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AIS - Automatic Identification System
The Automatic Identification System (AIS) is a broadcast system, operating in the VHF maritime mobile band. It is capable of sending ship information such as identification, position, course, speed and more, to other ships and to shore. It can handle multiple reports at rapid update rates and uses Self-Organising Time Division Multiple Access (SOTDMA) technology to meet these high broadcast rates and ensure reliable and robust ship-to-ship operation.

The AIS integrates standard VHF transceiver system, an electronic navigation system and additional sensors (like gyrocompass, rate turn sensor, etc.). Most of existing electronic navigation systems are supported: GPS, GLONASS, Loran-C, Galileo, etc.

The International Maritime Organization's (IMO) International Convention for the Safety of Life at Sea (SOLAS) requires AIS to be fitted all passenger ships and tankers, newly built ships (from 1 July 2002), aboard international voyaging ships with gross tonnage (GT) of 300 or more tons.

Basic overview
AIS transponders automatically broadcast information, such as their position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transponder. The information originates from the ship's navigational sensors, typically its global navigation satellite system (GNSS) receiver and gyrocompass. Other information, such as the vessel name and VHF call sign, is programmed when installing the equipment and is also transmitted regularly. The signals are received by AIS transponders fitted on other ships or on land based systems, such as VTS systems. The received information can be displayed on a screen or chart plotter, showing the other vessels' positions in much the same manner as a radar display. The AIS system defines following types of units:

  • Shipborne Station Class A Mandated for use on SOLAS Chapter V vessels (and others in some countries).
  • Shipborne Station Class B A lower cost derivative for leisure and non-SOLAS markets.
  • Aids-To-Navigation (AtoN) Station Can complement or replace an existing aid to navigation to provide identification, position, name and other
  • arameters in AIS network. Also can be virtual, which messages are broadcasted by Base Station under authorities control.
  • Search-And-Rescue (SAR) Station Installed on aircraft and helicopters involved in search-and-rescue operation.
  • Base Station Fitted on the shore to provide VTS Control Centres with AIS information and to control AIS network by authorities.
  • Repeater Station Used to extend coverage range of AIS Stations.

Broadcast Information
AIS Stations automatically broadcasts information on two parallel dedicated VHF maritime channels. The broadcasted information consists of static and dynamic information. Static information includes identity, name, call sign, dimensions, ETA, destination, and other parameters programmed and stored in the AIS transponder. Dynamic information covers position, course, speed, turn rate and other data, originating from sensors connected to the AIS transponder.

The received AIS information can be shown on a VTS Operator’s screen or an ECDIS display.

Position Reports
AIS Station sends position reports every 2 seconds to 3 minutes, depending on type of AIS transponder, speed and turn rate. The position report includes following data:

  • MMSI – Maritime Mobile Service Identity An unique nine digit identification number.
  • Position Latitude and Longitude got from position fixing device.
  • Position Accuracy Low (>10 m) or High (10 m).
  • COG – Course Over Ground Course over ground in 1/10 degrees relative to true north.
  • SOG – Speed Over Ground Speed over ground in 1/10 knot steps.
  • Navigation Status On the way, moored, at anchor, restricted manoeuvrability, engaged in fishing, etc.
  • Rate of Turn Measured in degrees per minute.
  • True Heading 0..359 degrees value obtained from e.g. gyrocompass .
  • Time Stamp UTC second when the report was generated by the electronic position system.

Static Information Reports
AIS Station Class A and B sends following static information every 6 minutes

  • IMO Number A seven digit number that remains unchanged upon transfer of the ship's registration to another country.
  • Call Sign International radio call sign, up to seven characters, assigned to the vessel by its country of registry.
  • Name 20 characters to represent the name of the vessel.
  • Type of ship and cargo Value that define type of vessel and cargo.
  • Dimensions of ship Defines dimensions of the vessel rounded to 1 meter and location of positioning system's (e.g. GPS) antenna onboard the
  • essel.
  • Type of Positioning System GPS, GLONASS, Loran-C, Chayka, Galileo, etc.
  • Maximum Present Static Draught 0.1 meter to 25.5 meters.
  • Destination Max 20 characters
  • ETA – Estimated Time of Arrival UTC month/date hour:minute of expected time of arrival to destination.

Other messages
In addition to position and static information reports AIS Stations can send following messages:

  • SRM – Safety Related Messages Text messages containing concerning safety information.
  • Binary Messages Contains binary data carrying any information.
  • Interrogation Messages Requests some information from remote vessel.
  • Control Messages Provide management of AIS network.
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