Marine electronics explained: Automatic Identification System (AIS)

Used by vessel traffic services (VTS) to automatically identify, locate and track ships, AIS uses local vessels equipped with AIS, fixed position base stations and satellite technology to provide important navigational and safety information.

Along with radar AIS is able to assist with collision avoidance but, primarily, provides both traceability to vessels at sea and important management information for marine traffic control and a ship’s crew in busy shipping zones.

AIS uses a standardized VHF transceiver along with other navigation technology such as a gyrocompass, rate of turn indicators, LORAN-C receivers or GPS. Being a VHF signal, the range is dependent upon the height of the antenna and, as such, also uses satellites fitted with AIS receivers to supplement the network when out of range of coastal base stations or other AIS equipped vessels.

A signal is broadcast every few seconds using two dedicated marine VHF channels with the interval period changing depending on the ship’s speed and course headings; faster vessels or those changing direction transmit data more frequently.

The benefits of using a VHF signal means that the frequency used is longer wave and can thus ‘see’ around bends in rivers or behind islands where radar may not. Not only is this important in being able to trace a ship but is invaluable in supplementing other navigational tools to locate vessels which may otherwise be ‘hidden’.

The transponder works continuously, irrespective of whether the vessel is moored or at sea, although a stationary vessel will not transmit it’s data as frequently.

There are two types of AIS transponders available; Class A relating to larger commercial vessels and Class B being developed for smaller boats. Being introduced after Class A, Class B transponders are designed to work with Class A traffic but not all Class A transponders can receive data from Class B signals.

AIS is required by international regulations (pursuant with the International Maritime Organisation and the Safety of Life at Sea convention) on all passenger carrying vessels and internationally voyaging ships over 300 tonnes (gross tonnage) with additional mandatory regulations being in place but varying by country.

However, AIS can be very useful for other vessels not covered by these statutory requirements; but how?

AIS is exceptionally useful to aid in the safe navigation around other vessels especially in crowded commercial harbours, busy shipping channels or other such situations. Whilst it should not replace a careful watch routine AIS, because it is constantly updated, provides a ‘belts-and-braces’ approach to marine traffic management. In addition, because the system identifies a vessel by name it provides specific information which can be used in ship-to-ship messaging via digital selective calling. Instead of radioing a generalised message to a ‘tanker off the starboard bow’ you can contact the bridge of the selected ship directly by using the MMSI number provided by AIS.

Although AIS isn’t required by law on smaller vessels, the benefits of having AIS on board your boat is particularly important if you regularly cross paths with large ships. Such vessels move fast and cannot stop quickly; combined with the fact that they can rarely see or look for smaller boats it makes these behemoths a very real and present danger. Having AIS installed gives a smaller vessel the crucial visibility it needs.

AIS can also be particularly helpful for navigational purposes during poor weather conditions or at night. VHF signals are received far better than the information provided by radar in squally winds or heavy rain; the latter showing only noise.

AIS receivers come with a wide range of functionality and the data format the information is produced in also varies by device. So, how do you choose the right device to suit your application?

Firstly you will want to find out whether your current navigation systems integrate with, and support, AIS data. Most modern chart plotters can receive an AIS signal and, typically, use this data to overlay triangles (representing a vessel) on an electronic chart. The vector image indicates the direction of travel with additional information being available from each specific vessel by moving a cursor over the target triangle.

In busy traffic, smaller screens can become very crowded with so much data being made available and the usefulness of the system can become lost. Fortunately most systems provides users with the option of programming alarms such as a CPA (closest point of approach) to warn about a ship’s potential to pass dangerously close. However, each system differs in the amount of alarms that can be set-up, how these can be filtered to avoid ‘nuisance-noise’ and other bells-and-whistles.

If you do not have a chart plotter or have one that does not support AIS then choosing a device suitable for your vessel comes down to several factors.

• Power consumption available – you can opt for receiving AIS data using a receiver in conjunction with a laptop that has charting software installed but the problem with this is the amount of power required to support a laptop on a continuous basis. Switching the laptop off means losing the ability to have real-time alarms; if you want to know more about a ship you have spotted then you will need to boot-up which could lose vital time. The benefit of a laptop is of course that the screen display can be much larger than other plotters and may already be part of your onboard ‘arsenal’ and so save you money. Be aware though that laptops, even with a reliable power supply, can be prone to failure and, as a portable device, can ‘travel’ when at sea. • Operating environment of your vessel – if you regularly use areas that have high amounts of traffic then choosing a device that has a large display screen is essential. In a busy harbour, with, say 100 targets being picked up, the ability to filter targets that pose no risk (such as stationary moored vessels) is important to be able to evaluate the information provided. • Purpose of AIS – are you using the AIS to supplement navigation or to be ‘seen’ by larger vessels? Ensuring that the system you choose is capable of receiving and broadcasting in both Class A and B is essential in selecting the right transponder.

Whichever type of AIS device you choose you will need to consider some of the following criteria to ensure you are selecting the best solution for your intended application of the technology:

• Is it waterproof? • Can the software be upgraded? • Does it support both Class A and Class B AIS data? • How much power does it use? • Is there a limit on the number of targets that can be tracked? • Can alarms be customised? • How large is the display? • Can targets be filtered?

Whichever type of AIS you require, find a fishing boat has a large selection of advertisers and suppliers who can help you find the right system for you.