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Envisat making first direct measurements of ocean surface velocities

For more than a decade space-based radar instruments have been routinely observing ocean surface phenomena including wind, waves, oil slicks, even the eyes of hurricanes. Now employing the same principle as police speed guns satellite radar has also begun to enable direct measurements of the speed of the moving ocean surface itself.

The oceans that cover 71% of the Earth's surface are constantly in motion. Ocean surface currents can lead to strong interaction with wind and waves, and through the transfer of heat influence the weather. Their dynamic range in the mesoscale is broad, with features such as current eddies and fronts, occurrences of filaments and jets, and wind-driven coastal upwelling or downwelling.

High-resolution surface current observations on a large scale are very difficult, although satellite altimeters permit monitoring of sea level anomalies that are related to surface currents. Sea surface temperature and ocean colour measurements also throw light on ocean circulation pattern and seasonal variations.

However Synthetic Aperture Radar (SAR) instruments such as the ASAR on ESA's Envisat have now been demonstrated to potentially offer a more direct and therefore very valuable source of high-resolution information for oceanographers.

SAR sensors record microwave radar backscatter in order to identify patterns of surface roughness. Indeed, over the ocean these patterns are linked to varying surface winds and currents in particular current shear and varying convergence zones that can focus wave energy, resulting in increased wave steepening and enhanced radar-detectable roughness changes. Unfortunately, interpreting radar images to accurately identify and quantify such signatures has been easier in theory than in practice.

"Due to a general lack of sufficient high-quality in-situ observations, the understanding of how these dynamic features contribute to the complicated surface roughness modulation pattern
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Contact: Mariangela D'Acunto
mariangela.dacunto@esa.int
39-069-418-0856
European Space Agency
27-Mar-2006


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