There are two classes of AIS transponders. The Class A unit meets all IMO requirements, while the Class B, intended for smaller vessels or those not requiring the more capable Class A device , lacks some of the IMO required features, but still provides vital data.

The Class A AIS broadcasts the nofollowing data every 2-10 seconds while underway, and every three minutes at anchor, at a power of 12.5 watts:

• MMSI number, a unique identification number.
• Navigation status: underway, anchored, not under command, etc.
• Rate of turn, right or left, to 720 degrees per minute.
• Speed over ground.
• Course over ground.
• Position accuracy; GPS, DGPS and whether RAIM is in operation.
• Lat. and long. to 1/10,000 minute.
• True heading, derived from gyro if installed.
• Time of report.

In addition, the Class A AIS will transmit every six minutes:

• MMSI number as above, links data above to vessel.
• IMO number, a unique identifier related to ship's construction.
• International call sign.
• Name of ship, to 20 characters.
• Type of ship and cargo, from list of types.
• Dimensions of ship, to nearest meter.
• Location on ship of reference point for position reports.
• Source of fix information: GPS, Loran, DR, undefined, etc.
• Draft of ship, to 0.1 meter; air draft is not defined.
• Destination, to 20 characters.
• ETA: month, day, hour, and minute in UTC.

Class B AIS capabilities are not yet specifically defined, but in general the Class B units will report less often, leave out certain information such as IMO number, destination, rate of turn, draft, and status, and are not required to transmit textual safety messages.

AIS has the potential of eventually replacing racons, since shore stations can transmit data on aids to navigation for display through the AIS system. This would enable aids to navigation to appear with appropriate text data on the display, instead of as simple unidentified blips.

IMO requirements specify various classes of ships that must commence use of AIS by certain dates under a phased schedule. In general, by 2007 all vessels operating under SOLAS V must have AIS equipment. Additionally, in the U.S., all vessels subject to the Bridge-to-bridge Radiotelephone Act may be required to carry AIS equipment. The U.S. Coast Guard will define the requirements for certification of U.S. vessels.

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