Time-frequency method to reduce contamination by ionospheric fast frequency modulation for HF skywave radars

The unstable and time-varying property of the ionosphere causes a frequency modulation on the high frequency sky-wave radar echo signals. When the modulation period is of 10 s or smaller, it is called the fast phase-path modulation. This sort of modulation will result in the broadening, the splitting and even the overlapping of the spectra, which makes target detection and sea-state remote sensing hard. Hence, it is necessary to decontaminate the echo signals before the coherent integration process. In this study, the pseudo Wigner-Ville distribution is proposed as a means of estimating the ionospheric frequency modulation. A frequency weighting method is used to obtain the instantaneous frequency (IF) from the time-frequency distribution and an adaptive low-pass filter is introduced to purify the IF. Then, the amplitude difference between the two first-order Bragg peaks is adopted in the weighting process to obtain the phase compensation function. To improve the compensation effect, an optimising process is proposed. The simulation results show that the method can improve the quality of the spectra for the target detection and remote sensing efficiently even in the cases of a high sea-state.