Ocean waves and winds from complex ERS SAR imagettes

ERS SAR (synthetic aperture radar) wave mode raw data are focussed to complex imagettes of 5x10 km size approximately. For the processing DLR's research SAR processor BSAR is used. The raw data imagettes are acquired globally every 200 km along the orbit over the ocean and over sea ice, usually acquisition is switched off over land, although a few scenes are available. Up to now the detected imagettes were further processed by ESA to so called UWA spectra, a dataset from which ocean wave spectra could be calculated using SAR inversion methods. Chapron and Kerbaol used some datasets of real valued imagettes to derive wind speed. The new complex imagettes give the opportunity to derive two looks separated in time by approximately half a second. Using these multilook techniques newly developed algorithms are taken to infer propagation direction of the waves, to calculate wind speed and to derive ocean wave spectra. The results are compared to buoy measurements, the WAM model and the Hasselmann algorithm, which is presently state of the art at the ECMWF. The cross correlation algorithm is used to derive wind speed. The algorithm relates the width of the cross correlation of two separate complex looks, which is a measure of the energy contained in the high frequency part of the ocean wave spectrum, to wind speed. Complex imagettes will be available from the ASAR on ENVISAT therefore these investigations are an ideal preparation for the planned operational use of ENVISAT imagette data at meteorological centers like ECMWF. Using the imagettes instead of spectra additionally yields the opportunity to study ocean surface features.