top of page


In 2000, IMO adopted a new requirement (as part of a revised new chapter V of Regulation 19 of SOLAS) for all ships, according to ship type, to carry automatic identification systems (AISs), through a phased implementation period spanning from 1 July 2002 to 1 July 2008, capable of providing information about the ship to other ships and to coastal authorities automatically.
The regulation requires that AIS shall:
• provide information - including the ship's identity, type, position, course, speed, navigational status and other safety-related information - automatically to appropriately equipped shore stations, other ships and aircraft;
• receive automatically such information from similarly fitted ships; · monitor and track ships;
• exchange data with shore-based facilities.

AIS is intended to enhance:
- safety of life at sea
- the safety and efficiency of navigation, and
- the protection of the marine environment.
Therefore, the purpose of AIS is to help identify ships, assist in target tracking, assist in search and rescue operation, simplify information exchange (e.g. reduce verbal mandatory ship reporting) and provide additional information to assist situation awareness. In general, data received via AIS will improve the quality of the information available to the OOW, whether at a shore surveillance station or on board a ship.



Very simply, the Automatic Identification System is a broadcast transponder system, operating in the VHF maritime mobile band.
It is capable of sending information such as identification, position, course, speed and more, to other ships and to shore. AIS operates principally on two dedicated VHF frequencies or channels:
AIS 1 - 161.975 MHz - channel 87B (Simplex, for ship to ship) and
AIS 2 - 162.025 MHz - channel 88B (Duplex for ship to shore).

AIS use Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation. It normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas, coastal or inland areas.
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.
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. 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.

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a
standard marine electronic communications link to shipboard display and sensor systems.

Position and timing information is normally derived from a built-in or external global GPS receiver,
including a differential GPS 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, 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 transmit following categories of information:
Static information
Dynamic information
Voyage related information
Short safety-related messages

Static information: (Every 6 min and on request)
 IMO number (where available)
 Call sign & name
 Length and beam
 Type of ship and
 Location of the position-fixing antenna
Dynamic information: (Dependent on speed and course alteration)
 Ship’s position with accuracy indication and integrity status
 Position time stamp (in UTC)
 Course over ground (COG)
 Speed over ground (SOG)
 Heading
 Navigational status (e.g. at anchor, underway, aground etc. And
 Rate of turn (where available).
Voyage related information (Every 6 min, when is data amended, or on request)
 Ship’s draught
 Hazardous cargo (type)
 Destination and ETA and
 Route plan (waypoints)
Short safety-related messages: Free format text message (sent as needed) addressed to one or more
specified destinations or to all stations in the area. The content should be relevant to safety messages
e.g. buoy missing, ice-berg sighted etc



There are 3 modes of operation of AIS:
Autonomous and continuous mode: In this mode AIS automatically defines its own reporting rate in
accordance with its navigational mode, speed and course. The unit also selects its own time slots. This
is normal mode for operation in all areas but the mode may be switched from/to Assigned or
Polled/Controlled mode by a competent authority via a base station on shore.
Assigned/ Controlled Mode: A competent authority responsible for traffic monitoring may remotely set
transmissions intervals and/or time slots for the vessel mobile stations. In Assigned mode the mobile
station does not change its reporting rate when changing course and speed.
Polled Mode: In this mode, mobile station will automatically respond to interrogation (polling)
messages from a ship or competent authority. The response is transmitted on the channel where the
interrogation message was received. Operation in Polled mode does not conflict with the operation in
other two modes.

AIS should always be on when underway or at anchor unless the Master feels the operation might
compromise with security, such as piracy areas. Entry in log book should be made stating time and
reasons. It must be switch on as soon as possible.

VTS centers may send information about vessels which are not carrying AIS and which are tracked only
by VTS radar, via the AIS to vessels equipped with AIS. These are Pseudo AIS. Any pseudo AIS target
broadcast by VTS should be clearly identified and particular care should always be taken when using
information which has been relayed by a third party. Accuracy of these targets may not be as accurate
as actual directly-received targets and the information content may not be as complete.
A Virtual AIS aid to navigation (Virtual AtoN) does not physically exist but is a digital information object
promulgated by an authorized service provider that can be presented on navigational Systems service
provider that can be presented on navigational systems.

AIS as an aid to navigation (AtoN) provide an opportunity to transmit position and status of buoys and
lights through the same VDL, which can then show up on an electronic chart, computer display or
compatible radar. This message (message 21 of the AIS reports) is used by ships as an aid to navigation
(AtoN) AIS station. It is generally transmitted autonomously at a rate of once every three minutes and
should not occupy more than two slots.

In coastal waters, shore side authorities may establish automated AIS stations to monitor the
movement of vessels through the area. These stations may simply monitor AIS transmissions from
passing ships, or may actively poll vessels via the AIS channels, requesting data such as identification,
destination, ETA, type of cargo and other information. Coast stations can also use the AIS channels for
shore-to-ship transmissions, to send information on tides, notices to mariners and local weather
Coastal nations may use AIS to monitor the movement of hazardous cargoes and control commercial
fishing operations in their territorial waters.
Multiple AIS coast stations and repeaters may be tied together into Wide Area Networks (WAN) for
extended coverage.
AIS data can be logged automatically for playback in investigating an accident, oil spill or other event.
AIS will also be a useful tool in search and rescue (SAR) operations, allowing SAR coordinators to
monitor the movements of all surface ships, aircraft and helicopters involved in the rescue effort.

AIS have potential to significantly contribute to safety of navigation. It provides positive identification of
targets fitted with AIS along with their static and dynamic information.
This enhances the navigational effectiveness and it can greatly improve situational awareness and
decision making abilities.
AIS also assist OOW in tracking and monitoring targets, as it also provides information on CPA and TCPA.
When using the AIS in the ship-to-ship mode for anti-collision purposes, the following should be borne
in mind:
1) Do not rely on AIS as the sole information system, making use of all safety-relevant information
2) AIS is an additional source for navigational information. AIS does not replace, but supports,
navigational systems such as radar target-tracking and VTS.
3) The use of AIS on board ship is not intended to have any special impact on the composition of the
navigational watch, which should continue to be determined in accordance with the STCW Convention.
The use of AIS does not negate the responsibility of the OOW to comply, at all times, with the Collision

Search and Rescue Transmitters using AIS can be used to assist in determining the location of a vessel in

Search and Rescue Aircraft may use AIS to assist in their operations.

1) The accuracy of AIS information received is only as good as the accuracy of the AIS information
2) Position of ships received on AIS display may not be referenced to WGS 84 datum.
3) Over reliance on AIS information for navigational safety should be avoided until steps can be taken to
ensure that all transmitting ships provide the necessary degree of data accuracy and integrity for all
connected sensors.
4) The users must be aware that transmission of wrong information implies a risk to other ships as well
as their own. The users remain responsible for all information entered into the system and the
information added by the sensors.
5) Certain elements of the dynamic information transmitted by AIS may be provided from sensors which
lack comprehensive integrity monitoring and whose accuracy has not been tested.
6) Not all ships will be fitted with AIS, particularly small craft and fishing boats. Other floating objects
which may give a Radar echo will not be detected by AIS.
Mariners should be aware of this Imitation when viewing information on ship borne navigational
display. Thus OOW should be on guard against making a picture of traffic scenario on the basis of AIS
7) The OOW should always be aware that other ships, fitted with AIS as a mandatory carriage
requirement, might switch off AIS under certain circumstances by professional judgment of the master.
8) It would not be prudent for the OOW to assume that the information received from other ships is of a
comparable quality and accuracy as that which might be available on own ship.

The carriage of AIS onboard ships is governed by SOLAS regulation V/19.2.4. The carriage of AIS is
mandatory for all ships of 300 gross tons and greater engaged on international voyages, cargo ships of
500 gross tons and greater not engaged on international voyages and all tankers and passengers ships.

Class A mandated by the IMO for vessels of 300 gross tonnage and upwards engaged on international
voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and
passenger ships irrespective of size.
Class B provides limited functionality and is intended for non-SOLAS vessels. It is not mandated by the
International Maritime Organization (IMO) and has been developed for vessels such as work craft and
pleasure craft.

The Class B is nearly identical to the Class A, except the Class B:
 Has a reporting rate less than a Class A (e.g. every 30 sec. when under 14 knots, as opposed to
every 10 sec. for Class A).
 Does not transmit the vessel's IMO number.
 Does not transmit ETA or destination.
 Does not transmit navigational status.
 Does not transmit rate of turn information.
 Is only required to receive, not transmit, text safety messages.

 Does not transmit maximum present static draught.


Base Stations are provided by an “aids to navigation authority” to enable the ship to shore / shore to
ship transmission of information. Networked AIS Base Stations can assist in providing overall maritime
domain awareness.
A virtual aid to navigation (Virtual AtoN) does not physically exist but is a digital information object
promulgated by an authorized service provider that can be presented on navigational Systems service
provider that can be presented on navigational systems.


capture 4.PNG
  • Facebook
  • Instagram
bottom of page