What is AIS?

AIS is a civil identification system for vessels, the purpose is to identify and monitor maritime traffic. The formal name is UAIS ("Universal Automatic Identification System"), but the shorter notation "AIS" is more commonly used.

Standards and regulation of UAIS are established by the International Maritime Organization(IMO).

Shipborne AIS

The AIS-unit aboard the vessel is used for both sending and receiving identification and navigation information. Exact navigation, position and timing information is obtained from an external or internal navigation system, for example a GPS receiver and a gyro unit. Identification information is stored more or less permanently (depending on the type of information) in the AIS unit. Other information can be obtained from shipboard equipment through standard data connections.

The information received from other vessels can be displayed, for example on a chart plotter, allowing ships to automatically track, identify, and exchange information navigation from one another. The technique is using the VHF maritime band, and is not quite as dependent of free sight between the vessels as radar and visual navigation, and is thus a better instrument for collision avoidance.

AIS Base Stations

As the VHF range is rather short (typically 20 nautical miles), the system is mainly intended for communication between near-by vessels. The AIS information can however be received on AIS base stations ashore, forwarded to neighbouring base stations, and retransmitted in order to extend the range of the signals.

The base stations can also forward the AIS information to regional or national AIS data providers, which serve as data sources for the CMR AIS Server.

What is broadcast by AIS?

The AIS equipment is classified according to class "A" and "B", and data from class "B" units can for all practical purposes be considered as a subset of class "A".

The following information is broadcast regularly (rate from 2 seconds 3 minutes depending on class ("A" or "B") and navigation status):

  • MMSI number (unique identification)
  • Navigation status
  • Rate of turn
  • Speed over ground (SOG)
  • Position accuracy
  • Longitude (Long) and Latitude (Lat)
  • Course over ground (COG)
  • True Heading
  • Time stamp

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

  • Name
  • Type of ship/cargo
  • Dimensions and draught of ship
  • Location on ship (where reference point for position reports is located)
  • Type of position fixing device
  • IMO number
  • Radio call sign
  • Destination
  • Estimated time of Arrival (ETA)

How is AIS data transmitted?

AIS is an autonomous transponder system in the the maritime VHF band, using HDLC packet protocols. The transponders are using the so-called Self-Organizing Time Division Multiple Access (SOTDMA) technology in order to enable a high broadcast rate and reliable operation. 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 required capacity according to the IMO performance standard is a minumum of 2000 time slots per minute, but the system provides is capable of handling well over 4,500 reports per minute and updates as often as every two seconds.

The SOTDMA broadcast mode allows the system to be heavily overloaded 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 practice, the capacity of the system is nearly unlimited, allowing for a great number of ships to be accommodated at the same time.