Experimental observations using superresolution DF and propagation path determination of additional off great circle paths due to the terminator

To resolve multi-component HF wave-fields from ionospherically propagated signals, the choice of radio direction finding algorithm is important, since many methods are not robust enough to cope with such a scenario. Algorithms which possess the ability to resolve highly correlated signals and are insensitive to the propagation induced modelling errors which are present when an ionospherically propagated signal is incident on an array of antennas, include the deterministic maximum likelihood estimator, (MLE), and the DOSE algorithm. Performing direction finding (DF) in beam-space rather than element space is known to improve the performance of many DF algorithms. Operating in beam-space also improves the resilience of a DF algorithm to modelling errors. The spatial filter implemented in beam-space processing not only suppress signals from particular directions of arrival, (DOAs), but since the filter is constructed assuming planar wavefronts, it will also suppress to some extent the non-planar components of a wavefield incident on an antenna array. The authors implement the MLE in beam-space in order to measure the DOA of an ionospherically propagated signal. Not only are typical diurnal variations seen, but close to the terminator additional off great circle paths are resolved. The DF hardware and processing used are described. The results are presented and the mechanism resulting in the additional off great circle paths is discussed. A method of determining the ray path(s) between given transmitter and receiver locations when off-great-circle propagation due to ionospheric tilts occurs is described.