Automatic Identification System (AIS) technology is now a mainstay of modern navigation, with its use spreading to applications far beyond those originally envisioned – some better than others. Dr Andy Norris looks at the strengths and weaknesses of modern usage of AIS, and how it might evolve in the future.
In principle, the Automatic Identification System can provide excellent navigational data and is also a unique source of information to support many governmental and private shore-based activities. The beauty of AIS is its universality. In fact, it is officially named as the Universal AIS (UAIS) but this nomenclature has not survived in everyday use, even though it has probably become more universal than ever originally envisaged. For instance, in the mid 1990s when AIS was being conceived, who would possibly have thought that AIS apps with quasiglobal coverage would be available at virtually zero cost to any smartphone user? In fact, probably few had even realised that AIS data would get used beyond ships, coastal stations and national security departments. Its present-day universality has made it of huge benefit to shipping offices and dockside facilities, as well as providing invaluable statistical data to enhance routeing facilities and to improve environmental protection. Unfortunately, use has also extended to crime-related activities, including piracy. The benefits of universality surely greatly outweigh its possible disreputable uses but it does mean that special precautions concerning AIS need to be taken by vessels in certain situations. However, despite the benefits of the present-day system it does suffer from a significant drawback that generally prevents the concept being used to its fullest possible extent. This is the relatively poor integrity of its transmitted data – which especially compromises its use as a navigational tool. When first introduced, the inherent integrity issues of AIS were underlined by numerous erroneous transmissions from ships, mainly resulting from actions or, indeed, lack of actions by inadequately trained bridge staff, in many cases exacerbated by improperly performed installations. Linked to the additional inadequacy of AIS for many aspects of navigation when used solely on an IMO-defined Minimum Keyboard and Display (MKD), the system created a very negative initial impact on many shipborne users. It is only when AIS data is displayed and used knowledgably on other navigational monitors, especially radar, that its navigation benefits become really useful.
Lack of integrity
There are a number of integrity issues. A major one is that the navigational inputs into the transmitting AIS – the dynamic data – are from individually operating sensors, which are not normally checked for consistency by any automatic system. Another is that the system is reliant upon operators remembering to correctly input voyage related data, such as the destination port. Furthermore, the static data, such as the vessel’s name, IMO number and GNSS antenna shipboard position, has to have been correctly set and maintained. Monitoring by coastal authorities has considerably improved data accuracy since the early days of AIS, particularly because many impose fines if action is not taken to correct anomalies that have been highlighted to the vessel. Even so, the system remains vulnerable to newly arising data errors and inaccuracies, which can occur at any time. Another issue that can affect the integrity of AIS is that it is technically quite easy to generate spoof signals, which, in the hands of subversive users, can potentially play havoc with the system, such as by generating false targets. All these inherent integrity issues have led IMO from the outset to caution its use for making collision avoidance decisions. AIS remains unmentioned in the COLREGs, in effect highlighting its relatively poor integrity – although Rule 7 implicitly requires AIS information to be used in assessing whether a risk of collision exists. Of course, radar also has integrity issues, such as when targets are obscured by clutter and with operator set-up problems. However, problematic scenarios are generally more easily recognised by users, not least because its operation is entirely self contained and does not rely on ‘cooperative targets’ – a highly significant advantage over AIS. Despite the real concerns for the integrity of AIS data it is perhaps ironic that its inherent accuracy and information content for vessel-to-vessel encounters normally easily beats that of marine radar. For instance, its range resolution and accuracy is typically around a few metres, compared to tens of metres for a standard radar. The latency of target turn information from AIS is normally of the order of a few seconds but is generally a minute or more from tracked radar data. AIS also gives a lot of extra information about the target that is not available to a radar based system and the use of VHF frequencies means that vessels separated by a moderate landmass can often still receive AIS signals from each other, whereas their mutual radar vision would be blocked. Of course, it is the combined and intelligent use of visual, radar and AIS derived information, together with other appropriate navigational data, that maximises the total benefits and minimises the possibility of an unnoticed integrity failing in any one system. In particular, it highlights the reliance that can be given to the perception of any actual situation, enabling decisions to take into account the degree of uncertainty. As a note of concern, some vessel traffic systems only have AIS information available to them, at least in some sectors of their coverage. Allegedly, this does not stop all VTS operators from giving ships detailed instructions based solely on such data – or is this hearsay?
AIS evolution
The future of AIS is intimately linked to that of e-navigation, which is based on relevant high integrity information always being available to ensure the safe passage of vessels. The navigational integrity inherent in enavigation thinking will ultimately ensure that the dynamic data input into AIS will be automatically assessed and perhaps even tagged with an integrity message. In addition, a separate onboard receiving system could continuously and automatically make integrity checks on the actually transmitted data from own ship. This would also flag up any nearby system attempting to spoof own ship’s transmissions. The increased integrity possible could even make AIS become the prime tool for avoiding ship-to-ship collisions and may considerably enhance the possibilities for a greater element of ‘sea traffic control’ in crowded waters. It is easy to dream up additional AIS services, especially with e-navigation in mind, and a host of new messages are already becoming available, including those for AIS Aids-to-Navigation. However, the currently allocated bandwidth will quickly become saturated in some parts of the world if all possibilities are used in earnest. In addition, as more and more AIS Class-B users come on line, they put considerable extra pressure on the network, which is likely to limit the effectiveness of these transmissions in some areas. However, it is in everybody’s interest for small craft to become AIS users, despite it requiring special techniques to reduce the display clutter that will inevitably arise. This particular topic is being specially discussed at a seminar being held by the Royal Institute of Navigation at Trinity House, London, on the 8th November. For all these reasons there is already talk about increasing the number of VHF channels dedicated to AIS. The current system is very cleverly thought out for today’s use, giving adequate performance to all users but using miniscule bandwidth resources. However, it is based on messages being compact and in the more distant future, as more and more information is considered necessary, it may require a fundamental rethink. In essence, AIS is just an automatic digital communications system that requires good availability and, in particular, relatively low transmission latency for some of its critical navigational data. Perhaps into a mature e-navigation future it can just become high priority data sent on future standard communications channels, whether satellite or terrestrial based. In principle, because the bandwidth is not so highly restricted as present day AIS, such channels offer very high levels of data security, greatly reducing the opportunities for spoofing. Aspects such as latency would have to be carefully controlled but current thinking on advanced digital communications systems recognises the need for giving data priority to certain categories of message. In fact, only a small class of AIS messages need such high priority. It is its transmission cost, rather than fundamental technology and knowhow, that prevents this prospect becoming viable in the relatively near future. But data costs are ever reducing ...
AIS evolution
The future of AIS is intimately linked to that of e-navigation, which is based on relevant high integrity information always being available to ensure the safe passage of vessels. The navigational integrity inherent in enavigation thinking will ultimately ensure that the dynamic data input into AIS will be automatically assessed and perhaps even tagged with an integrity message. In addition, a separate onboard receiving system could continuously and automatically make integrity checks on the actually transmitted data from own ship. This would also flag up any nearby system attempting to spoof own ship’s transmissions. The increased integrity possible could even make AIS become the prime tool for avoiding ship-to-ship collisions and may considerably enhance the possibilities for a greater element of ‘sea traffic control’ in crowded waters. It is easy to dream up additional AIS services, especially with e-navigation in mind, and a host of new messages are already becoming available, including those for AIS Aids-to-Navigation. However, the currently allocated bandwidth will quickly become saturated in some parts of the world if all possibilities are used in earnest. In addition, as more and more AIS Class-B users come on line, they put considerable extra pressure on the network, which is likely to limit the effectiveness of these transmissions in some areas. However, it is in everybody’s interest for small craft to become AIS users, despite it requiring special techniques to reduce the display clutter that will inevitably arise. This particular topic is being specially discussed at a seminar being held by the Royal Institute of Navigation at Trinity House, London, on the 8th November. For all these reasons there is already talk about increasing the number of VHF channels dedicated to AIS. The current system is very cleverly thought out for today’s use, giving adequate performance to all users but using miniscule bandwidth resources. However, it is based on messages being compact and in the more distant future, as more and more information is considered necessary, it may require a fundamental rethink. In essence, AIS is just an automatic digital communications system that requires good availability and, in particular, relatively low transmission latency for some of its critical navigational data. Perhaps into a mature e-navigation future it can just become high priority data sent on future standard communications channels, whether satellite or terrestrial based. In principle, because the bandwidth is not so highly restricted as present day AIS, such channels offer very high levels of data security, greatly reducing the opportunities for spoofing. Aspects such as latency would have to be carefully controlled but current thinking on advanced digital communications systems recognises the need for giving data priority to certain categories of message. In fact, only a small class of AIS messages need such high priority. It is its transmission cost, rather than fundamental technology and knowhow, that prevents this prospect becoming viable in the relatively near future. But data costs are ever reducing ...