The status of tropospheric radio propagation assessment is reviewed and recent advances in this area are described. Special emphasis is given to anomalous propagation in a marine environment. Modeling and measurements of ducting phenomena caused by the oceanic evaporation duct and by elevated refractive layers are discussed. The modeling involves extensive numerical calculations of waveguide propagation, ray-tracing techniques, and empirical relationships. Effects such as sea-surface roughness and horizontal inhomogeneity are also addressed. The models have been compared with extensive measurements in a variety of geographic locations. Worldwide radio refractivity climatologies have been compiled. A propagation assessment system, called the "Integrated Refractive Effects Prediction System" (IREPS), has been developed and successfully tested. It uses a small computer with interactive graphic displays and provides the user with performance assessment of communications and radar equipment based on prevailing atmospheric conditions. Environmental inputs into the system consist of surface measurements and profiles of atmospheric refractivity either calculated from radiosonde balloon data or measured by microwave refractometers. Status and shortcomings of direct and remote sensing techniques for measurement of atmospheric refractivity are also discussed.

Propagation theory along the lines of Budden's terrestrial waveguide formalism is reviewed. Special emphasis is placed on its application to the lower extremely low frequency (ELF) range for which a number of calculations have been made to demonstrate the relationship between propagation parameters and the physical constants of the ionosphere. Calculations performed with diffuse anisotropic ionospheres yield reasonable agreement with ELF test results.