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Capability for sea surface salinity observation was an important gap in ocean remote sensing in the last few decades of the 20th century. New technological developments during the 1990s at the European Space Agency led to the proposal of SMOS (Soil Moisture and Ocean Salinity), an Earth explorer opportunity mission based on the use of a microwave interferometric radiometer, MIRAS (Microwave Imaging Radiometer with Aperture Synthesis). SMOS, the first satellite ever addressing the observation of ocean salinity from space, was successfully launched in November 2009. The determination of salinity from the MIRAS radiometric measurements at 1.4 GHz is a complex procedure that requires high performance from the instrument and accurate modelling of several physical processes that impact on the microwave emission of the ocean’s surface. This paper introduces SMOS in the ocean remote sensing context, and summarizes the MIRAS principles of operation and the SMOS salinity retrieval approach. It describes the Spanish SMOS high-level data processing centre (CP34) and the SMOS Barcelona Expert Centre on Radiometric Calibration and Ocean Salinity (SMOS-BEC), and presents a preliminary validation of global sea surface salinity maps operationally produced by CP34., La imposibilidad de observar la salinidad superficial del mar desde el espacio era uno de los problemas importantes para la teledetección oceánica a finales del siglo XX. Nuevos desarrollos tecnológicos durante los años 90 en la Agencia Espacial Europea llevaron a formular la propuesta de SMOS (Soil Moisture and Ocean Salinity), una misión de oportunidad de observación de la Tierra basada en un radiómetro interferométrico de microondas, MIRAS (Microwave Imaging Radiometer with Aperture Synthesis). SMOS, el primer satélite que intenta observar la salinidad desde el espacio, fue lanzado en noviembre de 2009. La determinación de salinidad a partir de las mediciones de MIRAS a 1,4 GHz es un procedimiento complejo que requiere un excelente funcionamiento del instrumento y una modelización muy precisa de los procesos físicos que intervienen en la emisión de la superficie del océano en el dominio de las microondas. Este artículo presenta SMOS en el contexto de la teledetección oceánica, y resume los principios de operación de MIRAS y el enfoque utilizado en SMOS para obtener la salinidad. Describe el Centro español de Proceso de datos SMOS de alto nivel (CP34) y el Centro Experto SMOS en Barcelona para Calibración Radiométrica y Salinidad Oceánica (SMOS-BEC), así como presenta una primera validación de los mapas globales de salinidad superficial producidos operacionalmente por el CP34.

14 pages, 17 figures, This work describes the main effects that have to be taken into account to model the sea surface emission at L-band, and the existing approaches to perform the sea surface salinity retrieval from multiangular radiometric measurements. This manuscript reviews the activities carried out in these fields during the past years by the Universitat Politècnica de Catalunya (UPC) in collaboration with the Institut de Ciències del Mar (ICM - CSIC). They cover the precise measurement of the dielectric permittivity of the seawater at L-band, the sea surface emissivity (including the effect of wind-driven waves, swell, currents, rain, and oil spills), the comparison of experimental data with numerical models, and the development of sea surface salinity retrieval algorithms for SMOS using multiangular radiometric data. The first salinity retrievals using 2D aperture synthesis radiometry are also presented, This work has been supported by ESA Projects ESA, ESTEC contract 141888/00/NL/DC, CCN-1 and CCN2, ESA contract 18176/04/NL/CB, and Spanish Research projects and EU FEDER CICYT TIC 99-1050-C03-01, TIC2002-04451-C02-01, TEC2005-06863-C02-01, PNE-009/2001-C-02, ESP2002-11648-E, ESP2004-00671, and ESP2005-06823-C05

10 page, 2 tables, SMOS (Soil Moisture and Ocean Salinity) is a satellite mission of the European Space Agency (ESA) to be launched in 2007. One of its goals is the generation of global Sea Surface Salinity (SSS) maps. SMOS will carry an L-band (1.4 GHz) interferometric radiometer with full polarimetric capability. The airborne EuroSTARRS campaign was sponsored by ESA in 2001 to provide land and ocean data for the scientific studies supporting the SMOS mission, in particular to acquire ‘SMOS like’ data to advance in the knowledge of passive microwave multi-incidence observations for various surfaces. Some results of the campaign in the NW Mediterranean are presented and discussed here

The European Space Agency Soil Moisture and Ocean Salinity (SMOS) mission aims at obtaining global maps of soil moisture and sea surface salinity from space for large-scale and climatic studies. It uses an L-band (1400–1427 MHz) Microwave Interferometric Radiometer by Aperture Synthesis to measure brightness temperature of the earth’s surface at horizontal and vertical polarizations ( h and v). These two parameters will be used together to retrieve the geophysical parameters. The retrieval of salinity is a complex process that requires the knowledge of other environmental information and an accurate processing of the radiometer measurements. Here, we present recent results obtained from several studies and field experiments that were part of the SMOS mission, and highlight the issues still to be solved.

Soil moisture and ocean salinity at surface level can be measured by passive microwave remote sensing at L-band. To provide global coverage data of soil moisture and ocean salinity with three-day revisit time, the Earth Explorer Opportunity Mission SMOS (Soil Moisture and Ocean Salinity) was selected by ESA (European Space Agency) in May 1999. SMOS' single payload is a Y-shaped 2-D aperture synthesis interferometric radiometer called MIRAS (Microwave Imaging Radiometer by Aperture Synthesis). SMOS presents some particular imaging peculiarities: variation of incidence and azimuth angles, different radiometric sensitivity and accuracy at each direction (pixels), and geometric polarization mixing. Therefore, the accuracy of the geophysical parameter retrieval depends on the knowledge of the angular dependence of the emissivity over a wide range of incidence and azimuth angles. The accuracy of the sea surface salinity retrievals depends on our capability to correct the wind-induced variation of the brightness temperatures. To better understand wind effects, ESA sponsored the WInd and Salinity Experiment 2000 (WISE-2000) from November 15, 2000, to January 16, 2001, in the Casablanca oil rig, at 40 km off the coast of Tarragona (Spain). This paper is divided into two parts. First, it presents the derived sensitivities of the brightness temperatures at vertical and horizontal polarizations with wind speed, and compares to Hollinger's measurements and numerical simulations. Second, these results are applied to the SMOS sea surface salinity (SSS) retrieval problem for different tracks within the swath. It is shown that, except for low SSS and sea surface temperature (SST), the retrieved SSS has a RMS error of approximately 1 psu in one satellite pass.