Sea Clutter Suppression and Moving Target Detection Method Based on Clutter Map Cancellation in FRFT Domain Xiaolong Chen*, Yong Huang, Jian Guan*, You He
Department of Electronic and Information Engineering Naval Aeronautical and Astronautical University,NAAU Yantai,Shandong P.R. China,264001 Phone: 13562560769; Email:
[email protected].
[email protected] *
Abstract
In order to effectively detect moving target in strong sea clutter, a detection algorithm for moving target based on clutter map cancellation in fractional Fourier transform (FRFT) domain is proposed. Radar data in FRFT domain are noncoherently accumulated with the single pole feedback clutter map (SPFB-CM) in FRFT domain, which are used to suppress sea clutter and improve signal to clutter ratio (SCR). X-band real sea clutter of IPIX radar is used for verification and the cell-averaging CFAR (CA-CFAR) in time domain is compared. The results present that weak moving target can be detected in low SCR (SCR=-5dB) environment with high detection probability (Pd=0.9). The predetermination of the best transform order can be avoided and the poor detection performance of clutter map CFAR in time domain when detecting slow moving target can be improved greatly, which indicates the effectiveness and robustness of the algorithm. Keywords: Moving target detection; Fractional Fourier transform (FRFT); Sea clutter; Clutter map.
1. Introduction
Moving target detection within sea clutter is always a challenging subject in the field of radar signal processing and robust detection of moving target plays a significant role in military and civil fields. The radar echo of a moving ship with low radar cross section (RCS) is usually covered by heavy sea clutter and noise. Traditionally, sea clutter is regarded as random process and studied in terms of certain simple statistical features. The development of statistical models that properly characterize radar clutter processes is critical for designing optimum algorithms for detecting targets within radar clutter as well as for performance evaluation of those detectors. Sea clutter has often been observed to be highly non-Gaussian, and even spiky [I). However, sea clutter modelling is a very challenging problem and the non Gaussian character in heavy sea conditions makes it difficult for classical statistical mathematics to represent sea clutter accurately. Hence the space constant false alarm rate (CFAR) [2], such as cell averaging-CFAR (CA-CFAR) [3], order statistics CFAR (OS-CFAR) [4] often using parameters estimated from statistical distributions are not very effective for distinguishing sea clutter data with and without targets. A different approach to obtain CFAR based on clutter map exploits the intrinsic local homogeneity of the radar environment in which the detector output of each range
resolution cell is averaged over several scans in order to obtain an estimate of the background [5). The conventional clutter map CFAR (CM-CFAR) method in time domain is often used to detect targets in a temporally stationary and spatially non-homogeneous background, such as in land clutter and sea clutter [6). However, it is affected by self masking and heavy clutter, that is, a progressive reduction of the detection probability when a slow target at sea stays in the same range cell for several scans. The radar echo of moving target with constant velocity or constant acceleration can be modelled as linear frequency modulation (LFM) signal in some circumstances. The fractional Fourier transform (FRFT) is a newly developed generalized Fourier transform (FT), which has a good energy concentration property of LFM signal without cross-terms interference [7). The energy of moving target can be accumulated to the full extent in the best FRFT domain and the detection and parameter estimation for moving target in low signal-to-clutter ratio (SCR) environment is then applicable without extra prior knowledge of sea clutter[8). In this paper, we propose a novel method for moving target detection based on clutter map cancellation in FRFT domain. The remaining of the paper is organized as follows. In section 2,the time-shift properties of moving target and sea clutter in FRFT domain are analyzed. In section 3, we propose detection method based on the single pole feedback clutter map cancellation in FRFT domain. Then, real sea clutter is used for verification and detection performances are presented in section 4. Finally, some concluding remarks are presented in section 5. 2. Time-shift properties of moving target and sea clutter in FRFT domain
On the condition that radar transmits single frequency signal or LFM signal, radar echo of moving target can be approximated as LFM signal [9). As the radar cross section (RCS) of moving target varies with the visual angle,it will be more practical and effective using time-varying amplitude LFM signal model. The acceleration and initial velocity of targets are denoted as a, Vo and the instantaneous velocity at time t can be formulated as V=vo+at. Within observation time T, the model of moving target in the presence of sea clutter can be characterized as follows x(t) = set) + e(t) = A(t) exp[j21l"Jot + j1l"kt2] + e(t) , I tI :;:; T (1)
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where A(t) is amplitude of received signal,which is a function of time, the chirp rate is k=2 alA, A represents the wavelength of the radar, fo is the centre frequency corresponds to the
"0la ' velocity of the targets fo=2vo/A, and sea clutter is denoted as j t +j(2n/;-ucsca)t Le 2 c(t). i (6) The energy of sea clutter in FRFT domain is mainly distributed around order p=1, i.e. in frequency domain. Hence, -r)] = 2AaTL lsinc[( 21tJ; -ucsca)T]1 + Fa sea clutter can be regarded as the sum of single frequency Also, the energy of white noise in FRFT domain is signals and Gaussian noise [8). The time-shift properties of LFM signal and single frequency signal in FRFT domain are uniformly distributed and has close relation with the time delay. Hence, the FRFT modulus of sea clutter before and shown in Fig.l. after time delay is almost identical. Based on the above J analysis, clutter map cancellation method in FRFT domain is LFM Frequency obtained according to the method of clutter map in time domain domain. Then moving target can be detected and sea clutter is well suppressed with FRFT modulus subtraction between FRFT radar echo and its delayed signal due to the time-shift domain properties. Single frequency /I
dt+Fa[n(t-r)] [n(t
I
I
IFa[e(t-r)]I=Aa LT
, '
