Automatic identification system

How it works

Basic overview

AIS transceivers automatically broadcast information, such as their position, speed, and navigational status, at regular intervals via a VHF transmitter built into the transceiver. 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 transceivers 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. Data is transmitted via a tracking system which makes use of a Self-Organized Time Division Multiple Access (STDMA) datalink designed by Swedish inventor Håkan Lans.

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 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 12.5 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.^[20]

Class B

There are now the separate IMO specifications for Class B vessel-mounted AIS transceiver (transmit and receive) aimed at lighter commercial and leisure markets: a carrier-sense time-division multiple-access (CSTDMA) system and a system that uses SOTDMA (as in Class A).

In the original CSTDMA-based system, defined in ITU M.1371-0 and now called Class B “CS” (or unofficially as Class B/CS),^[22] 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 B “CS” transmits at 2 W and is not required to have an integrated display: Class B “CS” units can be connected to most display systems where the received messages will be displayed in lists or overlaid on charts. Default transmit rate is normally every thirty seconds, but this can be varied according to vessel speed or instructions from base stations. The Class B “CS” standard requires integrated GPS and certain LED indicators. Class B “CS” equipment receives all types of AIS messages.

The newer SOTDMA Class B “SO” system,^[23] sometimes referred to as Class B/SO or Class B+,^[24][25] leverages the same time slot finding algorithm as Class A, and has the same transmission priority as Class A transmitters, helping to guarantee that it will always be able to transmit. The Class B “SO” technology will also change its rate of transmission depending on the speed the vessel is going, up to every five seconds over 23 knots, instead of the constant rate of every thirty seconds in Class B “CS”.^[26] Finally Class B “SO” will also broadcast at a power of 5W instead of the previous 2W of Class B “CS”.^[24][27]

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 transceivers 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 transceiver (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 periodic use and only in emergencies due to its PATDMA-type operation which places stress on the slot map.

Specialist AIS transceivers

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.

Type testing and approval

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 been 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.

Message types

There are 27 different types of top level messages defined in ITU M.1371-5 (out of a possibility of 64) that can be sent by AIS transceivers.^[28][29]

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.^[30] 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.^[31] The International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA-AISM) now established a process for collection of regional application-specific messages.^[32]

Detailed description: Class A units

Each AIS transceiver 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.^[33] This internal receiver may also be used for position information. However, position is typically provided by an external receiver such as GPS, LORAN-C 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 destination, and ETA may also be provided.

An AIS transceiver normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. AIS transceivers 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 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 transceivers 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,^[34] 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.^[35]

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 4 and 8 minutes. 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 transceiver systems.^{[citation needed]}

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 TranspondereTech.^{[citation needed]} 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.

Broadcast information

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
• 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
• Type of ship/cargo
• 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 or 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 transceivers are 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, both alternating as 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.

Four messages are defined for class B units:

Message 14

Safety Related Message: This message is transmitted on request for the user – some transceivers 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 transceiver 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. 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.

Technical specification

RF characteristics

AIS uses the globally allocated Marine Band channels 87 & 88.

AIS uses the high side of the duplex from two VHF radio “channels” (87B) and (88B)

• Channel A 161.975 MHz (87B)
• Channel B 162.025 MHz (88B)

The simplex channels 87A and 88A use a lower frequency so they are not affected by this allocation and can still be used as designated for the maritime mobile frequency plan.

Most AIS transmissions are composed of bursts of several messages. In these cases, between messages, the AIS transmitter must change channel.

Before being transmitted, AIS messages must be NRZI encoded.

AIS messages are transmitted using GMSK modulation. The GMSK modulator BT-product used for transmission of data should be 0.4 maximum (highest nominal value).

The GMSK coded data should frequency modulate the VHF transmitter. The modulation index should be 0.5.

The transmission bit rate is 9600bit/s

Ordinary VHF receivers can receive AIS with the filtering disabled (the filtering destroys the GMSK data). However, the audio output from the radio would need to be then decoded. There are several PC applications that can do this.

Message organization

As there are a multitude of automatic equipment transmitting AIS messages, to avoid conflict, the RF space is organized in frames. Each frame lasts exactly 1 minute and starts on each minute boundary. Each frame is divided into 2250 slots. As transmission can happen on 2 channels, there are 4500 available slots per minute. Depending on the type and status of equipment and the status of the AIS slot map, each AIS transmitter will send out messages using one of the following schemes:

1. ITDMA (Incremental time division multiple access)
2. RATDMA (Random access time division multiple access)
3. FATDMA (Fixed access time division multiple access)
4. SOTDMA (Self-organizing time division multiple access)

The ITDMA access scheme allows a device to pre-announce transmission slots of non-repeatable character, ITDMA slots should be marked so that they are reserved for one additional frame. This allows a device to pre-announce its allocations for autonomous and continuous operation.

ITDMA is used on three occasions:

• data link network entry;
• temporary changes and transitions in periodical reporting intervals;
• pre-announcement of safety related messages.

RATDMA is used when a device needs to allocate a slot, which has not been pre-announced. This is generally done for the first transmission slot, or for messages of a non-repeatable character.

FATDMA is used by base stations only. FATDMA allocated slots are used for repetitive messages.

SOTDMA is used by mobile devices operating in autonomous and continuous mode. The purpose of the access scheme is to offer an access algorithm which quickly resolves conflicts without intervention from controlling stations.

Message format

An AIS slot is 26.66 ms long. The data modulation is 9600 bit/s, so each slot has a maximum capacity of 256 bits. The framing is derived from the HDLC standard, described in ISO/IEC 13239:2002.

Each slot is structured as such: <8 bit ramp up><24 bit preamble><8 bit start flag><168 bit payload><16 bit CRC><8 bit stop flag><24 bit buffer>

• 24 bit preamble: this is a sequence of 0101…
• Start flag: 0x7e
• 168 bit payload, this is the body of an AIS message. For messages requiring more data, several slots (Maximum of 5) must be used.
• 16 bit CRC-16-CCITT: 16-bit polynomial to calculate the checksum.
• Stop flag: 0x7e
• 24 bit buffer used for bit stuffing, synchronization jitter and distance delay.

Note that the signal on the VHF carrier is NRZI encoded and uses bit-stuffing to avoid unintentional stop-flags which may otherwise occur in the data. As such, the raw bits must first be decoded, and the stuffing bits removed, to arrive at the actual usable message format described above.

Messages

Messages sent and received over the air

All AIS messages transmit 3 basic elements of information:

1. The MMSI number of the ship or equipment that holds the transmitter (base station, buoy, etc.)
2. The identification of the message being transmitted (See below table)
3. A repeat indicator that was designed to be used for repeating messages over obstacles by relay devices.

The following table gives a summary of all the currently used AIS messages.

Ais message	Usage	Comments
Message 1, 2, 3: Position Report Class A	Reports navigational information	This message transmits information pertaining to a ships navigation: Longitude and latitude, time, heading, speed, ships navigation status (under power, at anchor…)
Message 4: Base Station Report	Used by base stations to indicate their presence	The message reports a precise position and time. It serves as a static reference for other ships
Message 5: Static and Voyage Related Data	Gives information on a ship and its trip	One of the few messages whose data is entered by hand. This information includes static data such as a ship’s length, width, draught, as well as the ship’s intended destination
Message 6: Binary Addressed Message	An addressed point-to-point message with unspecified binary payload.
Message 7: Binary Acknowledge Message	Sent to acknowledge the reception of a message 6
Message 8: Binary Broadcast Message	A broadcast message with unspecified binary payload.
Message 9: Standard Search and Rescue Aircraft Position Report	Used by an aircraft (helicopter or airplane) which is involved with search and rescue operation on the sea (i.e. search for and recovery of survivors of an accident at sea).	Sends out location (including altitude) and time information
Message 10: UTC/Date Inquiry	Obtain time and date from a base station	Request for UTC/Date information from an AIS base station. Used when a device does not have time and date locally, usually from GPS
Message 11: Coordinated universal time/date response	Response from message 10	Identical to message 4.
Message 12: Addressed Safety-Related Message	Used to send text messages to a specified vessel	Text message may be in plain English, commercial codes or even encrypted
Message 13: Safety related acknowledge	Response from message 12
Message 14: Safety related broadcast message	Identical to message 12, but broadcast
Message 15: Interrogation	Used by a base station to get the status of up to 2 other AIS devices
Message 16: Assigned mode command	Used by a base station to manage the AIS slots
Message 17: Global navigation-satellite system broadcast binary message	Used by a base station to broadcast differential corrections for GPS
Message 18: Standard class B equipment position report	A less detailed report than types 1-3 for vessels using Class B transmitters	Does not include navigation status nor rate of turn
Message 19: Extended class B equipment position report	For legacy class B equipment	Is replaced by message 18
Message 20: Data link management message	Used by a base station to manage the AIS slots	This message is used to pre-allocate TDMA slots within an AIS base station network
Message 21: Aids-to-navigation report	Used by an (AtN) aid to navigation device (buoys, lighthouse..)	Transmits precise time and location as well as the characteristics of the AtN
Message 22: Channel management	Used by a base station to manage the VHF link
Message 23: Group assignment command	Used by a base station to manage other AIS stations
Message 24: Static data report	Equivalent of a Type 5 message for ships using Class B equipment
Message 25: Single slot binary message	Used to transmit binary data from one device to another
Message 26: Multiple slot binary message with communications state	Used to transmit binary data from one device to another
Message 27: Long-range automatic identification system broadcast message	This message is used for long-range detection of AIS Class A and Class B vessels (typically by satellite).	Same as messages 1