Research On Ionosphere Contamination Of High Frequency Radar Echoes And Time-Frequency Analysis Technology

The skywave Over-the-Horizon Radar (OTHR) is a special type of radar which operates in high frequency band (3-30MHz) and employs the ionosphere to reflect signal to detect target from the top down. OTHR has some native advantages, such as the capability of detecting low-flying target and stealth target, avoiding the attack of anti-radiation missile, as well as providing wide area surveillance with longer early warning time than some other common radar. Therefore, OTHR play a more and more important role in modern air defense system and has potential significant contribution to military applications. However, OTHR's operating modes (high frequency operation band, ionospheric propagation medium and looking-down mode) bring some great challenges to the signal processing. For instance, owing to the phase distortion, the spectrum of the wave might be stretched, which would greatly influence the detection of the target; The multi-mode propagation makes the Doppler spectrum of the echo get ghost image and have more than two Bragg peaks, which also leads to the expansion of the clutter region and affects the target detection.The success in resolving these problems will be crucial to enhance OTHR's performance and extend its advantages.The summary of our research work is as follows:(1) The illumination of the influence of the ionosphere to the wave that travels in it. Since the subject of the thesis is about the wave in the ionosphere, it might be essential to perceive and comprehend how the ionosphere influences the wave. First, it briefly introduce the situations in the case of oblique incidence and scattering of the wave. Secondly, it emphases the influence of the ionosphere to the radar receiving wave, including the changes in the amplitude and the phase and the influence of the function of the HF radar.(2) The decontamination of ionospheric distortion is studied in detail. First, several decontamination methods of ionospheric phase disturbance are introduced and compared. The study of two techniques—the eigen decomposition and phase gradient autofocus (PGA)—to correct the phase distortion of the wave that passes the ionosphere and the simulation and discussion of the two methods. The correcting results of the SF signal that simulates the sea clutter and the receiving SF signal that appended with the target information, using the eigen decomposition technique and the PGA technique, and analyzes the performance of the two techniques.(3) Besides, for a complex case that mufti-mode echoes have different phase disturbances, we propose to use wavelet transformation time-frequency analysis technique to process the entire echoes. This new method not only resolves the contradiction between time-domain resolution and frequency-domain resolution of short time Fourier transform, but also does not yield the cross-terms. Thus, the new method has the capability to extract the detailed features of signal in time-frequency domain, that will benefit the target detection significantly.