Above and beyond: Practical ways to use AIS on board ship
Gregor Stevens (Senior Manager Nautical) and Arvind Natrajan (Senior Marine Advisor Crewing and Training), of the International Chamber of Shipping, look at some of the ways in which AIS can be used on board ship – and a few points to bear in mind
Possibly the biggest benefit of AIS is the ability to identify ships by name and callsign. This can be helpful to the OOW in areas of increased traffic, such as coastal passage and port approaches. When the AIS output is interfaced with radar, the OOW can label targets of interest on the radar, such as anything that could turn into a collision risk. Coupled with information about the destination and ETA of the target, this assists immensely in improving situational awareness.
The advantage of interfacing the AIS with ECDIS is that, from an OOW’s point of view, ECDIS will not only display own ship’s progress in real-time, but will also display the progress of all the targets. This enables the OOW to look ahead towards the traffic density in other areas in the passage plan, possibly planning their own ship’s progress more efficiently. Both radar and AIS can be interfaced with ECDIS. If there is an inherent error on the target ship, the AIS symbol and radar echo of the target will not match. In such situations, always rely on the target’s radar echo, whether on the radar or the ECDIS display, not on the AIS.
Data and situational awareness
Performance standards for AIS specify that transmission of data should take place with minimum involvement by the ship’s personnel. Navigating officers are expected to use the information from the AIS to supplement data from other bridge equipment and enhance situational awareness. For example, seeing the destination and ETA of a nearby vessel provided by AIS may help the Officer of the Watch (OOW) in predicting how that vessel is navigating, especially in a Traffic Separation Scheme (TSS) or restricted waters. However, remember that this information is only as good as the source it has been taken from – and this source may not be entirely accurate. AIS should never be used as the primary means of navigation; it should only ever be used as an aid.
AIS and visual lookout
Another AIS function is to provide the OOW with rate of turn (RoT) information of the target (depending on target ship size). On a radar, the heading information of a target is updated only when the radar processing unit has interpreted a series of consecutive echoes. In other words, heading information on a radar is accurate as long as the target is maintaining a constant heading. When a target is altering its heading (whether for collision avoidance or as part of a planned course manoeuvre), the heading information provided by the radar may be unreliable for several minutes. Since information from the target on the AIS is received via its GPS, the OOW can interpret the RoT information faster in order to understand if the target is manoeuvring. In normal visibility, visual observation of the target’s heading provides clearer evidence than the heading information given by the radar. This fact supports the important principle of maintaining a look out by ‘all available means’. However, in the event of restricted visibility, this is a good example of how AIS can be used to enhance situational awareness. It is also important to understand that AIS provides ground track information for the target. Even when a vessel has stopped its engine, AIS will still be able to show its progress due to prevalence of tide or current. The significance of this when manoeuvring in close proximity of other vessels or fixed-to-ground objects, such as navigation aids or shore objects, cannot be over-emphasised.
AIS and collision risk
Rule 5 of Colregs says that a vessel should use “sight and hearing” and “all available means” to assess the risk of collision. The OOW should use AIS to help them appraise the situation. However, AIS should not be relied on solely when making decisions about collision avoidance. It should be considered an aid to navigation and should always be used together with sight, hearing, radar etc. There have been some serious marine casualties in which the use of AIS by the OOW for collision avoidance purposes has been found to be a significant contributing factor.
Strengths and weaknesses of AIS
AIS is a carriage requirement for all vessels 300GT and above sailing on international voyages (SOLAS V). AIS must be active at all times, unless the Master has a specific reason to switch it off. Traditionally this would be in the case of a security threat to the vessel. However, we are seeing more occasions of AIS being turned off to make it harder to track vessels and their operations. The vast majority of merchant vessels do have an operating system that is switched on. However, the information that AIS transmits is only as good as the inputs that it receives. This means that it is not entirely reliable 100% of the time.
Despite this, AIS does give some very valuable information that may not be available from other sources. A ship’s static information, including length, breadth, name and callsign, is always being transmitted. Voyage data, such as draught, cargo and distance, should be regularly updated by the OOW. The range of AIS is also much greater than radar, and it can be detected at distances of up to 60NM depending on circumstance. This allows it to play a vital role in search and rescue operations, for example.
AIS transponders are often fitted to small craft like yachts and fishing boats that may not be easily picked up by radar – although bear in mind that many small vessels do not carry AIS, and it is important to keep a sharp lookout. AIS can also be used in river navigation in situations where vessels may be hidden by radar due to the landscape, but can still be tracked with AIS. All of the above enhances overall situational awareness.
Conversely, AIS does have some weaknesses that must be taken into account. As already discussed, the data it produces can only be as accurate as the inputs from the other vessel. If the other vessel has a GPS error, it will transmit a position incorrectly and the radar return and AIS target will not overlay correctly. Erroneous inputs would also display CPA and TCPA’s that do not correlate with the ARPA calculations.