Modeling And Suppression Of Sea Clutter For Shipborne HF Hybrid Sky-surface Wave Radar

High frequency(HF)hybrid sky-surface wave radar is proposed based on the combination of advantages of both the HF sky-wave radar and the HF surface-wave radar to improve the ability of radar system for the target detection.Regarding HF radar working with hybrid sky-surface propagation mode,the sky-wave transmit site is usually located inland.Depending on where the receiving platform is mounted,such a radar system can be classified as either land-based or ship-mounted.As an extension of the land-based HF hybrid sky-surface wave radar,a radar system mounted on a moving platform can be employed for agility and maneuverability to improve the capacity for oceanic observations of areas far from the coastline.The sea clutter as the important component of echoes scattered from the ocean surface is the major clutter background for detecting the vessel target with a low velocity.The sea clutter spectra influenced by the ionospheric transmit channel,the bistatic deployment and the shipborne platform motion are seen to spread severely and exhibit a complex signature.The suppression of the broadened sea clutter is an effective method for the target detection,which should be proposed based on the well understanding about the broadening mechanism and characteristics of sea clutter.The sea clutter modeling has laid the theoretical basis to address this problem.Therefore,the modeling and suppression of sea clutter for shipborne HF hybrid sky-surface wave radar has been the major purpose of this dissertation.First,the ionospheric space-time perturbation mechanism on the HF sky-wave radar signal is analyzed and then the corresponding simulation method is given.For the sake of convenience,the whole ionosphere is modeled to be comprised of the background ionosphere and the distorted ionosphere containing irregularities.Regarding the research on the phase path variations due to the reflection from the background ionosphere,the corresponding simulation method is proposed on the basis of the ray-tracing technique on the assumption that the background ionosphere is described by the combined result of the International Reference Ionosphere(IRI)model and quasi-parabolic(QP)model.According to the space-time perturbation mechanism caused by the ionospheric irregularities,the space-time correlation function is theoretically derived,which can measure the signal space-time coherence loss quantitatively.Together with the statistical distribution result of ionospheric perturbed phases,the simulation method of the ionospheric temporal perturbed phases caused by the ionospheric irregularities is given.Simulations and experiments have demonstrated the influence of ionospheric space-time perturbation.Afterwards,the characteristics of the broadened sea clutter spectra influenced by the bistatic angle and the shipborne platform motion are investigated.By analogy to the analysis process for the land-based bistatic HF surface waver radar(HFSWR),the introduction of the shape factor has completed the method for analyzing the broadening features of sea clutter due to the bistatic angle.With regard to the radar cross section of sea clutter,the first-and second-order cross section of the ocean surface are theoretically derived when the ship is assumed to keep the uniform linear motion.To proceed,based on the modified first-order electric field expression due to the ship motions in six degree of freedom(SDF),the first-order cross section of the ocean surface in consideration of the contributions from other forms of ship motions is given.The obtained results have laid the theoretical basis to study the broadened characteristics of sea clutter spectra for different radar operating parameters,the ship motion parameters and the sea state conditions.On the basis of these researches mentioned above,the sea clutter model under the compound factors including the ionospheric space-time perturbation,the bistatic angle and the ship motions is proposed for shipborne HF hybrid sky-surface wave radar.According to the main idea of parametric modeling method,the phase and amplitude parameters of the received first-order ocean wave echo are associated with the simulation results of ionospheric phase path variations and the cross section of sea clutter.In conjunction with the spectrum analysis method of random signals,the Doppler spectra of sea clutter can be thus calculated.Further,based on the simulated perturbed phases of radar signals when travelling through the ionosphere media,the ionospheric space-time perturbation matrix is constructed.In conjunction with the space-time modulation mechanism on the first-order sea clutter due to the platform motion,the space-time model of sea clutter is proposed.Based on the obtained sea clutter models,simulation results have studied the distribution characteristics of sea clutter for different ionosphere perturbation conditions and the ship motion parameters,which will be used to design the sea clutter suppression method.Finally,the sea clutter suppression method is investigated based on the study of sea clutter characteristics,which includes the ionospheric space-time compensation method and the space-time adaptive processing(STAP)based suppression method.In accordance with the mechanism analysis of the ionosphere perturbed phases on the sea clutter spectral broadening,the corresponding space-time compensation method is proposed,which is based on the covariance matching method and multicomponent decomposition technique,respectively.Regarding the problem of sea clutter suppression,the STAP based method is studied.On the basis of the intrinsic sparsity of sea clutter,the oblique projector based subspace sparse recovery STAP method is proposed,which realizes the effective sea clutter suppression with the training data collected from only one range cell.Then,in consideration of the nonstationarity involved in the training data,an improved knowledge-aided STAP method is given.By using the l1-norm regularization sparse constraint,the performance degradation because of the use of the inaccurate prior knowledge is reduced and the convergence performance of the proposed method is improved significantly in the case of the small training data support and the nonstationary clutter background.Simulated and semi-real radar data have demonstrated the effectiveness of the proposed sea clutter suppression method.