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Weather Radar, Object Transparency, Reflectivity, Scatter

Discussion in 'Misc Electronics' started by W. eWatson, Feb 15, 2009.

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  1. W. eWatson

    W. eWatson Guest

    I heard a public talk about doppler weather radar today, and have some
    questions. A fair amount of time was devoted to radar signals bypassing
    targets or reflecting off them. It was pretty clever. The dependency was
    generally the relationship of the object's size and the length of a wave.
    Polarization had some effect.

    Can someone give me an idea, a simple reason or example why, for example, if
    the wavelength is much shorter than the target's size, the wave will bounce
    off it, and scatter from a larger object? Apparently, polarization occurs
    when the wave length is much larger than the object. How does polarization
    help detect certain types of objects?

    Is there a fairly introductory book to these subjects or a website? The
    presenter made himself very clear about how this works without much use of
    math. One equation.
    --
    W. eWatson

    (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
    Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

    Web Page: <www.speckledwithstars.net/>
     
  2. ruddorthner

    ruddorthner

    3
    0
    Sep 9, 2009
    The book you want is Alan Bole, Bill Dineley and Alen Wall's “Radar and ARPA Manual”. But simply, when two reflector points are separated by a quarter wave-length across the polarisation vector then the two returns from both of these point reflections will reflect constructively and reinforce each other power as they will be in-phase with each other. If the two point are separated by more then a quarter wavelength then there will be a phase shift with respect to each other from these two reflection relative to the receiver and the resultant reflection not be constructive and contain some proportion of phase cancelation and therefore have a lower received power. As for Polarisation, you wish to arrange it a long the longest vector of your target: So for weather it is vertical to line up with vertical moving water droplets in storms or rain. In marine radar it is in Horizontal to line up with boat shape of the hull that parallel to the water line. Note there are some effects for sea and ground clutter too with polarisation.
     
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