Low-altitude, close-range, wide-band SAR squint imaging method, device, equipment, and medium

Abstract

The present application relates to a low-altitude, short-range, wide-swath SAR (Spatial SAR) squint imaging method, apparatus, device, and medium. Based on the idea of ​​combining range movement with along-track motion errors to uniformly correct the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The CS quadratic phase function is used to process target echo signals obtained by a SAR radar system in a low-altitude, short-range, wide-swath squint scenario when detecting a target, thereby obtaining an echo signal corrected for the space-variation of the range envelope. Subsequently, range envelope correction and compensation, as well as azimuth phase error compensation, are sequentially performed on the echo signal corrected for the space-variation of the range envelope based on INS measurement data. Focusing is then performed on the range and azimuth directions to obtain a complex image. The complex image is then processed using a phase gradient autofocusing method to obtain a final imaging result. This method can effectively improve imaging quality.

Description

Technical Field

[0001] The present application relates to the field of SAR imaging technology, and in particular to a method, device, equipment and medium for low-altitude, close-range and wide-band SAR squint imaging. Background Art

[0002] In traditional squint SAR imaging geometry, the squint angle is uniformly taken as the squint angle corresponding to the scene center, which provides good imaging results at long distances or in small scenes. However, in close-range, wide-area scenarios, due to the large imaging range of the target area and the relatively small distance between the platform and the scene center, the spatial squint angle corresponding to each point in the target area varies. Using conventional geometry will result in certain errors, resulting in poor imaging focusing.

[0003] Therefore, in the process of short-range wide-band SAR imaging, how to effectively solve the problem of space-time variation of oblique angle of view is the key to improving imaging quality. Summary of the Invention

[0004] Based on this, it is necessary to provide a low-altitude, close-range, wide-swath SAR squint imaging method, device, equipment and medium that can effectively improve imaging quality in response to the above technical problems.

[0005] A low-altitude, close-range, wide-band SAR squint imaging method, the method comprising:

[0006] Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario;

[0007] Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space variable correction.

[0008] According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image;

[0009] The complex image is processed using a phase gradient self-focusing method to obtain a final imaging result.

[0010] In one embodiment, the CS quadratic phase function is expressed as:

[0011]

[0012] In the above formula, t a Indicates the azimuth slow time, t rIndicates the distance to the fastest time, K r Indicates the distance modulation frequency, R s Indicates the reference center slope distance, R s (t a ; R s ) represents the reference instantaneous center slant distance, Δr(t a ; R s ) represents the distance variation factor, and c represents the speed of light.

[0013] In one embodiment, when performing range envelope correction and compensation:

[0014] Perform range-direction fast Fourier transform on the echo signal after range envelope space-variation correction to obtain the range frequency domain echo signal;

[0015] The range frequency domain echo signal is subjected to range movement correction and range space invariant envelope error compensation by using a range envelope movement compensation function to obtain a range frequency domain echo signal that has completed range envelope correction and compensation.

[0016] In one embodiment, after obtaining the range frequency domain echo signal with complete range envelope compensation, range matched filtering is performed on it, and the processed data is subjected to range inverse Fourier transform to obtain data that needs to be compensated for azimuth phase after range processing. When performing range matched filtering, a range matched filter adapted to the CS quadratic phase function is used to filter the corrected range frequency domain echo signal.

[0017] In one embodiment, the range-matched filter is expressed as:

[0018]

[0019] In the above formula, f r Indicates distance frequency.

[0020] In one embodiment, when performing phase error compensation:

[0021] Perform range space-variant phase error compensation and Doppler center space-variant correction on the data to be subjected to phase error compensation, to obtain space-variant corrected data;

[0022] After performing azimuth de-skew and residual video phase compensation on the space-variant corrected data through an azimuth matched filter, azimuth fast Fourier transform is performed to obtain the complex image.

[0023] In one embodiment, the position matched filter and the residual video phase compensation function are expressed as:

[0024]

[0025] In the above formula, the azimuth frequency modulation R varies with distance and slant angle, and represents the instantaneous slant range when the center of the beam illuminates the target.

[0026] In one embodiment, a distance-blocked phase gradient autofocusing method is adopted to process the complex image.

[0027] The present application also provides a low-altitude, close-range, wide-band SAR squint imaging device, comprising:

[0028] A target echo signal acquisition module is used to acquire target echo signals and related INS measurement data. The target echo signals are obtained by the SAR radar system detecting the target in a low-altitude, close-range, wide-width, and large-squint imaging scenario.

[0029] The range envelope space-variation correction module is used to uniformly correct the range envelope error space-variation based on the idea of ​​combining range movement with along-track motion error. The module constructs a CS quadratic phase function in the two-dimensional time domain and uses the CS quadratic phase function to process the target echo signal to obtain an echo signal after range envelope space-variation correction.

[0030] A complex image obtaining module is used to perform range envelope correction and compensation and azimuth phase error compensation on the echo signal after range envelope space variation correction according to the INS measurement data, and to focus the range and azimuth directions respectively to obtain a complex image;

[0031] The complex image focusing module is used to process the complex image using a phase gradient autofocusing method to obtain a final imaging result.

[0032] A computer device includes a memory and a processor, wherein the memory stores a computer program, and when the processor executes the computer program, the following steps are implemented:

[0033] Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario;

[0034] Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space variable correction.

[0035] According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image;

[0036] The complex image is processed using a phase gradient self-focusing method to obtain a final imaging result.

[0037] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the following steps:

[0038] Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario;

[0039] Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space variable correction.

[0040] According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image;

[0041] The complex image is processed using a phase gradient self-focusing method to obtain a final imaging result.

[0042] The above-mentioned low-altitude, short-range, wide-swath SAR squint imaging method, device, equipment and medium are based on the idea of ​​combining range movement and along-track motion errors to uniformly correct the spatial variable of the range envelope error. The CS quadratic phase function is constructed in the two-dimensional time domain. The CS quadratic phase function is used to process the target echo signal obtained by the SAR radar system in the low-altitude, short-range, wide-swath squint scenario when detecting the target to obtain an echo signal after the range envelope spatial variation correction. Then, according to the INS measurement data, the echo signal after the range envelope spatial variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and focusing processing is performed on the range and azimuth directions respectively to obtain a complex image. The complex image is then processed using the phase gradient autofocusing method to obtain the final imaging result. The use of this method can effectively improve the imaging quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic diagram of a radar squint SAR imaging geometric model in one embodiment;

[0044] Figure 2 Schematic diagram of a curve showing a change in squint angle versus distance in one embodiment;

[0045] Figure 3 Schematic diagram of the change of RCM with distance in one embodiment, where: Figure 3 (a) is a schematic diagram of the LRCM distance null variable. Figure 3(b) is a schematic diagram of the QRCM distance null variable;

[0046] Figure 4 1 is a flow chart of a low-altitude, close-range, wide-swath SAR squint imaging method according to an embodiment;

[0047] Figure 5 1 is a schematic diagram of a process for performing imaging according to the method of this invention in one embodiment;

[0048] Figure 6 A schematic diagram of the imaging scene in an experiment, where Figure 6 (a) is a schematic diagram of point target simulation distribution. Figure 6 (b) is a schematic diagram of the echo RCM trajectory;

[0049] Figure 7 Schematic diagram of the RCM trajectory after RCMC in an experiment, where Figure 7 (a) is a schematic diagram of the RCM trajectory after processing using the traditional method. Figure 7 (b) Schematic diagram of the RCM trajectory after processing using the proposed method;

[0050] Figure 8 This is a schematic diagram of the imaging and focusing results in an experiment, where: Figure 8 (a) is a schematic diagram of the imaging focusing result after processing using the traditional method. Figure 7 (b) is a schematic diagram of the imaging focusing result after processing using the method in this paper;

[0051] Figure 9 Schematic diagram of the motion error of each axis in an experiment;

[0052] Figure 10 This is a schematic diagram of the results after walking correction in an experiment;

[0053] Figure 11 This is a schematic diagram of the results after motion compensation in an experiment, where: Figure 11 (a) is a schematic diagram of the result after motion compensation without considering the distance variation. Figure 11 (b) Schematic diagram of the result after motion compensation considering distance variation

[0054] Figure 12 This is a schematic diagram comparing the azimuth profiles of imaging results at various points in an experiment, where: Figure 12 (a) is the azimuth profile of a close-range point target. Figure 12 (b) is the azimuth profile of a distant point target;

[0055] Figure 13 1 is a structural block diagram of a low-altitude, close-range, wide-band SAR squint imaging device according to an embodiment;

[0056] Figure 14FIG. 1 is a diagram showing the internal structure of a computer device in one embodiment. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0058] like Figure 1 As shown in Figure 1, the geometric relationship of the airborne high squint SAR echo acquisition process in actual situations. Ideally, the aircraft moves along the green straight line at a constant speed v. At this time, the instantaneous slant range between the target and the platform is R n (t a ; R). However, due to the existence of motion errors, the aircraft usually deviates from the ideal trajectory, such as Figure 1 The red curve in the middle is the actual flight path of the aircraft. Figure 1 In the model, the x-axis is the flight direction of the SAR radar system carrying platform, the y-axis is perpendicular to the flight direction, the z-axis is the height direction, and H is the platform height. a ), Δy(t a ), Δz(t a ) represent the motion errors in the x, y, and z directions respectively, so the actual position of the platform is [v x t a +Δx(t a ),Δy(t a ),H+Δz(t a )] T , the instantaneous slant distance between the target and the platform is R s (t a ; R), specifically expressed as:

[0059]

[0060] In formula (1), R n (t a ; R) represents the instantaneous slant distance between the target and the platform under ideal conditions, ΔR(t a ; R) represents the slope range error caused by motion error, t a , t c They represent the azimuth slow time and the beam crossing moment respectively, R represents the instantaneous slant range when the center of the beam illuminates the target, θ represents the spatial slant angle, and α represents the pitch angle, which is the angle between the closest slant range and the ground.

[0061] In traditional squint SAR imaging geometry, the squint angle is uniformly taken as the squint angle corresponding to the scene center. This provides good imaging results at long distances or in small scenes. However, in close-range, wide-swath SAR imaging, due to the large imaging range of the target area and the relatively small distance between the platform and the scene center, the spatial squint angles corresponding to each point in the target area vary. Using conventional geometry will produce certain errors, resulting in poor imaging focusing. Therefore, in the close-range, wide-swath SAR imaging process, effectively addressing the spatial variation of squint angles is the key to improving imaging quality.

[0062] Through analysis and verification, it is proved that the space-variant characteristics of the oblique viewing angle, such as Figure 2 As shown in the figure, it can be observed that when the imaging area is in the long-distance range (10-30 km), the slant angle is approximately fixed. However, in the close-range wide-band imaging area (0-10 km), the slant angle changes significantly. If the spatial variation of the slant angle is ignored, it will have a significant impact on the imaging processing and cause large errors. Therefore, the motion error of the slant angle due to the spatial variation of distance needs to be compensated.

[0063] Further imaging space-variant analysis is conducted. Considering that range walk is the main part of range migration when the image is highly squint, the influence of squint angle on the third and higher order RCM is ignored. The main analysis is on the space-variant error of Linear Range Cell Migration (LRCM) and Quadratic Range Cell Migration (QRCM). Figure 3 As shown, from Figure 3 (b) It can be seen that the QRCM changes very little, indicating that there is almost no need to consider the QRCM null variable in the large squint scene. Figure 3 As can be seen in (a), the LRCM varies significantly. When the distance from the reference point is greater than approximately 300m, the LRCM null variance reaches 1 / 2 of the range resolution unit, and the image quality gradually begins to deteriorate. This null variance in range poses a significant challenge to imaging processing.

[0064] Further analysis of the spatial variation of motion errors revealed that one-step motion compensation methods for space-varying range envelope errors rely on interpolation, which is computationally intensive and inefficient. Therefore, the signal is typically segmented by range to mitigate the spatial variation of the sub-block envelope error. This is accomplished by simply multiplying the range envelope by a linear phase in the frequency domain, avoiding complex interpolation. However, the precise range envelope of each ranging unit is not compensated. In particular, at sub-band boundaries, the phase compensation reference function mismatches the actual phase error, affecting the accuracy of phase compensation and, consequently, the focusing effect on the target.

[0065] In view of the above technical problems existing in the prior art in imaging and motion errors in low-altitude, close-range and wide-angle oblique scenes, in this application, Figure 4 As shown, a low-altitude, close-range, wide-swath SAR squint imaging method is provided, which specifically includes the following steps:

[0066] Step S100: Acquire a target echo signal and related INS measurement data. The target echo signal is obtained by detecting the target with a SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario.

[0067] In step S110, based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the empty variable of the range envelope error, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain an echo signal after the range envelope empty variable correction.

[0068] Step S120 , performing range envelope correction and compensation and azimuth phase error compensation on the echo signal after range envelope space-variation correction according to the INS measurement data, and performing focusing processing on the range and azimuth directions respectively to obtain a complex image.

[0069] Step S130 , processing the complex image using a phase gradient autofocus method to obtain a final imaging result.

[0070] In the present application, consideration of variable oblique angle is added to imaging processing and motion error. During imaging processing, the CS scaling function is first used to correct the distance envelope spatial variation. Compared with traditional imaging methods, this method performs compensation before low-resolution wide-band imaging, avoiding the distance unit migration azimuth spatial variation and subsequent compensation operations, and compensates the distance migration correction together with the motion error, without the need for interpolation operations, reducing the number of FFT operations, and providing the possibility for subsequent real-time imaging. In addition, the distance spatial variation line of sight motion error factor is improved in motion compensation, thereby improving the accuracy of motion compensation to obtain well-focused SAR images.

[0071] In step S100, a linear frequency modulation (LFM) signal is transmitted, and the target echo signal demodulated to the baseband is expressed as:

[0072]

[0073] In formula (2), K r Indicates the distance modulation frequency, represents the wavelength, f c Indicates the carrier frequency, R s (t a ; R) represents the instantaneous slant distance.

[0074] Firstly, for the problem of distance envelope spatial variation, based on the above analysis, the distance migration is mainly based on the distance movement, and the distance movement is combined with the along-track motion error to uniformly correct the distance envelope error spatial variation, and the instantaneous slant range R is converted to s (t a ; R) is written as:

[0075]

[0076] In formula (3), ΔR s (t a R) represents the total amount of distance envelope movement generated by the actual flight process of the platform, which can be further rewritten as:

[0077]

[0078] In formula (4), ΔR c (t a ; R o ), ΔR v (t a ; R) represent the distance envelope movement with the center point as the reference and the residual distance envelope space-varying error of any point target relative to the center point, Δr(t a ; R0) represents the distance variation factor.

[0079] In this application, the CS factor multiplication is constructed in the time domain to solve the problem of distance envelope space variation, remove the distance envelope space variation of the two end point targets in the scene relative to the scene center, and make the distance envelope of all point targets in the scene consistent for unified envelope compensation.

[0080] Specifically, the constructed CS quadratic phase function is expressed as:

[0081]

[0082] In formula (5), t a Indicates the azimuth slow time, t r Indicates the distance to the fastest time, K r Indicates the distance modulation frequency, R s Indicates the reference center slope distance, R s (t a ; R s ) represents the reference instantaneous center slant distance, Δr(t a ; R s ) represents the distance variation factor, and c represents the speed of light.

[0083] In step S110, after multiplying the CS quadratic phase function shown in formula (5) by the target echo signal in the time domain, that is, after performing distance CS correction on the target echo signal, the simplified result is:

[0084]

[0085] From formula (6), the third phase term is the residual phase caused by the CS function operation, which can be compensated in the subsequent azimuth compression operation. Comparing formula (2) and formula (6), it can be seen that the phase center moment is given by becomes That is, the distance space variability of the distance envelope term has been eliminated, and the remaining distance envelope non-empty variables are the motion errors caused by distance movement and actual flight trajectory, which are subsequently corrected uniformly using the envelope compensation function.

[0086] In step S120, when performing distance envelope error compensation: first, perform distance fast Fourier transform on the echo signal after the distance envelope space-variant correction to obtain the distance frequency domain echo signal, and then use the distance envelope movement compensation function to perform distance movement correction and distance space-invariant envelope error compensation on the distance frequency domain echo signal to obtain the corrected distance frequency domain echo signal.

[0087] Specifically, the distance envelope movement compensation function is expressed as:

[0088]

[0089] In this embodiment, after obtaining the range frequency domain echo signal with uniform range envelope error compensation, corresponding relevant range processing is performed on it, and the processed data is subjected to range inverse Fourier transform to obtain data to be compensated for azimuth phase error.

[0090] Specifically, the relevant range processing includes RCMC (range migration correction) and secondary range compression.

[0091] In this embodiment, when performing range compression, a range-matched filter adapted to the CS quadratic phase function is used to filter the corrected range frequency-domain echo signal.

[0092] In this embodiment, the range-matched filter is expressed as:

[0093]

[0094] In the above formula, f r Indicates distance frequency.

[0095] After further range processing, the echo signal to be compensated for azimuth phase error is expressed as:

[0096]

[0097] In formula (9), T prepresents the pulse width, which accurately compensates for the range envelope error. The fourth term is the additional phase caused by the CS operation, which is compensated together during azimuth compression.

[0098] Next, phase error compensation is performed in the two-dimensional time domain, including Doppler Center Correction (DCC), range-variant phase error compensation, and azimuth de-skew. Specifically, the data to be azimuthally phase-error-compensated undergoes range-variant phase error compensation and Doppler center-variant correction to obtain the corrected data. This corrected data is then de-skewed and subjected to residual video phase compensation using an azimuth matched filter. An azimuth fast Fourier transform is then performed to obtain a complex image.

[0099] In this embodiment, the data to be compensated for the phase envelope error is subjected to range space-varying phase error compensation and Doppler center space-varying correction, using the following formula:

[0100]

[0101] Furthermore, the position matching filter and the residual video phase compensation function are expressed as:

[0102]

[0103] In formula (11), the azimuth modulation frequency R varies with distance and slant angle, and represents the instantaneous slant range when the center of the beam illuminates the target.

[0104] In this embodiment, if the inertial navigation system has sufficient accuracy, data-based motion compensation is not required, and a well-focused image can be obtained after performing step S120. However, in actual flight, the output complex image is further estimated using the Phase Gradient Autofocus (PGA) algorithm to obtain a precisely focused image. Taking into account the space-variant nature of the error, the entire image is typically segmented into small blocks. Since the phase error under consideration is only space-variant in range, a range-based PGA algorithm is used on the output complex image to perform precise motion compensation.

[0105] like Figure 5 As shown in FIG, the processing flow of imaging the target echo signal according to the present method.

[0106] To verify the correctness and effectiveness of this method, point target simulation and measured data processing are used to compare the focusing effect with existing methods. The system parameters of the simulation and measured data are shown in Table 1.

[0107] Table 1 Simulation parameters

[0108]

[0109] The effectiveness of the improved algorithm is analyzed and evaluated through point target simulation experiments, and the parameters are shown in Table 1. The data set contains 3000 pulses and the synthetic aperture time is 3s.

[0110] In order to more comprehensively analyze the performance of the imaging and motion compensation algorithms proposed in this paper, the imaging scene is set to Figure 1 As shown in Figure 1, three point targets P0, P1, and P2 are set and arranged along the same beam center moment (with the same orientation in the imaging coordinate system) with different distance coordinates (0m±1000m from the scene center), as shown in Figure 1. Figure 6 As shown in the figure, P0 is the center point, P1 and P2 are the near point and the far point respectively.

[0111] Figure 6 The beam center crossing time of the three point targets in (a) is the same. The distance history of the point targets obtained after demodulation of the echo signal is as follows: Figure 6 As shown in (b), the three points have the same azimuth position but different slant ranges; and the range migration is mainly the distance movement, and the RCM trajectory varies with the slant angle and distance, so the three points have different RCM quantities.

[0112] First, point target echo data simulation under ideal conditions is carried out to verify the performance of the imaging algorithm. By comparing the focusing effect of the proposed method with that of the traditional configuration SAR imaging algorithm and conducting a detailed analysis, the effectiveness and efficiency of the proposed method in solving the problem are verified. Figure 7 、 8 shown.

[0113] Figure 7 The distance migration curves of the three point targets after RCMC for the two algorithms are shown in Figure 2. Figure 7 (a) It can be found that since P1 and P2 are not at the center slant range, there is still a relatively obvious residual distance movement after directly performing LRWC, and the residual distance movement of P1 is greater than that of P2, indicating that the change in the close range area is more obvious. After the echo data is preprocessed with the CS function by this method, the RCMC results are as follows Figure 7 As shown in (b), it can be seen that after the distance movement of the two edge targets P1 and P2 is compensated for, accurate correction is achieved, their energy is basically corrected to one range gate, and the RCM trajectory is straightened.

[0114] Figure 8 They are respectively the traditional squint imaging algorithm based on fixed squint angle geometry and the imaging focusing results after processing by the improved algorithm based on distance space variation in this paper. Figure 8As can be seen in (a), after the traditional method, only the center point P0 of the scene is accurately focused, and the two edge points P1 and P2 show different degrees of defocus. The uncompensated displacement causes the distance range of the point target to expand, and the inaccurate matching of the Doppler modulation frequency causes the image to be two-dimensionally defocused. After the method is used, Figure 8 (b) The comparison shows that the two edge points achieve finely focused imaging results in both distance and azimuth, and basically maintain the same good focusing performance as the center point of the scene.

[0115] The above simulation verifies that under ideal conditions without errors, the range variation can be eliminated by using the CS scaling function. However, in practical applications, motion errors are inevitable. Based on the trajectory error recorded by the actual inertial navigation, point target echo data simulation is performed to verify the compensation effect of the proposed method on motion errors. The three-dimensional trajectory deviation is as follows: Figure 9 The motion error caused by the platform is sufficient to affect the focusing effect of the SAR image, and motion error compensation is required.

[0116] The three-dimensional velocity set in the point target simulation experiment is the velocity provided by the inertial navigation. There is no additional motion error caused by insufficient velocity accuracy. Therefore, there is no need to use the PGA algorithm. Only the improved two-step motion compensation method based on inertial navigation data proposed in this paper (correcting the range envelope error and phase error separately) can be used to obtain a well-focused point target.

[0117] First, we verify the effectiveness of the motion compensation method for RCMC. Figure 10 The following figure shows the correction results and a zoomed-in image after space-variable range movement correction and range migration. As can be seen from the figure, the migration error caused by actual motion makes it impossible to accurately correct the RCM trajectory, and the echo energy spans multiple range gates.

[0118] Therefore, envelope correction is required. Figure 11 This is the correction result after motion compensation. Figure 11 (a) is the correction result after uniform motion compensation with the scene center as the reference point. The envelope error at the scene center is completely corrected, but there is still residual envelope error at the edge points. After using the distance-varying envelope correction method in this paper, the correction results are as follows: Figure 11 As shown in (b), the echo energy of the two edge points is basically corrected to the same range gate, which effectively solves the influence of range space variation on RCMC accuracy and realizes accurate RCM correction.

[0119] The influence of phase error on focusing effect is mainly manifested in azimuth. The azimuth profiles of the two edge targets P1 and P2 are compared. Figure 12 For the two-point target squint imaging, the algorithm proposed in this paper is used to calculate the formula C. ΔR'c (t a ; R0)=-Δx(t a )sinθ0-Δy(t a )cosα0cosθ0+Δz(t a )sinα0cosθ0 (not considering the space-variation of distance), formula B (considering only the space-variation of pitch angle with distance), and formula A (considering both the space-variation of pitch angle and squint angle with distance). Table 2 compares the imaging results of motion compensation using formula B (considering only the space-variation of pitch angle and squint angle with distance). Figure 13 After compensation using three motion compensation factors, the peak side lobe ratio (PSLR) and integrated side lobe ratio (ISLR) corresponding to the azimuth profile of the two-point target.

[0120] Due to the large motion error, using Formula C, which employs a unified compensation method for the scene's center motion error, resulted in significant azimuth defocusing of two point targets on the edge. Formula B, which accounts for the effects of pitch angle and range variations, significantly improved the focus of the two point targets after compensation, but the near-distance point remained severely defocused. However, using Formula A, the near-distance point was successfully focused and sidelobes were suppressed to a certain extent, resulting in significantly improved azimuth performance compared to Formula B.

[0121] Table 2 Comparison of target azimuth performance indicators after compensation of various methods

[0122]

[0123] In the above-mentioned low-altitude, close-range, wide-swath SAR squint imaging method, the range space-variation problem existing in the low-altitude, close-range, wide-swath SAR squint imaging is addressed. First, based on the speed and position parameters obtained by INS, the close-range, wide-swath SAR imaging geometry and slant range equivalent model are established. Based on theoretical analysis and compared with traditional algorithms, the consideration of the slant angle is added in the imaging processing and motion error. During the imaging processing, the CS scaling function is first used to correct the range envelope space-variation. Compared with the traditional imaging method, this method performs compensation before LRWC, avoiding the RCM azimuth space-variation and subsequent compensation operations. The range migration correction is compensated together with the motion error, eliminating the need for interpolation operations, reducing the number of FFT operations, and making it possible to achieve subsequent real-time imaging. In addition, the range space-variation line of sight motion error factor is improved in the motion compensation, thereby improving the accuracy of motion compensation to obtain well-focused SAR images.

[0124] It should be understood that although Figure 1The steps in the flowchart are shown in sequence as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified in this document, there is no strict order restriction for the execution of these steps, and these steps can be executed in other orders. In addition, Figure 1 At least part of the steps may include multiple sub-steps or multiple stages. These sub-steps or stages are not necessarily executed at the same time, but can be executed at different times. The execution order of these sub-steps or stages is not necessarily sequential, but can be executed in turn or alternately with other steps or at least part of the sub-steps or stages of other steps.

[0125] In one embodiment, Figure 13 As shown, a low-altitude, close-range, wide-band SAR squint imaging device is provided, comprising: a target echo signal acquisition module 200, a range envelope space-variation correction module 210, a complex image acquisition module 220, and a complex image focusing module 230, wherein:

[0126] The target echo signal acquisition module 200 is used to acquire the target echo signal and related INS measurement data. The target echo signal is obtained by the SAR radar system detecting the target in a low-altitude, close-range, wide-width, and large-squint imaging scenario;

[0127] The range envelope space-variation correction module 210 is used to uniformly correct the range envelope error space-variation based on the idea of ​​combining range movement with along-track motion error, construct a CS quadratic phase function in the two-dimensional time domain, and use the CS quadratic phase function to process the target echo signal to obtain an echo signal after range envelope space-variation correction;

[0128] A complex image obtaining module 220 is configured to perform range envelope correction and compensation and azimuth phase error compensation on the echo signal after range envelope space-variation correction according to the INS measurement data, and to perform focusing processing on the range and azimuth directions to obtain a complex image;

[0129] The complex image focusing module 230 is used to process the complex image using a phase gradient autofocusing method to obtain a final imaging result.

[0130] Regarding the specific definition of the low-altitude, close-range, wide-band SAR squint imaging device, please refer to the definition of the low-altitude, close-range, wide-band SAR squint imaging method above, and will not be repeated here. The various modules in the above-mentioned low-altitude, close-range, wide-band SAR squint imaging device can be implemented in whole or in part through software, hardware, or a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory of the computer device in software form, so that the processor can call and execute the operations corresponding to the above-mentioned modules.

[0131] In one embodiment, a computer device is provided. The computer device may be a terminal, and its internal structure diagram may be as follows: Figure 14 As shown. The computer device includes a processor, a memory, a network interface, a display screen and an input device connected via a system bus. The processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The network interface of the computer device is used to communicate with an external terminal via a network connection. When the computer program is executed by the processor, a low-altitude, close-range, wide-band SAR squint imaging method is implemented. The display screen of the computer device can be a liquid crystal display or an electronic ink display screen, and the input device of the computer device can be a touch layer covering the display screen, or a button, trackball or touchpad provided on the computer device housing, or an external keyboard, touchpad or mouse.

[0132] Those skilled in the art will understand that Figure 14 The structure shown in the figure is only a block diagram of a part of the structure related to the solution of the present application, and does not constitute a limitation on the computer device to which the solution of the present application is applied. The specific computer device may include more or fewer components than shown in the figure, or combine certain components, or have a different component arrangement.

[0133] In one embodiment, a computer device is provided, including a memory and a processor, wherein a computer program is stored in the memory, and when the processor executes the computer program, the following steps are implemented:

[0134] Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario;

[0135] Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space variable correction.

[0136] According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image;

[0137] The complex image is processed using a phase gradient self-focusing method to obtain a final imaging result.

[0138] In one embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the following steps are implemented:

[0139] Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario;

[0140] Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space variable correction.

[0141] According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image;

[0142] The complex image is processed by a phase gradient self-focusing method to obtain a final imaging result. The complex image is processed by a phase gradient self-focusing method to obtain a final imaging result.

[0143] Those skilled in the art will appreciate that all or part of the processes in the above-mentioned embodiments can be implemented by instructing the relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, database or other media used in the embodiments provided in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM).

[0144] 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.

[0145] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art could make various modifications and improvements without departing from the spirit of the present application, all of which fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A low-altitude, close-range, wide-band SAR squint imaging method, characterized in that: The method comprises: Acquire target echo signals and related INS measurement data, where the target echo signals are detected by a SAR radar system in a low-altitude, close-range, wide-swept, and high-squint imaging scenario; Based on the idea of ​​combining range movement with along-track motion error and uniformly correcting the range envelope error space variable, a CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal is processed using the CS quadratic phase function to obtain the echo signal after range envelope space-variation correction. The CS quadratic phase function is expressed as: In the above formula, Indicates the direction of slow time, Indicates distance to fast time, Indicates the distance modulation frequency, represents the reference center slope distance, Indicates the reference instantaneous center slope distance, represents the distance variable factor, represents the speed of light; According to the INS measurement data, the echo signal after the range envelope space variation correction is sequentially subjected to range envelope correction and compensation and azimuth phase error compensation, and the range and azimuth directions are respectively focused to obtain a complex image; The complex image is processed using a phase gradient self-focusing method to obtain a final imaging result.

2. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 1, characterized in that: When performing distance envelope correction and compensation: Perform range-direction fast Fourier transform on the echo signal after range envelope space-variation correction to obtain the range frequency domain echo signal; The range frequency domain echo signal is subjected to range movement correction and range space invariant envelope error compensation by using a range envelope movement compensation function to obtain a range frequency domain echo signal that has completed range envelope correction and compensation.

3. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 2, characterized in that: After obtaining the range frequency domain echo signal with complete range envelope compensation, range matched filtering is performed on it, and the processed data is subjected to range inverse Fourier transform to obtain data that needs to be compensated for azimuth phase after range processing. When performing range matched filtering, a range matched filter adapted to the CS quadratic phase function is used to filter the corrected range frequency domain echo signal.

4. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 3, characterized in that: The distance matched filter is expressed as: In the above formula, represents the distance frequency, Represents the distance-varying factor.

5. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 4, characterized in that: When performing phase error compensation: Perform range space-variant phase error compensation and Doppler center space-variant correction on the data to be subjected to phase error compensation, to obtain space-variant corrected data; After performing azimuth de-skew and residual video phase compensation on the space-variant corrected data through an azimuth matched filter, azimuth fast Fourier transform is performed to obtain the complex image.

6. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 5, characterized in that: The position matched filter and the residual video phase compensation function are expressed as: In the above formula, the azimuth frequency modulation With the distance and oblique angle of view, Indicates the instantaneous slant range when the center of the beam illuminates the target.

7. The low-altitude, close-range, wide-band SAR squint imaging method according to any one of claims 1 to 6, characterized in that: The complex image is processed using a distance-blocked phase gradient autofocusing method.

8. A low-altitude, close-range, wide-band SAR squint imaging device, characterized in that: The device comprises: A target echo signal acquisition module is used to acquire target echo signals and related INS measurement data. The target echo signals are obtained by the SAR radar system detecting the target in a low-altitude, close-range, wide-width, and large-squint imaging scenario. The range envelope space-variation correction module is used to uniformly correct the range envelope error space-variation based on the idea of ​​combining range movement with along-track motion error. The module constructs a CS quadratic phase function in the two-dimensional time domain and uses the CS quadratic phase function to process the target echo signal to obtain an echo signal after range envelope space-variation correction. The CS quadratic phase function is expressed as: In the above formula, Indicates the direction of slow time, Indicates distance to fast time, Indicates the distance modulation frequency, represents the reference center slope distance, Indicates the reference instantaneous center slope distance, represents the distance variable factor, represents the speed of light; A complex image obtaining module is used to perform range envelope correction and compensation and azimuth phase error compensation on the echo signal after range envelope space variation correction according to the INS measurement data, and to focus the range and azimuth directions respectively to obtain a complex image; The complex image focusing module is used to process the complex image using a phase gradient autofocusing method to obtain a final imaging result.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 7 are implemented.

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

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