A circular synthetic aperture radar imaging compensation method based on acceleration information

Abstract

The present invention provides a circular synthetic aperture radar imaging compensation method based on acceleration information, comprising: obtaining circular synthetic aperture radar motion data and generating a two-dimensional image; extracting the residual phase of the two-dimensional image by analyzing the spectrum of the two-dimensional image; determining the residual phase to be compensated in the two-dimensional image based on the acceleration information in the CSAR motion data; compensating the residual phase to be compensated to obtain a compensated two-dimensional image. The method improves the quality of the compensated image and realizes high-precision imaging of CSAR.

Description

Technical Field

[0001] The invention relates to the technical field of circular synthetic aperture radar imaging, and in particular to a circular synthetic aperture radar imaging compensation method based on acceleration information. Background Art

[0002] Circular Synthetic Aperture Radar (CSAR) imaging is a SAR working mode proposed in the 1990s. Circular SAR uses a radar platform to fly in a circular trajectory in the air, and the radar beam always illuminates the center of the scene to achieve all-round observation of the target object. The long-term circular synthetic aperture enables circular SAR to achieve sub-wavelength-level planar resolution and no longer depends on the system bandwidth. At the same time, circular SAR has the potential for three-dimensional imaging, breaking through the two-dimensional limitations of traditional SAR, and alleviating or even eliminating shadows, geometric deformations, and overlap problems.

[0003] Circular SAR has incomparable advantages over traditional linear SAR, but due to the complex motion trajectory of circular SAR, conventional imaging algorithms based on linear trajectory derivation are no longer applicable. For airborne SAR platforms, airflow disturbances and mechanical vibrations of the carrier will introduce imaging errors. The flight trajectory of circular SAR is complex, the synthetic aperture time is long, and the impact of motion errors on circular SAR cannot be ignored. Compared with traditional linear SAR, the motion compensation of circular SAR does not need to compensate irregular trajectories for ideal trajectories, but only needs to compensate for slant range errors and phase errors caused by factors such as trajectory measurement errors. The acquisition of high-resolution circular SAR images is restricted by the accuracy of the navigation system. To achieve high-quality imaging of circular SAR, the trajectory measurement error needs to be controlled within 1 / 16 of the wavelength. In addition, the range and azimuth directions of circular SAR are severely coupled, and the slant range expression is more complicated, which makes the motion compensation of circular SAR more difficult.

[0004] In general, there are two types of motion compensation for circular SAR: one is based on the high-precision position and orientation system (POS), and the other is based on echo data. Motion compensation based on POS is generally difficult to meet the accuracy requirements of high-resolution circular SAR. The method of motion compensation based on the estimation of echo signals is relatively complex and easily disturbed by clutter signals. The long synthetic aperture makes the CSAR motion error more complex, which is unmatched by non-linear SAR. The classic autofocus algorithm is difficult to directly apply to circular SAR. Summary of the invention

[0005] 1. Technical issues to be resolved

[0006] In view of the above shortcomings, the main purpose of the present invention is to provide a circular synthetic aperture radar imaging compensation method based on acceleration information, which utilizes the acceleration information of the carrier to perform circular SAR motion compensation and achieve high-precision circular SAR imaging.

[0007] (II) Technical solution

[0008] In order to achieve the above-mentioned object, the present invention provides a circular synthetic aperture radar imaging compensation method based on acceleration information, comprising: acquiring circular synthetic aperture radar (CSAR) motion data and generating a two-dimensional image; extracting the residual phase of the two-dimensional image by performing spectrum analysis on the two-dimensional image; determining the residual phase to be compensated in the two-dimensional image based on the acceleration information in the CSAR motion data; and compensating the residual phase to be compensated to obtain a compensated two-dimensional image.

[0009] In the above scheme, the spectrum analysis of the two-dimensional image includes: data interception of the calibration points in the two-dimensional image; and two-dimensional Fourier transformation of the intercepted two-dimensional image to obtain two-dimensional wavenumber domain information of the two-dimensional image.

[0010] In the above scheme, the residual phase of the two-dimensional image is extracted by analyzing the two-dimensional image spectrum, and it also includes: extracting the center frequency phase based on the two-dimensional wavenumber domain information of the two-dimensional image; removing the linear phase in the phase at the center frequency to obtain the residual phase.

[0011] In the above scheme, the residual phase to be compensated in the two-dimensional image is determined based on the acceleration information in the CSAR motion data, including: based on the acceleration information in the CSAR motion data, using the least squares method to determine the relationship between the acceleration and the residual phase; based on the relationship between the acceleration and the residual phase, determining the residual phase to be compensated in the two-dimensional image.

[0012] In the above scheme, the residual phase to be compensated is compensated to obtain a compensated two-dimensional image, including: corresponding the residual phase to the two-dimensional wavenumber domain information to establish a time-frequency relationship; based on the time-frequency relationship, the residual phase to be compensated is compensated to obtain a compensated two-dimensional image.

[0013] In the above scheme, the residual phase is matched with the two-dimensional wave number domain information to establish a time-frequency relationship, including: defuzzifying the two-dimensional wave number domain information; matching the residual phase with the frequency domain information in the two-dimensional wave number domain information after the defuzzification process to establish a time-frequency relationship.

[0014] In the above scheme, CSAR motion data includes high-precision position and orientation system (POS) data.

[0015] In the above solution, the acceleration information in the CSAR motion data includes the acceleration information of the POS.

[0016] (III) Beneficial effects

[0017] The technical solution of the embodiment of the present invention has at least the following beneficial effects:

[0018] By using POS acceleration information, motion compensation is performed on the target object in the wavenumber domain. After compensation, the peak sidelobe ratio and integrated sidelobe ratio of the image are effectively improved, the image quality is improved, and high-precision imaging of CSAR is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A flow chart of a circular synthetic aperture radar imaging compensation method based on acceleration information according to an embodiment of the present invention is shown;

[0020] Figure 2 A schematic diagram of circular SAR observation geometry according to an embodiment of the present invention is shown;

[0021] Figure 3 A schematic diagram of a circular SAR motion model according to an embodiment of the present invention is shown;

[0022] Figure 4 A schematic diagram of the deformation of a lever arm according to an embodiment of the present invention is shown;

[0023] Figure 5 A phase error estimation diagram during a verification process according to an embodiment of the present invention is schematically shown. DETAILED DESCRIPTION

[0024] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0025] Figure 1 A flow chart of a circular synthetic aperture radar imaging compensation method based on acceleration information according to an embodiment of the present invention is shown.

[0026] like Figure 1 As shown, the circular synthetic aperture radar imaging compensation method based on acceleration information specifically includes operations S1 to S4.

[0027] In operation S1 , circular synthetic aperture radar (CSAR) motion data is acquired and a two-dimensional image is generated.

[0028] Specifically, the circular synthetic aperture radar (CSAR) movement is achieved by having a radar platform fly in a circular trajectory in the air. The radar beam always illuminates the center of the scene, thereby achieving all-round observation of the target object.

[0029] Figure 2 A schematic diagram of circular SAR observation geometry according to an embodiment of the present invention is shown.

[0030] like Figure 2 As shown in the figure, the flight direction of the carrier is defined as the azimuth direction, the radar line of sight direction is defined as the range direction, and the azimuth angle formed by the carrier movement with the positive direction of the x-axis as the relative starting point is θ. The radar transmission carrier frequency is f c The chirp signal has a pulse width of T P , the frequency is K r , the expression is:

[0031]

[0032] The antenna is pointing straight sideways, and the echo signal of a single point target object located at P can be expressed as follows after demodulation:

[0033]

[0034] Among them, A0 is the backscattering coefficient of the target object P, 𝝉 is the distance to fast time, θ is the azimuth, and w r (·) is the range gate function, λ is the radar operating wavelength, R p (θ) is the distance between the radar and the point target object P when the radar is in the azimuth θ.

[0035] The signal after pulse compression is:

[0036]

[0037] When the airborne platform is performing circular flight, it is affected by external factors and there is a deviation between the radar measurement coordinates obtained by the navigation system and the actual coordinates.

[0038] Figure 3 A schematic diagram of a circular SAR motion model according to an embodiment of the present invention is shown.

[0039] like Figure 3 As shown, the radar position coordinates obtained by the navigation system are A, and the actual radar coordinates are A ’For point P in the scene, its motion error can be expressed as the change in slant range. In the azimuth direction θ, the slant range from the actual radar coordinates to the target object can be expressed as:

[0040]

[0041] Among them, R p (θ) is the measured slope distance, 𝜟R p (θ) is the slant range variation caused by motion error. Without considering the reconstruction of the azimuth signal, the echo after range compression can be expressed as:

[0042]

[0043] Furthermore, its frequency domain expression is:

[0044]

[0045] It can be seen that the data with motion errors is essentially the ideal error-free data multiplied by an error phase.

[0046] Therefore, by observing the circular SAR motion, CSAR (i.e. circular SAR) motion data is obtained, and then a two-dimensional image is generated.

[0047] It should be noted that CSAR motion data includes high-precision position and orientation system (POS) data.

[0048] In operation S2, a residual phase of the two-dimensional image is extracted by performing spectrum analysis on the two-dimensional image.

[0049] In an embodiment of the present invention, data is firstly intercepted at the calibration points in the two-dimensional image, and then a two-dimensional Fourier transform is performed on the intercepted two-dimensional image to obtain two-dimensional wavenumber domain information of the two-dimensional image.

[0050] Specifically, the two-dimensional image is first windowed and the calibration points in the two-dimensional image are intercepted to suppress the influence of surrounding clutter and obtain the image. Then perform a two-dimensional Fourier transform and transform it to the frequency domain to obtain the two-dimensional wave number domain information. .

[0051] In an embodiment of the present invention, based on the two-dimensional wave number domain information of the two-dimensional image, the center frequency phase is extracted, and then the linear phase in the phase at the center frequency is removed to obtain the residual phase.

[0052] For example, assuming that the image with phase error is represented as , then the spectrum obtained after two-dimensional Fourier transform is ,

[0053]

[0054]

[0055] Where α is the radar incident angle, θ is the azimuth, and k x is the wave number in the x direction, k y is the wave number in the y direction. Let the target object position be (x p ,y p ), under ideal conditions, the two-dimensional spectrum phase only contains the initial phase of the target object and the linear phase related to the position of the target object, ignoring the initial phase and the spectrum envelope of the transmitted signal, and we get:

[0056] (9)

[0057] and The phase difference is the phase error.

[0058] It can be seen from formula (5) that the main factor affecting imaging is the phase error at the center wave number. Therefore, the residual phase can be expressed as:

[0059] (10)

[0060] In operation S3, a residual phase to be compensated in the two-dimensional image is determined based on acceleration information in the CSAR motion data.

[0061] In an embodiment of the present invention, firstly, based on the acceleration information in the CSAR motion data, the relationship between the acceleration and the residual phase is determined using the least squares method; then, based on the relationship between the acceleration and the residual phase, the residual phase to be compensated in the two-dimensional image is determined.

[0062] Specifically, first of all, in general, for airborne SAR platforms, the center of the inertial measurement unit (IMU) is fixedly connected to the radar system and placed on the belly of the aircraft. The three-dimensional position relationship between the two changes very little, but the antenna of the satellite navigation system must be installed on the outside of the flying vehicle because it needs to receive satellite signals, which is far away from the IMU. The circular SAR flight trajectory is complex, and the aircraft attitude changes frequently. During the flight, there is a lack of rigid connection between the inertial measurement unit and the satellite navigation system, which will produce relative displacement. In the POS solution process, this part of the factor is easily ignored. Therefore, there is a measurement error in the obtained radar position information, which makes the circular SAR image defocused.

[0063] Figure 4 A schematic diagram of the deformation of a lever arm according to an embodiment of the present invention is shown.

[0064] In the embodiment of the present invention, the acceleration information in the CSAR motion data includes the acceleration information of the POS. The change in acceleration will cause the lever arm to be subjected to tension and deform. When the aircraft acceleration changes, the lever arm will be subjected to a tension related to the acceleration, that is,

[0065] (11)

[0066] Where m can be understood as the mass of the object hanging on the arm, a is the acceleration, and further according to Hooke's law, the deformation is proportional to the force, that is,

[0067] (12)

[0068] Where k is the deformation coefficient of the lever arm, and l is the deformation amount. In fact, the deformation amount is the slope distance error that this system needs to solve, and there is a linear relationship between acceleration and deformation error. Therefore, the slope distance change can be estimated by using acceleration, and then phase compensation can be performed. Figure 4 As shown, when acceleration a≠0m / s 2 The lever arm deforms.

[0069] It can be seen that the estimation of the residual phase is essentially to calculate the slant range error. The slant range error is mainly caused by the deformation error caused by the change of system acceleration. Therefore, the relationship between acceleration and slant range error is estimated using least squares. After obtaining the parameter relationship between acceleration and slant range error, the acceleration can be used to obtain the motion compensation amount.

[0070] In operation S4, the residual phase to be compensated is compensated to obtain a compensated two-dimensional image.

[0071] In an embodiment of the present invention, the process of compensating the residual phase to be compensated to obtain a compensated two-dimensional image specifically includes: corresponding the residual phase with the two-dimensional wavenumber domain information to establish a time-frequency relationship; based on the time-frequency relationship, compensating the residual phase to be compensated to obtain a compensated two-dimensional image.

[0072] Specifically, the two-dimensional wavenumber domain information is first defuzzified, and then the residual phase is matched with the frequency domain information in the two-dimensional wavenumber domain information after the defuzzification process to establish a time-frequency relationship.

[0073] For example, when the imaging grid is large, that is, when the grid spacing required for full-aperture imaging is not met, the obtained time domain image presents periodic blurring in the spectrum. Therefore, when establishing the two-dimensional wave number domain (K x , K y ) and the time-frequency relationship of the angle wavenumber domain (θ, K), it is necessary to perform defuzzification. In order to avoid the trouble of subsequent processing, it is recommended that the pixel interval should meet the sampling rate when dividing the grid, that is:

[0074] (13)

[0075] Where 𝜟𝜙 is the coherent accumulation angle of the synthetic aperture. At this time, the spectrum only has periodic ambiguity, and the correct spectrum information can be obtained by solving the periodic ambiguity. The two-dimensional spectrum support domain of the radar signal should satisfy:

[0076] (14)

[0077] in, , B is the system bandwidth. The radar spectrum should be , When the imaging grid 𝜟x=𝜟y, the two-dimensional Fourier transform is x Direction and f y The frequency axis range in the direction is , calculate the fuzzy period as

[0078] (15)

[0079] Among them, M X f x The fuzzy number of the direction, M y f y After defuzzification, the frequency domain information can be matched with the angle, and the corresponding acceleration can be found by interpolation based on the angle information, the deformation error can be calculated, and then compensation can be performed in the frequency domain to obtain the compensated result:

[0080] (16)

[0081] in is the two-dimensional wavenumber domain of the error image, is the deformation error obtained by acceleration estimation.

[0082] Based on the above-mentioned circular synthetic aperture radar imaging compensation method based on acceleration information, the effectiveness of this method was verified in this example.

[0083] Specifically, the characteristics of the calibration point are analyzed, and the ground calibration point used is composed of four triangular reflectors. According to the opening direction of the calibration point, this embodiment selects one of the sub-apertures for phase compensation verification.

[0084] Window the ground calibration points to suppress the influence of surrounding clutter and obtain the image . Perform a two-dimensional Fourier transform and transform it to the frequency domain For the spectrum phase, we only need to take the residual phase at the center of the spectrum, that is, the position The phase at the center frequency is extracted, and phase unwrapping is performed to remove the linear phase to obtain the residual phase that needs to be compensated.

[0085] Figure 5 A phase error estimation diagram during a verification process according to an embodiment of the present invention is schematically shown.

[0086] In the embodiment of the present invention, the motion error is actually not directly related to the azimuth moment. However, according to the verification of this embodiment, it is found that the residual phase has a certain relationship with the radial acceleration, that is, the radial acceleration change trend is consistent with the residual phase change trend, and the correlation coefficient is calculated to be 0.9. The phase error is estimated using the acceleration information through the least squares method, and the estimation result is as follows: Figure 5 shown.

[0087] Usually, the impulse response width (IRW), peak side lobe ratio (PSLR) and integral side lobe ratio (ISLR) are used to measure the quality of radar images. IRW represents the resolution of the SAR image, while PSLR and ISLR measure the contrast and clarity of the SAR image. IRW is the 3 dB main lobe width of the impulse function, also known as image resolution, usually in pixels or meters (m). The lower the PSLR, the smaller the side lobe interference, the better the image quality, and the stronger the target object recognition and resolution capabilities.

[0088] In order to further measure the effectiveness of the algorithm, the error obtained by acceleration calculation is used for phase compensation. The compensation results show that the compensated image becomes focused and the spectrum becomes uniform. The characteristics of the target object are analyzed and the results are shown in Table 1. It can be seen that the proposed algorithm can effectively improve the peak sidelobe ratio and the integrated sidelobe ratio, improve the image quality, and verify the effectiveness of the method.

[0089] Table 1 Analysis of target object characteristics

[0090]

[0091] Through the embodiments of the present invention, the present invention analyzes the application of acceleration information in CSAR high-resolution imaging, and proposes to use the acceleration information in POS data to perform residual phase compensation for CSAR, effectively improving the peak sidelobe ratio and integrated sidelobe ratio of the target object image, thereby realizing high-precision imaging and all-round observation of CSAR motion.

[0092] It will be appreciated by those skilled in the art that, although the present invention has been shown and described with reference to specific exemplary embodiments of the present invention, it will be appreciated by those skilled in the art that, without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents, various changes in form and detail may be made to the present invention. Therefore, the scope of the present invention should not be limited to the above-described embodiments, but should be determined not only by the appended claims, but also by the equivalents of the appended claims.

[0093] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A circular synthetic aperture radar imaging compensation method based on acceleration information, characterized in that: The method comprises: Acquire circular synthetic aperture radar CSAR motion data and generate two-dimensional images; Extracting the residual phase of the two-dimensional image by analyzing the spectrum of the two-dimensional image; Determining a residual phase to be compensated in the two-dimensional image based on acceleration information in the CSAR motion data; Compensating the residual phase to be compensated to obtain a compensated two-dimensional image; The determining, based on the acceleration information in the CSAR motion data, the residual phase to be compensated in the two-dimensional image includes: Based on the acceleration information in the CSAR motion data, determining the relationship between the acceleration and the residual phase using a least squares method; Based on the relationship between the acceleration and the residual phase, the residual phase to be compensated in the two-dimensional image is determined.

2. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 1, characterized in that: The spectrum analysis of the two-dimensional image includes: Performing data interception on the calibration points in the two-dimensional image; Perform a two-dimensional Fourier transform on the intercepted two-dimensional image to obtain two-dimensional wavenumber domain information of the two-dimensional image.

3. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 2, characterized in that: The step of extracting the residual phase of the two-dimensional image by analyzing the spectrum of the two-dimensional image further includes: Extracting a center frequency phase based on the two-dimensional wavenumber domain information of the two-dimensional image; The linear phase in the phase at the center frequency is removed to obtain a residual phase.

4. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 2, characterized in that: The step of compensating the residual phase to be compensated to obtain a compensated two-dimensional image includes: The residual phase is matched with the two-dimensional wave number domain information to establish a time-frequency relationship; Based on the time-frequency relationship, the residual phase to be compensated is compensated to obtain a compensated two-dimensional image.

5. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 4, characterized in that: The step of making the residual phase correspond to the two-dimensional wave number domain information to establish a time-frequency relationship includes: Defuzzification processing is performed on the two-dimensional wavenumber domain information; The residual phase is matched with the frequency domain information in the two-dimensional wavenumber domain information after the deambiguation process to establish a time-frequency relationship.

6. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 1, characterized in that: The CSAR motion data includes high-precision position and attitude measurement system POS data.

7. The circular synthetic aperture radar imaging compensation method based on acceleration information according to claim 6, characterized in that: The acceleration information in the CSAR motion data includes the acceleration information of the POS.

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