Doppler characteristics of sea clutter. Page: 25 of 39
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3.2.2. High Grazing Angles
In or der t o e xamine t he D oppler s pectrum a t hi gher grazing angles, a irborne s ystems a re
typically us ed, s ince 1 and-based s ystems t end t o be 1 imited t o s hallow gr azing a ngles. For
grazing a ngles a bove 1 0 de grees, t he f ollowing obs ervations hol d. The ba ndwidth of t he
Doppler spectrum tends to minimally decrease with increasing frequency and grazing angle [9].
According t o reference [9], which di d a s tudy at X -band ( 10.1GHz c enter frequency w ith
200MHz bandwidth) at 30-degree grazing, sea clutter is still composed of all three scattering
components. However at middle grazing angles (from about 20 to 70 degrees), Bragg scattering
is m uch s tronger t han whitecap and burst s cattering. Burst s cattering occurs ma inly in the
upwind direction, but also randomly at several azimuth angles that are not upwind. Bragg and
whitecap amplitudes a re s imilar, but the w hitecap scattering time is mor e s pread-out f or
horizontal polarization. For vertical polarization, the Bragg scattering is much stronger with a
smaller temporal spread. T his results in decorrelation times for horizontal pol arization being
slightly less than that of the vertical polarization (by -2ms). Furthermore, the decorrelation time
changes s lightly f or horizontal a nd ye rtical pol arizations as a function of a zimuth. A 11 ot her
observations of the D oppler s pectrum t end t o be s imilar in be havior t o the 1 ow g razing angle
case. For example, Figure 9 shows a 0.2 Hz spectrum of sea clutter at X-band with a 35 degree
grazing angle and vertical polarization from reference [9]. The wind speed was 8 m/s (or about
15.6 knots). T he w avelength at X -band i s approximately 0.03 m. W e therefore expect our
Doppler spectrum bandwidth and shift from the discussion in section 3.2.1 to be:
2(0.15) U
WDoppler VV - ~ 80 Hz, and
Radar
fDoppler VV 2[0.25+0.18 U] 113 Hz,
radar
which can be verified in the figure where the actual Doppler shift is supposedly -116Hz. Note
that t he 1 arge Doppler shift at about 10s econds in the spectrum is due to a br eaking wave.
Likewise, we expect a Ku-band radar at a wavelength of about 18mm with vertical polarization
to have spectrum characteristics when interrogating the ocean upwind with a 15 knot wind speed
of: WDoppler _VV 129 Hz and fDoppler _vv ~ 182 Hz.25
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Raynal, Ann Marie & Doerry, Armin Walter. Doppler characteristics of sea clutter., report, June 1, 2010; United States. (https://digital.library.unt.edu/ark:/67531/metadc1014834/m1/25/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.