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    Re: Fatal interaction between yacht and ferry.
    From: Paul Hirose
    Date: 2007 May 08, 20:52 -0700

    George Huxtable wrote:
    > In the case of a radar reflector, with wavelengths of a few cm., and the
    > dimensions of any mirror being only a few wavelengths, then "diffraction",
    > which some of you may recall from schooldays, plays a major part. The result
    > is that even the best reflector, unless is dimensions are absolutely
    > immense, reflects its energy in a spread-out, diffuse, maner, with a
    > beam-width of many degrees.
    
    I'm guessing that the beamwidth (radians) is on the order of 1 over the
    width of the reflector expressed in wavelengths. So at X band (3 cm
    wavelength), a reflector 30 cm aross will have a beamwidth of roughly
    1/10 radian, or 6 degrees.
    
    > But the benefit, the only benefit, that you will get from the situation of
    > reflectors being small(ish) measured in wavelengths, is that because the
    > reflection is so diffuse, there's no call, at all, to get any high precision
    > in the relative angling of the corner faces. I would go so far as to say
    > that if it looks, by eye, to be about 90 degrees between the faces, that's
    > good enough.
    
    In optics it is said that an image will not fall seriously short of
    perfection if the peak to valley errors on the wavefront do not exceed
    1/4 wave. Perhaps that applies to the geometry of corner cube reflectors
    as well. That is, if all the surfaces coincide with a perfect corner
    cube within some fraction of a wavelength, then the final reflection
    will be "diffraction limited".
    
    
    A companion report comparing the performance of several radar reflectors
      is now available at the MAIB:
    
    http://www.maib.gov.uk/publications/investigation_reports/2007/ouzo.cfm
    
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