Automatic Identification System

Contents

Introduction            3

1 Applications and limitations         4

     1.1 Collision avoidance         4

     1.2 Vessel traffic services        4

     1.3 Aids to navigation         4

     1.4 Search and rescue         5

     1.5 Accident Investigation        5

     1.6 Binary messages         5

     1.7 Computing and networking        5

     1.8 AIS data on the Internet        6

     1.9 Range limitations and space-based tracking     6

     1.10 Type Testing and Approval       7

2 How AIS works            8

     2.1 Basic overview         8

     2.2 Message Types         9

     2.3 Detailed description: Class A units       9

     2.3.1 Broadcast information        11

     2.4 Detailed description: Class B units       12

     2.4.1 Message 14: Safety Related Message

     2.4.2 Message 18: Standard Class B CS Position Report

     2.4.3 Message 19: Extended Class B Equipment Position Report

     2.4.4 Message 24: Class B CS Static Data Report

     2.5 Detailed description: AIS Receivers      13

Summary                                                                                        14

Attachement           16

Vocabulary            20 
 
 
 
 
 
 
 
 
 
 
 

Introduction

The Automatic Identification System (AIS) is an automated 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. AIS information supplements marine radar, which continues to be the primary method of collision avoidance for water transport.

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

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 gross tonnage (GT) of 300 or more tons, and all passenger ships regardless of size. It is estimated that more than 40,000 ships currently carry AIS class A equipment. In 2007 the new Class B AIS standard was introduced which enabled a new generation of low cost AIS transceivers. This has triggered multiple additional national mandates from Singapore, China, Turkey and North America affecting hundreds of thousands of vessels.

The Automatic Identification System (AIS) was originally developed to aid the Vessel Traffic Services (VTS) by use of a VHF transponder working on Digital Selective Call (DSC) at VHF CH70, and is still in use along the UK coastal areas and others. Some time later 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). The system operates in three 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). It 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 VHF channels 87B and 88B are commonly used and in addition there are local AIS frequencies. 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. 

1. Applications and limitations^[1]

1.1 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).

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.

1.2 Vessel traffic services ^[5]

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.

1.3 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.

1.4 Search and rescue ^[4]

For coordinating resources on scene of marine search and 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.

1.5 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

1.6 Binary messages

The Saint Lawrence Seaway uses 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.

1.7 Computing and 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.

1.8 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 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.

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.

1.9 Range limitations and space-based tracking

Shipboard AIS transponders have a horizontal range that is highly variable but typically only about 74 kilometers (46 miles). They reach much further vertically, up to the 400 km orbit of the International Space Station (ISS). Space-based AIS is referred to as S-AIS.

On April 28, 2008, Canadian company COM DEV International, became the first company to launch a space-based AIS nano-satellite designed to detect AIS signals from space^[4], and is currently deploying a full micro-satellite constellation, global ground network and centralized data processing center in order to offer global AIS data services. The service is operational and available worldwide as of mid-2010 through exactEarth, COM DEV's data services subsidiary. Exact Earth uses a patent-pending ground and space-based processing technology to minimize interference of collided AIS signals, therefore dramatically improving detection compared with all other satellite-based systems. As more satellites are launched, refresh rates will continue to increase as well.

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 two different AIS receivers, one from Luxspace (GdL), one from 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 June of 2008, ORBCOMM launched six new low-earth orbit (LEO) satellites for their machine-to-machine communications constellation. These new satellites were also equipped with the capability to collect AIS data. Unfortunately, during the next 18 months all but one of these six satellite failed on orbit and the the final satellite was declared a total constructive loss for insurance purposes. Additionally, ORBCOMM plans to incorporate the ability to receive, collect and forward AIS data in the design of the next 18 ORBCOMM Generation 2 (0G2) satellites under development. As additional satellites are launched, ORBCOMM will increase its capability by providing greater redundancy and more frequent updates of AIS data. In 2009, LUXSPACE, a Grand Duchy of Luxembourg based company has launched PathFinder2, (ex-Rubin) and is now the only European company to have an operational system in orbit providing data from all over the world on a daily basis. The satellite is operated in cooperation with SES ASTRA and REDU Space Services. 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.

In July 2009, SpaceQuest launched AprizeSat-3 and AprizeSat-4 with AIS receivers. These receivers have been able to pick up the USCG's SART search and rescue 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 woudl 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 20 x 20 x 20 cm. It weighs six kilograms and is shaped like a cube.

1.10 Type Testing and Approval

Class A AIS products are for ships over 300 tonnes or which are SOLAS vessels and are required to meet the guidelines set out in the Marine Equipment Directive.

Class B AIS products are for non-SOLAS vessels. They need testing and certification under the R&TTE Directive for the European Union and FCC and Industry Canada certification for North America. 
 
 
 

2.0 How AIS works ^[1]

System Overview from US Coast Guard

2.1 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 comprises several sub-standards 'Types' which 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^[3]:

• Class A – Vessel mounted AIS transceiver (transmit & receive) which operates using SOTDMA. Class A's must have an integrated display, transmit at 12W, 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 & receive) which operates using CSTDMA. Class B's transmit at 2W and are not required to have an integrated display: Class B's can be connected to most display systems which the received messages will be displayed in lists or overlayed on charts. Default transmit rate is normally every 30 seconds, but this can be varyed according to vessel speed or instructions from base stations. The Class B type standard requires integrated GPS and certain LED indicators. Class B's receive all types of AIS messages.
• Base Station – Shore based AIS transceiver (transmit & 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 tranceiver (transmit & receive) which operates using FATDMA. Designed to collect and transmit data related to sea and weather conditions as well as relay AIS messages to extend network coverage.

AIS receivers are NOT specified in the AIS standards. This is because they do not transmit. The main threat to the integrity of any AIS system are non-compliant AIS transmissions hence the 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.

2.2Message Types

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

2.3 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 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, though 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 achored 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 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 is 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 provides 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, 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, essentially depending on the height of the antenna, but is slightly better due to the use of digital VHF instead of analog VHF. Its propagation is better than that of radar, due to the longer wavelength, so it is possible to “see”  around bends and behind islands if the land masses are not too high. The “look ahead” distance 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 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 70 computer servers in three main regions. Hundreds of shore based users, including about 25 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 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.

2.3.1 Broadcast information^[6]

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, 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, for example from a gyro compass
• Time stamp – UTC time accurate to the 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 (e.g. GPS) antenna onboard 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

2.4 Detailed description: Class B units

Class B transponders are designed for carriage by sub-SOLAS vessels. 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 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:

2.4.1 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.

2.4.2 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

2.4.3 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.

2.4.4 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

2.5 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 some messages, but coming in at a lower price). In general they will output RS232, NMEA, USB or UDP data for display on electronic chart plotters or computers. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Summary

The required ship reporting capacity according to the IMO performance standard is a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute.

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

For coordinating resources on scene of marine search and 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.

Automatic Identification System (AIS) is very important part of the navigation equipment of the bridge. By means of this system we can avoid the collision with other vessels, take part at the search and resque operations. 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 IMO SOLAS requires AIS to be fitted aboard all ships greater than/equal to 300 gross tons for international voyages.

AIS transponders automatically transmit/broadcast the position and velocity of the ship at regular intervals via a VHF radio built into the AIS. The position and velocity originate from the ship's GPS or, if that fails, from an integral GPS receiver. The AIS also receives heading information from the ship's compass and transmits this at the same time. Other information, such as the vessel name and VHF call sign, is entered when installing the equipment and is transmitted less frequently. The signals are received by AIS transponders fitted on other ships or on land based systems, such as VTS systems. In order to ensure that the VHF transmissions of different AIS do not occur at the same time they are time multiplexed, using a patented technology termed STDMA (Self organising Time Division Multiple Access). (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 conventional marine radar. The time reference is derived from the GPS system.