Propagation and low grazing angle (LGA) scattering of centimetre-scale electromagnetic waves in a marine environment is the subject of scientific research for decades. For such a problem, a duct, which is described by the variations of the refractive index profile with the altitude near the sea surface, can substantially affect the performance of a radar system. This is a joint problem of propagation and scattering, especially for low grazing angles. To solve it, the well-known Parabolic Wave Equation (PWE) method, combined with the Split-Step Fourier resolution technique (SSF) [1], is commonly used. This fast method can be applied for any refractive index profile, but its main drawback is the use of heuristic boundary conditions. In this article, an alternative method, based on the Physical Optics (PO) approximation to derive the boundary conditions, is developed. Moreover, this method will be validated by a rigorous method based on the solution of Boundary Integral Equations (BIE) [2] solved by the Method of Moments (MoM) and accelerated by the Forward-Backward (FB) method.