Global assessment of directional effects in the inter-calibration of optical satellite instruments with the TRUTHS mission

Upcoming SI-Traceable Satellite (SITSat) missions such as TRUTHS aim to achieve an unprecented accuracy for SI-traceable measurements of the Earth-reflected radiation. These measurements will support the generation of low uncertainty climate records and significantly improve the calibration of other sensors. In such a context, the uncertainty will be limited by the calibration transfer process rather than the reference sensor. This study presents an end-to-end global inter-calibration simulator capable of assessing the potential uncertainty for multiple scenarios that considers the interrelation of different error sources and match-ups. We first define the sensor-to-sensor match-ups through an orbital analysis that is followed by a top-of-atmosphere (TOA) radiance modelling of each match-up. Finally, we calculate the radiometric uncertainty based on different error sources combined globally. In this first implementation we have calculated the match-ups of TRUTHS against observations by the Copernicus Sentinel-2A satellite over land areas throughout a year. We calculate the angular mismatch for both viewing differences and solar changes from different overpass time. We define multiple inter-calibration scenarios based on temporal, angular or cloud constraints. These first results show that considering overpasses up to 15 minute difference, low cloud probability and matching field-of-view (FoV), within 5°, we sample most land areas with a mean error <0.1% and bias regression <0.5%. We have also restricted the sun zenith angle (SZA) to 60° to minimise solar angle and viewing azimuthal dispersion over the poles. This also results in data-gaps of several months that might be complemented with dedicated manouevres or a dedicated processing of these polar-region match-ups.