This satellite mission will use new algorithms to try to forecast weather and estimate climate change from satellite measurements of the Earth's surface.

International audienceMicrowave radiometry at low frequencies (L-band: 1.4 GHz, 21 cm) is an established technique for estimating surface soil moisture and sea surface salinity with a suitable sensitivity. However, from space, large antennas (several meters) are required to achieve an adequate spatial resolution at L-band. So as to reduce the problem of putting into orbit a large filled antenna, the possibility of using antenna synthesis methods has been investigated. Such a system, relying on a deployable structure, has now proved to be feasible and has led to the Soil Moisture and Ocean Salinity (SMOS) mission, which is described. The main objective of the SMOS mission is to deliver key variables of the land surfaces (soil moisture fields), and of ocean surfaces (sea surface salinity fields). The SMOS mission is based on a dual polarized L-band radiometer using aperture synthesis (two-dimensional [2D] interferometer) so as to achieve a ground resolution of 50 km at the swath edges coupled with multiangular acquisitions. The radiometer will enable frequent and global coverage of the globe and deliver surface soil moisture fields over land and sea surface salinity over the oceans. The SMOS mission was proposed to the European Space Agency (ESA) in the framework of the Earth Explorer Opportunity Missions. It was selected for a tentative launch in 2005. The goal of this paper is to present the main aspects of the baseline mission and describe how soil moisture will be retrieved from SMOS data

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| 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