Performance of ENVISAT ASAR for Operational Ocean Wind Field Retrieval

Today, several scatterometers (SCATs) are in orbit, which enable to measure wind fields with a resolution of up to 25 km on a global and operational basis independent on daylight and cloudiness. The SCATs were originally not designed to measure high resolution wind fields and therefore make it difficult to measure the highly spatially variable winds, which are especially important to measure in coastal areas. Since the launch of the European remote sensing satellites ERS-l, ERS-2 and ENVISAT as well as the Canadian satellite RADARSAT-1, synthetic aperture radar (SAR) images of the ocean surface with a resolution in the order of 10 m have been acquired on a continuous basis over the last 13 years. Their high resolution together with the large spatial coverage make them a valuable tool for measuring geophysical parameters such as ocean surface winds, waves, and sea ice. All the above mentioned SARs operate at Cband (5.3 GHz) with either vertical (VV) or horizontal (HH) polarization in transmit and receive at moderate incidence angles between 15° and 50°. For this electromagnetic wavelength and range of incidence angles the backscatter of the ocean surface is primarily caused by the small-scale surface roughness (in the range of 5 to 10 cm), which is strongly influenced by the local wind field and therefore allows the backscatter to be empirically related to the wind. In this paper an algorithm for wind field retrieval from satellite borne SARs (WiSAR) operating at C-band with either VVor HH-polarization is introduced. The algorithm is applied to retrieve wind fields from the advanced SAR (ASAR) aboard the ENVISAT satellite. To demonstrate the applicability of WiSAR, SAR retrieved wind fields are compared to results of a numerical atmospheric model.