Observations of the directional characteristics of ionospherically propagated HF radio channel sounding signals over two high latitude paths

The author presents measurements of the directional characteristics of the Doppler and time delay components of a pulsed channel sounding signal received over two high latitude paths. A variation in bearing with Doppler frequency was frequently evident, an effect attributed to Doppler shifts imposed on the signal when scattered from ionospheric irregularities drifting across the reflection points. For a transauroral path, for which the ionospheric reflection points were subauroral, standard deviations of up to ~2.5° were observed in the azimuthal power distribution of the received energy. Much greater disturbances with azimuthal standard deviations of up to ~35° were recorded on signals received over a path contained entirely within the polar cap path