Ionospheric factors affecting the performance of HF sky-wave sea-state radars at high latitudes

Sky-wave radars provide a means of ocean surveillance beyond line of sight and beyond the ranges achievable with a ground-wave radar. However, the ionosphere introduces a number of important propagation limitations. Firstly, it is necessary to provide illumination of the areas of interest, which may place restrictions on radar sitting and periods of operation. Then, backscattered signals must have sufficient amplitude to provide adequate discrimination above the naturally occurring and interference backgrounds. A radar measures time-of-flight and such information needs transforming to equivalent ground range in terms of the prevailing ionospheric path. Finally, ionospheric motions create Doppler shifts and spreads of the returned signals which prevent spectral resolution of Bragg scattered components. The high-latitude ionosphere is characterised by features which exhibit marked spatial and temporal variability. Specific problems are discussed and possible ways of alleviating some of these considered.