Seeing things (or not)

roly_voya

roly_voya

Well-Known Member
Joined
5 Feb 2004
Messages
1,050
Location
Pembrokeshire Wales
Visit site
This is just to illustrate the piont about radar visibility. Over the B/H weekend we abandonded plans to go to Cornwall due to bad weather and insted did some bad weather exercises and nav practice. On the Monday we where going down the river and just clearing Pembroke dock with the Mate below with all the curtains closed piloting on Radar. The radar is a commercial grade 2ft job that had shown just about every mooring bouy or other hazard down the river and can normally be relied on to spot a tender at about a mile in flat water (engine block shows up). As we headed straight for the last two channel makers a large cat came the other way on a course very close to a collision course with us. I waited for instructions to change course but as we got closer I started to get distinctly nervious. Wondering if the mate was having problems I slid the hatch back to check and asked if there where any new targets - No, all clear, was the response and we both watched in amazement as at least 45ft of cat passed winth a cable of us and sailed sereanely down the river without ever registering so much as a flicker on the radar. Don't know if the skipper was aware that they where on a stealth cat but it oppened my eyes.
 
Last Friday, this Fisheries Protection vessel was patrolling the Kentish Knock area.
Here is a picture of the boat, and also a radar picture. The interesting thing is that the boat, which you will see was very close, had the same radar image as the Kentish Knock Cardinal buoy.
Note the standard signature of a local Coaster about the same size on the 3 mile ring.
Hmm.

P1010258.jpg


P1010263.jpg
 
Yes, good to expose the myth of the all-seeing radar.

As a professional seafarer I was trained to use radar as an aid, not a solution.

GRP hulls do not reflect radar waves well, even when fitted with the standard (reasonably useless) reflectors widely sold in chandlers.

Unless/until I could afford the 'See-Me' transponder I shall always assume that my boat is invisible on anyone's radar.

Whether that makes my Beneteau a stealth ship I doubt, but we certainly sail defensively. /forums/images/graemlins/smile.gif
 
The 'size' of the return is partly related to it's distance from your radar as the beam has a width from edge to edge like a cone. The same size target will paint a bigger picture if the range is greater, provided the return itself is strong enough not to vanish at the greater range. The cat with engine(s) low down, maybe partly below the waterline might well be a good stealth shape, especially bow or stern on. Side on the cat might be better seen but grp isn't a good reflector, the mast is better but it's round shape will tend to scatter the return.

In flat calms, we can 'see' a rubber tender without engine easily and even see seagulls in the air. It doesn't take much sea at all however for these to vanish. Also, very weak targets are best viewed (or looked for) at closer ranges and the gain and tune need to be adjusted correctly to get the best picture/range, the sttings will be different on each range chosen. We watch on 6mls in open waters and drop to 3mls (or less) if needs be and periodically anyway especially with small boats about. With a small LCD set also it can help to have target expansion turned on.
 
Good explanation Robin, but the bugger never had a larger target than that, which is why I dug out the camera and waited til it got near a similar distance buoy. The coaster you see at 3 miles was close to the protection vessel, because he had gone over to have a nose, which is when I first saw the 2 visually, and they were similar size, but not target.
 
There are various controls that affect the processing of the return. Sea and rain clutter, for example. Also, there is tuning and gain. Most people leave their tuning and gain on auto but depending on whether the sea or rain clutter are engaged you can get into a terrible pickle.

When navigating blind in the conditions you were simulating (we do this for real quite often and keep a careful listen out as well) I frequently adjust the gain to check that nothing is lurking unseen. With my present set I leave the tuning on auto but they vary, you need to know the set.

In very still conditions you need the clutter controls off and then turn the gain up from time to time to see plenty of noise (= grain on the display). Under normal viewing conditions your screen should look grainy otherwise you might well be missing targets. It would be very disturbing if you could not get your radar to see the cat and it would be worth your while doing some more checks or getting a professional to check the set out.
 
Thanks for all the suggestions for adjusting the set but in this case that was not the problem, conditions where ideal, the set was on the best range, sea and rain clutter where at zero and gain was high to get clear bank definition. The radar picture was exellent clearly showing all the nav marks and a range of moored boats of different sizes and shapes, even the unoccupied mooring bouys where showing up well. The radar is, as I said, a comercial 10" CRT set with 2ft dome 6m above the water level.

It may be that the cat showed its minimum image as she was comming fairly straight at us and passed close enough to be below the radar when she was side on, she may also have had a carbon mast or other reasons for having minimal radar reflection.

Yes it is probable that with further tuning for very week targets we could have got an image of the cat but only at the expence of swamping out the rest of the picture or maybe going down to 1/2 or 1/4 mile ranges. It may be that a more practiced opperator would have spotted the target

The piont is that had it been foggy (which is what the exercise was simulating) there would have been no indication that the biggest target in the area was in fact the most difficult to see. It was an eye opener for me to see how poor an image some yachts can have and I would think the skipper of the cat was also unaware of the problem or I am sure they would have done something about it.

I would recomend that unless you have already done it next time you get a chance get someone to report on your radar signature just to make sure you don't have a similar problem.
 
Did you have the AGC on? In a close quarters situation such as a river there will be many very strong signals and the AGC, if on, will cut back. Then there is a real risk of losing a weak signal.

In these situations you cannot safely leave the set hands-off. You have to keep tweaking it. At least that's been the case with every radar I have used.
 
So far I am finding the opposite, the autocontrols are much better than I am at getting a good standard picture and work well if left on for general monitoring. Do find I can often fet better detail in specific features such as bank detail or a specific target by hand tuning but only by degrading other areas of the image. Its a lot like using an automatic camera exellent for snap shots but not so good for a backlit siluette. Having said that when I tried out some radars before deciding what to buy I found that the cheaper LCD sets did need continuose tweeking to get a good picture and even then it was difficult to interpret. This was mainly due to poor heading stability but also lack of detail gradation between strong and week targets. Would definatly say if you are going to fit it get the best you can afford.
 
Your analogy with photography is almost perfect. Going up a river your magnetron is pushing out power sufficient to give a return from many miles blasting out pulses that are being returned by objects only a few cables away. It's the equivalent of having a huge old-fashioned flash bulb going off on every picture. In a photograph, this blows the highlights. In a camera, the average light is measured and adjusted to be the right level. But the average is the sum of a very light object on one side of the frame that you are not interested in, and a perhaps much darker subject on the other side of the frame. On occasions you are forced to blow the highlights to get adequate light from the subject. As in your radar, you have got "better detail in specific features such as bank detail or a specific target by hand tuning but only by degrading other areas of the image." It has to be done sometimes.

With the radar, as with the camera, there is no setting that is good for all types of target under these extreme conditions and you are going to have to keep tweaking the gain if you want to see all targets. As I said earlier, under those conditions I keep adjusting the set to make sure that there is nothing lurking. Also try introducing some sea clutter control in close quarters as it will help reduce the returns from the very local environment (surface of the water, birds, small buoys, craft, etc.). With a decent set you can adjust the sea clutter, so just add a tad to start with.
 
[ QUOTE ]
Radar range was 1.5 miles and clearly showing the chanel bouys either side of the cat as it pased between them


[/ QUOTE ]

Pretty amazing,the cat must have been the perfect stealth shape .

On a motorboat I use with a furuno radar by going down to the quarter mile range we can pick out plastic mooring buoys ,even a group of floating seagulls.It is sobering how poorly some yachts show up even with reflectors.

You need to alternate the range to see everything.
 
Fascinating post and one that I find very interesting as I have been drawing up a shortlist of what equipment I should have on my boat when I get it.
From previous posts radar was high on my list but not so sure now. Especially if single handed not sure if should spend time fiddling with radar to get the best picture or spend my time using the mark 1 eyeball and ear
 
I find my radar a must have now, esp when single handing. You dont spend all your time fiddling with it, and mine doubles as a chartplotter so already there on the binnacle.
 
You must log in or register to reply here.

Share this page

Top