# Automatic Identification System The Automatic Identification System (AIS) is a standardized and unencrypted self-reporting maritime surveillance system. ### How does this work? 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.

Image from Marinelink

### Types of AIS messages 1. **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). 2. **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. ### Limitations of AIS signals 1. Signals from vessels are lost. 2. 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: ## References 1. Brousseau, M. (2022). *A comprehensive analysis and novel methods for on-purpose AIS switch-off detection* \[Master’s thesis, Dalhousie University]. DalSpace. 2. Kazim, T. (2016, November 14). *A definitive guide to AIS*. MarineLink. Retrieved May 14, 2025, from --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://aisviz.gitbook.io/documentation/tutorials/automatic-identification-system.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.