AIS (Automatic Identification System)

What is AIS?

AIS is the maritime equivalent of an aircraft transponder. A small VHF transceiver on the bridge, paired with a dedicated GPS antenna, broadcasts a digital data packet every two to ten seconds containing the yacht's Maritime Mobile Service Identity (MMSI), name, call sign, position, heading, speed over ground, rate of turn, and static dimensions. The same unit receives identical broadcasts from every AIS-equipped vessel within VHF range - typically 20 to 40 nautical miles ship-to-ship.

Targets are plotted directly onto the chartplotter or ECDIS display as triangles with vector arrows, alongside live closest-point-of-approach (CPA) and time-to-CPA calculations. The captain can interrogate any target with a click and call it by name on the VHF.

The system has two classes. Class A, mandated by SOLAS Chapter V Regulation 19 on vessels of 300 GT and above on international voyages and on all passenger ships, transmits at 12.5 W with rapid update rates. Class B, intended for smaller commercial craft and recreational yachts, transmits at 2 to 5 W with slower update rates. Class B SO is a higher-power variant. Principal manufacturers: Furuno, Raymarine, em-trak, McMurdo.

Why it matters for yacht owners

For a buyer, AIS is the single most useful piece of bridge electronics for safety and situational awareness. A SOLAS-coded yacht above 300 GT, or any yacht operating commercially on international voyages, must carry type-approved Class A; finding only a Class B is a flag-state non-conformity.

The second issue is privacy. AIS data is harvested by public tracking sites such as MarineTraffic, meaning anyone with a browser can watch a yacht's movements. SOLAS permits the AIS to be switched off only where continuous operation would compromise the safety or security of the vessel - the time and reason must be logged. Discretionary silent mode for privacy alone is not a sanctioned exception on SOLAS-coded yachts.

Key facts

• AIS broadcasts MMSI, name, position, course, speed, heading and dimensions over marine VHF every 2-10 seconds.
• SOLAS V/19 mandates Class A on vessels of 300 GT and above on international voyages, cargo ships of 500 GT and above on domestic voyages, and all passenger ships.
• Class A transmits at 12.5 W with SOTDMA; Class B transmits at 2-5 W with CSTDMA or SOTDMA on Class B SO.
• The MMSI is a unique 9-digit identifier issued by the flag state.
• Ship-to-ship range is typically 20-40nm depending on antenna height; satellite AIS extends coverage globally.
• AIS targets are plotted on chartplotter/ECDIS with live CPA and TCPA collision-avoidance calculations.
• Silent mode is permitted only where continuous operation would compromise safety or security; the event must be logged and reported.
• Principal manufacturers: Furuno, Raymarine, em-trak, McMurdo.