Estimation of the remote-sensing reflectance from above-surface measurements

TLDR

The remote‑sensing reflectance Rrs is not directly measurable, and various methodologies have been employed in its estimation. The study reviews radiative transfer foundations of common Rrs estimation methods and evaluates errors from removing surface‑reflected sky radiance using the Hydrolight model. The analysis examines how the sea surface reflectance factor ρ depends on sky conditions, wind speed, solar zenith angle, and viewing geometry, and presents curves for determining ρ as a function of solar zenith angle and wind speed for higher wind speeds. The simulations show that inaccurate ρ leads to significant errors in Rrs, especially for near‑zenith Sun positions and high wind speeds, and recommend a viewing direction of 40° from nadir and 135° from the Sun, with ρ ≈ 0.028 acceptable only for wind speeds below 5 m s⁻¹, while overcast skies allow ρ ≈ 0.028 at all wind speeds.

Abstract

The remote-sensing reflectance Rrs is not directly measurable, and various methodologies have been employed in its estimation. I review the radiative transfer foundations of several commonly used methods for estimating Rrs, and errors associated with estimating Rrs by removal of surface-reflected sky radiance are evaluated using the Hydrolight radiative transfer numerical model. The dependence of the sea surface reflectance factor ρ, which is not an inherent optical property of the surface, on sky conditions, wind speed, solar zenith angle, and viewing geometry is examined. If ρ is not estimated accurately, significant errors can occur in the estimated Rrs for near-zenith Sun positions and for high wind speeds, both of which can give considerable Sun glitter effects. The numerical simulations suggest that a viewing direction of 40 deg from the nadir and 135 deg from the Sun is a reasonable compromise among conflicting requirements. For this viewing direction, a value of ρ ≈ 0.028 is acceptable only for wind speeds less than 5 m s-1. For higher wind speeds, curves are presented for the determination of ρ as a function of solar zenith angle and wind speed. If the sky is overcast, a value of ρ ≈ 0.028 is used at all wind speeds.