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SMOS, a L Band radiometer using aperture synthesis to achieve a good spatial resolution, was successfully launched on November 2, 2009. It was developed and made under the leadership of the European Space Agency (ESA) as an Earth Explorer Opportunity mission. It is a joint program with the Centre National d'Etudes Spatiales (CNES) in France and the Centro para el Desarrollo Teccnologico Industrial (CDTI) in Spain.

SMOS carries a single payload, an L band 2D interferometric, radiometer in the 1400-1427 MHz h protected band. This wavelength penetrates well through the vegetation and the atmosphere is almost transparent enabling to infer both soil moisture and vegetation water content over land and sea surface salinity over the oceans. SMOS achieves an unprecedented spatial resolution of 50 km at L-band maximum (43 km on average) with multi angular-dual polarized (or fully polarized) brightness temperatures over the globe and with a revisit time smaller than 3 days.

SMOS as been now acquiring data for over one year after the end of the commissioning phase. The data quality exceeds what was expected, showing very good sensitivity and stability. The data is however very much impaired by man made emission in the protected band, leading to degraded measurements in several areas including parts of Europe and of China. However, many different international teams are now addressing cal val activities in various parts of the world, with notably large field campaigns either on the long time scale or over specific targets to address the specific issues. These campaigns take place in various parts of the world, in different environments from the Antarctic plateau to the deserts, from rain forests to deep oceans. Actually SMOS is a new sensor making new measurements paving the way to new applications. However it also requires a very fine analysis of the data so as to validate both the approach and the retrieval quality, as well as for monitoring the evolution of the sensor. To achieve such goals it is very important to link efficiently ground measurement to satellite measurements through field campaigns and related airborne acquisitions as well as with other existing sensors.

This paper thus gives an overview of the science goals of the SMOS mission, a description of the main mission elements, and a foretaste of the first results including performances at brightness temperature as well as at geophysical parameters. It will include how the grond campaigns were elaborated to address the main cal Val activities accounting for SMOS specificities, in what context they were organized as well as the most significant results.