| SST-NC - Algorithm Specification |
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Algorithm Specification
The calculation of the sea surface temperature from (A)ATSR brightness temperatures is based on the established split-window/dual-view algorithms. These algorithms use a linear combination of the brightness temperatures measured in two or three spectral bands. The differences between the measurements in the bands can be attributed to the absorbing and emitting effects of the atmosphere and used for correction. During night-time, the radiance emitted by the earth is measured in three bands, at 3.7, 11.5 and 12.5 µm. By day, the band at 3.7 µm measures both the radiances emitted by the earth and reflected sun light, so that it cannot be used to retrieve the sea surface temperature.
The (A)ATSR views a given location on the earth twice, once in nadir view and once under a certain angle. The differences in the measurements are due to the different path-lengths through the atmosphere. Also these differences are used to correct for atmospheric effects.
Basically, the fundamental form of the SST algorithm is:
By day
nadir view: SST = a0 + a1 * T11 + a2 * T12
dual view: SST = c0 + c1 * T11, nadir + c2 * T12, nadir + c3 * T11,fwrd + c4 * T12, fwrd
By night
nadir view: SST = b0 + b1 * T11 + b2 * T12 + b3 * T37
dual view: SST = d0 + d1 * T11, nadir + d2 * T12, nadir + d3 * T37, nadir + d4 * T11, fwrd + d5 * T12, fwrd + d6 * T37, fwrd
Where a……..d are coefficients, given in look-up tables, which depend upon latitude and on viewing angle and T(i) the specific channel number.
In the SNAP implementation a dependency on the viewing angle is foreseen, but not a dependency on the latitude. The idea of this implementation is to provide a tool for users who have their own sets of coefficients available, which is typically the case for regional applications. For example, a dedicated data set for the North Sea may be used. In such a case, the latitude dependency is not meaningful.