| Based on the propagation mode associated with sky-wave transmitting and surface-wave receiving, HF hybrid sky-surface wave radar not only maintains the superiority of HF skywave radar which have a long detection rang and wide coverage, the advantage of HF surface wave radar which have a stable propagation channel, but also keeps a good invisibility and anti-interference ability. However, the sea clutter under this combined propagation mode with sky-wave transmitting and shore-based receiving is influenced by composite factors such as the bistatic angle and ionospheric phase disturbance. Therefore, the width and shape of sea clutter Doppler spectrum will be broaden and complicated, giving rise to the severe restriction of ship target detection. Firstly, the internal mechanism and characteristics of the broadening sea clutter based on the detection principle of HF hybrid sky-surface wave radar are studied deeply and carefully in this dissertation. Secondly, the broadening first-order sea clutter model for HF hybrid sky-surface wave radar is established. The theoretical analysis of sea clutter characteristics and broadening sea clutter model are examined by measured data. Eventually, based on the preceding established broadening sea clutter model and characteristics, the techniques to suppress the broadening sea clutter are studied. The research in this dissertation will provide the theory and technology foundation to detect the target over the horizon under the broadening sea clutter in HF hybrid sky-surface wave radar.Aiming at above problems, this dissertation will spread the study, the main content of the whole dissertation as follows:1. The internal mechanism and characteristics of the broadening sea clutter for HF hybrid sky-surface wave radar are studied deeply and carefully, which is the theoretical basis for following broadening sea clutter modeling and suppression. Firstly, according to the geometry configuration and detection characteristics of this new system radar, the first-order sea clutter broadening mechanism is analyzed on the basis of the scattering resonance mechanism of HF bistatic radar. Secondly, the first-order sea clutter Bragg frequency expression and the range of Bragg frequency broadening are derived based on the comprehensive considerations of the compound effect of the various factors on first-order sea clutter. Then, the characteristics of first-order sea clutter frequency shift and broadening under the action of each single factor of ionosphere, grazing angle and bistatic angle and the composite action between them are analyzed. And the characteristics of first-order sea clutter frequency shift and broadening with different range gates and azimuth angles are simulated and analyzed. Finally, the results of theoretic analysis are examined with the experimental data based on the newly-developed integrated experimental system.2. The simulation method of bistatic broadening sea clutter spectrum using wide beam for HF hybrid sky-surface wave radar is studied based on Bragg scattering theory in depth. The sea clutter spectrum broadening using wide beam is mainly caused by the bistatic angles at different space positions, which is not affected by the ionosphere. The sea clutter spectrum model using narrow beam will not be able to fully reflect the real sea clutter spectrum shape. In order to comprehensive analyze and simulate the sea clutter spectrum characteristics using wide beam in HF hybrid sky-surface wave radar, the geometric distribution and the scattering frequency characteristics of first-order sea clutter using narrow beam are firstly analyzed in the case of the fixed sum of distance. Then, when the ideal sky wave signals illuminated the detecting sea area, because of the sea clutter spectrum broadening characteristics induced by the different bistatic angles of each resolution cell in the range of received wide beam, the sea clutter power spectral densities model using wide beam is proposed based on the Bragg scattering theory and equiangular segmentation method of the scattering region. Finally, the bistatic distribution and broadening characteristics of the first-order sea clutter spectrum are simulated and analyzed according to the system layout. The research of this part will provide the theory foundation to establish the final broadening sea clutter model under the action of bistatic angle and ionospheric phase disturbance.3. The influence of the ionospheric phase disturbance on sea clutter spectrum is analyzed based on the ionospheric transmission characteristics and disturbance mechanism. The essence of ionospheric phase disturbance is electron concentration time-varying which lead to phase path time-varying and spectral broadening. Therefore, the ionospheric media characteristics are analyzed firstly. Then, the ionospheric phase path disturbance mechanism is deeply studied. And a simulation method of ionospheric phase path disturbance is proposed using the analytical ray tracing technique and the ionospheric mixed structure model, which is based on the ionospheric transmission characteristics. Based on the proposed simulation method of ionospheric phase disturbance, the influence of the different ionosphere state on sea clutter spectrum is analyzed and simulated. Finally, the limits of ionosphere on the size of sea clutter resolution cell and minimum detectable ship size of radar cross section(RCS) are quantitatively analyzed. The simulation method of ionospheric phase disturbance will provide the theory foundation for following broadening sea clutter model and suppression technology.4. According to the previous analysis of the simulation method of ionospheric phase disturbance based on the ionospheric transmission characteristics and the broadening sea clutter spectrum model using ideal wide beam for bistatic radar, the characteristics of sea clutter broadening under the action of ionospheric phase disturbance and bistatic angle are analyzed, and the final sea clutter broadening model for HF hybrid sky-surface wave radar is established. Then, according to the specific bistatic radar system layout and ionospheric medium parameters, the influence of bistatic angle and ionospheric phase disturbance on sea clutter spectrum broadening are simulated and analyzed based on the broadening sea clutter model. Finally, the first-order sea clutter distribution and broadening characteristics, and the proposed first-order sea clutter broadening model are examined respectively by measured data. The proposed first-order sea clutter broadening model will provide the theory foundation for following sea clutter suppression technology.5. The broadening sea clutter suppression techniques are discussed from the cascaded processing and space-time joint suppression methods based on the characteristics and model of the broadening first-order sea clutter. Firstly, the cascaded suppression processing scheme is given. In the cascaded processing method, a cascaded ionospheric phase disturbance suppression method based on S2-method is proposed to correct the ionospheric phase disturbance. Afterwards, a novel forward backward linear prediction(FBLP) algorithm is proposed to suppress the broadening bistatic sea clutter caused by bistatic angles, which is based on the distribution and multidimensional characteristics of the first-order sea clutter. Then, based on the space-time coupling characteristics of the first-order sea clutter spectrum, the space-time two-dimensional joint suppression technology is used to suppress the broadening sea clutter using the space-time adaptive processing(STAP)-joint domain localized algorithm(JDL). The influence of ionosphere on the performance of JDL algorithm is analyzed. And a sea clutter suppression method based on prior knowledge is proposed. Finally, the foresaid proposed methods are examined respectively by measured data.
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