What is (AIS)?
Automatic Identification System (AIS)

Transponders aboard vessels coupled with computer software allow users to graphically view vessels and their associated information within Bay waters. Future enhancements and development point to the system becoming a fixture locally and globally.

(The following exert is from Automatic Information System in theSan Francisco Bay Region, prepared by the Marine Exchange)

AIS can be characterized as a great mirror in the sky. If the sky was a mirror, and it was a very clear day, one could look up and see a reflection of one�s surroundings. You could see all other vessels as they move in relation to your own vessel providing a single situational picture. Also, all the other users of the system could also look up and see that very same situational picture. AIS is this and much more. It not only provides a display to all the system users of the physical vessel traffic situation; it also provides key navigational and operational information as per the requirements of the users. For example all the users can display static information of each display target such as the name, type and size of vessel data that is not usually available through standard radar. They can also obtain real time information such as the speed over ground (SOG), course over ground (COG) and the destination of other vessels. In short, AIS is a system that provides each user a picture of the navigational information that all other participants are using.

How does the basic AIS perform this task? It starts with the Global Positioning System (GPS). The GPS is a network of many satellites that are in orbit around the earth and they transmit a signal similar to a pulse or a clock. The shipboard GPS transponder/radio receives that signal and calculates the time differences between the various satellites pulses, at least six for accuracy within 10 meters, to determine an exact location of the vessel in longitude and latitude. That data is then directly input into the ship�s AIS computer. The AIS computer combines the GPS location information with other vessel related data such as name, course and speed and transmits it, via VHF radio, to the AIS Vessel Traffic Controller (VTC). The VTC is a communications hub, normally located on an appropriate mountain peak (Mt. Tamalpais in the San Francisco Bay Area) that receives, organizes and combines all the transmissions of all the AIS vessels and then rebroadcasts the data back out so that all the vessels share the same data. When the AIS vessel receives the data from the VTC, it then displays it on the computer monitor with an Electronic Navigation Chart (ENC). If for anyreason the VTC site fails to work properly, all the AIS vessel transponders automatically shift to work in ship-to-ship mode."