Automatic Identification System (AIS) is an automatic radio broadcast system which is used to exchange data ship-to-ship and with shore-based facilities. It was perceived by the International Maritime Organization (IMO) to enhance safety of life at sea, enhance the safety and efficiency of navigation and enhance the protection of the marine environment by helping in the identification of ships within a particular area, assisting in tracking targets, simplifying information exchange (through reducing the requirement to make verbal ship reports) and to provide additional information to assist situational awareness.
The installation of AIS became mandatory under Regulation 19.2.4 of the revised SOLAS Chapter V for 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 all passenger ships irrespective of size. The majority of affected ships were required to be compliant by 31 Dec 2004. However, ships not engaged on international voyages and constructed before 01 Jul 2002 (i.e. the date when the revision to SOLAS Chapter V entered into force) were given to 01 Jul 2008 to meet the carriage requirement.
AIS operates in the Very High Frequency (VHF) band. A “Class A” AIS shipborne installation (i.e. one that is mandated under the SOLAS Convention) consists of a VHF transmitter, two VHF receivers (AIS 1 and AIS 2) and a VHF Digital Selective Calling (DSC) receiver operating on VHF Channel 70. The system operates on the principle of Self-Organised Time Division Multiple Access (SOTDMA) for users to share the two internationally-assigned VHF frequencies – known as Channel 87B and Channel 88B. Each of the frequencies is divided up into 2250 time slots that are repeated every 60 seconds. Each ship sends a position message during one of the time slots and, at the same time, reserves another time slot for the next message. Other ships and shore stations equipped with AIS and within VHF range should be able to receive the transmitted messages.
The AIS transceiver needs to be interfaced to other on-board systems to receive positional information, heading, speed and rate of turn (if available). Although the AIS has its own built-in Global Satellite Navigation System (GNSS) receiver, this is mainly used to provide the precise timing needed to regulate the whole system. It can, nonetheless, also be used to provide positional information in the event of failure of the ship’s primary electronic position-fixing system.
Originally it was perceived that the AIS information received from another ship would be viewed on a dedicated screen (known as the Minimum Keyboard and Display) but nowadays it is common to also display the information on the radar and/or ECDIS.
An animation reconstructed from AIS data can be viewed by clicking here.
Scroll down the page to read a short summary of various documents relating to AIS. View and download the full document by clicking on the relevant link.
Amongst other things the Performance Standards specifies the categories of information provided by the AIS, namely:
Static Information (Reporting interval every 6 minutes)
- IMO Number (where available)
- Call sign and name
- Length and beam
- Type of ship
- Location of he position-fixing antenna on the ship
Dynamic Information (Reporting interval dependent upon speed and course alterations according to Table I)
- Ship’s position with accuracy indication and integrity status
- Time (UTC)
- Course over ground (COG)
- Speed over ground (SOG)
- Heading
- Navigational status
- Rate of turn (where available)
Voyage-Related Information (Reporting interval every 6 minutes or when data has been amended)
- Ship’s draught
- Hazardous cargo (type)
- Destination and ETA (at Master’s discretion)
Short Safety-Related Message (Reporting interval as required)
These messages can be in fixed or free text format and addressed either to a specific ship or shore-based facility or to all ships in the immediate area. The length of the message is limited to a maximum of 158 characters.
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A.917(22) (25 Jan 2002) – Guidelines on the Operational Use of AIS
The Guidelines were prepared by IMO, in advance of the entry into force of the revised SOLAS Chapter V on 01 Jul 2002, to promote the safe and effective use of AIS and to raise awareness, particularly to mariners, about the operational use, limits and potential uses of the system. It reminds mariners in charge of a navigational watch that not all ships will carry AIS (e.g. leisure craft, fishing boats, warships etc.) and that even where a ship is subject to a mandatory carriage requirement, there may be circumstances when the system has been switched off on the basis of the Master’s professional judgement.
SN/Circ. 227 (06 Jan 2003) – Guidelines on the Installation of Shipborne AIS
The underlying message in these Guidelines is that, as the information given by the AIS may be used for the navigation of the ship, it is essential that the information is reliable.
One issue which is frequently overlooked, in our opinion, relates to the reference point of position.
The Guidelines advise that the AIS stores two reference points – an “external” reference point for an external GNSS antenna position and an “internal” reference point for the built-in GNSS (which provides the precise timing sequences but may also be used as a fall-back for position reporting). The locations of these reference points have to be set up during the AIS installation using values A, B, C & D where A= distance from the bow, B = distance from the stern, C= distance from the port side and D = distance from the starboard side. As a check, A + B should correspond to the ship’s length and C + D to the ship’s beam.
The external reference point may also be assigned to a Consistent Common Reference Point (CCRP).
In many cases that we have come across, it is, however, apparent that the reference point of position has been incorrectly set up and that, for example, although the ship is using an external GNSS which is interfaced to the AIS, the positional information transmitted to other ships appears to originate from the internal reference point. This means that the validity of the transmitted positional information may have been significantly compromised due to the use of an incorrect reference point of position and is a commonly encountered problem when using AIS data in the reconstruction of certain types of incidents or accidents.
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SN/Circ.245 (15 Dec 2004) – Amendments to the Guidelines on the Installation of Shipborne AIS
The short amendment advises that the AIS should be connected to an uninterrupted power supply (UPS) to the ship’s main power supply.
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The purpose of the annual test is to determine that the AIS is operational in accordance with the performance standards set out in Annex 4 of MSC. 74(69). The test should be carried out by a qualified radio inspector authorised by the ship’s flag administration or a recognised organisation (RO) acting on the administration’s behalf.
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