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Old 19 December 2005, 15:24   #61
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OK. Thanks for that Des. But, I don't see why the target would look enlarged more than normal. I have a feeling something else was happening in your example. The 3 boats may have given targets large enough to join on the display or the side lobes of the scanner beam were active on the targets or, perhaps, the gain was set a little too high for that situation and the targets smeared into each other. There are many possibilities to consider before presuming the enhancer is the cause.
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Old 20 December 2005, 10:46   #62
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Quote:
Originally Posted by jwalker
.......There are many possibilities to consider before presuming the enhancer is the cause.
JW Im confused (isnt the first time and wont be the last ) If you look at a large ship on your radar it generally gives you a large bright blob, if you look at an islet of a similar size it gives you a large diffuse blob. A boat with a radar reflector gives you a larger blob than one of a similar size without. The brightness is all down to the quality of the returning signal and not the size. Or have I got it wrong Des
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Old 20 December 2005, 11:06   #63
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Quote:
Originally Posted by Scary Des
JW Im confused (isnt the first time and wont be the last ) If you look at a large ship on your radar it generally gives you a large bright blob, if you look at an islet of a similar size it gives you a large diffuse blob. A boat with a radar reflector gives you a larger blob than one of a similar size without. The brightness is all down to the quality of the returning signal and not the size. Or have I got it wrong Des
Des,

It's both. A big flat sided tanker will give a large, high quality return from loads of surfaces and will appear as a bright big blob because it's reflection back to the source is so big, from a load of different places on the hull/superstructure. A smaller ship will reflect less of the energy back to the source - smaller blip.

It's the opposite to some of the stealth technology - think of that Stealth Fighter with all the facets on it to send the radar energy anywhere but at where it came from - or a modern warship where the topsides are canted inwards to reflect surface radar upwards rather than back at the target.

An island wouldn't reflect as well because of what it is made of but would still give you lots of returns from different surfaces just not strong (bright) ones - but gives enough of a return that you can see it on a screen. If the Isle of Wight was made of steel, you'd struggle to see anything else on radar in the entire channel!!

If you look at a coastline on a radar screen, do you get 'speckling' along the coast from stronger returns amongst the weak 'land' returns (i.e. cars, or buildings etc) ?

D...
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Old 20 December 2005, 11:28   #64
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Not sure if the following helps clarify at all.

The test specifies the brightness of the blip increased not the size. This was in 1998 but I'm guessing the current models use the same principle.

http://www.ussailing.org/safety/Stud...arenhancer.htm

"Seventeen of these responses rated the improvement as "much brighter," while the six others chose "brighter" as their subjective qualifier."
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Old 20 December 2005, 12:56   #65
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Des, I'll reply a wee bit later....just had a big glass of wine with me tea....
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Old 20 December 2005, 13:01   #66
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Originally Posted by jwalker
Des, I'll reply a wee bit later....just had a big glass of wine with me tea....
I think Ill join you
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Old 20 December 2005, 15:49   #67
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....coming back through Hurst Narrows one afternoon we couldnt understand why we could see three yachts about a mile ahead of us through the window and only one large one on the radar. Once they passed the narrows they went there separate ways and became separated blips again on the radar even though one remained far bigger than the others, someone suggested that this was due to AR.
Des, if you already no what I'm going to say then just ignore me. Radar scanner beams have a width and they are slightly diverging. Mine has a beam width of approx 4. Let's say a target is side on at 2 miles and it represents a viewing angle of 6. The 4 beam will begin to see it 2 early, it will scan by and it will still see it 2 after. Therefore, the total size of the target painted on the screen will be 2+6+2 degrees, 10.

In your example of the 3 boats getting closer together as they pass through the narrows, there will come a point where the images on the screen will overlap but the boats will still be 4 apart. Obviously, as the boats separate once through, they will become 3 targets again.

A very narrow target, say, a pole, will have a minimum width on the screen which equals the beam width of the scanner and this is what I would expect the radar enhancer to produce.

There are other variables which will change the width of the image on the screen and most of these can be controlled using the unit's settings. One, which I use extensively, is target enhancement and this artificially enlarges the image on the screen and it is used so one's eye will notice an otherwise small blip on the screen. If it is used where traffic is dense, it will plainly cause overlap of the images and confusion.

The wider the beam, the bigger the image on the screen.

The 63sq metre I think is misleading. My understanding is that the enhancer gives a return strength equivalent to a target of that area not necessarily a target of any particular width.
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Old 20 December 2005, 16:00   #68
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Quote:
Originally Posted by jwalker

The wider the beam, the bigger the image on the screen.

.
Actually a typical Yacht radar will have a horizontal beam width of around 6 degrees .
This will have the following effects on your picture

Less transmitted power
A wider beam will make gaps appear closed due to the example Jw gave
Targets close together will merge Sary Des's 3 Yachts

When I teach my students I mention echo stretch is to be used carefully it does enhance the picture but enlarging the picture can tend to hide details it is not recomended for pilotage and accurate navigation.
The bigger the scanner the narrower the beam the narrower the beam the better picture painted by the scanner transmitting and receiving .
Think of the size of a warship scanner
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Old 20 December 2005, 16:37   #69
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..This will have the following effects on your picture...
Except for the less transmitted power, I agree but your comments make radar appear somewhat less attractive, however, there are other things which have a bearing on the screen image and hence its usefulness.

As you get closer to your 3 boats they will separate. Similarly, approaching a gap will unmerge the edges. Lowering the gain can separate targets by dropping the weaker edge returns. Adjusting the clutter settings can also enhance the image. You do need to be careful not to mis-adjust so that things which are actually there disappear.

Because radar it essentially a ranging device gaps can be seen through, eg. a harbour can still be seen even if the front edge appears closed at that particular range. A couple of apparently joined boats can be seen past and, say, a third boat is able to be seen behind the gap of the apparently joined pair and the gap will open up as you approach.

It takes a bit of learning but it is my favourite instrument.

One other thing, which I don't understand, although I said the minimum target width will equal the beam width, this was for simplicity. I frequently have targets which are considerably less wide than the beam width. Possibly the gain level just drops off the edges but I'm guessing.

Radar can be very sensitive and it is often easy to see birds flying buy at a mile or so.

It doesn't always see the curvy, rubbery bow of a police rib following behind......
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Old 20 December 2005, 16:50   #70
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Quote:
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Except for the less transmitted power
A antennas job is to focus the transmitted signal into a narrow beam the bigger the scanner the more focused the beam wil be, it gives the effect of greater power. A narrower beam will be better for target discrimination less chance of things merging i.e entrance to rivers and targets close together buoy's and yachts.
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Brilliance
Gain
Range (typical 2/3 miles and focus on a weak target)
Tuning
is te correct way to set up your radar most people just use the auto control but you get better results if you fine tune the controls your self abit like a photographer doing manual focus
Gain set to high screen is too fuzzy turn it down to low and you risk blotting out weak targets
Sea clutter adjustment will weaken the signals around the centre of the picture .
and Rain clutter will adjust the whole of the screen and can weaken the return from gently shelving coastlines .Depending on the age of your set Rain clutter is either on or off
You are absolutley right it is a very useful aid to navigation but it is important to learn how to interpret the picture . I prefer to teach this afloat rather than in the classroom as we have lots of places in the Solent that can demonstrate beam width , Racons and parrallel indexing and of course rule 19 and the use of it and how the rules of the road change if you have radar fitted . RTE are excellent but they work on the x band and some of the big boys only have s band .
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