Directional characteristics of ionospherically propagated HF radio signals and their effect on HF DF systems

The performance of high frequency direction finding systems is known to be related to the mode content of the incoming signal and to the frequency of operation, both of which depend on the electron density distribution in the ionosphere. Due to tilts and gradients in the electron density distribution, signals often arrive at the receiver over paths well displaced from the great circle. Deviations of a few degrees are associated with tilts in the ionospheric electron density profile due, for example, to the solar terminator and to travelling ionospheric disturbances. Very large deviations are particularly prevalent in the high latitude regions where signals often arrive at the receiver with bearings displaced from the great circle direction by up to ±100° or more. A systematic study of these features has been undertaken and the magnitudes of the bearing errors observed in the various ionospheric regions and their likely causes are summarised. The paper also reviews some of the work undertaken at the University of Leicester on the direction of arrival (DOA) of HF signals propagated over two high latitude paths, one along the subauroral trough and the other contained within the polar cap. These two paths are illustrated together with an indication of the position of the subauroral trough produced by the model of Halcrow and Nisbet (1977)