AIS Class B Filtering
by Andy Norris, 20 December 2010
Although AIS is a highly useful system for ships and small craft the warnings that I gave in 2006 to AIS Class B users still remain. You cannot rely on your AIS transmissions being picked up and appropriately reacted upon by any vessel. There are numerous reasons for this. The knotty problem of AIS Class B target filtering is only one of these and is probably near the bottom of the list in terms of the potential issues that can cause problems.
Since 2008 all new ship radars have had to include AIS display functionality. It is an IMO requirement that means for filtering of sleeping AIS targets are included on such radars 'to reduce display clutter'. In the list of example filtering modes 'AIS target class A/B' is mentioned. The requirements for filtering are not particularly explicit, especially with regards to its interaction with automatic activation algorithms, which are themselves left to manufacturers to decide upon. It may have been at the back of the mind of some legislators that innovation by manufacturers would be the best way to evolve both filtering and acquisition strategies in these relatively early days of AIS/radar integration. Maybe, in the future, more explicit functionality could then be statutorily defined. Until then, manufacturers will be implementing their own best ideas in these areas.
The problem with AIS Class B targets with regards to filtering is, depending on scenario, you may wish to have a filter that prevents all Class B targets being shown either as sleeping or activated, or you may wish to activate filtered targets under certain special conditions. Class B targets typically apply widely different safety zones compared to interactions between Class A targets, because of their differences in size and manoeuvrability. Particularly in busy areas, small craft often pass closer to ships than is generally considered safe for ship-to-ship encounters, even though needing particular alertness by the small craft skipper.
For this reason, especially in areas that are crowded with small craft but that also have appreciable shipping movements - such as in the Solent area of the UK - it could well be the case that any activation of Class B targets will cause almost constant activation of the Closest Point of Approach (CPA) alarm on the ship - continually distracting the navigating officer/pilot. A 1.0 NM CPA may be appropriate for ship-to-ship encounters in such an area, but many small craft skippers will be quite happy approaching ships at very much closer distances. Therefore, filtering of all AIS Class B targets, together with preventing their activation, may be the appropriate strategy in such areas to avoid possibly dangerous alarm distraction of the bridge team.
Many such areas are found around the world, justifying the inclusion of such a mode in manufacturers' equipment. It should not be forgotten that the bridge windows form the most widely used navigational aid. When operating in busy areas in reasonable visibility they normally form the primary collision avoidance tool.
In inclement weather in such waters it would generally be the correct practice to switch off the AIS Class B filter, considerably improving the probability of identifying small craft in poor visibility and bad radar clutter conditions. In general, there would be fewer Class B targets in such conditions. These would naturally wish to keep a greater distance from ships and, in any case, any detrimental over-alarming of the ships system would, in these conditions, be compensated by the benefits of increased probability of target detection. Of course, in other than crowded waters in good visibility, the AIS Class B filter should generally be switched off.
No normal ship would ever want to ignore the presence of small craft. In most situations AIS Class B transmissions are a useful additional detection aid. In some circumstances, however, Class B filtering is essential to avoid unnecessary and distracting alarms.
Over the next few years it will be interesting to see the strategies that evolve for AIS filtering and activation, especially when combined with evolving radar/AIS association algorithms. Combining radar and AIS data enhances navigational integrity. Used on their own, both AIS and radar have significant integrity issues but the overall integrity of navigation is greatly improved when used appropriately together - and can be enhanced by further integration with other navigational aids. Navigation sensor integration continues to form a highly interesting and relevant research area.
- See more at: Panbo: The Marine Electronics Hub: Class B AIS filtering, the word from Dr. Norris