Measurements of the direction of arrival, time dispersion and frequency dispersion of HF signals received over a path along the mid-latitude trough

The electron depletion in the mid-latitude trough has a severe impact on the maximum frequency of HF signal that can be reflected by the ionosphere along the great circle path (GCP). Also, the associated horizontal gradients in the electron density distribution displace propagation away from the GCP. The observed signal characteristics on a 1400 km link oriented along the trough between Sweden and the UK are reported. As anticipated, the delay and Doppler spread are mostly affected at night and in the winter. It is shown that Doppler spread is often large when GCP propagation is absent and reflections are from the trough´s north wall.