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Old 31-08-2015, 12:49   #76
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Re: AIS benefits

Originally Posted by Kenomac View Post

Point of clarification.

Here's a picture of the radar reflectors fitted on our Oyster and the other two Oysters pictured in previous posts. Two weeks ago, Jon made the same statements disparaging the reflector pictured when I stated on another thread that a large ferry skipper stated over the radio when contacted during a very windy night from five miles out, "no problem, we see you just fine... Don't worry about anything." We first saw him on radar, then contacted him via the AIS information on our receive only system, just to be sure he saw us.
Further point of clarification...

Until my post of a couple of weeks ago, you were not even aware of what kind of reflector you had on your own boat...


I'll let the reader decide, as to how "disparaging" my comments actually might have been ;-)

Originally Posted by Jon Eisberg View Post


Originally Posted by Kenomac

This kind. It's 40ft up the mast, don't know the brand. It's the white gizmo above the radar.
Appears to be a Firdell Blipper...

While no doubt your 53-footer produces a pretty strong return as far as most sailing yachts go, you may not be getting as much additional benefit from that one as you might think...


"The poor performance of the Firdell Blipper was surprising, given its popularity and reputation..."

"The Firdell Blipper 2107 narrowly failed to meet ISO8729 during this testing,
but showed good peak and average RCS performance. The Blipper is priced at
£130 and weighs 1.8kg; the main drawback was a RCS drop-off above an
elevation angle of 10̊."

Originally Posted by Kenomac View Post
Apparently, not all radar reflectors are created equal. Not trying to pick a fight with Jon, but I think we're each talking about two different types of reflectors, but each time the subject of radar reflectors comes up.... Out come the remarks condemning all reflectors. I just want to clear this up for some folks who might be thinking of purchasing a reflector.

I dunno, seems to me I was pretty specific about which type of reflectors I was referring to, in each instance...

After all, how likely that I would "condemn all reflectors", after I've spent a fair chunk of change to put one on my own little tub?



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Old 31-08-2015, 13:15   #77
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Re: AIS benefits

Originally Posted by ka4wja View Post
If anyone would like to see THE definitive study on marine radar reflectors, it is available on-line for free!!!
The UK gov't (UK MAIB) commissioned this study in 2006, and it was published in 2007....(this is what Jon was referring to as the "us sailing study")
I was mistaken there, what I was actually referring to in that case was the study commissioned by West Marine back in 1995, and conducted by Jim Corenman, Chuck Hawley, and Stan Honey... That was the one I linked to (along with the one in 2007 used by MAIB) in my previous exchange with Ken, in which the performance of the Firdell was found to be "poor", and "disappointing"...

Originally Posted by ka4wja View Post
3) Okay, enough about radar reflectors!!
How about we put to rest the oft-heard myth that commercial vessels "filter out" Class B targets! (sorry Jon!)
Fact #1 :
The Class A AIS transponder does NOT filter anything receives all AIS transponder data that is within range, Class A and Class B, all the time!!!
This cannot be disabled / filtered!!
I dunno, sounds like there are some out there who might disagree with that take on it...

I think we can probably agree, it likely depends on what the meaning of "filtering" is...


SetSail » Blog Archive » Class B AIS Filtering – The “Myth” Is Real

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

Class B AIS - Filtering Of Targets By Ships - gCaptain

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Old 31-08-2015, 13:29   #78
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Re: AIS benefits

Re AIS filtering:

The TRANSPONDER does not filter.
The mandatory Class-A "Minimum Keyboard Device" does not filter.
The display device (chartplotter, ECDIS, etc) may filter. Since the display device is what watch-standers are probably looking at, this is the important one.
Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
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Old 31-08-2015, 13:47   #79
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Re: AIS benefits

Originally Posted by Mike OReilly View Post

Collision avoidance is my responsibility. Just like on the motorcycle, my operating assumption is that drivers (be they cars or boats) don't see me. It's great when they do, but I have no way of knowing what they know. All I can control is me, and so I do.
Well, I would also add that I am dubious if all the sport fishers I see flying around at 30 or 40 knots in the bahamas are keeping an appropriate lookout all the time.
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Old 31-08-2015, 16:50   #80
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Re: AIS benefits

My reflector is mounted high and on the shroud very near the mast at the top on the port side. The ship was coming from the starboard side so he must have seen a small radar signature/image plus my running lights and possibly sails. It was near 10pm.

He was headed outbound and had less light pollution etc and an experience pilot onboard watching for folks not paying attention so he may have seen me visually also as I got close.

Next time I will be a lot more careful. Thimble Shoal Channel is always busy but at night the ships just aren't as visible unless you are looking closely among all the other lights. On the other hand, many times you will see no ships at all.

(But, the truckers are moving containers at least 12 hours per day everyday it seems. That's how busy it is)

Day time is different:
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Old 31-08-2015, 17:24   #81
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Re: AIS benefits

Originally Posted by Kenomac View Post
Then how do you explain the pontoon reflection in comparison to the 62ft Oyster at 11 o'clock which are a similar distance away from my boat where the Oyster appears 1/3 the size of the pontoon containing 30 or more boats? I'm not trying to be a wise guy... I'd like to know?


No probs....
Lets say you have a scanner like this Garmin 18inch
It has a beam width of 5* so lets assume a very small ( 0.0001* )but very high return target at 000*.... your display will start to 'paint' it when the scanner is pointing 357.5* and keep on painting it until the scanner is pointing 002.5* so you will get a 'blob' 5* wide... of course the 5* is smaller in width the closer you are to the target.

Leaving aside the angle at which the pontoon is lying let us assume it is at right angles to you and covers an arc of 20 *.... it will show up as an arc of 25* and so does not appear hugely bigger than it really is.
I think that makes sense to me.....hope it does to you.

Another little bit... shorter the antenna wider the beam width ( 5* in the Garmin above) .... longer the antenna narrower the beam width down to maybe 1.5* or less on big ship stuff.

This all ignores 3cm and 10cm differences.

That is a nice picture on your Furuno...CRT or LCD?
Mine is a crappy 14 yo Anritsu with a very 'soft' LCD display.....

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Old 04-09-2015, 05:54   #82
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Re: AIS benefits

I don't have a class B transponder yet, but will be getting one (operating in the lower Chesapeake), and will display the results on the Garmin 94SV GPS Chart plotter.

The Furuno NavNet VX2 GPS 10.5 plotter/Radar has a different map database, and is used to display radar returns in a GPS derived position-heading frame on the chart.

Lets say that two or more boats wanted to stay somewhat close to each other on a crossing between islands in the South Pacific. If the boats all had class B's, then maybe up to a 10 mile range you could see them in relation to you, along with their course and speed, and they could, likewise. Seems like a reasonable expectation, and a good side benefit.

You could do this with radar too, but AIS does not pull much current for what it does, (not the same range as radar, but no clutter-sea state issues), has less moving parts, and is most likely more reliable in its intended operation, having less parts.
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Old 04-09-2015, 07:43   #83
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Re: AIS benefits

Can you expand a little on the transponders, the RTE's I think you called them?
Do you know if any of them will detect the emissions of a FMCW Radar and reply?
Does your own Radar affect them?
I understand the principle, why I called them transponders, but have zero experience with them, but an active transponder will trump any passive measure I believe if the intent is to be seen.

On edit I found out my 4G Radar is indeed X band, I would have thought a higher frequency, K or Ka maybe.

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