The main ionospheric trough can be formed as result of horizontal convection and field-aligned plasma transport, changes in the electric and magnetic fields, precipitation of electrons, interaction of ionospheric plasma with neutral components and recombination and ionisation processes occurring in the ionosphere and inner magnetosphere. Recently the position of the minimum, and poleward and equatorward edges of the main ionospheric trough has been determined by a hybrid method based on wave diagnostics across the whole frequency band, together with electron and density temperature measurements. For terrestrial HF systems, the electron density depletion in the trough region reduces the maximum frequency that can be reflected by the ionosphere along the great circle (GC) path. The gradients in electron density associated with the trough walls and embedded ionospheric irregularities often result in propagation in which the signal path is well displaced from the GC direction with directions of arrival (DOA) at the receiver offset by up to 100°. Deviations from the GC direction impact not only on radiolocation systems for which estimates of a transmitter location are obtained by triangulation from a number of receiving sites, but also on any radio communications system in which directional antennas are employed. In this paper, in order to determine the mechanisms that are responsible for the off GC behaviour described above, HF signals propagating along the Uppsala-Leicester path are examined in the context of DEMETER observations of electron density structure and particle precipitation in the ionospheric trough region for a number of case studies.