Satellite ocean colour radiometry provides an indispensable global perspective for monitoring ocean health and ecosystems, but rigorous validation using in situ data is crucial for ensuring the accuracy and precision required for climate-quality records.

The study, led by Silvia Pardo of NEODAAS and hosted at Plymouth Marine Laboratory, focused on assessing the radiometric products of three key satellite instruments: Ocean and Land Colour Instrument (OLCI) aboard Sentinel-3A and 3B, Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua, and Visible Infrared Imaging Radiometer Suite instrument (VIIRS) aboard Suomi NPP and NOAA-20.

The research used high-quality Fiducial Reference Measurements (FRM) collected during four Atlantic Meridional Transect (AMT) field campaigns from 2016 to 2019. These FRM measurements, which conform to satellite protocols and metrology standards, provide a robust basis for validation of ocean-colour satellite data.

The AMT field campaigns provide a unique platform for acquiring accurate in situ radiometric measurements in a wide range of oceanic environments, including the rarely-sampled oligotrophic, low chlorophyll-a waters in the remote North and South Atlantic Ocean gyres.

One of the significant findings was the comparison of atmospheric correction models for the OLCI radiometric products from Sentinel-3A and 3B. The study found that the POLYMER model yielded the best results in retrieving water-leaving radiances. Furthermore, the research revealed that Suomi-VIIRS and MODIS-Aqua performed better than NOAA-20 VIIRS, and comparably to OLCI-3B standard products.

The study also compared the performance of OLCI-3A and OLCI-3B instruments during their tandem phase, uncovering systematic differences in radiances across the spectrum. In addition, it was observed that the OLCI-3A standard product outperformed similar products from NASA sensors.

Lead author, Silvia Pardo, of NEODAAS and Plymouth Marine Laboratory said:

"The Atlantic Meridional Transect provides an invaluable platform to collect high-quality in situ radiometric measurements across a range of oceanic regimes. By pairing these state-of-the-art field measurements with rigorous validation protocols, we have achieved an unprecedented assessment of the radiometric accuracy for key ocean colour missions including OLCI, MODIS and VIIRS. Our results give us confidence that these satellites are accurately measuring water-leaving radiance in wide range of oceanic environments, which increases our ability to monitor ocean health and productivity from space".

Dr Gavin Tilstone, Principal Investigator for the AMT4CO2Flux project and Bio-optical Oceanographer at Plymouth Marine Laboratory, said:

"This is the most comprehensive assessment of OLCI radiometric products in open-ocean waters to date."

This study presents a robust methodology for validating ocean-colour satellite instruments using high-quality in situ measurements, contributing to our understanding of their accuracy and performance. The results highlight the strengths and weaknesses of different satellite instruments and atmospheric correction models, aiding in the enhancement of ocean colour remote sensing capabilities for monitoring marine ecosystems and climate change effects.