A Practical Evaluation of Earth‐Backscatter Simulation and an Estimate of the HF Ground‐Scatter Coefficient

Abstract

An attempt has been made in this work to perform an appraisal of the degree to which fixed‐frequency amplitude versus time‐delay simulations can be made to agree in detail with actual HF‐radar ground‐back‐scatter observations taken with azimuthally broad beam antennas. The physical constraints and effects of propagation phenomena such as the spatial distribution of the ionospheric electron density, D‐layer absorption, the antenna pattern, rough‐surface scattering of the surface of the earth, and cross‐polarization losses are discussed.A composite backscatter‐simulation method, based upon a weighted sum of individual simulations (separate raytraces and backscatter simulations) from different electron density distributions, is used to account for downrange geographical variation in electron density profiles, and the multiplicity of propagation modes which may occur during conditions of nonblanketing sporadic‐E or spread‐F ionization. Tabular descriptions of electron density versus height, from actual ionospheric soundings, are used instead of ionospheric models. Both isotropic and anisotropic scattering models are used. Order of magnitude estimates are given for the radar cross section per unit area (σ0) of land illuminated by one‐hop 16‐MHz radiation westward from Washington, D.C.