HF Skywave Propagation Model for Interference Prediction

Many computer models are in existence that are designed to predict the properties of radio signals received via skywave modes of ionospheric propagation. In general, the prime objective of these models is to optimize communication circuits rather than to analyze interference. The computer model described in this paper was developed as an interference analysis tool. The parameters which are necessary for analysis of a parabolic layer over a spherical earth are obtained from (1) predicted F2-layer ionospheric characteristics, (2) available E-layer characteristics, and (3) a recently developed quasi-empirical determination of layer semithickness. Ray-tracing equations are used to provide critical angles and hop distances for mode selection. Possible modes are identified. Losses are calculated. Experimental statistics of loss and probability of occurrence are used to provide an output directly applicable to interference analysis. Techniques employed in this model, unlike most models, provide identification of possible stable and unstable modes. In addition, the analysis is performed before antenna characteristics are considered in the overall technique. These factors and statistical considerations provide a usable HF skywave propagation model for interference analysis.