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

Abstract

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 ) $Lr(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 ) $Lr(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 ) $Lr(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°.