AUTOMATIC IDENTIFICATION SYSTEM
Automatic Identification System (AIS) is a system initially intended to help ships avoid collisions, as well as assisting port authorities to better control sea traffic. 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.
AIS transponders on board vessels include a GPS (Global Positioning System) receiver, which collects position and movement details. It includes also a VHF transmitter, which transmits periodically this information on two VHF channels (frequencies 161.975 MHz and 162.025 MHz – old VHF channels 87 & 88) and make this data available to the public domain. Other vessels or base stations are able to receive this information, process it using special software and display vessels locations on a chart plotter or on a computer.
Read more on Wikipedia : http://en.wikipedia.org/wiki/Automatic_Identification_System