An improved Kirchhoff formula for reflection loss at a rough ocean surface at low grazing angles

The calculation of specular reflection loss at a rough ocean surface is a time-consuming procedure if the boundary perturbation or mode-coupling methods are used. The simple Kirchhoff formula is often used as an estimate of reflection loss, but this is shown to severely underestimate the loss at small grazing angles. A formula is derived using the large correlation length limit of the boundary perturbation calculation, and is shown to be a more accurate estimate of the reflection loss at small grazing angles than the Kirchhoff formula. The new formula is identical with the Kirchhoff formula at large grazing angles.