The SAR Moving Targets Detection Based On Fractional Fourier Transform

As a basic fuction of Synthetic Aperture Radar(SAR),moving targets detection plays an important role in both martial field and civilian field.Presently there are two kinds of methods in the moving targets detection approximately.The first one which mainly apply to single-channel SAR system is described according to the characteristics of the moving targets' echoes, including the method based on doppler filtering,the method based on doppler frequency filtering and the method based on parameter estimation.The second one which mainly apply to the multi-channel SAR system is considered according to clutter suppression,including detecting the moving targets according to temporal and spatial characteristics of the moving tragets and adopting interference treatment.The fractional Fourier transform which is the Fourier transform in broad sense is explained to the representation in fractional Fourier domain of the signal rotates arbitrary angle around the origin in the time-frequency plane.It can obtain the distribution of most concentrated energy in fractional Fourier domain if rotates proper angle and it is the basic principle for detecting the moving targets.The fractional Fourier transform which is a linear transform does not bring in the cross-term just like the Wigner-Ville distribution in detecting multiple moving targets.In addition,the numerical calculation of fractional Fourier transform can use the fast Fourier transform(FFT),so its computation is not too heavy.The main tasks of this paper are as follows:1.Detecting the moving targets based on fractional Fourier transform.For the single moving target,a method of two-dimensional search is presnted which can find the transform order of the most concentrated energy.For the multiple moving targets and when each component signal energy is larger difference,a separation technology in fractional Fourier domain is presented,which can suppress the strong signal and detect all the signals one by one according to the size of the signals' energy.2.The signal will be submerged by the noise when using fractional Fourier transform directly if the signal to noise ratio is relatively low.The LMS adaptive filtering algorithm based on fractional Fourier transform is presented in this paper,which is the LMS adaptive algorithm in time domain in broad sense.The basic principle of the LMS adaptive filtering algorithm in fractional Fourier domain is introduced and the convergency speed and stable error of this algorithm is analyzed.It compares the lerning curve between in time domain,frequency domain and fractional Fourier domain.The LMS adaptive filtering algorithm of fixed step size in fractional Fourier domain could not resolve the contradiction between the convergency speed and stable error,so a LMS adaptive filtering algorithm of variable step size in fractional Fourier domain is presented in this paper.