AIS

What is AIS?

AIS is an acronym and stands for Automatic Identification System.

AIS was developped as a system for collision avoidance. The use of AIS however, is not limited to ship-to-ship usage anymore. The data send out by AIS became increasingly valuable to port authorities, vessel traffic servcies (VTS), coast guards and other naval forces. More recently AIS tracking data is also used by other parties like ship owners, logistic companies or the financial world to get an overview of the world’s fleet.

How AIS works
Especially while off-shore, there were no navigational aids to the navigators to prevent collissions. AIS provides an additional means to get a clear view on critical area around the vessel and helped to judge the situation. It provides usefull information about vessels like their speed, GPS-location, course, heading, name, callsign etc.

 
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Since 31 December 2004 ships are required by IMO (the International Maritime Organisation) to have an AIS system. This rule applies to all vessels travelling internationally with more than 300 gross tonnage and all commercial passenger vessels irrespective of their size. Since 2008 also non-internationally travelling vessels above 500 gross tonnage are included.

The system is automatically sending and receiving via the standard vessel’s radio units. It uses two VHF channels simultaniously, channel 87B – 161.975 MHz and channel 88B – 162.025 MHz. To order a lot of vessels sending out their data, the AIS transmition protocol works with 2250 slots per minute which can be used by different senders to transmit their information. Self-Organized Time Division Multiple Access (SOTDMA) is used to autonomously devide the available timeslots between different senders. Due to the limited range of VHF and the different transmission intervals of senders the number of slots is sufficient and collissions hardly occur.

 
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An AIS system sends out different types of information in varying time intervals. In total, AIS is able to communicate 27 different message types. Message #1 e.g. is the position of the vessel. Not all messages are regularly used.

Here is a short overview of the data which is commonly transmitted:

Dynamic information, transmitted every 2-10 second when underway and every 3 minutes whens tanding still:

  • Maritime Mobile Service Identity (MMSI) – a unique nine digit identification number.
  • Position Timestamp – in UTC seconds
  • Rate of turn – in degree per minute
  • Speed over ground – in knots
  • Positional – in longitude & latitude
  • Course over ground – in deree
  • True heading – in degree
  • True bearing at own position – in degree

Static information, independently transmitted every 6 minutes:

  • IMO number
  • Call sign
  • Ship name
  • Ship type
  • Ship dimensions
  • Location of the position reference point on board the ship
  • Type of positioning system
  • Draught – in meter
  • Destination – free text
  • ETA (estimated time of arrival) at destination – UTC Timestamp

Whereas the the dynamic information is entered automatically, the static information like ship name or draught have to be entered automatically by the crew.

 
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Within AIS there are different classes of devices:

Class A AIS is what is found on commercial vessels. These devices transmit with a power of 12.5 watts and are fully integrated with the vessels onboard sensor network.
Class B AIS devices send with less power than the class A devices: 5 watts. Its range therefore is limited compared to a class A device. The devices are cheaper. They are mostly found on vessels for which the AIS requirements do not apply like fishing vessels or pleasure boats.
AIS aids to navigation are devices that can be integrated into existing navigational aids like lighthourses or bouys for them to be vissible within the AIS network.
AIS base stations are used by maritime authorities to control and monitor vessel traffic. Base stations have special functionallity to configure remote devices to e.g. increase the transmission time interval.
AIS SART (search and rescue transponder) are AIS devices that are not able to receive any signals. They send with very low power, 1 watt, continously for up to 96 hours. These devices are used assist locally bound rescue activities, e.g. to find vessels in distres. These devices can also be integrated into the crew’s livevests.

 
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For more information see the website of the IMO