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    Re: Fatal interaction betweeu yacht and ferry.
    From: George Huxtable
    Date: 2007 May 7, 21:50 +0100

    Gary LaPook makes several interesting points about radar reflectors.
    
    Radar target reflection is a complex business, and I'm not convinced by the
    methodology shown in that report. Some of the factors seem to have been
    thrown in based largely on guesswork, though dressed up by much computer
    simulation. Not that I can claim to do any better.
    
    But one factor seems to have been ignored, though it was given attantion in
    a study done for (I think) Practical Boat Owner, a few years ago. It's this-
    
    The radar reflection from a boat, illuminated by a coherent signal (which
    radar is), is the sum of all the reflections, that return toward the radar
    scanner, from all the different bits of the boat, summed up togeter. And
    when you sum them, you have to take account of the phase shift in each case,
    from each bit of boat, because the double-distance, from scanner to
    reflecting surface back to scanner, is different for each bit. The detected
    returned power, the integral of them all, will depend critically on the
    boat's geometry and the angle of the boat's attitude to the beam. There's
    nothing to prevent that returned signal summing up to zero, or near-zero, at
    a particular attitude, simply because that jumble of returned signals
    happens to cancel out to zero, a null-reflection, because of those differing
    phase shifts. Luckily, a small yaw in the boat, or changing direction from
    the radar antenna, will make it visible again. That's one reason for the
    complexity of radar image behaviour; the way they tend to "twinkle".
    
    But set that to one side for now, and imagine that the geometry, at a
    particular moment, is such that the boat is producing a measurable
    reflection, and makes a visible target. All those bits of the boat are
    combining together to prduce a net positive reflection back to the scanner,
    as though it was from a particular point, somewhere on the boat, which we
    can think of as the effective centre of reflection.
    
    Now hang up a radar reflector, somewhere on the boat. That, also, will
    reflect power back to the scanner. And its distance, from the scanner, will
    not in general be the same as that effective centre of reflection from the
    boat itself. The phases of the two reflections will then differ. Sometimes
    it will be that they add, enhancing the reflected power. But there's a 50%
    chance that they will subtract, fighting each other in a mutual
    cancellation, to some extent at least. In the worst case they may combine to
    give a null, but a narrow one, which only needs a small change in angle to
    defeat it.
    
    As long as the radar cross-section of the reflector alone is always
    overwhelmingly greater than that of the boat, then the boat itself can never
    reflect enough energy to nullify it, and all is well. But if reflections
    from boat and reflector are comparable in energy (and that's one thing the
    radar analysis of the "Ouzo" event seems to imply), then the simple act of
    hanging up a radar reflector can, at certain angles, nullify your radar
    reflection rather than enhance it.
    
    Not that I am arguing against using a radar reflector: not at all. I'm
    simply emphasising the complexity involved, and the uncertainty in assuming
    that a small radar target will always be seen.
    
    These problems are inherent in using monochromatic coherent energy to obtain
    a reflection. One simple approach to minimising the cancellations effects
    I've described might be to dangle the reflector from a rather slack
    halliard, so that the spacing, and thus the phase difference, changes with
    the motion of the boat. Where the radar wavelengths involved are only a few
    centimetres, it doesn't need much relative shift to turn a null into an
    addition of power (and vice versa, of course).
    
    The thoughts, above, are no more than musings about the problems from a
    non-specialist, and I can offer no textbook backup. Don't take them as
    authoritative. It would be interesting to learn, from others who know more,
    whether they contain any sense.
    
    George
    
    contact George Huxtable at george---.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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