SMOS L2 OS ATBD - ARGANS

1. Introduction
1
ICM-CSIC
LOCEAN/SA/CETP
IFREMER
SMOS L2 OS
Algorithm Theoretical
Baseline Document
Doc: SO-TN-ARG-GS-0007
Issue: 3 Rev: 9
Date: 25 January 2013
Page: 1
The purpose of this Algorithm Theoretical Baseline Document is to establish the procedure
that will be used in the SMOS mission to generate the Sea Surface Salinity data from
brightness temperatures (Tb) recorded by the MIRAS radiometer. The output product (SMOS
SSS Level 2 product) will consist on files containing half-orbit data (from pole to pole) on
the ISEA grid defined at Level 1.
As it is not possible to transform the Tb into SSS through a univoque mathematical
expression, two different approaches are proposed to retrieve SSS values from SMOS
measurements. In the first one the L-band emission of the sea surface is computed using a
series of mathematical models that have as independent variables the different geophysical
parameters (including SSS) that determine this emission. These parameters are obtained from
sources external to SMOS, and for SSS a guessed value is considered. The computed Tb, for
all angular configurations that correspond to the specific satellite passage, are compared to
the measured ones, and then the independent variables are modified in an iterative process
until reaching the maximum similarity between both Tb values. The SSS that corresponds to
this situation is considered to be the value retrieved from SMOS.
In the second approach, the relationship between measured Tb and sea surface conditions is
established empirically through a neural network technique from a wide training data set.
Once the method is efficiently operational, SSS in a grid point will be retrieved from Tb
angular measurements at each satellite passage.
In section 2 an overview of the algorithm is presented, with a scheme of its application in the
case of iterative retrieval. Section 3 describes the tests that have to be performed every time
that the SSS retrieval is attempted, to select the measurements that are suitable to be used and
flag any special conditions that may occur. Section 4 is a detailed description of all the parts
of the algorithm, the different modules or sub-models that are used to compute the different
contributions to sea surface Tb as well as the procedures to compare it with the measured Tb
and the iterative convergence method. Section 5 describes the alternative approach based on
neural networks. Section 6 presents the contents and structure of the output information after
SSS retrieval. Five annexes provide additional information, as the geometry convention to
relate Earth and satellite reference frames, the implementation of a pseudo-dielectric constant
using the cardioid model, the possible strategies to compute the first Stokes parameter in full
polarisation mode, the secondary algorithm, and a table summarising the parts of the ATBD
that are still to be completed.
The Intergovernmental Oceanographic Commission adopted recently the new International
Thermodynamic Equation of Seawater - 2010 that describes salinity through the Absolute
Salinity definition in g/kg instead of Practical Salinity (PS Scale - 78) to take into account the
spatial variability of seawater composition and to use IS units. However, measuring systems,
both in situ and remotely sensed, will continue being based on conductivity, so all
instruments and data bases will deliver practical salinity as before. For historical reasons, all
the SMOS data processing algorithms presented here use practical salinity regardless of
labels in different modules (no units/psu/pss/pss-78). SMOS is a data provider and users may