AIS, the Automatic Identification System, is a newer tool to help avoid collisions and improve overall situational awareness at sea. Basically, it works by telling you the identification, position and intentions of other nearby commercial vessels. Most commercial ships are now required to install AIS transponders, which use VHF frequencies to broadcast critical navigation data.
Using a special receiver, this information can be decoded onboard and displayed graphically on many plotters or PC-based navigation programs. Other ships appear as "targets" you can track, and most plotters or programs will automatically calculate the Closest Point of Approach (CPA) and Time until Closest Point of Approach (TCPA) for each target. Alarms can be set to minimize the chance of a "close call". For an example, click here.
Because AIS uses VHF, it will display targets within about a 20 mile range. And unlike radar, AIS will "see" targets that are behind an obstruction, such as an island or harbor jetty.
