Abstract
This work describes a proposed radiometric cross calibration between the Landsat 8 Operational Land Imager (OLI) and Sentinel 2A Multispectral Instrument (MSI) sensors. The cross-calibration procedure involves (i) correction of the MSI data to account for spectral band differences with OLI and (ii) normalization of Bidirectional Reflectance Distribution Function (BRDF) effects in the data from both sensors using a new model accounting for the view zenith/azimuth angles in addition to the solar zenith/view angles. Following application of the spectral and BRDF normalization, standard least-squares linear regression is used to determine the cross-calibration gain and offset in each band. Uncertainties related to each step in the proposed process are determined, as is the overall uncertainty associated with the complete processing sequence. Validation of the proposed cross-calibration gains and offsets is performed on image data acquired over the Algodones Dunes site. The results of this work indicate that the blue band has the most significant offset, requiring use of the estimated cross-calibration offset in addition to the estimated gain. The highest difference was observed in the blue and red bands, which are 2.6% and 1.4%, respectively, while other bands shows no significant difference. Overall, the net uncertainty in the proposed process was estimated to be on the order of 6.76%, with the largest uncertainty component due to each sensor’s calibration uncertainty on the order of 5% and 3% for the MSI and OLI, respectively. Other significant contributions to the uncertainty include seasonal changes in solar zenith and azimuth angles, target site nonuniformity, variability in atmospheric water vapor, and/or aerosol concentration.
Highlights
Ensuring the quality and accuracy of an Earth-orbiting satellite sensor’s image data throughout its operating lifetime [1] requires regular monitoring of its radiometric and geometric performance
This paper proposes a technique to perform reflectance-based cross calibration between the Landsat 8 Operational Land Imager (OLI) and Sentinel 2A Multispectral Instrument (MSI) sensors with a proposed Bidirectional Reflectance Distribution Function (BRDF) model, using coincident pairs of OLI and MSI images acquired over the Libya 4, Libya 1, Niger 2, and Sudan 1 North African Pseudo-Invariant Calibration Sites (PICS)
To compare uncertainties relating to selection of a linear vs quadratic BRDF model, TOA reflectances normalized with the proposed four-angle linear model in Equation (6) were compared to the corresponding results obtained from (i) linear and quadratic models based on the spherical SZA alone (Figure 5a,b respectively) which is obtained from Reference [16] and (ii) a multi-linear four-angle model based on the Cartesian coordinates that accounted for second-order effects and allowed interactions between all angles as in Equation (8) and Equation (9)
Summary
Ensuring the quality and accuracy of an Earth-orbiting satellite sensor’s image data throughout its operating lifetime [1] requires regular monitoring of its radiometric and geometric performance. Sampled TOA reflectances of the calibrated reference sensor They tested the proposed approach on non-BRDF-corrected image data acquired over the CEOS Saharan desert sites and found an approximate 2% difference in the red and NIR bands. In a rigorous analysis using Monte Carlo and MODTRAN (MODerate resolution atmospheric TRANsmission) simulations, Gorrono et al (2017) considered different sources of spectral, spatial, and temporal uncertainties affecting cross calibration of the Sentinel 2A MSI [22] Their reference data source was hyperspectral data simulated for the upcoming Traceable Radiometry Underpinning Terrestrial and Helio Studies (TRUTHS) sensor. This paper proposes a technique to perform reflectance-based cross calibration between the Landsat 8 Operational Land Imager (OLI) and Sentinel 2A Multispectral Instrument (MSI) sensors with a proposed BRDF model, using coincident pairs of OLI and MSI images acquired over the Libya 4, Libya 1, Niger 2, and Sudan 1 North African Pseudo-Invariant Calibration Sites (PICS). Given Landsat 8’s revisit time of 16 days and Sentinel 2A’s revisit time of 10 days, opportunities for same-day, coincident acquisition by both sensors occur approximately every 80 days
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