class A units receive as well as transmit data, receiving when to transmit their own data, as well as receiving data from other ships, AtoNs or base stations and when their next reserved slot is in the AIS slot map.

Class A units have one VHF transmitter, two VHF TDMA receivers and one VHF Digital Selective Calling (DSC) receiver. Class A units must also include links to the shipboard display and sensor systems via standard marine electronic communications. The unit primarily uses GPS, LORAN, Glonas or an internal navigation system to determine its position. The unit connects to shipboard systems through data connections, eg. NMEA 0183/2000/USB, to display information received through AIS.

Access scheme
Transmitter power
Number of receivers
Reporting rate
Number of interfaces
3 (2 x AIS, 1 xDSC)
2 seconds - 3 minutes depending on the speed of the vessel
Mini Keyboard Display (MKD)
6 serial interfaces