Sky and ground - wave high frequency (HF) radars: challenges in modelling, simulation and application preface

Radar systems operating at high frequency (HF) are able to exploit a variety of propagation mechanisms to achieve line-of-sight (LOS) illumination as well as over-the-horizon coverage, thereby providing many opportunities for long range, wide area multi-sensor surveillance. The purpose of this special issue is to bring together latest research results on physical understanding, mathematical modelling, numerical simulation, and novel application challenges (e.g., tsunami detection) in the field of sky- and ground-wave HF radars. The challenges include understanding complex radar echo, environmental noise, sea/land/ionospheric clutter, sky- and ground-wave propagation; mechanisms, radar cross-section (RCS) in the resonance regime, frequency monitoring in highly crowded spectrum, antenna design and beam-forming, beam-steering, detection, tracking, classification, and identification algorithm development, dataand sensor-fusion, etc. This special issue is devoted to these topics