What is A.I.S. ?
Automatic Identification System is a way of identifying a vessel's name, course, speed, classification, call sign, registration number, and other information through VHF digital information. This information is displayed on your A.I.S. capable Chartplotter.
Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available.
With this information, you could call any ship over VHF radiotelephone by name, rather than by "ship off my port bow" or some other imprecise means. Or you could dial it up directly using GMDSS equipment. Or you could send to the ship, or receive from it, short safety-related email messages.
A.I.S. is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds.
How Does It Work?
All A.I.S. information is transmitted by vessels and A.I.S. navigational aids equipped with VHF transponders - a unit able to receive and transmit A.I.S. data. Transmitting vessels tend to be commercial ships legally required to transmit A.I.S. data.
Your A.I.S. receiver listens to those transmissions. The receiver sends appropriate data to your A.I.S. - Chartplotter and is presented on your chart.
Why do I need to receive A.I.S. Information?
Due to VHF's longer wavelength, A.I.S. has particular use in identifying targets that may be obscured by poor visibility, or behind larger targets like low islands and large ships that radar cannot penetrate.