A method for focusing synthetic aperture radar images

Abstract

The present invention relates to a synthetic aperture radar image focusing method, comprising the following steps: placing a calibrator in an imaging scene; performing coarse imaging of the original echo according to the measurement data of a motion sensor; extracting the defocused calibrator image in the image domain of the coarse imaging, and reconstructing the echo; estimating the phase error according to the reconstructed echo; compensating the original echo according to the phase error, and performing a first focusing operation of the image; performing a phase gradient self-focusing process to calculate the residual phase error, and performing a second focusing operation of the image to obtain a synthetic aperture radar image. The method of the present invention provides a synthetic aperture radar image focusing method based on the fact that the phase error caused by the motion error of the radar platform is space-invariant, and solves the problems of defocused imaging images and low radar image quality.

Description

Technical Field

[0001] The present invention relates to the technical field of radar image processing, and in particular to a synthetic aperture radar image focusing method. Background Art

[0002] In recent years, with the commissioning of stealth equipment, the method of conducting equipment stealth performance diagnosis and evaluation through RCS (radar cross-sectional area) near-field measurement in the maintenance site environment has received more and more attention. This method generally uses a two-dimensional plane scanning frame to obtain a two-dimensional scattering center distribution image at a specific attitude angle of the equipment. Compared with the traditional RCS near-field measurement method, the vehicle-mounted radar RCS near-field scanning measurement system is not limited by the lack of hardware conditions such as turntables / guide rails and low-scattering metal brackets in the test site, and has the advantages of strong mobility and low cost in the scattering characteristics test of full-size targets. However, the omnidirectional SAR (synthetic aperture radar) mode of the vehicle-mounted radar platform moving around the target is directly used for testing. Due to the limitation of the platform's motion accuracy on the ground, the direct imaging image will produce defocusing phenomenon; and the higher the resolution of the imaging, the more serious the impact of motion error. When the motion error reaches the wavelength level, the radar image quality will deteriorate sharply; therefore, motion error compensation has become an indispensable technology for obtaining high-quality radar images.

[0003] In view of the above shortcomings, it is necessary to provide a synthetic aperture radar image focusing method to solve the problems of image defocusing and low radar image quality. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the uncertainty of radar platform motion leads to motion error, thereby generating echo data phase error. In view of the defects in the prior art, based on the fact that the phase error caused by radar platform motion error is space-invariant, a synthetic aperture radar image focusing method is provided to solve the problems of image defocusing and low radar image quality.

[0005] In order to solve the above technical problems, the present invention provides a synthetic aperture radar image focusing method, which is characterized by comprising the following steps: placing a calibrator in an imaging scene; performing coarse imaging on the original echo according to the measurement data of a motion sensor; extracting the defocused calibrator image in the image domain of the coarse imaging, and reconstructing the echo; estimating the phase error according to the reconstructed echo; compensating the original echo according to the phase error, and performing a first focusing operation on the image; performing phase gradient self-focusing processing to calculate the residual phase error, and performing a second focusing operation on the image to obtain a synthetic aperture radar image.

[0006] Preferably, in the step of extracting the defocused calibrator image in the image domain of the coarse imaging and reconstructing the echo, the defocused calibrator image is intercepted in the image domain of the coarse imaging by a windowing method, and the echo is reconstructed.

[0007] Preferably, in the step of estimating the phase error according to the reconstructed echo, the phase error caused by the residual motion error is obtained by comparing the echo data of the defocused calibrator obtained by echo reconstruction with the echo data of the ideal point target located at the calibrator. It can be expressed as,

[0008]

[0009] Among them, S r (θ, K) is the echo data reconstructed from the complex image data of the intercepted defocused calibrator, H A xθ, K) is the echo data of the ideal point target located at the calibrator.

[0010] Preferably, the echo data S reconstructed from the intercepted defocused scaler complex image data r (θ, K) can be expressed as,

[0011]

[0012] Among them, g ∈ (x, y) is the intercepted calibrator error complex image, θ is the angle recorded by the motion sensor, K is the range wave number, R xy (θ) is the distance from the pixel point in the imaging area to the radar measurement position.

[0013] Preferably, the echo data H of the ideal point target located at the calibrator A (θ, K) can be expressed as,

[0014] H A (θ,K)=exp(-j2KR Ae (θ)),

[0015] Where θ is the angle recorded by the motion sensor, K is the range wave number, R Ae (θ) is the distance from the point target to the ideal motion trajectory.

[0016] Preferably, the original echo is compensated according to the phase error, and in the step of performing the first focusing operation of the image, the first focusing operation of conjugate multiplication with the phase error term is performed on the basis of the original echo to eliminate the influence of the residual motion error. The echo data S1 (θ, K) after the phase error compensation can be expressed as:

[0017] S1(θ,K)=S0(θ,K)·exp(-jφ ε (θ,K)),

[0018] Where S0(θ, K) is the original echo.

[0019] Preferably, after compensating the original echo according to the phase error and performing the first focusing operation on the image, the echo data S1 (θ, K) after the phase error compensation obtained by the first focusing operation is imaged by a trial filtering-back projection algorithm.

[0020] Preferably, the step of performing phase gradient self-focusing processing to calculate the residual phase error, performing a second focusing operation on the image, and obtaining a synthetic aperture radar image includes the following processes: acquiring complex image domain data, performing a circular shift operation on the complex image domain data; performing a windowing operation on the complex image domain data after the circular shift operation; estimating the phase gradient, and constructing a phase error function, and calculating the residual phase error through the phase error function; performing phase error compensation and imaging according to the residual phase error; and performing a cyclic iterative operation until the imaging meets the imaging standard to obtain a synthetic aperture radar image.

[0021] Preferably, the imaging standard is that the number of cycles of the cyclic iteration operation is 4 times.

[0022] Preferably, the imaging standard is that the residual phase error is less than π / 10, or the imaging standard is that the image entropy value of the imaging is less than a preset value.

[0023] The implementation of the synthetic aperture radar image focusing method of the present invention has the following beneficial effects: the motion sensor measurement data and the radar original echo data both contain information about the motion trajectory of the radar platform. By analyzing and processing the two, the phase error caused by the residual motion error can be estimated and compensated to the original echo, so that the image can be focused; and the redundancy of the azimuth error phase in the preliminary focused image can be further improved through phase gradient self-focusing processing, so that the image quality can be further improved and the focusing effect can be enhanced. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a flow chart of a synthetic aperture radar image focusing method according to an embodiment of the present invention;

[0025] Figure 2 It is the data acquisition geometry model of the synthetic aperture radar platform;

[0026] Figure 3 4 is a flow chart of a second focusing operation of the synthetic aperture radar image focusing method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] Figure 1 is a flow chart of a synthetic aperture radar image focusing method according to an embodiment of the present invention; Figure 1 As shown, the synthetic aperture radar image focusing method provided by the embodiment of the present invention includes the following steps:

[0029] Step S01: placing a calibrator in an imaging scene;

[0030] Step S02: performing coarse imaging of the original echo according to the motion sensor measurement data;

[0031] Step S03: extracting the defocused calibrator image in the image domain of the coarse imaging, and reconstructing the echo;

[0032] Step S04: estimating a phase error according to the reconstructed echo;

[0033] Step S05: compensating the original echo according to the phase error and performing a first focusing operation on the image;

[0034] Step S06: performing phase gradient self-focusing processing to calculate the residual phase error, performing a second focusing operation on the image, and obtaining a synthetic aperture radar image.

[0035] The embodiment of the present invention is a synthetic aperture radar (SAR) image focusing method based on image domain phase error estimation and phase gradient autofocusing (PGA), which uses a motion sensor with low measurement accuracy to measure the motion trajectory of the radar platform and perform coarse imaging of the SAR echo; a calibrator is placed in the imaging scene, and the defocused calibrator image in the image domain of the coarse imaging is extracted to reconstruct the echo; the phase error obtained by comparing the reconstructed echo with the ideal echo signal is compensated to the original echo to focus the image; the remaining phase error is estimated by phase gradient autofocusing to complete further focusing of the image and obtain a SAR image with good quality. The present invention combines the SAR image focusing method based on the motion sensor with the SAR image focusing method based on the echo data, further improving the image focusing effect of the algorithm and the imaging quality.

[0036] Both the motion sensor measurement data and the radar original echo data contain information about the motion trajectory of the radar platform. By analyzing and processing the two, the phase error caused by the residual motion error can be estimated and compensated to the original echo to achieve image focusing. Then, by using the redundancy of the azimuth error phase in the preliminary focused image, after PGA processing, the image quality can be further improved and the focusing effect can be enhanced. Motion error is the measurement error of the motion parameters of the radar platform, which is the main reason for the deterioration of radar imaging quality. According to radar imaging theory, when the radar motion platform deviates from the ideal motion trajectory, high-quality imaging can be achieved as long as the precise positions of the radar and the target are known. Figure 2 is the data acquisition geometric model of the synthetic aperture radar platform; Figure 2 As shown, P(x0, y0, 0) is a point target, A(x, y, z) is an ideal SAR position, and A1(x1, y1, z1) is an actual SAR position. The two are inconsistent due to the existence of motion errors. According to this idea, an embodiment of the present invention proposes an imaging method for coarse imaging based on differential GPS measurement data, that is, using the platform motion data recorded by a differential GPS module with low accuracy installed on the radar platform to perform phase compensation on the target echo signal, thereby reducing motion errors, improving imaging quality, and obtaining a coarsely focused SAR image.

[0037] The synthetic aperture radar image focusing method provided by the embodiment of the present invention extracts the defocused calibrator image in the image domain of the coarse imaging, and in the step of reconstructing the echo, the defocused calibrator image is intercepted in the image domain of the coarse imaging by a windowing method, and the echo is reconstructed. By placing a calibrator in the scene, the scene is first coarsely imaged, and the defocused calibrator image is windowed in the image domain to reduce the interference of clutter. Then, the phase error caused by the residual motion error is estimated according to the method of reconstructing the echo. Finally, the phase error caused by the residual motion error is compensated to the original echo to complete the focusing of the image. This method does not rely on the calibrator echo to have a high signal-to-noise ratio. The separation of signal and clutter is achieved by windowing in the image domain, thereby improving the estimation accuracy.

[0038] In an embodiment of the present invention, the motion sensor measurement data includes a radar trajectory (i.e., a measurement trajectory) recorded by the motion sensor, which is used to perform coarse imaging of the original echo, reconstruct the echo of the defocused calibrator image in the image domain of the coarse imaging, and compensate the original echo according to the phase error.

[0039] In the synthetic aperture radar image focusing method provided by the embodiment of the present invention, in the step of estimating the phase error according to the reconstructed echo, the phase error caused by the residual motion error is obtained by comparing the echo data of the defocused calibrator obtained by echo reconstruction with the echo data of the ideal point target located at the calibrator. It can be expressed as,

[0040]

[0041] Among them, S r (θ, K) is the echo data reconstructed from the complex image data of the intercepted defocused calibrator, H A (θ, K) is the echo data of the ideal point target located at the calibrator. The physical meaning of the above formula is that the phase error is the reconstructed echo S of the error image r (θ, K) and the reconstructed echo H of the ideal image A By comparing the echo data of the defocused calibrator obtained by echo reconstruction with the echo data of the ideal point target located at the calibrator, the phase error in the entire wave number domain K caused by the residual motion error can be obtained.

[0042] The echo data S reconstructed from the complex image data of the scaler with intercepted defocus r (θ, K) can be expressed as,

[0043]

[0044] Among them, g ∈ (x, y) is the intercepted calibrator error complex image, θ is the angle recorded by the motion sensor, K is the range wave number, R xy (θ) is the distance from the pixel point in the imaging area to the radar measurement position. The double integral of the above formula is equivalent to the calibrator error complex image g ∈ (x, y) regenerates the echo for the target.

[0045] The echo data H of the ideal point target located at the calibrator A (θ, K) can be expressed as,

[0046] H A (θ,K)=exp(-j2KR Ae (θ)),

[0047] Where θ is the angle recorded by the motion sensor, K is the range wave number, R Ae (θ) is the distance from the point target to the ideal motion trajectory. The above two echo reconstructions can obtain the reconstructed echo data of the defocused calibrator and the echo data of the ideal point target located at the calibrator.

[0048] In the synthetic aperture radar image focusing method provided by the embodiment of the present invention, the original echo is compensated according to the phase error. In the step of performing the first focusing operation of the image, the first focusing operation of conjugate multiplication with the phase error term is performed on the basis of the original echo to eliminate the influence of the residual motion error. The echo data S1(θ, K) after the phase error compensation can be expressed as:

[0049] S1(θ,K)=S0(θ,K)·exp(-jφ ε (θ,K)),

[0050] In the formula, S0(θ, K) is the original echo. Error compensation is to pre-calculate the phase error term based on the complete original echo S0(θ, K). The conjugate multiplication is used as the echo data for subsequent imaging processing to eliminate the influence of residual motion errors.

[0051] The synthetic aperture radar image focusing method provided by the embodiment of the present invention compensates the original echo according to the phase error, and after the step of performing the first focusing operation of the image, the echo data S1(θ, K) after the phase error compensation obtained by the first focusing operation is imaged by a trial filtering-back projection algorithm to obtain a SAR image with a better focusing effect.

[0052] Figure 3 FIG. 4 is a flow chart of a second focusing operation of the synthetic aperture radar image focusing method according to an embodiment of the present invention. Figure 3 As shown, the synthetic aperture radar image focusing method provided by the embodiment of the present invention performs phase gradient self-focusing processing to calculate the residual phase error, performs a second focusing operation on the image, and obtains the synthetic aperture radar image, including the following process:

[0053] Step S061: Acquire complex image domain data, and perform a circular shift operation on the complex image domain data;

[0054] Step S062: performing a windowing operation on the complex image domain data after the circular shift operation;

[0055] Step S063: estimating the phase gradient, constructing a phase error function, and calculating the residual phase error through the phase error function;

[0056] Step S064: performing phase error compensation and imaging according to the residual phase error;

[0057] Step S065: perform the loop iteration operation of step S061 to step S064 until the imaging satisfies the imaging standard, and obtains a synthetic aperture radar image.

[0058] Wherein, in step S061, the complex image domain data is obtained through the first focusing operation, or is obtained by imaging in step S064.

[0059] The synthetic aperture radar image focusing method provided by the embodiment of the present invention has an imaging standard that the number of cycles of the cyclic iterative operation is 4 times, or an imaging standard that the residual phase error is less than π / 10, or an imaging standard that the image entropy value of the imaging is less than a preset value.

[0060] In summary, the implementation of the synthetic aperture radar image focusing method of the present invention has the following beneficial effects: the motion sensor measurement data and the radar original echo data both contain information about the motion trajectory of the radar platform. By analyzing and processing the two, the phase error caused by the residual motion error can be estimated and compensated to the original echo, so that the image can be focused; and by utilizing the redundancy of the azimuth error phase in the preliminary focused image, the image quality can be further improved through phase gradient self-focusing processing, thereby enhancing the focusing effect.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A synthetic aperture radar image focusing method, characterized in that: The steps include: placing a calibrator in the imaging scene; Performing coarse imaging of the original echoes according to the motion sensor measurement data; extracting a defocused calibrator image in the image domain of the coarse imaging and reconstructing an echo; estimating a phase error based on the reconstructed echo; Compensating the original echo according to the phase error to perform a first focusing operation on the image; Phase gradient self-focusing processing is performed to calculate the residual phase error, and a second focusing operation of the image is performed to obtain a synthetic aperture radar image.

2. The synthetic aperture radar image focusing method according to claim 1, characterized in that: In the step of extracting the defocused calibrator image in the image domain of the coarse imaging and reconstructing the echo, the defocused calibrator image is intercepted in the image domain of the coarse imaging by a windowing method, and the echo is reconstructed.

3. The synthetic aperture radar image focusing method according to claim 2, characterized in that: In the step of estimating the phase error according to the reconstructed echo, the phase error caused by the residual motion error is obtained by comparing the echo data of the defocused calibrator obtained by echo reconstruction with the echo data of the ideal point target located at the calibrator. It can be expressed as, Among them, S r (θ, K) is the echo data reconstructed from the complex image data of the intercepted defocused calibrator, H A (θ, K) is the echo data of the ideal point target located at the calibrator.

4. The synthetic aperture radar image focusing method according to claim 3, characterized in that: The echo data S reconstructed from the complex image data of the scaler with intercepted defocus r (θ, K) can be expressed as, Among them, g ∈ (x, y) is the intercepted calibrator error complex image, θ is the angle recorded by the motion sensor, K is the range wave number, R xy (θ) is the distance from the pixel point in the imaging area to the radar measurement position.

5. The synthetic aperture radar image focusing method according to claim 3, characterized in that: The echo data H of the ideal point target located at the calibrator A (θ, K) can be expressed as, H A (θ,K)=exp(-j2KR Ae (i)), Where θ is the angle recorded by the motion sensor, K is the range wave number, R Ae (θ) is the distance from the point target to the ideal motion trajectory.

6. The synthetic aperture radar image focusing method according to claim 3, characterized in that: In the step of compensating the original echo according to the phase error and performing the first focusing operation of the image, a first focusing operation of conjugate multiplication with the phase error term is performed on the basis of the original echo to eliminate the influence of the residual motion error. The echo data S1(θ, K) after the phase error compensation can be expressed as: S1(θ,K)=S0(θ,K)·exp(-jφ ε (θ,K)), Where S0(θ, K) is the original echo.

7. The synthetic aperture radar image focusing method according to claim 6, characterized in that: After the step of compensating the original echo according to the phase error and performing the first focusing operation on the image, the echo data S1 (θ, K) after the phase error compensation obtained by the first focusing operation is imaged using a trial filtering-back projection algorithm.

8. The synthetic aperture radar image focusing method according to any one of claims 1 to 7, characterized in that: The step of performing phase gradient self-focusing processing to calculate the residual phase error, performing a second focusing operation on the image, and obtaining a synthetic aperture radar image includes the following process: Acquire complex image domain data, and perform a circular shift operation on the complex image domain data; Performing a windowing operation on the complex image domain data after the circular shift operation; estimating a phase gradient and constructing a phase error function, and calculating a residual phase error through the phase error function; Performing phase error compensation and imaging according to the residual phase error; The loop iteration operation is performed until the imaging meets the imaging standard to obtain a synthetic aperture radar image.

9. The synthetic aperture radar image focusing method according to claim 8, characterized in that: The imaging standard is that the number of cycles of the cyclic iteration operation is 4 times.

10. The synthetic aperture radar image focusing method according to claim 8, characterized in that: The imaging standard is that the residual phase error is less than π / 10, or the imaging standard is that the image entropy value of the imaging is less than a preset value.

Images