Space Reflecto, November 4 th -5 th 2013, Plouzané Characterization of scattered celestial signals in SMOS observations over the Ocean J. Gourrion 1, J. Tenerelli 2, S. Guimbard 1, V. Gonzalez 1 1 SMOS-BEC, ICM/CSIC 2 CLS

Space Reflecto, November 4 th -5 th 2013, Plouzané an Earth Explorer mission (ESA) for Soil Moisture and Ocean Salinity Sun synchronous orbit, 3-days repeat cycle, 32.5º tilt MIRAS payload : a polarimetric and interferometric radiometer at L-band, 72 receivers SMOS Field of View Concept

Space Reflecto, November 4 th -5 th 2013, Plouzané Retrieving salinity

Space Reflecto, November 4 th -5 th 2013, Plouzané Retrieving salinity

Space Reflecto, November 4 th -5 th 2013, Plouzané Forward model

Space Reflecto, November 4 th -5 th 2013, Plouzané The problem Spatio-temporal systematic biases in SMOS salinities sun-synchronous orbit  the orbital plane makes one complete rotation relative to the stars per year. the instrument views roughly the same portion of the sky in successive orbits.

Space Reflecto, November 4 th -5 th 2013, Plouzané Spatio-temporal systematic biases in Aquarius salinities Version 2.0 Inner beam Descending passes The problem

Space Reflecto, November 4 th -5 th 2013, Plouzané GEOMETRICAL OPTICS GALACTIC MODEL Fitted galactic model High-frequency limit of the Kirchhoff approximation for scattering cross sections: Express in terms of slope probability distribution: Assume Gaussian slope pdf. Then fit the slope variance (MSS):

Space Reflecto, November 4 th -5 th 2013, Plouzané SMOS descending passes example SSS bias using the old (pre-launch) galatic model GO model impact

Space Reflecto, November 4 th -5 th 2013, Plouzané SMOS descending passes example SSS bias using the new galatic model GO model impact

Space Reflecto, November 4 th -5 th 2013, Plouzané GO model impact AQUARIUS inner beam (ascending passes only) Aquarius galactic reflection model

Space Reflecto, November 4 th -5 th 2013, Plouzané AQUARIUS inner beam (ascending passes only) GO model impact SMOS ascending pass fit galactic reflection model

Space Reflecto, November 4 th -5 th 2013, Plouzané DescendingAscending GO model adjustment to SMOS data Geometrical optics fitted to the data - slope variance : increases with wind speed (expected) changes with incidence angle (unexpected) changes with pass orientation (unexpected)

Space Reflecto, November 4 th -5 th 2013, Plouzané 1 - Systematic underestimation at high incidence angle  Imperfect model physics (GO, Gaussian slope pdf ?) Remaining issues with the GO mss fit the fitted mss depends on incidence angle and pass orientation

Space Reflecto, November 4 th -5 th 2013, Plouzané Remaining issues with the GO mss fit 2 - Potential phasing between biases and model errors  Need to uncouple bias estimation and model improvement tasks Estimate biases from expected low model errors dataset Update galactic model using a corrected dataset

Space Reflecto, November 4 th -5 th 2013, Plouzané Faraday : Dielectric : Roughness : Atmosphere : Direct sun : Radiometric noise : Celestial reflections : use 1 st Stokes parameter cancel contribution using Klein-Swift model latitude band with low SST seasonal variations thin interval of wind speed values thin bands of latitude (sea state) model for emission/attenuation contributions remove sun alias and sun tails (0.05) average over 18-days and longitude [ -180º -100º ] ? Variability sourceReduction approach 1 - phasing between T B biases and model errors Variability reduction strategy

Space Reflecto, November 4 th -5 th 2013, Plouzané 1 st Stokes T B time series : an example centered (zero mean) Galactic (KA) 1 - phasing between T B biases and model errors

Space Reflecto, November 4 th -5 th 2013, Plouzané Faraday : Dielectric : Roughness : Atmosphere : Direct sun : Radiometric noise : Celestial reflections : use 1 st Stokes parameter cancel contribution using Klein-Swift model latitude band with low SST seasonal variations thin interval of wind speed values thin bands of latitude (sea state) model for emission/attenuation contributions remove sun alias and sun tails (0.05) average over 18-days and longitude [ -180º -100º ] select xi/eta points with low annual variations (as predicted by the KA) Variability sourceReduction approach Variability reduction strategy 1 - phasing between T B biases and model errors

Space Reflecto, November 4 th -5 th 2013, Plouzané highly consistent throughout the field of view Temporal bias estimates 1 - phasing between T B biases and model errors

Space Reflecto, November 4 th -5 th 2013, Plouzané 2 - Underestimation at high incidence angle 55º incidence angle

Space Reflecto, November 4 th -5 th 2013, Plouzané 2 - Underestimation at high incidence angle A fully-empirical correction of the bistatic coefficients

Space Reflecto, November 4 th -5 th 2013, Plouzané 2 - Underestimation at high incidence angle 55 º incidence, 8 m/s data GO modified GO

Space Reflecto, November 4 th -5 th 2013, Plouzané 2 - Underestimation at high incidence angle 55 º incidence, 4 m/s 55 º incidence, 12 m/s data GO modified GO

Space Reflecto, November 4 th -5 th 2013, Plouzané Non gaussian slope distribution The model is still physically inconsistent : the slope variance is still incidence angle dependent the slope variance is still a filtered value (the surface slope pdf is frequency-dependent) Preliminary results for a more physically-based approach … Use a non gaussian pdf which variance is based on Cox and Munk’s results

Space Reflecto, November 4 th -5 th 2013, Plouzané Non gaussian slope distribution 35 º incidence, 8 m/s data GO non Gaussian GO 45 º incidence, 8 m/s 55 º incidence, 8 m/s using Cox and Munk’s results mss = for all incidence angles

Space Reflecto, November 4 th -5 th 2013, Plouzané Summary Accurate modeling of the scattered celestial signal is of importance for salinity retrieval at L-band The pre-launch Kirchhoff scattering model strongly underpredicts the scattered brightness near the galactic plane and overpredicts the brightness away from the galactic plane under most surface roughness conditions A gaussian GO model fitted to SMOS residuals significantly improves the scattering description near the specular lobe BUT remaining inaccuracy at high incidence angle AND physically inconsistent slope variance values A methodology was developed to characterize separately T B biases and sea surface scattered celestial signal  consistent SMOS celestial residuals It is shown that problems come from off-specular contributions modeling Preliminary results suggest that most inconsistencies may be removed when accounting for non-gaussian slope statistics

Space Reflecto, November 4 th -5 th 2013, Plouzané Incidence 55º 45º 35º FOV/latitude location of the low galactic time series

Space Reflecto, November 4 th -5 th 2013, Plouzané 1 - phasing between biases and model errors Objective : uncouple bias estimation and model improvement tasks estimate the temporal “instrumental” bias from a specific dataset with low model errors (to build a corrected dataset) Strategy : build T B signals with reduced environmental variability low environmental (geophysical + foreign sources) variability  expected to be constant in a perfect world Environmental variability is reduced through either data selection or correction based on reliable model components Data 30-month time series (June’10 to Jan’13, Pacific) processed at BEC Noise reduction: 18-days and 5 o x 80 o averages, circular apodisation