The radio system through which ships broadcast their identity, position, course and speed to everyone around them.
AIS, the Automatic Identification System, lets vessels broadcast who they are and where they're going. A transponder on board sends its data automatically over two dedicated VHF channels, and every AIS receiver within radio range picks it up and shows it on the chartplotter.
What a target transmits
- Identity: MMSI number, name, call sign, ship type and size.
- Movement: position, course over ground, speed over ground, heading and rate of turn.
- Voyage (Class A): destination, ETA, draught and navigational status, such as "at anchor".
From this, the plotter calculates the CPA (closest point of approach) and TCPA (time to it) for each target and can sound an alarm when a ship comes too close.
Class A and Class B
- Class A is mandatory for larger commercial ships and passenger ships. It transmits with more power and more often.
- Class B is the version for leisure boats: cheaper, with less power and longer intervals between reports.
- A receiver only listens: you see others, but they don't see you.
Limits
AIS only shows vessels that carry it and have it switched on. Many small craft don't, and VHF range drops behind land. AIS supplements a proper lookout and radar; it doesn't replace them. AIS receivers usually pass their data on as NMEA 0183 (!AIVDM sentences) or NMEA 2000.