Transmission types

AIS (Automatic Identification System) transmission types refer to the different ways AIS transponders send and receive data for vessel tracking and navigation. AIS uses VHF radio frequencies to broadcast information about ships, including position, speed, and identification details. The transmission types can be categorized based on functionality, mode, and protocol.

1. Based on AIS Class

• Class A AIS: Used by commercial ships and larger vessels. It transmits more frequently and with higher power.

• Class B AIS: Used by smaller vessels, leisure boats, and some coastal stations. It has lower transmission power and a longer reporting interval.

2. Based on Transmission Mode

• Self-Organizing Time-Division Multiple Access (SOTDMA): Used by Class A AIS. It automatically assigns time slots to avoid data collisions.

• Carrier-Sense Time-Division Multiple Access (CSTDMA): Used by Class B AIS. It listens for an available time slot before transmitting, which can lead to delays in busy networks.

• Fixed Access Time-Division Multiple Access (FATDMA): Used by shore stations and AIS AtoN (Aids to Navigation) to reserve time slots for transmission.

3. Based on Data Type

• Position Reports: Transmits ship position, speed, course, and heading at regular intervals.

• Static and Voyage Data: Includes ship name, MMSI number, cargo type, and destination.

• Safety Messages: Used for broadcasting navigational warnings and emergency alerts.

4. Based on Communication Method

• Broadcast Transmission: Most AIS messages are sent to all nearby vessels and shore stations.

• Addressed Transmission: Some AIS messages are directed to specific ships or shore stations (e.g., AIS Application-Specific Messages - ASM).

• Interrogation Responses: A vessel or shore station can request AIS information from another AIS unit.

5. Based on AIS Frequency and Range

• Terrestrial AIS (T-AIS): Uses VHF radio waves to transmit signals between ships and shore stations. It has a range of 10-50 nautical miles.

• Satellite AIS (S-AIS): Uses satellites to receive AIS signals from ships beyond the range of terrestrial AIS, covering open oceans.