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| Titel |
Two dimensional estimates from ocean SAR images |
| VerfasserIn |
J. M. Caillec, R. Garello, B. Chapron |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 3, no. 3 ; Nr. 3, no. 3, S.196-215 |
| Datensatznummer |
250001022
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| Publikation (Nr.) |
 copernicus.org/npg-3-196-1996.pdf |
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| Zusammenfassung |
Synthetic Aperture Radar (SAR)
images of the ocean
yield a lot of information on the sea-state surface
providing
that the mapping process between the surface and the
image is
clearly defined. However it is well known that SAR
images
exhibit non-gaussian statistics and that the motion
of the
scatterers on the surface, while the image is being
formed,
may yield to nonlinearities.
The detection and quantification of these
nonlinearities are
made possible by using Higher Order Spectra (HOS)
methods and
more specifically, bispectrum estimation. The
development of
the latter method allowed us to find phase relations
between
different parts of the image and to recognise their
level of
coupling, i.e. if and how waves of different
wavelengths
interacted nonlinearly. This information is quite
important
as the usual models assume strong nonlinearities
when the
waves are propagating in the azimuthal direction (i.e.
along
the satellite track) and almost no
nonlinearities when
propagating in the range direction. In this
paper, the
mapping of the ocean surface to the SAR image is
reinterpreted
and a specific model (i.e. a Second Order Volterra
Model) is
introduced. The nonlinearities are thus explained as
either
produced by a nonlinear system or due to waves
propagating
into selected directions (azimuth or range) and
interacting
during image formation.
It is shown that quadratic nonlinearities occur for
waves
propagating near the range direction while for
those
travelling in the azimuthal direction the
nonlinearities, when
present, are mostly due to wave interactions but are
almost
completely removed by the filtering effect coming
from the
surface motion itself (azimuth cut-off). An
inherent
quadratic interaction filtering (azimuth high pass
filter) is
also present. But some other effects, apparently
nonlinear,
are not detected with the methods described here,
meaning that
either the usual relation developed for the Ocean-to-SAR transform is
somewhat
incomplete, although the mechanisms leading to its
formulation
seem to be correct, or that these nonlinearities
cannot be
detected in the classical bispectrum theory. |
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