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AIS, The Automatic Identification System

Basic Operations of the Automatic Identification System


In very simple terms an Automatic Identification System is a transceiver designed to collect information from instruments on board a vessel and convert it to an internationally standard message that can be decoded by other AIS unit on other vessels or shore stations.

AIS systems are one of the most versatile instruments in the maritime sphere. Ships, shore stations, and even aids to navigation can transmit and receive a wide variety of information.

Not only can the position of a ship be plotted in real time, the units also transmit data like Course Over Ground (COG), Rate of Turn (ROT), vessel speed, compass heading, and IMO number.

One of the greatest benefits of this system is the ability to expand as new technologies emerge. By using a binary data stream it is possible to encode almost any type of information. One of the first uses of AIS binary data was the transmission of lock status along the Saint Lawrence Seaway. The Panama Canal also uses AIS data to transmit wind information to ships as they make passage.

Several International Maritime Organization (IMO) regulations dictate what sort of information is broadcast by these devices. The two most significant are SOLAS, which stands for Safety of Life at Sea and COLREGS, which is a convention to address collisions at sea. The IMO also sets hardware standards for the devices.

There are three common types of AIS hardware.

AIS Class A – This is a standalone device that interfaces with other instruments but has its own display. The specified transmission power is 12 watts. Transmissions are sent approximately every three seconds. This class of AIS can display information on an integrated display which shows other data like radar or chart plotting information. For more information see the full AIS Class A Standards.

AIS Class B – These are sometimes called black box devices because they do not require their own display. This class of AIS is designed to be integrated into a complete instrument system. These systems allow the operator to view less screens while on watch and therefore have less fatigue. The problem is these screens can fail more easily than a self-contained unit. Transmission rates for this class are approximately every thirty seconds. Class B AIS is less versatile than Class A but the cost is much less. As always, see the full AIS Class B standards for more information.

AIS Receivers – These are not regulated like AIS transceivers because they are unable to transmit information and therefore cannot corrupt the system with bad data. They are only mentioned here because of the broad availability of the devices for the recreational and small vessel market. Considering the value and safety they provide all vessels should have one of these devices on board if the budget allows.

AIS Data Flow

A simple example of data flow through an AIS system is easy to understand. In reality there are many more steps that occur to validate the data as it makes its way through the system.

  • Data is sent from position instruments like GPS (Global Positioning System), or GNSS (Global Navigational Satellite System) to the AIS
  • AIS queries additional instruments like the gyrocompass, rudder angle gauge, and other sensors then packages all of this data for transmission.
  • The data is transmitted in a standard format over Marine VHF radio frequencies.
  • The AIS the returns to listen mode. The process then begins again.

AIS Range

AIS transmits on standard marine VHF radio frequencies and is therefore subject to the same limits as other terrestrial radio transmissions in the same bandwidth. A reputable source of technical information regards the range of AIS to 74 KM or 46 Miles.

What Vessels Must Use AIS

Once again we are giving simple answers here so readers who are unfamiliar can gain a basic understanding of the system. The IMO dictates the international rules regarding who must use AIS and there is no substitution for the actual documentation when implementing this type of system.

In general all vessels that carry passengers or exceed 300 gross tons must be fitted with certified AIS equipment.

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