AIS - Automatic Identification System
Last updated
Last updated
The Automatic Identification System (AIS) is a standardized and unencrypted self-reporting maritime surveillance system.
The protocol operates by transmitting one or more of 27 message types from an AIS transponder onboard a vessel at fixed time intervals. These intervals depend on the vessel’s status—stationary vessels (anchored or moored) transmit every 3 minutes, while fast-moving vessels transmit every 2 seconds.
These VHF radio messages are sent from the vessel’s transponder and received by either satellite or ground-based stations, enabling more detailed monitoring and analysis of maritime traffic.
Dynamic messages convey the vessel's real-time status, which can vary between transmissions. These include data such as Speed Over Ground (SOG), Course Over Ground (COG), Rate of Turn (ROT), and the vessel’s current position (latitude and longitude).
Static messages, on the other hand, provide information that remains constant over time. This includes details like the Maritime Mobile Service Identity (MMSI), International Maritime Organization (IMO) number, vessel name, call sign, type, dimensions, and intended destination.
Signals from vessels are lost.
Terrestrial base stations are limited by their physical range, while satellite AIS receivers are limited based on their position globally.
To learn more about AIS, refer to:
Brousseau, M. (2022). A comprehensive analysis and novel methods for on-purpose AIS switch-off detection [Master’s thesis, Dalhousie University]. DalSpace.
Kazim, T. (2016, November 14). A definitive guide to AIS. MarineLink. Retrieved May 14, 2025, from