The AIS system is making our oceans a safer place to navigate.



The English Channel, Le Manche


OVERVIEW: The Automatic Identification System (AIS) was originally developed to aid the Vessel Traffic Services (VTS) by use of VHF transponder working on Digital Selective Call (DSC) at VHF Channel 70 and is still in use along the UK coastal areas and others. Afterward the IMO developed a Universal AIS using the new sophisticated technology called Self-Organized Time Division Multiple Access (SOTDMA) based on a VHF Data Link (VDL). This system is synchronized with GPS time to avoid conflict among multiple users (IMO minimum 2000 reports per minute and IEC requires 4500 reports on two channels).

The system operates in 3 modes - autonomous (continuous operation in all areas), assigned (data transmission interval remotely controlled by authority in traffic monitoring service) and polled (in response to interrogation from a ship or authority). The VHF channels 87B and 88B are commonly used and in addition there are local AIS frequencies.The shipborne AIS transponders exchange various data as specified by the IMO and ITU on either frequency automatically set up by the frequency management telecommand received by the DSC receiver on ship. VHF transmit power is also set up for 12.5 W or 2 W automatically. All ships broadcast Static and Dynamic information (autonomous and continuous mode). 



The Automatic Identification System (AIS) is an automatic tracking system used on ships and by vessel traffic services (VTS) around the world to identify and locate vessels by electronically exchanging data with other nearby ships, AIS base stations, and satellites. 


When satellites are used to detect AIS signatures then the term Satellite-AIS (S-AIS)is used. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport. To our mind this is most important method of communication, potentially providing secure passage to ships in blue water.


While the "Identification" may be automatic, AIS is at the present time reliant on human operators to interpret communications for traffic control purposes. We at Bluebird Marine Systems are looking to take AIS to the next level, akin to Super-AIS, in that we are developing and lobbying for acceptance of what is in effect Super-AIS, or an SAIS collision avoidance system 'SAIS CAS,' an AIS system that incorporates an autonomous navigation element - to enhance a ships ability to avoid collisions, with another layer of sensory perception, which is superior to human sight and hearing, and more importantly is not subject to fatigue. We call this system SNAV.


SNAV may reduce the workload at shore based marine traffic control stations, where those stations could rely on ships that are fitted with the SAIS-CAS layer to take all the necessary navigational precautions and interpret what is shown on the Coastguard's (or Port Authority's) screen to avoid collisions, only do it quicker and automatically.







Information provided by AIS equipment, such as unique identification, position, course, and speed, can be displayed on a screen or an ECDIS. AIS is intended to assist a vessel's watchstanding officers and allow maritime authorities to track and monitor vessel movements. AIS integrates a standardized VHF transceiver with a positioning system such as a LORAN-C or GPS receiver, with other electronic navigation sensors, such as a gyrocompass or rate of turn indicator. Vessels fitted with AIS transceivers and transponders can be tracked by AIS base stations located along coast lines or, when out of range of terrestrial networks, through a growing number of satellites that are fitted with special AIS receivers which are capable of de-conflicting a large number of signatures.

The International Maritime Organization's International Convention for the Safety of Life at Sea requires AIS to be fitted aboard international voyaging ships with gross tonnage (GT) of 300 or more tons, and all passenger ships regardless of size.


Based on the number of Class A vessels in service in 2008—it is estimated that there are more than 40,000 ships currently using AIS Class A equipment on an ongoing basis.

In 2007, the Class B AIS standard was introduced which enabled a new generation of economical AIS transceivers to be used by smaller vessels. The introduction of Class B transponders has triggered multiple additional national mandates from Singapore, China, Turkey, and North America affecting hundreds of thousands of vessels. However, neither the US nor Canada (as of 2012) demand that pleasure craft in Inland (non-Ocean) Waterways and Lakes have AIS transponders. Potentially 10 million Class B vessels worldwide should have Class B transponders, with respect to inland waters.

In 2010, most commercial vessels operating on the EU inland waterways were mandated to fit an inland-waterway (modified and approved) AIS Class A device. The entire EU fishing fleet over 15 meters was given until 2014 to do the same. Additionally, a number of other countries, including China, India, the United States, and Singapore, have started AIS mandate programs which require large numbers of vessels to fit an approved AIS device for safety and national security purposes.


The commercial ability to decode AIS messages using a constellation of satellites has been continuously demonstrated by a number of commercial and government operators, including ORBCOMM, exactEarth and Spacequest since 2008. Through this research and development, AIS signals can now be detected by a satellite in a low earth orbit and provide a global capability for monitoring all AIS-equipped vessels.

S-AIS & Sensor Correlation


Correlating optical and radar imagery with S-AIS signatures enables the end-user to identify rapidly all manner of vessel. The greatest operational strength of S-AIS is the ease with which it can be correlated with information received from sources such as radar, optical sources, ESM and more SAR related tools such as GMDSS SARSAT and AMVER.


Space-based radar and other sources can contribute to maritime surveillance by detecting all vessels in specific maritime areas of interest, a particularly useful attribute when trying to co-ordinate a long range rescue effort or when dealing with VTS issues.




AIS was developed to avoid collisions among large vessels at sea that are not within range of shore-based systems. 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 the risk of collision rather than as an automatic collision avoidance system, in accordance with the International Regulations for Preventing Collisions at Sea.

When a ship is navigating at sea, information about 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 (e.g., unaided, binoculars, and night vision), audio exchanges (e.g., whistle, horns, and VHF radio), and radar or Automatic Radar Plotting Aid are historically used for this purpose. These preventative mechanisms, however, sometimes fail due to time delays, radar limitations, miscalculations, and display malfunctions and can result in a collision.

While requirements of AIS are to display only 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 information about the configuration and movements of ships.

Maritime Security

AIS enables authorities to identify specific vessels and their activity within or near a nation's Exclusive Economic Zone. When AIS data is fused with existing radar systems, authorities are able to differentiate between vessels more easily.

AIS improves maritime domain awareness and allows for heightened security and control. Additionally, AIS can be applied to freshwater river systems and lakes.

Aids to navigation

AIS was developed with the ability to broadcast the positions and names of objects other than vessels, such as navigational aid and marker positions and dynamic data reflecting the marker's environment (e.g., currents and climatic conditions). These aids can be located on shore, such as in a lighthouse, or on water, platforms, or buoys. The U.S. Coast Guard has suggested that AIS might replace racon (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 a 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 such as submerged rocks or a shipwreck. 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.


For coordinating on-scene resources of a marine search and rescue (SAR) operation, it is imperative to have data on the position and navigation status of other ships in the vicinity. In such cases, AIS can provide additional information and enhance awareness of available resources, even if the 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 their position.

To aid SAR vessels and aircraft in locating people in distress, the specification (IEC 61097-14 Ed 1.0) for an AIS-based SAR transmitter (AIS-SART) was developed by the IEC's TC80 AIS work group. AIS-SART was added to Global Maritime Distress Safety System regulations effective January 1, 2010.

AIS-SARTs have been available on the market since at least 2009.

Recent regulations have mandated the installation of AIS systems on all Safety Of Life At Sea (SOLAS) vessels and vessels over 300 tons.


AIS information received by VTS is important for accident investigation since it provides accurate data on time, identity, GPS-based position, compass heading, course over ground, speed (by log/SOG), and rates of turn, rather than the less accurate information provided by radar.

A more complete picture of the events could be obtained by Voyage Data Recorder (VDR) data if available and maintained on board 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 twelve hours storage by IMO requirement.

Other reference: Automatic Identification System (AIS): A Human Factors Approach

Binary messages

AIS messages 6, 8, 25, and 26 provide "Application Specific Messages" (ASM), that allow "competent authorities" to define additional AIS message subtypes. There are both "addressed" (ABM) and "broadcast" (BBM) variants of the message. Addressed messages, while containing a destination MMSI, are not private and may be decoded by any receiver.

One of the first uses of ASMs was the Saint Lawrence Seaway use of AIS binary messages (message type 8) to provide information about water levels, lock orders, and weather. The Panama Canal uses AIS type 8 messages to provide information about rain along the canal and wind in the locks. In 2010, the International Maritime Organization issued Circular 289 that defines the next iteration of ASMs for type 6 and 8 messages. Alexander, Schwehr and Zetterberg proposed that the community of competent authorities work together to maintain a regional register of these messages and their locations of use. The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA-AISM) now established a process for collection of regional application specific messages.


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 radiotelephone, tuned to the AIS frequencies (Channel 87 – 161.975 MHz and Channel 88 – 162.025 MHz) into AIS data.

Some programs can re-transmit the AIS information to a local or global network (via TCP or UDP) 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.

Computer AIS monitoring programs, by definition do not possess AIS transponders. Most AIS physical devices (like USB VHF radio dongles) do not contain AIS transponders.

With these monitoring systems your position (or your vessel's position) will not be transmitted. However, these devices may be used as an inexpensive alternative to AIS devices for smaller vessels if no other viable alternative can be found.
Ship enthusiasts also use such systems to track and find vessels to add to their photo collections.

AIS mobile apps are also becoming common for use with Android and iOS devices. These apps use GPS positioning and Internet access on the mobile device (smartphones or tablets) to provide near real-time position data to and from the AIS network. As the devices use a mobile (cellular) telephone network for the data transmission, the useful distance offshore will be limited and subject to network availability. Note also that the mobile device is not a transponder and accordingly a craft using the Internet-based AIS network will not be visible to other vessels on their AIS systems unless they also use data from the same land based AIS networks.

AIS data on the Internet

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

In relation to the issue of freely available automatic identification 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.

Some argue 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.

However, binoculars need line of sight and clear visibility and so do not help in fog or where a land mass obstructs the view, unlike AIS radio waves (though both fog and large land masses can interfere with the VHF signals used by AIS, too). Vessels can turn off AIS to attempt to be invisible but, since they can still be seen by radar, the absence of an AIS signal would highlight them as unusual and worthy of investigation where security or piracy are priorities.

Range limitations drive S-AIS space race

Shipboard AIS transponders have a horizontal range that is highly variable, but typically only up to about 74 kilometres (46 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 two antennas—an AIS VHF antenna, and an Amateur Radio antenna to the Columbus module of the ISS. Both antennas were built in cooperation between ESA and the ARISS team (Amateur Radio on ISS). Starting from May 2010 the European Space Agency is testing an AIS receiver from Kongsberg Seatex (Norway) in a consortium lead by FFI (Norway) in the frame of technology demonstration for space-based ship monitoring. This is a first step towards a satellite-based AIS-monitoring service.

In 2008, ORBCOMM launched AIS enabled satellites in conjuction with a US Coast Guard contract to demonstrate the ability to collect AIS messages from space. In 2009, Luxspace, a Luxembourg-based company, launched the RUBIN-9.1 satellite (AIS Pathfinder 2). The satellite is operated in cooperation with SES and REDU Space Services. In late 2011 and early 2012, ORBCOMM and Luxspace launched the Vesselsat AIS microsatellites, one in an equatorial orbit and the other in a polar orbit.

In 2007, the U.S. tested space-based AIS tracking with the TacSat-2 satellite. However, the received signals were corrupted because of the simultaneous receipt of many signals from all over the world.

In July 2009, SpaceQuest launched AprizeSat-3 and AprizeSat-4 with AIS receivers. These receivers were successfully able to receive the U.S. Coast Guard's SART test beacons off of Hawaii in 2010. In July 2010, SpaceQuest and exactEarth of Canada announced an arrangement whereby data from AprizeSat-3 and AprizeSat-4 would be incorporated into the exactEarth system and made available worldwide as part of their exactAIS(TM)service.

On July 12, 2010, The Norwegian AISSat-1 satellite was successfully launched into polar orbit. The purpose of the satellite is to improve surveillance of maritime activities in the High North. AISSat-1 is a nano-satellite, measuring only 20x20x20 cm, with an AIS receiver made by Kongsberg Seatex. It weighs six kilograms and is shaped like a cube.


AIS is a technology which has been developed under the auspices of the IMO by its technical committees. The technical committees have developed and published a series of AIS product specifications. Each specification defines a specific AIS product which has carefully created to work in a precise way with all the other defined AIS devices, thus ensuring AIS system interoperability worldwide. Maintenance of the specification integrity is deemed critical for the performance of the AIS system and the safety of vessels and authorities using the technology. As such most countries require that AIS products are independently tested and certified to comply with a specific published specification. Products that have not been tested and certified by a competent authority, may not conform to the required AIS published specification and therefore may not operate as expected in the field. The most widely recognized and accepted certifications are the R&TTE Directive, the U.S. Federal Communications Commission, and Industry Canada, all of which require independent verification by a qualified and independent testing agency.



If OS wants to know information about ship 1, OS shall send an interrogation in polling mode; then ship 1 will transmit her response on the same VHF channel without operator intervention. As and when a streamlined low-cost version becomes available for pleasure craft, the system is sure to reduce accidents at sea.





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 comprises several substandards called "types" that specify individual product types. The specification for each product type provides a detailed technical specification which ensures the overall integrity of the global AIS system within which all the product types must operate. The major product types described in the AIS system standards are:

Class A

Vessel-mounted AIS transceiver (transmit and receive) which operates using self-organised time-division multiple-access (SOTDMA). Targeted at large commercial vessels, SOTDMA requires a transceiver to maintain a constantly updated slot map in its memory such that it has prior knowledge of slots which are available for it to transmit. SOTDMA transceivers will then pre-announce their transmission, effectively reserving their transmit slot. SOTDMA transmissions are therefore prioritised within the AIS system. This is achieved through 2 receivers in continuous operation. Class A's must have an integrated display, transmit at 12 W, interface capability with multiple ship systems, and offer a sophisticated selection of features and functions. Default transmit rate is every few seconds. AIS Class A type compliant devices receive all types of AIS messages.

Class B

Vessel-mounted AIS transceiver (transmit and receive) which operates using either carrier-sense time-division multiple-access (CSTDMA)or SOTDMA; there are now 2 separate IMO specifications for Class B. Aimed at lighter commercial and leisure markets. CSTDMA transceivers listen to the slot map immediately prior to transmitting and seek a slot where the 'noise' in the slot is the same or similar to background noise, thereby indicating that the slot is not being used by another AIS device. Class Bs transmit at 2 W and are not required to have an integrated display: Class Bs can be connected to most display systems where the received messages will be displayed in lists or overlayed on charts. Default transmit rate is normally every 30 seconds, but this can be varied according to vessel speed or instructions from base stations. The Class B type standard requires integrated GPS and certain LED indicators. Class B equipment receives all types of AIS messages.

Base station

Shore-based AIS transceiver (transmit and receive) which operates using SOTDMA. Base stations have a complex set of features and functions which in the AIS standard are able to control the AIS system and all devices operating therein. Ability to interrogate individual transponders for status reports and or transmit frequency changes.

Aids to navigation (AtoN)

Shore- or buoy-based transceiver (transmit and receive) which operates using fixed-access time-division multiple-access (FATDMA). Designed to collect and transmit data related to sea and weather conditions as well as relay AIS messages to extend network coverage.

Search And Rescue Transponder (SART)

Specialist AIS device created as an emergency distress beacon which operates using pre-announce time-division multiple-access (PATDMA), or sometimes called a "modified SOTDMA". The device randomly selects a slot to transmit and will transmit a burst of eight messages per minute to maximize the probability of successful transmission. A SART is required to transmit up to a maximum of five miles and transmits a special message format recognised by other AIS devices. The device is designed for perodic use and only in emergencies due to its PATDMA-type operation which places stress on the slot map.

Specialist AIS Transponders

Despite there being IMO/IEC published AIS specifications, a number of authorities have permitted and encouraged the development of hybrid AIS devices. These devices seek to maintain the integrity of the core AIS transmission structure and design to ensure operational reliability, but to add a range of additional features and functions to suit their specific requirements. The "Identifier" AIS transceiver is one such product where the core Class B CSTDMA technology is designed to ensure that the device transmits in complete compliance with the IMO specifications, but a number of changes have been made to enable it to be battery powered, low cost and more easy to install and deploy in large numbers. Such devices will not have international certification against an IMO specification since they will comply with a proportion of the relevant specification. Typically authorities will make their own detailed technical evaluation and test to ensure that the core operation of the device does not harm the international AIS system.

AIS receivers are not specified in the AIS standards, because they do not transmit. The main threat to the integrity of any AIS system are non-compliant AIS transmissions, hence careful specifications of all transmitting AIS devices. However, it is well to note that AIS transceivers all transmit on multiple channels as required by the AIS standards. As such single-channel, or multiplexed, receivers will not receive all AIS messages. Only dual-channel receivers will receive all AIS messages.

Message types

There are 27 different types of top level messages defined in ITU 1371-4 (out of a possibility of 64) that can be sent by AIS transceivers.

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 (transmission scheduling) for a Class A unit. Therefore, every unit is required to have an internal time base, synchronized to a 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, position (latitude and longitude), "speed over ground", and rate of turn are normally provided by all ships equipped with AIS. Other information, such as angle of heel, pitch and roll, destination, and ETA may also be provided.

An 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. AIS transponders use two different frequencies, VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz), and use 9.6 kbit/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, Håkan Lans. Moreover, the United States Patent and Trademark Office (USPTO) canceled all claims in the original patent on March 30, 2010.

In order to make the most efficient use of the bandwidth available, vessels that are anchored or moving slowly transmit less frequently than those that are moving faster or are maneuvering. The update rate ranges from 3 minutes for anchored or moored vessels, to 2 seconds for fast moving or maneuvering vessels, the latter being similar to that of conventional marine radar.

Each AIS station determines its own transmission schedule (slot), based upon data link traffic history and an awareness of probable future actions by other stations. A position report from one station fits into one of 2,250 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 is a minimum of 2,000 time slots per minute, though the system provides 4,500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provides nearly 100% throughput for ships closer than 8 to 10 nmi 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, which are of greater 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. The range of any VHF radio is determined by multiple factors, the primary factors are: the height and quality of the transmitting antenna and the height and quality of the receiving antenna. Its propagation is better than that of radar, due to the longer wavelength, so it is possible to reach around bends and behind islands if the land masses are not too high. The look-ahead distance at sea is nominally 20 nmi (37 km). With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.

The system is backward 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 2003 and 2007 and was delivered by Saab TransponderTech. 


The entire Chinese coastline is covered with approximately 250 base stations in hot-standby configurations including seventy computer servers in three main regions. Hundreds of shore based users, including about twenty-five vessel traffic service (VTS) centers, are connected to the network and are able to see the maritime picture, and can also communicate with each ship using SRM's (Safety Related Messages). All data are in real time. 


The system was designed to improve the safety and security of ships and port facilities. It is also designed according to an 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 the beginning of 2007, a new worldwide 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 affect users, but system builders need to upgrade software to accommodate the new standard. A standard for AIS base stations has been long awaited. Currently 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.



An AIS transceiver sends the following data every 2 to 10 seconds depending on a vessel's speed while underway, and every 3 minutes while a vessel is at anchor:
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, from 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)

Positional accuracy:

Longitude – to 0.0001 minutes

Latitude – to 0.0001 minutes

Note that this positional accuracy is not adequate to tell 2 personal watercraft apart.

Course over ground – relative to true north to 0.1°

True heading – 0 to 359 degrees (for example from a gyro compass)

True bearing at own position. 0 to 359 degrees

UTC Seconds – The seconds field of the UTC time when these data were generated. A complete timestamp is not present.

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


Dimensions of ship – to nearest meter
Location of positioning system's (e.g., GPS) antenna on board the vessel - in meters aft of bow and meters port of starboard
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
optional : high precision time request, a vessel can request other vessels provide a high precision UTC time and datestamp

Detailed description: Class B units

Class B transponders smaller, simpler and lower cost than Class A transceivers. Each consists of one VHF transmitter, two VHF Carrier Sense Time Division Multiple Access (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 this data is seldom transmitted. Output is the standard AIS data stream at 38.400 kbit/s, as RS232 and/or NMEA formats. To prevent overloading of the available bandwidth, transmission power is restricted to 2 W, giving a range of about 5–10 mi.

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

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 one 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. These may have two receivers, for monitoring both frequencies simultaneously, or they may switch between frequencies (thereby missing messages on the other channel, but at reduced price). In general they will output RS232, NMEA, USB or UDP data for display on electronic chart plotters or computers.



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December 2000 amendments

Types of Automatic Identification Systems". U.S. Coast Guard Navigation Center.

Maritime Navigation and Radiocommunication Equipment and Systems IEC


Circular 289: Guidance On the Use of AIS Application-Specific Messages

Establishing an IALA AIS Binary Message Register: Recommended Process IALA Conference

AIS Application Specific Messags IALA-AISM.

EMC Analysis of Universal Automatic Identification and Public Correspondence Systems in the Maritime VHF Band

Vessel Tracker Community.

Maritime security – AIS ship data 79th session: 1–10 December 2004. IMO Maritime Safety Committee.

Atlantis leaves Columbus with a radio eye on Earth’s sea traffic ESA. 4 December 2009.

LUXSPACE Sarl - LuxSpace successfully launches AIS satellite on PSLV LuxSpace

ESA satellite receiver brings worldwide sea traffic tracking within reach". Euro Space Agency. 23 April 2009

News Room - SpaceQuest, Ltd

SpaceQuest receiving AIS SART messages from orbit". Kurt Schwehr. 29 April 2010

Successful launch of Norwegian satellite The Norwegian Space Centre

Technical characteristics for AIS using time division multiple access VHF maritime band (Recommendation ITU-R M.1371-4)

AIS Messages U.S. Coast Guard Navigation Center.

IEC 61993-2 Clause 6.2

US patent 5506587, Lans, Håkan, "Position indicating system", issued 1996-04-09, assigned to GP&C Systems International AB

USPTO ex-parte reexamination certificate (7428th), issued on March 30, 2010

AIS Advice


AIS references

Commissioners of Irish Lights Oct-11 - AIS Home page

IMO AIS transponders information Oct-11

Further AIS, VTS and VTMIS information

US Coast Guard Automatic Identification System Overview Mar-13




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