AIS - Ship Movements

AIS - Automatic Identification System is an aid to navigation to help ships avoid collisions, as well as assisting the rescue services and port authorities to better control sea traffic.
All commercial vessels over 300 Gross Tonnes are fitted with AIS transponders which allow other vessels to see who is sailing near to them. Smaller vessels can also be fitted with AIS to help them see and be seen. The transponders send data about the vessel in digital format using VHF radio with a GPS (Global Positioning System) receiver, which collects position and movement details.
The map is using AIS data collected by Marine Traffic from "Sharers" who send the AIS data they receive to the Marine Traffic server to give an overall view of shipping around the globe.