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Yin X.,University Pierre and Marie Curie | Boutin J.,University Pierre and Marie Curie | Font J.,CSIC - Institute of Marine Sciences | Reul N.,French Research Institute for Exploitation of the Sea | And 3 more authors.
2014 31th URSI General Assembly and Scientific Symposium, URSI GASS 2014

The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission, launched in November 2009, has been providing global maps of sea surface salinity (SSS) since 2010. The SMOS SSS derived with version 5 reprocessing senses realistic SSS variability at various time and space scales, although some biases remain a basin and seasonal scale. In this paper, after providing an overview of the SMOS version 5 SSS quality derived from comparisons with in situ measurements, we provide examples of the observed variability associated with tropical instability waves during 2010 and 2013. Then we show improvements expected in future SMOS level 2 reprocessing coming from a new method for retrieving Total Electron Content (TEC). We also estimate the SMOS SSS uncertainties due to uncertainties in a priori sea surface temperature (SST) and wind speed (WS), especially in the tropical Pacific Ocean where there are significant and sometimes coupled variations of SST and WS due to strong seasonal upwelling, zonal surface currents and the development of tropical instability waves. © 2014 IEEE. Source

Yin X.,University Pierre and Marie Curie | Boutin J.,University Pierre and Marie Curie | Spurgeon P.,ARGANS
IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing

More than two years after the launch of the Soil Moisture and Ocean Salinity (SMOS) mission in November 2009, the level 1C brightness temperatures (TB) reprocessed with the up-to-date ESA level 1 processing version (the level 1 processor V5.04 and V5.05) have been released. Systematic biases of several Kelvins are still observed between averaged TB measurements and simulations, and depend on the location of the measurement in the field of view (FOV). The systematic biases may originate from imperfections in instrument calibration, image reconstruction, TB forward model, and in removing influence of external sources (Sun, galaxy, etc.). These systematic biases are monitored using the so-called 'Ocean Target Transformation' (OTT) and are analyzed during two years (May 2010-April 2012). The peak to peak variations in OTTs are higher than 1 K in magnitude. We find large variations of OTTs where Sun contaminations are expected, in locations affected by Sun aliases and Sun tails. The seasonal variation of OTTs computed over descending passes is opposite to the seasonal variation of the physical antenna patch temperature of the noise injection radiometer (Tp7), whereas the link between the temporal variation of OTTs during ascending passes with the one of Tp7 is less clear. In addition, there are no clear correlations between seasonal variations in OTTs in terms of Stokes 1 and geophysical parameters, such as scattered galactic signal. This suggests that the seasonal variations in OTTs is likely to come from the instrument heating and the imperfect L1 image reconstruction. © 2013 IEEE. Source

Vergely J.-L.,ACRI ST | Waldteufel P.,LATMOS | Boutin J.,University Pierre and Marie Curie | Yin X.,University Pierre and Marie Curie | And 2 more authors.
Journal of Geophysical Research C: Oceans

The European Space Agency (ESA)-led SMOS (Soil Moisture and Ocean Salinity) mission aims at monitoring both soil moisture (SM) and ocean surface salinity (OS) on a global scale. The SMOS instrument is a microwave interferometric radiometer, which provides visibilities, from which brightness temperatures (TB) maps are reconstructed in the spacecraft' antenna reference frame. In this study, we investigate how to improve the retrieval of salinity thanks to a better knowledge of the ionospheric total electron content (TEC). We show how both the SMOS bias correction (the so-called Ocean Target Transformation, OTT) and the half orbit TEC profile can be obtained from SMOS third Stokes parameter A3 using a location on the SMOS field of view (FOV) where the sensitivity of TB to TEC is highest. The resulting TEC global maps compare favorably with those built from the International Global navigation satellite system Service observations. TEC values obtained from A3 are next used to optimize the OTT estimation for every polarization, and proved to provide more stable values. Finally, improvements achieved in the salinity retrieved from SMOS data are reported. © 2014. American Geophysical Union. All Rights Reserved. Source

Yin X.,French National Center for Scientific Research | Boutin J.,French National Center for Scientific Research | Martin N.,French National Center for Scientific Research | Spurgeon P.,ARGANS | And 2 more authors.
Remote Sensing of Environment

The wind speed (WS) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) is used to initialize the retrieval process of WS and Sea Surface Salinity (SSS) obtained by the Soil Moisture and Ocean Salinity (SMOS) mission. This process compensates for the lack of onboard instrument providing a measure of ocean surface WS independent of the L-band radiometer measurements. The SMOS-retrieved WS in the center of the swath (±300km) is adjusted regarding to its a priori estimate. The quality of the SMOS-retrieved SSS (SSSSMOS) is better at the center of the swath than at the edge of the swatch because the larger number of brightness temperature measurements available at the center of the swath reduces the effects of noise and because the greater variety of incidence angles provides more scope for adjusting the WS. This highlights the advantage of using a multi-parameter retrieval with respect to a SSS-only retrieval in which the WS would be entirely prescribed. Systematic inconsistencies between the atmospheric WS modeled using ECMWF and the WS sensed by radiometers are observed. These inconsistencies in the WS are reduced by the retrieval scheme but they still lead to residual biases in the SSSSMOS, especially in the eastern equatorial Pacific ocean if the ECMWF WS is used as an a priori estimate. © 2013 Elsevier Inc. Source

Yin X.,University Pierre and Marie Curie | Boutin J.,University Pierre and Marie Curie | Spurgeon P.,ARGANS
IEEE Transactions on Geoscience and Remote Sensing

The Soil Moisture and Ocean Salinity (SMOS) mission carries the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) instrument. It is the first time that an interferometric radiometer is in orbit. The objective of this paper is to assess the quality of the brightness temperatures (TBs) derived from this novel instrument, as processed with the SMOS operational chain at the end of the SMOS commissioning phase. Extensive comparisons have been conducted between reconstructed TBs derived from MIRAS measurements (MIRAS TB) and TBs simulated using the default radiative transfer model implemented in the European Space Agency SMOS ocean salinity processor and the European Centre for Medium-Range Weather Forecast forcings. At first order, the North-South variability of MIRAS TB due to geophysical variations of temperature, salinity, and wind speed over the ocean is consistent with the simulated L-band signal, and the standard deviation of the MIRAS TB minus the model simulations is close to the theoretical radiometric resolution. On the other hand, biases of several Kelvins, that depend on the location in the field of view, are observed between averaged MIRAS TB and simulations. After these biases are removed, the North-South gradient of sea surface salinity is well sensed by MIRAS except at high wind speed. © 2012 IEEE. Source

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