The calibration and performance of SLSTR is monitored using analysis of orbital and long-term trends of key instrument parameters:
- Instrument temperature trends
- Blackbody thermometers, signals, noise, NEDT analysis
- VISCAL signal trends
- Visible channel signals and noise
- Gain and offset variations
- Contamination monitoring
- Scan mirror jitter statistics
As a primarily thermal infrared instrument, the thermal stability and uniformity of the instrument, baffles and detectors, as well as the blackbody calibration sources, is critical to the radiometric calibration. In addition, the stability of the VISCAL source is critical for the calibration of the visible/SWIR channels. The status of these key performance parameters is reported in the SLSTR Data Product Quality Reports.
VISCAL signal trend for VIS channels (nadir view)
The visible channels show oscillations in their radiometric response due to the build-up of ice on the optical path within the focal plane assembly (FPA). Similar oscillations were observed for the corresponding channels on ATSR-2 and AATSR. Periodic decontamination of the infrared FPA is necessary to remove the water ice contamination. The radiometric responses of the SWIR channels appear to be more stable and not affected by the build-up of water ice contamination, although there is a seasonal cycle of the response that could be caused by variations in the solar zenith angle on the diffuser or partial vignetting of the Sun's disc by the VISCAL baffle.
It should be noted that the data from the VISCAL unit and blackbodies calibrates the signal and counteracts the degradation of the optics and other variations in signal.
Regular monitoring of the geolocation performance in Level-1 images is carried out using the GeoCal Tool implemented at the SLSTR Mission Performance Centre. This monitors the geolocation performance by correlation with ground control point (GCP) imagettes. Each Level-1 granule typically contains several hundred GCPs, which are filtered based on signal-to-noise to obtain a daily average in the across and along track directions.
The radiometric calibration of the visible and SWIR channels is monitored using the S3ETRAC service. The S3ETRAC service extracts OLCI and SLSTR Level-1 data and computes associated statistics over 49 sites corresponding to different surface types (desert, snow, ocean maximising Rayleigh signal, and ocean maximising sunglint scattering). These S3ETRAC products are used for the assessment and monitoring of the VIS and SWIR radiometry by the SLSTR Expert Support Laboratory.
The latest status of the SLSTR calibration and performance can be found in the SLSTR Data Product Quality Reports.