3 SMOS and Aquarius/SAC-D Missions
39
Fig. 3.2 Brightness Temperatures T BV (left) and T BH (right) as a function of SSS (contours) and
SST (abscissa) for typical ocean surface conditions, and incidence angle of 37.8 ◦ (as an example
from one of the Aquarius/SAC-D satellite viewing angles). Salinity can be determined from either
polarization when SST is known (dashed lines). Calculations based on the dielectric model Klein
and Swift (1977)
new set of laboratory measurements is presently being carried out (Lang, 2008).
The dielectric model, combined with in-situ SSS and SST validation measurements
will provide a consistent calibration reference for present and future salinity satellite
missions.
Satellite remote sensing of salinity is done in the protected L-band frequency
centered at 1.413 GHz to avoid radio interference. At this band, the brightness temperature change relative to a change in salinity, although small, is nevertheless
enough to make SSS remote sensing possible, given sufficiently sensitive radiometric measurements. Figure 3.2 shows the relationship between T BH and T BV with
SST and SSS for a particular viewing angle. The contour lines are for salinities
ranging from 32 to 37 psu. It is easy to see that a unique value of salinity can be
retrieved when SST and either T BH or T BV are known. This is the essence of how
salinity remote sensing is achieved, although it is more complicated in practice. The
SMOS and Aquarius instruments approach the salinity retrieval in very different
ways, based on sensor design, as will be explained further below.
The dynamic range of brightness temperature is about 5 K over the range of typical open ocean surface salinity and temperature conditions. At a given temperature,
brightness temperature decreases as salinity increases, whereas the tendency with
respect to temperature changes sign. The salinity contours are spread farther apart
for V polarization than for H, and therefore V is slightly more sensitive to salinity changes. The sensitivity is strongly affected by temperature, being largest at
the highest temperatures and yielding better measurement precision in warm versus
cold ocean conditions. Corrected brightness temperature will need to be measured
to 0.02–0.08 K precision to achieve 0.1 pss salinity resolution. The difference in
sensitivity between polarizations also increases with incidence angle (not shown).
Temporal and spatial averaging can reduce random error. The degraded measurement precision in higher latitudes will be partly offset by averaging with the greater
sampling frequency from a polar orbiting satellite.
39
Fig. 3.2 Brightness Temperatures T BV (left) and T BH (right) as a function of SSS (contours) and
SST (abscissa) for typical ocean surface conditions, and incidence angle of 37.8 ◦ (as an example
from one of the Aquarius/SAC-D satellite viewing angles). Salinity can be determined from either
polarization when SST is known (dashed lines). Calculations based on the dielectric model Klein
and Swift (1977)
new set of laboratory measurements is presently being carried out (Lang, 2008).
The dielectric model, combined with in-situ SSS and SST validation measurements
will provide a consistent calibration reference for present and future salinity satellite
missions.
Satellite remote sensing of salinity is done in the protected L-band frequency
centered at 1.413 GHz to avoid radio interference. At this band, the brightness temperature change relative to a change in salinity, although small, is nevertheless
enough to make SSS remote sensing possible, given sufficiently sensitive radiometric measurements. Figure 3.2 shows the relationship between T BH and T BV with
SST and SSS for a particular viewing angle. The contour lines are for salinities
ranging from 32 to 37 psu. It is easy to see that a unique value of salinity can be
retrieved when SST and either T BH or T BV are known. This is the essence of how
salinity remote sensing is achieved, although it is more complicated in practice. The
SMOS and Aquarius instruments approach the salinity retrieval in very different
ways, based on sensor design, as will be explained further below.
The dynamic range of brightness temperature is about 5 K over the range of typical open ocean surface salinity and temperature conditions. At a given temperature,
brightness temperature decreases as salinity increases, whereas the tendency with
respect to temperature changes sign. The salinity contours are spread farther apart
for V polarization than for H, and therefore V is slightly more sensitive to salinity changes. The sensitivity is strongly affected by temperature, being largest at
the highest temperatures and yielding better measurement precision in warm versus
cold ocean conditions. Corrected brightness temperature will need to be measured
to 0.02–0.08 K precision to achieve 0.1 pss salinity resolution. The difference in
sensitivity between polarizations also increases with incidence angle (not shown).
Temporal and spatial averaging can reduce random error. The degraded measurement precision in higher latitudes will be partly offset by averaging with the greater
sampling frequency from a polar orbiting satellite.