AUTHOR=Jordan Thomas M. , Simis Stefan G. H. , Selmes Nick , Sent Giulia , Ienna Federico , Martinez-Vicente Victor 

TITLE=Spatial structure of in situ reflectance in coastal and inland waters: implications for satellite validation

JOURNAL=Frontiers in Remote Sensing

VOLUME=4

YEAR=2023

URL=https://www.frontiersin.org/journals/remote-sensing/articles/10.3389/frsen.2023.1249521

DOI=10.3389/frsen.2023.1249521

ISSN=2673-6187

ABSTRACT=<p>Validation of satellite-derived aquatic reflectance involves relating meter-scale <italic>in situ</italic> observations to satellite pixels with typical spatial resolution ∼ 10–100 m within a temporal “match-up window” of an overpass. Due to sub-pixel variation these discrepancies in measurement scale are a source of uncertainty in the validation result. Additionally, validation protocols and statistics do not normally account for spatial autocorrelation when pairing <italic>in situ</italic> data from moving platforms with satellite pixels. Here, using high-frequency autonomous mobile radiometers deployed on ships, we characterize the spatial structure of <italic>in situ</italic><italic>R</italic><sub><italic>rs</italic></sub> in inland and coastal waters (Lake Balaton, Western English Channel, Tagus Estuary). Using variogram analysis, we partition <italic>R</italic><sub><italic>rs</italic></sub> variability into spatial and intrinsic (non-spatial) components. We then demonstrate the capacity of mobile radiometers to spatially sample <italic>in situ</italic><italic>R</italic><sub><italic>rs</italic></sub> within a temporal window broadly representative of satellite validation and provide spatial statistics to aid satellite validation practice. At a length scale typical of a medium resolution sensor (300 m) between 5% and 35% (median values across spectral bands and deployments) of the variation in <italic>in situ</italic><italic>R</italic><sub><italic>rs</italic></sub> was due to spatial separation. This result illustrates the extent to which mobile radiometers can reduce validation uncertainty due to spatial discrepancy via sub-pixel sampling. The length scale at which <italic>in situ</italic><italic>R</italic><sub><italic>rs</italic></sub> became spatially decorrelated ranged from ∼ 100–1,000 m. This information serves as a guideline for selection of spatially independent <italic>in situ</italic><italic>R</italic><sub><italic>rs</italic></sub> when matching with a satellite image, emphasizing the need for either downsampling or using modified statistics when selecting data to validate high resolution sensors (sub 100 m pixel size).</p>