阅读说明：本技术 基于移变构型的双基前视sar波数域成像方法 (Bistatic forward-looking SAR wave number domain imaging method based on shift configuration ) 是由 李亚超 张廷豪 王宇 张磊 郭亮 黄平平 左磊 熊涛 于 2020-12-30 设计创作，主要内容包括：本发明公开了一种基于移变构型的双基前视SAR波数域成像方法,主要解决现有技术难以实现曲线轨迹状态下大场景目标精确聚焦,其运算量大的问题。其实现方案是：首先获得目标回波信号的二维频谱；对二维频谱进行距离压缩补偿校正；设计插值因子对距离压缩补偿校正后的信号进行插值；对插值后的信号进行方位空变滤波校正；对方位空变滤波校正后的信号进行方位因子重构校正聚焦得到二维成像图。本发明具有成像精确高,运算量小的优点,可用于移变构型双基前视SAR的高分辨成像。(The invention discloses a bistatic forward-looking SAR wave number domain imaging method based on a shift configuration, which mainly solves the problems that the prior art is difficult to realize the accurate focusing of a large-scene target in a curve track state and the calculation amount is large. The implementation scheme is as follows: firstly, obtaining a two-dimensional frequency spectrum of a target echo signal; performing distance compression compensation correction on the two-dimensional frequency spectrum; designing an interpolation factor to interpolate the signal after the distance compression compensation correction; carrying out azimuth space-variant filtering correction on the interpolated signal; and performing orientation factor reconstruction, correction and focusing on the signals subjected to the orientation space-variant filtering correction to obtain a two-dimensional imaging image. The invention has the advantages of high imaging accuracy and small operand, and can be used for high-resolution imaging of the shift-variant double-base forward-looking SAR.)

1. A bistatic forward-looking SAR wave number domain imaging method based on a shift configuration is characterized by comprising the following steps:

(1) obtaining a baseband echo signal, and performing linear walk correction and two-dimensional Fourier transform on the baseband echo signal to obtain a two-dimensional frequency spectrum S of a target echo signal_r,a(f_τ,f_η)，f_τRepresenting the distance frequency, f_ηRepresenting the azimuth frequency;

(2) according to a correction factor H_src(f_τ,f_η) For two-dimensional frequency spectrum S_r,a(f_τ,f_η) Distance compression compensation correction is carried out to obtain a signal S after distance compression compensation correction_cr,ca(f_τ,f_η)；

(3) Design of interpolation factor f_strExpressed as follows:

in the formula, E_2i,E_3i,E_4iI 1 and 2 are respectively the signals S after the distance compression compensation correction_cr,ca(f_τ,f_η) Second, third, and fourth order phase residue coefficients of (1) with respect to Δ R_resLinear coefficient of construction of, Δ R_res＝R₀-R_s+ΔR，ΔR_resRepresenting target point bistatic sum of slopesCompensated residual amount, R₀Representing the sum of the target bipolarity slopes at the initial moment, R_sRepresenting the sum of the dual base slant distances of the scene center reference point, ar representing the amount of residual distance warping, λ being the wavelength,denotes the wave number, f_cCarrier frequency, c is speed of light;

(4) using interpolation factor f_strCompensating the corrected signal S for distance compression_cr,ca(f_τ,f_η) Performing two-dimensional linear interpolation to obtain interpolated signal S_str,r,a(f_str,f_η)；

(5) For the interpolated signal S_str,r,a(f_str,f_η) Performing orientation space-variant filtering correction to obtain an orientation space-variant filtering-corrected signal s_s,r(t_τ,t_η) Wherein, t_τFor a fast time, t_ηIs a slow time;

(6) signal s corrected by space-variant filtering of the orientation_s,r(t_τ,t_η) And performing azimuth reconstruction, correction and focusing to obtain a two-dimensional imaging image.

2. The method according to claim 1, wherein the two-dimensional spectrum S of the target echo signal in (1)_r,a(f_τ,f_η) Expressed as follows:

where γ is the frequency modulation of the chirp signal, t_nIs the target focusing time, R₀Represents the sum of the two base skews of the target at the initial moment, mu₂,μ₃,μ₄Formed for f, respectively, of Taylor expansion coefficients of the target slope distance model_ηSecond, third and fourth order phase coefficients of (f)_ηIndicating the azimuth frequency.

3. The method of claim 1, wherein the correction factor H in (2)_src(f_τ,f_η) Expressed as follows:

wherein γ is the frequency modulation rate of the chirp signal, μ'₂,μ′₃,μ′₄Second, third and fourth phase correction coefficients R formed by Taylor expansion coefficients of the center reference point slope distance model respectively_sIs the dual base slant sum of the scene center reference point.

4. The method of claim 1, wherein (2) is performed on a two-dimensional spectrum S_r,a(f_τ,f_η) The distance compression compensation correction is carried out by converting the two-dimensional frequency spectrum S_r,a(f_τ,f_η) And a correction factor H_src(f_τ,f_η) Multiplying to obtain a signal S after distance compression compensation correction_cr,ca(f_τ,f_η)：

In the formula, t_nIs the target focusing time, Δ_jJ is 2,3,4, which indicates second, third, and fourth phase residue coefficients, respectively.

5. The method of claim 1, wherein the interpolation factor f is designed in (3)_strThe implementation is as follows:

(3a) a linear regression model was constructed, represented as follows:

in the formula,. DELTA.₂,Δ₃,Δ₄Compensating the corrected signal S for range compression_cr,ca(f_τ,f_η) Second, third and fourth order phase residue coefficients;

(3b) substituting a linear regression model into the distance compression compensation-corrected two-dimensional spectrum signal S_cr,ca(f_τ,f_η) Obtaining a reconstructed two-dimensional spectrum S_nr,na(f_τ,f_η) Is represented as follows:

in the formula, t_nIs the target's focus time;

(3c) for the reconstructed two-dimensional frequency spectrum signal S_nr,na(f_τ,f_η) Extracting the formula S_nr,na(f_τ,f_η) In the following expression form:

(3d) assuming an interpolation factor of f_strUsing a hypothetical interpolation factor f_strFor the two-dimensional spectrum signal S after extracting the formula_nr,na(f_τ,f_η) Interpolation is carried out, decoupling is finished, and a signal S after interpolation is obtained_tr,ta(f_τ,f_η) Comprises the following steps:

in the formula (I), the compound is shown in the specification,representing the wave number, f, of the electromagnetic radiation_cIs the carrier frequency;

(3e) by the signal S in pair (3c)_nr,na(f_τ,f_η) Signal S in (3d) and (3d)_tr,ta(f_τ,f_η) The contrast derivation of the expression is to obtain an interpolation factor f_strThe expression of (a) is:

6. the method of claim 1, wherein (4) utilizes an interpolation factor f_strCompensating the corrected signal S for distance compression_cr,ca(f_τ,f_η) Interpolation is carried out to obtain an interpolated signal S_str,r,a(f_str,f_η) Is represented as follows:

in the formula, t_nIs the time of focus of the object,representing the electromagnetic radiation wavenumber.

7. The method of claim 1, wherein the interpolated signal S is paired in (5)_str,r,a(f_str,f_η) Performing orientation space-variant filtering correction to obtain an orientation space-variant filtering-corrected signal s_s,r(t_τ,t_η)：

Where B is the transmission signal bandwidth and R_bfFor signal sampling position, f_pFor the focus position of the target point in the frequency domain,representing the wave number, h, of the electromagnetic radiation_j2(ΔR_res) J-2, 3,4 respectively denote the doppler frequency modulation term with f_pLinearly varying part with respect to slow time t_ηSecond, third, and fourth order coefficients.

8. The method of claim 1, wherein (6) the corrected signal s is filtered for space variant in orientation_s,r(t_τ,t_η) And performing azimuth reconstruction, correction and focusing to obtain a two-dimensional imaging image, wherein the following steps are realized:

(6a) the structural orientation reconstruction relation is expressed as follows:

wherein the content of the first and second substances,representing the wave number, h, of the electromagnetic radiation_j2(ΔR_res) J-2, 3,4 respectively denote the doppler frequency modulation term with f_pLinearly varying part with respect to slow time t_ηSecond, third and fourth order coefficients of (f)_pFor the focus position of the target point in the frequency domain, t_ηηIs an orientation reconstruction factor;

(6b) construction of phase term exp (j2 π f) using orientation reconstruction relations_pt_ηη)；

(6c) With the constructed phase term exp (j2 π f)_pt_ηη) Substitution of the azimuth space-variant filtered corrected signal s_s,r(t_τ,t_η) The phase term in the method is used for obtaining a signal s after the azimuth factor is reconstructed_s,a(t_τ,t_ηη) Is represented as follows:

s_s,a(t_τ,t_ηη)＝sinc(B(R_bf-ΔR_res))exp{j2πf_pt_ηη}

where B is the transmission signal bandwidth and R_bfSampling positions for the signals;

(6d) to the directionFactor reconstructed signal s_s,a(t_τ,t_ηη) And carrying out azimuth focusing to obtain a two-dimensional imaging image.

Technical Field

The invention belongs to the technical field of radars, and further relates to a bistatic forward-looking SAR imaging method which can be used for high-resolution imaging of bistatic SAR under a shift-variable geometric configuration.

Background

One of the important applications of the bistatic synthetic aperture radar SAR is bistatic forward-looking SAR, under the configuration, the positions of a transmitting and receiving platform are separated, a receiver forwards looks to receive a target echo, the two-dimensional spatial resolution of the receiver in a forward-looking area is orthogonal or approximately orthogonal, and forward-looking two-dimensional imaging can be realized. The bistatic forward-looking SAR has great potential advantages in the aspect of high-resolution forward-looking two-dimensional imaging, and can be used for forward-looking imaging under the background of aircraft blind landing and complex terrain, and the like. However, in the above-described situation, which has a velocity and an acceleration in a three-dimensional direction during flight, it is difficult to obtain an accurate echo two-dimensional spectrum. Therefore, various researchers at home and abroad propose various solutions for the problem of obtaining the bistatic SAR echo spectrum.

Hee-Sub Shin in the paper "Omega-K Algorithm for Airborne Spatial initiative Bistatic Spotlight SAR imaging" (IEEE trans. Geosci. remote Sens., Vol.47, No.1,238 + 250, Jan.2009.) the echo two-dimensional spectrum is Taylor expanded, then approximate conversion is carried out, Bistatic parallel isovelocity SAR is equivalent to single-base SAR, and finally the imaging processing is carried out by adopting the Omega-K Algorithm. The method is only suitable for a level flight mode without acceleration, and bistatic SAR forward-looking imaging under a curve track cannot be realized.

Baochang Liu et al adopts a Series inversion Method to obtain a two-dimensional spectrum of a Bistatic SAR in a published paper "Bistatic SAR Data Focusing Using an Omega-K Algorithm Based on Method of Series conversion" (IEEE trans. geosci. remote Sens., Vol.47, No.8,2899-2912, Aug.2009.) and performs approximate processing in a beam pointing direction to realize linearization of the spectrum in a distance direction, and finally adopts an interpolation Method to realize Bistatic side view SAR imaging with a large squint angle and a wide field. The method has the advantages of low imaging precision and large calculation amount, and is not suitable for the double-base model with the acceleration.

Disclosure of Invention

The invention aims to provide a bistatic forward-looking SAR wave number domain imaging method under a shift-variant geometric configuration, so that the wave number domain algorithm is combined with motion compensation, the constraint of acceleration is avoided, the bistatic SAR forward-looking two-dimensional high-resolution imaging is realized, and the operation amount is reduced.

In order to achieve the purpose, the technical scheme of the invention comprises the following steps:

(1) obtaining a baseband echo signal, and performing linear walk correction and two-dimensional Fourier transform on the baseband echo signal to obtain a two-dimensional frequency spectrum S of a target echo signal_r,a(f_τ,f_η)，f_τRepresenting the distance frequency, f_ηRepresenting the azimuth frequency;

(2) according to a correction factor H_src(f_τ,f_η) For two-dimensional frequency spectrum S_r,a(f_τ,f_η) Distance compression compensation correction is carried out to obtain a signal S after distance compression compensation correction_cr,ca(f_τ,f_η)；

(3) Design of interpolation factor f_strExpressed as follows:

in the formula, E_2i,E_3i,E_4iI 1 and 2 are respectively the signals S after the distance compression compensation correction_cr,ca(f_τ,f_η) Second, third, and fourth order phase residue coefficients of (1) with respect to Δ R_resLinear coefficient of construction of, Δ R_res＝R₀-R_s+ΔR，ΔR_resRepresenting the biradical slope distance of the target point and the compensated residual amount, R₀Representing the sum of the target bipolarity slopes at the initial moment, R_sRepresenting the sum of the dual base slant distances of the scene center reference point, ar representing the amount of residual distance warping, λ being the wavelength,denotes the wave number, f_cCarrier frequency, c is speed of light;

(4) using interpolation factor f_strCompensating the corrected signal S for distance compression_cr,ca(f_τ,f_η) Performing two-dimensional linear interpolation to obtain interpolated signal S_str,r,a(f_str,f_η)；

(5) For the interpolated signal S_str,r,a(f_str,f_η) Performing orientation space-variant filtering correction to obtain an orientation space-variant filtering-corrected signal s_s,r(t_τ,t_η) Wherein, t_τFor a fast time, t_ηIs a slow time;

(6) signal s corrected by space-variant filtering of the orientation_s,r(t_τ,t_η) And performing azimuth reconstruction, correction and focusing to obtain a two-dimensional imaging image.

Compared with the prior art, the invention has the following advantages:

firstly, the invention realizes the imaging processing of the shift-configuration bistatic forward-looking SAR under the sub-aperture condition by two-dimensional interpolation and azimuth factor reconstruction correction focusing.

Secondly, the invention corrects the orientation space-variant of the Doppler frequency modulation coefficient, so that the wave number domain algorithm can be combined with motion compensation, and wave number domain focusing is adopted according to the characteristic of sub-aperture imaging, thereby avoiding the zero filling of the orientation of the traditional wave number domain algorithm, and reducing the operation amount compared with the traditional wave number domain imaging method.

Thirdly, the invention breaks through the limitation of acceleration on wave number domain algorithm imaging, solves the two-dimensional space variation of the Doppler frequency modulation coefficient under the configuration of the shift variation, effectively improves the focusing effect of a scene target point, and has good scene applicability.

Drawings

FIG. 1 is a flow chart of an implementation of the present invention;

FIG. 2 is an imaging geometry and scene layout diagram of a bistatic forward-looking SAR;

FIG. 3 is a graph of the results of imaging a point target using the method of the present invention;

FIG. 4 is a sectional view of the pulse pressure at each point of the imaging process using the prior Omega-K algorithm;

FIG. 5 is a cross-sectional view of the pulse pressure at each point of the imaging process using a prior art series inversion method;

FIG. 6 is a cross-sectional view of the pulse pressure at each point of the imaging process using the method of the present invention;

FIG. 7 is a two-dimensional contour map of points imaged using a prior Omega-K algorithm;

FIG. 8 is a two-dimensional contour plot of points imaged using a prior art series inversion method;

figure 9 is a two-dimensional contour plot of points imaged using the method of the present invention.

Detailed Description

The embodiments and effects of the present invention will be described in further detail below with reference to the accompanying drawings.

Referring to fig. 1, the double-base forward-looking SAR wave number domain imaging method based on the shift configuration, provided by the invention, comprises the following implementation steps:

step 1, calculating and obtaining a two-dimensional frequency spectrum of an echo signal.

(1.1) the transmitted signal is a chirp signal, and for the convenience of description, the invention does not specifically analyze the change of the window function in the echo signal, so that the receiver obtains a baseband echo signal s after frequency mixing_ta(t_τ,t_η) Expressed as follows:

s_ta(t_τ,t_η)＝exp(jπγ(t_τ-τ(t_η))²)exp(-j2πf_cτ(t_η))

where γ is the frequency modulation of the chirp signal, t_τFor a fast time, t_ηIs a slow time, f_cFor the carrier frequency, τ (t)_η) Target bistatic echo time delay;

(1.2) for the base band echo signal s_ta(t_τ,t_η) Carrying out range-to-Fourier transform to obtain an echo signal S of a range frequency domain-an azimuth time domain_s(f_τ,t_η)：

S_s(f_τ,t_η)＝exp[-j2πKcτ(t_η)]

In the formula (I), the compound is shown in the specification,denotes the wave number, f_cCarrier frequency, c is speed of light;

(1.3) echo Signal S for distance frequency domain-azimuth time domain_s(f_τ,t_η) The linear walking correction is carried out by using the Taylor first-order expansion coefficient mu of the slope distance model of the scene central reference point_r1Construction of a Linear correction Filter H_l(f_τ,t_η) Expressed as follows:

H_l(f_τ,t_η)＝exp(2jπKμ_r1t_η)；

(1.4) for the base band echo signal s_ta(t_τ,t_η) Linear walk correction is performed by applying a baseband echo signal s_ta(t_τ,t_η) And a linear correction filter H_l(f_τ,t_η) Multiplying to obtain a signal S after the linear walk correction is finished_srl(f_τ,t_η) Comprises the following steps:

(1.5) correction of the Linear-Walking Signal S_srl(f_τ,t_η) Performing azimuth Fourier transform to obtain two-dimensional frequency spectrum S of echo signal_r,a(f_τ,f_η)：

Wherein f is_τRepresenting the distance frequency, f_ηRepresenting the azimuth frequency, t_nIs the target focusing time, R₀Represents the target dual-base slant sum at the initial moment,represents the wave number, mu₂,μ₃,μ₄Each of f being formed by Taylor expansion coefficients of the target slope distance model_ηSecond, third, and fourth order phase coefficients.

And 2, distance compression compensation correction.

(2.1) the following correction factor H is selected_src(f_τ,f_η)：

In the formula (f)_τDenotes distance frequency, μ'₂,μ′₃,μ′₄Second, third and fourth phase correction coefficients R formed by Taylor expansion coefficients of the center reference point slope distance model respectively_sThe sum of the dual-base slant distance of the scene center reference point is shown;

(2.2) Using the correction factor H_src(f_τ,f_η) For two-dimensional frequency spectrum S_r,a(f_τ,f_η) Correction for distance compression compensation, i.e. two-dimensional spectrum S_r,a(f_τ,f_η) And a correction factor H_src(f_τ,f_η) Multiplying to obtain a compensated and corrected signal S_cr,ca(f_τ,f_η)：

In the formula,. DELTA.R_res＝R₀-R_s+ΔR，ΔR_resRepresenting the biradical slope distance of the target point and the compensated residual amount, R₀Representing the sum of the target bibase pitches at the initial time, Δ R representing the residual distance warping amount, Δ_iAnd i is 2,3 and 4, which respectively represent second, third and fourth phase residue coefficients.

Step 3, designing an interpolation factor f_str。

The subsequent imaging processing requires compensation of the corrected signal S for distance compression_cr,ca(f_τ,f_η) Interpolation is carried out to complete two-dimensional decoupling, however, under the condition of a large imaging scene, the two-dimensional space variation of Doppler frequency modulation characteristics in echo signals cannot be ignored, and the echo signals need to be subjected to space variation correction in the azimuth direction. In order to solve the problems existing in the traditional interpolation imaging method, the interpolation factor f needs to be redesigned_str。

(3.1) obtaining the distance compression compensated corrected signal S_cr,ca(f_τ,f_η) Middle second, third and fourth phase residue coefficients delta₂,Δ₃,Δ₄The residue delta R after the double base slope distance process with the target point is compensated_resThe linear relationship of (1):

signal S corrected by distance compression compensation_cr,ca(f_τ,f_η) The phase residual coefficient in (1) is only related to the position between the transceiving platform of the bistatic SAR and the target point, and is related to delta R_resThere is no linear relation between them, and the interpolation factor f is designed_strThe phase residue coefficient and Δ R must be obtained_resThus constructing a linear regression model as follows:

in the formula, E_2i,E_3i,E_4iI 1 and 2 are two-dimensional frequency spectrums S after distance compression compensation correction_cr,ca(f_τ,f_η) Second, third, and fourth order phase residue coefficients of (1) with respect to Δ R_resLinear construction coefficients of (a);

(3.2) substituting the linear regression model into the two-dimensional frequency spectrum signal S after distance compression compensation correction_cr,ca(f_τ,f_η) Obtaining a reconstructed two-dimensional spectrum S_nr,na(f_τ,f_η)：

(3.3) pair of the reconstructed two-dimensional spectrum signal S in (3.2)_nr,na(f_τ,f_η) Extracting the formula S_nr,na(f_τ,f_η) In the following expression form:

(3.4) assume an interpolation factor of f_strUsing a hypothetical interpolation factor f_strFor the two-dimensional spectrum signal S after the formula is extracted from (3.3)_nr,na(f_τ,f_η) Interpolation is carried out, decoupling is finished, and a signal S after interpolation is obtained_tr,ta(f_τ,f_η) Comprises the following steps:

in the formula (I), the compound is shown in the specification,representing the wave number, f, of the electromagnetic radiation_cIs the carrier frequency;

(3.5) extracting a formula from the two-dimensional spectrum signal S in the (3.3)_nr,na(f_τ,f_η) And (3.4) interpolated signal S_tr,ta(f_τ,f_η) By contrast derivation of the expression(s) of (c), to obtain a factor f_strThe expression of (a) is:

and 4, performing two-dimensional linear interpolation.

Utilizing the interpolation factor f designed in the step 3_strCompensating the corrected signal S for distance compression_cr,ca(f_τ,f_η) Performing two-dimensional interpolation to obtain envelope termSeparate from the azimuth phase term to obtain the interpolated signal S_str,r,a(f_str,f_η) Comprises the following steps:

from the interpolated signal S_str,r,a(f_str,f_η) It can be seen that the phase term exp (-j2 π t)_nf_η) Indicating the position of the target point at the azimuthal focusing instant. Phase termRepresenting the focusing position of the target point in the distance direction, and performing inverse Fourier transform on the signal in the distance direction to realize focusing in the distance direction, wherein the residual phase item comprises azimuth frequency f_ηThe second order and high order terms of (2) represent compensated azimuth residual frequency modulation terms, and the terms imply azimuth space-variant of the Doppler frequency modulation terms.

And 5, correcting the orientation space-variant filter.

Step 4, completing the two-dimensional decoupling operation in the imaging processing process, and interpolating the signal S_str,r,a(f_str,f_η) The middle azimuth frequency modulation term coefficient is related to the position of the target point, namely, the Doppler frequency modulation two-dimensional space-variant characteristic exists. For the interpolated signal S_str,r,a(f_str,f_η) The space-variant correction is carried out, firstly, the signal S needs to be corrected_str,r,a(f_str,f_η) Performing two-dimensional inverse Fourier transform to obtain a two-dimensional time domain signal s_rt(t_τ,t_η) Then to a two-dimensional time-domain signal s_rt(t_τ,t_η) The azimuth space-variant phase in (1) is subjected to azimuth space-variant filtering correction, and the method is realized as follows:

(5.1) on the interpolated signal S_str,r,a(f_str,f_η) Performing two-dimensional inverse Fourier transform to obtain a two-dimensional time domain signal s_rt(t_τ,t_η)：

In the formula, t_ηIs a slow time, t_τFor fast time, B is the transmission signal bandwidth, R_bfFor signal sampling position, f_pFor the focus position of the target point in the frequency domain, h_j1(ΔR_res) J-2, 3,4 respectively indicate that the space-variant part of the doppler frequency modulation term with distance is related to the slow time t_ηSecond, third and fourth order coefficients of (h)_j2(ΔR_res) J-2, 3,4 respectively denote the doppler frequency modulation term with f_pLinearly varying part with respect to slow time t_ηSecond, third, and fourth order coefficients;

(5.2) for h_j1(ΔR_res) Partially correcting to construct an orientation space-variant filter H_d(t_η) Comprises the following steps:

(5.3) converting the two-dimensional time domain signal s_rt(t_τ,t_η) And azimuth space-variant filter H_d(t_η) Multiplying to obtain a signal s after azimuth space-variant filtering correction_s,r(t_τ,t_η)：

And 6, reconstructing azimuth, correcting and focusing.

Signal s corrected by space-variant filtering_s,r(t_τ,t_η) Middle h_j2(ΔR_res) J-2, 3,4 denotes the doppler frequency modulation term with f_pLinearly changing part, f_pRepresenting the frequency domain focus position of the target point, which can be corrected by the orientation factor reconstruction, is implemented as follows:

(6.1) constructing an orientation reconstruction relation formula as follows:

wherein the content of the first and second substances,representing the wave number, h, of the electromagnetic radiation_j2(ΔR_res) J-2, 3,4 respectively denote the doppler frequency modulation term with f_pLinearly varying part with respect to slow time t_ηSecond, third and fourth order coefficients of (t)_ηηIs an orientation reconstruction factor;

(6.2) constructing a phase term exp (j2 π f) using an orientation reconstruction relation_pt_ηη)；

(6.3) Filtering corrected Signal s for the space variant of the orientation_s,r(t_τ,t_η) Performing azimuth factor reconstruction, namely using the phase term exp (j2 pi f) constructed in (6.2)_pt_ηη) Substitution s_s,r(t_τ,t_η) The phase term in the method is used for obtaining a signal s after the azimuth factor is reconstructed_s,a(t_τ,t_ηη)：

s_s,a(t_τ,t_ηη)＝sinc(B(R_bf-ΔR_res))exp{j2πf_pt_ηη}

In the formula, t_τFor fast time, B is the transmission signal bandwidth, R_bfFor signal sampling position, f_pThe focusing position of the target point in the frequency domain;

(6.4) reconstruction of the orientation factor of the signal s_s,a(t_τ,t_ηη) And carrying out azimuth focusing to obtain a two-dimensional imaging image.

The effect of the present invention can be further illustrated by the following simulation experiments:

simulation conditions

The signal carrier frequency of the double-base forward-looking SAR radar system based on the shift configuration is set to be 17GHz, the pulse repetition frequency is set to be 10KHz, and both the receiver and the transmitter move along a curved track. The imaging geometry and scene layout of the bistatic forward-looking SAR are shown in fig. 2, wherein fig. 2(a) is a three-dimensional schematic diagram of the imaging geometry of the bistatic forward-looking SAR, fig. 2(b) is a schematic diagram of the scene layout, and as can be seen from fig. 2(b), the initial layout of the imaging domain is a 5 × 5 rectangular lattice.

(II) simulation content

Simulation 1, under the above conditions, the method of the invention is used for simulating scene imaging of the bistatic forward-looking SAR radar system based on the migration configuration, and an imaging result is obtained, as shown in fig. 3.

As can be seen from FIG. 3, the focusing effect of the point target is good, and meanwhile, the imaging result focused by the target point is a five-row five-column rectangular dot matrix, the horizontal direction represents the distance direction, the longitudinal direction represents the azimuth direction, and the imaging result is consistent with the initial point distribution diagram, so that the accuracy of the algorithm provided by the invention is proved.

Simulation 2, selecting edge points 1 and 3 and a center point 2 in an imaging scene, performing imaging simulation on the three points by Using an Omega-K Algorithm proposed by a document "Bistatic SAR Data Focusing Using an Omega-K Algorithm Based on Method of Series conversion", wherein an azimuth pulse pressure profile of each point obtained by simulation is shown in FIG. 4, wherein FIG. 4(a) shows an azimuth pulse pressure profile of the edge point 1, FIG. 4(b) shows an azimuth pulse pressure profile of the center point 2, and FIG. 4(c) shows an azimuth pulse pressure profile of the edge point 3.

As can be seen from fig. 4, the edge points 1 and 3 of the scene processed by the existing Omega-K algorithm cannot be focused precisely in the azimuth direction, because when the echo is processed by the existing Omega-K algorithm, the dual-base platform has an upward altitude speed and an acceleration in the three-dimensional direction, so that the linearization operation of the doppler frequency modulation term in the echo on the distance space-variant has a large phase error.

And 3, selecting edge points 1 and 3 and a center point 2 in an imaging scene, performing imaging simulation on the three points by using a grade inversion method provided by the literature 'missile-borne double-base forward-looking SAR extended imaging algorithm design', wherein an azimuth pulse pressure section diagram of each point obtained by simulation is shown in fig. 5, wherein fig. 5(a) shows the azimuth pulse pressure section diagram of the edge point 1, fig. 5(b) shows the azimuth pulse pressure section diagram of the center point 2, and fig. 5(c) shows the azimuth pulse pressure section diagram of the edge point 3.

As can be seen from fig. 5, the azimuth peak-to-peak side lobe ratio of the scene edge points 1 and 3 imaged by the conventional series inversion method is too large, so that a defocus phenomenon exists, which indicates that when the scene width is large, the residual doppler modulation azimuth space-variant term is not negligible.

And 4, selecting edge points 1 and 3 and a central point 2 in an imaging scene, performing imaging simulation on the three points by using the method of the invention, and obtaining an azimuth pulse pressure cross-sectional view of each point through simulation as shown in fig. 6, wherein fig. 6(a) shows the azimuth pulse pressure cross-sectional view of the edge point 1, fig. 6(b) shows the azimuth pulse pressure cross-sectional view of the central point 2, and fig. 6(c) shows the azimuth pulse pressure cross-sectional view of the edge point 3.

As can be seen from fig. 6, the distance space-variant characteristic and the azimuth space-variant item of the doppler frequency modulation item are considered in each target point subjected to the simulation processing by the algorithm provided by the present invention, the scene edge points 1 and 3 and the scene center point 2 both exhibit good focusing effects, the peak side lobes of each point are low, the main lobe and the first side lobe are clearly distinguished, and the focusing depth is good.

Simulation 5, selecting edge points 1 and 3 and a center point 2 in an imaging scene, and performing imaging simulation on the three points by Using an Omega-K algorithm proposed by the document "Bistatic SAR Data Focusing Using an Omega-K Algorithm based on Method of Series Reversion", wherein a two-dimensional contour map of each point is shown in FIG. 7, wherein FIG. 7(a) shows a two-dimensional contour map of the edge point 1; fig. 7(b) shows a two-dimensional contour diagram of the center point 2; fig. 7(c) shows a two-dimensional contour diagram of the edge point 3;

simulation 6, selecting edge points 1 and 3 and a center point 2 in an imaging scene, performing imaging simulation on the three points by using a grade inversion method provided by a literature 'missile-borne double-base forward-looking SAR extended imaging algorithm design', wherein a two-dimensional contour map of each point is shown in fig. 10, wherein fig. 8(a) shows the two-dimensional contour map of the edge point 1; fig. 8(b) shows a two-dimensional contour diagram of the center point 2; fig. 8(c) shows a two-dimensional contour diagram of the edge point 3;

as can be seen from fig. 7 and 8, the two-dimensional main lobes at the edge points 1 and 3 are distorted, which indicates that there is a residual amount for the distance migration correction at these two points, and there is significant coupling between the main lobe and the side lobe.

Simulation 7, selecting edge points 1 and 3 and a center point 2 in an imaging scene, performing imaging simulation on the three points by using the method of the invention, and obtaining a two-dimensional contour map of each point as shown in fig. 9, wherein fig. 9(a) shows the two-dimensional contour map of the edge point 1; fig. 9(b) shows a two-dimensional contour diagram of the center point 2; fig. 9(c) shows a two-dimensional contour diagram of the edge point 3;

as can be seen from fig. 9, both the scene edge points 1 and 3 and the scene center point 2 exhibit a good "cross" effect.

In conclusion, the method combines the wave number domain imaging algorithm and the motion compensation through two-dimensional interpolation and azimuth factor reconstruction correction focusing, realizes the accurate focusing of the double-base forward-looking SAR based on the shift configuration on the scene target point, and verifies the accuracy and the effectiveness of the method.