Keywords Synthetic Aperture Radar (SAR); Range Doppler Algorithm (RDA); Range Cell Migration (RCM); remote sensing; Radarsat1 Y 0

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1 Sedighe Mirbolouk * Deptment of Electricl Volume 3, Issue 5, My 013 ISSN: 77 18X Interntionl Journl of Advnced Reserch in Computer Science nd Softwre Reserch Pper Avilble online t: Synthetic Aperture Rdr Dt Processing Mhrokh Mghsoodi Deptment of Electricl Emd Torbi Deptment of Electricl Mehregn Mhdvi Deptment of Computer Abstrct The initil lgorithm tht hs been used for processing of rw dt synthetic perture rdr (SAR) is Rnge Doppler Algorithm (RDA). This dt is relted to spceborne or irborne SARs tht often dt gthering is done by stellite. Due to SAR sensor motion with respect to sttic or moving trgets in ground encounter with rnge cell migrtion (RCM) phenomenon in spceborne. In the ircrft cse RCM is negligible becuse the pltform velocity is lower. In this pper we implement the RDA on spceborne rel dt gthered from RADARSAT1 to generte SAR imge. Keywords Synthetic Aperture Rdr (SAR); Rnge Doppler Algorithm (RDA); Rnge Cell Migrtion (RCM); remote sensing; Rdrst1 I. INTRODUCTION Airborne or spceborne Synthetic perture rdr, developed in the erly 1950s, drws on cpbilities in generting rdr imge with fine fetures in ddition to its position nd velocity. Synthetic perture rdr by utilizing the movement of the ntenn with respect to the trget without sophisticted post processing obtin high resolution imge. The common SAR processing lgorithms in stripmp mode re RDA, Chirp scling lgorithm (CSA) [1] nd spectrl nlysis (SPECAN) []. The RDA ws developed in for processing SEASAT dt[3]. The prefer of RDA is ccurte nd effective ccommodtion of rnge vrying prmeters. This lgorithm hs not computtionl complexity nd it isn t time-consuming nd use of fst Fourier trnsforms (FFT s) effectively. In this pper we implement the rnge Doppler lgorithm on input imge in irborne SAR nd correct the effect of rnge cell migrtion. II. SIGNAL MODEL The purpose of processing of rw SAR dt tht its energy is spred in rnge nd zimuth is focusing. In other words, we wnt collect this dispersed energy by the LFM trnsmitted pulse s durtion in rnge nd synthetic perture length, or illumintion time of trget in zimuth, into single pixel in the output imge. When SAR pltform moves on the scene, t first the distnce of pltform with respect to trget in scene decreses then increses so the instntneous slnt rnge chnges tht this cuses chnging of phse in echo s function of cross-rnge (zimuth) tht is clled rnge cell migrtion phenomenon nd hve to is corrected before zimuth compression. For processing of zimuth frequency spectrum we didn t estimte Doppler centroid here nd is ssumed it hs lredy been performed. Aircrft Flight Pth L X C X 0, X C X 0 y Y 0 L Rdr x 4B+Yo Dy Y 0 Trget ri of interi B -L Rdr Rdition Pttern Xo Fig. 1 Geometry of stripmp SAR [4] 013, IJARCSSE All Rights Reserved Pge 805

2 My - 013, pp In Fig.1 is demonstrted two-dimensionl SAR imging geometry. Object is composed of rdr sctters tht re rryed in Crtesin type coordinte system spce x nd y. The x nd y xes denotes the rnge nd cross-rnge directions respectively. SAR pltform moves long y xis tht is clled the synthetic perture domin or the slow time domin. The rdr height determines the ground swth in the x domin. Length of ntenn SAR long the flight pth determines its footprint in the y domin tht shifts s the rdr is moved. This is due to the rdr motion in the y domin. The min lobe of the rdr rdition pttern in stripmp SAR system is not focused on specific trget region for ll the vilble slow time vlues. In this figure B mens the hlf bemwidth nd the trget rnge domin re X C X 0, X C X 0 is clled the rdr swth. L is the synthetic perture intervl [4]. Assume the pltform move in stright line with constnt forwrd velocity to trget, X C is t the middle point in the flight. The trnsmitted chirp signl is Vp long the zimuth direction nd minimum rnge tr str rect exp fctr jkrtr 1 T r Where T r, Kr, tr, t, f nd B0 denotes the pulse durtion, chirp rte, fst time, slow time, crrier frequency nd bndwidth respectively. The received signl is time-delyed nd ttenuted tht in following explin bout it. III. RANGE DOPPLER ALGORITHM The RDA processes the rw SAR dt to produce the SAR focused imge. The min steps bsic RDA re shown in Fig.. RDA is common lgorithm for low squint cses. We consider point trget reflection locted t re tht hs been pointed in fig. 1 nd follow lgorithm steps. Rw Dt of rdr Rnge Compression Azimuth FFT RCMC Azimuth IFFT De-rmping And FFT Compressed Dt Fig. Block digrm of the bsic RDA A. Rnge Compression A common technique in communictions is mtched filtering tht is min technique in RDA. A mtched filter is filter tht provides the mximum output SNR when the signl is corrupted by white Gussin noise. Mtched filtering is the correltion of reference signl with n unknown signl, which is the equivlent of convolution of n unknown signl with time reversed reference, to detect the presence of the reference signl in the unknown signl. The chirp signl often is used in the trnsmitted rdr signl construction due to there is more informtion embedded for detection. For to trnsform the rnge reference nd rw dt functions to frequency domin from time domin use fst Fourier trnsform (FFT) then in frequency domin every rw dt element is multiplied by the complex conjugte of the corresponding rnge reference function element. Finlly inverse Fourier trnsform (IFFT) is done. While mtch filtering implementtion we use smoothed window such s Kiser for lower spectrl lekge nd to reduce side ripples. The rnge-compressed signl of single point trget fter trnsform to bsedbnd cn be expressed by R t 4R t, sinc exp s tr t K KrTr KrTr tr w t tc j c K nd c re constnt reflectivity of the trget nd the speed of light. Bem pttern, wt t c, is pproximtely squre of sinc function. t c, referenced to the time of zero Doppler. The loction of trget fter rnge compression is founded. Prmeters of SAR simultion is denoted in tble. 1. TABLE I POINT TARGET SIMULATION PARAMETERS Prmeter Symbol vlue Illumintion time Pltform velocity 013, IJARCSSE All Rights Reserved Pge 806 T p V p 3 seconds 00 m/s Pulse Repetition Frequency PRF 300 Crrier frequency f c 5 GHz

3 My - 013, pp The point trget fter rnge compression is denoted in fig. 4. Fig.4 The point trget fter rnge compression B. Rnge cell migrtion correction nd Azimuth Compression Due to the hyperbolic trend with respect to zimuth time of the instntneous slnt rnge RCMC is needed tht it performed prior zimuth compression in the zimuth frequency nd rnge time domin. After Azimuth Fourier trnsform on (), the moving trget signl in rnge-doppler domin cn be obtined by pplying the principle of sttionry phse (POSP) [5] s S r t 0 3 s r r r k 4 c km Tpkm k m m vr The Doppler centroid, Doppler rte of the moving nd sttic trget t the sme slnt rnge is fd, z K rtr f ts f f d, sinc t fd f t f s s d t f A K T t R rect exp j p v p v v km nd ks respectively. We consider sttic trget so fd 0 nd km ks. The energy long the R0 R0 zimuth is shifted to counter cell migrtion, in the rnge direction [6]. We ssume squint is zero. The residul RCM cn be derived in RD domin tht it is negligible here. Fig.5 shows the compressed sttic trget fter RCMC. Fig. 5 the compressed trget fter RCMC 013, IJARCSSE All Rights Reserved Pge 807

4 My - 013, pp The zimuth reference function is like to the rnge reference function but due to slnt rnge vrition, frequency on the echo chnges in the zimuth direction. If pltform is moving towrd the trget, frequency increse nd while it moves wy, frequency decrese tht is referred to s Doppler effect, is kind of chirp. In Low Signl to noise, Doppler centroid hve to is estimted ccurtely. Using FFT zimuth reference function is converted to frequency domin then every frequency domin element is multiplied by the complex conjugte of the corresponding frequency domin reference element. An inverse fst Fourier trnsform (IFFT) then completes the compression exp j f t 4 R t 4R t str, t K KrTr sinckrtr tr wt exp j ts c After rnge cell migrtion correction the signl is compressed in zimuth domin tht imge generted from the two dimensionl simultion of single point trget is shown in Fig. 7 Fig. 7 Finl Processed SAR Imge of Single Point Trget IV. SIMULATION OF REAL DATA Now we implement the rnge Doppler lgorithm on spceborne rel dt gthered from RADARSAT1 to generte SAR imge. Dt set nd its prmeters hs been exploited from [6]. The RADARSAT1 stellite ws lunched by Cndin Spce Agency in It opertes t C-bnd, nd its min use is the dily mpping of Arctic ice. Its min innovtion is scnning mode of opertion, clled ScnSAR, whereby very wide swths re imged by scnning the rdr bem to different elevtion ngles within the synthetic perture time [6]. Fig. 8 shows the originl SAR imge of Vncouver, Cnd nd Fig. 9 shows the processed imge by RDA. As cn be detected from processed imge there is speckle noise in it. Speckle is multiplictive noise nd it is n inherent nd chrcteristic feture of rdr imges [7]. Multilook processing is performed to reduce the speckle noise tht it didn t pplied to RDA lgorithm here. V. CONCLUSIONS We described Rnge Doppler Algorithm simultion for sttic trget nd implement this lgorithm for rel dt gthered from RADARSAT1 tht the effectiveness of this lgorithm is illustrted by RADARSAT-1 fine bem dt. The Rnge Doppler Algorithm compred to other SAR processing lgorithm is less computtionl complexity nd it isn t timeconsuming. Fig. 8 Originl SAR imge 013, IJARCSSE All Rights Reserved Pge 808

5 My - 013, pp Fig. 9 Processed SAR imge References [1] A. Moreir, Y. Hung, Chirp scling lgorithm for processing SAR dt with high squint ngle nd motion error, SAR Dt Processing for Remote Sensing, 68 /SPIE Vol. 316, 1994 [] J. Holzner, R. Bmler, Burst-Mode nd ScnSAR Interferometry, IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 00 [3] I. G. Cumming nd J. R. Bennet., Digitl Processing of SEASAT SAR Dt, In IEEE 1979 Interntionl Conference on Acoustics, Speech nd Signl Processing, Wshington, D. c., April -4, 1979 [4] M. Soumekh, Synthetic Aperture Rdr Signl Processing with MATLAB Algorithms, nd ed. New York, United Sttes of Americ: Wiley, [5] J. Xu, Y. Zuo, B. Xi, X.-G. Xi, Y.-N, Peng, Y.-L. Wng, Ground Moving Trget Signl Anlysis in Complex Imge Domin for Multichnnel SAR IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 50, NO., FEBRUARY 01 [6] I.G. Cumming, nd F.H. Wong,. Digitl Processing of Synthetic Aperture Rdr Dt: Algorithms nd implementtion. Artech House, 660p., 005 [7]. Oliver nd S. Quegn. Understnding Synthetic Aperture Rdr Imges. Artech House, Norwood, MA, , IJARCSSE All Rights Reserved Pge 809