RESUMEN
The Three-Component Reflectance Model (3C) was primarily developed to improve the determination of the remote-sensing reflectance (Rrs) from above-water radiometric hyperspectral measurements performed during sub-optimal conditions (i.e., cloudy sky, variable viewing geometry, high glint perturbations, low illumination conditions). In view of further validating the model and showing its broad range of uses, this work presents the application of 3C to above-water radiometry data collected in oceanic and coastal waters with a variety of measurement conditions. Rrs derived from measurements performed during optimal and slightly sub-optimal conditions exhibit equivalence with Rrs obtained with an established above-water method that is commonly used to support ocean color validation activities. Additionally, the study shows that 3C can still provide relevant Rrs retrievals from field data characterized by low-light illumination, high glint perturbations and variable measurement geometries, for which the established method cannot be confidently applied. Finally, it is shown that the optimization residual returned by the 3C full-spectrum inversion procedure can be a potential relative indicator to assess the quality of derived Rrs.
RESUMEN
The equivalence of two radiometric methods relying on a single nadir-view optical sensor to determine the water-leaving radiance LW, namely the Single Depth Approach (SDA) and the Sky-Blocked Approach (SBA), was investigated applying identical hyperspectral radiometers operated on the same deployment platform. Values of LW from SDA and SBA measurements performed in the Black Sea across a variety of waters during ideal illumination conditions and with low-to-slight sea state, exhibited mean absolute differences within 0.5% in the blue-green spectral region and 2% in the red. This result, benefitting of a comprehensive parameterization of optical processes in combination with the characterization of sensors non-linearity, in-water response and reproducibility of absolute radiometric calibrations, indicated ample equivalence of the two near-surface methods in terms of performance and data reduction needs.
RESUMEN
Deployment platforms such as ships, towers or buoys may affect the accuracy of nearby radiometric measurements. Aiming at expanding the know-how on platform perturbations in above-water radiometric measurements, this study investigated the spectral impact of the Acqua Alta Oceanographic Tower (AAOT) on the remote-sensing reflectance RRS as a function of the distance d between the tower and the sensor footprint at the sea surface. This was accomplished by exploiting measurements performed with radiometers operated on deployment rigs extending beyond the AAOT superstructure with sensor viewing angle θ = 40° and relative azimuth Ï = 90° between sensor and sun. AAOT perturbations were also investigated by increasing the reflectance of the tower through white sheets covering part of its superstructure. Results indicate a spectral dependence of perturbations in RRS more pronounced in the near infrared, significantly increasing with the tower reflectance and decreasing with the inverse square of the distance d. In particular, for distances approaching the platform height, AAOT perturbations are found to be generally well below 1% for measurements performed in the visible spectral region and exceed 2% beyond 800 nm. However, with identical measurement geometry, but increasing the AAOT reflectance through the white cover, perturbations approach 1% in the blue-green spectral region and exceed 2% beyond approximately 600 nm. These findings, yet derived from a distinct tower and for specific measurement conditions, raise awareness on spectral perturbations of deployment platforms in above-water radiometry and additionally provide practical elements for the implementation of measurement protocol allowing to constrain these perturbations below required thresholds.
RESUMEN
The effects of non-nadir viewing geometry in above-water radiometry data were investigated using field measurements and two different correction approaches: one centered on chlorophyll-a concentration (Chla) developed for Case-1 waters, and the other relying on seawater inherent optical properties (IOP) proposed for any water type. With specific reference to data from the Ocean Color component of the AErosol RObotic NETwork (AERONET-OC), the study focused on the assessment of the uncertainties affecting corrections for non-nadir view of data collected with 40° in-air viewing angle and with 90° relative azimuth between viewing direction and sun. The study analyzed AERONET-OC water-leaving radiance data from different European seas to determine differences between corrections performed with the Chla- and the IOP-based approaches. Additionally, data collected in waters characterized by different optical complexity and comprising water-leaving radiances measured at nadir and with 28.6° in-water viewing angle (corresponding to 40° in-air) and 90° relative azimuth, were used to investigate the uncertainties of the two correction approaches. Results from the analysis of data from AERONET-OC sites characterized by a variety of optically complex waters, indicate corrections with uncertainties between 20% and 35% from 412 nm to 667 nm for the IOP-based approach. Conversely, uncertainties for the Chla-based one largely vary with wavelength and water type, with values of approximately 55% at 412 nm, 20-40% between 490 nm and 551 nm, and exceeding 60% at 667 nm.
RESUMEN
The spectral resolution requirements for in situ remote sensing reflectanceRRS measurements aiming at supporting satellite ocean color validation and System Vicarious Calibration (SVC) were investigated. The study, conducted using sample hyperspectral RRS from different water types, focused on the visible spectral bands of the ocean land color imager (OLCI) and of the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) satellite sensors. Allowing for a ±0.5% maximum difference between in situ and satellite derived RRS solely due to the spectral band characteristics of the in situ radiometer, a spectral resolution of 1 nm for SVC of PACE is needed in oligotrophic waters. Requirements decrease to 3 nm for SVC of OLCI. In the case of validation activities, which exhibit less stringent uncertainty requirements with respect to SVC, a maximum difference of ±1% between in situ and satellite derived data indicates the need for a spectral resolution of 3 nm for both OLCI and PACE in oligotrophic waters. Conversely, spectral resolutions of 6 nm for PACE and 9 nm for OLCI appear to satisfy validation activities in optically complex waters.
RESUMEN
The polarimetric characteristics of a class of hyperspectral radiometers commonly applied for above-water radiometry have been investigated by analyzing a sample of sensors. Results indicate polarization sensitivity increasing with wavelength and exhibiting values varying from sensor to sensor. In the case of radiance sensors, the maximum differences increase from approximately 0.4% at 400 nm to 1.3% at 750 nm. In the case of irradiance sensors, due to depolarizing effects of the diffusing collector, the maximum differences between horizontal and vertical polarization sensitivities vary from approximately 0.3% at 400 nm to 0.6% at 750 nm. Application of the previous results to above-water radiometry measurements performed in sediment dominated waters indicates that neglecting polarization effects may lead to uncertainties not exceeding a few tenths of a percent in remote sensing reflectance RRS determined in the 400-570 nm spectral interval. Conversely, uncertainties spectrally increase toward the near infrared, reaching several percent at 750 nm in the case of oligotrophic waters.
RESUMEN
Stray light perturbations are unwanted distortions of the measured spectrum due to the nonideal performance of optical radiometers. Because of this, stray light characterization and correction is essential when accurate radiometric measurements are a necessity. In agreement with such a need, this study focused on stray light correction of hyperspectral radiometers widely applied for above-water measurements to determine the remote-sensing reflectance (RRS). Stray light of sample radiometers was experimentally characterized and a correction algorithm was developed and applied to field measurements performed in the Mediterranean Sea. Results indicate that mean stray light corrections are appreciable, with values generally varying from -1% to +1% in the 400-700 nm spectral region for downward irradiance and sky radiance, and from -1% to +4% for total radiance from the sea. Mean corrections for data products such as RRS exhibit values that depend on water type varying between -0.5% and +1% in the blue-green spectral region, with peaks up to 9% in the red in eutrophic waters. The possibility of using one common stray light correction matrix for the analyzed class of radiometers was also investigated. Results centered on RRS support such a feasibility at the expense of an increment of the uncertainty typically well below 0.5% in the blue-green and up to 1% in the red, assuming sensors are based on spectrographs from the same production batch.
