Some observations of the directions of arrival of an ionospherically reflected HF radio signal on a very high latitude path

The high latitude ionosphere is a disturbed region containing irregularities which is often regarded as providing a rough reflecting surface for obliquely propagating HF radio waves. As a result of this roughness, signals associated with each propagation mode may arrive at the receiver over a range of angles in both azimuth and elevation. A single ionospheric mode is therefore often modelled as a single ray specularly reflected from a smooth ionosphere surrounded by a cone of rays produced by the roughness of the ionosphere. Ionospheric movements close to the reflection points impose Doppler shifts and Doppler spreads onto the signal. Furthermore, the presence of large gradients within the polar cap ionosphere, due, for example, to the presence of convecting patches of enhanced ionisation, often lead to propagation well displaced from the great circle path. In this paper, observations of the direction of arrival of a narrow band pulsed channel sounding signal propagated over a polar cap path made with a multi-channel receiver system connected to a wide aperture antenna array are reported. The signals received on each antenna were processed to provide a measure of the relative times of flight of the propagating modes and their associated Doppler spectra. In this way, the signal was split into components distinguished by antenna position, time of flight and Doppler frequency. A direction finding algorithm was then applied to each of the signal components in turn in order to determine the directional characteristics of the received signal.