I. Introduction

One of the principal goals of a propagation engineer is to develop an effective numerical propagation tool that accurately calculates the path loss between any two points specified on a digital map of the area of interest. The accurate modeling of the groundwave propagation over the Earth's surface is usually a challenging task because of several complex processes occurring during the propagation. In particular, the terrain irregularities reflect and diffract energy in a complicated way and have a significant impact on the radiowave path loss. Such terrain effects become even more pronounced when coupled with atmospheric refraction because an inhomogeneous atmosphere may redirect energy and cause multiple interactions with the ground. This, in turn, certainly requires a good understanding of electromagnetic (EM) wave behavior in the presence of irregular terrain and inhomogeneous atmosphere. Therefore, a generally applicable all-purpose propagation prediction method/tool must be able to predict the effect of such complex environmental factors/uncertainties on the performance of radio communication and radar systems [1]. However, since the complexity of the realistic propagation scenarios may place limitations in the range of validity of the prediction models/tools, the fidelity of the models to the underlying physics must be assessed by means of, for example, validation, verification, and calibration (VV&C) tests. This process of determining whether the right model is built (or solving the right equations) is called “validation,” whereas the “verification” assessment examines if the model is built right. The final step is the “calibration,” which is the process of quantitatively defining the performance of the model with respect to known and controlled models (such as exact solutions and/or other numerical methods).