Global measurements of wind using complex synthetic aperture radar images

A global set of single look complex (SLC) synthetic aperture radar (SAR) images is processed, using the B-SAR processor developed at Deutsches Zentrum für Luft und Raumfahrt (DLR), from wave mode raw data, which where acquired by the European remote sensing satellite ERS-2. Every day about 1000 SAR images of 5 x 10 km size every 200 km along the satellite track are available thus yielding global and continuous coverage of the oceans. This dataset is used to derive wind speed and direction.

The CMOD algorithm, originally derived by the European Space Agency (ESA) for scatterometer (SCAT) wind speed measurements, is applied on the mean value of the amplitude images using additional information on wind direction. The direction is taken from wind streaks visible on the images or respective SCAT measurements. Global statistics are derived and compared to model results of the European Center for Medium Range Weather Forecast (ECMWF). Using images instead of image power spectra, that are delivered as a standard product by ESA (the so called UWA spectra), allows in addition to study ocean surface features caused by natural slicks, sea ice, or atmospheric processes. The impact of these phenomena on SCAT measurements is discussed.

The complex SAR images give the opportunity to apply multilook techniques in range as well as in azimuth. Such methods are used to reduce speckle or to analyze the time evolution of the ocean surface cross section during SAR integration time. Possibilities of operational applications are discussed using a case study of hurricane FRAN. These operational application will be possible after the start of ENVISAT planned for mid 2001, when the ASAR data will be available in real time.