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Old 29-12-2010, 05:02   #1
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AIS Class B Can Be Filtered Out by Big Ship Radars

I see over on www.panbo.com that the the issue on big ships ignoring or potentially ignoring class B transmissions has received some expert opinion that basically days that yes they can. This was prompted by Steve dashew finding a " ignore class b" option on his updated software on his furuno radar. It would seem this myth is true after all.

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Old 29-12-2010, 06:51   #2
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It's worth noting that this filtering feature does not mean a class B transceiver is useless. I would still recommend a class B transceiver over a receive-only unit, or no unit.

What it does mean is that you cannot count on the AIS to ensure the ship's watchkeeper knows you're there. It is still up to you, the boat captain, to hail the ship (on DSC VHF, of course, since you have his AIS info), ensure that he is aware of you, and ensure that you will pass well clear of him.

Display filters have their place. A big ship entering a busy port will have CPA alarms going off all over the place, rendering the system almost useless, unless it can suppress smaller targets. The danger is that these filters will be left on once the ship is in the traffic lanes, or offshore, and that information isn't conveyed to the new watchkeeper at shift change. The only way we can guard against that situation (ship watchkeeper thinks he sees it all on the screen, when many targets are actually suppressed) is to take it upon ourselves, when out in the open, to call any ship that appears to present a possible risk on the AIS display and ensure that he is seeing our transmission and that his intent is clear.

The original three articles, for reference:
SetSail » Blog Archive » AIS A or B – A Disturbing Discovery
Panbo: The Marine Electronics Weblog: Steve Dashew's IMO radar, an AIS myth resurfaces
Panbo: The Marine Electronics Weblog: Class B AIS filtering, the word from Dr. Norris
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Old 29-12-2010, 07:26   #3
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Radars can also be set to ignore smaller targets if sea clutter settings are excessive, I don't think that fact renders a radar reflector useless per se. Same applies for my AIS transceiver
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Old 29-12-2010, 08:02   #4
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I turn off my AIS alarms as I enter a crowded harbor area and turn them on again when I get out in the open ocean. I can only hope that the ships do the same. Coming down the East Coast over the last month, I have had a half-dozen ships change course to avoid me at about the time they should have picked up my class B AIS signal. It ain't perfect, but the AIS system works better than radar.
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Old 29-12-2010, 15:02   #5
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yes I agree, its a good system despite some of the inconsistencies

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Old 03-01-2011, 03:06   #6
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Interesting discussion! Associated British Ports (my former employer) once carried out an analysis of their radar plot of the West Solent (near Southampton UK) on a summer Saturday. There were over 6000 (yes six thousand) radar targets, mostly yachts. Unless ships were able to filter our class B transmissions, there would be a strong case for banning any AIS transmissions from yachts. Therefore the ability to filter is right and necessary. We just have to learn how to manage it.

However, I also believe there is a need to be able to oerpate the set in a 'receive only' mode. I do not like the idea of the bureaucrats knowing where I am all the time.
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Old 03-01-2011, 04:18   #7
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The problem is that conceptually this is trying to solve one problem at the expense of "the little guy".

If filtering means that large ships do not consider the consequences of running down a small boat, then those in small boats with a safety feature which is intended to get them noticed is turned off.

If watch keeping devolves to looking at cockpit displays lots of small boats will get run down by larger boats in the years to come and AIS will end up as a safety feature for the big guys and a disaster for the little guys who place a false sense of security in these devices.

The always will be additional safety with AIS since even receive only gives the little guy the ability to identify and contact with GREATER ease and RELIABILITY the big guy who us about to run him down and the little guy can then use the CPA data HE HAD from AIS to take evasive measures.

Bottom line is that watch keepers need to watch and evaluate all the data from the environment and use it to avoid loss of life and property.

Filtering simply means to me that when you switch that on you MUST RAMP UP THE TRADITIONAL WATCH KEEPING. That sadly is not a feature of AIS, but one that a prudent mariner should have in his own "software".
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Old 03-01-2011, 04:47   #8
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Quote:
Originally Posted by GrahamMYC View Post
However, I also believe there is a need to be able to oerpate the set in a 'receive only' mode. I do not like the idea of the bureaucrats knowing where I am all the time.
You can switch all transceivers Ive seen off any time..
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Old 03-01-2011, 04:52   #9
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Visual watch essential!

DefJef.

Your last para is the vital one. I quote
Filtering simply means to me that when you switch that on you MUST RAMP UP THE TRADITIONAL WATCH KEEPING. That sadly is not a feature of AIS, but one that a prudent mariner should have in his own "software".
Unquote

The point is that in places like the Solent, if ALL yachts had AIS transmission, the system would be swamped.

I once showed the head of Maritime Safey for the European Union round the Solent. It has cruise ships, tankers, warships, ferries, high speed ferries, transiting vessels, container ships (up to about 11000teu) - all mixed up with yachts. At one point in the Solent there are 80000 ship movements per annum, excluding leisure craft. He shook his head and said "This should not work". That it works at all is a tribute to the common sense and patience of the skippers, crews and pilots, and well as to the yacht crews.

Once again, you are absolutely right with your final comment - to which I woul add that common sense has to be an ingredient too..
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Old 03-01-2011, 05:09   #10
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Quote:
Originally Posted by defjef View Post

Bottom line is that watch keepers need to watch and evaluate all the data from the environment and use it to avoid loss of life and property.

Filtering simply means to me that when you switch that on you MUST RAMP UP THE TRADITIONAL WATCH KEEPING. That sadly is not a feature of AIS, but one that a prudent mariner should have in his own "software".

In crowded waters, the AIS and ARPA CPA limits are set to minimum levels, but at the same time, the number of watchkeepers on the bridge are doubled or even tripled.
Modern bridge systems are have numerous alarms, and they really are distracting.
Good routine is that once in open waters, alarms limits and filters are reset to proper levels.
(Speaking for my self and the vessels I sail on, remember there are watchkeepers out there who should not be
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Old 03-01-2011, 07:06   #11
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This has been beaten in another thread, so I'll keep it brief. While I don't like the idea that I can be "filtered". I can see the necessity in crowded area's. One big thing to note is, in crowded bays or inlets most bigger ships are 'C'onstrained 'B'y 'D'raft, and as a CBD have the right of way under Coltregs. SO,... use YOUR AIS to stay out of the Bludi way!!! If you have AIS you have their DSC # and can call them to ask intentions. I have never heard of an instance of any big ship refusing to answer a DSC call.
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Old 29-01-2012, 06:03   #12
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Re: AIS Class B Can Be Filtered Out by Big Ship Radars

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Originally Posted by capn_billl View Post
This has been beaten in another thread, so I'll keep it brief. While I don't like the idea that I can be "filtered". I can see the necessity in crowded area's. One big thing to note is, in crowded bays or inlets most bigger ships are 'C'onstrained 'B'y 'D'raft, and as a CBD have the right of way under Coltregs. SO,... use YOUR AIS to stay out of the Bludi way!!! If you have AIS you have their DSC # and can call them to ask intentions. I have never heard of an instance of any big ship refusing to answer a DSC call.
Cap.Bill

I agree with all you say.

Save one thing.....the bloody big ships...shouldn't be able to come in full steam.

If it's one thing I see brewing , it is the size of their wake in respect to the width of the fairway.

I have been swamped by 4 in a row of super freighter, not to mention super tanker.

MY ISSUE IS:

When they are transiting a narrow fairway in proportion to their wake.......THEY NEED TO SLOW WAY BACK.

I'm all for commerce..but not to the point of citizens bedamed

constricted in seattle
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Old 29-01-2012, 07:08   #13
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Re: AIS Class B Can Be Filtered Out by Big Ship Radars

Or... you can just install a class-A transponder

ciao!
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Old 29-01-2012, 07:44   #14
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Re: AIS Class B Can Be Filtered Out by Big Ship Radars

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Or... you can just install a class-A transponder

ciao!
Nick.
Hi Nick,

That would mean the citizens don't count....

unless they we required to go twittle their thumbs.

I see em on radar, I even go out of their way...but when they come in the PS, one after another...drafting between 4 and 10 foot swells in a one mile wide fairway (i mean wakes...in the US we are responsible for our wakes....that is unless your super big) depending on how close...our thumb twittle could be up to an hour.

Yea I know trying to time 4 freighters down a 20 mile streatch... would mean some big corp or 2.... is going to loose a couple of dollars.

But hey we all have to wave something in life right?

Lloyd
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Old 29-01-2012, 08:09   #15
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Re: AIS Class B Can Be Filtered Out by Big Ship Radars

It seems to me this issue is more paranoia than reality. There are some very practical reasons why Class B alarms might overwhelm the crew (see comments too):

Panbo: The Marine Electronics Weblog: Class B AIS filtering, the word from Dr. Norris

False alarms of any type are just as dangerous because they can get ignored or switched off permanently. The goal of professional mariners is to use class B data smartly, not just block it out.

And after all, you're watching that ship on your AIS/radar right? You know its name, heading, speed and its MMSI. If there is something urgent going on, like you are becalmed/disabled, you can call the bridge directly via the MMSI and alert them. Assuming you're keeping watch....
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