Abstract
The performance of the dark spectrum fitting (DSF) atmospheric correction algorithm is evaluated using matchups between metre- and decametre-scale satellite imagery as processed with ACOLITE and measurements from autonomous PANTHYR hyperspectral radiometer systems deployed in the Adriatic and North Sea. Imagery from the operational land imager (OLI) on Landsat 8, the multispectral instrument (MSI) on Sentinel-2 A and B, and the PlanetScope CubeSat constellation was processed for both sites using a fixed atmospheric path reflectance in a small region of interest around the system's deployment location, using a number of processing settings, including a new sky reflectance correction. The mean absolute relative differences (MARD) between in situ and satellite measured reflectances reach <20% in the Blue and 11% in the Green bands around 490 and 560 nm for the best performing configuration for MSI and OLI. Higher relative errors are found for the shortest Blue bands around 440 nm (30-100% MARD), and in the Red-Edge and near-infrared bands (35-100% MARD), largely influenced by the lower absolute data range in the observations. Root mean squared differences (RMSD) increase from 0.005 in the NIR to about 0.015-0.020 in the Blue band, consistent with increasing atmospheric path reflectance. Validation of the Red-Edge and NIR bands on Sentinel-2 is presented, as well as for the first time, the Panchromatic band (17-26% MARD) on Landsat 8, and the derived Orange contra-band (8-33% MARD for waters in the algorithm domain, and around 40-80% MARD overall). For Sentinel-2, excluding the SWIR bands from the DSF gave better performances, likely due to calibration issues of MSI at longer wavelengths. Excluding the SWIR on Landsat 8 gave good performance as well, indicating robustness of the DSF to the available band set. The DSF performance was found to be rather insensitive to (1) the wavelength spacing in the lookup tables used for the atmospheric correction, (2) the use of default or ancillary information on gas concentration and atmospheric pressure, and (3) the size of the ROI over which the path reflectance is estimated. The performance of the PlanetScope constellation is found to be similar to previously published results, with the standard DSF giving the best results in the visible bands in terms of MARD (24-40% overall, and 18-29% for the turbid site). The new sky reflectance correction gave mixed results, although it reduced the mean biases for certain configurations and improved results for the processing excluding the SWIR bands, giving lower RMSD and MARD especially at longer wavelengths (>600 nm). The results presented in this article should serve as guidelines for general use of ACOLITE and the DSF.
Highlights
A multitude of metre- and decametre-scale optical satellite sensors have been collecting regular imagery of the earth in the last few years, including Landsat 8 (2013–present), Sentinel-2 (2 units, 2015–present and 2017–present), RapidEye (5 units, 2012–present) and PlanetScope (100’s of units since 2015)
Imagery from the operational land imager (OLI) on Landsat 8, the multispectral instrument (MSI) on Sentinel-2 A and B, and the PlanetScope CubeSat constellation was processed for both sites using a fixed atmospheric path reflectance in a small region of interest around the system’s deployment location, using a number of processing settings, including a new sky reflectance correction
Scatterplots of the matchups between Sentinel-2A and B (S2) and PANTHYR for the Dark Spectrum Fitting (DSF)+NoSWIR+SkyTOA processing are shown in Fig. 5, and between Landsat 8 (L8) and PANTHYR in Fig. 6, with points coloured according to site
Summary
A multitude of metre- and decametre-scale optical satellite sensors have been collecting regular imagery of the earth in the last few years, including Landsat 8 (2013–present), Sentinel-2 (2 units, 2015–present and 2017–present), RapidEye (5 units, 2012–present) and PlanetScope (100’s of units since 2015). The DSF as applied to a number of high resolution satellites, in particular Landsat 8, Sentinel-2 A/B, and PlanetScope, is evaluated using matchups with two autonomously measuring PANTHYR systems [40] in two sites featuring distinct water types, one in the Belgian coastal zone, and one in the Adriatic Sea. This paper is intended as an update of validation results as previously presented [4,5,27], and includes a sensitivity analysis to various processing settings in order to make general recommendations to the users. This study provides the first validation of Panchromatic and Orange contra-band [13] water reflectances as derived from the Operational Land Imager (OLI) on board Landsat 8, and extends the validation to the Red-Edge and NIR bands on Sentinel-2 previously not possible with multispectral in situ measurements
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