Minimize Calibration

On-board calibration of the SLSTR is based on the approach successfully implemented in the (A)ATSR series of instruments.

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 crossed by both the nadir and along track scan cones. Each target is observed once per cycle by each scan cone. One black body is held at a temperature lower than the expected range of surface brightness temperature, and the other at a temperature higher than the expected range, so that the interval between the two black body temperatures covers the expected range of sea surface brightness temperature. The physical temperatures of the two black bodies are continuously monitored, and so for each scan the linear calibration 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 diffuse calibration (VISCAL) target 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. Measurements of this target when it is illuminated by the sun enable the linear calibration relationship between the measured signal in each channel and the surface reflectance to be determined. If applicable, corrections for any detector non-linearity measured during the pre-launch characterisation of the instrument may be applied based on look-up tables.

SLSTR Visible Calibration Unit (credit Selex Galileo/TNO)