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

The remote-sensing reflectance R(rs) 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 R(rs), and errors associated with estimating R(rs) by removal of surface-reflected sky radiance are evaluated using the Hydrolight radiative transfer numerical model. The dependence of the sea surface reflectance factor rho, which is not an inherent optical property of the surface, on sky conditions, wind speed, solar zenith angle, and viewing geometry is examined. If rho is not estimated accurately, significant errors can occur in the estimated R(rs) 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 rho approximately 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 rho as a function of solar zenith angle and wind speed. If the sky is overcast, a value of rho approximately 0.028 is used at all wind speeds.