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Gary:

You seem to know a lot about radar reflectors, so I'm surprised you
didn't mention what many consider the most crucial aspect of radar
reflector construction:  that the angles between the plates must be an
accurate 90 degrees.  Since the radar signal bounces off one plate and
then another, only by having a very accurate 90 degree corner is it
guaranteed that the incoming radar pulse will be returned to the radar
that emitted it!   For this reason many experts advise against home-brew
radar reflectors (I can't tell you the number of times I've read letters
to the editor of various sailing publications with people bragging about
constructing a reflector out of bits of cardboard covered with aluminum
foil).

Comments welcome.

Lu Abel

Gary LaPook wrote:
> Gary wrote:
>
> I read this entire report which was very interesting. What will put the
> fear of god in you is the finding that even with their Davis radar
> reflector mounted, that the probability of detection was still minimal.
> Allot of sailors rely on those things.
>
> I looked at the issue of radar cross section in the past and built my
> own radar reflector. The standard reflector, like the one in this
> incident consists of three mutually perpendicular plates which form 8
> corners so as to reflect radar energy back in the direction that it came
> from, these corners are aligned to cover the horizon. Unfortunately,
> there are deep nulls in the reflective pattern so there are many gaps in
> the pattern.
>
> The Davis reflector on this boat is 12.5 inches in diameter meaning that
> the edge where the plates meet to form each corner is only 6.25 inches.
> Only one of the corners is effective for any particular radar beam so
> the rest of the reflector has no value, the strength of the echo is
> determined by the one corner or "bucket" facing the radar transmitter.
> If you do the calculation for a bucket with a 6.25 inch edge circular
> edge (such as the davis) for x-band radar (9.4 ghz, lambda or wavelength
> of 3.2 cm) you find the radar cross section has a maximum value of  9.7
> meters ^2. If you do the same calculation for an s-band radar (3 ghz,
> 10cm wavelength), the kind used by large ships when off soundings,  you
> find only 1.0 meters^2 since the effectiveness of a corner reflector
> varies inversely with the square of the wavelength.
>
> The effectiveness also varies with the shape of the plates and with the
> length of edge where the plates are joined. A corner reflector made of
> plates that are square rather than round, as in the davis, returns an
> echo 2.42 times as strong than the round plates. But the main
> determinant of strength is the length of the edge. The strength of the
> echo varies with the length of the edge to the fourth power. If you
> double the length of the edge you increase the echo 16 times! So if you
> combine these two factors and make a radar reflector with square plates
> that is twice as big you increase the strength of the echo by almost 39
> times. So if you make a reflector that has 12.5 inch buckets the RCS is
> 379 meters^2 for an x-band radar and 38.4 at s-band.
>
> O.K. this would be too big to hoist aloft.
>
> So what I decided to do was to make a radar reflector with just one
> bucket that I could aim at any ship that I thought might be a hazard.
> This would be much more effective than shinning a flashlight on the
> sails since it would set off the automatic alarm on the radar scope.  I
> got 3 sheets of aluminum 12 inches square and joined them with two
> hinges so that it can fold flat when not in use. It is a simple matter
> to take it out and unfold it and aim it at the ship. Opened, it is
> slightly smaller than the Davis radar reflector but returns an echo in
> the direction of the radar of 318 meters^2 at x-band and 32.6 meters^2
> at s-band, thirty three (33) times stronger than the standard radar
> reflector.
>
> I haven't been run over yet.
>
> gl
> George Huxtable wrote:
>
>
>>Fatal interaction between yacht and ferry.
>>
>>There's quite a lot to ponder, in the highly workmanlike report of the UK's
>>Marine Accident Investigation Branch, into the sinking last Summer of the
>>yacht "Ouzo", off the Isle of Wight, resulting in the death of all three
>>aboard her.
>>
>>Go to-
>>
>>http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm
>>
>>and from that download the fifty-odd page report Ouzo_.pdf at about 1.4 MB.
>>Other options are a short synopsis, and a bulky technical appendix, mainly
>>about photochromic spectacles and radar detection of small craft.
>>
>>The MAIB brief is to find out what happened, and it seems to me they have
>>bent over backward to avoid apportioning blame.
>>
>>The second officer of the P & O ferry, "Pride of Bilbao", is due to stand
>>trial for manslaughter, later this year, as a result.
>>
>>My reaction, and I suspect that of many other sailors, is to think "There,
>>but for the grace of God, go I". And no doubt many ship's officers will,
>
>>from another viewpoint, think exactly the same, .
>
>>It surprises me that the master, though he was below at the time, appears
>>not to be bearing any of the responsibilty.
>>
>>I won't comment further, to avoid prejudicing those that prefer to make up
>>their own minds. Read it yourself.
>>
>>George.
>>
>>contact George Huxtable at george@huxtable.u-net.com
>>or at +44 1865 820222 (from UK, 01865 820222)
>>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>
>>
>>
>>
>>contact George Huxtable at george@huxtable.u-net.com
>>or at +44 1865 820222 (from UK, 01865 820222)
>>or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
>>
>>
>>
>>
>>
>>
>
>
>
> >
>

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