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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 [email protected] , [email protected]. Concept. SMOS. - PowerPoint PPT Presentation
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Space Reflecto, November 4th -5th 2013, Plouzané
Space Reflecto, November 4th -5th 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 4th -5th 2013, Plouzané
Retrieving salinity
Space Reflecto, November 4th -5th 2013, Plouzané
Retrieving salinity
Space Reflecto, November 4th -5th 2013, Plouzané
Forward model
Space Reflecto, November 4th -5th 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 4th -5th 2013, Plouzané
Spatio-temporal systematic biases in Aquarius salinities
The problem
Space Reflecto, November 4th -5th 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 4th -5th 2013, Plouzané
SMOSSMOS descending passes example descending passes exampleSSS bias using the SSS bias using the old old (pre-launch) galatic model (pre-launch) galatic model
GO model impact
Space Reflecto, November 4th -5th 2013, Plouzané
SMOSSMOS descending passes example descending passes exampleSSS bias using the SSS bias using the newnew galatic model galatic model
GO model impact
Space Reflecto, November 4th -5th 2013, Plouzané
GO model impact
AQUARIUSAQUARIUS inner beam (ascending passes only) inner beam (ascending passes only)
AquariusAquarius galactic reflection model galactic reflection model
Space Reflecto, November 4th -5th 2013, Plouzané
AQUARIUSAQUARIUS inner beam (ascending passes only) inner beam (ascending passes only)
GO model impact
SMOSSMOS ascending pass fit galactic reflection model ascending pass fit galactic reflection model
Space Reflecto, November 4th -5th 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 4th -5th 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 4th -5th 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 4th -5th 2013, Plouzané
Faraday :
Dielectric :
Roughness :
Atmosphere :
Direct sun :
Radiometric noise :
Celestial reflections :
• use 1st 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 source Reduction approach
1 - phasing between TB biases and model errors
Variability reduction strategy
Space Reflecto, November 4th -5th 2013, Plouzané
1st Stokes TB time series : an example
centered(zero mean)
Galactic(KA)
1 - phasing between TB biases and model errors
Space Reflecto, November 4th -5th 2013, Plouzané
Faraday :
Dielectric :
Roughness :
Atmosphere :
Direct sun :
Radiometric noise :
Celestial reflections :
• use 1st 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 source Reduction approach
Variability reduction strategy
1 - phasing between TB biases and model errors
Space Reflecto, November 4th -5th 2013, Plouzané
highly consistent throughout the field of view
Temporal bias estimates
1 - phasing between TB biases and model errors
Space Reflecto, November 4th -5th 2013, Plouzané
2 - Underestimation at high incidence angle
55º incidence angle
Space Reflecto, November 4th -5th 2013, Plouzané
2 - Underestimation at high incidence angle
A fully-empirical correction of the bistatic coefficients
Space Reflecto, November 4th -5th 2013, Plouzané
2 - Underestimation at high incidence angle
55º incidence, 8 m/s dataGOmodified GO
Space Reflecto, November 4th -5th 2013, Plouzané
2 - Underestimation at high incidence angle
55º incidence, 4 m/s
55º incidence, 12 m/sdataGOmodified GO
Space Reflecto, November 4th -5th 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 4th -5th 2013, Plouzané
Non gaussian slope distribution
35º incidence, 8 m/sdataGO
non Gaussian GO
45º incidence, 8 m/s
55º incidence, 8 m/s
using Cox and Munk’s results
mss = 0.044
for all incidence angles
Space Reflecto, November 4th -5th 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 TB 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