Self-shading associated with a skylight-blocked approach system for the measurement of water-leaving radiance and its correction

On the measurement of remote sensing reflectance by a traditional above-water approach in small water bodies

Small water bodies are an important part of the Earth's freshwater system, protecting biodiversity and providing ecosystem services. Because of various surrounding features, it is unknown to what extent we can obtain accurate remote-sensing reflectance (R_rs) of such an environment by the conventional above-water approach (AWA). In this study, we used both AWA and the skylight-blocked approach (SBA) side-by-side to measure R_rs in a typical small water body. It was found that the variation of R_rs in the UV-blue domain from AWA is around 50% and is inconsistent with the variation of the total absorption coefficient (a_t) obtained from water samples; on the contrary, the variation of R_rs obtained from SBA is highly consistent, with a coefficient of variation under ∼5%. These results highlight the large uncertainties in the measured R_rs from AWA due to the complexity of such an environment and further echo the robustness of SBA to measure R_rs in the field, even in such challenge environments.

Direct measurement system of water-leaving albedo in the field by the skylight-blocked approach: Monte Carlo simulations

Water-leaving albedo (αw(λ)) is an important component of the ocean surface albedo. Direct measurement of αw(λ) in the field is not yet available due to difficulties in removing the contribution of surface-reflected solar radiation. Following the concept of the skylight-blocked approach (SBA), a novel system, termed α_wSBA, is proposed in this study to directly measure Ew(λ), where a wide-angle black cone is used to block the surface-reflected radiance. The shading errors associated with the cone and the measuring system are examined via Monte-Carlo (MC) simulations for a wide range of water inherent optical properties (IOPs), solar zenith angle, and different configurations of the α_wSBA system (i.e., half cone angle, and the length of supporting arm). Based on sensitive analysis using MC simulations, an optimal configuration of α_wSBA is recommended. We further propose a mathematical expression to parameterize the shading error (ɛ), along with an error correction scheme (α_wOPT). It is found that, with the optimal configuration and α_wOPT, the uncertainties of obtained αw(λ) by α_wSBA are generally less than 7% for a wide range of waters with different IOPs and particulate scattering phase functions.

Evaluating Atmospheric Correction Algorithms Applied to OLCI Sentinel-3 Data of Chesapeake Bay Waters

Satellite remote sensing permits large-scale monitoring of coastal waters through synoptic measurements of water-leaving radiance that can be scaled to relevant water quality metrics and in turn help inform local and regional responses to a variety of stressors. As both the incident and water-leaving radiance are affected by interactions with the intervening atmosphere, the efficacy of atmospheric correction algorithms is essential to derive accurate water-leaving radiometry. Modern ocean color satellite sensors such as the Ocean and Land Colour Instrument (OLCI) onboard the Copernicus Sentinel-3A and -3B satellites are providing unprecedented operational data at the higher spatial, spectral, and temporal resolution that is necessary to resolve optically complex coastal water quality. Validating these satellite-based radiance measurements with vicarious in situ radiometry, especially in optically complex coastal waters, is a critical step in not only evaluating atmospheric correction algorithm performance but ultimately providing accurate water quality metrics for stakeholders. In this study, a regional in situ dataset from the Chesapeake Bay was used to evaluate the performance of four atmospheric correction algorithms applied to OLCI Level-1 data. Images of the Chesapeake Bay are processed through a neural-net based algorithm (C2RCC), a spectral optimization-based algorithm (POLYMER), an iterative two-band bio-optical-based algorithm (L2gen), and compared to the standard Level-2 OLCI data (BAC). Performance was evaluated through a matchup analysis to in situ remote sensing reflectance data. Statistical metrics demonstrated that C2RCC had the best performance, particularly in the longer wavelengths (>560 nm) and POLYMER contained the most clear day coverage (fewest flagged data). This study provides a framework with associated uncertainties and recommendations to utilize OLCI ocean color data to monitor the water quality and biogeochemical dynamics in Chesapeake Bay.

Experimental analysis of the measurement precision of spectral water-leaving radiance in different water types

The on-water radiometric approach employs a unique provision to obtain water-leaving radiance from nadir (Lw(λ)) which can be used for the calibration of ocean color satellites. In this effort, we address the measurement precision associated with Lw(λ) from a single on-water instrument, which is an important aspect of measurement uncertainty. First, we estimated the precision as the ratio of the standard deviation of the means of repeated measurements to the mean of these measurements. We show that the measurement precision for Lw(λ) is within 2.7-3.7% over 360-700 nm. The corresponding remote sensing reflectance spectra (Rrs(λ)) from the same instrument also exhibit a high precision of 1.9-2.8% in the same spectral domain. These measured precisions of radiance and reflectance over the 360-700 nm range are independent of the optical water type. Second, we quantified the consistency of on-water Lw(λ) and Rrs(λ) from two collocated systems for further insight into their measurement repeatability. The comparison reveals that Lw(λ) measurements in the 360-700 nm agree with each other with an absolute percentage difference of less than 3.5%. The corresponding Rrs(λ) data pairs are subjected to increased differences of up to 8.5%, partly due to variable irradiance measurements (Es(λ)). The evaluation of measurement precision corroborates the reliability of the on-water acquisition of radiometric data for supporting satellite calibration and validation.

Experimental evaluation of the self-shadow and its correction for on-water measurements of water-leaving radiance

Accurate determination of the water-leaving radiance (L_w) is key to correctly interpret in-water optical properties and to validate the atmospheric correction schemes in ocean color studies. Among the various approaches adopted to measure L_w in the field, the skylight-blocked approach (SBA) is the only scheme that can potentially measure L_w directly. However, the apparatus associated with an SBA system will introduce self-shading effects to the measured L_w, which is required to be corrected for an accurate L_w determination. In this study, we experimentally evaluate several factors that could contribute to the self-shading effects of the SBA-measured L_w, including solar zenith angle (∼18^∘-64^∘), water's optical properties, and cone size (radius of 22 mm and 45 mm). For waters with the total absorption coefficient at 440 nm as high as ∼6.0m^-1, the normalized root-mean-square difference between the SBA-measured L_w after shade correction and the "true" L_w is generally between ∼5% and ∼10% for wavelengths in the range of 400-750 nm. These results suggest that SBA can obtain highly accurate and precise L_w in nearly all natural aquatic environments.

On the equivalence of near-surface methods to determine the water-leaving radiance

The equivalence of two radiometric methods relying on a single nadir-view optical sensor to determine the water-leaving radiance L_W, 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 L_W 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.

Impact of ship on radiometric measurements in the field: a reappraisal via Monte Carlo simulations

The presence of a ship in water disturbs the ambient light field and propagates errors to radiometric measurements. This study investigated the ship perturbation via Monte Carlo simulations with a reflective 3D ship. It is found that the height of ship could cause significant perturbation. However, these perturbations could be compensated by the reflection of the ship's hull, where such compensations vary from sun angle to hull's reflectance. Further, as a rule of thumb, to keep the perturbation on water-leaving radiance under ∼3% from an operating ship, a look-up table is generated with the requirements of viewing angle for the radiometers operated at the deck and for the deployment distance of floating and profiling instruments.

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.

Uncertainty in global downwelling plane irradiance estimates from sintered polytetrafluoroethylene plaque radiance measurements

Global downwelling plane irradiance is a necessary variable to normalize water-leaving radiance measurements, reducing the magnitude and spectral variabilities introduced by the incident light field. As a result, the normalized measurements, known as remote sensing reflectance, have higher correlation with the inherent optical properties of the water body and so to the composition of optically active water components. For in situ measurements, the global downwelling plane irradiance can be estimated from the exitant radiance of sintered polytetrafluoroethylene plaques or other diffuse reflectance standards. This allows use of a single spectrometer to measure all necessary variables to estimate the remote sensing reflectance, reducing cost in acquisition and maintenance of instrumentation. However, despite being in use for more than 30 years, the uncertainty associated with the method has been only partially evaluated. In this study, we use a suite of sky radiance distributions for 24 atmospheres and nine solar zenith angles in combination with full bidirectional reflectance distribution function determinations of white and gray plaques to evaluate the uncertainties. The isolated and interactive effects of bidirectional reflectance distribution, shadowing, and tilt error sources are evaluated. We find that under the best-performing geometries of each plaque, and with appropriate estimation functions, average standard uncertainty ranges from 1% to 6.5%. The simulated errors are found to explain both previous empirical uncertainty estimates and new data collected during this study. Those errors are of the same magnitude as uncertainties of plane irradiance sensors (e.g., cosine collectors) and overlap with uncertainty requirements for different uses of in situ data, which supports the continued use of the plaque method in hydrologic optics research and monitoring. Recommendations are provided to improve the quality of measurements and assure that uncertainties will be in the range of those calculated here.