Wikipedia

Marinate

Joined 9 January 2007
A United States Coast Guard Operations Specialist using AIS and RADAR to manage vessel traffic.

The Automatic Identification System (AIS) is a system used by ships and Vessel Traffic Services principally for identification and locating vessels. AIS helps to resolve the difficulty of identifying ships when not in sight (e.g. in fog, at distance, etc.) by providing a means for ships to exchange identification, 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 an electronic navigation system, such as a LORAN-C or Global Positioning System receiver, and other navigational sensors on board ship (gyrocompass, rate of turn indicator, etc.).

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 of 300 or more gross tonnage, and all passenger ships regardless of size. It is estimated that more than 40,000 ships currently carry AIS class A equipment.[citation needed]

Applications and limitations

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Collision avoidance

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AIS is used in navigation primarily for collision avoidance. Due to the limitations of radio characteristics, and because not all vessels are equipped with AIS, the system is meant to be used primarily as a means of lookout and to determine risk of collision rather than as an automated collision avoidance system, in accordance with the International Regulations for Preventing Collisions at Sea (COLREGS).

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

When a ship is navigating at sea, the movement and identity of other ships in the vicinity is critical for navigators to make decisions to avoid collision with other ships and dangers (shoal or rocks). Visual observation (unaided, binoculars, night vision), audio exchanges (whistle, horns, VHF radio), and radar or Automatic Radar Plotting Aid (ARPA) are historically used for this purpose. However, a lack of positive identification of the targets on the displays, and time delays and other limitation of radar for observing and calculating the action and response of ships around, especially on busy waters, sometimes prevent possible action in time to avoid collision.

While requirements of AIS are only to display a very basic text information, the data obtained can be integrated with a graphical electronic chart or a radar display, providing consolidated navigational information on a single display.

Vessel traffic services

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

Aids to navigation

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AIS was developed with the ability to broadcast positions and names of things other than vessels, namely it can serve to transmit navigation 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.[1]

The ability to broadcast navigation aid positions has also created the concept of Virtual AIS, also known as Synthetic AIS or Artificial AIS. The terms can mean two things; 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 exist, or a concern which is not visible (i.e. submerged rocks, or a wrecked ship). Although such virtual aids would only be visible to AIS equipped ships, the low cost of maintaining them could lead to their usage when physical markers are unavailable.

Search and rescue

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For coordinating resources on scene of marine search & rescue operation, it is important to know the position and navigation status of ships in the vicinity of the ship or person in distress. Here AIS can provide additional information and awareness of the resources for on scene operation, even though AIS range is limited to VHF radio range. The AIS standard also envisioned the possible use on SAR Aircraft, and included a message (AIS Message 9) for aircraft to report position.

Binary messages

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The Saint Lawrence Seaway uses AIS binary messages (message type 8) to provide information about water levels, lock orders, and weather in its navigable system.[2]

Concern over web-based data

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In December 2004, the International Maritime Organization's (IMO) Maritime Safety Committee condemned the use of freely available AIS data published irresponsibly with the following statement.[3]

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

How AIS works

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Basic overview

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

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

Detailed description: Class A units

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Each AIS transponder consists of one VHF transmitter, two VHF TDMA receivers, one VHF Digital Selective Calling (DSC) receiver, and links to shipboard display and sensor systems via standard marine electronic communications (such as NMEA 0183, also known as IEC 61162). Position and timing information is typically derived from an integral or external global navigation satellite system (e.g. GPS) receiver, but may also use a LORAN receiver or an inertial navigation system. Other information broadcast by the AIS, if available, is electronically obtained from shipboard equipment through standard marine data connections. Heading information and course and speed over ground would normally be provided by all AIS-equipped ships. Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided.

The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. Transmissions use 9600-bps Gaussian minimum shift keying (GMSK) modulation over 25 or 12.5 kHz channels using the High-level Data Link Control (HDLC) packet protocol. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other ships. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.

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

Each station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations. A position report from one AIS station fits into one of 2250 time slots established every 60 seconds on each frequency. AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions. Slot selection by an AIS station is randomized within a defined interval, and tagged with a random timeout of between 0 and 8 frames. When a station changes its slot assignment, it announces both the new location and the timeout for that location. In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels.

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% through sharing of slots, and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only targets further away will be subject to drop-out, in order to give preference to nearer targets that are a primary concern to ship operators. In practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.

The system coverage range is similar to other VHF applications, essentially depending on the height of the antenna, but slightly better due to digital VHF and not analog VHF. Its propagation is better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles (37 km). With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.

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

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

Class A Broadcast information

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

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

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

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

Detailed description: Class B units

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A specification for Class B AIS transponders was introduced to allow development of a lower cost AIS transponder for leisure vessels and non-SOLAS commercial vessels. The Class B AIS is backwards compatble with the Class A system however Class B transponders differ from Class A transponders in several ways:

  • Class B transponders transmit at a lower power (2W) compared to Class A (12W)
  • Class B transponders use a different scheme to multiplex their transmissions into a VHF channel. The scheme is known as CSTDMA or 'Carrier Sense' TDMA and is described in more detail below.
  • Class B transponders are designed to be 'polite' to Class A transponders. Priority is always given to Class A transponder transmissions in order to ensure operation for SOLAS vessels.
  • Class B transponders do not have to incorporate a separate DSC receiver. DSC reception can be 'time shared' with one of the AIS recievers.
  • ....

Carrier Sense TDMA (CSTDMA)

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A Class B(CS) AIS utilizes Carrier-Sense TDMA (CSTDMA) instead of the slot mapping utilized by SOTDMA Class A units. The CSTDMA unit listens to the AIS network to determine if a slot is being used. Once the unit determines that the slot is open, it powers up its transmitter and broadcasts a 1 slot position report. The unit will wait until it finds an open slot before it transmits. The Class B AIS is also required to listen for reservations from base stations and comply with these reservations. This behavior ensures that a CSTDMA unit is a "polite" user in the AIS network. If the unit can not find an open slot, it will not broadcast. [4]

Class B Broadcast information

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Class B messaging here...

Standards

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AIS Transponders are approved to internationally agreed standards to ensure interoperability. The main standards are:

IEC62287-1 Class B IEC ITU-R M.1371


See also

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References and Footnotes

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  1. ^ US Coast Guard Navigation Center, Types of AIS
  2. ^ Great Lakes St. Lawrence Seaway System, AIS Project. Accessed December 15, 2006.
  3. ^ "Maritime security - AIS ship data". 79th session: 1-10 December 2004. IMO Maritime Safety Committee. Retrieved 2007-01-08.
  4. ^ http://webstore.iec.ch/webstore/webstore.nsf/artnum/035858?opendocument
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AIS related
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Category:Navigational equipment Category:Water transport Category:Technology systems

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