Automatic Identification System

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A United States Coast Guard Operations Specialist using AIS and RADAR to manage vessel traffic.
A United States Coast Guard Operations Specialist using AIS and RADAR to manage vessel traffic.

Automatic Identification System (AIS) is a system used by ships and vessel traffic systems (VTS) principally for identification of vessels at sea. AIS helps to resolve the difficulty of identifying ships when not in sight (e.g. at night, in fog, in radar blind arcs or shadows or at distance) by providing a means for ships to exchange ID, position, course, speed and other ship data with all other nearby ships and VTS stations. It works by integrating a standardized VHF transceiver system with a GPS receiver and other navigational equipment on board ship (Gyro compass, Rate of turn indicator, etc.).

The International Maritime Organization (IMO) SOLAS requires AIS to be fitted aboard all ships greater than/equal to 300 gross tonnage for international voyages. It is estimated that more than 40,000 ships currently carry AIS class A equipment.

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[edit] Applications and limitations

AIS is used in navigation primarily for collision avoidance. Because of the limitations of radio characteristics, however—and because not all vessels are equipped with AIS—the system is meant to be used primarily as a means of lookout and to determine risk of collision in accordance with ColRegs rather than as an automated collision avoidance system.

Vessel AIS display listing nearby vessels' range, bearing, and name
Vessel AIS display listing nearby vessels' range, bearing, and name

When a ship is navigating at sea, the movement and identity of other ships in the vicinity are very important to navigators to make decisions to avoid collision with other ships and dangers (shoal or rocks). Visual (eye, binoculars) / audio means (whistle, VHF radio), radar and ARPA were primarily used for this purpose. However, a lack of positive identification of the targets on the displays, and time delays and other limitation of radar for observing and calculating the action and response of ships around, especially on busy waters, sometimes prevent possible action in time to avoid collision.

While AIS provides only a very basic graphical display, the data obtained can be integrated with ECDIS or a radar, providing most navigational information on a single display.

However for navigators to know the full traffic situation in the vicinity of their ships is very difficult in busy waters, although VTS may provide traffic management services if available in some harbor or river areas. AIS is a very useful aid for navigators to know a much better traffic situation of what kind of other ships and their movement with identities of targets around much earlier and accurate than just by other methods.

There are also many new ideas for the application of AIS, e.g. virtual AIS for aids to navigation and supplement to the on scene coordination for search and rescue (see also GMDSS).

Use of virtual AIS for marking important navigational information provides significant improvement to indicate the position of new wrecks, other new dangers or even navigational warning in relation to the ship's position. Virtual marking of dangers can be transmitted by virtual AIS stations (e.g. VTS) and ships can know the position of dangers on the AIS display much quickly, accurate and easier than physical navigational marks and traditional safety message boardcast by VHF Radio, Navtex, SafetyNet or Notice to Mariners. Of course such virtual aids would only be visible to AIS equipped ships but it is most important for large ocean going ships.

For coordinating resources on scene of marine search & rescue operation, it is important to know the position and navigation status of ships in the vicinity of the ship or person in distress. With the aid of AIS, which could provide much better picture of the resources for on scene operation even the AIS range is limited to VHF radio range, usually about 30 to 50 nautical miles (60 to 90 km).

Recent trends also includes Airborne AIS for SAR (Search and Rescue) activities and surveillance. An airborne AIS in addition also transmit altitude (based on GPS) and has a fixed position rate of every 10th second. In high altitude surveillance platforms it is possible to get a stable range better than 150 nautical miles.

[edit] Binary Messages

The Saint Lawrence Seaway uses AIS Binary Messages (message type 8) to provide information about water levels, lock orders, and weather in its navigable system.[1]

[edit] Concern over web-based data

The International Maritime Organization (IMO) Maritime Safety Committee condemned the use of freely available AIS data published irresponsibly with the following statement.[2]

In relation to the issue of freely available automatic information system (AIS)-generated ship data on the world-wide web, the publication on the world-wide web or elsewhere of AIS data transmitted by ships could be detrimental to the safety and security of ships and port facilities and was undermining the efforts of the Organization and its Member States to enhance the safety of navigation and security in the international maritime transport sector.

[edit] How AIS works

[edit] 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.

Currently (early 2007) there are two major classes of AIS units: Class A - mandated for use on SOLAS Chapter V vessels (and others in some countries). Class B - low power, lower cost derivative for leisure and non-SOLAS markets.

Other variants are under development specifically for Base Stations, Aids to Navigation and SART's, though they will all be derived from one of the existing standards and interoperate with them.

[edit] Detailed description: Class A units

Each AIS transponder consists of one VHF transmitter, two VHF TDMA receivers, one VHF Digital Selective Calling (DSC) receiver, and links to shipboard display and sensor systems via standard marine electronic communications (such as NMEA 0183, also known as IEC 61162). Position and timing information is typically derived from an integral or external global navigation satellite system (e.g. GPS) receiver, but may also use a LORAN receiver or an inertial navigation system. 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.

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 9600-bps Gaussian minimum shift keying (GMSK) modulation over 25 or 12.5 kHz channels using the High-level Data Link Control (HDLC) packet protocol. 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.

In order to ensure that the VHF transmissions of different transponders do not occur at the same time the signals are time multiplexed using a technology called Self-Organized Time Division Multiple Access (SOTDMA). The design of this technology is patented, and whether this patent has been waived for use by SOLAS vessels is a matter of debate between the manufacturers of AIS systems and the patent holder. In order to make the most efficient use of the bandwidth available, vessels which are anchored or are moving slowly transmit less frequently than those that are moving faster or are maneuvering. The update rate of fast maneuvering vessels is similar to that of a conventional marine radar. The time reference is derived from the navigation system.

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 on each frequency. 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 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, but slightly better due to digital VHF and not analog VHF. Its propagation is 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 (37 km). With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.

The system is backwards compatible with digital selective calling systems, allowing shore-based GMDSS systems to inexpensively establish AIS operating channels and identify and track AIS-equipped vessels, and is intended to fully replace existing DSC-based transponder systems.

Shore-based AIS network systems are now being built up around the world. One of the biggest fully-operational, real time systems with full routing capability is in China, operated by China MSA and delivered by Saab TransponderTech in Sweden. The entire coastline is covered with approximately 150 base stations and 50 computer servers. Hundreds of shore based users, including ca 25 VTS centers, are then connected to the network and been able to see the maritime picture, but also to communicate with the ship with SRM:s (Safety Related Messages). All data is in real time and will full safety and security of ships and port facilities.

[edit] Broadcast information

AIS transceiver sends the following data every 2 to 10 seconds depending on vessels speed while underway, and every 3 minutes while vessel is at anchor. This data includes:

  • MMSI number of vessel - vessel's unique identification
  • Navigation status - "at anchor", "under way using engine(s)", "not under command", etc
  • Rate of turn - right or left, 0 to 720 degrees per minute
  • Speed over ground - 0.1 knot resolution from 0 to 102 knots
  • Position accuracy
  • Longitude - to 1/10000 minute and Latitude - to 1/10000 minute
  • Course over ground - relative to true north to 0.1 degree
  • True Heading - 0 to 359 degrees from eg. gyro compass
  • Time stamp - UTC time accurate to nearest second when this data was generated

In addition, the following data is broadcast every 6 minutes:

  • MMSI number - vessel's unique identification
  • IMO number - number remains unchanged upon transfer of the ship to other flag(s).
  • Radio call sign - international radio call sign assigned to vessel
  • Name - Name of vessel, max 20 characters
  • Type of ship/cargo
  • Dimensions of ship - to nearest meter
  • Location of positioning system's (eg. GPS) antenna onboard the vessel
  • Type of positioning system - usually GPS or DGPS
  • Draught of ship - 0.1 meter to 25.5 meters
  • Destination - max 20 characters
  • ETA (estimated time of arrival) at destination - UTC month/date hour:minute

[edit] See also

[edit] References and Footnotes

  1. ^ Great Lakes St. Lawrence Seaway System, AIS Project. Accessed December 15, 2006.
  2. ^ Maritime security - AIS ship data. 79th session: 1-10 December 2004. IMO Maritime Safety Committee. Retrieved on January 8, 2007.

[edit] External links

Graphical AIS Tracking

Note: These sites may use JavaScript or Flash to display AIS information

Other AIS related

Category:Navigational equipment

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