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3.3.
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Validation of Geophysical Data Evaluation
The mechanism whereby a SAR images ocean waves is a topic
of considerable interest (for example [3]). As a consequence of
long imaging times for the formation of the synthetic aperture
and motion of the sea surface during this interval, the sea image
is often confused. Other factors such as resolution effects and
coherent speckle can contribute to make extraction of geophysical
data, ocean wavelength and direction, more difficult.
In order to provide an experimental tool for the data user,
a procedure has been developed by Marconi Research Centre for the
analysis of sea images generated by SAR. The theory is well
developed [4].
The procedure has been incorporated into a software package
which is now installed at ESRIN. The package corrects the image
for: :
- resolution effects,
- speckle, and
- system modulation transfer function.
The corrections are applied in the frequency domain of the image.
After correction the image can be transformed back into the time
domain, or measurements can be made on the sea wave spectrum in
the frequency domain.
The applied corrections are described briefly. After
transforming the image into the frequency domain, because of
resolution effects, the spectrum is generally dome shaped. This
shape is calculated by reference to an image where no structure
is visible. The spectral form is then the static system transfer
function, denoted by Qo(k).
À version of the function Qo(k)2, smoothed to reduce the
effects of speckle and noise, is then used to divide the image
power spectrum.
The next stage is to determine the mean speckle background,
which shows as a flat noise background over the signal spectrum.
The speckle is removed by thresholding, but alternatives are
being sought.
The final stage, before inversion, is to correct for the
dynamic modulation transfer function. This function takes
account of the different scattering mechanisms for the SAR, and
their dependences on the directional motion of the wave
patterns.
