AIS Transponder on Defiance
AIS Transponder on Defiance

AIS on-board

Defiance has been fitted with an Automatic Identification System or AIS for short.  It is not compulsory for vessels to have AIS. Only larger vessels of 300 tons or more and passenger vessels are required to have it. AIS is an additional safety system on the boat and it has been fitted to improve the safety of passengers on-board.

Uses of AIS

The information can be used in several ways:

  1. Collision Avoidance
    The speed, direction and location of vessels close by can be used to calculate if they are on a collision course with you. AIS receivers have alarms to warn if there is a possible risk to enable early action to be taken. With divers in the water the identity of the approaching vessel can be used to directly call them to avoiding action. A great safety tool.
  2. Vessel Traffic Services, Fishing monitoring and control, Maritime Security
    Not very relevant to smaller craft but large vessels can be monitored and tracked through busy shipping areas.
  3. Aid to Navigation
    Ports, Buoys and other navigation aids such as lighthouses are often fitted with AIS transmitters to aid navigation. These positions will show up on plotters to help with navigation. It is possible for virtual transmissions to be sent to mark points that will show up on the ships plotter
  4. Search and Rescue
    Not only is the last position and data of a vessel recorded to help in search and rescue. The system also identifies ships in the area that can be used to assist in any emergency.
  5. Accident investigation
    The tracks of vessels leading up to a collision or other incident is vital to understanding the actions of the captains and a great help in accident investigation.

How does AIS Work?

The system contains three major parts linked into a box of tricks:

  1. A position location receiver such as a GPS receiver, Galileo or Loran
  2. A VHF Receiver to receive details from other vessels
  3. A VHF Transmitter to send details of the vessel

The system uses the GPS position of the vessel to calculate the speed and direction of the boat. This information is then broadcast by a VHF transmitter to other craft in the area. Likewise all other vessels transmit similar information. So the basic information gathered for all vessels is:

  • Identity including the unique Marine Mobile Identity Service Number (MMSI)
  • Time of message
  • Position sent as latitude and longditude
  • Speed and Heading
AIS track of Defiance during testing. Shown on Marine Traffic Website
AIS track of Defiance during testing. Shown on Marine Traffic Website

This information can be picked up and displayed for all to see. For example on Marine Traffic Website. Applications for moble phones as well as most modern chart plotters can display AIS information. Most systems have some type of display, but this is not actually necessary.

In addition to the basic information the name, size and destination along with much more information is possible.

This information is transmitted regularly by the boat usually every 30 seconds but more complex systems can alter this depending on speed. e.g. at anchor they will only transmit every 3 minutes but at high speed may transmit every 2 seconds, which is as fast as conventional Radar systems. These systems use a special system called SOTDMA ( Self-Organized Time Division Multiple Access) which books time slots when they will transmit data. Simpler systems use carrier-sense time-division multiple-access (CSTDMA) whereby they look for a free slot to transmit just before making their transmission.

AIS uses VHF channels 87B (161.975 MHz) and 88B (162.025 MHz). The Class A transmitters transmit at 12.5W which are the type used on large vessels. Class B transmitters are used on smaller craft and transmit at 2W meaning they can broadcast their information between 5-10 miles. Large vessels are required to have some type of visual display on board, but this is not true for Class B transmitters.