Rough surface backscatter and statistics via extended parabolic integral   equation
  This paper extends the parabolic integral equation method, which is very effective for forward scattering from rough surfaces, to include backscatter. This is done by applying left-right splitting to a modified two-way governing integral operator, to express the solution as a series of Volterra operators; this series describes successively higher-order surface interactions between forward and backward going components, and allows highly efficient numerical evaluation. This and equivalent methods such as ordered multiple interactions have been developed for the full Helmholtz integral equations, but not previously applied to the parabolic Green's function. In addition, the form of this Green's function allows the mean field and autocorrelation to be found analytically to second order in surface height. These may be regarded as backscatter corrections to the standard parabolic integral equation method. 
Mark Spivack and Orsola Rath Spivack
physics.optics