Abstract
The Sentinel-3 SLSTR instrument is primarily designed to measure Sea and Land Surface Temperatures for meteorological and climate research applications. As well as spectral bands in the thermal infrared for measurement of surface temperatures, the radiometer is equipped with channels in the visible to short wavelength infrared range, primarily for daytime cloud screening and scene classification. However, the VIS-SWIR channels are used in the retrievals of Fire Radiative Power (FRP) and land applications when used in synergy with the OLCI instrument data. Furthermore, when combined with the dual view capability of the radiometer, the data from the VIS-SWIR channels are used for measurements of aerosol and cloud properties not possible with a single view instrument.
The use of the VIS-SWIR channels in Sentinel-3 level-2 data products, demands that they be radiometrically calibrated to standards traceable to SI. Demonstrating direct traceability to SI is not so straightforward because the L1 processing has many inputs, and several assumptions are made about the instrument model. Analysis of the radiometric model used in the L1 processing shows that there are several key factors that affect the radiometric performances, for instance:
• Calibration of the diffuser-based calibrator
• Long term stability of the calibrator
• Response non-linearity
• Ground-orbit changes – e.g., where there are differences between the pre-launch test conditions and flight operations.
Analysis has been performed by several groups to assess the on-orbit performance of the VIS-SWIR channels using Pseudo Invariant Calibration Sites (PICS). An earlier analysis performed by RAL for the ESA-funded Sentinel-3 Mission Performance Centre (MPC) established calibration corrections for users to apply to the Level-1 data products [1]. Since then, further activities have continued to monitor the calibration performance by RAL using comparisons to measurements of historical sensors, and by ACRI-ST using the DIMITRI toolkit. In addition, EUMETSAT using MICMICS intercalibration and calibration (radiative transfer) modules, and CNES using the SADE database and MUSCLE (associated) tools have provided further independent measurements. Although each group uses the same reference sites and input Level-1 data, differences in the analysis methods, in particular the reference sensor used, can result in small differences in the results. A working group has been set up to compare the results of the different methods, with a view to providing updated coefficients for the absolute radiometric calibration and the long-term stability of the calibration. As the various methodologies use different references, a key challenge is to define and align to a common reference sensor that covers the full spectral range of SLSTR.
References:
1. Sentinel-3 SLSTR VIS and SWIR Channel Vicarious Calibration Adjustments - Sentinel-3 Mission Performance Centre
Merging Multiple Analyses of SLSTR Vis-SWIR Vicarious Calibration Results
The Sentinel-3 SLSTR instrument is primarily designed to measure Sea and Land Surface Temperatures for meteorological and climate research applications. As well as spectral bands in the thermal infrared for measurement of surface temperatures, the radiometer is equipped with channels in the visible to short wavelength infrared range, primarily for daytime cloud screening and scene classification. However, the VIS-SWIR channels are used in the retrievals of Fire Radiative Power (FRP) and land applications when used in synergy with the OLCI instrument data. Furthermore, when combined with the dual view capability of the radiometer, the data from the VIS-SWIR channels are used for measurements of aerosol and cloud properties not possible with a single view instrument.
The use of the VIS-SWIR channels in Sentinel-3 level-2 data products, demands that they be radiometrically calibrated to standards traceable to SI. Demonstrating direct traceability to SI is not so straightforward because the L1 processing has many inputs, and several assumptions are made about the instrument model. Analysis of the radiometric model used in the L1 processing shows that there are several key factors that affect the radiometric performances, for instance:
• Calibration of the diffuser-based calibrator
• Long term stability of the calibrator
• Response non-linearity
• Ground-orbit changes – e.g., where there are differences between the pre-launch test conditions and flight operations.
Analysis has been performed by several groups to assess the on-orbit performance of the VIS-SWIR channels using Pseudo Invariant Calibration Sites (PICS). An earlier analysis performed by RAL for the ESA-funded Sentinel-3 Mission Performance Centre (MPC) established calibration corrections for users to apply to the Level-1 data products [1]. Since then, further activities have continued to monitor the calibration performance by RAL using comparisons to measurements of historical sensors, and by ACRI-ST using the DIMITRI toolkit. In addition, EUMETSAT using MICMICS intercalibration and calibration (radiative transfer) modules, and CNES using the SADE database and MUSCLE (associated) tools have provided further independent measurements. Although each group uses the same reference sites and input Level-1 data, differences in the analysis methods, in particular the reference sensor used, can result in small differences in the results. A working group has been set up to compare the results of the different methods, with a view to providing updated coefficients for the absolute radiometric calibration and the long-term stability of the calibration. As the various methodologies use different references, a key challenge is to define and align to a common reference sensor that covers the full spectral range of SLSTR.
References:
1. Sentinel-3 SLSTR VIS and SWIR Channel Vicarious Calibration Adjustments - Sentinel-3 Mission Performance Centre