OTH Radar Phenomenology Figure 5. Poincare sphere representation of the progressive state transformation of the radar signal for a skywave radar. In this example the transmit and receive antennas are assumed to have vertical polarization; the target scattering matrix is responsible for the transformation shown in magenta. where the echoes are interpreted as coming from the shaded region. Figure 4b shows an alternative mechanism that delivers energy to the receiver from the same direction and at the same group delay as the echo from the patch of interest; this energy is therefore superimposed on echoes that did arrive from the patch of interest. Special techniques are required if one is to deal with this problem other than by redesigning the radar [7]. Another instance of the rich phenomenology of HF propagation arises with consideration of polarization. Traditionally the polarization transformation resulting from propagation through the magnetoionic medium of the ionosphere was modelled as repolarization, that is to say, conversion of one pure polarization state to another. This is depicted schematically as mappings on the surface of the Poincare sphere, as shown in Figure 5. An example of the impact of spatial variation of the polarization transformation is shown in Figure 6, along with the modelled scattering matrix elements of the sea surface for this instance. Figure 6. (a) Polarization fringes evident in skywave sea clutter as a consequence of the strong dependence of the surface scattering coefficient on polarization. (b) Modelled Doppler spectra for a specific sea state for the elements of the power scattering matrix. 8 IEEE A&E SYSTEMS MAGAZINE DECEMBER 2017