What AIS is and why it exists
AIS is an automated VHF radio-based transponder system that exchanges vessel identity, position, course, speed and other voyage data between ships and shore stations. Its primary purpose is collision avoidance support and traffic monitoring — it gives the OOW situational awareness that radar alone cannot provide, particularly in name-identification of contacts and their intentions.
How the system works
AIS uses two dedicated VHF channels (AIS 1 and AIS 2, 87B and 88B) and a protocol called SOTDMA (Self-Organised Time Division Multiple Access), which divides transmission time into slots and allows multiple vessels to broadcast without continuously colliding with each other's signals. Each transponder determines its own position via an internal or external GPS source and broadcasts that data automatically at intervals that vary with vessel speed and rate of turn — a vessel manoeuvring transmits more frequently than one on a steady course. This dynamic reporting means a fast-altering contact updates more often, which is exactly when you need current information.
Classes of AIS and what each means operationally
Class A is the IMO-mandated standard fitted to SOLAS vessels. It transmits at higher power and higher update rates, and it receives all other AIS traffic.
Class B is the standard for non-SOLAS vessels including most yachts. It transmits at lower power, at lower update rates, and — importantly — Class B transmissions have lower priority; they can be interrupted by Class A traffic during busy periods. As OOW, you must understand that a Class B contact may disappear temporarily from your display not because the vessel has altered course, but because its transmission was suppressed.
Modes of operation
- Autonomous/continuous mode — normal at sea; the transponder transmits and receives automatically without operator input.
- Assigned mode — a competent authority (e.g. VTS) commands the transponder to transmit on a different schedule.
- Polling mode — a shore station or another vessel requests specific data from your transponder.
The OOW does not normally change modes; awareness of their existence is required to explain unexpected behaviour.
Limitations — the critical exam territory
This is where candidates must be precise. AIS is a navigational aid, not a collision-avoidance system, and must never replace radar watchkeeping or a visual lookout.
- Data integrity: positions displayed are only as accurate as the transmitting vessel's GPS and the accuracy of manually entered data (static data such as vessel dimensions and draught is entered by the crew and may be wrong or outdated).
- Non-equipped vessels: fishing vessels, small craft, warships and many leisure craft carry no AIS. Absence of a target does not mean the water is clear.
- Switched off or malfunctioning: a vessel may have AIS disabled — deliberately or through failure.
- Display lag: the chart overlay position may lag true position depending on update interval.
- Over-reliance: treating AIS as a primary collision avoidance tool violates COLREGS Rule 5 (look-out) and Rule 7 (risk of collision). Radar and visual observation remain the primary tools.
MSN 1858 addresses carriage requirements and certification; the applicable yacht code (the Red Ensign Group Yacht Code, REG YC Part A, which superseded LY3) and SOLAS provide the operational and technical standards governing AIS use.