On-board calibration of the SLSTR is performed continuously, as described below.

For the thermal infra-red channels, provision for their continuous on-board calibration is an integral part of the instrument design. The instrument includes two stable and high precision black-body targets, which are intersected by both the nadir and along track scan cones. Each target is observed by all detectors once per cycle for each scanner. One black body is close to the optics temperature of ~260K and the other is heated to ~302K, so that the interval between the two black body temperatures covers the expected range of sea surface brightness temperatures. The physical temperatures of the two black bodies are continuously monitored, and so for each scan the linear relationship between the infra-red radiance and the measured signal counts can be determined from the measurements of the black body signals.

The use of two black bodies in this way ensures that the calibration is optimised over the normal range of SST, and the effects of detector non-linearity over this range are minimised, maximising the accuracy of SST measurement. The same two-point calibration approach is used for the fire channels, on the basis of the same range of SST as that used for the other TIR channels.

The calibration scheme for the short-wave, near infra-red, and visible channels is based on a diffuser based calibration VISCAL system of accurately known reflectance which is illuminated by the sun over a short segment of the orbit, and which is intersected by the instrument scans. Using the signals from the VISCAL when it is illuminated by the Sun, provides a calibration reference to convert the measured signal in each channel to the surface reflectance.

Corrections for any detector non-linearity are applied using look-up tables derived from the pre-launch characterisation of the instrument.

SLSTR Visible Calibration Unit (credit Selex Galileo/TNO)


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