The Automatic Identification System - AIS

AIS and RADAR are essential to manage vessel traffic. 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. It is estimated that more then 40,000 ships currently carry AIS class A equipment.

 

Applications and limitations

AIS is used in navigation primarily for collision avoidance. Because of 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 in accordance with ColRegs rather than as an automated collision avoidance system.

Vessel AIS display listing nearby vessels' range, bearing, and name. When a ship is navigating at sea, the movement and identity of other ships in the vicinity are very important to navigators to make decisions to avoid collision with other ships and dangers (shoal or rocks). Visual (eye, binoculars) / audio means (whistle, VHF radio), radar and ARPA were primarily 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 AIS provides only a very basic graphical display, the data obtained can be integrated with ECDIS or a radar, providing most navigational information on a single display.

However for navigators to know the full traffic situation in the vicinity of their ships is very difficult in busy waters, although VTS may provide traffic management services if available in some harbor or river areas. AIS is a very useful aid for navigators to know a much better traffic situation of what kind of other ships and their movement with identities of targets around much earlier and accurate than just by other methods. There are also many new ideas for the application of AIS, e.g. virtual AIS for aids to navigation and supplement to the on scene coordination for search and rescue (see also GMDSS).

Use of virtual AIS for marking important navigational information provides significant improvement to indicate the position of new wrecks, other new dangers or even navigational warning in relation to the ship's position. Virtual marking of dangers can be transmitted by virtual AIS stations (e.g. VTS) and ships can know the position of dangers on the AIS display much quickly, accurate and easier than physical navigational marks and traditional safety message boardcast by VHF Radio, Navtex, SafetyNet or Notice to Mariners. Of course such virtual aids would only be visible to AIS equipped ships but it is most important for large ocean going ships.

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. With the aid of AIS, which could provide much better picture ofthe resources for on scene operation even the AIS range is limited to VHF radio range, usually about 30 to 50 nautical miles (60 to 90 km).

Broadcast information

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

MMSI number 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:

MMSI number - vessel's unique identification
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 - usually GPS or DGPS
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

AIS Frequencies

The International Telecommunications Union World Radio Conference in 1997 designated two VHF radio frequencies: 161.975 MHz (AIS1, or channel 87B) and 162.025 MHz (AIS2, or channel 88B) for AIS.

How AIS works

Overview

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 organizing Time Division Multiple Access, also called SOTDMA). (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 GPS system.

Detailed description

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and standard marine electronic communications links (IEC 61162/NMEA 0183) to shipboard display and sensor systems (AIS Schematic). Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. 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 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels using HDLC packet protocols. 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. 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. 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 pre-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. The biggest fully operational real time, with full routing capability, is in China operated by China MSA delivered by Saab TransponderTech in Sweden.[citation needed] 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.

IMO condemned[2] the publication on the world-wide web, or elsewhere, of AIS data transmitted by ships and urged Member Governments, subject to the provisions of their national laws, to discourage those who make available AIS data to others for publication on the world-wide web, or elsewhere from doing so.Recent trends also includes Airborne AIS for SAR (Search and Rescue) activities and surveillance. An airborne AIS in addition also transmit altitude (based on GPS) and has a fixed position rate of every 10th second. In high altitude surveillance platforms it is possible to get a stable range better than 150 nautical miles.