Nonlinear scattering at HF: Prospects for exploitation in OTH radar systems

HF skywave ‘over-the-horizon’ radars experience exceptionally high levels of clutter from the earth’s surface as a consequence of their ‘look-down’ propagation geometry. While Doppler processing is effective in separating aircraft echoes from the clutter, thereby enabling detection, the same does not always hold for slow moving targets. In particular, ship echoes occupy the same region of Doppler space as the sea clutter, so the likelihood of target echo obscuration is high. By exploiting the known dependence of the characteristic spectral form of the clutter Doppler spectrum on radar frequency and other parameters, HF radars can maximise the prospects for detectability against particular ranges of target parameters. Nevertheless, even assuming that a radar makes use of all available a priori knowledge to optimise its parameters, small ships and boats often escape detection because their echoes are still masked by clutter in the same Doppler subspace. This is particularly likely to occur if the vessels concerned wish to remain undetected, and hence use the properties of sea clutter to their advantage. In this paper we examine the feasibility of a novel approach to the problem of detection against sea clutter – detection via nonlinear scattering mechanisms. The nonlinear scatterers of concern here include electronic systems as well as metal-oxide-metal junctions formed by corrosion in the marine environment. This so-called ‘rusty bolt’ effect has previously been considered in the context of electromagnetic compatibility of communication systems. Here we explore the possibility of exploiting it to achieve a form of ‘clutter-free’ radar.