Groundwave (HF)

Groundwave is the surface-following propagation path of MF and HF radio that travels along the curvature of the sea rather than reflecting off the ionosphere. Over salt water its conductivity gives useful ranges at the lower frequencies, a few hundred miles at MF, which is why marine direction-finding and distress watch historically used the ground wave for stable, single-path bearings. The competing sky wave reflects off the ionosphere and, where the two arrive together at night, causes the night effect that corrupts radio DF bearings. Direction finders favored the ground wave for reliable bearings near dawn and dusk.