Automatic identification system

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

An AIS radar like view to present the bearing and distance of other ships in vicinity from own ship

A graphical display of ship's AIS in Dover Straits

The Automatic Identification System (AIS) is a short range coastal tracking system used on ships and by Vessel Traffic Services (VTS) for identifying and locating vessels by electronically exchanging data with other nearby ships and VTS stations. Information such as unique identification, position, course, and speed can be displayed on a screen or an ECDIS. AIS is intended to assist the vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements, and integrates a standardized VHF transceiver system such as a LORAN-C or Global Positioning System receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator.

The International Maritime Organization's (IMO) International Convention for the Safety of Life at Sea (SOLAS) requires AIS to be fitted aboard international voyaging ships with 300 or more GT, and all passenger ships regardless of size. It is estimated that more than 40,000 ships currently carry AIS class A equipment.

Ships outside AIS radio range can be tracked with the Long Range Identification and Tracking (LRIT) system with less frequent transmission.

Applications and limitations

Collision avoidance

AIS is used in navigation primarily for collision avoidance. Due to the limitations of VHF radio communications, 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 rather than as an automated collision avoidance system, in accordance with the International Regulations for Preventing Collisions at Sea (COLREGS).

A vessel's text-only AIS display, listing nearby vessels' range, bearings, and names

When a ship is navigating at sea, the movement and identity of other ships in the vicinity is critical for navigators to make decisions to avoid collision with other ships and dangers (shoal or rocks). Visual observation (unaided, binoculars, night vision), audio exchanges (whistle, horns, VHF radio), and radar or Automatic Radar Plotting Aid (ARPA) are historically 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 requirements of AIS are only to display a very basic text information, the data obtained can be integrated with a graphical electronic chart or a radar display, providing consolidated navigational information on a single display.

Vessel traffic services

In busy waters and harbors, a local Vessel Traffic Service (VTS) may exist to manage ship traffic. Here, AIS provides additional traffic awareness and provides the service with information on the kind of other ships and their movement.

Aids to navigation

AIS was developed with the ability to broadcast positions and names of objects other than vessels, like navigational aid and marker positions. These aids can be located on shore, such as in a lighthouse, or on the water, on platforms or buoys. The US Coast Guard suggests that AIS might replace RACON, or radar beacons, currently used for electronic navigation aids.

The ability to broadcast navigational aid positions has also created the concepts of Synthetic AIS and Virtual AIS. In the first case, an AIS transmission describes the position of physical marker but the signal itself originates from a transmitter located elsewhere. For example, an on-shore base station might broadcast the position of ten floating channel markers, each of which is too small to contain a transmitter itself. In the second case, it can mean AIS transmissions that indicate a marker which does not exist physically, or a concern which is not visible (i.e. submerged rocks, or a wrecked ship). Although such virtual aids would only be visible to AIS equipped ships, the low cost of maintaining them could lead to their usage when physical markers are unavailable.

Search and rescue

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. Here AIS can provide additional information and awareness of the resources for on scene operation, even though AIS range is limited to VHF radio range. The AIS standard also envisioned the possible use on SAR Aircraft, and included a message (AIS Message 9) for aircraft to report position. To aid SAR vessels and aircraft in locating people in distress a standard for an AIS-SART AIS Search and Rescue Transmitter is currently being developed by the International Electrotechnical Commission (IEC), the standard is scheduled to be finished by the end of 2008 and AIS-SARTs will be available on the market from 2009.

Accident Investigation

AIS information received by VTS is important for accident investigation to provide the accurate time, identity, position by GPS, compass heading, course over ground (COG), Speed (by log/SOG) and rate of turn (ROT) of the ships involved for accident analysis, rather than limited information (position, COG, SOG) of radar echo by radar.

The maneuvering information of the events of the accident is important to understand the actual movement of the ship before accident, particularly for collision, grounding accidents.

A more complete picture of the events could be obtained by Voyage Data Recorder (VDR) data if available and maintained onboard for details of the movement of the ship, voice communication and radar pictures during the accidents. However, VDR data are not maintained due to the limited 12 hours storage by IMO requirement.

Other reference:

• Automatic Identification System (AIS): A Human Factors Approach

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.

Computing & networking

Several computer programs have been created for use with AIS data. Some programs (such as ShipPlotter and gnuais) use a computer to demodulate the raw audio from a modified marine VHF radio telephone when tuned to the AIS broadcast frequency (Channel 87 & 88) into AIS data. Some programs can re-transmit the AIS information to a local or global network allowing the public or authorized users to observe vessel traffic from the web. Some programs display AIS data received from a dedicated AIS receiver onto a computer or chartplotter. Most of these programs are not AIS transmitters, thus they will not broadcast your vessel's position but may be used as an inexpensive alternative for smaller vessels to help aid navigation and avoid collision with larger vessels that are required to broadcast their position. Ship enthusiasts also use receivers to track and find vessels to add to their photo collections.

AIS data on the Internet

AIS position data are available on the Internet through many privately operated geographic information systems. In December 2004, the International Maritime Organization's (IMO) Maritime Safety Committee condemned the Internet publication of AIS data as follows:

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.

Others have countered that AIS provides the same information that can be obtained with a pair of binoculars and that ships have the option of turning off AIS when they are in areas with security concerns.

Range limitations and space-based tracking

Shipboard AIS transponders have a horizontal range that is highly variable but typically only about Template:Km to mi. They reach much further vertically, up to the 400 km orbit of the International Space Station (ISS).

In November 2009, the STS-129 space shuttle mission attached the Norwegian-built "NORAIS" VHF antenna to the Columbus module of the ISS. NORAIS is intended to serve as a test and technology demonstration for space-based ship monitoring, as a first step towards a satellite-based AIS-monitoring service. In 2007, a previous test of space-based AIS tracking by the U.S. TacSat-2 satellite suffered from signal corruption because the many AIS signals interfered with each other.

How AIS works

System Overview from US Coast Guard

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 standard describes two major classes of AIS units:

• Class A - mandated for use on SOLAS Chapter V vessels (and others in some countries).
• Class B - a low power, lower cost derivative for leisure and non-SOLAS markets.

Other variants are under development specifically for base stations, aids to navigation and search and rescue, though they will all be derived from one of the existing standards and inter-operate with them.

Message Types

There are 26 different types of messages capable of being sent by an AIS transponder.

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). Timing is vital to the proper synchronization and slot mapping for a Class A unit. Therefore, every unit is required to have an internal global navigation satellite system (e.g. GPS) receiver. This internal receiver may also be used for position information. However, position is typically provided by an external receiver such as GPS, LORAN or an inertial navigation system and the internal receiver is only used as a backup for position information. 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 are found on two frequencies, VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz) and use 9600 bit/s 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 (STDMA). 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. This system was built between the years 2003–2007 and delivered by Saab TransponderTech. The entire coastline is covered with approximately 250 base stations in hot-standby configurations including 70 computer servers in three main regions. Hundreds of shore based users, including ca 25 VTS centers, are connected to the network and are able to see the maritime picture, but also to communicate with the ship using SRM's (Safety Related Messages). All data are in real time and will improve safety and security of ships and port facilities. It is also designed according to a SOA architecture with socket based connection and using IEC AIS standardized protocol all the way to the VTS users. The base stations have hot-standby units (IEC 62320-1) and the network is the third generation network solution.

By beginning of 2007 a new world wide standard for AIS Base Stations was approved - the IEC 62320-1 standard. The old IALA recommendation and the new IEC 62320-1 standard are in some functions incompatible and therefore attached network solutions have to be upgraded. This will not impact users, but system builders have to upgrade software to accommodate this. A standard for AIS base stations has been long awaited. Currently many ad-hoc networks exist with class A mobiles. Base stations can control the AIS message traffic in a region, which will hopefully reduce the number of packet collisions.

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. These data include:

• The vessel's Maritime Mobile Service Identity (MMSI) - a unique nine digit identification number.
• 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 (0.19 km/h) resolution from 0 to 102 knots (189 km/h)
• Position accuracy:
  • Longitude - to 1/10000 minute
  • 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 these data were generated

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

• IMO ship identification number - a seven digit number that remains unchanged upon transfer of the ship's registration to another country
• Radio 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/cargo
• Dimensions of ship - to nearest meter
• Location of positioning system's (eg. GPS) antenna onboard the vessel
• Type of positioning system - such as GPS, DGPS or LORAN-C
• 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

Detailed description: Class B units

Class B transponders are designed for carriage by sub-SOLAS vessels. Each consists of one VHF transmitter, two VHF CSTDMA receivers, one of which is multiplexed with the VHF Digital Selective Calling (DSC) receiver, and a GPS active antenna. Although the data output format supports heading information, in general units are not interfaced to a compass, so these data are seldom transmitted. Output is the standard AIS data stream at 38400bps, as RS232 and/or NMEA formats. To prevent overloading of the available bandwidth, transmission power is restricted to 2W, giving a range of about 5 – 10 miles.

At the time of writing (November 2009) almost all Class B units use boards from Software Radio Technology (SRT). Exceptions to this are Furuno and AMEC.

Four messages are defined for class B units:

Message 14: Safety Related Message

This message is transmitted on request for the user - some transponders have a button that enables it to be sent, or it can be sent through the software interface. It sends a pre-defined safety message.

Message 18: Standard Class B CS Position Report

This message is sent every 3 minutes where Speed over Ground (SOG) is less than 2 knots, or every 30 seconds for greater speeds.

MMSI, Time, SOG, COG, Longitude, Latitude, True Heading

Message 19: Extended Class B Equipment Position Report

This message was designed for the SOTDMA protocol, and is too long to be transmitted as CSTDMA. However a coast station can poll the transponder for this message to be sent.

MMSI, Time, SOG, COG, Longitude, Latitude, True Heading, Ship type, Dimensions.

Message 24: Class B CS Static Data Report

This message is sent every 6 minutes, the same time interval as for Class A transponders. Because of its length, this message is divided into two parts, sent within a minute of each other.

Note that this message was defined after the original AIS specifications, so some Class A units may need a firmware upgrade to be able to decode this message.

MMSI, Boat Name, Ship Type, Call Sign, Dimensions, and Equipment vendor ID

Detailed description: AIS Receivers

A number of manufacturers offer AIS receivers, designed for monitoring AIS traffic, either from a shore station or for use on board a vessel that does not carry a Class A or Class B unit. These may have two receivers, for monitoring both frequencies simultaneously, or they may switch between frequencies (thereby missing some messages, but coming in at a lower price). In general they will output RS232 or NMEA data for display on an electronic chart plotter or on a computer.

See also

• Automatic Packet Reporting System
• Håkan Lans - The inventor of STDMA, used as the communication protocol in AIS.
• Ship location mapping service - uses AIS input data from water vessels worldwide.
• TCAS

External links

• AIS Conference's Presentations from 2006
• Kordia Maritime AIS Services Kordia’s Automatic Identification System (AIS) provides maritime coverage of New Zealand’s coastal waters.
• Vesper Marine Information on AIS and dedicated AIS instrumentation
• Commissioners of Irish Lights AIS information
• Hobbyist decoding of AIS
• UAIS.org - Articles and information about AIS
• Web-based AIVDM (NMEA) sentence decoder
• Yacht Insure's Guide to AIS
• US Coast Guard's AIS Overview
• International Maritime Organization's AIS transponders information
• Template:Pt icon AIS presentations in Portuguese language, and a project report on AIS in English

Free AIS data sharing

• Volunteer AIS data sharing center. The data are shared in raw NMEA format.
• AIS data sharing community based on NOAA ShipPlotter application

Graphical AIS Tracking

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

• worldwide AIS tracking Website
• AIS vessel tracking based on standalone VT Explorer application
• www.ios-hellas.gr/ais Vessels from around the world update every sec!!
• Mariweb.gr On-Line tracking of vessels from around the world, weather reports and live cameras!
• Vessels near Portugal
• Vessels in Northern Europe
• Template:Fi icon Vessels in the Gulf of Finland
• Vessels in the Gulf of Mexico and Texas Ports
• Vessels in the Irish Sea / United Kingdom
• Vessels around Norway
• Vessels in Seattle, WA
• Vessels in San Francisco bay
• Vessels around Southampton, UK
• Vessels around Öckerö, Sweden
• Vessels around Great Britain and Ireland
• Vessels in and around the River Orwell, UK + Vancouver BC
• Vessels in and around the Aegean Sea, Greece & Mediterranean
• Template:Es icon Vessels around the world
• Vessels near Ios Island in Greece Limited coverage
• Vessels around Denmark
• Vessels around the port of Rostock
• MarineTraffic.com Vessels from around the world!
• Live AIS @ Landsort Vessels in Northern Baltic Sea around Landsort!
• Vessels around the port of Koper and Trieste