a thought on AIS

oldvarnish said:
I posted a couple of days ago an interesting assessment of AIS http://www.ybw.com/forums/showthread.php?463876-Class-B-AIS-pros-and-cons and there were some thoughtful replies.

However, I've notice that whenever we discuss AIS the emphasis seems to be on boasts such as "I've received ships at 100 miles and yachts at 30", as if that was the most important thing.

I'm not so certain.

When crossing the Channel, for example, AIS is God's second best gift after GPS, but I wonder if it quite as valuable in closer situations.

This summer I was approaching Peterhead at dawn in thick fog with the radar spinning. I asked the HM for permission to enter, was given it, and he advised me that there was a yacht, with an AIS transmitter, leaving the marina on a reciprocal track to me, and I should keep a sharp lookout.

The yacht was clearly seen on the radar but not on my receive-only AIS (Vesper, and very reliable - I too can get ships many, many miles away)

The visibility was about 30 metres, quite thick. I proceeded dead slow. Then the AIS bleeped a collision alarm and the yacht appeared for the first time on the screen. Less than ten seconds later the yacht appeared out of the fog. We passed port to port, waved, and all was well. I used the radar to set a course for the marina but noticed the AIS was still showing a collision alarm, and did so for some time even though I knew for certain she was by now well astern of me. I was alongside and secure before the AIS jumped and showed the yacht's true position, now a mile out to sea.

So don't worry about how far you can see, worry more about what might be so close that AIS class B is just too slow to deal with it.
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I don't think AIS is so slow as that . But certainly its not wise to rely 100% on the position shown on your screen. Possibly in the case you describe the antenna on the boat could be faulty such that your AIS receives intermittently .
 
Happy Dayz said:
after finally announcing in very stern terms he was a USA warship making his guns ready and might open fire on them, he was told he was talking to a light house keeper, and advised he should change coarse.

If true as i hope, just a Classic lol!
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It's a very old joke :)

Pete
 
prv said:
It's a very old joke :)

Pete
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Oh well, another bubble bursts. along with the winabago story and driver beleiving it was auto steer, steps in the back to makd a coffee on a freeway.

I know i heard them on a radio station - i'm never going to trust them fools again lol!

Alan
 
oldvarnish said:
I posted a couple of days ago an interesting assessment of AIS http://www.ybw.com/forums/showthread.php?463876-Class-B-AIS-pros-and-cons and there were some thoughtful replies.

However, I've notice that whenever we discuss AIS the emphasis seems to be on boasts such as "I've received ships at 100 miles and yachts at 30", as if that was the most important thing.
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I'm not aware that long reception range has ever been a "boast". It's just been cited as one of the benefits of using an active splitter and a masthead antenna, which it is.

There, of course, are other benefits / disadvantages.

Richard
 
MartynG said:
I don't think AIS is so slow as that . But certainly its not wise to rely 100% on the position shown on your screen. Possibly in the case you describe the antenna on the boat could be faulty such that your AIS receives intermittently .
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Class B AIS on a boat travelling slowly operates on a 3 minute transmission schedule. If the yacht in question had been moored close to the marina entrance, it could easily have got as far as the mouth before you saw the first AIS report.
 
My experience of using AIS in the Adriatic for the last 5 years is:

For peace of mind in shipping lanes it's fantastic. CPA info has pretty much been excellent but I still have a hand bearing compass which I use to determine if my bearing to the big floaty things is changing and to confirm if I'm going to pass in front of or behind them.

In my experience, not all ships show up on AIS.

I wish private boats would not use class B, it creates clutter in crowded areas and the courses leisure boats follow are not steady enough for useful collision alarms, CPAs etc. Best example is around Fenoliga, South of Pula where everything weaves its way through the Lighthouses, island and Hazard buoys. By all means turn it on in fog if it gives peace of mind but I think it's a pain - especially when it's all overlaid on a zoomed out plotter. Over the last 5 years class B transponders have been getting more and more popular and IMO it gets in the way of avoiding the ships which really matter. I haven't found a way of filtering out class B transmissions on my equipment, if I could, I would.
 
I can agree AIS is not reliable.
I'm in the privaliged position of having two separate AIS receivers connected to two separate AIS enabled chart plotters.

A Vesper Transever to Garmin GPS750s plotter displaying Garmin G2 blue charts and a Standard Horizon AIS VHF Matrix receiver linked to a CP500 Standard Horizon plotter displaying C-Map Max charts.

It is amazing the number of times ships / boats will be showing on one but not the other. And its not because the AIS receiver has gone down because quite often it's close by ships that will vanish while ones miles away are still being consistently shown,

For example a couple of weeks traveling ago on the Spanish coast between Sant Carles and Barcelona a designated anchoring area with around 10 oil tankers moored and a couple moving about. On this occasion all ships were appearing on the Vesper / Garmin combination, however the Std Horizon combination was only showing ships moored in the port further away in one direction and fishing boats miles out to sea in the other direction, it wasn't showing up any of the anchored or moving ships as we threaded our way through the large anchorage, passing within 100 meters or so of some.

This was the worst and most obvious example I have come across, not dangerous as perfect visibility and calm sea. There are times when this situation is reversed and the Garmin will be missing boats that are shown on the Standard Horizon.

I'm in the unique position of having the two plotter screens only inches apart which makes is obvious when differences occur. (Not just AIS but also chart differences. The two AIS systems and plotters are completely separate, not linked in any way, they have seperate antennas and even get their power from different batteriey banks, a belt and braces back up for each other.
 
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blampied said:
I can agree AIS is not reliable. I have two separate AIS receivers connected to seperate AIS enabled chart plotters.
(A vesper Transever to Garmin plotter and an Icom AIS VHF Matrix receiver linked to a Standard Horizon plotter)

It is amazing the number of times Ships will be showing on one but not the other. And its not because the AIS receiver has gone down because quite often close by ships will vanish while ones miles away are being shown,

For example a couple of weeks ago on the Spanish coast between Sant Carles and Barcelona an area with around 10 oil tankers moored and a couple moving about. On this occasion all were appearing on the Vesper Garmin combination, however the Icon Std Horizon combination was showing ships in the port further away in one direction and fishing boats miles out to sea in the other direction but wasn't showing up any of the moored or moving ships as we threaded our way through the large anchorage, pasding within 100 meters or so of many.

There are times when this situation is reversed and the Garmin will be missing boats that are shown on the standard horizon.

(The two AIS systems and plotters are completely separate, not linked in any way, they have Seperste antennas and even get their power from different batteries, a belt and braces back up for each other)
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That's a phenomenon I had not observed until I recently connected my VesperMarine AIS transceiver to my eldely C80 plotter with NMEA. The VM display has always been rock-solid and I've never seen a target appearing or disappearing on the screen. However, targets on the C80 sometimes do appear and disappear for no apparent reason although this has never happened with a target which is actually on a collision course.

I suspect that this something to do with the software rather than the actual signal reception as the VM software is much smarter than the elderly Raymarine software such that whenever a target is on a collision course I always use to the VM display as it is much more informative.

Richard
 
RichardS said:
That's a phenomenon I had not observed until I recently connected my VesperMarine AIS transceiver to my eldely C80 plotter with NMEA. The VM display has always been rock-solid and I've never seen a target appearing or disappearing on the screen. However, targets on the C80 sometimes do appear and disappear for no apparent reason although this has never happened with a target which is actually on a collision course.

I suspect that this something to do with the software rather than the actual signal reception as the VM software is much smarter than the elderly Raymarine software such that whenever a target is on a collision course I always use to the VM display as it is much more informative.

Richard
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This is a window from OpenCPN for an AIS target.

You will notice a field "Report Age". This time increases until the next report is received which then resets to zero.

This is used of a basis for removing a target that has been lost of gone out of range, together with a "lost target time out setting"

4Target-Query1.png


If therefore for some reason a transmission or 2 is lost the plotter could think that the target was lost and the piotter would remove the symbol from the screen but if the target transmission was received a short time later the plotter would reinstate the target on the screen .
 
I had a similar situation, going through the Sound of Iona in early June this year.
Thick fog visibility less than 30m, AIS and Radar running.
The AIS showed the ferry leaving Iona, and I soon picked it up on the Radar and tracked with MARPA, but the AIS target was well behind the Radar return, because of the delay caused by time interval between the AIS transmissions.
If I'd relied on the AIS we'd have been on a collision course.
However, the AIS was useful because I could see the ferry moving close to the shore where the ferries radar return had merged into the return from the coast line.
But I can see how it would give a dangerous sense of false security if the AIS was relied upon for close quarters collision avoidance.
 
Ian_Edwards said:
But I can see how it would give a dangerous sense of false security if the AIS was relied upon for close quarters collision avoidance.
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The mca agrees with you...

https://mcanet.mcga.gov.uk/public/c4/solas/solas_v/Annexes/Annex17.htm

Collision avoidance must be carried out in strict compliance with the COLREGs. There is no provision in the COLREGs for use of AIS information therefore decisions should be taken based primarily on visual and/or radar information.
 
GHA said:
The mca agrees with you...

https://mcanet.mcga.gov.uk/public/c4/solas/solas_v/Annexes/Annex17.htm

Collision avoidance must be carried out in strict compliance with the COLREGs. There is no provision in the COLREGs for use of AIS information therefore decisions should be taken based primarily on visual and/or radar information.
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AIS is only one of the tools available and as colregs also state lookout by ALL means available.

I one thing I like is with AIS and DSC radio it is possible to call the vessel in question directly or at least by name without resorting to calling "Vessel at position XX YY" or such vague statements.
 
oldvarnish said:
So don't worry about how far you can see, worry more about what might be so close that AIS class B is just too slow to deal with it.
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I responded to the last AIS thread with a tale about coming down the Chenal Du Four in fog.

It was reassuring to know that small boats were out there and to have some idea of their range and course but, as you suggest, as an absolute tool for avoiding collision class B is dodgy. Even allowing for the lag in transmission it was difficult not to expect a target to appear on the bow and a surprise to see a boat materialise on the beam, and past you before you could blink.
I am fitting a transponder, in that way both boats at least will know the full score and can act accordingly. It is not difficult to think of cases where this may make this sort of close quarter thing more tricky though... At least with one boat blind only one vessel will be taking action to avoid a close situation.

Generally, in open water, both boats are steering a fairly steady course and ranges are in terms of miles rather than yards, so things are easier.

.
 
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