1、 2lIntroduction to the SA-FBSAR and its nonlinear RMC l Nonlinear RCM compensation methodl Simulation resultslConclusions and further workSpaceborne/Airborne Forward-Looking Bistatic SAR(SA-FBSAR)l Platforms:Transmitter and receiver of SA-FBSAR are low earth orbit(LEO)satellite and aircraft,respecti
2、vely.l Working Modes:Transmitter antenna works in side-looking or squint-looking mode;receiver antenna in forward-looking mode.l Target imaging scene:Target scene is along the receivers forward-looking directiontransmitterreceiverImaging scene Diversity of target information High immunity to attacks
3、 Low cost Wide coverage,high SNR Platform flexibility Power saving wide band repeated observation attractive potential for aircraft landing and navigationIn Nov.2009,FGAN(German Aerospace Center)launched the first experiment to test the feasibility of SA-FBSAR.Fig.1 Imaging result of the first SA-FB
4、SAR feasibility experiment in 2009Satellite height：500-800kmAircraft height：1-5kmSatellite velocity：7.4-7.6km/sAircraft velocity：100m/sSatellite:side-lookingAircraft:forward-lookingRange cell migration(RCM)features are:Vary with the targets range and azimuth locationexhibits significant nonlinearity
5、 with targets range locationSevere distortion and nonlinear misregistration will occur,if such RCM is not properly compensatedFig2.Imaging result of point targets(a)original point scatterers(b)without RCM compensation9xy(a)original area target(b)Without RCMCFig3.Imaging result of area targets Purpos
6、e:find a nonlinear two-dimensional RCM compensation method for SA-FBSAR in frequency domain Main idea:1.Set up SA-FBSAR response spectrum model2.Deduce nonlinear RCM analytic formula3.Propose SA-FBSAR nonlinear RCM compensation methodFig.4 SA-FBSAR system geometry00,:denote transmitter and receiver
7、platforms,respectively:reference point scatterer located at 0,0:non-reference point scatterer located at,0,:velocity of platforms,:range and SPS PAyAx yv vr T00 00 00000 00 0000azimuth time distance of A from ,:closest range from platforms to ,:closest range from platforms to ,:azimuth time when is
8、closest to platforms,:azimuth tSPSPSPSPArrAr rAttAttime when is closest to platforms ,:the depression angles of platforms antennaSPA 12220002200000002.Receiver closest range:Due to its,targets along range direction are.Sincsinforward-lookinge sin modesymmet,we have .So trically situated2he varsinSPP
9、PSPPPSrrrrrrrrr0ianceof receivers closest range on is.not linear but quadricPrr2200 022S0 01.Transmitter closeset range:Transmitter operates in,and it is asymmetrical with targets along range direction,the condito ctgside-looking modectnholds.the g2vari n aSSSSrrrrrrrr00 0ce of the is about proporti
10、onal with target transmis range tters closest aposition,i.e.pproachlinearly.SSrrr.0The,which is directly affected by,is also .-variance of the range history in SA-FBSARnonlinearly variant with range locationPrr 2The SA-FBSAR system response after range compression is,;,expexp (1),;,2,;,where is rang
11、e freqency,is Doppler frequency,is the range of thedddddf fr THf fjdrdTjf fr Tf fr TffR t 0220 00 02220 SA-FBSAR system about scatterer 2,;,;,;,;,bbbddt tdt tdt tdSbdSdSSAfftR tf tf fr Ttcttf fr Tf fr Tttfrrtf fr TtTfffvcvAfter multiplying with conjugate of reference scatterers spectrum,we get:;,exp
12、2 (2);where RDdADdADdRDdRDffrTHf fjdrdTTffr2212120112222220 00 00 00 0022320;,;(3)1,2sin11,1,2sindRDdRDdRDdRDdRDdSRDdRDdRDdSSPfdPSPPPSRDdADADPZSPfdPZSSfrfrfrfrfrfrfvFfffvcFcrfattvavrrffc rcrfrvfFvc 222220 00 00 00 02220 00 00 0,ddSPfdPPSPSSPZPPSPSPffrrfrvttvvFrrvttavv22In(2)(3),due to the forward-lo
13、oking mode,the coefficients of range-dependent terms and are significant comparing with the linear terms.For example,in the SA-FBSAR syquadstem of simruicRDRDlation systemwhen 300 ,the ratio between the quadric term and linear term is almost 0.1.SA-FBSAR,RCM not only depends on targets range locatio
14、n(RD-RCM)and azimuth location(AD-RCM);but also varmries with the range location nonlinearly.The nonlinearity in RD-RCM is not just slight deviation from the linear part as the monostatic spaceborne side-looking SAR;it exhibits evident nonlinear deviation in RCM trajectory.Fig.5 flow chart of nonline
15、ar RCM compensation method for SA-FBSAR RDRAADTr0;RARDdfdfr0;RDdRDfddfrf,ParametersParametersTransmitterReceiverHeight(km)5143 velocity(m/s)7600100azimuth beam width(degree)0.332.9maximum steering angle(degree)0.7515depression angle(degree)3768beam velocity(m/s)2100700integration duration(s)0.43puls
16、e width(s)2central frequency of transmitting signal(GHz)9.65bandwidth of transmitting signal(MHz)60pulse repetition frequency(Hz)2500Point scatterers(a)original point scatterers(b)without RCM compensation(d)with the proposed method(c)with RCMC Method in Ref1Ref1:X.Qiu,D.Hu and C.Ding,IEEE Geosci.Rem
17、ote Sens.Lett.,4,735-739,2008.Fig.6 Imaging results of 15 point scatters19Point scatterers(a)error in range position(b)error in azimuth position area targetxyFig.7 Imaging results of area target(a)original area target(b)Without RCMC(c)With the proposed RCM compensation2116.416.416.416.816.816.817.21
18、7.217.217.617.617.618181818.418.418.418.818.818.819.219.219.219.619.619.620202020.420.420.420.820.820.821.221.221.221.621.621.6222222x/my/m-500-400-300-200-1000100200300400500-100-50050100Fig.8 two-dimensional resolution performance x/my/mAi=16.20m2A=16.34m2500100rax/my/mAi=18.72m2A=19.55m200ra(a)Co
19、ntour of ideal resolution cells area(unit:m2)(b)target located at(500,100)(c)target located at(0,0)From the above simulation results,we could find that:Uncompensated RCM could deteriorate imaging result severely,cause nonlinear distortionRCM compensation method designed for other FBSAR system could
20、not compensate the nonlinear RCM,thus could not be applied to SA-FBSAR.The proposed RCM compensation method could effectively compensate the nonlinear RCM in SA-FBSAR,and all targets are arranged in their originally correct positions.RCM in SA-FBSAR not only depends on the targets two-dimensional sp
21、ace location,but also varies with its range location nonlinearly.If not properly corrected,RCM would cause nonlinear distortion in the image and greatly degrade the imaging quality.We propose a two-dimensional nonlinear RCMC method for SA-FBSAR.The validity of the proposed method is verified.Further improvement on resolution performance is under research24Thank you
