SMOS L2 OS ATBD - ARGANS

4.13. Sum of contributions
167
ICM-CSIC
LOCEAN/SA/CETP
IFREMER
4.13.1. Theoretical description
4.13.1.1. Physics of the problem
SMOS L2 OS
Algorithm Theoretical
Baseline Document
Doc: SO-TN-ARG-GS-0007
Issue: 3 Rev: 9
Date: 25 January 2013
Page: 167
This module consists in the addition of all the sub-models used to compute the brightness
temperature of a specific ocean grid point at antenna level. Then it includes the forward
model for L-band emissivity of a flat sea, plus correction for surface roughness (3 different
options are considered by now, that can incorporate additional terms for foam, swell effects
or others), the introduction of galactic noise contamination, atmospheric effects, and finally
transport from ground level to antenna level. With the present development, measurements
affected by sun glint and moon contamination are flagged and discarded for retrieval.
However, if in the future algorithms to adequately correct for these effects can be obtained,
the measurements will be kept and the corresponding corrections will be introduced in the
sum of contributions.
The input to module 13 are the equations provided by modules 1 to 12 and will be applied to
those grid points that have been selected as adequate for SSS retrieval in the Measurement
discrimination step.
4.13.1.1.1. Note on first Stokes parameter computation
As information to the rest of modules, we indicate here the detail of the first Stokes parameter
(I=Th+Tv=A1+A2) computation from a complete SMOS measurement (2.4 s), in fact a pair
of two consecutive measurements in orthogonal polarisations:
As A1 and A2 are measured in two consecutive (in case of dual-pol) 1.2 s snapshots, the
incidence angle for a specific grid point is not exactly the same for both, but will differ (in
case of dual-pol) in some 0.6º (approx. 8 km displacement at 756 km height). A little bigger
in case of full-pol.
Then a correction was assumed to be done routinely by shifting both measurements to a value
that corresponds to the angle that is just in the middle of θx and θy (“mean angle”). To do
that, we planned to add to each A the difference between modelled values for the two angles.
However, the modelisation of both polarisations at the antenna reference frame requires
introducing the Faraday rotation angle. And this is the step we want to avoid by computing
the first Stoke parameter. Moreover, this correction would be for sure much below the noise
of the measurement, and then was not expected to have a very significant impact.
Consequently it was decided (October 2006) not to perform any correction to A1 and A2 but
to compute directly a ‘Pseudo’ First Stokes parameter: