How tracking ship movements with AIS works

Published: 08th April 2010
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Automatic Identification Systems are in use around the world, though unless you're a sailor you'll probably never have heard of them.

Even so, tracking ship movements with AIS has made the seas and oceans a safer place for everyone. The technology has made it possible to save hundreds of lives in maritime disasters, helped to keep busy ship lanes accident free and revolutionised maritime insurance and accident investigation.

Basically, AIS is a short range system where vessels and coastal tracking services can identify and locate nearby ships using data transmitted by a VHF transceiver. Navigational data from GPS systems, gyro-compasses or other such tools is integrated with a unique ship ID, making it possible for ships to see who else is in their vicinity and what direction they're going.

Currently AIS transponders have a horizontal range of about 74 kilometres, though this can differ considerably depending on conditions. This means that generally, tracking is conducted by coastal systems or from fixed platforms at sea. Global tracking systems make the best of this by using the most up-to-date navigation information to estimate a ships heading and ETA, correcting the estimate as a vessel appears on other AIS receivers.

However, this could soon change; AIS transponders have a vertical reach that extends as far as the International Space Station's 400km orbit. In 2009, the space-shuttle Atlantis attached two experimental AIS antenna's to the ISS; starting in May 2010, the European Space Agency will be testing a satellite-based, real-time monitoring service for ship movements.

Thanks to an international convention (specifically, SOLAS: the International Convention for the Safety Of Life At Sea) an AIS system is required on board a lot of the ships that regularly undertake international voyages. Every passenger ship has to have one fitted, as is any cargo ship that has a gross tonnage of 300 or more.

However, AIS is so useful that many ships that are smaller than the requirements also choose to have it fitted.

Tracking ship movements with AIS is most often used in collision avoidance; AIS responders are often placed in navigational buoys or on-shore bases to transmit warnings around navigational hazards. Also larger ships - who must be much more careful when navigating - can clearly broadcast their planned movements and smaller vessels can navigate around them by integrating the data with a electronic map or display. Busy harbours and shipping lanes also use AIS to track the amount of traffic in the area and to co-ordinate it as best they can.

One of the most important applications of AIS technology has been in the field of Search & Rescue operations. As you might imagine, knowing the position of a vessel in distress is fairly useful.

Even if the ship in distress doesn't have its own AIS system, knowing the navigational status and location of any ships in the area can be hugely significant - literally a matter of life and death. Any captain worth his license will offer to help out a ship in distress and identifying whose in the best position to help means rescue services can move quickly and effectively.

Finally, a more prosaic use of the technology can be found in cargo haul logistics and maritime insurance. Firstly, multinational companies can track a cargo vessels progress via AIS systems, ensuring that they know the ETA of goods and make arrangements accordingly. Secondly, if a ship goes down, then AIS data can be invaluable in substantiating or debunking an insurance claim. It can provide a snap-shot of the GPS position, the compass heading, the speed and the rate of turn of any vessel involved in a collision.

Perhaps the only area where AIS hasn't been used to its full extent is in automated collision avoidance. Strangely, people worry about leaving such things to a complicated radio...

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