Video SAR fast imaging method and system based on mobile platform

Abstract

The present invention discloses a video SAR fast imaging method and system based on a mobile platform, which relates to the field of radar imaging technology. The method comprises: establishing an echo signal model, obtaining a corresponding echo signal through the model, performing sub-aperture segmentation on the echo signal, generating a first frame of image using the first sub-aperture signal, and separating an overlapping aperture signal spectrum from its spectrum, performing self-focusing error compensation on a new non-overlapping aperture signal spectrum and an overlapping aperture signal spectrum of a previous frame, respectively, restoring the phase coherence of the two using a phase correlation method, fusing the spectrum after the coherence is restored, generating a new frame of image, and realizing continuous video SAR imaging by repeatedly performing the separation of overlapping aperture signal spectrum and the fusion of new non-overlapping aperture signal spectrum. The present invention solves the problem that the video SAR imaging method in the prior art cannot simultaneously realize high frame rate, high resolution and fast imaging.

Description

Technical Field

[0001] The present invention relates to the field of radar imaging technology, and in particular to a video SAR fast imaging method and system based on a mobile platform. Background Art

[0002] Video synthetic aperture radar (ViSAR) is an advanced radar imaging technology that combines the high-resolution imaging capability of traditional SAR with the continuity of video surveillance. It can generate sequential frame images by continuously observing the target area, and realize the dynamic change monitoring of the ground or target. Mobile platform SAR has a flexible configuration and is usually in a large squint situation. Combined with video SAR, it can greatly enhance the target detection capability of the mobile platform. Therefore, unlike traditional SAR that can only obtain static images, mobile platform video SAR can capture the motion information of the target and has a wide range of applications in disaster monitoring, traffic management and other fields.

[0003] There are two main categories of existing mobile platform video SAR imaging methods: frequency domain algorithms and time domain algorithms. Among frequency domain algorithms, the PFA algorithm is an early method for imaging frequency domain, which was subsequently widely used in SAR imaging. Its characteristic is that it uses fast Fourier transform to perform efficient focusing in the wavenumber domain, but it is based on the plane wave assumption, which limits the range of imaging scenes. Other frequency domain algorithms, such as RDA, RMA, CSA, etc., can be applied to different imaging needs, especially in complex scenes with higher adaptability and operational flexibility, but most of these algorithms are based on the direct flight mode of the radar platform. The time domain algorithm can adapt to any trajectory, including traditional back projection algorithm, recursive back projection algorithm, and fast factorization back projection algorithm. The traditional back projection algorithm has high accuracy but extremely high computational complexity; the recursive back projection algorithm has error accumulation limitations; the fast factorization back projection algorithm used for video SAR imaging divides the imaging scene into the region of interest (ROI) and the general region (GR), and uses different aperture lengths for imaging in different regions, thereby improving the resolution of the ROI area. At the same time, the computational complexity of video SAR imaging is reduced by the recursive method, but the computational burden still exists when processing large-scale data.

[0004] Therefore, current mobile platform video SAR imaging methods cannot achieve fast high-resolution and high-frame rate imaging at the same time. Summary of the invention

[0005] In view of this, an object of the present invention is to provide a video SAR fast imaging method and system based on a mobile platform, aiming to solve the problem that the video SAR imaging method in the prior art cannot simultaneously achieve fast high-resolution and high frame rate imaging.

[0006] An object of the present invention is to provide a video SAR fast imaging method based on a mobile platform, the method comprising:

[0007] Establish an echo signal model of the video SAR based on the mobile platform, and use the echo signal model to obtain the corresponding echo signal according to the signal emitted by the video SAR system of the mobile platform;

[0008] Perform sub-aperture segmentation on the echo signal, use the first sub-aperture signal to complete full-aperture imaging to generate the first frame image, and then separate the overlapping aperture signal spectrum from the spectrum of the first frame image using the time-frequency correspondence relationship;

[0009] The self-focusing error compensation is performed on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame are restored by using the phase correlation method;

[0010] The new non-overlapping aperture signal spectrum after restoring coherence is fused with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image. The overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame are repeated to achieve the image generation of the next frame to form video SAR imaging.

[0011] Furthermore, in the above-mentioned video SAR rapid imaging method based on a mobile platform, the steps of performing sub-aperture segmentation on the echo signal, using the first sub-aperture signal to complete full-aperture imaging to generate a first frame of image, and then separating the overlapping aperture signal spectrum from the spectrum of the first frame of image using the time-frequency correspondence relationship include:

[0012] Sub-aperture segmentation is performed using a preset overlapping frame rate, and the overlapping frame rate is expressed as;

[0013] ;

[0014] in, Indicates the azimuth resolution, Indicates the carrier frequency, represents the platform equivalent speed, represents the central beam slant angle, represents the shortest slope distance, represents the overlap ratio, c represents the speed of light;

[0015] Based on the stationary phase principle, the first sub-aperture signal is transformed into the range frequency domain through Fourier transform, and the GCBP algorithm is used to image the first sub-aperture signal received to obtain the signal representation of the first frame image:

[0016] ;

[0017] Among them, there is any point in the scene , The Jacobian determinant introduced for the substitution of integral variables is, and denote the magnitude of the wave number vector in the azimuth and range directions, respectively. j represents an imaginary unit;

[0018] The first frame image is corrected by using a preset spectrum space-varying correction function to obtain a two-dimensional image spectrum, and then the overlapping aperture signal spectrum is separated from the two-dimensional image spectrum of the first frame image by using the time-frequency correspondence relationship.

[0019] Furthermore, in the above-mentioned video SAR fast imaging method based on a mobile platform, the spectrum space-varying correction function expression is:

[0020] ;

[0021] in, j represents the imaginary unit, , and They represent the azimuth position, distance position and height corresponding to the sub-aperture center azimuth moment, respectively. represents the beam vector center, represents the wavelength of the carrier frequency, Represents the range spectrum of the target after the first step of spectrum correction, represents the range spectrum of the target, represents the center of the target's distance spectrum, Indicates the scene distance center;

[0022] After the first step of spectrum correction, the spectrum is shifted in distance and azimuth using the translation function, where the translation function is expressed as:

[0023] ;

[0024] in, and Respectively represent the corrected azimuth and distance spectra, An oblique viewing angle.

[0025] Furthermore, in the above-mentioned video SAR rapid imaging method based on a mobile platform, the step of separating the overlapping aperture signal spectrum from the two-dimensional image spectrum of the first frame image by using the time-frequency correspondence relationship comprises:

[0026] By establishing the center coordinates of the imaging scene The corresponding rectangular coordinate system azimuth spectrum and distance spectrum , calculate the spectrum width, expressed as:

[0027] ;

[0028] in, and represents the broadening coefficient;

[0029] Calculate the spectrum corresponding to the grid point through the spectrum width:

[0030] ;

[0031] in, and Represents the azimuth and distance resolutions, respectively. and are the central frequency points of the range spectrum and azimuth spectrum of the sub-aperture image, respectively. is the azimuthal grid spectrum, is the distance grid spectrum;

[0032] The time-frequency correspondence is any azimuth time Spectrum with azimuth grid The corresponding relationship is expressed as:

[0033] ;

[0034] in, represents the center coordinates of the imaging scene, Indicates any time at any location The effective transmission frequency, Indicates any time at any location The distance grid point corresponds to the frequency, Indicates any time at any location The corresponding aperture azimuth position, Indicates the instantaneous slant range from the instantaneous ideal antenna phase center to the target;

[0035] Based on the spectrum first-in-first-out, the aperture overlap ratio that meets the video SAR frame rate and high resolution is used to calculate the non-overlapping aperture starting azimuth time. and end position time Corresponding to the spectrum grid position interval, the image spectrum part within the spectrum grid position interval is separated therefrom to separate the overlapping aperture signal spectrum.

[0036] Furthermore, in the above-mentioned video SAR rapid imaging method based on a mobile platform, the steps of respectively performing self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame, and restoring the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using a phase correlation method include:

[0037] Extract the spectrum corresponding to the new non-overlapping aperture signal through the time-frequency correspondence Spectrum of the overlapped aperture signal from the previous frame , the spectrum and spectrum Move to the same spectrum center;

[0038] By calculating the spectrum and spectrum The cross power spectrum between them is obtained and inverse Fourier transform is performed to locate the pulse peak position to obtain the offset between the aperture segment images. Subsequently, linear phase multiplication is applied in the frequency domain of the image to achieve spatial alignment of the image in order to restore the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame.

[0039] Furthermore, in the above-mentioned video SAR rapid imaging method based on a mobile platform, the step of fusing the new non-overlapping aperture signal spectrum after restoring coherence with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image comprises:

[0040] The new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame are converted to the time domain by two-dimensional inverse Fourier transform, and multiplied by the phase , so that it is moved to the central frequency position of the next frame aperture, and then converted to the frequency domain to achieve additive fusion, and then a two-dimensional inverse Fourier transform is performed to generate a new frame of image;

[0041] in, The expression is:

[0042]

[0043] in, j represents the imaginary unit, represents the beam vector center, represents the wavelength of the carrier frequency, , and Respectively represent The frame aperture azimuth moment corresponds to the azimuth position, distance position and altitude.

[0044] Furthermore, in the above-mentioned video SAR rapid imaging method based on a mobile platform, the step of establishing an echo signal model of the video SAR based on the mobile platform and obtaining a corresponding echo signal according to a signal emitted by the video SAR system of the mobile platform using the echo signal model comprises:

[0045] The signal emitted by the video SAR system of the mobile platform is demodulated and range compressed, and the corresponding echo signal is expressed as:

[0046] ;

[0047] in, Indicates the distance to the fast time, Indicates the slow time of the direction, represents the signal bandwidth, represents the wavelength of the carrier frequency, represents the speed of light, Indicates the target The azimuth window, Indicates the instantaneous ideal antenna phase center to the target The instantaneous slope distance, j Represents an imaginary unit.

[0048] Another object of the present invention is to provide a video SAR fast imaging system based on a mobile platform, the system comprising:

[0049] An acquisition module is used to establish an echo signal model of a video SAR based on a mobile platform, and obtain a corresponding echo signal according to a signal transmitted by the video SAR system of the mobile platform using the echo signal model;

[0050] A segmentation module is used to perform sub-aperture segmentation on the echo signal, complete full-aperture imaging using the first sub-aperture signal to generate a first frame of image, and then separate the overlapping aperture signal spectrum from the spectrum of the first frame of image using the time-frequency correspondence relationship;

[0051] A compensation module is used to perform self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and to restore the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using a phase correlation method;

[0052] The imaging module is used to fuse the new non-overlapping aperture signal spectrum after restoring coherence with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image, and repeatedly perform the overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame to realize the image generation of the next frame, so as to form video SAR imaging.

[0053] Another object of the present invention is to provide a readable storage medium having a computer program stored thereon, wherein the program implements the steps of the above method when executed by a processor.

[0054] Another object of the present invention is to provide an electronic device, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, wherein the processor implements the steps of the above method when executing the program.

[0055] The present invention separates and fuses the spectrum through the time-frequency correspondence. Before the spectrum is fused, the two parts of the signal are compensated for the self-focusing error to eliminate the influence of the phase error on the imaging. After the error compensation, the spectrum of the overlapping part is used for phase correlation processing to restore the coherence characteristics, effectively reducing the discontinuity and defocusing phenomenon in the imaging, thereby ensuring that the high-frame rate and high-resolution image is well focused and output stably, and the correlation characteristics between continuous frame images are used to improve the efficiency of fast imaging in the time domain, and a "sliding" update is realized in the frequency domain, thereby avoiding the amount of calculation caused by the interpolation operation in the image domain, and realizing fast high-frame rate and high-resolution imaging. The problem that fast high-frame rate and high-resolution imaging cannot be realized at the same time in the prior art is solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is a flow chart of a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0057] Figure 2 It is a geometric structure diagram of a mobile platform video SAR in a spotlight mode in a mobile platform-based video SAR fast imaging method in one embodiment of the present invention;

[0058] Figure 3 It is a schematic diagram comparing the conventional SAR and video SAR imaging modes in the video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0059] Figure 4 It is a schematic diagram of image spectrum space-varying correction under GCBP in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0060] Figure 5 It is a schematic diagram of phase error coherent recovery in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0061] Figure 6 It is a schematic diagram of spectrum separation and fusion in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0062] Figure 7 It is a schematic diagram of spectrum separation and fusion of a point target simulation experiment in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0063] Figure 8 It is a schematic diagram of imaging results before and after restoring phase coherence in a point target simulation experiment in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0064] Fig. 9It is a schematic diagram of the first frame and the second frame imaging results of a surface target simulation experiment in a video SAR fast imaging method based on a mobile platform in one embodiment of the present invention;

[0065] Fig.10 It is a structural block diagram of a video SAR rapid imaging system based on a mobile platform in the second embodiment of the present invention.

[0066] The following specific implementation manner will further illustrate the present invention in conjunction with the above-mentioned drawings. DETAILED DESCRIPTION

[0067] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. Several embodiments of the present invention are given in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive.

[0068] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0069] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0070] Embodiment 1

[0071] See also Figure 1 , which shows a video SAR rapid imaging method based on a mobile platform in a first embodiment of the present invention, and the method includes steps S10 to S13.

[0072] Step S10: establishing an echo signal model of the video SAR based on the mobile platform, and using the echo signal model to obtain a corresponding echo signal according to a signal emitted by the video SAR system of the mobile platform.

[0073] The embodiment of the present invention is based on the mobile platform video SAR configuration in the spotlight mode. Figure 2 As shown in the figure, during the data collection process, the radar beam of the mobile platform always points to the center of the scene. In the ground Cartesian coordinate system, represents the slow time of the azimuth angle, then the instantaneous ideal antenna phase center can be expressed as , there is any point in the scene ,in, x 0 , y 0 Respectively represent the horizontal and vertical coordinate values, or represent the azimuth and distance directions, the instantaneous ideal antenna phase center to the target The instantaneous slope distance is , which is:

[0074] ;

[0075] In the specific implementation, it is assumed that the video SAR system transmits a linear frequency modulation signal (LFM). After demodulation and range compression, the target The echo signal can be expressed as:

[0076] ;

[0077] in, Indicates the distance to the fast time, Indicates the slow time of the direction, represents the signal bandwidth, represents the wavelength of the carrier frequency, represents the speed of light, Indicates the target The azimuth window, Indicates the instantaneous ideal antenna phase center to the target The instantaneous slope distance, j Represents an imaginary unit.

[0078] Step S11, performing sub-aperture segmentation on the echo signal, using the first sub-aperture signal to complete full-aperture imaging to generate a first frame image, and then separating the overlapping aperture signal spectrum from the spectrum of the first frame image using the time-frequency correspondence relationship.

[0079] Among them, Figure 3 As shown, the video SAR of the embodiment of the present invention is different from the traditional SAR mode. In the traditional SAR, in order to achieve the ideal azimuth resolution, a long time of aperture synthesis is required, which may cause obvious inter-frame delay and image discontinuity in video imaging. Video SAR ensures high coherence and smooth transition between images by using highly overlapping synthetic apertures in each frame imaging, so that high-resolution and high-frame-rate image capture can be achieved even in rapidly changing scenes.

[0080] First, the relationship between the azimuth resolution and frame rate of each frame is analyzed. It is known that the non-overlapping frame rate of video SAR is the inverse of the synthetic aperture time, which can be expressed as:

[0081] ;

[0082] in, Indicates the azimuth resolution, Indicates the carrier frequency, represents the platform equivalent speed, represents the central beam slant angle, represents the shortest slope distance, c represents the speed of light;

[0083] In order to ensure the high resolution in azimuth, the above formula can obtain a higher frame rate by increasing the carrier frequency and reducing the ratio of the shortest slant range to the velocity. However, for micro-SAR with low carrier frequency and limited imaging conditions, the imaging frame rate can be increased by overlapping sub-apertures to achieve high resolution requirements. Therefore, the overlapping frame rate of video SAR can be expressed as:

[0084] ;

[0085] in, represents the overlap ratio of apertures, Indicates the azimuth resolution, Indicates the carrier frequency, represents the platform equivalent speed, represents the central beam slant angle, represents the shortest slope distance, c Represents the speed of light.

[0086] Therefore, for a video SAR with a given frame rate, the higher the azimuth resolution of the frame image, the greater the aperture overlap. Similarly, when the azimuth resolution is the same, the higher the frame rate, the greater the aperture overlap.

[0087] Specifically, in SAR imaging, the fast back projection algorithm (CFBP) of rectangular coordinates is to establish an imaging grid on an inclined plane, which may cause geometric deformation and increase in the amount of calculation in the coordinate conversion from the slant range plane to the ground range plane. Therefore, the embodiment of the present invention uses the fast back projection algorithm (GCBP) of ground rectangular coordinates to establish an imaging grid on the ground, and the center of the imaging grid is located at the center of the beam. Like the CFBP algorithm, the image is superimposed by a simple translation of the local rectangular coordinate system, avoiding the interpolation processing in the polar coordinate system. And by introducing the wave number spectrum compression kernel in the rectangular coordinate system, more efficient resolution improvement and image synthesis are achieved, reducing the computational burden. Wave number representation is necessary for the effective estimation of motion error. The essence of motion error is the dynamic offset of the geometric relationship between the radar and the target. Such error is mainly manifested as phase distortion in the echo signal, and the frequency-space coupling characteristics of the wave number domain provide key support for the accurate modeling of phase error. Among them, the imaging process includes spectrum separation and fusion.

[0088] Therefore, based on the stationary phase principle (POSP), the first sub-aperture signal is transformed into the range frequency domain through Fourier transform:

[0089] ;

[0090] in, represents the range frequency, represents the center frequency, represents the signal bandwidth, is the goal The azimuth window, Indicates the instantaneous ideal antenna phase center to the target The instantaneous slope distance, j represents the imaginary unit, Indicates the speed of light, wave number is the magnitude of the wave number vector of the echo signal, .

[0091] The GCBP algorithm is used to perform imaging processing on the first sub-aperture signal received to obtain the signal representation of the first frame image:

[0092] ;

[0093] Among them, the wave number is the magnitude of the wave number vector of the echo signal, j represents the imaginary unit, It represents the instantaneous slant distance from the instantaneous ideal antenna phase center to a target, and the wave number Decompose along the three-dimensional space coordinate axis direction, that is , can be obtained at any time , the target's azimuth wavenumber and distance wavenumber can be expressed as and Then, the above formula is transformed into an integral to obtain n The signal representation of the frame image is as follows:

[0094] ;

[0095] Among them, and It can be seen that the two-dimensional wave number domain is not only related to the azimuthal (sub-aperture length) and the position of the target point (scene range). In polar coordinates, it can only be determined by the sub-aperture length, so the influence of the scene size needs to be removed. In addition, the existence of aliasing in the spectrum will make the azimuth sampling rate high, and the spectrum cannot be directly processed uniformly to achieve the separation and fusion of aperture signals. Therefore, a spectrum space-variant correction function is designed to align the image spectrum along the azimuth direction, providing favorable conditions for achieving unified spectrum processing. The overlapping aperture signal spectrum is then separated from the two-dimensional spectrum of the first frame of the image using the time-frequency correspondence.

[0096] The two-dimensional wave number can be regarded as a continuously changing process during the subaperture accumulation time, so the subaperture two-dimensional spectrum center of the target can be regarded as determined by the two-dimensional wave number corresponding to the subaperture center moment. For the n The antenna phase center (APC) at the moment when the frame image corresponds to the aperture center, represents the sub-aperture azimuth center moment, and the designed spectrum space-varying correction function is expressed as follows:

[0097] ;

[0098] in, j represents the imaginary unit, , and They respectively represent the azimuth position, distance position and height corresponding to the sub-aperture center azimuth moment, represents the beam vector center, represents the wavelength of the carrier frequency, Represents the range spectrum of the target after the first step of spectrum correction, represents the range spectrum of the target, represents the center of the target’s distance spectrum, Indicates the scene distance center.

[0099] go through , Two-step calibration, step one The purpose is to align the center of the wavenumber support area of ​​all targets with the coordinate origin, which can be directly compensated in the image generated by BP. The purpose is to correct the space-variant azimuth spectrum spread caused by spectrum tilt and compensate it in the range frequency domain and azimuth time domain.

[0100] Due to the flexibility of the trajectory of the mobile platform, the analysis based on the squint focusing mode brings higher resolution. After the initial first step of spectrum correction, the spectrum will be tilted, and its tilt angle is approximately equal to the squint angle. , which is the angle between the slant distance and the y-axis. The slant angle is expressed as:

[0101] ;

[0102] Therefore, two translations in the range and azimuth directions are realized by multiplying the phase in the range and azimuth frequency domains respectively, instead of spectrum rotation for correction. The translation function can be expressed as:

[0103] ;

[0104] in, and They represent the corrected azimuth and distance spectra respectively. After the spectrum rotation correction and the second step correction, the wavenumber spectrum width in the rectangular coordinate system is the same as that in the polar coordinate system, which is only related to the length of the sub-aperture and is no longer affected by the scene size.

[0105] The corrected two-dimensional spectrum of the image can be expressed as:

[0106] ;

[0107] in, and Respectively represent the target P The azimuth and distance spectra after spectrum correction;

[0108] Specifically, Figure 4 As shown, Figure 4 (a) is a schematic diagram of the uncorrected aliasing spectrum. Figure 4 (b) is a schematic diagram of the image spectrum distribution after the first step of correction. Figure 4 (c) is a schematic diagram of spectrum distribution after spectrum rotation. Figure 4 (d) is a schematic diagram of the image spectrum distribution after the second step of correction. After correction, the distribution range of the spectrum no longer varies with the position of the point target, and is aligned along the azimuth. The corrected spectrum facilitates the rapid separation and fusion of the spectrum corresponding to the synthetic aperture when each frame of the subsequent video SAR image is updated.

[0109] The above analysis shows that the spectra of all grid points are aligned after spectrum compression. Assume that the center coordinates of the imaging scene are , then the central frequency points of the range spectrum and azimuth spectrum of the sub-aperture image are and , the corresponding rectangular coordinate system is and the distance spectrum is , the spectrum width is calculated and can be expressed as:

[0110] ;

[0111] in, and Represents the broadening coefficient. The azimuth distance grid resolution is obtained through the spectrum width, and the spectrum corresponding to the grid point is calculated:

[0112] ;

[0113] in, and are the central frequency points of the range spectrum and azimuth spectrum of the sub-aperture image, respectively. is the azimuthal grid spectrum, is the distance grid spectrum, and Represents the azimuth and distance resolution respectively. Since the distance spectrum has spatial variability, but the variability is small, the range of the distance spectrum does not change after spectrum compression, and the effective emission frequency can be calculated by the range of the distance spectrum of the image. Therefore, through the two-dimensional spectrum after two-step correction, the corresponding relationship between the azimuth spectrum and the azimuth time can be obtained as follows:

[0114] ;

[0115] in, represents the center coordinates of the imaging scene, Indicates any time at any location The effective transmission frequency, Indicates any time at any location The distance grid point corresponds to the frequency, Indicates any time at any location The corresponding aperture azimuth position, It represents the instantaneous slant distance from the instantaneous ideal antenna phase center to the target. , They respectively represent the azimuth position and distance position corresponding to the sub-aperture center azimuth moment.

[0116] According to the time-frequency correspondence of the above formula, the grid position of the non-overlapping part of the aperture of the previous frame can be determined by calculating the mapping of the azimuth and time, thereby achieving effective separation of the spectrum.

[0117] Specifically, based on the spectrum first-in-first-out, using the aperture overlap rate that meets the video SAR frame rate and high resolution, the spectrum grid position interval corresponding to the starting azimuth time and the ending azimuth time of the non-overlapping aperture is calculated, and the image spectrum part within the spectrum grid position interval is separated to separate the overlapping aperture signal spectrum.

[0118] Specifically, based on the spectrum first input first output (FIFO), the aperture overlap ratio that meets the video SAR frame rate and high resolution is used to calculate the non-overlapping aperture starting azimuth time. and end position time The corresponding frequency spectrum grid position interval separates the image spectrum part within the interval. n The frame aperture position time interval can be expressed as:

[0119] ;

[0120] in, represents the first frame full synthetic aperture length, represents the platform equivalent speed, is the overlap ratio.

[0121] Its separated azimuth spectrum interval:

[0122] ;

[0123] in, , represents the window function.

[0124] Step S12, performing self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and restoring the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using the phase correlation method.

[0125] Among them, Figure 5 As shown, before fusion, the new non-overlapping aperture signals are Compared with the original partial aperture signal The self-focusing error is estimated and expressed in the phase history domain as and After error compensation, the influence of aperture phase error on imaging is eliminated, and the phase correlation method is used to restore the phase coherence characteristics of the new aperture and the previous frame image, so that the entire aperture is continuous.

[0126] Furthermore, the error compensation implementation process is as follows:

[0127] The distribution range of the image spectrum before correction varies with the position of the target point. Therefore, the phase error function also has corresponding space-varying characteristics in the phase history domain, which makes it difficult to directly estimate and compensate for the phase error. Therefore, after the space-varying compensation of the image spectrum, the distribution of the spectrum is aligned along the azimuth, and the phase error function also has consistency along the azimuth. The accurate effective spectrum support area can be selected through the time-frequency correspondence, and the spectrum data phase outside the effective area can be discarded. This method does not require error estimation for the complete aperture image of each frame, but only requires error analysis and correction for the current update frame, thereby significantly improving computational efficiency and resource utilization. The present invention uses a phase gradient autofocus (PGA) algorithm to obtain the phase error function of each frame of the aperture image, which can be expressed as:

[0128] ;

[0129] in, Indicates the azimuth time corresponding to the spectrum range interval, represents the instantaneous slant range error value estimated by the phase gradient autofocusing algorithm, Represents the center of the beam vector. After error compensation, the original image spectrum information can be restored, and finally it can be transformed into two-dimensional time domain through azimuthal inverse Fourier transform.

[0130] Furthermore, the phase correlation method is used to restore coherence as follows:

[0131] Since the autofocus compensation method is performed in the phase history domain of the first frame aperture image and the subsequent updated frame aperture sub-images, the error estimation and compensation process will most likely destroy the phase coherence between the images, causing discontinuity and defocusing of the next frame image. Therefore, it is necessary to restore the phase error coherence between adjacent frame apertures. The restored coherence is mainly manifested as the difference in the linear phase component left by adjacent frame apertures after error compensation, which can be expressed as ,in and Represents the displacement of adjacent frame images along the X-axis and Y-axis in space.

[0132] The present invention uses the phase correlation method to realize the spatial alignment of adjacent frame aperture images, and extracts the first The spectrum corresponding to the new non-overlapping aperture signal of the frame The previous frame The frame image corresponds to the aperture that retains a small part of the overlapping spectrum , the spectrum and spectrum Move to the same spectrum center;

[0133] By calculating the spectrum and spectrum The cross power spectrum between them is obtained and inverse Fourier transform is performed to locate the pulse peak position to obtain the offset between the aperture segment images. Then, linear phase multiplication is applied in the frequency domain of the image to achieve precise spatial alignment of the image. Specifically, the calculation formula of the cross power spectrum function is:

[0134] ;

[0135] in, Represents the conjugate operation. The peak position obtained by the phase correlation method is often limited to the pixel level. The polynomial fitting method in the local area can be used to refine the position to determine the precise offset. and . Linear phase multiplication is then applied in the frequency domain of the image to achieve accurate spatial alignment of the image, which can be expressed as:

[0136] ;

[0137] in, represents the Fourier transform, Represents the inverse Fourier transform. The above operation restores the phase consistency between adjacent frames.

[0138] Step S13, after the coherence is restored, the new non-overlapping aperture signal spectrum is fused with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image, and the overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame are repeated to realize the image generation of the next frame, so as to form video SAR imaging.

[0139] Specifically, by calculating the mapping of the non-overlapping part of the aperture of the previous frame in azimuth and time, its grid position in the spectrum space is determined, thereby achieving effective separation of the spectrum. On this basis, these spectral components are multiplied by the phase , can be translated to the center frequency position of the next frame aperture to achieve seamless splicing and integration with the current updated frame spectrum. This method not only maintains the integrity of time-frequency information, but also improves the accuracy and efficiency of spectrum splicing. The phase is expressed as follows:

[0140]

[0141] in, , and Respectively represent The frame aperture azimuth moment corresponds to the azimuth position, distance position and altitude, j represents the imaginary unit, represents the beam vector center.

[0142] Next, it is converted to the frequency domain to achieve additive fusion, and then a two-dimensional inverse Fourier transform is performed to generate a new frame of image, as follows:

[0143] ;

[0144] in, Indicates n Time domain image after frame correction, represents the two-dimensional Fourier transform, Represents the two-dimensional inverse Fourier transform.

[0145] Further, such as Figure 6 As shown in the figure, the high frame rate time domain fast imaging algorithm designed by the present invention is based on the spectrum first-in first-out (FIFO) update processing. The concept of FIFO comes from the execution mode of the queue in the data structure, that is, the data that enters the queue first also leaves the queue first. It is roughly divided into three steps: 1) Perform GCBP imaging processing on the non-overlapping aperture signals corresponding to the updated frame image to obtain a sub-image , and then respectively n Frame Image and sub-image Perform spectrum space-variant correction to align the spectrum distribution along the azimuth direction; 2) According to the time-frequency correspondence, separate the image spectrum part corresponding to the aperture signal to be removed, and then 3) Perform a two-dimensional Fourier inverse transform of the synthesized image spectrum signal to the image domain, and restore the phase by referring to the conjugate form of the correction function, and finally obtain the new n +1 frame image.

[0146] From n The frame image is updated to n The computation time of the +1 frame image is mainly concentrated in the GCBP imaging processing of the non-overlapping aperture signals of the two apertures, while the other operations only include fast Fourier transform and complex multiplication and addition. In the high frame rate imaging mode, the non-overlapping aperture length is usually much smaller than the entire aperture length. Therefore, compared with the GCBP processing of the full aperture signal, the designed spectrum FIFO processing method can significantly shorten the first n +1 frame of image imaging time, processing efficiency is significantly improved.

[0147] In addition, the technical effects of the embodiments of the present invention are described in detail below in combination with point and surface target simulation experiments.

[0148] 1. Simulation Experiment

[0149] Some parameters used in the simulation of the present invention are shown in Table 1.

[0150] Table 1

[0151]

[0152] The band is Ku, the bandwidth is 180MHz, the sampling frequency is 200MHz, the pulse repetition frequency is 600Hz, and the radar platform is based on Figure 1 The curve flight shown has a flight altitude of about 5000m, a radar platform movement speed of 200m / s, a platform downward speed in the Z direction of height of 5m / s, and a scene size of about 335m×335m (X direction×Y direction). System environment: 64-bit Windows 10, CPU: i7-9700; Memory: 16GB.

[0153] 2. Experimental results

[0154] Figure 7 (a) shows the image spectrum after the aperture separation of the first frame and the space-variant correction; Figure 7 (b) shows the image spectrum of the second frame updated sub-image after space-variant correction; Figure 7 (c) shows the spectrum of the two spectra after the phase coherence is restored; Figure 8 (a) and (b) in the figure show the target imaging result before restoring the phase error coherence and the focusing effect of the target point. Figure 8 (c) and (d) show the point target imaging result after restoring the phase error coherence and the focusing effect diagram of the target point. Fig. 9 (a) shows the first frame of SAR complex image of the surface target experiment using the proposed method; Fig. 9 (b) in the figure shows the second frame SAR complex image that is updated.

[0155] The video SAR imaging method based on a mobile platform in an embodiment of the present invention designs a new time-domain fast imaging algorithm according to the signal characteristics of the mobile platform video SAR and the correlation characteristics between continuous frame images, thereby improving the high frame rate imaging performance of the mobile platform video SAR. Specifically: the original echo signal after range pulse compression is subjected to sub-aperture segmentation, and the first sub-aperture signal is subjected to full-aperture imaging to generate the first frame image. Subsequently, it is necessary to separate the original partial aperture signal spectrum from the previous frame image, and then fuse the new non-overlapping aperture signal spectrum to the previous frame image to generate a new frame image. Before fusion, the two parts are respectively compensated for the self-focusing error to eliminate the influence of the new aperture phase error on the imaging. The fused image may be discontinuous and defocused. The phase correlation method is used to restore the phase coherence characteristics of the new aperture and the previous frame image. Finally, the two apertures are spliced ​​and fused after removing the overlapping parts.

[0156] The advantage of this method is that it constructs an easy-to-process time-frequency correspondence in the image frequency domain, thereby performing signal separation and fusion processing in the frequency domain, thereby avoiding the increase in the amount of calculation caused by the interpolation operation in the image domain; and uses phase correlation processing to restore the phase coherence characteristics destroyed by the self-focusing error compensation. In the simulation test process, the feasibility and effectiveness of the method proposed in the embodiment of the present invention were verified.

[0157] In summary, the video SAR rapid imaging method based on a mobile platform in the above embodiment of the present invention separates and fuses the spectrum through the time-frequency correspondence relationship. Before the spectrum is fused, the two parts of the signal are compensated for the self-focusing error to eliminate the influence of the phase error on the imaging; after the error compensation, the spectrum of the overlapping part is used for phase correlation processing to restore the coherence characteristics, effectively reducing the discontinuity and defocusing phenomenon in the imaging, thereby ensuring that the high-frame rate and high-resolution image is well focused and stably output, and the correlation characteristics between continuous frame images are used to improve the efficiency of time-domain rapid imaging, and "sliding" updates are realized in the frequency domain, thereby avoiding the amount of calculation caused by the interpolation operation in the image domain, and realizing fast high-frame rate and high-resolution imaging. The problem that fast high-frame rate and high-resolution imaging cannot be realized at the same time in the prior art is solved.

[0158] Embodiment 2

[0159] See also Fig.10 , which is a video SAR rapid imaging system based on a mobile platform in a second embodiment of the present invention, the system comprises:

[0160] An acquisition module is used to establish an echo signal model of a video SAR based on a mobile platform, and obtain a corresponding echo signal according to a signal transmitted by the video SAR system of the mobile platform using the echo signal model;

[0161] A segmentation module is used to perform sub-aperture segmentation on the echo signal, complete full-aperture imaging using the first sub-aperture signal to generate a first frame of image, and then separate the overlapping aperture signal spectrum from the spectrum of the first frame of image using the time-frequency correspondence relationship;

[0162] A compensation module is used to perform self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and to restore the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using a phase correlation method;

[0163] The imaging module is used to fuse the new non-overlapping aperture signal spectrum after restoring coherence with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image, and repeatedly perform the overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame to realize the image generation of the next frame, so as to form video SAR imaging.

[0164] The functions or operation steps implemented when the above modules are executed are substantially the same as those in the above method embodiments, and will not be repeated here.

[0165] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0166] Those skilled in the art will appreciate that the logic and / or steps represented in the flowchart or otherwise described herein, for example, may be considered as an ordered list of executable instructions for implementing logical functions, and may be specifically implemented in any computer-readable storage medium for use by an instruction execution system, device or apparatus (such as a computer-based system, a system including a processor, or other system that can fetch instructions from an instruction execution system, device or apparatus and execute instructions), or in conjunction with such instruction execution systems, devices or apparatuses. For purposes of this specification, a "computer-readable storage medium" may be any device that can contain, store, communicate, propagate or transmit a program for use by an instruction execution system, device or apparatus, or in conjunction with such instruction execution systems, devices or apparatuses.

[0167] More specific examples (a non-exhaustive list) of computer-readable storage media include the following: an electrical connection with one or more wires (electronic device), a portable computer disk case (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable and programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disk read-only memory (CDROM). In addition, the computer-readable storage medium may even be a paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, deciphering or, if necessary, processing in another suitable manner, and then stored in a computer memory.

[0168] It should be understood that the various parts of the present invention can be implemented by hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or a combination thereof: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0169] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0170] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A video SAR fast imaging method based on a mobile platform, characterized in that: The method comprises: Establish an echo signal model of the video SAR based on the mobile platform, and use the echo signal model to obtain the corresponding echo signal according to the signal emitted by the video SAR system of the mobile platform; Perform sub-aperture segmentation on the echo signal, use the first sub-aperture signal to complete full-aperture imaging to generate the first frame image, and then separate the overlapping aperture signal spectrum from the spectrum of the first frame image using the time-frequency correspondence relationship; The self-focusing error compensation is performed on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame are restored by using the phase correlation method; The new non-overlapping aperture signal spectrum after restoring coherence is fused with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image. The overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame are repeated to achieve the image generation of the next frame to form video SAR imaging.

2. The video SAR rapid imaging method based on a mobile platform according to claim 1 is characterized in that: The steps of performing sub-aperture segmentation on the echo signal, completing full-aperture imaging using the first sub-aperture signal to generate a first frame of image, and then separating the overlapping aperture signal spectrum from the spectrum of the first frame of image using the time-frequency correspondence relationship include: Sub-aperture segmentation is performed using a preset overlapping frame rate, and the overlapping frame rate is expressed as; ； in, Indicates the azimuth resolution, Indicates the carrier frequency, represents the platform equivalent speed, represents the central beam slant angle, represents the shortest slope distance, represents the overlap ratio, c represents the speed of light; Based on the stationary phase principle, the first sub-aperture signal is transformed into the range frequency domain through Fourier transform, and the GCBP algorithm is used to image the first sub-aperture signal received to obtain the signal representation of the first frame image: ； Among them, there is any point in the scene , The Jacobian determinant introduced for the substitution of integral variables is, and denote the magnitude of the wave number vector in the azimuth and range directions, respectively. j represents an imaginary unit; The first frame image is corrected by using a preset spectrum space-varying correction function to obtain a two-dimensional image spectrum, and then the overlapping aperture signal spectrum is separated from the two-dimensional image spectrum of the first frame image by using the time-frequency correspondence relationship.

3. The video SAR rapid imaging method based on a mobile platform according to claim 2, characterized in that: The spectrum space-varying correction function expression is: ; in, j represents the imaginary unit, , and They respectively represent the azimuth position, distance position and height corresponding to the sub-aperture center azimuth moment, represents the beam vector center, represents the wavelength of the carrier frequency, Represents the range spectrum of the target after the first step of spectrum correction, represents the range spectrum of the target, represents the distance spectrum center of the target, Indicates the scene distance center; After the first step of spectrum correction, the spectrum is shifted in distance and azimuth using the translation function, where the translation function is expressed as: ； in, and Respectively represent the corrected azimuth and distance spectra, An oblique viewing angle.

4. The video SAR rapid imaging method based on a mobile platform according to claim 3 is characterized in that: The step of separating the overlapping aperture signal spectrum from the two-dimensional spectrum of the first frame image by using the time-frequency correspondence relationship comprises: By establishing the center coordinates of the imaging scene The corresponding rectangular coordinate system azimuth spectrum and distance spectrum , calculate the spectrum width, expressed as: ； in, and represents the broadening coefficient; Calculate the spectrum corresponding to the grid point through the spectrum width: ； in, and represent the azimuth and distance resolutions, respectively. and are the central frequency points of the range spectrum and azimuth spectrum of the sub-aperture image, respectively. is the azimuthal grid spectrum, is the distance grid spectrum; The time-frequency correspondence is any azimuth time Spectrum with azimuth grid The corresponding relationship is expressed as: ; in, represents the center coordinates of the imaging scene, Indicates any time at any location The effective transmission frequency, Indicates any time at any location The distance grid point corresponds to the frequency, Indicates any time at any location The corresponding aperture azimuth position, Indicates the instantaneous slant range from the instantaneous ideal antenna phase center to the target; Based on the spectrum first-in-first-out, the aperture overlap ratio that meets the video SAR frame rate and high resolution is used to calculate the non-overlapping aperture starting azimuth time. and end position time Corresponding to the spectrum grid position interval, the image spectrum part within the spectrum grid position interval is separated therefrom to separate the overlapping aperture signal spectrum.

5. The video SAR rapid imaging method based on a mobile platform according to claim 4 is characterized in that: The steps of respectively performing self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame, and restoring the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using the phase correlation method include: Extract the spectrum corresponding to the new non-overlapping aperture signal through the time-frequency correspondence Spectrum of the overlapped aperture signal from the previous frame , the spectrum and spectrum Move to the same spectrum center; By calculating the spectrum and spectrum The cross power spectrum between them is obtained and inverse Fourier transform is performed to locate the pulse peak position to obtain the offset between the aperture segment images. Then linear phase multiplication is applied in the frequency domain of the image to achieve spatial alignment of the image to restore the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame.

6. The video SAR rapid imaging method based on a mobile platform according to claim 5, characterized in that: The step of fusing the new non-overlapping aperture signal spectrum after coherence restoration with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image comprises: The new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame are converted to the time domain by two-dimensional inverse Fourier transform, and multiplied by the phase , so that it is moved to the central frequency position of the next frame aperture, and then converted to the frequency domain to achieve additive fusion, and then a two-dimensional inverse Fourier transform is performed to generate a new frame of image; in, The expression is: in, j represents the imaginary unit, represents the beam vector center, represents the wavelength of the carrier frequency, , and Respectively represent The frame aperture azimuth moment corresponds to the azimuth position, distance position and altitude.

7. The video SAR rapid imaging method based on a mobile platform according to claim 1, characterized in that: The step of establishing an echo signal model based on the video SAR of the mobile platform and obtaining a corresponding echo signal according to a signal emitted by the video SAR system of the mobile platform by using the echo signal model comprises: The signal emitted by the video SAR system of the mobile platform is demodulated and range compressed, and the corresponding echo signal is expressed as: ; in, Indicates the distance to the fast time, Indicates the slow time of the direction, represents the signal bandwidth, represents the wavelength of the carrier frequency, represents the speed of light, Indicates the target The azimuth window, Indicates the instantaneous ideal antenna phase center to the target The instantaneous slope distance, j Represents an imaginary unit.

8. A video SAR rapid imaging system based on a mobile platform, characterized in that: The system comprises: An acquisition module is used to establish an echo signal model of a video SAR based on a mobile platform, and obtain a corresponding echo signal according to a signal transmitted by the video SAR system of the mobile platform using the echo signal model; A segmentation module is used to perform sub-aperture segmentation on the echo signal, complete full-aperture imaging using the first sub-aperture signal to generate a first frame of image, and then separate the overlapping aperture signal spectrum from the spectrum of the first frame of image using the time-frequency correspondence relationship; A compensation module is used to perform self-focusing error compensation on the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame respectively, and to restore the phase coherence characteristics of the new non-overlapping aperture signal spectrum and the overlapping aperture signal spectrum of the previous frame by using a phase correlation method; The imaging module is used to fuse the new non-overlapping aperture signal spectrum after restoring coherence with the overlapping aperture signal spectrum of the previous frame to generate a new frame of image, and repeatedly perform the overlapping aperture signal spectrum separation of the previous frame and the non-overlapping aperture signal spectrum fusion of the updated frame to realize the image generation of the next frame, so as to form video SAR imaging.

9. A readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 7 are implemented.

10. An electronic device, characterized in that: The method comprises a memory, a processor and a computer program stored in the memory and running on the processor, wherein the processor implements the steps of the method according to any one of claims 1 to 7 when executing the program.

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