The Wonders Of AIS

 

I and my watchkeeping partner were on the Morning Watch on an overcast, and otherwise pleasant morning, but with only moderate visibility, and with not much in the way of traffic movements and I, as is frequently my choice, was standing on the West side of the Watch Tower thus enabling me to keep an all round lookout whilst also monitoring the AIS traffic movements as shown on the Ship Plotter. On one of my routine assessments of what AIS traffic was showing up on the screen I spotted the AIS data of a yacht on the Swashway apparently entering Portsmouth Harbour. Nothing unusual in that except that I could not make out any yacht in that given position even with the aid of my high power binoculars

However, on my next and subsequent checks of this AIS target of a yacht on the Ship Plotter I observed that the AIS target had apparently altered course to the North. This gave me no cause for concern as I just assumed that the Skipper had decided to change his approach track from the Swashway to the Inner Swashway and that she was doing a fair six knots on the incoming spring tide but that she was safe as she was well to the westward of the Hamilton Bank. Nonetheless I decided to keep a watchful eye on this target, especially as I had still not, as yet, seen her visually.

As this, still unseen, AIS target yacht approached the South side of the Inner Swashway channel without showing any intention of altering course to the Northeast to proceed along that channel, I attempted to sight her with the aid of the optic on the Southwest wall, on failing that I called Solent Coastguard on the telephone to advise them as to what I was observing on my Ship Plotter making clear my concern that if the target did not alter course very soon she would ground on The Fort Blockhouse Seawall at about six knots. They reported that they could see no such AIS target on their screens, but would I keep them advised.

Well the AIS target of the yacht, still with no visual sighting, continued on its Northerly course, making no attempt to reduce speed, until it arrived at the Fort Blockhouse Seawall in the vicinity of the Radar Tower where it continued OVER the wall and the road until it arrived in Haslar Marina and was, apparently, safely moored alongside its berth with the AIS continuing to transmit.

I recontacted the Solent Coastguard to whom I recounted all that I had seen on our Ship Plotter, with additional advice to them that my only explanation was that this had been a spurious AIS target all along and that they should stand down with my apologies for having called them in the first place. They requested that I try to contact the owners of the yacht so that they could confirm that all was well with those on board.

This I eventually achieved only, perhaps by now not surprisingly, apart from apologising to having left their AIS switched on overnight, they were all fine and safely moored at their berth where they had been for the previous 24 hours. This fact I passed on to a querulous Coastguard Officer.

The Moral of this story for Watchkeeps is to only trust their own eyes aided by binoculars and optics and NEVER believe solely what the AIS data on the Ship Plotter shows, no matter how strongly it may appear.

[Editors note:- this article was sent in by a Watchkeeper at NCI Gosport.  It looks as if the vessels GPS had lost contact with satellites somehow, and it’s eventual position fix was progressively becoming more accurate.  And of course, anyone looking at an AIS display should verify what they see by using the old Mk 1 eyeball].