Analysis of radio signal interference effects due to ionized layer around a re-entry vehicle

Abstract

The theoretical section of this paper discusses the electromagnetic properties of the ionized shock layer about a missile re-entering the atmosphere; a treatment is then given of the problem of predicting the transmission, through the shock layer, of electromagnetic waves originating on the vehicle. The major portion of this section is given to outlining the important quantitative aspects for calculating the extent of the attenuation problem. Theoretical curves are given which allow rapid prediction of the attenuation effects due to air ionization once temperature and density are specified in the shock layer. If other sources of free electrons are significant, e.g. ablation products, separate determination of the density distribution must be made. Specific results are given on certain Polaris configurations and trajectories as computed using this method; these predictions are based upon two specific transmitter frequencies. Although the method utilizes the Lorentz theory and the plane wave-plane homogeneous sheet assumption is made, the resulting estimates are shown to agree satisfactorily with signal strength records of actual re-entry tests. A quantitative discussion is given concerning the approximations inherent with the use of the Lorentz theory of conductivity in view of “strong fields” near a transmitter and the velocity dependence of collision cross-sections. A portion is given to possible solutions of the transmission problem suggested by the parameters available in various ranges of temperature and density; particular attention is given to transmitter frequency and position of the antenna. A method is demonstrated for extrapolating the results of a minimum of predictions for a given combination of radiator and re-entry body to include predictions for a wide range of altitudes and velocities.