Project update  |   30 November 2023

New publication on spatial structure of in situ reflectance in coastal and inland waters

A new paper has been published as part of the HyperBOOST project in the journal Frontiers in Remote Sensing entitled "Spatial structure of in situ reflectance in coastal and inland waters: implications for satellite validation".

This is the first publication partially funded by the European Space Agency's HyperBOOST project. With collaboration from scientists at Plymouth Marine Laboratory, UK and the Marine and Environmental Science Centre, Faculdade de Ciências da Universidade de Lisboa, Lisboa, Portugal.

The study consists of deployments at three different water bodies (Western English Channel - UK, Tagus Estuary - Portugal, Lake Balaton - Hungary). In each case, a So-Rad system was mounted onboard a ship-of-opportunity undergoing operational tasks whilst capturing radiometric data and ship transects.

Abstract

Validation of satellite-derived aquatic reflectance involves relating meter-scale in situ 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 in situ data from moving platforms with satellite pixels. Here, using high-frequency autonomous mobile radiometers deployed on ships, we characterize the spatial structure of in situ Rrs in inland and coastal waters (Lake Balaton, Western English Channel, Tagus Estuary). Using variogram analysis, we partition Rrs variability into spatial and intrinsic (non-spatial) components. We then demonstrate the capacity of mobile radiometers to spatially sample in situ Rrs 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 in situ Rrs 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 in situ Rrs became spatially decorrelated ranged from ∼ 100–1,000 m. This information serves as a guideline for selection of spatially independent in situ Rrs 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).

Read the full publication on the Frontiers in Remote Sensing website


Reference

Jordan, TM, Simis, SGH, Selmes, N, Sent, G, Ienna, F and Martinez-Vicente, V. 2023. Spatial structure of in situ reflectance in coastal and inland waters: implications for satellite validation. Frontiers in Remote Sensing, 4. https://doi.org/10.3389/frsen.2023.1249521