Automatic Identification Systems

AIS

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.

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

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 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.

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. Its propagation is slightly 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.

AMS provides the following: