Ray-tracing investigation of direction of arrival observations of HF radio waves

A high frequency radio direction-finding experiment involving a transmitter near Houston, Texas and a receiver near Urbana, Illinois has been under progress on a continuous basis for 1 week per month since March 1967. A network of five vertical incidence ionosondes located near and around the midpoint of the great circle path permit the evaluation of electron density gradients along the propagation path from true height analysis of ionograms recorded by this network. These gradients are used in conjunction with a three-dimensional ray-tracing computer program to compute the expected directions of arrival of the radio waves. Changes in bearing resulting from temporal variations in the electron density distribution and seasonal variations in the sunrise tilts are investigated. The results obtained by ray tracing are found to be in excellent agreement with the measured bearing deviations. Effective layer tilts are computed from the results of ray tracing.