CN110806577A - Focusing imaging method and device of synthetic aperture radar, equipment and storage medium - Google Patents

Focusing imaging method and device of synthetic aperture radar, equipment and storage medium Download PDF

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CN110806577A
CN110806577A CN201911077115.XA CN201911077115A CN110806577A CN 110806577 A CN110806577 A CN 110806577A CN 201911077115 A CN201911077115 A CN 201911077115A CN 110806577 A CN110806577 A CN 110806577A
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synthetic aperture
aperture radar
echo data
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radar image
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CN110806577B (en
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闫志朔
张毅
张衡
王宇
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Institute of Electronics of CAS
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/41—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using analysis of echo signal for target characterisation; Target signature; Target cross-section
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/52—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00
    • G01S7/52017—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S15/00 particularly adapted to short-range imaging
    • G01S7/52046—Techniques for image enhancement involving transmitter or receiver
    • G01S7/52047—Techniques for image enhancement involving transmitter or receiver for elimination of side lobes or of grating lobes; for increasing resolving power

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Abstract

The embodiment of the application discloses a focusing imaging method, a device, equipment and a storage medium of a synthetic aperture radar, wherein the method comprises the following steps: enveloping and aligning the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, wherein the target echo data is used for imaging the target object; performing phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image; and inhibiting side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.

Description

Focusing imaging method and device of synthetic aperture radar, equipment and storage medium
Technical Field
The embodiment of the invention relates to an imaging method of a synthetic aperture radar, in particular to but not limited to a focusing imaging method, a device, equipment and a storage medium of the synthetic aperture radar.
Background
The Synthetic Aperture Radar (SAR) has a very wide application in imaging a target object, for example, the SAR can monitor a specific sea area, bay and important port by focusing imaging of a sea vessel, and can be used for early warning of a sea emergency, which is beneficial to identification and detection of the vessel.
Synthetic aperture radars generally use synthetic apertures generated by the motion of the radar itself to image, and the imaging object is a stationary target, so when a single SAR processing method is used to image a moving ship on the sea surface, the ship is affected by the motion of itself and waves, and various problems of image quality influence such as defocusing, blurring, and dislocation of the target ship can occur.
The problem affecting the image quality is caused by the components of the ship motion and the Radar motion, however, the Inverse Synthetic Aperture Radar (ISAR) technology can only solve the problem of the image quality caused by the ship motion when the ship images, but cannot solve the problem caused by the components of the Radar motion. Therefore, the problem that image quality is affected in the existing imaging technology cannot be completely solved only by compensating the self motion of the ship through the ISAR technology, and a focusing image of the ship moving on the sea surface is generated.
Disclosure of Invention
The embodiment of the invention provides a focusing imaging method, a device, equipment and a storage medium of a synthetic aperture radar.
The technical scheme of the embodiment of the application is realized as follows:
in one aspect, an embodiment of the present application provides a method for focused imaging of a synthetic aperture radar, where the method includes:
enveloping and aligning the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, wherein the target echo data is used for imaging the target object;
performing phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
and inhibiting side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
In another aspect, an embodiment of the present application provides a focusing imaging apparatus for a synthetic aperture radar, the apparatus including:
the envelope alignment module is used for carrying out envelope alignment on the target echo data of the obtained target object in the distance direction to obtain aligned target echo data;
the phase compensation module is used for carrying out phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
and the side lobe suppression module is used for suppressing side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
In another aspect, an embodiment of the present application provides a focusing imaging apparatus for synthetic aperture radar, the apparatus including: a memory storing a computer program operable on a processor and a processor implementing the steps of the method when executing the program.
In yet another aspect, the present application provides a computer-readable storage medium, on which a computer program is stored, and the computer program, when executed by a processor, implements the steps of the method.
In the embodiment of the application, the target echo data of the obtained target object is subjected to envelope alignment in the distance direction, so that the distance alignment can be realized while the distance error is restrained, and the image blurring problem caused by the distance error is avoided; and phase compensation is carried out on the aligned target echo data, so that the problem of image defocusing caused by phase errors can be avoided, and focusing imaging of the synthetic aperture radar is realized.
Drawings
Fig. 1A is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present application;
FIG. 1B is a schematic diagram illustrating the Doppler effect of the movement of a target vessel in accordance with an embodiment of the present application;
fig. 1C is a schematic flowchart of a focusing imaging method of a synthetic aperture radar according to an embodiment of the present application;
fig. 1D is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present disclosure;
fig. 2 is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present application;
fig. 3 is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present application;
fig. 4 is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present application;
FIG. 5 is a schematic diagram of a range-Doppler domain image and a ship image according to an embodiment of the present application;
FIG. 6 is a diagram illustrating comparison of results of processing true complex image data obtained by high resolution three times with different focusing algorithms according to an embodiment of the present application;
fig. 7A is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application;
fig. 7B is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application;
fig. 7C is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application;
fig. 7D is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application;
fig. 7E is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application;
fig. 7F is a schematic structural diagram of a focusing and imaging apparatus of a synthetic aperture radar according to an embodiment of the present application.
Detailed Description
The embodiment provides a focusing imaging method of a synthetic aperture radar, as shown in fig. 1A, the method includes:
step 110, enveloping and aligning the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, wherein the target echo data is used for imaging the target object;
here, for the target echo data, envelope alignment in the range direction is performed, and range alignment of the target echo data is performed.
The distance alignment is used for eliminating the dislocation of adjacent echoes in a range image caused by the translation of a target relative to a radar, and is the basis of phase compensation. If the accuracy of the distance alignment is not high, the result of the subsequent phase compensation is affected. Methods of distance alignment include, but are not limited to, global minimum entropy, envelope cross correlation, and super-resolution methods. Here, the envelope cross correlation method is used to perform envelope alignment on the target echo data in the distance direction.
Step 120, performing phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
here, the phase compensation process may include: performing phase compensation by using a rank-phase estimation method or performing phase compensation by using the improved rank-phase estimation method provided by the embodiment of the application; wherein: and after the phase error of the echo data is estimated by adopting an improved rank-phase estimation method, multiplying the estimated phase error by a conjugate function to offset the phase error to obtain a focused inverse synthetic aperture radar image.
And step 130, suppressing side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
In a focused inverse synthetic aperture radar image, the sidelobe level of a reflected echo of a strong scattering target may be higher than the main lobe level of a reflected echo of a weak scattering target, so that the weak scattering target is submerged by the strong scattering target, and the identification effect of a target object of the focused image is influenced. Therefore, a sidelobe suppression operation is required for the focused inverse synthetic aperture radar image to obtain an image in which the target object is accurately focused.
In the embodiment of the application, the target echo data is subjected to envelope alignment in the distance direction, so that the distance alignment can be performed while the distance error is restrained, and the image blurring problem caused by the distance error is avoided; and phase compensation is carried out on the aligned target echo data, so that the problem of image defocusing caused by phase errors can be avoided, and focusing imaging of the synthetic aperture radar is realized.
In some embodiments, see fig. 1C: prior to step 110, the method further comprises: step 101A: carrying out SAR imaging processing on original echo data obtained by a synthetic aperture radar to obtain complex image data;
step 102A: and extracting the target object from the complex image data to obtain initial echo data of the target object.
Then, step 103, performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object.
Because the storage space and the number transmission bandwidth of the satellite-borne SAR imaging system are limited, in general, the system reduces the data volume of the echo data by using a technology of compressing the original data, and therefore, the target echo data for focusing imaging can be obtained only by performing azimuth back compression on the initial echo data. The azimuth Inverse compression can be performed by Inverse Fourier Transform (IFFT).
Thereafter, step 110 is performed, comprising: enveloping and aligning the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, wherein the target echo data is used for imaging the target object;
in other embodiments, see fig. 1D: prior to step 110, the method further comprises: step 101B: extracting a target object from original echo data obtained by the synthetic aperture radar to obtain original echo data of the target object;
step 102B: performing range compression on the original echo data of the target object to obtain target echo data of the target object; step 110 is thus performed. From the above, it can be seen that the target echo data of fig. 1C can also be obtained by performing range-wise compression on the original echo data of the target object, as shown in fig. 1D, step 101B and step 102B.
In the implementation process, after step 103, this embodiment further includes: determining a translational doppler shift of the target echo data, correspondingly, step 110 includes: and carrying out envelope alignment on the obtained target echo data of the target object in the distance direction to obtain aligned target echo data. That is, the alignment of the envelope of the target echo data in the range up direction can be performed according to the translational doppler shift of the target echo data.
The steps of determining the translational doppler shift of the target echo data are as follows:
assuming that the number of scattering centers of the moving target object is P, the backscattering of the target echo after the distance compression can be expressed as formula (1-1):
Figure BDA0002262831320000051
in the formula (1-1), ApIs the backscattering coefficient, p, of the p-th scattering pointrIs distance sinc envelope curve, tau is round-trip delay time of distance echo signal, r (t) is distance from scattering point p to radar, c is light speed 3X 108m/s，ωaIs the sine envelope of the azimuth, t is the azimuth observation time, tcAs beam center offset time, f0For the carrier frequency of the SAR sensor system, backscattering is the scattering observed from the direction of incidence in synthetic aperture radars.
FIG. 1B is a Doppler effect diagram illustrating the movement of a target vessel according to an embodiment of the present invention, as shown in FIG. 1B, where P (x, y) is the target vessel, R (t) is the translational distance of the target to the radar, and u is the angle of rotation of the target relative to the line of sight (RLOS) axis of the radar. The radar is divided into an airborne radar and a satellite-borne radar, and the radar A in the figure is replaced by an airplane.
Eliminating phase term exp (-j4 pi f) in formula (1-1) by multiplying formula (1-1) by phase term conjugate0r (t)/c) with the target center as the origin O, r (t) is approximated by the formula (1-2):
in equation (1-2), R (t) is the translational distance from the target to the radar, θ (t) is the angle of rotation of the target relative to the radar line of sight (RLOS) u-axis, x is the x-direction coordinate of the target vessel P (x, y), and y is the y-direction coordinate of the target vessel P (x, y).
Taylor expansion of the translational distance of the target object to the radar, the Taylor expansion of the rotation angle of the target relative to the radar line of sight (RLOS) u-axis, as in formulas (1-3):
in the formula (1-3), R0Is the initial distance, v, of the target objecttIs the translation speed of the target object, atIs the acceleration of the target object. Also, theta0Is the initial angle of the target object relative to the RLOS axis. OmegarIs the angular velocity of the target object, arIs the angular acceleration of the target object. The distance r (t) from the scattering point p to the radar can be obtained by combining the formulas (1-1), (1-2) and (1-3), and further the distance a from the scattering point p to the radar can be deducedt0 and arWhen 0, the doppler shift caused by motion.
At at0 and arAt 0, the motion-induced doppler shift can be expressed as the time derivative of the phase by equation (1-4):
Figure BDA0002262831320000062
in the formula (1-4), λ is the wavelength of the radar, and φ is the phase of the scattering point.
The translational doppler shift and the rotational doppler shift can be expressed as equations (1-5):
in the formula (1-5), the metal oxide,
Figure BDA0002262831320000072
in order to shift the doppler shift in frequency,
Figure BDA0002262831320000073
is a rotational doppler shift. By step 110, the translation of echo data required for envelope alignment in the distance up may be obtainedDoppler shift.
The embodiment provides a focusing imaging method of a synthetic aperture radar, as shown in fig. 2, the method includes:
step 200, carrying out SAR imaging processing on original echo data obtained by the synthetic aperture radar to obtain complex image data;
the raw echo data can be processed by SAR imaging to obtain two-dimensional distribution of scattering coefficients of a target region, namely complex image data. The SAR imaging process includes, but is not limited to, a Chirp-Scaling (CS) algorithm and a Range-Doppler (Range-Doppler RD) algorithm.
Step 201, extracting the target object from the complex image data to obtain the initial echo data;
the complex image data obtained by the SAR imaging processing often includes a plurality of target objects. A commonly used target object may be a marine sport vessel or the like. Since the respective motion trajectories of the plurality of target objects are complex and have different motion characteristics, it is complicated to directly use the obtained complex image data for image focusing imaging. Therefore, before focusing imaging, it is necessary to extract echo data of a target object from complex image data.
The echo data of the target object is extracted from the complex image data in a parametric and non-parametric manner, wherein:
when the parameter mode is used for extraction, the echo data of the target object can be extracted according to the basic system parameters of the SAR sensor for obtaining the complex image data. The basic system parameters comprise a transmission signal carrier frequency, time width and bandwidth, azimuth synthetic aperture length, azimuth sampling rate, antenna length and the like of the SAR sensor system, and the position of a target object.
The nonparametric way may be an interception way, that is: and intercepting the target object from the complex image data to obtain initial echo data.
202, performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object;
step 203, performing cross correlation on the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data;
the target echo data is divided into a plurality of range bins in the range direction. The traditional envelope cross-correlation method is as follows: and (4) assuming that the complex envelopes of adjacent one-dimensional directions do not change greatly, obtaining distance offset through cross correlation between echo data envelopes in adjacent distance units, and thus completing distance alignment. Unlike conventional methods, the cross-correlation here is the range alignment of the target echo data with the echo data of the first range bin in the imaging time.
Step 204, performing phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
and 205, utilizing frequency domain windowing to suppress side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
The frequency domain windowing method is also called a frequency domain weighting method, and performs windowing processing on an image and amplitude weighting by a fixed weighting function. The frequency domain windowing firstly transforms the image data to a frequency domain, and then smoothes a frequency spectrum in the frequency domain by adopting a weighting function, so that the leakage of main lobe energy is reduced, strong scattering echo side lobes are inhibited, and a focused image of a target object is obtained.
In the embodiment of the application, when envelope alignment is performed, a method for performing cross correlation between target echo data and echo data of a first range cell in imaging time is used, so that drift errors of aligned range cells caused by errors of adjacent range cells when envelope alignment is performed between the target echo data and the echo data of the adjacent range cells can be avoided. Therefore, the envelope alignment method in this embodiment can eliminate the misalignment of adjacent echoes in the distance direction caused by the translation of the target relative to the radar, suppress drift errors while effectively performing distance alignment, and improve the focusing imaging effect of the synthetic aperture radar.
The embodiment provides a focusing imaging method of a synthetic aperture radar, as shown in fig. 3, the method includes:
step 300, performing SAR imaging processing on original echo data obtained by the synthetic aperture radar to obtain complex image data;
step 301, extracting the target object from the complex image data to obtain the initial echo data;
step 302, performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object;
303, performing cross correlation between the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data;
304, performing initial phase correction on the aligned target echo data by using a Doppler centroid tracking algorithm to obtain phase-corrected target echo data;
the Doppler centroid tracking algorithm is to perform weighted average on complex exponential functions of phase differences of range gates of adjacent aligned target echo data according to amplitude products to obtain initial phase estimation. And performing phase correction through initial phase estimation to obtain target echo data after phase correction. Wherein the radar quantifies the distance between the target object and the radar as a range gate.
305, performing azimuth inverse Fourier transform on the target echo data after the phase correction to obtain an inverse synthetic aperture radar image;
step 306, performing circular shift and direction Fourier transform (FFT) on the inverse synthetic aperture radar image in sequence to obtain an inverse synthetic aperture radar image of a range-Doppler domain;
for the phase compensation, the motion component in the rotation direction affects the phase compensation effect in the translation direction, and therefore, the influence of the rotation component needs to be reduced by setting the zero doppler frequency.
When the initial zero point Doppler is set, the ISAR image is circularly shifted, namely the initial zero point Doppler is set through the cyclic shift of the salient points in each distance unit, so that the blindness of setting the initial zero point Doppler is avoided, and the efficiency is higher. After circular shift, the ISAR image is subjected to azimuth FFT, and the ISAR image is transformed to a range-Doppler domain.
Step 307, performing rank-phase estimation on the inverse synthetic aperture radar image in the range-doppler domain to obtain the focused inverse synthetic aperture radar image;
in this embodiment, steps 305 to 307 are an improved Rank-phase Estimation method (IROPE), and compared with the conventional Rank-phase Estimation method (ROPE), the method preprocesses the target echo data, and reduces the influence of the initial phase error and the rotational component.
And 308, suppressing side lobes of the focused inverse synthetic aperture radar image by using frequency domain windowing to obtain a focused image of the target object.
In the embodiment of the application, the inverse synthetic aperture radar image is circularly shifted, so that the blindness of setting initial zero point Doppler through the cyclic shift of the salient point in each distance unit can be eliminated; during error estimation, an improved rank-phase estimation method is used for preprocessing target echo data, so that the influence of initial phase errors and rotation components is reduced, and the defocusing problem of the target echo data caused by the rotation components is avoided. Therefore, the focusing imaging method of the embodiment can improve the focusing imaging effect of the synthetic aperture radar, solve the problem of image defocusing, and generate the focused image of the target object.
The embodiment provides a focusing imaging method of a synthetic aperture radar, which comprises the following steps:
a0, carrying out SAR imaging processing on original echo data obtained by synthetic aperture radar to obtain complex image data;
step A1, extracting the target object from the complex image data to obtain the initial echo data;
step A2, performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object;
step A3, performing cross correlation on the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data;
a4, performing initial phase correction on the aligned target echo data by using a Doppler centroid tracking algorithm to obtain phase-corrected target echo data;
step A5, performing azimuth inverse Fourier transform on the target echo data after phase correction to obtain an inverse synthetic aperture radar image;
step A6, performing circular shift and direction Fourier transform (FFT) on the inverse synthetic aperture radar image in sequence to obtain an inverse synthetic aperture radar image of a range-Doppler domain;
step A7, performing rank-phase estimation on the inverse synthetic aperture radar image of the range-Doppler domain to obtain a phase error of the inverse synthetic aperture radar image;
the process of performing rank-phase estimation on the inverse synthetic aperture radar image comprises the following steps: and (3) estimating the phase error by using a ROPE algorithm on the range-Doppler inverse synthetic aperture radar image through two-step convergence. The ROPE algorithm estimates the first difference of the phase error and the doppler frequency at the same time, and can better avoid the influence of the rotating phase component compared with other algorithms. In an embodiment, compared with the PGA algorithm that employs windowing operation, the frequency is limited to a low frequency, which causes a problem of missing more information, the ROPE algorithm does not perform windowing operation, and high-frequency errors and random errors can be avoided, so that the influence of rotating phase components can be better avoided, and the phase error of the inverse synthetic aperture radar image can be obtained.
Step A8, performing phase error compensation according to the phase error of the inverse synthetic aperture radar image to obtain a first compensated inverse synthetic aperture radar image;
here, the inverse synthetic aperture radar image is subjected to phase error compensation in a conjugate manner by multiplying a phase term, and a first compensated inverse synthetic aperture radar image is obtained.
Step A9, judging the focusing effect of the first compensated inverse synthetic aperture radar image according to the image evaluation index;
the image quality evaluation method is divided into a subjective evaluation method and an objective evaluation method, and the objective evaluation method usually adopts some digital characteristics of the image as main objective standards for quality evaluation, such as brightness, contrast, brightness value error, spectral flatness, mean square error, peak signal-to-noise ratio, information entropy, gradient entropy, and the like of the image.
The entropy of the image information reflects the richness of the image information. Generally, the larger the entropy of image information is, the richer the information amount thereof is, and the better the quality is. When the entropy change of the image information is small, the change of the image information amount is small, the quality is good, and the image is focused. Therefore, the image quality objective evaluation index is adopted to judge the focusing effect of the first compensated inverse synthetic aperture radar image.
Step a10, when the first compensated inverse synthetic aperture radar image is in focus, determining the first compensated inverse synthetic aperture radar image as the focused inverse synthetic aperture radar image;
and A11, utilizing frequency domain windowing to inhibit side lobes of the focused inverse synthetic aperture radar image, and obtaining a focused image of the target object.
In the embodiment of the application, the image quality objective evaluation index is adopted to judge the focusing effect of the first compensated inverse synthetic aperture radar image, the image focusing effect can be judged according to the index transformation, and the image focused by the target object can be obtained.
The embodiment provides a focusing imaging method of a synthetic aperture radar, which comprises the following steps:
b0, carrying out SAR imaging processing on the original echo data obtained by the synthetic aperture radar to obtain complex image data;
step B1, extracting the target object from the complex image data to obtain the initial echo data;
step B2, performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object;
step B3, performing cross correlation on the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data;
b4, performing initial phase correction on the aligned target echo data by using a Doppler centroid tracking algorithm to obtain phase-corrected target echo data;
b5, performing azimuth inverse Fourier transform on the target echo data after phase correction to obtain an inverse synthetic aperture radar image;
step B6, performing circular shift and direction Fourier transform (FFT) on the inverse synthetic aperture radar image in sequence to obtain an inverse synthetic aperture radar image of a range-Doppler domain;
step B7, performing rank-phase estimation on the inverse synthetic aperture radar image of the range-Doppler domain to obtain a phase error of the inverse synthetic aperture radar image;
step B8, performing phase error compensation according to the phase error of the inverse synthetic aperture radar image to obtain a first compensated inverse synthetic aperture radar image;
step B9, judging the focusing effect of the first compensated inverse synthetic aperture radar image according to the image evaluation index;
step B10, when the first compensated inverse synthetic aperture radar image is not focused, performing circular shift and azimuth Fourier transform on the first compensated inverse synthetic aperture radar image to obtain an inverse synthetic aperture radar image of a range-Doppler domain;
step B11, performing rank-phase estimation on the inverse synthetic aperture radar image of the range-Doppler domain to obtain a phase error of the first compensated inverse synthetic aperture radar image;
when the index such as the image information entropy is judged to be changed according to the image quality objective evaluation index, the first compensated inverse synthetic aperture radar image is not focused, and at the moment, the image needs to be further estimated according to a rank-one phase estimation method to obtain the phase error of the first compensated inverse synthetic aperture radar image.
Step B12, performing phase error compensation according to the phase error of the first compensated inverse synthetic aperture radar image to obtain a second compensated inverse synthetic aperture radar image;
step B13, judging the focusing effect of the second compensated inverse synthetic aperture radar image according to the image evaluation index;
step B14, determining said second compensated inverse synthetic aperture radar image as said focused inverse synthetic aperture radar image when said second compensated inverse synthetic aperture radar image is focused;
and step B15, utilizing frequency domain windowing to inhibit side lobes of the focused inverse synthetic aperture radar image, and obtaining a focused image of the target object.
In the embodiment of the application, the compensated inverse synthetic aperture radar image is judged according to the objective evaluation index of the image quality, further error estimation and compensation are carried out on the unfocused image until the evaluation index is reached, the image quality is judged by using the evaluation index, so that the image can be further converged, the image with better focusing imaging effect is obtained, and the problem of defocusing of the image is avoided.
Fig. 4 is a schematic flowchart of a method for focusing and imaging a synthetic aperture radar according to an embodiment of the present application, and as shown in fig. 4, the method includes:
step 401, extracting and separating complex image data, that is: extracting echo data of a target object from the complex image data;
because the multi-target motion is complex and has different motion characteristics, however, the current refocusing algorithm is specific to a single moving ship target, and therefore, the echo data of the single moving ship needs to be extracted before subsequent processing.
The echo data of the moving vessel is extracted from the complex image data in a parametric and non-parametric manner. In one embodiment, when the parametric extraction is used, the echo data of the target object may be extracted according to basic system parameters of the SAR sensor that obtains the complex image data. The basic system parameters include the carrier frequency, time width and bandwidth of the transmitted signal, the length of the azimuth synthetic aperture, the azimuth sampling rate, the length of the antenna, and the like, and the position of the sea surface moving ship. In another embodiment, where non-parametric extraction is used, a single target moving vessel may be extracted from the complex image data in a truncated manner.
Step 402, azimuth counter-compression, namely: performing azimuth back compression on echo data of a single sea surface moving ship to obtain echo data required by an imaging method;
the echo data of a single sea moving vessel is obtained as frequency domain data by step 401, while phase compensation of the echo data in step 403 needs to be performed based on the time domain echo data of the single sea moving vessel. Thus, in step 402, time domain echo data for a single sea moving vessel is obtained by counter compressing the azimuth of the image of the single sea moving vessel in the frequency domain. Wherein the azimuth inverse compression is azimuth IFFT transformation.
Obtaining a translational doppler shift of the echo data required for coarse phase compensation from the target echo: assuming that the number of scattering centers of the moving target object is P, the backscattering of the target echo after the distance compression can be expressed as in equation (4-1):
Figure BDA0002262831320000141
in the formula (4-1), ApIs the backscattering coefficient, p, of the p-th scattering pointrIs distance sinc envelope curve, tau is round-trip delay time of distance echo signal, r (t) is distance from scattering point p to radar, c is light speed 3X 108m/s，ωaIs the sine envelope of the azimuth, t is the azimuth observation time, tcAs beam center offset time, f0For the carrier frequency of the SAR sensor system, backscattering is the scattering observed from the direction of incidence in synthetic aperture radars.
Fig. 1B is a schematic diagram illustrating doppler effect of the movement of a target vessel according to an embodiment of the present application. As shown in fig. 1B, where P (x, y) is the target vessel, r (t) is the target to radar translation distance, and u is the target rotation angle relative to the radar line of sight (RLOS) axis. The radar is divided into an airborne radar and a satellite-borne radar, and the radar A in the figure is replaced by an airplane.
By multiplying by equation (4-1)Eliminating phase term exp (-j4 pi f) in formula (4-1) in a way of phase term conjugation0r (t)/c) with the target center as the origin O, r (t) is approximated by the following equation (4-2):
Figure BDA0002262831320000142
in equation (4-2), R (t) is the translational distance from the target to the radar, θ (t) is the angle of rotation of the target relative to the radar line of sight (RLOS) u-axis, x is the x-direction coordinate of the target vessel P (x, y), and y is the y-direction coordinate of the target vessel P (x, y).
Taylor expansion of the translational distance of the target object to the radar, the angle of rotation of the target with respect to the radar line of sight (RLOS) u-axis, as in equation (4-3):
Figure BDA0002262831320000151
in the formula (4-3), R0Is the initial distance, v, of the target objecttIs the translation speed of the target object, atIs the acceleration of the target object. Also, theta0Is the initial angle of the target object relative to the RLOS axis. OmegarIs the angular velocity of the target object, arIs the angular acceleration of the target object. The distance r (t) from the scattering point p to the radar can be obtained by combining the formulas (4-1), (4-2) and (4-3), and further the distance a from the scattering point p to the radar can be deducedt0 and arWhen 0, the doppler shift caused by motion.
At at0 and arAt 0, the motion-induced doppler shift can be expressed as the time derivative of the phase by equation (4-4):
Figure BDA0002262831320000152
in the formula (4-4), λ is the wavelength of the radar, and φ is the phase of the scattering point.
The translational doppler shift and the rotational doppler shift can be expressed as equation (4-5):
Figure BDA0002262831320000153
in the formula (4-5), the metal oxide,
Figure BDA0002262831320000154
in order to shift the doppler shift in frequency,
Figure BDA0002262831320000155
is a rotational doppler shift. Through step 402, the translational doppler shift of the echo data required for coarse phase compensation can be obtained.
Step 403, aligning the distance envelopes, that is: performing coarse phase compensation on the echo data to obtain target echo data after the coarse phase compensation;
the coarse phase compensation method aims at performing range-wise alignment on echo data. The distance direction alignment usually adopts an envelope cross correlation method. The target echo data and the echo data of the first distance unit in the imaging time are subjected to cross correlation, so that the method is a real-time and effective envelope alignment method, the drift error is inhibited, the distance alignment of the target echo can be completed, and the target echo data after coarse phase compensation is obtained.
Step 404, fine phase compensation, namely: carrying out fine phase compensation on the echo data after the coarse phase compensation to obtain a refocused image;
step 404 fine phase compensation, is the most important step in the precision imaging method. The step provides an improved Rank-phase estimation method based on mixed SAR/ISAR processing, namely IROPE (improved Rank One phase estimation). In order to obtain the Phase error, a rope (rank One Phase estimation) iterative two-step convergence method is adopted in the IROPE, wherein the fine Phase compensation can be realized by the following steps:
step 441, obtain a preliminary phase error, that is: firstly, a Doppler centroid tracking algorithm (DCT) is used for obtaining a preliminary phase error;
through the DCT algorithm, the initial phase error estimation can be carried out on the echo data after the coarse phase compensation.
Step 442, compensating the phase error, namely: compensating the obtained phase error;
correction is performed by the initial phase error estimated in advance.
Step 443, obtaining an ISAR image preliminarily, that is: performing azimuth IFFT on the echo data subjected to the preliminary phase error compensation to obtain an ISAR image;
step 444, circular shift, namely: circularly moving the ISAR image;
when the initial zero point Doppler is set, the ISAR image is circularly moved, the initial zero point Doppler is set through the cyclic shift of the salient points in each distance unit, the blindness of setting the initial zero point Doppler is avoided, and the efficiency is higher.
Step 445, azimuth FFT, that is: performing azimuth FFT (fast Fourier transform) on the circularly moved image, and returning the image to a range-Doppler domain;
step 446, two steps of ROPE iteration, namely: for the range-Doppler image, two-step iteration is carried out by using an ROPE algorithm, and phase errors are estimated;
and (3) using an ROPE algorithm for the range-Doppler image, and obtaining the refocused Doppler image through two-step convergence estimation and phase error compensation. The ROPE algorithm estimates the first difference of the phase error and the doppler frequency at the same time, and can better avoid the influence of the rotating phase component compared with other algorithms. In an embodiment, compared to the PGA algorithm that employs windowing to limit the frequency to a low frequency, which causes a problem of missing more information, the ROPE algorithm does not perform windowing to avoid high-frequency errors and random errors, thereby better avoiding the influence of the rotating phase component.
Step 447, phase error compensation, i.e.: performing phase error compensation according to the obtained phase error to obtain a refocused Doppler image;
step 448, determining convergence, namely: judging the convergence of the algorithm according to the Doppler image effect after refocusing; when the algorithm converges, proceed to step 449 azimuth focus; when the algorithm does not converge, the process of steps 444 to 448 is repeated. In the iterative process, the residual phase error can be further estimated, and the ISAR image quality is improved.
Step 449, azimuth focusing, namely: completing azimuth focusing of the ISAR image;
step 405, sidelobe suppression, namely: suppressing side lobes of the refocused ISAR image;
sidelobe suppression is performed on the refocused ISAR image by using a Hanning window, and sidelobe levels are reduced by weighting processing so that sidelobes with high response can be reduced.
Step 406, refocusing the image, namely: resulting in a refocused image.
FIG. 5 is a schematic diagram of a range-Doppler domain image and a ship image according to an embodiment of the present application, and as shown in FIG. 5, (a) the graph and (b) the graph are the results of conventional SAR processing using raw echo data; it can be seen that (a) the image is a range-doppler domain image, and (b) the image of the ship obtained after conventional SAR processing. (c) Graphs (d) and (d) are the results of the hybrid SAR/ISAR processing with the modified ROPE algorithm. Wherein, (c) is a range-Doppler domain image, and (d) is a ship image obtained after the mixed SAR/ISAR processing of an improved ROPE algorithm.
Fig. 6 is a schematic diagram illustrating comparison of results of processing high resolution three acquired real complex image data by different focusing algorithms according to an embodiment of the present application, as shown in fig. 6, (a) the diagram is an original defocused SAR image; (b) the image is a refocused image processed by a sub-aperture correlation algorithm (Map-drift, MD); (c) the image is a refocused image processed by the ROPE algorithm; (d) the figure is a refocused image processed by the algorithm proposed by this patent.
EXAMPLE seven
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, where the apparatus includes modules and units included in the modules, and may be implemented by a processor in an imaging device; of course, it may also be implemented by logic circuitry; in implementation, the processor may be a Central Processing Unit (CPU), a Microprocessor (MPU), a Digital Signal Processor (DSP), a Field Programmable Gate Array (FPGA), or the like.
Fig. 7A is a schematic structural diagram of a focusing imaging apparatus of a synthetic aperture radar according to an embodiment of the present application, and as shown in fig. 7A, the apparatus 700 includes an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where:
an envelope alignment module 710, performing envelope alignment on the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, where the target echo data is used to image the target object; a phase compensation module 720, configured to perform phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image; and a side lobe suppression module 730, configured to suppress a side lobe of the focused inverse synthetic aperture radar image, so as to obtain a focused image of the target object.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7B, the apparatus 700 includes a processing module 701, a first extracting module 702, a back compression module 703, an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where:
a processing module 701, configured to perform SAR imaging processing on original echo data obtained by a synthetic aperture radar to obtain complex image data; a first extraction module 702, configured to extract a target object from the complex image data to obtain initial echo data of the target object; the decompression module 703 is configured to perform azimuth decompression on the initial echo data of the target object to obtain target echo data of the target object; an envelope alignment module 710, configured to perform envelope alignment on obtained target echo data of a target object in a distance direction to obtain aligned target echo data, where the target echo data is used to image the target object; a phase compensation module 720, configured to perform phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image; and a side lobe suppression module 730, configured to suppress a side lobe of the focused inverse synthetic aperture radar image, so as to obtain a focused image of the target object.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7B, the apparatus 700 includes a processing module 701, a first extracting module 702, a back compression module 703, an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where:
a processing module 701, configured to perform SAR imaging processing on original echo data obtained by a synthetic aperture radar to obtain complex image data; a first extraction module 702, configured to extract the target object from the complex image data to obtain the initial echo data; the decompression module 703 is configured to perform azimuth decompression on the initial echo data of the target object to obtain target echo data of the target object; the envelope alignment module 710 is further configured to perform cross-correlation between the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data; a phase compensation module 720, configured to perform phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image; the sidelobe suppression module 730 is further configured to suppress sidelobes of the focused inverse synthetic aperture radar image by using frequency domain windowing, so as to obtain a focused image of the target object.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7C, the apparatus 700 includes a second extracting module 704, a range-direction compressing module 705, an envelope aligning module 710, a phase compensating module 720, and a sidelobe suppressing module 730, where:
a second extraction module 704, configured to extract a target object from original echo data obtained by the synthetic aperture radar, so as to obtain original echo data of the target object; a distance direction compression module 705, configured to perform distance direction compression on the original echo data of the target object to obtain target echo data of the target object; an envelope alignment module 710, configured to perform envelope alignment on obtained target echo data of a target object in a distance direction to obtain aligned target echo data, where the target echo data is used to image the target object; a phase compensation module 720, configured to perform phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image; and a side lobe suppression module 730, configured to suppress a side lobe of the focused inverse synthetic aperture radar image, so as to obtain a focused image of the target object.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7D, the apparatus 700 includes a processing module 701, a first extracting module 702, a back compression module 703, an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where the phase compensation module 720 includes an initial phase correction unit 721, an inverse fourier transform unit 722, a circular shift unit 723, and a rank-one phase estimation unit 724, where:
a processing module 701, configured to perform SAR imaging processing on original echo data obtained by a synthetic aperture radar to obtain complex image data; a first extraction module 702, configured to extract the target object from the complex image data to obtain the initial echo data; the decompression module 703 is configured to perform azimuth decompression on the initial echo data of the target object to obtain target echo data of the target object; the envelope alignment module 710 is further configured to perform cross-correlation between the target echo data and echo data of a first distance unit in imaging time to obtain aligned target echo data; an initial phase correction unit 721, configured to perform initial phase correction on the aligned target echo data by using a doppler centroid tracking algorithm, so as to obtain phase-corrected target echo data; an inverse fourier transform unit 722, configured to perform azimuth inverse fourier transform on the phase-corrected target echo data to obtain an inverse synthetic aperture radar image; a circular shift unit 723, configured to perform circular shift and directional fourier transform on the inverse synthetic aperture radar image in sequence, so as to obtain an inverse synthetic aperture radar image in a range-doppler domain; a rank-phase estimation unit 724, configured to perform rank-phase estimation on the inverse synthetic aperture radar image in the range-doppler domain to obtain the focused inverse synthetic aperture radar image; the sidelobe suppression module 730 is further configured to suppress sidelobes of the focused inverse synthetic aperture radar image by using frequency domain windowing, so as to obtain a focused image of the target object.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7E, the apparatus 700 includes a processing module 701, a first extraction module 702, a back compression module 703, an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where the phase compensation module 720 includes an initial phase correction unit 721, an inverse fourier transform unit 722, a circular shift unit 723, and a rank-one phase estimation unit 724, where the rank-one phase estimation unit 724 further includes a first error estimation subunit 7241, a first phase error compensation subunit 7242A, a first evaluation subunit 7243A, and a first determination subunit 7244A, where:
a first error estimation subunit 7241, configured to perform rank-phase estimation on the inverse synthetic aperture radar image in the range-doppler domain to obtain a phase error of the inverse synthetic aperture radar image;
a first phase error compensation subunit 7242A, configured to perform phase error compensation according to the phase error of the inverse synthetic aperture radar image, to obtain a first compensated inverse synthetic aperture radar image;
a first evaluation subunit 7243A configured to determine a focusing effect of the first compensated inverse synthetic aperture radar image according to an image evaluation index;
a first determining subunit 7244A configured to determine the first compensated inverse synthetic aperture radar image as the focused inverse synthetic aperture radar image when the first compensated inverse synthetic aperture radar image is focused.
Based on the foregoing embodiments, the present application provides a focusing imaging apparatus for synthetic aperture radar, as shown in fig. 7F, the apparatus 700 includes a processing module 701, a first extraction module 702, a back compression module 703, an envelope alignment module 710, a phase compensation module 720, and a sidelobe suppression module 730, where the phase compensation module 720 includes an initial phase correction unit 721, an inverse fourier transform unit 722, a circular shift unit 723, and a rank-one phase estimation unit 724, where the rank-one phase estimation unit 724 further includes a second phase error compensation subunit 7242B, a second evaluation subunit 7243B, and a second determination subunit 7244B, where:
a second phase error compensation subunit 7242B, configured to perform phase error compensation according to the phase error of the first compensated inverse synthetic aperture radar image, to obtain a second compensated inverse synthetic aperture radar image;
a second evaluation subunit 7243B configured to determine a focusing effect of the second compensated inverse synthetic aperture radar image according to an image evaluation index;
a second determining subunit 7244B, for determining the second compensated inverse synthetic aperture radar image as the focused inverse synthetic aperture radar image, when the second compensated inverse synthetic aperture radar image is focused.
The above description of the apparatus embodiments, similar to the above description of the method embodiments, has similar beneficial effects as the method embodiments. For technical details not disclosed in the embodiments of the apparatus of the present application, reference is made to the description of the embodiments of the method of the present application for understanding.
It should be noted that, in the embodiment of the present application, if the above-mentioned focus imaging method of the synthetic aperture radar is implemented in the form of a software functional module and sold or used as a standalone product, it may also be stored in a computer readable storage medium. Based on such understanding, the technical solutions of the embodiments of the present application may be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for causing an imaging device to execute all or part of the methods described in the embodiments of the present application. And the aforementioned storage medium includes: various media capable of storing program codes, such as a usb disk, a removable hard disk, a Read Only Memory (ROM), a magnetic disk, or an optical disk. Thus, embodiments of the present application are not limited to any specific combination of hardware and software.
Correspondingly, the embodiment of the application provides a focusing imaging device of a synthetic aperture radar, and the device comprises: a memory storing a computer program operable on a processor and a processor implementing the steps of the method when executing the program.
Embodiments of the present application provide a computer-readable storage medium, on which a computer program is stored, and the computer program, when executed by a processor, implements the steps in the above method.
Here, it should be noted that: the above description of the storage medium and device embodiments is similar to the description of the method embodiments above, with similar advantageous effects as the method embodiments. For technical details not disclosed in the embodiments of the storage medium and apparatus of the present application, reference is made to the description of the embodiments of the method of the present application for understanding.
It should be appreciated that reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present application. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It should be understood that, in the various embodiments of the present application, the sequence numbers of the above-mentioned processes do not mean the execution sequence, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present application. The above-mentioned serial numbers of the embodiments of the present application are merely for description and do not represent the merits of the embodiments.
It should be noted that, in this document, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element.
In the several embodiments provided in the present application, it should be understood that the disclosed apparatus and method may be implemented in other ways. The above-described device embodiments are merely illustrative, for example, the division of the unit is only a logical functional division, and there may be other division ways in actual implementation, such as: multiple units or components may be combined, or may be integrated into another system, or some features may be omitted, or not implemented. In addition, the coupling, direct coupling or communication connection between the components shown or discussed may be through some interfaces, and the indirect coupling or communication connection between the devices or units may be electrical, mechanical or other forms.
The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units; can be located in one place or distributed on a plurality of network units; some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment. In addition, all functional units in the embodiments of the present application may be integrated into one processing unit, or each unit may be separately regarded as one unit, or two or more units may be integrated into one unit; the integrated unit can be realized in a form of hardware, or in a form of hardware plus a software functional unit.
Those of ordinary skill in the art will understand that: all or part of the steps for realizing the method embodiments can be completed by hardware related to program instructions, the program can be stored in a computer readable storage medium, and the program executes the steps comprising the method embodiments when executed; and the aforementioned storage medium includes: various media that can store program codes, such as a removable Memory device, a Read Only Memory (ROM), a magnetic disk, or an optical disk.
Alternatively, the integrated units described above in the present application may be stored in a computer-readable storage medium if they are implemented in the form of software functional modules and sold or used as independent products. Based on such understanding, the technical solutions of the embodiments of the present application may be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for causing a device to perform all or part of the methods described in the embodiments of the present application. And the aforementioned storage medium includes: a removable storage device, a ROM, a magnetic or optical disk, or other various media that can store program code.
The above description is only for the embodiments of the present application, but the scope of the present application is not limited thereto, and any person skilled in the art can easily conceive of changes or substitutions within the technical scope of the present application, and shall be covered by the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims (11)

1. A method of focused imaging for synthetic aperture radar, the method comprising:
enveloping and aligning the obtained target echo data of the target object in the distance direction to obtain aligned target echo data, wherein the target echo data is used for imaging the target object;
performing phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
and inhibiting side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
2. The method according to claim 1, characterized in that it comprises:
carrying out SAR imaging processing on original echo data obtained by a synthetic aperture radar to obtain complex image data;
extracting a target object from the complex image data to obtain initial echo data of the target object;
and performing azimuth back compression on the initial echo data of the target object to obtain target echo data of the target object.
3. The method according to claim 1, characterized in that it comprises:
extracting a target object from original echo data obtained by the synthetic aperture radar to obtain original echo data of the target object;
and performing range compression on the original echo data of the target object to obtain target echo data of the target object.
4. The method of claim 1, wherein the envelope aligning the target echo data of the acquired target object in a distance direction to obtain aligned target echo data comprises:
and performing cross correlation on the target echo data and the echo data of the first distance unit in the imaging time to obtain aligned target echo data.
5. The method of claim 1, wherein the phase compensating the aligned target echo data to obtain a focused inverse synthetic aperture radar image comprises:
performing initial phase correction on the aligned target echo data by using a Doppler centroid tracking algorithm to obtain phase-corrected target echo data;
performing azimuth inverse Fourier transform on the target echo data after the phase correction to obtain an inverse synthetic aperture radar image;
sequentially performing circular shift and azimuth Fourier transform on the inverse synthetic aperture radar image to obtain an inverse synthetic aperture radar image in a range-Doppler domain;
and performing rank-phase estimation on the inverse synthetic aperture radar image of the range-Doppler domain to obtain the focused inverse synthetic aperture radar image.
6. The method of claim 5, wherein said performing rank-phase estimation on said inverse synthetic aperture radar image in range-Doppler domain to obtain said focused inverse synthetic aperture radar image comprises:
performing rank-phase estimation on the inverse synthetic aperture radar image of the range-Doppler domain to obtain a phase error of the inverse synthetic aperture radar image;
performing phase error compensation according to the phase error of the inverse synthetic aperture radar image to obtain a first compensated inverse synthetic aperture radar image;
judging the focusing effect of the first compensated inverse synthetic aperture radar image according to the image evaluation index;
determining the first compensated inverse synthetic aperture radar image as the focused inverse synthetic aperture radar image when the first compensated inverse synthetic aperture radar image is focused.
7. The method of claim 5, wherein said performing rank-phase estimation on said inverse synthetic aperture radar image in range-Doppler domain resulting in said focused inverse synthetic aperture radar image, further comprises:
when the first compensated inverse synthetic aperture radar image is not focused, performing circular shift and azimuth Fourier transform on the first compensated inverse synthetic aperture radar image to obtain an inverse synthetic aperture radar image in a range-Doppler domain;
performing rank-phase estimation on the inverse synthetic aperture radar image in the range-doppler domain to obtain a phase error of the first compensated inverse synthetic aperture radar image;
performing phase error compensation according to the phase error of the first compensated inverse synthetic aperture radar image to obtain a second compensated inverse synthetic aperture radar image;
judging the focusing effect of the second compensated inverse synthetic aperture radar image according to the image evaluation index;
determining the second compensated inverse synthetic aperture radar image as the focused inverse synthetic aperture radar image when the second compensated inverse synthetic aperture radar image is focused.
8. The method of any of claims 1 to 7, wherein said suppressing sidelobes from said focused inverse synthetic aperture radar image to obtain a focused image of said target object comprises:
and suppressing side lobes of the focused inverse synthetic aperture radar image by using frequency domain windowing to obtain a focused image of the target object.
9. A focusing imaging apparatus for synthetic aperture radar, the apparatus comprising:
the envelope alignment module is used for carrying out envelope alignment on the target echo data of the obtained target object in the distance direction to obtain aligned target echo data;
the phase compensation module is used for carrying out phase compensation on the aligned target echo data to obtain a focused inverse synthetic aperture radar image;
and the side lobe suppression module is used for suppressing side lobes of the focused inverse synthetic aperture radar image to obtain a focused image of the target object.
10. A focusing imaging device for synthetic aperture radar comprising a memory and a processor, said memory storing a computer program operable on the processor, characterized in that the processor realizes the steps in the method of any of claims 1 to 8 when executing said program.
11. A computer-readable storage medium, on which a computer program is stored which, when being executed by a processor, carries out the steps of the method of any one of claims 1 to 8.
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