Polarized reflectance and transmittance distribution functions of the ocean surface

Automatic Suppression Method for Water Surface Glints Using a Division of Focal Plane Visible Polarimeter

To address the problem of water surface detection imaging equipment being susceptible to water surface glints, this study demonstrates a method called De-Glints for suppressing glints and obtaining clear underwater images using a division of focal plane (DoFP) polarimeter. Based on the principle of polarization imaging, the best polarization angle and the image corresponding to the minimal average gray level of each pixel are calculated. To evaluate the improvement in image quality, the index E was designed. The results of indoor and outdoor experiments show that the error of the angle calculation of this method is within 10%, and the minimum error is only 3%. The E index is positively improved and can be relatively improved by 8.00 under the interference of strong outdoor glints, and the method proposed in this paper shows a good adaptive ability to the dynamic scene.

Transfer model to determine the above-water remote-sensing reflectance from the underwater remote-sensing ratio

Remote-sensing reflectance, R_rs(λ, θ, Δϕ, θ_s), contains the spectral color information of the water body below the sea surface and is a fundamental parameter to derive satellite ocean color products such as chlorophyll-a, diffuse light attenuation, or inherent optical properties. Water reflectance, i.e., spectral upwelling radiance, normalized by the downwelling irradiance, can be measured under- or above-water. Several models to extrapolate this ratio from underwater "remote-sensing ratio", r_rs(λ), to the above-water R_rs, have been proposed in previous studies, in which the spectral dependency of water refractive index and off-nadir viewing directions have not been considered in detail. Based on measured inherent optical properties of natural waters and radiative transfer simulations, this study proposes a new transfer model to spectrally determine R_rs from r_rs for different sun-viewing geometries and environmental conditions. It is shown that, compared to previous models, ignoring spectral dependency leads to a bias of ∼2.4% at shorter wavelengths (∼400 nm), which is avoidable. If nadir-viewing models are used, the typical 40°-off nadir viewing geometry will introduce a difference of ∼5% in R_rs estimation. When the solar zenith angle is higher than 60°, these differences of R_rs have implications for the downstream retrievals of ocean color products, e.g., > 8% difference for phytoplankton absorption at 440 nm and >4% difference for backward particle scattering at 440 nm by the quasi-analytical algorithm (QAA). These findings demonstrate that the proposed r_rs-to-R_rs model is applicable to a wide range of measurement conditions and provides more accurate estimates of R_rs than previous models.

Estimation for a complex index of refraction based on the Stokes vector of the reflected light under active polarized detection

An estimation method for a complex index of refraction based on the Stokes vector of the reflected light under active polarized detection is presented. In this method, the difference of the Stokes vector of the reflected light is used to introduce the three new estimation indices for the L-M nonlinear least squares algorithm to estimate the complex index of refraction n and k of the target, which does not require either interpretation of the diffuse reflection part or calculation of every polarized bidirectional reflection distribution function (pBRDF) matrix element. Aluminum is chosen to be the sample to prove the principle of this method, and the result shows that this method can estimate the complex index of refraction more precisely compared with the Hyde method. The result further shows that the estimation index containing elements related to circular polarization has better performance than the index only containing elements related to linear polarization.

Calculation of Stokes vector of laser backscattering from typical geometric rough surfaces at a long distance

The polarization bidirectional reflectance distribution function is key to establishing the relationships between incident and backscattering Stokes vectors. For analytical calculation of Stokes vectors of backscattering light from rough surfaces of objectives at long distances, we treat complicated objectives as a combination of several typical geometric surfaces. The analytical calculation forms of Mueller matrices of typical geometric rough surfaces at different sizes and geometric parameters are presented using a microfacet model, and thus, the backscattering Stokes vectors are determined. Experimental results of four types of geometric forms show good agreement with theoretical simulation, except when the incident angle is larger than about 60° at a wavelength of 532 nm. Further studies should be focused on improving the microfacet model for fitting the experimental results at large incident angles, and effects of multiple reflections between different geometric surfaces cannot be neglected when the combination of typical geometric surfaces is considered.

Sea-surface reflectance factor: replicability of computed values

The sea-surface reflectance factor ρ required for the determination of the water- leaving radiance from above-water radiometric measurements is derived from radiative transfer simulations relying on models of the sky-radiance distribution and sea-surface statistics. This work primarily investigates the impact on ρ of various sky-radiance and sea-surface models. A specific replicability analysis, restricted to the 550 nm wavelength, has been performed with the Monte Carlo code for Ocean Color Simulations (so-called MOX) accounting for the measurement geometry recommended in protocols for the validation of satellite ocean color data and commonly applied for operational measurements. Results indicate that the variability of ρ increases with wind speed and reaches the largest values for sun elevations close to the zenith or approaching the horizon. In particular, a variability up to about 2% is observed for wind speeds below 4 ms^-1 and sun zenith angles larger than 20°. Finally, the benchmark of the ρ values from this study with those formally determined with the Hydrolight code and widely utilized by the ocean color community, exhibits systematic differences. The source of these differences is discussed and the implications for field measurements are addressed.

Sensitivity analysis of the dark spectrum fitting atmospheric correction for metre- and decametre-scale satellite imagery using autonomous hyperspectral radiometry

The performance of the dark spectrum fitting (DSF) atmospheric correction algorithm is evaluated using matchups between metre- and decametre-scale satellite imagery as processed with ACOLITE and measurements from autonomous PANTHYR hyperspectral radiometer systems deployed in the Adriatic and North Sea. Imagery from the operational land imager (OLI) on Landsat 8, the multispectral instrument (MSI) on Sentinel-2 A and B, and the PlanetScope CubeSat constellation was processed for both sites using a fixed atmospheric path reflectance in a small region of interest around the system's deployment location, using a number of processing settings, including a new sky reflectance correction. The mean absolute relative differences (MARD) between in situ and satellite measured reflectances reach <20% in the Blue and 11% in the Green bands around 490 and 560 nm for the best performing configuration for MSI and OLI. Higher relative errors are found for the shortest Blue bands around 440 nm (30-100% MARD), and in the Red-Edge and near-infrared bands (35-100% MARD), largely influenced by the lower absolute data range in the observations. Root mean squared differences (RMSD) increase from 0.005 in the NIR to about 0.015-0.020 in the Blue band, consistent with increasing atmospheric path reflectance. Validation of the Red-Edge and NIR bands on Sentinel-2 is presented, as well as for the first time, the Panchromatic band (17-26% MARD) on Landsat 8, and the derived Orange contra-band (8-33% MARD for waters in the algorithm domain, and around 40-80% MARD overall). For Sentinel-2, excluding the SWIR bands from the DSF gave better performances, likely due to calibration issues of MSI at longer wavelengths. Excluding the SWIR on Landsat 8 gave good performance as well, indicating robustness of the DSF to the available band set. The DSF performance was found to be rather insensitive to (1) the wavelength spacing in the lookup tables used for the atmospheric correction, (2) the use of default or ancillary information on gas concentration and atmospheric pressure, and (3) the size of the ROI over which the path reflectance is estimated. The performance of the PlanetScope constellation is found to be similar to previously published results, with the standard DSF giving the best results in the visible bands in terms of MARD (24-40% overall, and 18-29% for the turbid site). The new sky reflectance correction gave mixed results, although it reduced the mean biases for certain configurations and improved results for the processing excluding the SWIR bands, giving lower RMSD and MARD especially at longer wavelengths (>600 nm). The results presented in this article should serve as guidelines for general use of ACOLITE and the DSF.

Determination of the remote-sensing reflectance from above-water measurements with the "3C model": a further assessment

The Three-Component Reflectance Model (3C) was primarily developed to improve the determination of the remote-sensing reflectance (R_rs) 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. R_rs derived from measurements performed during optimal and slightly sub-optimal conditions exhibit equivalence with R_rs 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 R_rs 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 R_rs.

Field Intercomparison of Radiometer Measurements for Ocean Colour Validation

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 ( E d ), 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 ( L s k y ) 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 ( L t ), the difference was <3.5% with an RMS <0.009 mWm−2 nm−1 sr−1. For remote-sensing reflectance ( R r s ), 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-Hyperspectral Surface Acquisition System (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 E d accounted for the largest fraction of the variance in R r s , which suggests that minimizing the errors arising from this measurement is the most important variable in reducing the inter-group differences in R r s . 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 ( L w n ) was therefore compared to AERONET-OC SeaPRiSM L w n as an alternative reference measurement. For the TriOS-RAMSES and Seabird-HyperSAS sensors the differences were similar across the visible spectra with 4.7% and 4.9%, respectively. The difference between SeaPRiSM L w n 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.

Exploring the limits for sky and sun glint correction of hyperspectral above-surface reflectance observations

Above-surface radiance observations of water need to be corrected for reflections on the surface to derive reflectance. The three-component glint model (3C) [Opt. Express25, A742 (2017)OPEXFF1094-408710.1364/OE.25.0000A1] was developed to spectrally resolve contributions of sky and sun glint to the surface-reflected radiance signal $ {L_r}(\lambda ) $L_r(λ), and for observations recorded at high wind speed and with fixed-position measurement geometries that frequently lead to significant sun glint contributions. Performance and limitations of 3C are assessed for all relevant wind speeds, clear sky atmospheric conditions, illumination/viewing geometries, and sun glint contamination levels. For this purpose, a comprehensive set of $ {L_r}(\lambda ) $L_r(λ) spectra was simulated with a spectrally resolved sky radiance distribution model and Cox-Munk wave slope statistics. Reflectances were also derived from an extensive four-year data set of continuous above-surface hyperspectral observations from the Long Island Sound Coastal Observatory, allowing to corroborate 3C processing results from simulations and measurements with regard to sky and sun glint contributions. Simulation- and measurement-derived $ {L_r}(\lambda ) $L_r(λ) independently indicate that spectral dependencies of the sky light distribution and sun glint contributions may not be neglected for observations recorded at wind speeds exceeding $ 4\, m/s $4m/s, even for sun glint-minimizing measurement geometries (Sun-sensor azimuth angle $ \Delta \phi = 90 {-} {135° } $Δϕ=90-135°). These findings are in accordance with current measurement protocols for satellite calibration/validation activities. In addition, it is demonstrated that 3C is able to reliably derive water reflectance for wind speeds up to 8 m/s and $ \Delta \phi { \gt 20° } $Δϕ>20°.

Polarization Properties of Reflection and Transmission for Oceanographic Lidar Propagating through Rough Sea Surfaces

Over the past few years, oceanographic lidar was applied to many fields, and polarization lidar could provide extra information for marine particles. To retrieve the water properties, many simulation models and inversion methods were developed. However, few of them account for the depolarization effect of a rough sea surface. In this study, we develop a model to calculate reflection and transmission Mueller matrices, coupled with the lidar observation geometry. Compared with another operational method, our model has a satisfactory performance. This model also considers the shadowing effects of wave facets. Then, we analyze the polarized properties in different azimuth and zenith angles and find that the reflection of sea surface has a crucial effect on the polarization properties of lidar. For unpolarized light, the reflected light tends to be partially polarized. However, for lidar light that is completely polarized, there is an obvious depolarization owing to multiple scattering, and the depolarization is not negligible at small incident angles. The retrieval of properties of ocean constituents can be effectively improved, becoming more accurate by accounting for the depolarization effects of sea surfaces based on our method.

A Review of Protocols for Fiducial Reference Measurements of WaterLeaving Radiance for Validation of Satellite Remote-Sensing Data over Water

This paper reviews the state of the art of protocols for measurement of waterleaving radiance in the context of fiducial reference measurements (FRM) of water reflectance for satellite validation. Measurement of water reflectance requires the measurement of waterleaving radiance and downwelling irradiance just above water. For the former there are four generic families of method, based on: 1) underwater radiometry at fixed depths; or 2) underwater radiometry with vertical profiling; or 3) abovewater radiometry with skyglint correction; or 4) onwater radiometry with skylight blocked. Each method is described generically in the FRM context with reference to the measurement equation, documented implementations and the intramethod diversity of deployment platform and practice. Ideal measurement conditions are stated, practical recommendations are provided on best practice and guidelines for estimating the measurement uncertainty are provided for each protocolrelated component of the measurement uncertainty budget. The state of the art for measurement of waterleaving radiance is summarized, future perspectives are outlined, and the question of which method is best adapted to various circumstances (water type, wavelength) is discussed. This review is based on practice and papers of the aquatic optics community for the validation of water reflectance estimated from satellite data but can be relevant also for other applications such as the development or validation of algorithms for remote-sensing estimation of water constituents including chlorophyll a concentration, inherent optical properties and related products.

Spectral band adaptation of ocean color sensors for applicability of the multi-water biogeo-optical algorithm ONNS

The ocean color algorithm ONNS (OLCI Neural Network Swarm) is designed to process remote-sensing reflectances at 11 Sentinel-3/OLCI bands in order to derive water quality parameters for most natural waters [Hieronymi et al., Front. Mar. Sci.4, 140 (2017)]. The present work introduces a spectral band-shifting procedure, which allows exploitation of atmospherically corrected input from SeaWiFS, MODIS, MERIS, OCM-2, VIIRS, SGLI, GOCI-2, EnMAP, or PACE/OCI and corresponding utilization of ONNS or other ocean color algorithms. The performance of the band adapters is evaluated in view of diverse optical water types. In the spectral range between 400 and 600 nm, the mean percentage retrieval error is mostly <5%. The band adaptation is a tool for cross-mission Earth observation and uncertainty estimate, as well as for extended possibilities of algorithm validation.

Vector radiative transfer model for coupled atmosphere and ocean systems including inelastic sources in ocean waters

Inelastic scattering plays an important role in ocean optics. The main inelastic scattering mechanisms include Raman scattering, fluorescence by colored dissolved organic matter (FDOM), and fluorescence by chlorophyll. This paper reports an implementation of all three inelastic scattering mechanisms in the exact vector radiative transfer model for coupled atmosphere and ocean Systems (CAOS). Simulation shows that FDOM contributes to the water radiation field in the broad visible spectral region, while chlorophyll fluorescence is limited in a narrow band centered at 685 nm. This is consistent with previous findings in the literature. The fluorescence distribution as a function of depth and viewing angle is presented. The impacts of fluorescence to the degree of linear polarization (DoLP) and orientation of the polarization ellipse (OPE) are studied. The DoLP is strongly influenced by inelastic scattering at wavelengths with strong inelastic scattering contribution. The OPE is less affected by inelastic scattering but it has a noticeable impact, in terms of the angular region of positive polarization, in the backward direction. This effect is more apparent for deeper water depth.

Polarized transfer functions of the ocean surface for above-surface determination of the vector submarine light field

A method is developed to determine the underwater polarized light field from above sea surface observations. A hybrid approach combining vector radiative transfer simulations and the Monte Carlo method is used to determine the transfer functions of polarized light for wind-driven ocean surfaces. Transfer functions for surface-reflected skylight and upward transmission of light through the sea surface are presented for many common viewing and solar geometries for clear-sky conditions. Sensitivity of reflection matrices to environmental conditions is examined and can vary up to 50% due to wind speed, 25% due to atmospheric aerosol load, and 10% due to radiometer field-of-view. Scalar transmission is largely independent of water type and varies a few percent with wind speed, while polarized components can change up to 10% in high winds. Considerations for determining the water-leaving radiance (scalar or vector) are discussed.