A direction-finding ionosonde consisting of an interferometer using seven-element, 50- and 150-meter crossed baselines, has been developed to provide group path delay, azimuth, and elevation of each resolved mode over the sounding frequency range. Direction-finding (DF) ionograms have been acquired automatically, using various noncooperative, compatible sounder transmissions. Diurnal sequences of DF ionograms were obtained for mid-latitude paths at nominal ranges of 55-, 1000-, 2300- and 3500-km. Data measured to characterize the DF properties of each path include group delay, azimuth, and elevation versus frequency, as well as an azimuth/elevation scatter plot showing the distribution of angular occupancy for the path. The results show modes arriving closest to the great circle bearing (i.e., with lowest azimuth error) arrive via the lowest elevation angles and with least group delay. Fractional degree bearing standard deviations were commonly observed for one-hop E and one-hop F low-ray modes, decreasing with increasing range. Variable bearing standard deviations between 1° and 6° were observed for one-hop F high-ray and two-hop F modes. Bearing standard deviation of 50° were seen at near vertical incidence (55-km range).
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