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Channeling phenomenon is a type of forward scattering and multipath that occasionally occurs when a well-formed sea wave briefly reflects or focuses the out-of-plane energy toward the receiver, yielding a higher than expected, broadband power gain. A series of X-band, forward-scattering measurements in a large wave tank observed multiple channeling events at low grazing angles. The wave tank simulated a one-dimensional, 1/10th-scale Pierson-Moskowitz surface at sea states 0, 3, and 5. During the measurements, channeling events were observed under crosswind wave conditions when a properly constructed wave trough, coinciding with the radar's line of sight, channeled or focused the out-of-plane, forward-scattered energy toward the receiver. Presented is a summary of the channeling events observed and a physical model of the channeling phenomenon based on experimental observations and numerical simulations.Index Terms-Electromagnetic scattering by rough surfaces, physical optics (PO), sea surface electromagnetic scattering.

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Several techniques are considered for the analysis of electromagnetic scattering from rough ocean surfaces. A rigorous Multi Level Fast Multipole Method (MLFMM) i� employed, as well as a high-frequency ray-based solution. The MLFMM analysis is implemented in scalable form, allowing consideration of scattering from very large surfaces (in excess of 100,4, x lOOk, where ,4, represents the electromagnetic wavelength). Plane-wave incidence is assumed, and a key aspect of the MLFMM study involves investigating techniques' for rough-surface truncation. The rough surface is modeled as a target placed in the presence of an infinite half-space background; to minimize edge effects, the surface is smoothly tapered into the planar half space. We also consider the technique of employing a resistive taper on the edges of the rough surface. These two truncation techniques are compared in accuracy, memory requirements (RAM), and in computational time (CPU). The MLFMM results are used to validate an approximate ray-based high-frequency model that allows rapid analysis of large surfaces. The computational results are compared to measured forward-scattering data from scaled laboratory measurements, used to simulate scattering from an ocean surface.

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The moments via integral transform method (MITM) is a technique to analytically reduce the 2-D method of moments (MoM) impedance double integrals into single integrals. By using a special integral representation of the Green’s function, the impedance integral can be analytically simplified to a single integral in terms of transformed shape and weight functions. The reduced expression requires fewer computations and reduces the fill times of the MoM impedance matrix. Furthermore, the resulting integral is analytic for nearly arbitrary shape and weight function sets. The MITM technique is developed for mixed boundary conditions and predictions with basic shape and weight function sets are presented. Comparisons of accuracy and speed between MITM and brute force are presented. [Work sponsored by ONR and NSWCCD ILIR Board.]