CN107544068A - A kind of image area synthetic wideband method based on frequency domain BP - Google Patents

A kind of image area synthetic wideband method based on frequency domain BP Download PDF

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CN107544068A
CN107544068A CN201710575477.6A CN201710575477A CN107544068A CN 107544068 A CN107544068 A CN 107544068A CN 201710575477 A CN201710575477 A CN 201710575477A CN 107544068 A CN107544068 A CN 107544068A
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distance
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张晓玲
贾淑培
胡克彬
韦顺军
师君
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University of Electronic Science and Technology of China
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Abstract

The invention discloses a kind of image area synthetic wideband method based on frequency domain BP.It is to be based on frequency domain BP imaging theories, Range compress first is carried out to the sub-band echo signal received, then by the echo spectrum rear orientation projection after Range compress to image space frequency domain, then the image spatial spectrum to being obtained after rear orientation projection carries out two-dimentional inverse Fourier transform and obtains subband spatial domain picture.Obtained sub-band images are entered with row distance to translate to spatial spectrum, finally the sub-band images after translation are entered with row distance and obtains synthetic wideband image to spatial spectrum is cumulative.It is an advantage of the invention that the more traditional synthetic wideband method performance of the image area synthetic wideband method based on frequency domain BP is more preferably, it can be good at solving phase discontinuous problem, and system and signal transacting difficulty need not be increased；Due to using frequency domain BP imaging algorithms, imaging efficiency is high, and can carry out spectrum truncation automatically according to systematic parameter when subband is overlapping.

Description

A kind of image area synthetic wideband method based on frequency domain BP
Technical field
The invention belongs to Radar Technology field, its more particularly to Step Frequency synthetic aperture radar (SAR) technical field of imaging.
Background technology
Due to the performance with the round-the-clock imaging of round-the-clock, synthetic aperture radar (SAR) is widely used in many necks The fields such as domain, such as military surveillance, environmental monitoring, disaster alarm, refer to document " I.G.Cumming, F.H.wong. synthetic aperture Radar --- algorithm is with realizing [M] Beijing：Electronic Industry Press, 2009 ".Because 2 dimension resolution ratio directly affect SAR knowledge Other ability, so improving the important directions that resolution ratio is always SAR researchs.In the case of other conditions identical, orientation Closely related to resolution ratio and synthetic aperture time, range resolution is related to transmitting LFM signal bandwidths.Therefore orientation point Resolution is generally improved by synthesizing large aperture；Range resolution is improved then to realize by increasing transmitted signal bandwidth.So And common SAR is by system instant bandwidth and sample rate due to being constrained, its signal bandwidth improves ability and is extremely restricted.
Step Frequency SAR (SFSAR) is an important technology for improving range resolution, and this technology can not improve Improve range resolution on the premise of system instant bandwidth and sample rate, refer to document " Deng, Y.K, H.F.Zheng, R.Wang, J.Feng, and Y.Liu.Internal Calibration for Stepped-frequency Chirp SAR Imaging.IEEE Geoscience and Remote Sensing Letters 8(6)：1105-1109.”.Step Frequency SAR By sequential transmitting and reception sub-band burst, the sub-band echo signal received is then synthesized into width by signal processing means Band signal, so as to reach the purpose for improving range resolution.For Step Frequency SAR, mainly there are three kinds of synthetic wideband sides at present Method：Time domain broadband synthesis, frequency domain synthetic wideband and image area synthetic wideband.Time domain broadband synthesis mainly passes through frequency displacement and time shift Time domain broadband signal is rebuild, it has preferable synthetic wideband ability, but the compensation of oblique distance error is extremely difficult and efficiency is non- It is often low.Frequency domain synthetic wideband be intended to rebuild echo signal broadband reflection spectrum, refer to document " A.J.Wikinson, R.T.Lord, M.R.Inggs.Stepped-frequency processing by reconstruction of target reflectivity spectrum[C].Proceeding of the 1998 South African Symposium on Communica-tions and Signal Processing, 1998,101-104 ".Frequency domain synthetic wideband closes than time domain broadband It is more succinct into step, but the design of matched filter and the compensation of oblique distance error are still highly difficult.Image area synthetic wideband is first to obtain Several sub-band images are obtained, then sub-band images are handled, obtain high-resolution synthetic wideband image, have scholar by this side Method is referred to as SCA (Subimage Coherent Accumulation) method.The essence of SAR imagings is recovered from echo-signal The scattered information of target, and target scattering information is unrelated with position of platform.So if first passing through imaging obtains subband figure Picture, then oblique distance error intangibly be compensate in imaging process, then sub-band images are carried out into synthetic wideband, i.e., It can not have to consider oblique distance error, therefore, be had a clear superiority for the more other two methods of image area synthetic wideband method.
In terms of imaging algorithm, rear orientation projection's (Back Projection, BP) imaging algorithm as a kind of time domain SAR into As algorithm, it, then will be corresponding to the round trip time delay between antenna phase center by calculating each scattering point in imaging region Time domain echo-signal carries out coherent accumulation, so as to recover the scattering coefficient information of each scattering point.Due to high with accuracy, Suitable for any scene of any configuration SAR imaging, imaging plane can be selected arbitrarily, result does not have any geometric distortion etc. excellent Point, BP algorithm are increasingly paid close attention to and applied in scientific research and engineering practice.Time domain BP algorithm is applied into image area In synthetic wideband method, high performance high-resolution composograph can be obtained, but simultaneously it should also be noted, that time domain BP algorithm Realize that orientation focuses on by the way of matching point by point, the time is very long, less efficient.Therefore need to be improved BP algorithm, Efficiency is improved while image quality is ensured.
The content of the invention
The present invention proposes a kind of figure for being based on frequency domain BP (Frequency-domain Back Projection, FDBP) Image field synthetic wideband method.This method is based on frequency domain BP imaging theories, first enters row distance pressure to the sub-band echo signal received Contracting, then by the echo spectrum rear orientation projection after Range compress to image space frequency domain, then the image to being obtained after rear orientation projection Spatial spectrum carries out two-dimentional inverse Fourier transform and obtains subband spatial domain picture.Enter row distance to obtained sub-band images to put down to spatial spectrum Move, i.e., travel frequency axle is so that each sub-band images distance is mutually aligned to frequency axis.Finally the sub-band images after translation are carried out Distance obtains synthetic wideband image to spatial spectrum is cumulative.When spatial frequency stepping-in amount is less than sub-band images distance to bandwidth, figure As will appear from graing lobe, and side lobe performance can deteriorate, and at this moment also need progress image spatial spectrum to block, in the broadband based on frequency domain BP In synthesis, it can realize that image spatial spectrum is blocked automatically by known parameters.
In order to facilitate description present disclosure, make following term definition first：
Define 1, signal and rise the method for sampling.
Increase the process of time-domain signal sample rate by frequency domain interpolation, interpolation only nulling interpolation here need to be with sparse letter The interpolation of number reconstructed analog signal is distinguished.
Define 2, standard synthetic aperture distance by radar compression method
Standard synthetic aperture distance by radar compression method refers to the transmission signal parameters using polarization sensitive synthetic aperture radar system, raw Into Range compress reference signal, and the mistake being filtered using matched filtering technique to the distance of synthetic aperture radar to signal Journey.Refer to document " radar imaging technology ", protect it is polished etc. write, Electronic Industry Press publishes.
Define 3, synthetic aperture radar slow time and fast time
The synthetic aperture radar slow time refers to the time that radar platform is flown over required for a synthetic aperture.Radar system with Certain repetition period transmitting receives pulse, therefore when the slow time can be expressed as a discretization using the repetition period as step-length Between variable, each of which discrete-time variable value is a slow moment.
The synthetic aperture radar fast time refers to that radar emission receives the time of a cycle of pulse.Because radar is received back to Ripple is sampled with sample rate, then the fast moment can be expressed as the time variable of a discretization, each discrete variable value For a fast moment.
Refer to document " synthetic aperture radar image-forming principle ", skin, which also rings, etc. writes, and publishing house of University of Electronic Science and Technology publishes.
Define 4, synthetic aperture radar projection imaging space
Synthetic aperture radar projection imaging space refers to the imaging space chosen when data of synthetic aperture radar is imaged, and closes Need echo data projecting to the imaging space into aperture radar imaging and be focused processing.
Define 5, the most short oblique distance of Step Frequency SAR radar systems
The most short oblique distance of Step Frequency SAR radar systems refers to that antenna is in length of synthetic aperture in Step Frequency SAR radar systems Between position when to scene center distance, in the present invention the most short oblique distance of Step Frequency SAR radar systems be designated as R0。
Define 6, standard synthetic aperture radar original echo emulation mode
Standard synthetic aperture radar original echo emulation mode refer to given radar system parameters, platform trajectory parameters and Observe under the Parameter Conditions needed for scenario parameters etc., obtain believing with SAR echoes based on synthetic aperture radar image-forming principles simulation The method of the original echoed signals of number characteristic, detailed content refer to document：" Step Frequency SAR echo signal is ground with system emulation Study carefully ", Zhang Jianqi, Harbin Institute of Technology's Master's thesis.
A kind of image area synthetic wideband method based on frequency domain BP provided by the invention, it includes following steps：
Step 1, initialization Step Frequency SAR imaging radar system parameters：
Step Frequency SAR imaging radar system parameters are initialized, including：The spread speed of light, it is designated as c, radar carrier wave fundamental wave Wavelength, it is designated as λ0, radar platform primary antenna transmitted signal bandwidth, it is designated as Br, radar transmitted pulse time width, it is designated as Tr, radar sampling Frequency, it is designated as Fs, radar pulse repetition frequency, PRF is designated as, radar beam width, is designated as Ba, base load frequency, it is designated as f0, frequency steps Input, Δ f is designated as, subband number, is designated as N, platform movement velocity vector is designated as V, and radar system distance is remembered to sampling number For Nr, radar system orientation sampling number, it is designated as Na, Step Frequency SAR radar system antenna initial positions, it is designated as Pt(0), field Scape centre coordinate, is designated as Pc, R0=| Pt(0)-Pc| it is most short oblique distance.Original time in slow time η and fast time t of n-th of subband Wave number evidence, is designated as Em(t, η, n), t=1,2 ..., Nr, η=1,2 ..., Na, n=1,2 ..., N, wherein t, η and n are natural number, NrTo initialize obtained Step Frequency SAR radar systems distance to sampling number, NaTo initialize obtained Step Frequency SAR radars For system attitude to sampling number, N is the sequential sub-band burst number for the Step Frequency SAR radar systems transmitting that initialization obtains；
Step 2, Range compress is carried out to Step Frequency SAR radar system antennas raw radar data：
Using standard synthetic aperture distance by radar compression method to the raw radar data E that is obtained in step 1m(t, η, n) Range compress is carried out, the echo data after compression is designated as Sr(t, η, n), t=1,2 ..., Nr, η=1,2 ..., Na, n=1, 2 ..., N.
Step 3, the echo data after compression is transformed to apart from frequency domain and carries out echo liter sampling
Using gauged distance to Fourier transformation method, the echo data S after the Range compress that step 2 is obtainedr(t, η, N) signal obtained after frequency domain, conversion is transformed to, is designated as Sf(k, η, n), k=1,2 ..., Nr, η=1,2 ..., Na, n= 1,2 ..., N, wherein k are the sampled point apart from frequency domain；
The method of sampling is risen using standard signal, the echo-signal S after frequency domain will be transformed tof(k, η, n) rise and adopted Sample, the signal after sampling is risen, is designated as Sff(k ', η, n), k '=1,2 ..., Nr× Times, η=1,2 ..., Na, n=1,2 ..., N, wherein Times represent to rise sampling multiple.
Step 4, using frequency domain BP imaging algorithms respectively to each subband carry out projection imaging processing：
Time domain BP algorithm is to image space domain by the time domain echo rear orientation projection after Range compress, and frequency domain BP algorithm is then By the echo spectrum rear orientation projection after Range compress to image space frequency domain.Therefore for frequency domain BP algorithm, projection imaging is empty Between be image space frequency domain.
Step 4.1 is using formula Δ kx=2 π/R0, Δ ky=2 π/Y0, the distance of spatial spectrum grid is calculated respectively to space Frequency interval Δ kxWith orientation spatial frequency space Δ ky, wherein X0And Y0Respectively distance is to the scene size with orientation；
Using formulaRespectively calculate spatial spectrum grid distance to space frequency Rate points NxWith orientation spatial frequency points Ny, then spatial spectrum grid share Nx×NyIndividual Frequency point, ρxAnd ρyRespectively distance To the image resolution ratio with orientation,Expression rounds up operation.
Step 4.2 remembers that the coordinate of any one Frequency point is (nx, ny, n), nx∈ { 0,1 ..., Nx, ny∈ 0,1 ..., Ny, n ∈ { 1,2 ..., N }；
Using formulaCalculate frequency (nx, ny, n) Index of the rear orientation projection in k domains, whereinIt is image distance to centre frequency, fc(n)=f0+(n-1)×Δf For the centre frequency of n-th subband,f1=fc(n)-Fs/ 2 be distance to first frequency cells, fΔ=Fs/Nr,Expression rounds up operation；
Step 4.3, using formulaCalculate Frequency point (nx, ny, n) corresponding to it is backward Projection value, wherein φ1=exp {-jkyY (η) },R0 is the most short oblique of step 1 definition Away from SffThe liter obtained for step 3 samples back echo signal Sff(k ', η, n), k '=1,2 ..., Nr× Times, η=1,2 ..., Na, n=1,2 ..., N；
Using formulaCalculate Frequency point (nx, ny, n) corresponding to image Space spectrum；
Step 4.4 is using step 4.2~step 4.3 methods described to spatial spectrum grid Nx×NyEach in individual Frequency point Frequency point performs same operation, obtains the image spatial spectrum H (n), n=1,2 ... of n-th of subband, N；
Step 4.5 carries out two-dimentional IFFT using standard two-dimensional inverse Fourier transform method to H (n), obtains spatial domain picture Im (n), n=1,2 ..., N.
Step 5, row distance is entered to spatial spectrum frequency displacement to obtained each sub-band images：
Using formulaIn the spatial domain picture Im (n) obtained to step 4 by Compensated apart from phase caused by variable, obtain Im_m (n)；
Using formula Im_s (n)=Im_m (n) × exp (jkshift) each sub-band images are entered with row distance to spatial spectrum frequency Move, obtain the spatial domain picture after frequency displacement and be designated as Im_s (n), n=1,2 ..., N, wherein frequency shift amountIt is empty Between number of frequency steps
Step 6, row distance is entered to each sub-band images after frequency displacement added up to spatial spectrum：
Spatial frequency stepping-in amount kΔLess than or equal to sub-band images distance to bandwidth Kr.Work as kΔ=KrWhen, sub-band images away from Descriscent spatial spectrum main band will not be overlapping, synthesizes the amplitude of spectrum and will not also be elevated, it means that composograph is not in Graing lobe.But work as kΔ＜ KrWhen, composograph will appear from graing lobe, and side lobe performance can deteriorate.Eliminate composograph graing lobe simultaneously A kind of effective and succinct method for improving side lobe performance is exactly to carry out image spatial spectrum to block, and removes overlapping spectra to synthesis performance Influence.It is as follows that sub-band images spatial spectrum blocks step automatically：
Step 6.1, the spatial domain picture Im_s (n) for being obtained step 5 to Fourier transformation using normal orientation transform to away from From wave-number domain, the image Im_w (n) after being converted；
Using formulaIt is corresponding to bandwidth and number of frequency steps to calculate image distance Spatial frequency points NbAnd NΔ, whereinExpression rounds up,Expression rounds downwards,For wave-number domain subband Distance is to bandwidth, BrRadar platform primary antenna transmitted signal bandwidth, Δ kxIt is distance to the spatial frequency sampling interval, For spatial frequency stepping-in amount.
Step 6.2, using formulaN-th of sub-band images distance is calculated to space Compose the Frequency point d of truncated positionc(n), NxCounted for distance to spatial frequency.
Step 6.3, interception spatial frequency point are located at n-th of sub-band images metric space spectrum at d, d ∈ Dn, for difference Sub-band images, DnFor
According to DnEnter row distance to different sub-band image Im_w (n) to intercept to spatial spectrum, n=1,2 ..., N, then will interception Image afterwards is added up to obtain Im_a, i.e., is composed in distance to the standard rectangular for obtaining broadening；
Step 7, distance are to inverse Fourier transform：
Distance is transformed to by spatial domain to data Im_a of the spatial spectrum after cumulative to inverse Fourier transform using gauged distance, most Step Frequency SAR image field width band compound imaging is realized eventually.
The innovative point of the present invention is to be combined Step Frequency SAR image area synthetic wideband method with frequency domain BP algorithm, is proposed Step Frequency SAR image field width band synthetic method based on frequency domain BP.This method first obtains several subbands using frequency domain BP algorithm Image, synthetic wideband then is carried out to obtained sub-band images in image area, so as to improve range resolution.
It is an advantage of the invention that the synthetic wideband method performance that the image area synthetic wideband method based on frequency domain BP is more traditional More preferably, it can be good at solving phase discontinuous problem, and system and signal transacting difficulty need not be increased；Due to using frequency domain BP imaging algorithms, imaging efficiency is high, and can carry out spectrum truncation automatically according to systematic parameter when subband is overlapping.
Brief description of the drawings
Fig. 1 is the schematic process flow diagram of method provided by the present invention；
Fig. 2 is system emulation parameter list used by the specific embodiment of the invention；
Embodiment
It is of the invention mainly to be verified that all steps, conclusion are all soft in MATLABR2014b using the method for emulation experiment Verified on part correct.Specific implementation step is as follows：
Step 1, initialization Step Frequency SAR imaging radar system parameters：
Step Frequency SAR imaging radar system parameters are initialized, including：Spread speed c=3 × 10 of light8M/s, radar are put down Platform antenna transmitted signal bandwidth Br=3 × 108Hz, radar transmitted pulse time width Tr=1 × 10-6S, radar sampling frequency Fs= 1.3×Br=1.3 × 3 × 108Hz=3.9 × 108Hz, radar pulse repetition frequency PRF=800HZ, base load frequency f0=1 × 1010Hz, number of frequency steps Δ f=280 × 106Hz, subband number N=4, platform movement velocity vector V=[0,100,0] m/s, Radar system distance is to sampling number Nr=1024, radar system orientation sampling number Na=2000, Step Frequency SAR radars system Unite antenna initial position Pt(0)=[0,0,4000] m, scene center coordinate Pc=[3000,0,0] m.Most short oblique distance R0=| Pt (0)-Pc|=5000m.N-th of subband is designated as E in slow time η and fast time t raw radar datam(t, η, n), t=1, 2 ..., 1024, η=1,2 ..., 2000, n=1,2,3,4, wherein t, η and n are natural number, NrTo initialize obtained Step Frequency SAR radar systems distance is to sampling number, NaTo initialize obtained Step Frequency SAR radar system orientation sampling numbers, N is Initialize the sequential sub-band burst number of obtained Step Frequency SAR radar systems transmitting；
Step 2, Range compress is carried out to Step Frequency SAR radar system antennas raw radar data：
Using standard synthetic aperture distance by radar compression method to the raw radar data E that is obtained in step 1m(t, η, n) Range compress is carried out, the echo data after compression is designated as Sr(t, η, n), t=1,2 ..., 1024, η=1,2 ..., 2000, n= 1,2,3,4.
Step 3, the echo data after compression is transformed to apart from frequency domain and carries out echo liter sampling
Using gauged distance to Fourier transformation, by the echo data S after Range compressr(t, η, n) transforms to distance frequency Domain, the signal after conversion, is designated as Sf(k, η, n), k=1,2 ..., 2014, η=1,2 ..., 2000, n=1,2,3,4, wherein k For apart from the sampled point of frequency domain；
The method of sampling is risen using standard signal, the echo-signal S after frequency domain will be transformed tof(k, η, n) carries out appropriate The liter sampling of multiple, rises the signal after sampling, is designated as Sff(k ', η, n), k '=1,2 ..., 1024 × Times, η=1,2 ..., 2014, n=1,2,3,4, wherein Times=8 represent to rise sampling multiple.
Step 4, using frequency domain BP imaging algorithms respectively to each subband carry out projection imaging processing：
Time domain BP algorithm is to image space domain by the time domain echo rear orientation projection after Range compress, and frequency domain BP algorithm is then By the echo spectrum rear orientation projection after Range compress to image space frequency domain.Therefore for frequency domain BP algorithm, projection imaging is empty Between be image space frequency domain.
Step 4.1, using formula Δ kx=2 π X0/ π==2/, Δ ky=2 π/Y0=2 π/30m=0.2094/m, respectively Spatial spectrum grid distance is calculated to the spatial frequency space with orientation, wherein X0=30 and Y0=30m be respectively distance to and side Position to scene size.Using formula
Count respectively Spatial spectrum grid distance is calculated to the spatial frequency points with orientation, then spatial spectrum grid shares 300 × 300=9 × 104Individual frequency Point, wherein ρx=0.1m and ρy=0.1m is respectively distance to the image resolution ratio with orientation,Expression rounds up operation.
The coordinate of any one Frequency point is (n in n-th step 4.2, note of subbandx, ny, n), nx∈ 0,1 ..., 300 }, ny∈ { 0,1 ..., 300 }, n ∈ { 1,2 ..., 4 }；
Using formulaCalculate frequency (nx, ny, n) Index of the rear orientation projection in k domains, whereinIt is image distance to centre frequency, fc(n)=f0+(n-1)×Δf For the centre frequency of n-th subband,f1=fc(n)-Fs/ 2 be distance to first frequency cells, fΔ=Fs/Nr,Expression rounds up operation；
Step 4.3, using formulaCalculate Frequency point (nx, ny, n) corresponding to it is backward Projection value, wherein φ1=exp {-jkyY (η) },R0=5000m is that step 1 defines Most short oblique distance, SffThe liter obtained for step 3 samples back echo signal Sff(k ', η, n), k '=1,2 ..., 1024 × 8, η= 1,2 ..., 2014, n=1,2,3,4；
Using formulaCalculate Frequency point (nx, ny, n) corresponding to image Space spectrum；
Step 4.4, using step 4.2~step 4.3 methods described to spatial spectrum grid 9 × 104It is each in individual Frequency point Individual Frequency point performs same operation, obtains the image spatial spectrum H (n), n=1,2 of n-th of subband, and 3,4；
Step 4.5, two-dimentional IFFT carried out to H (n) using standard two-dimensional inverse Fourier transform method, obtain spatial domain picture Im (n), n=1,2,3,4.
Step 5, row distance is entered to spatial spectrum frequency displacement to obtained each sub-band images：
Using formulaThe phase as caused by apart from variable in image Im (n) is compensated, Obtain Im_m (n).
Using formula Im_s (n)=Im_m (n) × exp (jkshift) each sub-band images are entered with row distance to spatial spectrum frequency Move, obtain the spatial domain picture after frequency displacement and be designated as Im_s (n), n=1,2, wherein 3,4, frequency shift amountSpatial frequency stepping-in amount
Step 6, row distance is entered to each sub-band images after frequency displacement added up to spatial spectrum：
Spatial frequency stepping-in amount kΔLess than or equal to sub-band images distance to bandwidth Kr.Work as kΔ=KrWhen, sub-band images away from Descriscent spatial spectrum main band will not be overlapping, synthesizes the amplitude of spectrum and will not also be elevated, it means that composograph is not in Graing lobe.But work as kΔ＜ KrWhen, composograph will appear from graing lobe, and side lobe performance can deteriorate.Eliminate composograph graing lobe simultaneously A kind of effective and succinct method for improving side lobe performance is exactly to carry out image spatial spectrum to block, and removes overlapping spectra to synthesis performance Influence.It is as follows that sub-band images spatial spectrum blocks step automatically：
Step 6.1, using normal orientation spatial domain picture Im_s (n) is transformed to apart from wave-number domain to Fourier transformation, become Image after changing is designated as Im_w (n).Using formulaCalculate image distance to bandwidth and Spatial frequency points N corresponding to number of frequency stepsbAnd NΔ, whereinExpression rounds up,Expression rounds downwards,It is wave-number domain subband distance to bandwidth, Br=3 × 108Hz radar platforms primary antenna transmitting letter Number bandwidth, Δ kx=0.2094/m be distance to the spatial frequency sampling interval,For space frequency Rate stepping-in amount.
Step 6.2, using formulaN-th of sub-band images distance is calculated to space Compose the Frequency point d of truncated positionc(n), Nx=300 be that distance is counted to spatial frequency.
Step 6.3, interception spatial frequency point are located at d, d ∈ DnN-th of sub-band images metric space spectrum at place, for difference Sub-band images, DnFor
According to DnTo different sub-band image Im_w (n), n=1,2,3,4 enter row distance intercepts to spatial spectrum, then by after interception Image added up to obtain Im_a, i.e., composed in distance to the standard rectangular for obtaining broadening
Step 7, distance are to inverse Fourier transform：
Distance is transformed to by spatial domain to data Im_a of the spatial spectrum after cumulative to inverse Fourier transform using gauged distance, most Step Frequency SAR image field width band compound imaging is realized eventually.
By embodiment it can be seen that the more traditional synthetic wideband method energy of the image area synthetic wideband method based on frequency domain BP It is enough to solve phase discontinuous problem well, and system and signal transacting difficulty need not be increased；Due to being imaged using frequency domain BP Algorithm, imaging efficiency is high, and can carry out spectrum truncation automatically according to systematic parameter when subband is overlapping.

Claims (1)

  1. A kind of 1. image area synthetic wideband method based on frequency domain BP, it is characterized in that it includes following steps：
    Step 1, initialization Step Frequency SAR imaging radar system parameters：
    Step Frequency SAR imaging radar system parameters are initialized, including：The spread speed of light, is designated as c, radar carrier wave fundamental wavelength, It is designated as λ0, radar platform primary antenna transmitted signal bandwidth, it is designated as Br, radar transmitted pulse time width, it is designated as Tr, radar sampling frequency, It is designated as Fs, radar pulse repetition frequency, PRF is designated as, radar beam width, is designated as Ba, base load frequency, it is designated as f0, number of frequency steps, Δ f is designated as, subband number, is designated as N, platform movement velocity vector is designated as V, and radar system distance is designated as N to sampling numberr, Radar system orientation sampling number, is designated as Na, Step Frequency SAR radar system antenna initial positions, it is designated as Pt(0), scene center Coordinate, it is designated as Pc, R0=| Pt(0)-Pc| it is most short oblique distance；N-th of subband is in slow time η and fast time t original echo number According to being designated as Em(t, η, n), t=1,2 ..., Nr, η=1,2 ..., Na, n=1,2 ..., N, wherein t, η and n are natural number, NrFor Obtained Step Frequency SAR radar systems distance is initialized to sampling number, NaTo initialize obtained Step Frequency SAR radar systems Orientation sampling number, N are the sequential sub-band burst number for the Step Frequency SAR radar systems transmitting that initialization obtains；
    Step 2, Range compress is carried out to Step Frequency SAR radar system antennas raw radar data：
    Using standard synthetic aperture distance by radar compression method to the raw radar data E that is obtained in step 1m(t, η, n) enters line-spacing Tripping contracts, and the echo data after compression is designated as Sr(t, η, n), t=1,2 ..., Nr, η=1,2 ..., Na, n=1,2 ..., N；
    Step 3, the echo data after compression is transformed to apart from frequency domain and carries out echo liter sampling
    Using gauged distance to Fourier transformation method, the echo data S after the Range compress that step 2 is obtainedr(t, η, n) becomes The signal obtained after frequency domain, conversion is changed to, is designated as Sf(k, η, n), k=1,2 ..., Nr, η=1,2 ..., Na, n=1, 2 ..., N, wherein k are the sampled point apart from frequency domain；
    The method of sampling is risen using standard signal, the echo-signal S after frequency domain will be transformed tof(k, η, n) carries out a liter sampling, rises Signal after sampling, is designated as Sff(k', η, n), k'=1,2 ..., Nr× Times, η=1,2 ..., Na, n=1,2 ..., N, its Middle Times represents to rise sampling multiple；
    Step 4, using frequency domain BP imaging algorithms respectively to each subband carry out projection imaging processing：
    For frequency domain BP algorithm, projection imaging space is image space frequency domain；
    Step 4.1 is using formula Δ kx=2 π/R0, Δ ky=2 π/Y0, the distance of spatial spectrum grid is calculated respectively between spatial frequency Every Δ kxWith orientation spatial frequency space Δ ky, wherein X0And Y0Respectively distance is to the scene size with orientation；
    Using formulaThe distance of spatial spectrum grid is calculated respectively to spatial frequency point Number NxWith orientation spatial frequency points Ny, then spatial spectrum grid share Nx×NyIndividual Frequency point, ρxAnd ρyRespectively distance to The image resolution ratio of orientation,Expression rounds up operation；
    Step 4.2 remembers that the coordinate of any one Frequency point is (nx,ny,n),nx∈{0,1,...,Nx},ny∈{0,1,...,Ny}, n∈{1,2,…,N}；
    Using formulaCalculate frequency (nx,ny, n) and it is backward The index being projected in k domains, whereinIt is image distance to centre frequency, fc(n)=f0+ (n-1) × Δ f is The centre frequency of n-th of subband,f1=fc(n)-Fs/ 2 be distance to first frequency cells,fΔ =Fs/Nr,Expression rounds up operation；
    Step 4.3, using formulaCalculate Frequency point (nx,ny, n) corresponding to rear orientation projection Value, wherein φ1=exp {-jkyY (η) },R0 is the most short oblique distance that step 1 defines, SffThe liter obtained for step 3 samples back echo signal Sff(k', η, n), k'=1,2 ..., Nr× Times, η=1,2 ..., Na, n =1,2 ..., N；
    Using formulaCalculate Frequency point (nx,ny, n) corresponding to image space Spectrum；
    Step 4.4 is using step 4.2~step 4.3 methods described to spatial spectrum grid Nx×NyEach frequency in individual Frequency point Point performs same operation, obtains the image spatial spectrum H (n) of n-th of subband, n=1,2 ..., N；
    Step 4.5 carries out two-dimentional IFFT using standard two-dimensional inverse Fourier transform method to H (n), obtains spatial domain picture Im (n), n =1,2 ..., N；
    Step 5, row distance is entered to spatial spectrum frequency displacement to obtained each sub-band images：
    Using formulaBy distance in the spatial domain picture Im (n) obtained to step 4 Phase compensates caused by variable, obtains Im_m (n)；
    Using formula Im_s (n)=Im_m (n) × exp (jkshift) each sub-band images are entered with row distance to spatial spectrum frequency displacement, obtain Spatial domain picture after to frequency displacement is designated as Im_s (n), n=1,2 ..., N, wherein frequency shift amountSpatial frequency Stepping-in amount
    Step 6, row distance is entered to each sub-band images after frequency displacement added up to spatial spectrum：
    Spatial frequency stepping-in amount kΔLess than or equal to sub-band images distance to bandwidth Kr；Work as kΔ=KrWhen, sub-band images distance to Spatial spectrum main band will not be overlapping, synthesizes the amplitude of spectrum and will not also be elevated, it means that composograph is not in graing lobe； But work as kΔ＜ KrWhen, composograph will appear from graing lobe, and side lobe performance can deteriorate；Eliminate composograph graing lobe and improve side A kind of effective and succinct method of valve performance is exactly to carry out image spatial spectrum to block, and removes shadow of the overlapping spectra to synthesis performance Ring；It is as follows that sub-band images spatial spectrum blocks step automatically：
    Step 6.1, the spatial domain picture Im_s (n) for being obtained step 5 to Fourier transformation using normal orientation are transformed to apart from ripple Number field, the image Im_w (n) after being converted；
    Using formulaCalculate empty corresponding to image distance to bandwidth and number of frequency steps Between frequency points NbAnd NΔ, whereinExpression rounds up,Expression rounds downwards,For wave-number domain subband distance To bandwidth, BrRadar platform primary antenna transmitted signal bandwidth, Δ kxIt is distance to the spatial frequency sampling interval,For sky Between number of frequency steps；
    Step 6.2, using formulaN-th of sub-band images distance is calculated to cut to spatial spectrum The Frequency point d at disconnected placec(n), NxCounted for distance to spatial frequency；
    Step 6.3, interception spatial frequency point are located at n-th of sub-band images metric space spectrum at d, d ∈ Dn, for different sons Band image, DnFor
    <mrow> <msub> <mi>D</mi> <mi>n</mi> </msub> <mo>=</mo> <mfenced open = "{" close = ""> <mtable> <mtr> <mtd> <mn>1</mn> <mo>&amp;le;</mo> <mi>d</mi> <mo>&amp;le;</mo> <msub> <mi>d</mi> <mi>c</mi> </msub> <mo>(</mo> <mi>n</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> <mo>-</mo> <mn>1</mn> <mo>,</mo> <mi>n</mi> <mo>=</mo> <mn>1</mn> </mtd> </mtr> <mtr> <mtd> <msub> <mi>d</mi> <mi>c</mi> </msub> <mo>(</mo> <mi>n</mi> <mo>-</mo> <mn>2</mn> <mo>)</mo> <mo>&amp;le;</mo> <mi>d</mi> <mo>&amp;le;</mo> <msub> <mi>N</mi> <mi>x</mi> </msub> <mo>,</mo> <mi>n</mi> <mo>=</mo> <mn>4</mn> </mtd> </mtr> <mtr> <mtd> <msub> <mi>d</mi> <mi>c</mi> </msub> <mo>(</mo> <mi>n</mi> <mo>-</mo> <mn>2</mn> <mo>)</mo> <mo>&amp;le;</mo> <mi>d</mi> <mo>&amp;le;</mo> <msub> <mi>d</mi> <mi>c</mi> </msub> <mo>(</mo> <mi>n</mi> <mo>-</mo> <mn>1</mn> <mo>)</mo> <mo>-</mo> <mn>1</mn> <mo>,</mo> <mi>n</mi> <mo>=</mo> <mi>o</mi> <mi>t</mi> <mi>h</mi> <mi>e</mi> <mi>r</mi> <mi>s</mi> </mtd> </mtr> </mtable> </mfenced> </mrow>
    According to DnEnter row distance to different sub-band image Im_w (n) to intercept to spatial spectrum, n=1,2 ..., N, then by the figure after interception As being added up to obtain Im_a, i.e., composed in distance to the standard rectangular for obtaining broadening；
    Step 7, distance are to inverse Fourier transform：
    Distance is transformed to by spatial domain to data Im_a of the spatial spectrum after cumulative to inverse Fourier transform using gauged distance, it is final real Existing Step Frequency SAR image field width band compound imaging.
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