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Wind field estimation from SAR ocean images

Two new methods for estimating wind parameters from SAR image spectra is discussed and tested. Both methods apply the azimuth cut-off strongly dependent on both the large and the small scale of the ocean wave system, where only the small scale part (typical smaller than the SAR resolution) is directly connected to the wind speed. The simplest method is similar to the correlation approach, but instead of estimating the decorrelation length the azimuth cut-off is estimated from the high wavenumber part of the SAR image spectrum. A new method for separating the small scale dependence from the large scale dependence is presented. This method is unique as it estimates both wind speed and direction without any calibration or in-situ knowledge. Both methods are tested on collocated wind and ERS-2 SAR measurements. The agreement with in-situ measurements is good, especially for the wind speed showing an uncertainty of 1.6 m/s in a range of wind speeds from 1.5 m/s to 17 m/s. It is concluded that both wind speed and direction now can be extracted from the SAR ocean images without use of any a priori information.