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What is AIS:

Picture a shipboard radar or an electronic chart display that includes a symbol for every significant ship within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship "symbol" can reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By "clicking" on a ship symbol, you can learn the ship name, course and speed, classification, call sign, registration number, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, can also be available.

With this information, you can call any ship over VHF radiotelephone by name, rather than by "ship off my port bow" or some other imprecise means. Or you can dial it up directly using GMDSS equipment. Or you can send to the ship, or receive from it, short safety-related email messages.

The AIS is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band, that is capable of handling well over 4,500 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation.

What are the differences between a Class A and Class B transponder? 

There are several important functional differences between AIS Class A and Class B transponders.  
  - Class A meets the requirements of the IMO AIS carriage requirements and is standardized in SOLAS, while the Class B provides capabilities which are not fully compliant with IMO requirements. 
  - A main difference is the data communication technology; Class A units utilize Self-Organizing Time-Division Multiple Access (SOTDMA) technology.  This technology ensures that VHF transmissions of different transponders do not occur at the same time by synchronizing their data transmission to an exact timing standard, providing that no transmissions are missed.  Class B units utilize Carrier-Sense TDMA (CSTDMA), which listens to the AIS network to determine if a slot is being used. Once the unit determines that the slot is open, it powers up its transmitter and broadcasts a 1 slot position report.  The Class B is also required to listen for reservations from base stations and comply with these reservations. If the unit cannot find an open slot, it will not broadcast.  
  - The reporting intervals are lower with a Class B transponder than a Class A transponder. 
  - The transmitted AIS data from a Class B transponder is limited; a Class A transponder transmits and receives all required AIS data.

How AIS works:

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and standard marine electronic communications links (IEC 61162/NMEA 0183) to shipboard display and sensor systems (AIS Schematic). Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. 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.

AIS 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 over 25 or 12.5 kHz channels 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.

Each station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations. 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 an 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 benefits of AIS...

Operating in the VHF maritime band, the AIS (Automatic Identification System) system enables the wireless exchange of navigation status between vessels and shore-side traffic monitoring centers. Commercial ships, ocean-going vessels and recreational boats equipped with AIS transceivers broadcast AIS messages that include the vessel's name, course, speed and current navigation status.

  • Transmit your position. Fitting a Class A or Class B AIS transceiver ensures that you are seen by other AIS equipped vessels.
  • Vessel Protection. As part of a suitably configured network, AIS enables owners to be alerted to unauthorized vessel movements.
  • Port management. AIS can be used as a highly effective port management tool allowing easy identification, control and direction of vessels.
  • Coastal surveillance. AIS and radar can be fused to create effective and efficient coastal tracking, surveillance and safety systems.

The Class A receiver and transmitter is approved to deep sea and inland waterway standards and offers a highly intuitive user interface.

The class B AIS transceiver allow you to transmit your data to other AIS equipped vessels, and receive their data for viewing right on your multifunction display's radar or chartplotter screen

AIS Requirements:

All ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with AIS, as follows:

  • Ships constructed on or after 1 July 2002
  • Ships engaged on international voyages constructed before 1 July 2002
  • In the case of passenger ships, not later than 1 July 2003
  • In the case of tankers, not later than the first "safety equipment survey" after 1 July 2003
  • In the case of ships, other than passenger ships and tankers, of 50,000 gross tonnage and upwards, not later than 1 July 2004
  • In the case of ships, other than passenger ships and tankers, of 10,000 gross tonnage and upwards but less than 50,000 gross tonnage, not later than 1 July 2005
  • In the case of ships, other than passenger ships and tankers, of 3,000 gross tonnage and upwards but less than 10,000 gross tonnage, not later than 1 July 2006
  • In the case of ships, other than passenger ships and tankers, of 300 gross tonnage and upwards but less than 3,000 gross tonnage, not later than 1 July 2007

The United States Coast Guard also requires AIS on certain vessels not subject to SOLAS under new USCG rules which take effect March 2016.