| Ground/maritime moving target indication is the main mission for the surveillance of airborne/spaceborne radar.When the moving platform observes down,the clutter may be severe to affect the detection and identification of the target.When the radar observes the sea,the property of the sea clutter is much different with that of the ground clutter.Therefore,it is necessary to model the sea clutter exactly for processing according to the movement of the sea surface.Conformal array has some advantages,such as decreasing the load and air resistance of the airplane and increasing the effective radar aperture.It is necessary to use the conformal array to detect the weak small targets.For the problems of the ground/sea clutter property and the maritime information acquiring,this dissertation covers the clutter processing of conformal array,the statistical analysis and space-time model of the sea clutter,the current estimation and the target focusing.The main contents are summarized as follows:1?In the conventional space time signal model,the statistical amplitude of the clutter is assumed to be specific distribution.For sea clutter,a pulse-to-pulse correlation matrix is added to the temporal covariance matrix to describe the motion of the sea surface.However,such sea clutter model cannot reflect the property of the time-varying of the sea clutter directly.Aimed at the time-varing property of sea surface,A space time model for the sea clutter is presented based on the physical sea surface model,in which the clutter amplitude is a function of the time and space,instead of statistic.The reflectivity and the radial velocity of the sea clutter for any specific position and time are computed based on the physical sea surface.Both the aforementioned two factors vary with time,which corresponds to the time-variation of the sea surface.Besides,the spatial channel decorrelation is modeled,which has an effect on the spatial covariance matrix of sea clutter.The simulated angle Doppler spectra and the signal to clutter-plus-noise ratio(SCNR)loss show that the reflectivity of the sea clutter makes the space-time adaptive processing(STAP)performance degrade and the radial velocity of the sea clutter results in a more significant spread of the clutter power spectrum in sea state 4 than sea state 2.The Doppler spectrum can be acquired by the space-time model proposed in the dissertation instead of the experimental Doppler model for sea clutter.Finally,the real sea data is used to validate the effectiveness of the model and clutter supression performance of several typicla STAP methods is analyzed.2?In the condition of conformal array,the minimum variance spectrum with sampling matrix inversion(SMI)extends much.This has an influence on the target detection.So the method of using the gain spectrum to analyze is proposed.In the method the gain weight was used to process the clutter and the element gain was contained in the gain weight to match the actual clutter better.The effects of the central angle and element main-lobe width on eigenvalue,the minimum variance spectrum and the gain spectrum of the clutter were mainly analyzed on the basis of this method.The performance analysis provides theoretical support for optimal conformal array design.The simulation results indicate that a larger main-lobe width makes the minimum variance spectrum extends smaller and a bigger central angle makes the gain spectrum get better performance.Besides,for the case of clutter in main-lobe,the gain weight can be used to effectively improve the performance of the target detection.When the central angle is large,the gain weight can evidently get better processing result than traditional SMI.Moreover,the gain weight is also effective for sea clutter and can decrease the spectrum width of sea clutter evidently.3?For radar target detection,the clutter distribution model needs to be known or assumed before CFAR processing.The goodness of fit(Go F)between the original data and the assumed distribution can be used to choose the proper distribution model.Generally,the Go F is obtained using data histogram and theoretical distribution curve,and then the distribution model is judged via Go F.However,when the sample number is small,the histogram is rough and fluctuating,affecting the analysis of Go F.For the small sample,the graphical characteristic is obtained with the sample data according to some computaiton metric to choose the most fitting distribution to the data in this dissertation.The graphical characteristic is acquired by a simpler process,that is,the original data is directly set as the test statistics,avoiding computing and sorting of other statistics.In this dissertation,the real airborne circular SAR data under different scan angles are analyzed using the Go F corresponding to histogram and graphical Go F,respectively.The results show that when the sea clutter data histogram is close to two distributions,different distribution model results may be obtained according to traditional Go F.In this case,the distribution model is difficult to judge,but can be acquired by graphical representation.In addition,the sea data with different sight angles have different match properties.It is seen that the sea data is closer to Rayleigh distribution in side-looking mode than in big squint-angle mode,while Weibull distribution and K distribution show equal fitting performance to sea clutter under variant radar sight angles.4?A method of current velocity estimation using airborne sea data is proposed in the dissertation.The sea data are collected by circular synthetic aperture radar(SAR).The range Doppler image can be acquired after range and azimuth compression for each beam scan angle.The corresponding average Doppler spectrum can be obtained and fitted to Gaussian shape well.So the Doppler center shift can be represented by the mean of Doppler spectrum.It is observed that the Doppler center varies with the beam scan angle and the variation is approximately a sinusoid-shape function.The velocity and direction of current in a local area are fixed so the radial velocity caused by the current is a harmonic function of the scan angle.Therefore,the current velocity can be obtained by the amplitude of the harmonic and the current direction can be acquired by multi Doppler shifts.Actually,the radial velocity does not only include the current,but also includes the surface wave motion.However,the method in this letter can obtain the real current velocity directly,avoiding adopting the radial velocity of the scatterer.The estimation of the current velocity is in accord with the measured value,which demonstrates the effectiveness of the method.5?A ground moving target focusing and velocity estimation method based on first-order discrete polynomial-phase transform(DPT)for airborne synthetic aperture radar(SAR)is introduced in this dissertation.Firstly,DPT is used to reduce the target phase order.Secondly,Keystone transform(KT)is applied to correct the residual range walk,and then the target second-order motion parameter can be estimated after performing the azimuth fast Fourier transform(FFT).Finally,the target first-order motion parameter can be estimated after the Doppler frequency migration(DFM)compensation and a well-focused result of a moving target can be obtained.Compared with the conventional SAR imaging methods,the proposed method can eliminate the effects of Doppler ambiguity and azimuth spectrum split.In addition,the proposed method is computationally efficient since the target motion parameter searching procedure is avoided,which can satisfy the target real-time imaging requirements.Real data processing results are provided to validate the effectiveness of the proposed algorithm. |
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