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AIS transmissions from small craft

Stuart Carruthers on the growing concern that ships are filtering out AIS Class B transmissions from small craft.

Apart from some exemptions, IMO (International Maritime Organisation) requires all ships over 300 gross tonnage and upwards engaged on international voyages, cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of their size to be fitted with AIS Class A.

However, when the IMO regulation was first implemented on 31 December 2004, it did not include a requirement to display AIS ‘targets’ on a screen. The regulation simply required a display to be capable of showing a minimum of three ships at any one time together with bearing, range and name; furthermore, even this scant information was not monitored on a routine basis.

AIS targets better displayed?

Since 1 July 2008, IMO has required the integration and display of AIS data on all new radars. Arguably this means that AIS targets are now better displayed and that AIS has a greater role in improving navigational integrity and accuracy than was initially envisaged by improving information available to the mariner.

Anecdotal evidence would suggest that many bigger vessels have now converted to equipment capable of displaying AIS data. But and it’s a big but, a significant number of existing radars may not have been upgraded, particularly on coastal shipping and it will be many years before AIS data can be used effectively for navigation on many compulsory carriage ships.

With the introduction of AIS Class B aimed at the leisure boating market, much has been written about its pros and cons. Those who have taken the time and trouble to understand these and therefore appreciate that you cannot rely on AIS transmissions being received or the correct action taken by any vessel will no doubt find that it is a very useful aid to navigation.

Growing concern that ships are able to filter out AIS Class B transmissions

However, more recently, the Royal Institute of Navigation and indeed the RYA has noted growing concern among the leisure boating community that despite the revised radar performance standard, ships are able to ‘filter out’ AIS Class B transmissions from small craft. Such a facility is open to misuse and ultimately makes the fitting of anything other than a receiver a fairly pointless and expensive exercise.

So the question is can ships filter out AIS Class B transmissions?

I recently read that such filtering is a myth and after some research will stick my neck out and say that it is not available as part of the AIS Class A performance standard. Although I will admit the technical standard is not an easy read. I have spoken to a number of manufacturers who would confirm this view and are not themselves aware of any AIS equipment that has the facility to filter out AIS transmissions.

But and it’s another big but, the revised performance standard requiring the integration and display of AIS data on radar displays includes a requirement to be able to reduce clutter on displays so that detection performance is not adversely affected.

The same standard goes on to state that ‘Reported targets provided by the AIS may be filtered according to user-defined parameters’; to you and me that’s the man on the bridge and he is not going to be drilling down through layers of menu levels to find the filter!

Requirements for filtering are not explicit

The problem with the revised performance standard is that the requirements for filtering are not particularly explicit, and appear to have been left to manufacturers to decide upon. Certainly, a number of manufacturers have incorporated Class B filtering on their products and I currently write this from a vessel that has this feature available at the flick of a switch!

According to Professor Andy Norris, Ocean Yachtmaster™ and Chairman of the International Electrotechnical Commission Technical Committee responsible for international equipment standards for ship borne navigational and communications equipment, ‘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’.

AIS map


Clutter likely to become a significant distraction

Clearly as more and more leisure boaters invest in low cost AIS transponders, the problems of Class B clutter on navigation displays for those navigating large vessels is likely to be a significant distraction.

Under such circumstances, filtering of all AIS Class B targets and supressing alarms might be necessary to avoid distracting those on the bridge. It is a fact that the Class B AIS Update Rate is too slow and recreational craft frequently navigate too closely for Class B information to be useful to larger vessels in busy, congested and confined waters.

However, it is at these times that when the most pairs of eyes will be looking out of the bridge window!

No single bit of equipment will guarantee that you will be seen

So there you have it, modern radar displays are certainly capable of filtering AIS and the view is that it does not contravene international guidelines. However, while the filter is clearly intended to avoid distraction on the bridge under certain conditions, it is worth remembering that the man who switched the filter on may just forget to switch it off.

Which leaves the obvious question: "Is it worth investing in AIS Class B if ships are going to switch it off?"

My view is that AIS is another aid to navigation and no single bit of equipment is going to guarantee that you will be seen. However, like decent navigation lights and radar reflectors, an AIS transponder will increase your chances of being spotted from the bridge and that can only be a good thing!

This and other AIS Class B issues will be explored in a Royal Institute of Navigation -day seminar to be held at Trinity House, London on Tuesday 8 November 2011. The seminar will be chaired by Professor Norris and is supported by the RYA, Nautical Institute and Trinity House.

Furthermore, the seminar will also discuss what advice on AIS filtering and other AIS matters should be given to the International Electrotechnical Commission which is currently reviewing the ships’ marine radar standard, IEC 62388.

What are your views on AIS?

Stuart Carruthers, RYA Cruising Manager


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