Abstract
Few weeks after its launch in April 2018, Sentinel-3B of the European Space Agency has been put in a tandem phase with its twin Sentinel-3A already in orbit. Both platforms were on the same track with the same geometrical conditions to gather acquisitions over the same targets only thirty seconds apart. This tandem phase lasted from early June to mid October 2018 to provide a unique opportunity for each S-3 sensors to increase knowledge of payload differences, reduce uncertainties when comparing data and to homogenise differences by defining appropriate adjustments. The inter-unit consistency is critical for the mission.
The outcome of the tandem phase analysis provides a strong reference for assessing other cross-calibration methodologies, one of those being based on the use of Deep Convective Clouds (DCCs). Whereas a physical model of DCC reflectance must be provided to compare Ocean and Land Colour Instrument (OLCI) measurements with an absolute reference, DCC observations are rather used for their whiteness, brightness, and large spatial extent, for interband monitoring. In this presentation, we present and validate a DCC-based radiometric validation methodology adapted to OLCI with a specific emphasis on its ability to accurately monitor the cross-calibration of the independent sensors. We base the analysis on a careful analysis of the OLCI DCC reflectance measurements with a sensitivity assessment of the data selection employed (use of SLSTR synergetic brightness temperature or reflectance in absorption bands, analysis and handling of saturated pixels) as well as a cautious analysis of the FOV-dependency of the results.
Performance is assessed by comparisons with the cross-calibration reference of the tandem analysis, in and out of the tandem phase acquisition period. The methodology covers the complete OLCI spectrum (to the exception of absorption bands) with precision less than about 1%.
Deep Convective Clouds for Sentinel-3 OLCI Cross-Calibration Monitoring
Few weeks after its launch in April 2018, Sentinel-3B of the European Space Agency has been put in a tandem phase with its twin Sentinel-3A already in orbit. Both platforms were on the same track with the same geometrical conditions to gather acquisitions over the same targets only thirty seconds apart. This tandem phase lasted from early June to mid October 2018 to provide a unique opportunity for each S-3 sensors to increase knowledge of payload differences, reduce uncertainties when comparing data and to homogenise differences by defining appropriate adjustments. The inter-unit consistency is critical for the mission.
The outcome of the tandem phase analysis provides a strong reference for assessing other cross-calibration methodologies, one of those being based on the use of Deep Convective Clouds (DCCs). Whereas a physical model of DCC reflectance must be provided to compare Ocean and Land Colour Instrument (OLCI) measurements with an absolute reference, DCC observations are rather used for their whiteness, brightness, and large spatial extent, for interband monitoring. In this presentation, we present and validate a DCC-based radiometric validation methodology adapted to OLCI with a specific emphasis on its ability to accurately monitor the cross-calibration of the independent sensors. We base the analysis on a careful analysis of the OLCI DCC reflectance measurements with a sensitivity assessment of the data selection employed (use of SLSTR synergetic brightness temperature or reflectance in absorption bands, analysis and handling of saturated pixels) as well as a cautious analysis of the FOV-dependency of the results.
Performance is assessed by comparisons with the cross-calibration reference of the tandem analysis, in and out of the tandem phase acquisition period. The methodology covers the complete OLCI spectrum (to the exception of absorption bands) with precision less than about 1%.