On satellite measurements, the centroid of the measured Doppler spectrum is dominated by the platform along-track velocity relative to the rotating Earth. On ENVISAT, thanks to the very stable satellite orbit and attitude it is possible to estimate the precise satellite motion and antenna pointing contributions to this centroid. Subtracting this expected contribution from the measured centroid yields a centroid anomaly related to ocean surface motions only. For a simple target of fixed shape moving along the surface with a given velocity, defined positive (negative) when the target moves away from (toward) the radar, the so-called radial velocity can be deduced from the Doppler anomaly estimate.
The method works best for images with quasi-uniform radar cross-section at moderate to higher winds, predominantly used in this study, and yield estimates with a resolution (azimuth, range) of about 8km by 4km for WSM images with 30% overlap in azimuth. Prior to geophysical interpretations, corrections are applied to compensate along-track large cross section variations and biases are further removed using land surface references.
The measured radial velocity results of a combination of wind/waves/currents interactions. In the Gulf Stream and Aghulas area where the sea surface currents are expected to be steady and strong the wind effect is removed based on prior wind information (given by ECMWF) and a transfer function between wind and radial velocity. The radial Doppler velocity is thus expected to be mainly induced by the sea surface current. Over Europe, the wind correction is not applied to let the user discover the containt of the full Doppler velocity. As the current effects are expected to be smaller in this area, the radial velocity is expected to be mainly induced by wind here. When the radial velocity is corrected from the wind effect it is indicated in the legend. The NetCDF file always contains the corrected and uncorrected from the wind products.
Once the current product selected, in addition of the standard information related to the level 1 product, the following details about the current product are available: range of incidence angles, the processing center, software and date.
In radial surface current field, as produced in Soprano, colors stand for current radial speed intensity, and image brightness is proportional to the normalized sea surface rougness. The resolution of radial surface current field is about 4 km accross track and 8km along track. On graphical display, this is resampled on a higher resolution sea surface rougness map. All these information on the native 4x8km grid can be downloaded via a netcdf file directly available on Soprano surface current details webpage (available for any ENVISAT ASAR wide swath product).