The effect of the high latitude ionosphere on measurements of the direction of arrival of HF signals using super-resolution direction finding

Measurements of the direction of arrival of a 9.3-MHz signal propagated over a 2100 km path within the polar cap are presented. Several effects are evident in the measurements and particular attention is drawn to the following: a) large swings in bearing of up to around 70/spl deg/ were observed. These were particularly prevalent when the operating frequency was close to the maximum usable frequency. b) Multiple traces were apparent in the calculated bearings. Whilst these may arise from the presence of multiple reflectors (patches of enhanced ionisation) closely tracking one another, it was found that the precise nature of these multiple traces are dependent upon the array geometry. Whilst this does not preclude the presence of multiple reflectors, care needs to be taken in interpreting such observations since the measurements clearly contain some artefacts of the data processing. c) The average bearings calculated using various array geometries differ. This effect may be a result of the signal energy arriving at the receiving array over a range of directions due to the "rough reflector" nature of the high latitude ionosphere. This combined with the asymmetrical nature of the array beam pattern would result in maximum energy being received when the array look direction was offset from the direction of maximum signal power.