Abstract
The dark spectrum fitting (DSF) atmospheric correction method for aquatic application of metre-scale resolution optical satellite imagery is adapted to Landsat and Sentinel-2 (L/S2), including an automated tiled processing of full scene imagery and an optional image based glint correction. The DSF uses multiple dark targets in the subscene to construct a “dark spectrum” which is used to estimate the atmospheric path reflectance (ρpath) according to the best fitting aerosol model. This method is fully automated and can be used for full mission archive processing, as demonstrated here for a study region in the North Sea. The new approach overcomes common issues with the SWIR based exponential extrapolation approach (EXP). An evaluation of both methods is presented using Lw measurements from 19 sites in the AERONET-OC network over a 15 year period and 5 satellite sensors. Overall, the DSF performs better than the EXP, with a notable improvement in the blue spectral region. The tiled processing allows for a smooth ρpath estimation for full and merged L/S2 scenes, over clear and turbid coastal waters, inland waters, and land. The DSF selects the most appropriate band automatically, i.e. the one giving the lowest atmospheric path reflectance, and hence largely avoids amplification of glint and adjacency effects in the atmospheric correction. After application of the DSF, sun glint reflectance can be estimated from the SWIR bands, and the application of a sun glint correction significantly improves data availability for these nadir viewing sensors. A consistent processing across sensors allows for the exploitation of the >30 year L/S2 archive, including Landsat 5 imagery dating back to 1984. A practical application of the DSF and the L/S2 archive is presented, where the remotely sensed water turbidity from 5 satellites is compared with in situ measurements from a long-term (2000–present) monitoring station in the southern North Sea.
Highlights
The launch of Landsat 8 (L8) in 2013 and Sentinel-2 (S2) A and B in 2015 and 2017 sparked the interest of both the water and land remote sensing communities with the unprecedented spatial, spectral, and temporal coverage combined with impressive radiometric quality and free data access
Several atmospheric correction (AC) algorithms were developed for land and water applications of these systems, as evidenced by the 14 algorithms participating in the L8 and S2 Atmospheric Correction Intercomparison Exercise (Doxani et al, 2018)
The dark spectrum fitting (DSF) aerosol selection process is illustrated for a S2A scene over Zeebrugge in Fig. 2, showing the ρpath according to several τa steps in the look-up table (LUT) for the Continental and Maritime model, coloured from yellow to red with increasing τa
Summary
The launch of Landsat 8 (L8) in 2013 and Sentinel-2 (S2) A and B in 2015 and 2017 sparked the interest of both the water and land remote sensing communities with the unprecedented spatial, spectral, and temporal coverage combined with impressive radiometric quality and free data access. Coupled with easy data access through open portals such as USGS EarthExplorer, the Copernicus Science Hub, and several private initiatives such as Google Earth Engine (GEE), for the first time the entire image archive is available for free to individual researchers. This includes L5 imagery dating back to 1984, and data from earlier Landsats (1 through 4) dating back to the early 1970s. The aerosol optical thickness can be imposed from external measurements or model results (Harmel et al, 2018), or can be derived by spectral unmixing of end-member spectra (De Keukelaere et al, 2018) Other methods model both the water and atmospheric components simultaneously through iterative fitting (Steinmetz et al, 2011)
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