Doppler Signature Analysis of Mixed O/X-Mode Signals in Over-The-Horizon Radar

We analyze the Doppler signatures of local multi-path signals in an over-the-horizon radar in the presence of both ordinary (O) and extraordinary (X) polarization modes. As the ionospheric signal reflection for the two polarization modes varies from each other, the existing local multipath model developed for a single polarization mode must be extended to account for such a propagation environment. In this paper, we focus on the case with small delays between the signals corresponding to the two propagation modes. We exploit the multipath signal model considering the mixed O/X mode signals and analyze the variation in the resulting Doppler signatures. The analytical as well as numerical results show that the existence of both O/X polarization modes renders more signal components with close Doppler signatures. In the underlying situation with small delays between the two modes, the mixed O/X-mode signals corresponding to each local multipath signal component are unresolvable and yield time-varying fading magnitude. Accurate parameter estimation is still achieved using fractional Fourier transform over a longer coherent processing time.