Time-frequency mixed domain SAR (Synthetic Aperture Radar) imaging method

Abstract

The invention discloses a time-frequency mixed domain synthetic aperture radar (SAR) imaging method. The method comprises the following steps: acquiring an SAR echo signal; the method comprises the following steps: carrying out range pulse pressure processing on an SAR echo signal, carrying out cross coupling compensation on a range frequency domain echo signal subjected to range pulse pressure through azimuth angle preprocessing, then carrying out range direction first-order space-variant correction on the echo signal subjected to cross coupling compensation by adopting a time-frequency scaling conversion method, and finally, carrying out range direction first-order space-variant correction on the echo signal subjected to range direction first-order space-variant correction on the echo signal subjected to range direction first-order space-variant correction. The method comprises the following steps of: performing azimuth direction first-order space-variant correction on an echo signal subjected to range direction first-order space-variant correction through an extended keystone transformation technology, then performing high-order space-variant correction on the echo signal subjected to azimuth direction first-order space-variant correction through a filter bank compensation technology, and obtaining an SAR image by using a correction result. According to the method, the significant suppression of two-dimensional cross coupling and the improvement of space-variant correction performance are realized, the imaging quality of the SAR is significantly improved, and the method can effectively adapt to a complex imaging scene under a large diving track of a mobile platform.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of radar, and particularly relates to a time-frequency hybrid domain SAR imaging method. BACKGROUND

[0002] Synthetic Aperture Radar (SAR) is a remote sensing technology that can provide high-resolution radar images under any lighting and weather conditions. With the increasing application demand and diversification of monitoring tasks, SAR systems have been installed on a variety of platforms such as aircraft, satellites, and mobile aircraft. Unlike spaceborne SAR and airborne SAR, mobile aircraft SAR can provide rapid response, flexible trajectory adjustment, and wide-area coverage, which makes it valuable in tasks such as emergency rescue.

[0003] Mobile aircraft SAR usually works in a diving mode, and the flight height of the platform changes greatly with slow time. When the mobile aircraft SAR platform moves along a large diving trajectory, its significant vertical speed destroys the assumption of azimuth invariance relied on by traditional SAR imaging, resulting in the failure of conventional imaging models. Moreover, the large diving trajectory has a strong nonlinear characteristic, and this flight trajectory deviation cannot be simply regarded as motion error, which makes the traditional motion compensation algorithm no longer applicable. Therefore, high-order motion parameters such as acceleration and jerk must be considered in the distance history modeling process. In addition, at the end of the flight of the mobile aircraft SAR platform, the platform flies along a curved path to the area of interest, the beam pointing is close to the flight direction, and the radar's azimuth angle and incidence angle are small, which makes the signal space sampling non-uniform, the data recording space non-planar, and the slant range history fast time-varying, resulting in serious signal cross-coupling and significant space variation characteristics, which leads to limited focusing depth or even failure of the imaging algorithm.

[0004] In existing diving trajectory SAR imaging algorithms, researchers have derived wave number domain imaging algorithms and time domain variable scale algorithms based on equivalent distance models without acceleration, but these methods only consider the impact of vertical speed and ignore the nonlinear characteristics of flight trajectory. For example, the constant acceleration wave number domain algorithm proposed by Li Zhenyu et al. eliminates the impact of acceleration on the spectrum, but does not solve the space variation problem caused by acceleration. Party Yanfeng et al. proposed a distance balancing method to reduce the azimuth space variation of distance cells caused by acceleration, but this algorithm does not consider the impact of residual high-order motion parameters on imaging quality. Wang Fengfei proposed a polar coordinate imaging method based on a spatial polar coordinate slant range model, which greatly eliminates the first-order azimuth space variation of the signal, but does not solve the residual high-order space variation problem in mobile diving trajectories. These algorithms have great approximations in model and algorithm under large diving trajectories, resulting in limited space variation correction capability and insufficient algorithm accuracy.

[0005] Therefore, there is an urgent need for a high-precision maneuvering platform large-dive trajectory SAR imaging method, which is of great significance to improve the imaging performance and adaptability of the maneuvering platform SAR system in complex environments. SUMMARY

[0006] In order to solve the above problems existing in the prior art, the present application provides a time-frequency hybrid domain SAR imaging method.

[0007] The technical problem to be solved by the present application is solved by the following technical scheme: A time-frequency hybrid domain SAR imaging method, comprising acquiring a SAR echo signal; performing range pulse compression processing on the SAR echo signal to obtain a range pulse compressed range frequency domain echo signal; performing cross-coupling compensation on the range frequency domain echo signal through azimuth pre-processing to obtain a cross-coupling compensated range frequency domain echo signal; performing range first-order space-variant correction on the cross-coupling compensated range frequency domain echo signal by using a time-frequency variable scaling conversion method to obtain a range first-order space-variant corrected range frequency domain echo signal; performing azimuth first-order space-variant correction on the range first-order space-variant corrected range frequency domain echo signal by using an extended keystone transformation technology to obtain an azimuth first-order space-variant corrected range frequency domain echo signal; performing high-order space-variant correction on the azimuth first-order space-variant corrected range frequency domain echo signal by using a filter bank compensation technology, and obtaining a SAR image by using the correction result.

[0008] Optionally, the range first-order space-variant correction on the cross-coupling compensated range frequency domain echo signal by using the time-frequency variable scaling conversion method comprises: multiplying a variable scaling function with the cross-coupling compensated range frequency domain echo signal to obtain a variable scaling converted range frequency domain echo signal; performing convolution reconstruction on the variable scaling converted range frequency domain echo signal to obtain a reconstructed time domain echo signal; multiplying a first-order space-variant compensation function with the reconstructed time domain echo signal to obtain a range first-order space-variant corrected time domain echo signal; performing range Fourier transform on the range first-order space-variant corrected time domain echo signal to obtain a range first-order space-variant corrected range frequency domain echo signal.

[0009] Optionally, the variable scaling function is represented as: ; wherein, To represent a scaled function, Indicates the range frequency. Indicates slow time. Represents the imaginary unit. Indicates the time-frequency scaling conversion coefficient. , express The first-order distance-to-space variation coefficient, The slant range history at time zero is the first... Taylor expansion coefficients.

[0010] Optionally, the convolution function used for the convolution reconstruction is: ; in, This represents the convolution function.

[0011] Optionally, the first-order spatially variable compensation function is expressed as: ; in, Indicates a fast time. Indicates the carrier frequency. Represents the linear compensation coefficient. , This represents the first-order spatially variable compensation function.

[0012] Optionally, in the extended keystone transformation technique, a new keystone transformation relationship is defined, expressed as: ; in, Indicates the range frequency. Indicates slow time. Indicates the slow time after the transformation. Indicates the carrier frequency. Represents the velocity vector of the SAR platform. This represents the modulo operation. express and The included angle, This represents the distance vector from the SAR platform to the central reference point at the azimuth center time. express The first-order azimuth spatial variation coefficient, express The first-order distance-to-space variation coefficient, The slant range history at time zero is the first... Taylor expansion coefficients, It represents the imaginary unit.

[0013] Optionally, the high-order spatially variant correction on the azimuth first-order spatially variant corrected range frequency domain echo signal is performed by using a filter bank compensation technique, and a SAR image is obtained by using a correction result, comprising: performing inverse Fourier transform on the azimuth first-order spatially variant corrected range frequency domain echo signal to obtain a range time domain echo signal; performing correction on the range time domain echo signal by using a range high-order compensation filter to obtain a range high-order spatially variant corrected time domain echo signal; performing azimuth Fourier transform on the range high-order spatially variant corrected time domain echo signal to obtain a coarse focusing image, performing block division on the coarse focusing image along the azimuth direction to obtain an azimuth sub-block, and performing inverse azimuth Fourier transform on a signal of the azimuth sub-block to obtain an azimuth direction sub-signal in the azimuth time domain; performing correction on the azimuth direction sub-signal by using an azimuth high-order spatially variant compensation filter to obtain an azimuth high-order spatially variant corrected azimuth direction sub-signal; performing block division on the azimuth high-order spatially variant corrected azimuth direction sub-signal along the range direction to obtain a range sub-block, and performing inverse range Fourier transform on a signal of the range sub-block to obtain a range time domain sub-signal; performing correction on the range time domain sub-signal by using a cross-coupling spatially variant compensation filter to obtain a cross-coupling spatially variant corrected range time domain sub-signal; performing range Fourier transform on the cross-coupling spatially variant corrected range time domain sub-signal to obtain a focusing sub-image, and splicing the focusing sub-image to obtain a SAR image.

[0014] Optionally, the range high-order compensation filter is represented as: ; wherein, c represents a speed of light, a second-order range spatially variant coefficient of a second-order range spatially variant coefficient of a third-order range spatially variant coefficient of a third-order range spatially variant coefficient of a third-order range spatially variant coefficient of a third-order range spatially variant coefficient of and respectively represent a second-order Taylor expansion coefficient and a third-order Taylor expansion coefficient of a slant range history at a zero time point, represents a distance from a SAR platform to an arbitrary target point at a center time point of an azimuth angle, , represents a function relationship of a slow time and represents a range high-order compensation filter.

[0015] ​Optionally, the azimuth high-order space-variant compensation filter is represented as: ; wherein, represents a second-order azimuth space-variant coefficient of represents a second-order azimuth space-variant coefficient of represents a third-order azimuth space-variant coefficient of represents an azimuth space-variant variable of a center point of an azimuth sub-block, represents an azimuth high-order space-variant compensation filter.

[0016] Optionally, the cross-coupling space-variant compensation filter is represented as: ; wherein, represents a cross-coupling item of represents an azimuth space-variant variable from a center point of a sub-block, represents a reference slant distance from the center point of the sub-block, represents a cross-coupling space-variant compensation filter.

[0017] The application provides a time-frequency hybrid domain SAR imaging method, which eliminates cross-coupling effects caused by vertical velocity, acceleration and high-order motion parameters through azimuth angle preprocessing technology, corrects first-order space variations in the range direction and the azimuth direction caused by large diving motion through a time-frequency variable conversion method and an extended keystone transformation technology, eliminates two-dimensional space variations and Doppler parameters of first-order space variations, corrects high-order azimuth space variations and range space variations by using a filter bank, and removes high-order two-dimensional space variations and cross-coupling space variations of Doppler parameters. The application realizes significant suppression of two-dimensional cross-coupling and improvement of space variation correction performance, significantly improves the imaging quality of SAR, and can effectively adapt to complex imaging scenes under large diving trajectories of a mobile platform.

[0018] The application will be further described in detail below with reference to the drawings and the application. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a flowchart of a time-frequency hybrid domain SAR imaging method provided by an embodiment of the application; Figure 2 is a geometric model of a SAR imaging system in a time-frequency hybrid domain SAR imaging method provided by an embodiment of the application; Figure 3 is a result graph of a point target spread function of a scene upper left edge point in a time-frequency hybrid domain SAR imaging method provided by an embodiment of the application; Figure 4 is a result graph of a point target spread function of a scene center point by a time-frequency hybrid domain SAR imaging method provided by an embodiment of the present application; Figure 5 is a result graph of a point target spread function of a scene right lower edge point by a time-frequency hybrid domain SAR imaging method provided by an embodiment of the present application; Figure 6 is a result graph of a point target spread function of a scene left upper edge point by a conventional SAR imaging method; Figure 7 is a result graph of a point target spread function of a scene center point by a conventional SAR imaging method; Figure 8 is a result graph of a point target spread function of a scene right lower edge point by a conventional SAR imaging method; Figure 9 is a result graph of a conventional SAR imaging method; Figure 10 is a result graph of a time-frequency hybrid domain SAR imaging method provided by an embodiment of the present application. DETAILED DESCRIPTION

[0020] The present application will be further described in details below in combination with specific embodiments, but the embodiments of the present application are not limited thereto.

[0021] In order to overcome the serious signal cross-coupling and significant space-variant characteristics caused by a SAR platform of a mobile aircraft moving along a large diving trajectory, so as to realize high-precision SAR imaging of a complex imaging scene under a large diving trajectory of a mobile SAR platform, an embodiment of the present application provides a time-frequency hybrid domain SAR imaging method applied to a mobile aircraft SAR platform moving along a large diving trajectory, referring to Figure 1 The method comprises the following steps: S10, acquiring a SAR echo signal.

[0022] Specifically, a SAR radar on the mobile aircraft SAR platform transmits a linear frequency modulation signal, and receives an echo signal of a ground target wherein represents a fast time, represents a slow time.

[0023] Since when the mobile aircraft SAR platform moves along a large diving trajectory, its significant vertical speed destroys the azimuth invariance assumption relied on by a conventional SAR imaging, resulting in failure of a conventional imaging model. Therefore, the present application first constructs a geometric model of a large diving trajectory mobile aircraft SAR imaging system, i.e. a distance-angle polar coordinate model, in which an instantaneous slant range is approximated as a fourth-order Taylor series. The geometric model is specifically as follows: Referring to Figure 2 , the SAR platform of the maneuverable aircraft follows a curved path flight, with a velocity vector , an acceleration vector , and a jerk vector , the SAR platform is located at point at time is set as the azimuth center time, is the distance vector from the center reference point , represents the distance vector from any target to , is the dive angle, is the sweep angle, is the azimuth angle, is the angle between and , represents the angle between and , represents the angle between and , and , , is the azimuth air vector.

[0024] When the height of the SAR platform of the maneuverable aircraft is , the instantaneous coordinates of the platform can be represented as: ; wherein , and represent the axis coordinate, axis coordinate and axis coordinate of the SAR platform of the maneuverable aircraft at the slow time .

[0025] The slant range history of any target in the scene can be represented as: ; wherein represents the slant range history, is the coordinate of point , and according to the geometric relationship in Figure 2 , we have: .

[0026] The coordinate of point is brought into the above slant range history expression, and a fourth-order Taylor series expansion is performed, so that the slant range history can be represented as: ; wherein, denotes the first Taylor expansion coefficient of the slant range history at zero time, denotes the first Taylor expansion coefficient of the slant range history at zero time, denotes the slant range history.

[0027] Specifically, the present application is directed to the scenario that the SAR platform of the maneuverable aircraft moves along a large-dive trajectory, and constructs a range-angle polar coordinate model on the imaging plane defined by the reference slant range vector and the velocity vector. The model helps to analyze and correct the spatial variation of the imaging parameters on the new imaging plane, and effectively matches the cross-range signals after eliminating the linear coupling term between the range and the azimuth angle, without the need for model correction. Meanwhile, the influence of high-order motion parameters is considered, providing higher precision and greater universality.

[0028] S20, the SAR echo signal is range pulse compression processed to obtain a range pulse compression processed range frequency domain echo signal.

[0029] Specifically, the SAR echo signal is range pulse compression processed to obtain a range pulse compression processed range frequency domain echo signal, including: S201, the received echo signal is subjected to a range direction Fourier transform to obtain a range frequency domain echo signal ; ; wherein, denotes the range direction frequency, denotes the range frequency modulation, denotes the imaginary unit, denotes the speed of light, denotes the carrier frequency.

[0030] S202, the range pulse compression function is multiplied by the range frequency domain echo signal to perform range pulse compression processing to obtain a range pulse compression processed range frequency domain echo signal .

[0031] Here, can be expressed as: ; wherein, denotes the range pulse compression function; then can be expressed as: ; wherein, denotes the range frequency domain echo signal with a range direction frequency and a slow time .

[0032] S30, cross-coupling compensation is performed on the range frequency domain echo signal through azimuth angle preprocessing, to obtain a range frequency domain echo signal after cross-coupling compensation.

[0033] Specifically, the azimuth angle preprocessing function is multiplied by the range frequency domain echo signal , so that the coupling term in the echo signal is eliminated, and a range frequency domain echo signal after cross-coupling compensation is obtained. In the present application, the azimuth angle preprocessing function is constructed as: ; wherein, denotes the azimuth angle preprocessing function, denotes the slant range history of the central reference point , denotes the nth order expansion coefficient of the slant range history of the central reference point , , denotes the instantaneous Doppler frequency of the central reference point .

[0034] At this time, the range frequency domain echo signal after cross-coupling compensation is expressed as: ; wherein, is , which denotes the nth order Taylor expansion coefficient of .

[0035] S40, a time-frequency variable conversion method is used to perform range first-order space variation correction on the range frequency domain echo signal after cross-coupling compensation, to obtain a range frequency domain echo signal after range first-order space variation correction.

[0036] Here, the first-order Taylor expansion coefficient of the slant range history at zero time in step S10 is subjected to two-dimensional Taylor series decomposition, to obtain: ; wherein, , and denote constant terms, denotes the first-order range space variation coefficient of , denotes the first-order azimuth space variation coefficient of , ​​​​denotes the cross-coupling term.

[0037] Specifically, the time-frequency variable conversion method is adopted to perform first-order range space correction on the range frequency domain echo signal after cross-coupling compensation , to obtain the range frequency domain echo signal after first-order range space correction, including: S401, multiplying the variable conversion function and the range frequency domain echo signal after cross-coupling compensation to obtain the range frequency domain echo signal after variable conversion .

[0038] In the application, the variable conversion function is represented as: ; wherein, denotes the variable conversion function, denotes the range frequency, denotes the slow time, denotes the imaginary unit, denotes the time-frequency variable conversion coefficient, , denotes the first-order range space coefficient of the range frequency domain echo signal, denotes the first Taylor expansion coefficient of the slant range history at zero time.

[0039] S402, convoluting the range frequency domain echo signal after variable conversion to obtain the reconstructed time domain echo signal . Here, wherein denotes the convolution operation, denotes the convolution function, denotes the range frequency domain echo signal with the fast time and the slow time .

[0040] In the application, the convolution function adopted by the convolution reconstruction is: ; wherein, denotes the convolution function.

[0041] S403, multiplying the first-order space compensation function and the reconstructed time domain echo signal to obtain the time domain echo signal after first-order range space correction .

[0042] In the application, the first-order space compensation function is represented as:​​ ; wherein, denotes fast time, denotes carrier frequency, denotes linear compensation coefficient, , denotes the first-order space-variant compensation function.

[0043] Thus, the time-domain echo signal after distance direction first-order space-variant correction may be expressed as: .

[0044] S404, the time-domain echo signal after distance direction first-order space-variant correction is subjected to distance direction Fourier transform to obtain the distance direction first-order space-variant corrected distance frequency domain echo signal .

[0045] Specifically, the distance direction Fourier transform is to map from two-dimensional time domain to time-frequency domain, here, for specific implementation of distance direction Fourier transform, reference can be made to related technologies of existing SAR imaging method, which will not be described herein.

[0046] S50, the distance direction first-order space-variant corrected distance frequency domain echo signal is subjected to azimuth direction first-order space-variant correction through extended keystone transform technology to obtain the azimuth direction first-order space-variant corrected distance frequency domain echo signal.

[0047] Wherein, in the extended keystone transform technology, the present application defines a new keystone transform relationship, expressed as: ; wherein, denotes distance direction frequency, denotes slow time, denotes transformed slow time, denotes carrier frequency, denotes velocity vector of SAR platform, denotes modulus operation, denotes and angle, denotes distance vector from SAR platform to central reference point at azimuth angle center time, denotes the first-order azimuth direction space-variant coefficient of , denotes the first-order distance direction space-variant coefficient of , denotes the first order Taylor expansion coefficient, denotes the imaginary unit.

[0048] From the above formula, that is where denotes the function relationship of the slow time and .

[0049] After that, the extended keystone transform is performed by SINC interpolation, so that the range direction first-order spatially variant corrected range frequency domain echo signal is corrected in the azimuth direction first-order spatially variant, to obtain the azimuth direction first-order spatially variant corrected range frequency domain echo signal .

[0050] Specifically, first, for each range direction frequency and azimuth time , the non-uniform sampling points are calculated according to , and then the SINC interpolation kernel (such as a windowed SINC function) is used to resample to obtain the azimuth direction first-order spatially variant corrected range frequency domain echo signal ; wherein denotes .

[0051] S60, the azimuth direction first-order spatially variant corrected range frequency domain echo signal is corrected by filter bank compensation technology, and the SAR image is obtained by using the correction result.

[0052] Here, the distance spatially variant filter and the two azimuth spatially variant phase filters are combined to remove the high-order two-dimensional spatially variant and cross-coupling spatially variant of the Doppler parameters, and through the filter combination, the second-order, third-order and fourth-order two-dimensional spatially variant of the Doppler parameters can be effectively eliminated, and the spatial variation correction ability with strong robustness is obtained.

[0053] Specifically, the azimuth direction first-order spatially variant corrected range frequency domain echo signal is corrected by filter bank compensation technology, and the SAR image is obtained by using the correction result, specifically including: S601, the azimuth direction first-order spatially variant corrected range frequency domain echo signal is subjected to inverse Fourier transform in the range direction, to obtain the range time domain echo signal .

[0054] Specifically, the azimuth direction first-order spatially variant corrected range frequency domain echo signal Performing an inverse Fourier transform on the range data converts the range frequency domain to the range time domain. For the specific implementation of the range inverse Fourier transform, please refer to relevant techniques in existing SAR imaging methods; this invention will not elaborate further.

[0055] S602, Employs a high-order range compensation filter for the range time-domain echo signal. After correction, the time-domain echo signal after higher-order spatial variation correction is obtained. .

[0056] Specifically, by using a higher-order distance compensation filter With distance-time domain echo signal Multiplication can eliminate distance-time domain echo signals. The higher-order range spatial variation term in the equation yields the time-domain echo signal after higher-order range spatial variation correction. ,Right now .

[0057] In this invention, the higher-order distance compensation filter is represented as: ; in, Represents the speed of light. express The second-order distance-to-space variable coefficient, express The second-order distance-to-space variable coefficient, express The third-order distance-to-space variation coefficient, and Let represent the second-order Taylor expansion coefficients and the third-order Taylor expansion coefficients of the slant range history at time zero, respectively. This represents the distance from the SAR platform to any target point at the center of the azimuth angle. , Indicates slow time and The functional relationship, This indicates a high-order compensation filter for distance.

[0058] S603, Time-domain echo signal after distance high-order spatial variation correction A coarse focused image is obtained by performing an azimuth Fourier transform. The coarse focused image is then divided into blocks along the azimuth direction to obtain azimuth sub-blocks. An inverse azimuth Fourier transform is then performed on the signals of the azimuth sub-blocks to obtain the azimuth sub-signals in the azimuth time domain.

[0059] Specifically, the time-domain echo signal after high-order spatial variation correction. Perform a Fourier transform along the azimuth direction (slow time dimension) to form a coarsely focused image. Then, divide the coarsely focused image into several azimuth sub-blocks along the azimuth direction (column direction) and perform an inverse Fourier transform along the azimuth direction on the signal of each azimuth sub-block independently to convert the Doppler domain signal back to the azimuth time domain, thus obtaining the azimuth sub-signal in the azimuth time domain.

[0060] S604. The azimuth sub-signal is corrected by using a high-order spatial variable compensation filter to obtain the azimuth sub-signal after high-order spatial variable correction.

[0061] Specifically, the high-order azimuth spatial variation compensation filter is constructed based on the center point position of each azimuth sub-block. By multiplying the high-order azimuth spatial variation compensation filter with each azimuth sub-signal, the high-order azimuth spatial variation term can be eliminated.

[0062] In this invention, the azimuth high-order spatial variation compensation filter used is represented as: ; in, express The second-order azimuth spatial variation coefficient, express The second-order azimuth spatial variation coefficient, express The third-order azimuth spatial variation coefficient, An empty variable representing the orientation of the center point of the orientation sub-block. This indicates a high-order spatial variation compensation filter for azimuth.

[0063] S605. Divide the azimuth sub-signal after high-order spatial variation correction along the range direction to obtain range sub-blocks. Perform range-direction inverse Fourier transform on the signals of the range sub-blocks to obtain range time-domain sub-signals.

[0064] Specifically, the azimuth sub-signal after high-order spatial variation correction is divided into several range sub-blocks along the range direction (i.e., the line direction). Then, an inverse Fourier transform is performed on the range data of each range sub-block to convert the range frequency domain to the range time domain, thereby obtaining the range time domain sub-signal. For the specific implementation of the range inverse Fourier transform, refer to relevant techniques in existing SAR imaging methods; this invention will not elaborate further.

[0065] S606. A cross-coupled air-varying compensation filter is used to correct the range-time sub-signal, resulting in a cross-coupled air-varying corrected range-time sub-signal.

[0066] Specifically, the cross-coupled spatial variation compensation filter is constructed based on the center point position of each distance sub-block. By multiplying the cross-coupled spatial variation compensation filter with each distance time-domain sub-signal, the cross-coupled spatial variation term can be eliminated.

[0067] In the present application, the cross-coupling space-variant compensation filter used is represented as: ; wherein, represents the cross-coupling term of , represents the azimuth space-variant variable distance from the center point of the sub-block, represents the reference slant distance from the center point of the sub-block, represents the cross-coupling space-variant compensation filter.

[0068] S607, the distance time domain sub-signal after cross-coupling space-variant correction is subjected to distance direction Fourier transform to obtain a focused sub-image, and the focused sub-images are spliced to obtain a SAR image.

[0069] Specifically, the focused sub-images are spliced to obtain a SAR image with good focusing.

[0070] The time-frequency mixed domain SAR imaging method provided by the present application is further described below through simulation experiments.

[0071] The specific parameter settings of the simulation experiment can be seen in the following table:

[0072] First, in the scene of the SAR platform of the mobile aircraft moving along a large diving trajectory, the focusing effects of the present application and the existing SAR imaging method (polar coordinate imaging method based on a space polar coordinate slant distance model) are compared. In this experiment, the upper left edge point target, the center point target and the lower right edge point target in the scene are selected for comparison, Figures 3-5 is a simulation result graph of the point target spread function of the present application, Figures 6-8 is a simulation result graph of the point target spread function of the existing SAR imaging method, wherein the horizontal axis represents the sampling point sequence in the azimuth direction, and the vertical axis represents the sampling point sequence in the range direction. It can be seen that the present application has good focusing for the reference target (i.e. the center point target) and the edge target, while the existing SAR imaging method is used to process the three targets, wherein the reference target has good focusing quality, but the edge point target is severely defocused. This is because the existing SAR imaging method ignores the high-order and cross-coupling space-variant phase error, resulting in a wide main lobe and degraded side lobe. This experiment shows that the present application can achieve good focusing effect in the scene of the large diving trajectory motion of the mobile aircraft SAR.

[0073] Secondly, due to the limited real data of the SAR system of the maneuverable aircraft, in order to evaluate the applicability of the present application, a semi-physical simulation experiment is carried out on the SAR echo data of the maneuverable aircraft with large diving trajectory, and the experimental parameters are set as follows: the carrier frequency is set to 17 GHz, the signal bandwidth is 150 MHz, and the azimuth angle is 14.88°. Referring to Figure 9 is an imaging result graph of the existing SAR imaging method (polar coordinate imaging method based on a spatial polar coordinate slant range model), Figure 10 is an imaging result graph of the present application. It can be seen that the entire image obtained by the present application has good focusing, while the existing SAR imaging method has general focusing effect.

[0074] The time-frequency hybrid domain SAR imaging method provided by the present application eliminates the cross-coupling effect caused by the vertical velocity, acceleration and high-order motion parameters through the azimuth angle preprocessing technology; the first-order range variation and the first-order azimuth variation caused by the large diving motion are corrected through the time-frequency variable scale conversion method and the extended keystone transformation technology, and the two-dimensional range migration and the first-order Doppler parameter of the first-order range variation are eliminated; the high-order azimuth variation and the high-order range variation are corrected by using the filter bank, and the high-order two-dimensional range variation and the cross-coupling variation of the Doppler parameter are removed. The present application realizes the significant suppression of two-dimensional cross-coupling and the improvement of the range variation correction performance, significantly improves the imaging quality of the SAR, and can effectively adapt to the complex imaging scene under the large diving trajectory of the maneuverable platform. In addition, the SAR imaging method of the present application has extremely high parallelism, and the operation amount of the algorithm is low, which is beneficial to the acceleration processing on the GPU and the DSP.

[0075] It should be noted that the terms "first", "second", and the like are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than those illustrated or described herein. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present application. Rather, they are merely examples of devices and methods consistent with some aspects of the present application.

[0076] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in the present application.

[0077] Although the present application has been described in connection with various embodiments thereof with reference to the drawings, it will be apparent to those skilled in the art that various changes in form and details can be made therein without departing from the scope of the application as set forth in the accompanying claims. In the description of the application, the word "comprising" does not exclude other components or steps, the word "a" or "an" does not exclude a plurality, and the word "multiple" means two or more, unless expressly stated otherwise. Furthermore, some measures can be described as being implemented in different embodiments, but this does not exclude that these measures can be combined in one embodiment.

[0078] The above description is further detailed in connection with specific preferred embodiments of the application, and it is not to be construed that the specific implementation of the application is limited to these descriptions. For those skilled in the art, some simple deductions or replacements can be made without departing from the concept of the application, and all of these should be considered as falling within the protection scope of the application.

Claims

1. A time-frequency hybrid domain SAR imaging method, characterized in that, include Acquire SAR echo signals; The SAR echo signal is subjected to range pulse compression to obtain the range-frequency domain echo signal after range pulse compression. The range frequency domain echo signal is cross-coupled and compensated by azimuth angle preprocessing to obtain the cross-coupled and compensated range frequency domain echo signal. The range frequency domain echo signal after cross-coupling compensation is corrected by first-order spatial variation in the range direction using the time-frequency scaling conversion method to obtain the range frequency domain echo signal after first-order spatial variation in the range direction. The range frequency domain echo signal after first-order spatial-variant correction in the range direction is corrected in the azimuth direction by the extended keystone transform technique, and the range frequency domain echo signal after first-order spatial-variant correction in the azimuth direction is obtained. The range-frequency domain echo signal after the first-order spatially calibrated azimuth direction is subjected to higher-order spatially calibrated correction using filter bank compensation technology, and the correction results are used to obtain SAR images.

2. The time-frequency hybrid domain SAR imaging method according to claim 1, characterized in that, The method of using time-frequency scaling conversion to perform first-order space-variant correction on the range-frequency domain echo signal after cross-coupling compensation, to obtain the range-frequency domain echo signal after first-order space-variant correction, includes: Multiply the scaling function with the cross-coupling compensated range-frequency domain echo signal to obtain the scaled-converted range-frequency domain echo signal; The range-frequency domain echo signal after scaling is reconstructed by convolution to obtain the reconstructed time-domain echo signal. Multiply the first-order spatial variation compensation function with the reconstructed time-domain echo signal to obtain the time-domain echo signal after first-order spatial variation correction in the range direction. The time-domain echo signal after first-order spatial variation correction in the range direction is subjected to range-direction Fourier transform to obtain the range-frequency domain echo signal after first-order spatial variation correction in the range direction.

3. The time-frequency hybrid domain SAR imaging method according to claim 2, characterized in that, The scaling function is expressed as: ； in, To represent a scaled function, Indicates the range frequency. Indicates slow time. Represents the imaginary unit. Indicates the time-frequency scaling conversion coefficient. , express The first-order distance-to-space variation coefficient, The slant range history at time zero is the first... Taylor expansion coefficients.

4. The time-frequency hybrid domain SAR imaging method according to claim 3, characterized in that, The convolution function used for the convolutional reconstruction is: ； in, This represents the convolution function.

5. The time-frequency hybrid domain SAR imaging method according to claim 4, characterized in that, The first-order spatially variable compensation function is expressed as: ； in, Indicates a fast time. Indicates the carrier frequency. Represents the linear compensation coefficient. , This represents the first-order spatially variable compensation function.

6. The time-frequency hybrid domain SAR imaging method according to claim 1, characterized in that, The extended keystone transformation technique defines a new keystone transformation relationship, expressed as follows: ； in, Indicates the range frequency. Indicates slow time. Indicates the slow time after the transformation. Indicates the carrier frequency. Represents the velocity vector of the SAR platform. This represents the modulo operation. express and The included angle, This represents the distance vector from the SAR platform to the central reference point at the azimuth center time. express The first-order azimuth spatial variation coefficient, express The first-order distance-to-space variation coefficient, The slant range history at time zero is the first... Taylor expansion coefficients, It represents the imaginary unit.

7. The time-frequency hybrid domain SAR imaging method according to claim 6, characterized in that, The step of performing high-order spatial-variant correction on the range-frequency domain echo signal after first-order spatial-variant correction in the azimuth direction using filter bank compensation technology, and obtaining a SAR image using the correction result, includes: Perform a range-direction inverse Fourier transform on the range frequency domain echo signal after the first-order spatial variation correction in the azimuth direction to obtain the range time domain echo signal; The time-domain echo signal is corrected by using a high-order range compensation filter to obtain a time-domain echo signal with high-order range spatial variation correction. A coarse focused image is obtained by performing an azimuth Fourier transform on the time-domain echo signal after the distance high-order spatial variation correction. The coarse focused image is then divided into blocks along the azimuth direction to obtain azimuth sub-blocks. The signals of the azimuth sub-blocks are then subjected to an inverse azimuth Fourier transform to obtain azimuth sub-signals in the azimuth time domain. The azimuth sub-signal is corrected by using a high-order spatial variation compensation filter to obtain the azimuth sub-signal after high-order spatial variation correction. The azimuth sub-signal after the higher-order spatial variation correction is divided into blocks along the range direction to obtain range sub-blocks. The range sub-blocks are then subjected to range-direction inverse Fourier transform to obtain range time-domain sub-signals. The range-time sub-signal is corrected by a cross-coupled space-variable compensation filter to obtain the cross-coupled space-variable corrected range-time sub-signal. The range-time sub-signal after cross-coupled spatial variation correction is subjected to range-to-Fourier transform to obtain a focused sub-image, and the focused sub-image is stitched together to obtain a SAR image.

8. The time-frequency hybrid domain SAR imaging method according to claim 7, characterized in that, The higher-order distance compensation filter is represented as follows: ； in, Represents the speed of light. express The second-order distance-to-space variable coefficient, express The second-order distance-to-space variable coefficient, express The third-order distance-to-space variation coefficient, and Let represent the second-order Taylor expansion coefficients and the third-order Taylor expansion coefficients of the slant range history at time zero, respectively. This represents the distance from the SAR platform to any target point at the center of the azimuth angle. , Indicates slow time and The functional relationship, This indicates a high-order compensation filter for distance.

9. The time-frequency hybrid domain SAR imaging method according to claim 8, characterized in that, The azimuth high-order spatial variation compensation filter is represented as follows: ； in, express The second-order azimuth spatial variation coefficient, express The second-order azimuth spatial variation coefficient, express The third-order azimuth spatial variation coefficient, An empty variable representing the orientation of the center point of the orientation sub-block. This indicates a high-order spatial variation compensation filter for azimuth.

10. The time-frequency hybrid domain SAR imaging method according to claim 9, characterized in that, The cross-coupled air-variable compensation filter is represented as follows: ； in, express Cross-coupling terms, An empty variable representing the orientation of the distance from the center point of the sub-block. This represents the reference slope distance from the center point of the sub-block. This indicates a cross-coupled air-varying compensation filter.

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