3 SMOS and Aquarius/SAC-D Missions
47
with respect the one that would be obtained if a perfect model existed. This includes
not only the mentioned problem of the surface roughness effect (how the roughness
is described with the available environmental parameters and how the roughness
modifies the ocean emission, with the additional impact caused by the presence of
foam at high winds), but also the determination of other GMF components as the
contribution to the signal of the polarized emission of galactic bodies reflected on
the roughened sea surface.
The particularities of the imaging capability of an interferometric radiometer
contribute to the performance of the salinity determination. We have mentioned the
weakness due to the need of performing an image reconstruction step, but SMOS
has remarkable strengths compared to measurements made by real aperture antennas. We have highlighted before the multi-angular observation that allows taking
advantage of the sensitivity of T B to the incidence angle to increase the robustness
of the inversion. Another fundamental feature is the high angular resolution that
allows imaging pixels of the order of 30 km and as a consequence identifying different elements within the FOV. This allows locating the pixels that include a direct,
or more likely reflected, image of the Sun. With the SMOS orientation, the Sun is
present in 97% of the snapshots; and considering the very high T B of the Sun L-band
emission, it is necessary to discard the few affected angular measurements instead
of attempting a correction.
Concerning the observation modes, if MIRAS is operating in dual-polarization
every 1.2 s either horizontal or vertical T B is acquired in consecutive snapshots.
Under full-polarization, some time is dedicated to acquire the cross-polarized components and then less data is available for each polarization and there is less noise
reduction. However, the additional information can be used to avoid the singularities of the transformation from the antenna to the Earth reference frame, to allow
improved RFI detection, to eventually identify azimuthal signals, or to estimate the
Faraday rotation. Both modes are to be tested during SMOS Commissioning Phase
(6 months after launch) to decide what is the nominal configuration to be used for
operations. The salinity retrieval can be performed using the two polarized T B separately or applying all the calculations to the first Stokes parameter, the sum of
both polarizations. Doing the latter the number of independent measurements to
integrate in the inversion is halved, then the noise reduction diminishes, but the
problem of polarization mixing by Faraday rotation is avoided. Other advantages
of this approach is that the uncertainties in the T B associated to angular dependencies of the sea water dielectric constant model and in the roughness correction
term are reduced, as well as the above mentioned singularities in the geometric
transformation disappear.
Idealized tests of the SMOS L2OP performance have been done under different
configurations and environmental conditions (Zine et al., 2008). Simulated scenes
are used to compute the polarized T B in the SMOS swath along an orbit. Then
radiometric noise is added according to the expected MIRAS performance, and the
processor is run with different errors and biases for the auxiliary parameters. These
tests show that the retrieved SSS values from one satellite overpass will be affected
by considerable noise, both from radiometric origin and from uncertainties in the
47
with respect the one that would be obtained if a perfect model existed. This includes
not only the mentioned problem of the surface roughness effect (how the roughness
is described with the available environmental parameters and how the roughness
modifies the ocean emission, with the additional impact caused by the presence of
foam at high winds), but also the determination of other GMF components as the
contribution to the signal of the polarized emission of galactic bodies reflected on
the roughened sea surface.
The particularities of the imaging capability of an interferometric radiometer
contribute to the performance of the salinity determination. We have mentioned the
weakness due to the need of performing an image reconstruction step, but SMOS
has remarkable strengths compared to measurements made by real aperture antennas. We have highlighted before the multi-angular observation that allows taking
advantage of the sensitivity of T B to the incidence angle to increase the robustness
of the inversion. Another fundamental feature is the high angular resolution that
allows imaging pixels of the order of 30 km and as a consequence identifying different elements within the FOV. This allows locating the pixels that include a direct,
or more likely reflected, image of the Sun. With the SMOS orientation, the Sun is
present in 97% of the snapshots; and considering the very high T B of the Sun L-band
emission, it is necessary to discard the few affected angular measurements instead
of attempting a correction.
Concerning the observation modes, if MIRAS is operating in dual-polarization
every 1.2 s either horizontal or vertical T B is acquired in consecutive snapshots.
Under full-polarization, some time is dedicated to acquire the cross-polarized components and then less data is available for each polarization and there is less noise
reduction. However, the additional information can be used to avoid the singularities of the transformation from the antenna to the Earth reference frame, to allow
improved RFI detection, to eventually identify azimuthal signals, or to estimate the
Faraday rotation. Both modes are to be tested during SMOS Commissioning Phase
(6 months after launch) to decide what is the nominal configuration to be used for
operations. The salinity retrieval can be performed using the two polarized T B separately or applying all the calculations to the first Stokes parameter, the sum of
both polarizations. Doing the latter the number of independent measurements to
integrate in the inversion is halved, then the noise reduction diminishes, but the
problem of polarization mixing by Faraday rotation is avoided. Other advantages
of this approach is that the uncertainties in the T B associated to angular dependencies of the sea water dielectric constant model and in the roughness correction
term are reduced, as well as the above mentioned singularities in the geometric
transformation disappear.
Idealized tests of the SMOS L2OP performance have been done under different
configurations and environmental conditions (Zine et al., 2008). Simulated scenes
are used to compute the polarized T B in the SMOS swath along an orbit. Then
radiometric noise is added according to the expected MIRAS performance, and the
processor is run with different errors and biases for the auxiliary parameters. These
tests show that the retrieved SSS values from one satellite overpass will be affected
by considerable noise, both from radiometric origin and from uncertainties in the