A SAR polar coordinate imaging method and device based on level flight equivalence

Abstract

The application discloses a SAR polar coordinate imaging method and device based on flat flight equivalence, and the method comprises the following steps: constructing a flat flight equivalence slant range model of a large oblique SAR to obtain a baseband echo signal formed by target area scattering; performing distance direction matched filter processing on the baseband echo signal to obtain a signal after range pulse compression; sequentially performing Dechirp processing, two-dimensional resampling processing and IFFT transformation on the signal after range pulse compression to obtain a SAR polar coordinate primary imaging result; and performing geometric correction on the primary imaging result based on reverse projection to obtain an imaging result without geometric deformation and with good focusing. The SAR polar coordinate imaging method based on flat flight equivalence provided by the embodiment avoids the problem of azimuth translation invariance caused by diving through slant conversion, solves the problem that the traditional PFA is not applicable to diving SAR, and successfully extends the PFA imaging algorithm without deformation to diving SAR by correcting the geometric deformation problem caused by diving.

Description

Technical Field

[0001] This invention belongs to the field of radar signal processing technology, specifically relating to a SAR polar coordinate imaging method and device based on level flight equivalence. Background Technology

[0002] Compared to optical systems, Synthetic Aperture Radar (SAR) offers all-weather, all-day operation, significantly enhancing battlefield awareness and holding significant value in the military field. It is now applied to various mobile platforms. However, due to the complexity of the mobile platform's trajectory, SAR platforms often need to operate in dive mode. In this mode, the vertical altitude between the radar platform and the ground target changes over time, causing the SAR echo signal to no longer satisfy azimuth translation invariance, thus preventing traditional level-flight algorithms from focusing. Furthermore, imaging under large squint conditions results in severe coupling between range and azimuth, leading to significant image spatial variability and severe geometric distortion in traditional imaging methods.

[0003] With the increasing demands for imaging accuracy in radar, spotlight imaging, as a high-resolution imaging mode, has received widespread attention and research. This mode, through beam pointing control, directs the beam coverage area towards a fixed target scene, thereby significantly improving the synthetic aperture accumulation time and overcoming the resolution limitations of stripe imaging. The PFA algorithm (polar coordinate algorithm), as one of the classic algorithms for high-resolution imaging in spotlight imaging, has advantages such as phase compensation in the time domain and automatic correction of linear motion, attracting considerable attention from researchers.

[0004] Currently, level-fly PFA primarily employs traditional geometric models for imaging. In these models, dive SAR imaging reflects altitude changes in the slant range history. Due to the unique characteristics of the dive slant range history, interpolation factors in traditional PFA algorithms are unsuitable; direct processing leads to severe defocusing, and the dive trajectory introduces azimuth translation invariance issues. Therefore, traditional PFA algorithms are not applicable to dive trajectories. Consequently, combining PFA algorithms with dive platform spotting SAR remains a pressing issue that needs to be addressed. Summary of the Invention

[0005] To address the aforementioned problems in the existing technology, this invention provides a SAR polar coordinate imaging method and apparatus based on level flight equivalence. The technical problem to be solved by this invention is achieved through the following technical solution:

[0006] A SAR polar coordinate imaging method based on level flight equivalence includes:

[0007] Construct a level flight equivalent slant range model for large slant-look SAR to obtain the baseband echo signal formed by scattering from the target area;

[0008] The baseband echo signal is subjected to range-direction matched filtering to obtain the range pulse compressed signal;

[0009] The signal after distance pulse compression is processed by Decirp to obtain the Decirp-compensated signal;

[0010] The Decirp-compensated signal is subjected to two-dimensional resampling to obtain a resampled two-dimensional wavenumber domain signal.

[0011] Perform an IFFT transform on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result;

[0012] The initial imaging results are geometrically corrected based on back projection to obtain imaging results with no geometric deformation and good focus.

[0013] In one embodiment of the present invention, the baseband echo signal is represented as:

[0014]

[0015] Where k is the linear frequency modulation frequency, t r For distance to fast time, t a For azimuth, time slows down, f c R(t) is the center frequency. a Instantaneous slant distance, w r (t r ), w a (t a ) are the time-domain expressions for the envelopes of the distance and azimuth window functions, respectively.

[0016] In one embodiment of the present invention, range-direction matched filtering is performed on the baseband echo signal to obtain a range pulse-compressed signal, including:

[0017] Perform an FFT transform on the baseband echo signal to obtain a range frequency domain signal;

[0018] A matched filter function is constructed and multiplied with the range frequency domain signal to obtain the range pulse compressed signal.

[0019] In one embodiment of the present invention, the matched filter function is expressed as:

[0020] H RMF (k r )=exp[j(k r -k rc ) 2 c 2 / (16πk)]

[0021] Where, k r k is the distance wavenumber.rc Let c be the distance from the wavenumber center and c be the speed of light.

[0022] In one embodiment of the present invention, the distance pulse compressed signal is subjected to Decirp processing to obtain a Decirp-compensated signal, including:

[0023] The Decirp function is constructed as follows:

[0024] H dechrip =exp(jk r R a (t a ))

[0025] Where, k r R is the distance wavenumber. a (t a () represents the instantaneous slant distance history of the scene center point;

[0026] The Decirp function is multiplied by the distance pulse-compressed signal to obtain the Decirp-compensated signal.

[0027] In one embodiment of the present invention, the Decirp-compensated signal is subjected to two-dimensional resampling processing to obtain a resampled two-dimensional wavenumber domain signal, including:

[0028] Perform a Taylor expansion on the phase of the Decirp-compensated signal;

[0029] Construct distance interpolation factor and azimuth interpolation factor;

[0030] The Decirp-compensated signal is resampled in both range and azimuth directions according to the range interpolation factor and the azimuth interpolation factor, respectively, to obtain the resampled two-dimensional wavenumber domain signal.

[0031] In one embodiment of the present invention, the distance interpolation factor is expressed as:

[0032]

[0033] The azimuth interpolation factor is expressed as:

[0034]

[0035] Where, k r For the distance wavenumber, t a R represents the azimuth time, v represents the radar speed, and R represents the radar velocity. s For reference slope distance, R a (t a () represents the instantaneous slant distance history of the scene center point. θa The angle between the center azimuth and the beam line of sight at that moment.

[0036] In one embodiment of the present invention, geometric correction based on back projection is performed on the primary imaging result to obtain an imaging result with no geometric deformation and good focus, including:

[0037] Construct a Cartesian coordinate system grid for the ground plane and calculate the positions of the grid points in the oblique plane;

[0038] Based on the PFA algorithm principle, the coordinates of the oblique plane are rotated and transformed to obtain the coordinate position of the primary imaging result on the ground plane;

[0039] The imaging results are resampled according to their coordinates on the ground plane to achieve geometric correction and obtain a final imaging result with no geometric deformation and good focus.

[0040] Another embodiment of the present invention also provides a SAR polar coordinate imaging device based on level flight equivalence, comprising:

[0041] The data acquisition module is used to construct a level flight equivalent slant range model for large slant-look SAR in order to obtain the baseband echo signal formed by scattering from the target area;

[0042] The data processing module is used to perform range-direction matched filtering on the baseband echo signal to obtain the range pulse compressed signal;

[0043] The compensation module is used to perform Decirp processing on the signal after distance pulse compression to obtain the Decirp-compensated signal;

[0044] The resampling module is used to perform two-dimensional resampling processing on the Decirp-compensated signal to obtain a resampled two-dimensional wavenumber domain signal.

[0045] The imaging module is used to perform IFFT transformation on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result;

[0046] The geometric correction module is used to perform geometric correction on the primary imaging result based on back projection to obtain an imaging result with no geometric deformation and good focus.

[0047] The beneficial effects of this invention are:

[0048] This invention improves the traditional PFA imaging geometric model by using the dive-flight equivalent method, avoiding the azimuth translation invariance problem caused by dive, thus solving the problem that traditional PFA is not applicable to dive SAR; at the same time, the geometric deformation problem in the level-flight equivalent PFA is corrected by back projection, successfully extending the deformation-free PFA imaging algorithm to dive SAR.

[0049] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of a SAR polar coordinate imaging method based on level flight equivalent provided by an embodiment of the present invention;

[0051] Figure 2 This is the geometric model for signal acquisition of the slant-look diving SAR platform provided in this embodiment of the invention;

[0052] Figure 3 Yes Figure 2 Simplify the geometric model;

[0053] Figure 4 This is a schematic diagram of another SAR polar coordinate imaging method based on level flight equivalence provided in an embodiment of the present invention;

[0054] Figure 5 This is a schematic diagram of a SAR polar coordinate imaging device based on level flight equivalent provided in an embodiment of the present invention;

[0055] Figure 6 It is the final imaging result of the simulation experiment;

[0056] Figure 7 It is the imaging result from the simulation experiment without geometric correction;

[0057] Figure 8 and Figure 9 This is for Figure 6 Two-dimensional contour map of edge points and center points. Detailed Implementation

[0058] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0059] Example 1

[0060] Please see Figure 1 , Figure 1 This is a schematic flowchart of a SAR polar coordinate imaging method based on level flight equivalence provided by an embodiment of the present invention, which includes:

[0061] Step 1: Construct a level flight equivalent slant range model for large slant-look SAR to obtain the baseband echo signal formed by scattering from the target area.

[0062] Specifically, we first construct the level flight equivalent slant range model for large slant-look SAR.

[0063] Please see Figure 2 , Figure 2 This is the geometric model for signal acquisition of a slant-down SAR platform provided in this embodiment of the invention. Figure 2 In this scenario, the projection O of the radar null position onto the ground plane is taken as the scene center point, and the projection of the flight direction along the ground plane is taken as the X-axis, establishing a rectangular coordinate system O-XYZ. During data acquisition, the radar operates in spotlight mode, and the platform descends at a constant speed v along the flight trajectory AC. AC lies in the YOZ plane, and the angle between AC and the Y-axis is α. Let the radar's pitch angle at null moment be β, the angle between OQ and the X-axis be γ, and the reference slant range be R. s The beam coverage area is defined as a gray elliptical region, with its scene center point being Q(R). s cosβsinγ,R s cosβcosγ,0).

[0064] Let the azimuth time be t. a Then for any point P(x) in the scene p ,y p The slant distance history of (,0) can be expressed as:

[0065]

[0066] Due to variations in radar platform altitude, directly applying traditional level-flight algorithms results in severe defocusing. Therefore, we need to start with an imaging slant-range model to observe... Figure 2 It can be observed that the platform dives downwards along line AC. Due to the change in altitude, it does not possess azimuth translation invariance. If the platform's flight direction is considered level flight, then the scene is essentially rotated. In this case, the signal acquisition along the BD direction within the BDE plane can be considered azimuth translation invariant, and a level flight algorithm can be used to process the signal. However, the actual imaging plane should be the XOY plane. Therefore, points in the actual imaging scene that are not on line DE will be projected onto the BDE plane according to the criterion of consistent slant range history, resulting in geometric model distortion.

[0067] Choosing the oblique plane BDE as the imaging plane, with Let X′ be the axis, and let the plane of BDE be perpendicular to X′. Let the Y′ axis be the coordinate axis. Transforming from the original three-dimensional rectangular coordinate system to a two-dimensional oblique plane coordinate system, the equivalent BDE plane for points P and Q is P′(x). p ′,y p ′),Q′(Rs cosθ a ,R s sinθ a Point ). The slant distance model at this point can be rewritten as:

[0068]

[0069] Where θ a Let be the angle between the center azimuth and the beamline, i.e., the angle between BD and BQ. From geometric relationships, we know that:

[0070]

[0071] Therefore, we can conclude that:

[0072]

[0073] set up The above equation can be simplified to:

[0074]

[0075] As can be seen from the formula, the slant range form is equivalent to the slant range model under level flight slant-look conditions, so traditional PFA can be used for imaging.

[0076] After the equivalent of a dive-to-level flight, the baseband echo signal scattered from the target area can be expressed as:

[0077]

[0078] Where k is the linear frequency modulation frequency, t r For distance to fast time, t a For azimuth, time slows down, f c R(t) is the center frequency. a Instantaneous slant distance, w r (t r ), w a (t a ) are the time-domain expressions for the envelopes of the distance and azimuth window functions, respectively.

[0079] Step 2: Perform range-direction matched filtering on the baseband echo signal to obtain the range pulse compressed signal.

[0080] First, perform an FFT transform on the baseband echo signal to obtain the range frequency domain signal, denoted as s(k r ,t a ), k r This represents the distance wavenumber.

[0081] Then, a matched filter function is constructed and multiplied with the range frequency domain signal to obtain the range pulse compressed signal.

[0082] Specifically, the matched filter function is expressed as:

[0083] H RMF (k r )=exp[j(k r -k rc ) 2 c 2 / (16πk)]

[0084] Where, k r k is the distance wavenumber. rc Let c be the distance from the wavenumber center and c be the speed of light.

[0085] Combine the distance frequency domain signal with H RMF Multiplying these components, we can obtain the time-domain expression for the range-frequency domain and azimuth of the matched-filtered signal as follows:

[0086] s(k r ,t a ) = W r (k r )w a (t a )exp[-jk r R(t a )]

[0087] Among them, W r (k r ) is the frequency domain expression of the envelope of the range-direction window function.

[0088] Step 3: Perform Decirp processing on the distance pulse compressed signal to obtain the Decirp-compensated signal.

[0089] First, construct the Decirp function as follows:

[0090] H dechrip =exp(jk r R a (t a ))

[0091] Where, k r R is the distance wavenumber. a (t a () represents the instantaneous slant distance history of the scene center point;

[0092] Then, Dechirp processing is performed on the signal according to the scene center point Q′. The Dechirp function is multiplied by the pulse-compressed signal to obtain the Dechirp-compensated signal, which is expressed as:

[0093] s(kr ,t a ) = W r (k r )w a (t a )exp{-jk r [R(t a )-R a (t a )]}

[0094] in,

[0095] Step 4: Perform two-dimensional resampling on the Decirp-compensated signal to obtain the resampled two-dimensional wavenumber domain signal.

[0096] 41) Perform a Taylor expansion on the phase of the Decirp-compensated signal.

[0097] Specifically, this implementation adopts the plane wavefront assumption for R(t) a )-R a (t a ) along x p ′,y p Taylor series expansion, ignoring x p ′,y p After adding the elevation and coupling terms, the slant distance difference can be written in the following form:

[0098]

[0099] Where, o(x p ′,y p ′) is a Taylor series expansion of second order or higher.

[0100] 42) Construct distance interpolation factor and azimuth interpolation factor.

[0101] Due to the unique nature of the dive slant range history, the interpolation factor in the traditional PFA algorithm is not applicable, and direct processing will lead to severe defocus. Therefore, this embodiment needs to reconstruct the interpolation factor.

[0102] In this embodiment, the distance interpolation factor and the azimuth interpolation factor are defined as follows:

[0103]

[0104]

[0105] in, θ a The angle between the center azimuth and the beam line of sight at that moment.

[0106] 43) The Decirp-compensated signal is resampled in two dimensions, namely range and azimuth, according to the range interpolation factor and the azimuth interpolation factor, respectively, to obtain the resampled two-dimensional wavenumber domain signal.

[0107] First, the signal is resampled along the range direction. The interpolated signal expression is:

[0108]

[0109] Then, the signal is resampled in the azimuth direction, and the interpolated signal can be represented as:

[0110] s(k y ,k x ) = W r (k y W a (k x )exp(jk x (x p cosθ a ′+y p sinθ a ′)+jk y (y p cosθ a ′-x p sinθ a ′))

[0111] make:

[0112] x pn =x p cosθ a ′+y p sinθ a ′

[0113] y pn =y p cosθ a ′-x p sinθ a ′

[0114] It can be seen that the position of the imaging point at this time is (x pn ,y pn This is equivalent to rotating the BDE plane rectangular coordinate system counterclockwise by θ within the plane. a Then, the resampled two-dimensional wavenumber domain signal can be expressed as:

[0115] s(k y ,k x ) = W r (k y )w a (t a )exp(jkx x pn +jk y y pn )

[0116] Step 5: Perform IFFT transform on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result.

[0117] Because the projection of any point P onto the BDE plane results in severe geometric distortion of the image after oblique transformation, this embodiment further includes the following after step 5:

[0118] Step 6: Perform geometric correction on the primary imaging results based on back projection to obtain imaging results with no geometric deformation and good focus.

[0119] 61) Construct a Cartesian coordinate system grid for the ground plane and calculate the positions of the grid points on the oblique plane. First, for Figure 2 Simplified imaging geometric deformation relationship as follows Figure 3 As shown, let along Let e1 be the unit vector, and e2 be the unit vector perpendicular to e1 in the BDE plane. Then, by geometric relations, e1(cosα, 0, -sinα). Since point P and P′ have the same slant range history, the 'projection' at this point is not a traditional geometric projection mapping, but rather a mapping relationship resulting from the rotation of the BDP plane cluster to the BDE plane, where the position of point P changes. After further rotation along the line of sight, the true position of P′ in the image is obtained.

[0120] Therefore, to obtain accurate imaging results without geometric deformation, it is first necessary to span a set of rectangular networks on the ground plane with X and Y coordinate axes, and then calculate the position of a point on the network within the coordinate system (x, y). pn ,y pn By projecting the original oblique plane data onto the ground plane grid using the location information below, a deformation-free imaging result can be obtained. The specific process is as follows:

[0121] After forming a rectangular grid, given the location information of point P, the first step is to project P onto the BDE plane to obtain point P′. Because Equivalent to Rotating around e1 as the axis, therefore and The projections along the e1 direction have the same magnitude. Let P(x) p ,y p ,0), B(0,0,R) s sinβ), thus the vector can be obtained. e1(cosα,0,sinα) has the following equation:

[0122]

[0123] The projection along the e2 direction can be expressed as:

[0124]

[0125] 62) Based on the PFA algorithm principle, the coordinates of the oblique plane are rotated and transformed to obtain the coordinate position of the primary imaging result on the ground plane.

[0126] According to step 61), P′(x) can be obtained. p ′,y p The position information of the polar primary image is obtained by rotating and transforming it to obtain the coordinate position of the polar primary image on the ground plane. The transformation formula is:

[0127] x pn =x p cosθ a ′+y p sinθ a ′

[0128] y pn =y p cosθ a ′-x p sinθ a ′

[0129] 63) Resample the primary imaging results according to their coordinates on the ground plane to achieve geometric correction of the imaging results and obtain the final imaging results with no geometric deformation and good focus.

[0130] Specifically, given x pn y pn Then, the distance and azimuth positions of a point on the ground plane in the imaging oblique plane can be determined. Based on this positional relationship, azimuth and distance resampling is used, and the grayscale information of the ground plane projection is obtained by interpolation.

[0131] Please see Figure 4 , Figure 4 This is a schematic diagram of another SAR polar coordinate imaging method based on level flight equivalence provided in an embodiment of the present invention.

[0132] The SAR polar coordinate imaging method based on level flight equivalent provided in this embodiment not only avoids the azimuth translation invariance problem caused by dive by using oblique ground transformation, thus solving the problem that traditional PFA is not applicable to dive SAR; it also corrects the geometric deformation problem in level flight equivalent PFA by using back projection, successfully extending the deformation-free PFA imaging algorithm to dive SAR.

[0133] Example 2

[0134] Based on Embodiment 1 above, this embodiment also provides a SAR polar coordinate imaging device based on level flight equivalence. Please refer to... Figure 5 , Figure 5 This is a schematic diagram of a SAR polar coordinate imaging device based on level flight equivalent provided in an embodiment of the present invention, including:

[0135] Data acquisition module 1 is used to construct a level flight equivalent slant range model for large slant-look SAR in order to obtain the baseband echo signal formed by scattering from the target area;

[0136] Data processing module 2 is used to perform range-direction matched filtering on the baseband echo signal to obtain the range pulse compressed signal;

[0137] Compensation module 3 is used to perform Decirp processing on the signal after distance pulse compression to obtain the Decirp-compensated signal;

[0138] Resampling module 4 is used to perform two-dimensional resampling processing on the Decirp-compensated signal to obtain a resampled two-dimensional wavenumber domain signal.

[0139] Imaging module 5 is used to perform IFFT transformation on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result.

[0140] The geometric correction module 6 is used to perform geometric correction on the primary imaging result to obtain an imaging result with no geometric deformation and good focus.

[0141] The SAR polar coordinate imaging device based on level flight equivalence provided in this embodiment can realize the SAR polar coordinate imaging method based on level flight equivalence provided in Embodiment 1 above. The detailed process will not be repeated here.

[0142] Therefore, the device provided in this embodiment not only solves the problem that traditional PFA is not suitable for dive SAR, but also corrects the geometric deformation problem in the level flight equivalent PFA, and successfully extends the deformation-free PFA imaging algorithm to dive SAR.

[0143] Example 3

[0144] The beneficial effects of the present invention will be verified and explained through simulation experiments below.

[0145] 1. Simulation parameters:

[0146] In this simulation experiment, the ground scenario uses Q(R) s cosβsinγ,R s cosβcosγ,0)

[0147] Centered on the X-axis and Y-axis, a 5x5 dot matrix is ​​arranged, with an azimuth width of 500m and a range width of 500m.

[0148] 2. Simulation Results and Analysis:

[0149] Please see Figure 6-9 , Figure 6 This is the final imaging result of the simulation experiment. Figure 7 These are imaging results from simulation experiments without geometric correction. Figure 8 and Figure 9 This is for Figure 6 Two-dimensional contour map of edge points and center points.

[0150] from Figure 7 It can be seen that the PFA imaging results without geometric correction have severe geometric distortion and cannot accurately reflect the distance and azimuth. Using the method in step 6 of the above embodiment, cubic spline interpolation is applied to correct the SAR image, and the correction result is as follows: Figure 6 As shown. At this point, the target distance and orientation dimensions can reflect the correct geometric relationships. A two-dimensional upsampling contour map is drawn using the upper right point and the center point; the result is as follows. Figure 8 , 9 As shown, the focusing effect on the center point and edge points of the target is good, proving the effectiveness of the method.

[0151] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A SAR polar coordinate imaging method based on level flight equivalence, characterized in that, include: Construct a level flight equivalent slant range model for large slant-look SAR to obtain the baseband echo signal formed by scattering from the target area; The baseband echo signal is subjected to range-direction matched filtering to obtain the range pulse compressed signal; The signal after distance pulse compression is processed by Decirp to obtain the Decirp-compensated signal; The Decirp-compensated signal is subjected to two-dimensional resampling to obtain a resampled two-dimensional wavenumber domain signal. Perform an IFFT transform on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result; The initial imaging results are geometrically corrected based on back projection to obtain imaging results with no geometric deformation and good focus.

2. The SAR polar coordinate imaging method based on level flight equivalence according to claim 1, characterized in that, The baseband echo signal is represented as: Where k is the linear frequency modulation frequency, t r For distance to fast time, t a For azimuth, time slows down, f c R(t) is the center frequency. a Instantaneous slant distance, w r (t r ),w a (t a ) are the time-domain expressions for the envelopes of the distance and azimuth window functions, respectively.

3. The SAR polar coordinate imaging method based on level flight equivalence according to claim 2, characterized in that, The baseband echo signal is subjected to range-direction matched filtering to obtain a range pulse-compressed signal, including: Perform an FFT transform on the baseband echo signal to obtain a range frequency domain signal; A matched filter function is constructed and multiplied with the range frequency domain signal to obtain the range pulse compressed signal.

4. The SAR polar coordinate imaging method based on level flight equivalence according to claim 3, characterized in that, The matched filter function is expressed as follows: H RMF (k r )=exp[j(k r -k rc ) 2 c 2 / (16πk)] Where, k r k is the distance wavenumber. rc Let c be the distance from the wavenumber center and c be the speed of light.

5. The SAR polar coordinate imaging method based on level flight equivalence according to claim 1, characterized in that, The distance pulse-compressed signal is subjected to Decirp processing to obtain a Decirp-compensated signal, including: The Decirp function is constructed as follows: H dechrip =exp(jk r R a (t a )) Where, k r R is the distance wavenumber. a (t a () represents the instantaneous slant distance history of the scene center point; The Decirp function is multiplied by the distance pulse-compressed signal to obtain the Decirp-compensated signal.

6. The SAR polar coordinate imaging method based on level flight equivalence according to claim 1, characterized in that, The Decirp-compensated signal is subjected to two-dimensional resampling to obtain a resampled two-dimensional wavenumber domain signal, including: Perform a Taylor expansion on the phase of the Decirp-compensated signal; Construct distance interpolation factor and azimuth interpolation factor; The Decirp-compensated signal is resampled in both range and azimuth directions according to the range interpolation factor and the azimuth interpolation factor, respectively, to obtain the resampled two-dimensional wavenumber domain signal.

7. The SAR polar coordinate imaging method based on level flight equivalence according to claim 6, characterized in that, The distance interpolation factor is expressed as: The azimuth interpolation factor is expressed as: Where, k r For the distance wavenumber, t a R represents the azimuth time, v represents the radar speed, and R represents the radar velocity. s For reference slope distance, R a (t a () represents the instantaneous slant distance history of the scene center point. θ a The angle between the center azimuth and the beam line of sight at that moment.

8. The SAR polar coordinate imaging method based on level flight equivalence according to claim 1, characterized in that, Geometric correction based on back projection is performed on the primary imaging result to obtain an imaging result with no geometric deformation and good focus, including: Construct a Cartesian coordinate system grid for the ground plane and calculate the positions of the grid points in the oblique plane; Based on the PFA algorithm principle, the coordinates of the oblique plane are rotated and transformed to obtain the coordinate position of the primary imaging result on the ground plane; The imaging results are resampled according to their coordinates on the ground plane to achieve geometric correction and obtain a final imaging result with no geometric deformation and good focus.

9. A SAR polar coordinate imaging device based on level flight equivalence, characterized in that, include: The data acquisition module (1) is used to construct the level flight equivalent slant range model of the large slant-look SAR in order to obtain the baseband echo signal formed by the scattering of the target area; The data processing module (2) is used to perform range-direction matched filtering on the baseband echo signal to obtain the range pulse compressed signal; The compensation module (3) is used to perform Dechirp processing on the signal after the distance pulse compression to obtain the Dechirp-compensated signal; The resampling module (4) is used to perform two-dimensional resampling processing on the Decirp-compensated signal to obtain a resampled two-dimensional wavenumber domain signal. The imaging module (5) is used to perform IFFT transformation on the resampled two-dimensional wavenumber domain signal to obtain the SAR polar coordinate primary imaging result; The geometric correction module (6) is used to perform geometric correction on the primary imaging result based on back projection to obtain an imaging result with no geometric deformation and good focus.

Images