Radar reflection

[bedouin]

Re: On reflection

If you really want a serious answer - then it is "NO" it would not work.

The key property of a good radar reflector is that it reflects the incident radar signal back towards the originating transmitter. To get the same size image with a random metallic surface would require 10m2 of metal of sufficient thickness to reflect the signal.

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[jimi]

Re: On reflection

I think we should arrange a test.

Two identical boats, one dressed and one plan and then see which one is most visible

<hr width=100% size=1>.. whit way roon should it be again ..

 

[MainlySteam]

Re: On reflection

I know some toy radars cannot see anything much at all, but buoys with an metallic area very much less than 10m2 are easily seen by radar. In fact some things with no metallic area at all can be seen eg waves.

The propellor on a prop aircraft has a very small area indeed, but it is usually the part of the aircraft having the largest radar cross section.

Hope you are not telling me I did all my mast climbing today for nothing /forums/images/icons/smile.gif?

John


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[ccscott49]

they do have them available already, you can buy them for inclusion in your raft, when serviced or purchased. Mine has one, I got it from thr raft supplier. Why would you want it to tow the raft, might be in the wrong direction?

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[bedouin]

Re: On reflection

Well - if it wasn't an Echomax then you probably did

10m2 is the size (surface area) of the metal sphere that would give the same reflected image. with a simple shape it is only surfaces that are at exactly 90 degrees to the incident radar beam that get reflected back at all so a small piece of metal can give a large image if (and only if) it is pointing in exactly the right direction.

I guess (but I don't know) that the reason a propellor gives a good radar reflection is that because it is spinning, there is always one part that is acting like a mirror pointing in the right direction.

Radar reflectors work in a similar way to cat's eyes in road in that they focus the beam back to the sender and so give a much greater reflected image.

The particular property of a good radar reflector is that it gives a good reflection from any direction.

Most buoys these days have a radar reflector fitted as standard, so they will show up in the same way as a boat would with a similar device; and if you get close enough anything will show up on radar.

In order for a ship to avoid you you have to present a radar image that is strong enough not to be filtered out with the wave clutter, and reliable enough to be tracked by the automated system a few miles out. I don't think that would necessarily apply to most GRP yachts without a reflector in any but the calmest of conditions.


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[Sybarite]

Towing as on surf boards where you can control direction.

John

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[Talbot]

Re: On reflection

Sorry to have to agree with you - you are correct - what you wrote is utter rubbish /forums/images/icons/smile.gif Actually not entirely. your 4 x 10m lengths will provide good reflection, but very little of it will be reflected back to the receiver. There is a possibility that in one specific direction, you will get a good return however the elevation of that is unlikely to be correct, and in more than 90% of the azimuth you will get zero. The whole point of lots of small bits is that:
the same length as the wavelength will provide the most reflection,
the scattering effect will provide a reflection from all round the cloud so direction of arrival is irrelevant, sufficient echo will be reflected back in both azimuth and elevation.

I am sure that it will not surprise you to know that the principle of the chaff cloud is well known and understood with scientific investigation and trials dating back to the second world war.

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[ccscott49]

you think you can cintrol the direction?? Hahahaha! Have you seen these guys, up and down the beach they go.

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[mickshep]

Re: On reflection

At the end of the day, the tried and tested self assembly flat plate type hoisted correctly must be one of the best, Most of the modern designs are as you say just copies with nice covers, I have a steel 32' sloop which I would hope gives a pretty strong return, but still have an 18" octahedral reflector permanantly rigged on the back stay. Having said that I still rely on the mk1 eyeball. If you assume everyone else is pissed/asleep/incompetent you can only be pleasantly surprised. Mike.

<hr width=100% size=1>"Just because I'm paranoid doesn't mean they're not out to get me."

 

[Talbot]

Re: On reflection

I view sailing as rather like motorbiking, always assuming that the other guy is incompetent and going to do what he should not /forums/images/icons/crazy.gif

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[Guest]

When I used to lighter off from MIssissippi at LOT .... occasionally USAF jobs would go over .... one time the foil was dropped ... actually it was like a small bomb that went off and this foil sort of spread out .... on the radar it was like a handful of rice in an area - it didn't actually block out strong returns ... but fazed it all up ....

The other point is the foil is rotating moving in 3 dimensions as it falls creating many different faces to the radar signal. The rope idea wouldn't be able to do this to the same extent ....

Remember it was not so long ago that someone actually in the mags put foil all around inside the yacht .... signal return was negilgible increase if I remember right ...

Good idea but needs work I think ...


<hr width=100% size=1>Nigel ...
So WHAT does the EU really stand for ????/forums/images/icons/cool.gif

 

[Guest]

And first lightning strike ?

nm

<hr width=100% size=1>Nigel ...
So WHAT does the EU really stand for ????/forums/images/icons/cool.gif

 

[Rabbie]

Re: On reflection

Why not? See - http://vintagesail.com/videos/Toyot on Para

<hr width=100% size=1>There is nothing wrong with sex on television. Unless you fall off!..

 

[ccscott49]

I always at sea assume nobody can see me, even in bright daylight, and act accordingly.

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[mickshep]

Re: On reflection

Exactly where I learned the same painfull lesson! With the gravel rash scars to prove it. Mike

<hr width=100% size=1>"Just because I'm paranoid doesn't mean they're not out to get me."

 

[MainlySteam]

Re: On reflection

<<<always assuming that the other guy is incompetent and going to do what he should not>>>

And after many years of experience one comes to learn that it is best to assume that both ones self and the other guy are both incompetent.

John

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[MainlySteam]

Re: On reflection

What I am interested in is where the 10 m2 of metal that has been quoted by a couple of posters as being necessary for painting on a radar comes from.

I am used to radars which will paint a buoy (without a radar reflector) very much less than 10m2. Similarly with radars I have been involved with on land, unless filtered out on speed, they will track cars and trucks on the road many miles away if the elevation of the road is sufficient to get them into the main part of the antenna's pattern, and they are much less than 10m2 in area. Most small aircraft paint just fine on primary radars (although they do have a spinning prop)?

As I also said I am used to radars that will paint non metallic things to - so no metal cross section at all. An easy example of that we are all familiar with is the common measurement of cricket ball bowling speed with radar (but I don't know what is inside cricket balls these days - perhaps Christmas tinsel /forums/images/icons/smile.gif.

I mostly understand why all these things are achievable, they are just examples which question the 10 m2 that has been quoted - I don't understand what basis that has in scientific fact.

John

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[bedouin]

Re: On reflection

The 10m2 figure is the minimum standard radar reflectors should comply with where the fitting is required by regulation (e.g. RORC or coded boats). I don't know how it is derived, but I know that many experienced ships officers say that yachts are hardly visible in adverse conditions.

That doesn't mean that the radar reflector needs to have 10m2 of reflective material (clearly they don't) but it must give the same radar image as a sphere of surface area 10m2 (i.e. about 90cms radius - not really that large). Where a metal object e.g. a buoy is predominately perpendicular to the radar beam it will give a much stronger image than the sphere would.

I am not a radar expert but I think that exactly the time when you need to be visible to radar e.g. in rain, fog or heavy seas is when radar is at it's least effective. Rain greatly reduces the effectiveness and sea clutter many mean that the sensitivity is also reduced.

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[MainlySteam]

Re: On reflection

The radar cross section of an aluminium sphere with a surface area of 10 m2 is actually only around 0.6 m2. The inference given by the earlier post that 10 m2 (not by you Bedouin as I think you mostly qualified it by stating a sphere) is very misleading. For things like chafe and reflection off other real objects, including Christmas tinsel, their radar cross section is dependant on their dimensions (in radar wavelength terms as well as physical), shape and motion (eg rotating propellor, your car wheels when snapped by a radar operated speed camera), and is not directly related to the metallic area of the surface of the reflecting object.

Whether Jimi made his post completely in jest or not I do not know. But either way, behind it is the possibility of an interesting case.

My 100 m of Christmas tinsel /forums/images/icons/smile.gif would probably have an area (surface, not radar cross section) of around 2.5 m2 if all the tinsel strips were aligned the same way, but would not have a radar cross section anything like this for a 10 cm radar as the foil is not close to resonant and is shorter than half a wavelength. But it might be interesting to see how it (or something similar) performed with a 3 cm radar as the little strips of foil would be approximately equal to or longer than 1/2 wavelength. However, even if Christmas tinsel did not work well, it is easy to imagine the same concept being applied using pieces of metallicised plastic of approximate resonant dimensions strung on a rope and that may give a far more interesting result. If ships tend to turn off their 3 cm radars while in the English Channel, for example (and I couldn't comment on that), then one would have to use a bushier rope in order to be seen with 10 cm radar.

Wouldn't suggest it for everyday use, even if it works, but for places like the Channel where fog can be a problem, a hoist of such on the flag halyard or whatever (as well as a proper reflector) may be interesting. I wonder if it has been tested on small boats?

So, I have spent today back down at the boat adding Christmas Tree lights up the shrouds and stays to go with the tinsel. We don't have to worry about fog here (maybe get some very small banks about one short couple of hours period every 2 years or so) but for those dark nights I will be lit up "like a Christmas tree"!

John


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[bedouin]

Re: On reflection

When you say 0.6m2 do you mean that it is equivalent to a flat surface of that area placed perpendicular to the radar beam? If so then you have to question the claims of the manufacturers of radar reflectors claims for their products since I don't think a passive reflector can out perform a flat surface normal to the beam, so the best an octohedral reflector can do is it's surface area - which is typlically only .25m2. These little tubes you can buy, only about 5cms diameter, are therefore a bit of a joke.

All comments about radar reflectors only relate to their use on the 3cm band - tests show that none of them are any use for 10cm radar - so a ship out there navigating on 10cm is not going to spot a yacht on radar.

As far as tinsel is concerned, firstly I doubt whether the plasticy stuff it is made of actually has any impact on radar at all - I think the beam would probably just pass straight through.

Even if it were reflective then I think the alignment of the strips would be counter productive since the first strips the beam encounters would be set at an angle and very little would get through to those which would reflect back to the transmitter. After all tinsel is tinselly because it reflects the light "randomly" rather than acting as a mirror.

It is fairly simple to do a basic experiment using light. In a darkened room shine a narrow beamed torch on to the tinsel and see how bright the reflection is

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