Automatic Identification System

The Automatic Identification System is a transponder fitted to ships that continuously and automatically exchanges standardised messages over dedicated VHF radio channels. Each transmission carries the ship's identity, including its name and MMSI number, together with dynamic data from its sensors (position, course over ground, speed and rate of turn) and voyage information such as destination and draught. Other ships and shore stations within VHF range receive these messages, so each vessel builds up a live picture of the traffic around it without anyone having to make a radio call.

AIS was introduced primarily as an aid to collision avoidance and is required by SOLAS on most commercial ships. On the bridge, AIS targets are displayed on the ECDIS or a dedicated screen, often overlaid with radar, letting a watchkeeper identify a nearby ship by name, see its intentions and call it directly if needed. Ashore, Vessel Traffic Services use AIS to monitor and manage traffic in busy waters, ports and approaches, and coastal authorities use it for safety and for search-and-rescue coordination. The same broadcasts, picked up by terrestrial receivers and satellites, feed the ship-tracking websites and fleet-monitoring services now widely used across the industry.

AIS has limitations that mariners must respect. It depends on the accuracy of the data entered and on other ships transmitting correctly, so it supplements but never replaces radar, visual lookout and the COLREGs; a target seen on AIS but not confirmed by other means must be treated with care, and AIS can be switched off or its data falsified. Different classes of equipment exist, with Class A carried by SOLAS ships and simpler Class B units used by smaller craft. Used sensibly alongside other tools, AIS has greatly improved situational awareness at sea and transformed the ability to track shipping worldwide.