Behaviour of superresolution direction finding algorithms for HF signals propagating through the high latitude ionosphere

The performance of high frequency direction finding systems is known to be related to the frequency of operation and to the propagation mode content of the incoming signal, which is dependent upon the electron density distribution in the ionosphere. In addition to large scale tilts that cause gross deviations of the signal from the great circle direction, irregularities in the ionospheric electron density distribution may be considered as providing a rough reflecting surface for HF radio waves. As a result, signals associated with each propagation mode arrive at the receiver over a range of angles in both azimuth and elevation. Measurements are presented of HF radio signals received over a disturbed, high latitude path. Multiple, closely separated traces were apparent in the time history of the calculated bearings, the precise nature of which depends upon the array geometry and algorithm employed. The presence of these multiple traces is attributed to the behaviour of the DF algorithms when the signals arrive over a range of azimuth and elevation angles due to the rough reflector nature of this disturbed region of the ionosphere