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This paper reviews IREPS and some of its major products, and follows with a description of a recent effort to improve IREPS signal-prediction capabilities in tropospheric ducting environments. The IREPS products discussed are the altitude-versus-range coverage display, the path-loss-versus-range display, and a new historical propagation-conditions summary display derived from a computer-based refractivity climatology. In the GHz range elevated tropospheric layers are multimode systems, and practical beyond-the-horizon field calculations for system applications such as IREPS require simplified procedures. These simplified procedures could involve approximations directed towards speeding up waveguide calculations, or could take the form of replacing waveguide methods by ray methods or by a hybrid mixture of waveguide and ray concepts. Involved in each of these alternatives, and within the context of the modified refractivity formalism, is the need for calculating plane-wave reflection coefficients or their equivalents for layered media. The modelling effort discussed in this paper deals with the feasibility of using asymptotic forms of the plane-wave reflection coefficients for the purpose of waveguide analysis for a trilinear ducting environment.