Field Intercomparison of Radiometer Measurements for Ocean Colour Validation

Type Article
Date 2020-05
Language English
Author(s) Tilstone Gavin1, Dall’olmo Giorgio1, 2, Hieronymi Martin3, Ruddick Kevin4, Beck Matthew4, Ligi Martin5, Costa Maycira6, D’alimonte Davide7, Vellucci Vincenzo8, Vansteenwegen Dieter9, Bracher Astrid10, Wiegmann Sonja10, Kuusk Joel5, Vabson Viktor5, Ansko Ilmar5, Vendt Riho5, Donlon Craig11, Casal Tânia11
Affiliation(s) 1 : Plymouth Marine Laboratory, Earth Observation Science and Applications, Plymouth PL1 3DH, UK
2 : National Centre for Earth Observations, Plymouth PL1 3DH, UK
3 : Institute of Coastal Research, Helmholtz-Zentrum Geesthacht (HZG), 21502 Geesthacht, Germany
4 : Royal Belgian Institute of Natural Sciences, 29 Rue Vautierstraat, 1000 Brussels, Belgium
5 : Tartu Observatory, University of Tartu, Tõravere EE-61602, Estonia
6 : Geography Department at the University of Victoria, Victoria, BC V8P 5C2, Canada
7 : Center for Marine and Environmental Research CIMA, University of Algarve, 8005-139 Faro, Portugal
8 : Sorbonne Université, CNRS, Institut de la Mer de Villefranche, IMEV, F-06230 Villefranche-sur-Mer, France
9 : Flanders Marine Institute (VLIZ), Wandelaarkaai 7, 8400 Ostend, Belgium
10 : Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Department of Climate Sciences, D-27570 Bremerhaven, Germany
11 : European Space Agency, 2201 AZ Noordwijk, The Netherlands
Source Remote Sensing (2072-4292) (MDPI AG), 2020-05 , Vol. 12 , N. 10 , P. 1587 (50p.)
DOI 10.3390/rs12101587
WOS© Times Cited 25
Note This article belongs to the Special Issue Fiducial Reference Measurements for Satellite Ocean Colour
Keyword(s) fiducial reference measurements, remote sensing reflectance, ocean colour radiometers, TriOS RAMSES, Seabird HyperSAS, field intercomparison, AERONET-OC, Acqua Alta Oceanographic Tower

A field intercomparison was conducted at the Acqua Alta Oceanographic Tower (AAOT) in the northern Adriatic Sea, from 9 to 19 July 2018 to assess differences in the accuracy of in- and above-water radiometer measurements used for the validation of ocean colour products. Ten measurement systems were compared. Prior to the intercomparison, the absolute radiometric calibration of all sensors was carried out using the same standards and methods at the same reference laboratory. Measurements were performed under clear sky conditions, relatively low sun zenith angles, moderately low sea state and on the same deployment platform and frame (except in-water systems). The weighted average of five above-water measurements was used as baseline reference for comparisons. For downwelling irradiance (), there was generally good agreement between sensors with differences of <6% for most of the sensors over the spectral range 400 nm–665 nm. One sensor exhibited a systematic bias, of up to 11%, due to poor cosine response. For sky radiance () the spectrally averaged difference between optical systems was <2.5% with a root mean square error (RMS) <0.01 mWm−2 nm−1 sr−1. For total above-water upwelling radiance (), the difference was <3.5% with an RMS <0.009 mWm−2 nm−1 sr−1. For remote-sensing reflectance (), the differences between above-water TriOS RAMSES were <3.5% and <2.5% at 443 and 560 nm, respectively, and were <7.5% for some systems at 665 nm. Seabird HyperSAS sensors were on average within 3.5% at 443 nm, 1% at 560 nm, and 3% at 665 nm. The differences between the weighted mean of the above-water and in-water systems was <15.8% across visible bands. A sensitivity analysis showed that accounted for the largest fraction of the variance in , which suggests that minimizing the errors arising from this measurement is the most important variable in reducing the inter-group differences in . The differences may also be due, in part, to using five of the above-water systems as a reference. To avoid this, in situ normalized water-leaving radiance () was therefore compared to AERONET-OC SeaPRiSM as an alternative reference measurement. For the TriOS-RAMSES and Seabird-Hyperspectral Surface Acquisition System (HyperSAS) sensors the differences were similar across the visible spectra with 4.7% and 4.9%, respectively. The difference between SeaPRiSM and two in-water systems at blue, green and red bands was 11.8%. This was partly due to temporal and spatial differences in sampling between the in-water and above-water systems and possibly due to uncertainties in instrument self-shading for one of the in-water measurements.

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Tilstone Gavin, Dall’olmo Giorgio, Hieronymi Martin, Ruddick Kevin, Beck Matthew, Ligi Martin, Costa Maycira, D’alimonte Davide, Vellucci Vincenzo, Vansteenwegen Dieter, Bracher Astrid, Wiegmann Sonja, Kuusk Joel, Vabson Viktor, Ansko Ilmar, Vendt Riho, Donlon Craig, Casal Tânia (2020). Field Intercomparison of Radiometer Measurements for Ocean Colour Validation. Remote Sensing, 12(10), 1587 (50p.). Publisher's official version : , Open Access version :