From Sanctions Wiki
The system is based on AIS (Automatic Identification System). As from December 2004, the International Maritime Organization (IMO) requires all vessels over 299GT to carry an AIS transponder on board, which transmits their position, speed and course, among some other static information, such as vessel’s name, dimensions and voyage details. For more information and looking for near real time location of vessels see Marine Traffic Database.
Retrieved from “http://sanctionswiki.org/Marine_Traffic”