Exploring the limits for sky and sun glint correction of hyperspectral above-surface reflectance observations
Philipp Groetsch, Robert E. Foster, Alexander Gilerson
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. Express 25 , A742 ( 2017 ) OPEXFF 1094-4087 10.1364/OE.25.0000A1 ] was developed to spectrally resolve contributions of sky and sun glint to the surface-reflected radiance signal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>r</mml:mi> </mml:msub> </mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mi>λ</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>4</mml:mn> <mml:mspace width="thinmathspace"/> <mml:mi>m</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>s</mml:mi> </mml:math> , even for sun glint-minimizing measurement geometries (Sun-sensor azimuth angle <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>ϕ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>90</mml:mn> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>135</mml:mn> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>°</mml:mo> </mml:mrow> </mml:mrow> </mml:math> ). 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>ϕ</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>></mml:mo> <mml:mn>20</mml:mn> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>°</mml:mo> </mml:mrow> </mml:mrow> </mml:math> .