The technology uses public broadcast data from the Automatic Identification System (AIS), which uses satellite and land-based receivers to track the movement of ships over time. Not all fishing vessels willingly broadcast their location, of course - particularly those intent on breaking the law - and vessels can switch off their trackers, potentially hindering the usefulness of the new technology. The United States and other countries already require vessels of a certain size to use the locator system, partly as a safety measure to avoid collisions at sea, and more countries are beginning to follow suit. Global Fishing Watch allows users to access that information to track specific vessels over time, going back to 2012.