Recent developments in HF skywave radar polarimetry

While active microwave radar polarimetry is an established technology for remote sensing, the exploitation of the polarisation domain in HF skywave radar systems has yet to be demonstrated operationally, being complicated by a number of factors which do not arise with conventional line-of-sight radars operating at higher frequencies. Of paramount importance is the role of the ionosphere, which acts as a polarisation transformer with seemingly stochastic properties. In addition, significant practical difficulties arise with skywave radar antenna design. Efficient antennas at HF are generally large and expensive, and the need to provide dual polarisations for transmit and receive functions, maintaining essential gain and phase coherence relationships, compounds the problem. Further challenges arise with the estimation of S-matrix and Mueller matrix structure through an intervening ionosphere, with establishing the polarimetric signatures of both discrete and extended targets by computation, and with the need to apply various forms of ameliorative signal processing. In this paper the authors review their progress towards overcoming the present limitations on HF skywave radar polarimetry as a tool for geophysical and other investigations