48
G. Lagerloef and J. Font
algorithm and auxiliary data. This error is of the order of 0.5–0.7 pss in the centre of the swath and degrades to about 1.5 on its borders. These results improve
with high SST scenes, but can be as bad as 1.2 pss (centre) and 2.4 pss (borders)
for SST = 5 ◦ C. Introducing biases on the auxiliary parameters produces also a
bias on the retrieved SSS that can reach up to 0.7 pss (centre) and 0.9 pss (border) when wind speed is biased by 2 m/s. More realistic conditions (e.g. using all
the measured antenna patterns for the different MIRAS elements) have been considered in some SMOS system end-to-end performance tests, and these indicate
that the above described accuracy of the salinity determination can be degraded by
about 50%.
It appears evident that the quality of salinity retrieval obtained from a SMOS orbit
(in grid points situated in an Icosahedral Snyder Equal Area projection, ISEA4H9,
15 km characteristic length scale, as interpolated during image reconstruction) will
not meet the mission scientific requirements. This is expected to be significantly
improved by performing spatio-temporal averages in the generation of global gridded maps (Boutin et al., 2003). A salinity error budget analysis (Sabia et al., 2010)
made in an open ocean region using different combinations of configurations and
auxiliary data uncertainties, concluded that an average of SMOS products over 30
days and 2 ◦ × 2 ◦ boxes would generate a SSS map with an error of 0.22 pss, very
close to the mission requirements. In further processing steps this can be improved
by introducing balancing terms in the cost function (Gabarró et al., 2009) and
by bias reduction through external calibration techniques using other sources of
salinity data.
3.5 Aquarius/SAC-D Mission
3.5.1 Early Configuration and Evolution of Design
The first significant step toward a NASA salinity mission began with the Salinity Sea
Ice Working Group (SSIWG), established in early 1998. The SSIWG included participation from United States as well as European scientists and engineers. During
that year the SMOS mission was also being formulated for proposal to ESA (see
above). The SSIWG became an international, voluntary and open forum, and held
workshops in 1998, 1999 and 2000 focusing on a range of scientific and technical
issues covered by the charter (see www.esr.org/ssiwg/mainssiwg.html). The SSIWG
provided the basic scientific framework and objectives for salinity remote sensing
and outlined basic measurement requirements (Lagerloef et al., 2008). An analyses
by Yueh et al. (2001) provided more rigorous assessment of the technical issues and
feasibility. During this time, parallel efforts continued in both Europe (with SMOS)
and in the United States. The NASA effort focused on satellite sensor concepts and
mission designs to measure salinity as a primary objective. Another team in the US
pursued a separate mission concept to measure soil moisture with science requirements that demanded much higher spatial and temporal resolution, but much less
radiometric accuracy, than needed for salinity.
G. Lagerloef and J. Font
algorithm and auxiliary data. This error is of the order of 0.5–0.7 pss in the centre of the swath and degrades to about 1.5 on its borders. These results improve
with high SST scenes, but can be as bad as 1.2 pss (centre) and 2.4 pss (borders)
for SST = 5 ◦ C. Introducing biases on the auxiliary parameters produces also a
bias on the retrieved SSS that can reach up to 0.7 pss (centre) and 0.9 pss (border) when wind speed is biased by 2 m/s. More realistic conditions (e.g. using all
the measured antenna patterns for the different MIRAS elements) have been considered in some SMOS system end-to-end performance tests, and these indicate
that the above described accuracy of the salinity determination can be degraded by
about 50%.
It appears evident that the quality of salinity retrieval obtained from a SMOS orbit
(in grid points situated in an Icosahedral Snyder Equal Area projection, ISEA4H9,
15 km characteristic length scale, as interpolated during image reconstruction) will
not meet the mission scientific requirements. This is expected to be significantly
improved by performing spatio-temporal averages in the generation of global gridded maps (Boutin et al., 2003). A salinity error budget analysis (Sabia et al., 2010)
made in an open ocean region using different combinations of configurations and
auxiliary data uncertainties, concluded that an average of SMOS products over 30
days and 2 ◦ × 2 ◦ boxes would generate a SSS map with an error of 0.22 pss, very
close to the mission requirements. In further processing steps this can be improved
by introducing balancing terms in the cost function (Gabarró et al., 2009) and
by bias reduction through external calibration techniques using other sources of
salinity data.
3.5 Aquarius/SAC-D Mission
3.5.1 Early Configuration and Evolution of Design
The first significant step toward a NASA salinity mission began with the Salinity Sea
Ice Working Group (SSIWG), established in early 1998. The SSIWG included participation from United States as well as European scientists and engineers. During
that year the SMOS mission was also being formulated for proposal to ESA (see
above). The SSIWG became an international, voluntary and open forum, and held
workshops in 1998, 1999 and 2000 focusing on a range of scientific and technical
issues covered by the charter (see www.esr.org/ssiwg/mainssiwg.html). The SSIWG
provided the basic scientific framework and objectives for salinity remote sensing
and outlined basic measurement requirements (Lagerloef et al., 2008). An analyses
by Yueh et al. (2001) provided more rigorous assessment of the technical issues and
feasibility. During this time, parallel efforts continued in both Europe (with SMOS)
and in the United States. The NASA effort focused on satellite sensor concepts and
mission designs to measure salinity as a primary objective. Another team in the US
pursued a separate mission concept to measure soil moisture with science requirements that demanded much higher spatial and temporal resolution, but much less
radiometric accuracy, than needed for salinity.