Estimation and Separation of Ionospheric Narrow-Band Unknown O and X Waves

The geomagnetic field splits an electromagnetic wave into two characteristic waves, termed the ordinary (O) and extraordinary (X) waves. For the narrow-band unknown high-frequency (HF) signals, we develop a model for the oblique propagation of the characteristic waves. The two waves travel through similar paths in the ionosphere and incident on a small aperture array with the same direction of arrival (DOA) and perpendicular polarizations. In a short period, there is only amplitude ratio and phase difference between the two waveforms, resulting in an elliptically polarized resultant (R) wave. Based on the maximum likelihood estimation, DOA and elliptically polarized parameters of the R, O, and X waves are estimated using the antenna’s vector effective length and the geomagnetic field. The Cramér–Rao Bounds are also presented. In addition, the O and X waveforms are separated based on polarization differences. Simulation and experiment validate the model and performance of estimators. DOA estimator for the R wave outperforms the estimator for the characteristic waves and the estimator for matching the energy of orthogonal polarizations. One of the separated characteristic waves presents smoother and shallower fading than the R wave.