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Automatic Identification System
Operator's Manual
AIS data transmissions utilize a robust 9.6 kbps FM IGMSK (Gaussian Minimum
Shift Keying) modulation technique, which is specified in ITU Recommendation
M.1371.1. The International Telecommunications Union (ITU) has designated
two dedicated frequencies for AIS. They are 161.975 MHz (marine band channel
87B) and 162.025 MHz (channel 88B). In some parts of the world, such as the
United States, where these frequencies may not be available for AIS, other chan-
nels may be designated.
The ship's AIS station has two independent VHF receivers, which are normally
tuned to the two AIS frequencies, as well as one transmitter, which alternates its
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transmissions back and forth between the two. The shipborne system can also
be retuned to other frequencies, for instance when operating under the control of
a shore-based VTS. This can be done either manually or remotely by the AIS
shore station;':'
AIS Message
AIS is designed to work autonomously and continuously in a ship-to-ship mode,
but the specifications provide for switchover to an "assigned mode" for opera-
tion in an
are~
subject to a competent authority responsible for traffic monitor-
ing, with the data transmission intervals and timeslots set remotely by the
shoreside authority. Alternatively, the AIS can work in a "polling mode" in which
the data transfer occurs in response to interrogation from another ship or shore
station.
Information provided by the AIS falls into several categories:
Static data
_IMO number (where available)
Call sign and name
Ship's name
Type of ship
Location of position-fixing antenna on the ship
(aft of bow and port or starboard of eenterIine)
Dynamic data
Ship's position with accuracy indication and integrity status
Time in UTC
Course over ground
Speed over ground
Heading
Navigational status (e.g., "at anchor," "not under command,"
manually entered)
Rate of turn (where available)
Voyage related data
Version 7.5
739
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