The SLSTR design is based on the heritage of the Along-Track Scanning Radiometers (ATSR) that were previously flown on ERS-1/2 and Envisat. The instrument design has the same spectral bands as used ATSR-2 and AATSR at wavelengths of 0.555µm (S1), 0.660µm (S2), 0.870µm (S3), 1.6µm (S5), 3.74µm (S7), 10.8µm (S8), and 12µm (S9) ensuring continuity of data, together with two new channels at 1.375µm (S4) and 2.25µm (S6) for improved cloud detection. SLSTR also includes the capability to measure active wildfires, achieved by extending the dynamic range of the 10.8µm channel (F2) and including dedicated detectors at 3.7µm (F1).

The principle design feature of the radiometer is the capability to view the same position on the Earth at two view zenith angles to provide a robust atmospheric correction. This is achieved through two conical scans, at nadir and oblique, performed by two independent scan mirrors rotating in opposite directions, each scanning at a rate of 200 scans per minute. This rotation rate corresponds to 2 km per scan. The width of the nominal image swath is 1400 km for the nadir view, and 740 km for the oblique view. The optical design ensures that the scanners view the on-board calibration sources once every scan cycle (two consecutive scans) so there is no need for special calibration modes.

The TIR detectors (S7-S9, F1, F2) have a resolution of 1km at nadir, and the VIS-SWIR channels (S1-S6) have a resolution of 0.5 km at nadir. The detectors are arranged 2x11km pixels (along track, across track) for the TIR, 4x1 0.5 km pixels for the VIS channels (S1-S3) and 4x2 0.5 km pixels for the SWIR (S4-S6). The detectors are optically co-registered so that they view the same area of the earth simultaneously. One of the columns of the SWIR detectors are optically co-aligned with the VIS detectors. This is the so-called ‘a' stripe.

The SLSTR Optical Scanning Unit. (Credit IABG)

SLSTR is separated into two physical units that are integrated onto a single plate: the SLSTR Optical Scanning Unit (see figure above) housing the Optomechanical Enclosure (OME) and Detection Assembly (DA), and a separate SLSTR control and processor electronics unit.

SLSTR Visible Channel Detector (Credit OSI)
Structural and Thermal Model of the SLSTR (showing the large kidney-shaped Earth-view baffles of nadir (bottom) and oblique (top) views) (Credit IABG)


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