Modelling of off-great circle propagation effects for HF radiowaves received over northerly paths
Both the high latitude ionosphere and mid --latitude trough region are essentially inhomogeneous and non-stationary media containing a multitude of large-scale irregularities. Electron density gradients associated with these irregularities form a diverse range of tilted reflection surfaces for high frequency (HF) radio waves. There are also a large number of smaller scale inhomogeneities that scatter the signal.;A series of measurements in the polar cap, auroral and sub-auroral zones of the Earth have been made by researchers at the University of Leicester over a number of years. It have been established that HF radio signals propagating through the high latitude ionosphere often arrive at the receiver over paths well displaced from the great circle direction, sometimes by up to 100Â°. Another common feature of high latitude HF propagation is the large Doppler and delay spreads imposed on the signal. These together with directional spread of the received signal energy are important parameters to be considered in the design of communication system and the associated signal processing methods.;The main outcome of the project described in this thesis is the design of a model of the high latitude ionosphere providing numerical investigation of off-great circle propagation of HF signals based on three-dimensional ray tracing. A large number of simulations were carried out for different types of propagation paths, and a comparison of the model results with observations indicates that the model is capable of reproducing the main features of HF signals propagating in the high latitude ionosphere.;A major outcome of the ray-tracing simulations is that paths other than those subject to experimental investigation can be assessed. It is anticipated that the results of this research will be incorporated into prediction tools for forecasting the effects of off-great circle propagation on any path impinging on the northerly ionosphere.