Satellite-borne synthetic aperture radar (SAR) data are highly valuable not only for observing the open ocean but also for monitoring seasonally or permanently ice-covered ocean regions in the Arctic, Antarctic, and other areas such as the Baltic Sea, the Bohai Sea, or the Sea of Okhotsk. Fundamentals of sea ice monitoring by SAR that address the following questions are introduced: Which sea ice properties influence the radar backscattering signal directly? Which geophysical sea ice parameters are retrieved from SAR images? Important fields of recent sea ice observation activities, such as ice type classification, ice drift retrieval, and melt detection, are described. Modern satellite sensor technologies offer possibilities for significant improvements of retrieval methods, especially for more complex tasks of sea ice monitoring such as determination of ice thickness. These technologies cover enhancements of the SAR instruments themselves (multipolarization and multifrequency systems), modified mission designs (satellite constellations), and the combined use of different passive and active sensors operated in the optical, thermal, and microwave regimes.

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