Time of flight and direction of arrival of HF radio signals received over a path along the midlatitude trough: Observations

Measurements of the time-of-flight, direction of arrival, and Doppler spread are presented for HF radio signals radiated on six frequencies between 4.6 and 18.4 MHz received over a subauroral path oriented along the midlatitude trough between Sweden and the UK. During the day, the signals usually arrived from the great circle direction whereas at night, especially during the winter and equinoctial months; the signals on frequencies between 7.0 and 11.1 MHz often arrived from directions well displaced from the great circle direction. In summer the deviations tended to be smaller (<5°) than those observed during the other seasons (several tens of degrees). The deviations were mainly to the north and often lasted all night, with the time of flight initially decreasing and then increasing, showing an approach and then recession of the reflection point. Southerly deviations were much less coherent and less frequent.