| Soil Moisture and Ocean Salinity, European Space Agency, is the first satellite mission addressing the challenge of measuring sea surface salinity from space. It uses an L-band microwave interferometric radiometer with aperture synthesis (MIRAS) that generates brightness temperature images, from which both geophysical variables are computed. The retrieval of salinity requires very demanding performances of the instrument in terms of calibration and stability. This paper highlights the importance of ocean salinity for the Earth's water cycle and climate; provides a detailed description of the MIRAS instrument, its principles of operation, calibration, and imagereconstruction techniques; and presents the algorithmic approach implemented for the retrieval of salinity from MIRAS observations, as well as the expected accuracy of the obtained results.Andrés Borges graduated in electrical engineering from the Polithecnic Universisty of Valencia, Spain, in 1989. He received a degree in business administration from UNED, Spain, and the M.B.A. degree from the Instituto de Empresa, Spain. He was a Software Engineer with the European Space Agency, specializing in software development for space robotics. In 1992, he joined EADS CASA Espacio, Madrid, Spain, and he worked during six years in several space projects as a Software and System Engineer. In 1998, he became Project Manager of the MIRAS Demonstrator Project, a technology development project for SMOS. Later, he continued as Project Manager throughout the development phases of the SMOS payload, MIRAS. At present, he is Project Manager of INGENIO, the first Spanish optical satellite. Manuel Martín-Neira (Senior Member, IEEE) received the M.S. and Ph.D. degrees in telecommunication engineering from the

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