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The causes of ambiguities have been analysed and future
improvements have been defined:
(
1) The algorithm selects the maximum value in the Fourier
spectrum to evaluate the wind direction. This method is
very sensitive to noise. The improvement remains in
locating the “sea of wind” frequencies in the Fourier
spectrum. The “sea of wind” frequencies are represented
in the Fourier space by an aggregate of high spectral
values (see Figure 3). The wind direction would then be
computed from the “sea of wind” orientation in the
Fourier spectrum.
Dutilleux, P. 1987. An implementation of the “algorithme à trous” to compute
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a
scatterometer wind algorithm for ERS-1 SAR. IEEE Transactions on
Geoscience and Remote Sensing, Vol. 36, No. 2, pp. 479–492.
Furevik, B., and Korsbakken, E. 2000. Comparison of derived wind speed
from SAR and scatterometer during the ERS tandem phase. IEEE
Transactions on Geoscience and Remote Sensing, Vol. 38, No. 2, pp. 1113–
1
121.
(
(
(
2) The homogeneity of the wind field must be taken into
account. The wind computation direction process at a
given scale can be led by the knowledge of the values at
lower resolution.
Horstmann, J., Lehner, S., Koch, W., and Rosenthal, W. 1997. Wind fields
from ERS SAR compared with a mesoscale atmospheric model near the
coast. In Proceedings of the 3rd ERS Symposium — Space at the Service of
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Noordwijk, The Netherlands. pp. 1205–1209.
3) Other prospects remain in using a two-dimensional
directional WT (Antoine et al., 1999) to obtain the wind
direction directly by applying the wavelet transformation
process.
Horstmann, J., Koch, W., Lehner, S., and Rosenthal, W. 1998. Ocean wind
fields and their variability derived from SAR. ESA Special Earth
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4) The influence of some other aspects must be taken into
account, such as the variation of the propagating
direction of the waves with bathymetry, known as
refraction.
Kerbaol, V. 1997. Analyse spectrale et statistique vent-vague des images radar
à ouverture synthétique (ROS) — Application aux données des satellites
ERS1/2. Thèse de doctorat, Université de Rennes I, Rennes, France.
1
85 pp.
Korsbakken, E., and Furevik, B. 1998. Wind field retrieval compared with
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Conclusions
In this paper, a method enabling the extraction of fine-scale
wind fields from SAR data has been exposed. This method
makes use of advanced signal processing techniques. The
originality of the method is that wind speed and wind direction
are extracted from a single image at high spatial resolution.
Data from the wind direction extraction method have been
compared with in situ data and shown to be efficient for areas of
high wind speed. These first results establish our confidence in
the applicability and validity of the method.
Mallat, S.G. 1989. A theory for multiresolution signal decomposition: the
wavelet representation. IEEE Transactions on Pattern Analysis and
Machine Intelligence, No. 11, pp. 674–693.
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Colon, P. 2001. Comparison of SAR-derived wind speed with model
predictions and ocean buoy measurements. IEEE Transactions on
Geoscience and Remote Sensing, Vol. 39, No. 12, pp. 2587–2600.
Moreover, improvements have been exposed to avoid the
ambiguities in areas of lower wind speed.
The ability to obtain fine-scale wind fields from SAR images
has been demonstrated. The method enables the use of SAR
data for wind parameter retrieval in an operational context.
Quilfen, Y., Bentamy, A., Queffeulou, P., and Chapron, B. 1995.
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Acknowledgments
This work is supported by l’Agence pour le Developpement
et la Maitrise de l’Energie (ADEME). We would like to
acknowledge Daniele Hauser (Centre d’étude des
Environnements Terrestre et Planétaires (CETP)) for the access
to data from the FETCH experiment.
Rosenthal, W., Lehner, S., Horstmann, J., and Koch, W. 1995. Wind
measurements using ERS1 SAR. In Proceedings of the 2nd ERS
Applications Symposium — Space at the Service of Our Environment.
Publication Division, European Space Agency, Noordwijk, The
Netherlands.
Stoffelen, A., and Anderson, D. 1993. Characterisation of ERS1 scatterometer
measurements and wind retrieval. In Proceedings of the 2nd ERS-1
Symposium: Space at the Service of Our Environment. Publication
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