There are two types of shipboard AIS available: Class A AIS using SOTDMA technology, for larger ships such as those subject to the Safety of Life at Sea Convention, and Class B AIS using CSTDMA technology, for smaller ships.  Each Class A AIS system consists of a 12.5W VHF transmitter, an integral global navigation satellite system (i.e. GPS) receiver, two VHF TDMA receivers, one VHF DSC receiver, and standard marine electronic data interface (IEC 61162/NMEA 0183) to shipboard display and sensor systems (AIS Schematic).   Position is normally derived from an external navigation receiver and timing information derived from the integral GPS receiver, although the integral receiver does provide position information as a backup.  Precise position information in coastal and inland waters is available too, from either a dGPS receiver or equivalent information broadcast by an AIS base station.   Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections.  Heading information and course and speed over ground would normally be provided by all AIS-equipped ships.  Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided.

Class B AIS generally consists of a 2W VHF transmitter, a GPS receiver and two VHF receivers, one of which is able to decode DSC transmissions as well as AIS.

The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas.  Transmissions use 9.6 kb GMSK FM modulation using HDLC packet protocols. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.  Class A AIS can tune over the whole 156.025 -162.025 MHz VHF maritime band, while Class B AIS is limited to 161.5 - 162.025 MHz. 

Each Class A AIS station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations.   Class B AIS is a "polite", listen-before-transmitting system that will transmit on the first available slot.  A position report from one AIS station fits into one of 2250 time slots established every 60 seconds.  AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions.   Slot selection by a Class A AIS station is randomized within a defined interval, and tagged with a random timeout of between 0 and 8 frames.  When a station changes its slot assignment, it pre-announces both the new location and the timeout for that location.  In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels. 

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.

The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles. With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.

Other types of AIS include base stations, aids to navigation stations, search and rescue transmitters (AIS-SART), and stations used on search and rescue aircraft.

What AIS Broadcasts >>