Study On Small Target Detection Under Sea Clutter Based On Microwave Doppler Radars

At present,small target detection technologies under sea clutter have great value in the field of military and civilian.With the rapid development of radio detection technology in marine field,how to achieve the goal of realizing effective target detection under sea clutter has become a research hotspot.However,small target detection technology under sea clutter seems complex and difficult:on one hand,due to the complexity of the marine environment,sea clutter modeling has become more difficult with its complicated statistical characteristics;on the other hand,small target embedded in sea clutter has small radar cross section(RCS)and very low echo energy,while its Doppler shift is extremely closed to or overlapping the clutter region because of its relatively slow speed.When it comes to microwave Doppler radars,important practical significance has been shown with its advantages such as lighter weight,higher measurement accuracy,smaller environmental interference and so on.Based on microwave Doppler radars,the dissertation discusses and analyzes the issue of small target detection under sea clutter,and the main contents of this paper are shown as follows:The basic theory of small target detection under the sea clutter is introduced in detail.Firstly,the basic principle of radar target detection is presented,and then the radar systems and hypothesis testing criterion are analyzed.Secondly,physical mechanism and statistical models of sea clutter are explained,while the statistical model and the fluctuation characteristics of the target backscattering cross section are discussed.Finally,this paper introduces and explains the measured data sets which are used in the dissertation.Classic and useful technologies of sea clutter suppression and target detection in the ocean are introduced and discussed.In the part of sea clutter suppression,traditional methods are presented,such as moving target indication(MTI)and moving target detection(MTD).Moreover,because of the robustness of the median operator,two adaptive clutter suppression filters are introduced.As to target detection algorithm,we introduce the traditional constant false alarm rate detection algorithm and matched filter detection algorithm.According to the characteristics of slow and stationary targets,target detection algorithm based on fractal technology and Hilbert spectral ridge are analyzed.On the aspect of fractal-based detection technology,self-similarity and scale deformation show the significant influence on sea clutter when targets exist.As for the detection algorithm based on Hilbert spectrum ridge,target detection can be achieved by the differences between target and pure sea clutter in the time-frequency domain.A new algorithm based on joint features is proposed to improve the detection performance when the target is embedded in sea clutter.In this algorithm,peak power,kurtosis and entropy are extracted from the test samples to form feature vectors,while training data from the residual sea clutter is utilized to determine a decision region by convex hull algorithm.Then,the difference between target and clutter is significantly enlarged by mixed product of the convex hull.Finally,the preliminary detection rule is whether the decision region surrounds the test vector,and false alarm trace points are excluded by constant false alarm rate threshold.Detection performance of the proposed method is analyzed by the way of Monte Carlo simulation.It demonstrates that the detector based on joint features outperforms the compared algorithms.As to empirical mode decomposition(EMD)algorithm,how to realize automatic selection and discrimination of modal functions is the key problem.In this paper,target detection method based on characteristic spectrums of modal functions are introduced to solve this problem.In the algorithm,the echoes of radar are preprocessed by empirical mode decomposition and characteristic spectrum is extracted from each intrinsic mode function.Then,distribution characteristics are obtained.Finally,target detection is realized adaptively based on the difference of distribution characteristics in all mode functions between target and sea clutter.This algorithm does not need clutter suppression,and it is able to realize automatic identification and screening of the modal function.In this way,target can be detected adaptively when empirical mode decomposition algorithm is used.