Novel techniques for the analysis of the TOA radiometric uncertainty

Abstract

In the framework of the European Copernicus programme, the European Space Agency (ESA) has launched the Sentinel-2 (S2) Earth Observation (EO) mission which provides optical high spatial -resolution imagery over land and coastal areas. As part of this mission, a tool (named S2-RUT, from Sentinel-2 Radiometric Uncertainty Tool) estimates the radiometric uncertainties associated to each pixel using as input the top-of-atmosphere (TOA) reflectance factor images provided by ESA. The initial version of the tool has been implemented -code and user guide available 1 -and integrated as part of the Sentinel Toolbox. The tool required the study of several radiometric uncertainty sources as well as the calculation and validation of the combined standard uncertainty in order to estimate the TOA reflectance factor uncertainty per pixel. Here we describe the recent research in order to accommodate novel uncertainty contributions to the TOA reflectance uncertainty estimates in future versions of the tool. The two contributions that we explore are the radiometric impact of the spectral knowledge and the uncertainty propagation of the resampling associated to the orthorectification process. The former is produced by the uncertainty associated to the spectral calibration as well as the spectral variations across the instrument focal plane and the instrument degradation. The latter results of the focal plane image propagation into the provided orthoimage. The uncertainty propagation depends on the radiance levels on the pixel neighbourhood and the pixel correlation in the temporal and spatial dimensions. Special effort has been made studying non-stable scenarios and the comparison with different interpolation methods.

A Radiometric Uncertainty Tool for the Sentinel 2 Mission

“…The script automatically reads certain calibration parameters as DS or relative gain coefficients. The version was presented in [16] and expanded to L1C in [17]. The effects of the contributors detailed in Sections 3.2.2 and 3.2.9 are not considered, since they are added linearly in Equation (13).…”

“…The validation up to L1C involves the propagation through the bi-spline radiometric interpolation as in [17]. The radiance levels in Figure 5 have been propagated assuming a perfectly flat 4 × 4 kernel grid of constant radiance values.…”

“…For example, the "spectral knowledge" will largely depend on the spectral signature of the measured scene whereas the uncertainty propagation through the orthorectification and the accuracy of the resampling will depend on the radiometric uniformity of the scene. The "novel methodologies" required to assess these contributors have already been discussed in [17] which provides a preliminary discussion of the impact of the Ortho-rectification uncertainty propagation and Spectral knowledge. The refinement of these methods might result in the implementation of these contributors in further versions of the S2-RUT.…”

“…The residual of this effect is accounted in Non-uniformity spectral residual contributor. See [17] for the preliminary assessment and first implementation of this contributor.…”

“…Recent missions, such as S2, include a bi-spline interpolation of the data. Research in [17] has proposed a preliminary implementation of the propagation of the uncertainty through the resampling process. Further assessment should also consider the accuracy of the resampling with respect to the real scene fluctuations.…”