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 exerp is from Automatic
Information System in the San 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 any reason the VTC site fails to work properly,
all the AIS vessel transponders automatically shift to work in ship-to-ship
mode."
