Some super-resolution DF measurements within the HF band

The unknown propagation mode content of signals monitored in some earlier super-resolution HF-DF experiments was a significant limitation in the interpretation of the measurements and inferences concerning the mode content had to be made from the processed data. In order to overcome this limitation, further experiments have been undertaken in which the propagation mode content of the signals was determined by oblique sounding. Measurements made over a 778 km path from Wick, Caithness to Cricklade, Wiltshire are reported. The complex amplitude distributions of the received signals were sampled by means of a wide aperture multi-element antenna array connected to a 6-channel receiving and data recording system. The directions of arrival were subsequently estimated by applying various super-resolution direction finding algorithms to the recorded data. Two super-resolution algorithms were employed in these studies: (a) the MUSIC algorithm and (b) an iterative null steering technique based on the IMP algorithm and the DOSE algorithm. Enhancements have been made to the null steering algorithm and a description of the modified algorithm is given.