Accuracy of wind field deduced from Envisat WSM SAR images along range

Since several years the Synthetic Aperture Radar (SAR) is used as a reliable tool for sea surface observation. SAR provides information about the sea surface roughness generated by wind. In order to extract wind field from SAR images the direction and the intensity of the wind vector must be considered separately. Particularly the well-established methods for determining the wind intensity required the wind direction as input. We analysed the Envisat WSM images: the wind direction has been deduced applying the Continuous Wavelet Transform (CWT) analysis to the periodic structures of the image. The obtained direction has been used as input of the CMOD5 model to calculate the intensity. The large swath width of an Envisat WSM image corresponds to incidence angles ranging from 15 to 45, involving specular and Bragg scatter mechanisms; the Normalised Radar Cross Section (NRCS) depends on the position along the range, on the wind intensity as well as on the angle between the radar beam and the wind direction. Here we present an analysis concerning the accuracy of the wind field deduced from Envisat WSM SAR images as a function of the position along the range. We have compared the data obtained in different position along the range, of about 100 images, with external data of wind. The accuracy of the outputs of the wind as a function of the different zones of the range are presented and the backscatter mechanisms and sensitivity concerning NRCS are discussed.