SMOS L2 OS ATBD - ARGANS

ICM-CSIC
LOCEAN/SA/CETP
IFREMER
4.16. Measurements selection (polarisation)
191
SMOS L2 OS
Algorithm Theoretical
Baseline Document
Doc: SO-TN-ARG-GS-0007
Issue: 3 Rev: 9
Date: 25 January 2013
Page: 191
This module describes how to select the successive MIRAS measurements for the
computation of SSS, according to the polarisation mode chosen.
To perform the iterative convergence, brightness temperature has to be taken in couples of H
and V polarisations if the 1 st Stokes parameter is used, otherwise the two polarisations are
included independently in the computations. The way to extract proper measurements is as
follows:
In case of Dual Polarisation: In a specific grid point all the snapshots that include it should be
used, and this will include consecutively perpendicular polarisations. However, some of them
can be invalid for SSS retrieval (either classified as bad by L1 or discarded in the L2
measurement discrimination) and as a consequence some pairs of horizontal (A1) and vertical
(A2) measurements will not be formed.
In case of using the 1 st Stokes parameter, and if the Scene Bias Correction (SBC, see module
4.11) is applied, the H+V pairs have always to be organised by taking first an H measurement
and its immediately consecutive V. Then if the first snapshot that includes a grid point
provides a V measurement, this will not be processed. In case of lack of one or more
measurements in the sequence, the order should not be changed and continue by coupling
first A1 and then A2. With this selection method it might happen that some measurements are
not used, in spite of providing good data.
The following figure represents a couple of examples (grid points with very small number of
measurements, for simplicity) from the first to the last view in a satellite overpass:
*
A1A2 A1A2 A1A2 A1 N N N N N N A2 A1A2 A1A2 A1A2 A1
*
A2 A1A2 A1A2 A1A2 A1 N N N A1A2 A1A2 A1A2 N A2 A1A2 A1A2 A1A2
The brackets show the formed pairs of perpendicular polarisations in the most restrictive case
(SBC with 1 st Stokes), while N means invalid measurements and red marks measurements
that will be lost due to the measurement selection (lack of companion polarisation). If SBC is
not applied, only measurements marked with * in the examples above are lost with 1 st Stokes
option (end of a string with odd number of good data), as the order of complementary
polarisations in a pair is not relevant. When the two polarisations are used separately, no
good data have to be discarded, as pairs formation is not needed.
When a correct measurement is not used in the retrieval due to this selection method, a flag
on the measurement should be raised (Fm_lost_data).
In case of Full Polarisation mode: do the same as dual polarisation, but taking into
consideration only HH and VV measurements, not the mixed ones.
The first Stokes value (I) is computed as described in 4.13.1.1.1 and 2 I = 2 A1+ 2 A2