Radar satellites such as ESA’s Envisat and ERS-2 maintain constant watch on the Earth’s surface, their signals able to cut through clouds, rain or darkness. This surveillance extends beyond the land to the 71% of the planet covered by ocean acquiring unique imagery of the ever-shifting face of the sea that is proving a boon to oceanographers.
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http://www.esa.int/esaEO/SEMGMCNZCIE_economy_0.html