46
G. Lagerloef and J. Font
produced by the existence of independent measurements (different incidence angles)
of the T B emitted by the ocean surface, which SSS does not change during the
satellite overpass.
3.4.4 Expected SMOS Performance, Error Analysis
The determination of ocean salinity by SMOS has a major drawback, compared
to the retrieval of soil moisture, in the much lower sensitivity of T B to salinity
changes, especially at low temperatures. This makes the instrument performance
more critical for salinity retrieval. The science requirements established for the
mission translated into quite strict radiometric requirements for MIRAS, but tests
made after completing the instrument development indicated these were met with
considerable margin. The Table 3.1 summarizes the MIRAS radiometric requirements and performances expressed in RMS of T B as measured in May–June 2007 at
the Maxwell Electromagnetic Chamber in ESTEC (European Space Technology
Centre, ESA, The Netherlands). The values presented in the table provide the worst
case, whenever several measurements were available (Font et al., 2010).
Besides the mentioned low range of T B values that correspond to the whole range
of salinity values in the world oceans, other key problems are impacting the quality
of SMOS retrieval of ocean salinity. First, although the radiometric performance of
the instrument is better than initially expected, the process of image reconstruction
from the correlations of the measurements made by the individual antenna elements
is introducing some errors (not well known yet) that would not exist if the measurement was directly made by a physical aperture antenna. Second, there is a need
for simultaneous auxiliary information on the sea surface properties (temperature,
roughness . . .) to be estimated from external sources, as they are not directly measured by SMOS itself. The inaccuracy of this information (in terms of bias and noise
in the auxiliary fields provided by the 3-hourly ECMWF forecasts) impacts on the
retrieved salinity, in spite of these being only taken as reference values in the convergence retrieval procedure. And third, the imperfections in the different modules
that constitute the GMF are also introducing degradations in the retrieval quality
Table 3.1 MIRAS system radiometric requirements and instrument-measured performances at
boresight and at the edge of the FOV (32 ◦ )
Required
Measured
Systematic error
1.5 K RMS (0 ◦ )
2.5 K RMS (32 ◦ )
0.9 K RMS
in AF-FOV
Land (T Bland = 220 K)
Radiometric sensitivity
3.5 K RMS (0 ◦ )
5.8 K RMS (32 ◦ )
2.23 K RMS (0 ◦ )
3.95 K RMS (32 ◦ )
Ocean (T Bocean = 150 K)
Radiometric sensitivity
2.5 K RMS (0 ◦ )
4.1 K RMS (32 ◦ )
1.88 K RMS (0 ◦ )
3.32 K RMS (32 ◦ )
Stability (1.2s interval)
4.1 K RMS (<32 ◦ )
4 . 0 3 K R M S
Stability (6 d interval)
0.03 K
<0.02 K
G. Lagerloef and J. Font
produced by the existence of independent measurements (different incidence angles)
of the T B emitted by the ocean surface, which SSS does not change during the
satellite overpass.
3.4.4 Expected SMOS Performance, Error Analysis
The determination of ocean salinity by SMOS has a major drawback, compared
to the retrieval of soil moisture, in the much lower sensitivity of T B to salinity
changes, especially at low temperatures. This makes the instrument performance
more critical for salinity retrieval. The science requirements established for the
mission translated into quite strict radiometric requirements for MIRAS, but tests
made after completing the instrument development indicated these were met with
considerable margin. The Table 3.1 summarizes the MIRAS radiometric requirements and performances expressed in RMS of T B as measured in May–June 2007 at
the Maxwell Electromagnetic Chamber in ESTEC (European Space Technology
Centre, ESA, The Netherlands). The values presented in the table provide the worst
case, whenever several measurements were available (Font et al., 2010).
Besides the mentioned low range of T B values that correspond to the whole range
of salinity values in the world oceans, other key problems are impacting the quality
of SMOS retrieval of ocean salinity. First, although the radiometric performance of
the instrument is better than initially expected, the process of image reconstruction
from the correlations of the measurements made by the individual antenna elements
is introducing some errors (not well known yet) that would not exist if the measurement was directly made by a physical aperture antenna. Second, there is a need
for simultaneous auxiliary information on the sea surface properties (temperature,
roughness . . .) to be estimated from external sources, as they are not directly measured by SMOS itself. The inaccuracy of this information (in terms of bias and noise
in the auxiliary fields provided by the 3-hourly ECMWF forecasts) impacts on the
retrieved salinity, in spite of these being only taken as reference values in the convergence retrieval procedure. And third, the imperfections in the different modules
that constitute the GMF are also introducing degradations in the retrieval quality
Table 3.1 MIRAS system radiometric requirements and instrument-measured performances at
boresight and at the edge of the FOV (32 ◦ )
Required
Measured
Systematic error
1.5 K RMS (0 ◦ )
2.5 K RMS (32 ◦ )
0.9 K RMS
in AF-FOV
Land (T Bland = 220 K)
Radiometric sensitivity
3.5 K RMS (0 ◦ )
5.8 K RMS (32 ◦ )
2.23 K RMS (0 ◦ )
3.95 K RMS (32 ◦ )
Ocean (T Bocean = 150 K)
Radiometric sensitivity
2.5 K RMS (0 ◦ )
4.1 K RMS (32 ◦ )
1.88 K RMS (0 ◦ )
3.32 K RMS (32 ◦ )
Stability (1.2s interval)
4.1 K RMS (<32 ◦ )
4 . 0 3 K R M S
Stability (6 d interval)
0.03 K
<0.02 K