A comparison of observed and modelled deviations from the great circle direction for a 4490 km HF propagation path along the midlatitude ionospheric trough

Measurements of the direction-of-arrival of signals propagating on a long (4490 km) path along the midlatitude trough show that the azimuth can be deviated by up to 100° from the great circle bearing. In this paper an attempt has been made to model the shift in azimuth through a ray tracing simulation. Two possible mechanisms which lead to changes in azimuth have been investigated: (1) propagation along the density gradients which form the equatorward wall of the trough and (2) side scatter from regions of the sea well to the south of the trough. Of these two mechanisms, sea scatter gives results which are much closer to those observed.