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An earlier, three-dimensional ray-tracing program has been upgraded to include magnetic field effects, variable step sizes, improved propagation loss computation, and efficient homing and focusing computation algorithms. A combination of fourth-order Runge-Kutta and Adams methods, with error control and extension to variable step size, has been included. The program is applied to an investigation of high-frequency near vertical incidence sky wave (NVIS) effects. Model simulations of oblique ionograms are found to be consistent with experiment. Implications for NVIS geolocation are discussed.