Low-altitude short-distance wide-width SAR squint imaging method, device, equipment and medium

Abstract

The invention relates to a low-altitude short-distance wide-width SAR squint imaging method, device, equipment and medium, based on the thought of combining range walk and along-track motion errors and uniformly correcting distance envelope error space variables, a CS quadratic phase function is constructed in a two-dimensional time domain, and the space variables of the distance envelope errors are corrected. A CS quadratic phase function is used for processing a target echo signal obtained by detecting a target by an SAR radar system in a low-altitude short-distance wide-width squint scene to obtain an echo signal after distance envelope space-variant correction; sequentially performing range envelope correction and compensation and azimuth phase error compensation on the echo signal after range envelope space-variant correction according to INS measurement data, respectively performing focusing processing on range and azimuth to obtain a complex image, and processing the complex image by adopting a phase gradient self-focusing method to obtain a final imaging result. The method can effectively improve the 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 the traditional squint SAR imaging geometric configuration, the squint angle is uniformly taken as the squint angle corresponding to the center point of the scene, which has good imaging effect at long distance or small scene. However, in the case of close-range and wide-band, due to the large imaging range of the target area and the relatively small distance between the platform and the center point of the scene, the spatial squint angle corresponding to each point in the target area is different. The conventional geometric configuration will produce certain errors, resulting in poor imaging focusing effect and other phenomena.

[0003] Therefore, in the process of short-range wide-band SAR imaging, how to effectively solve the problem of spatial variation of oblique angle and distance 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-band SAR squint imaging method, device, equipment and medium that can effectively improve the imaging quality in response to the above-mentioned 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, wherein the target echo signals are detected by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario;

[0007] Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction.

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

[0009] The complex image is processed by 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 fast 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 slope distance, Δr(t a ; R s ) represents the distance variable factor, and c represents the speed of light.

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

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

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

[0016] In one embodiment, after obtaining the range frequency domain echo signal with fully compensated range envelope, a range matching filter is performed on it, and the processed data is subjected to a range inverse Fourier transform to obtain data that needs to be subjected to azimuth phase compensation after range processing, wherein when performing range matching filtering, a range matching 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 the distance frequency.

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

[0021] Performing distance 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, an 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 modulation frequency R varies with distance and slant angle, and represents the instantaneous slant distance 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, short-range, wide-band SAR squint imaging device, the device comprising:

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

[0029] The distance envelope space-variation correction module is used to unify the idea of ​​correcting the distance envelope error space-variation based on the combination of distance movement and 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 the echo signal after the distance 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 the range envelope space-variation correction in sequence according to the INS measurement data, and to perform focusing processing on 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 self-focusing method to obtain a final imaging result.

[0032] A computer device comprises 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, wherein the target echo signals are detected by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario;

[0034] Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction.

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

[0036] The complex image is processed by 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, wherein the target echo signals are detected by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario;

[0039] Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction.

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

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

[0042] The low-altitude, close-range, wide-band SAR squint imaging method, device, equipment and medium are based on the idea of ​​combining range movement with along-track motion errors and uniformly correcting the empty variable of the range envelope error. The CS quadratic phase function is constructed in the two-dimensional time domain. The target echo signal obtained by the SAR radar system in the low-altitude, close-range, wide-band squint scene is processed by the CS quadratic phase function to obtain the echo signal after the range envelope space-variation correction. Then, according to the INS measurement data, the echo signal after the range envelope space-variation correction is sequentially corrected for the range envelope and compensated for the azimuth phase error. The range and azimuth directions are focused respectively to obtain a complex image. The complex image is then processed by the phase gradient self-focusing method to obtain the final imaging result. The 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 an embodiment;

[0044] Figure 2 A schematic diagram of a curve showing a change in the oblique angle with distance in one embodiment;

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

[0046] Figure 4 A schematic diagram of a process of a low-altitude, close-range, wide-band SAR squint imaging method in one embodiment;

[0047] Figure 5 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 an 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 A 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) is the schematic diagram of the RCM trajectory after being processed by the method proposed in this paper;

[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 being processed by the method in this paper;

[0051] Fig. 9 This is a schematic diagram of the motion errors of each axis in an experiment;

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

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

[0054] Fig.12 This is a schematic diagram of the comparison of the azimuth profiles of the imaging results at each point in an experiment, where: Fig.12 (a) is the azimuth profile of a close-range point target. Fig.12 (b) is the azimuth profile of a long-range point target;

[0055] Fig.13 1 is a structural block diagram of a low-altitude, short-range, wide-band SAR oblique imaging device in one embodiment;

[0056] Fig.14FIG. 4 is a diagram showing the internal structure of a computer device in one embodiment. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present 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 distance 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 figure 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 nearest slant range and the ground.

[0061] In the traditional squint SAR imaging geometric configuration, the squint angle is uniformly taken as the squint angle corresponding to the center point of the scene, which has good imaging effect at long distance or small scene. However, in the case of close-range wide-band, due to the large imaging range of the target area, the distance between the platform and the center point of the scene is relatively small, and there are differences in the spatial squint angles corresponding to each point in the target area. The use of conventional geometric configuration will produce certain errors, resulting in poor imaging focusing effect, etc. Therefore, in the process of close-range wide-band SAR imaging processing, how to effectively solve the problem of squint angle distance variation is the key to improving imaging quality.

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

[0063] Further imaging spatial variation analysis is performed, considering that range walk is the main part of range migration when large squint imaging is used, ignoring the influence of squint angle on the third and higher order RCM, and mainly analyzing the linear range cell migration (LRCM) spatial variation error and the quadratic range cell migration (QRCM) spatial variation error. 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 empty variable in the large squint scene. Figure 3 As can be seen in (a), the LRCM varies greatly. When the distance position is greater than about 300m from the reference point, the LRCM spatial variation reaches 1 / 2 range resolution unit, and the imaging quality will gradually begin to be affected. This distance spatial variation characteristic brings great challenges to imaging processing.

[0064] Further analysis of the motion error space variation found that for the distance space variation envelope error, the one-step motion compensation method is implemented through interpolation, which has a large amount of calculation and low processing efficiency. Therefore, the signal is usually divided into blocks by distance to ignore the distance space variation of the sub-block envelope error. At this time, it only needs to be multiplied by a linear phase in the distance frequency domain to avoid complex interpolation. However, the precise distance envelope of each ranging unit is not compensated, especially at the boundary of the sub-band, the phase compensation reference function is mismatched with the actual phase error, affecting the accuracy of the phase compensation, which directly affects the focusing effect of the target.

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

[0066] Step S100, acquiring a target echo signal and related INS measurement data, wherein 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] Step S110, based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance envelope error, a CS quadratic phase function is constructed in the two-dimensional time domain, and the target echo signal is processed using the CS quadratic phase function to obtain the echo signal after the distance 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 in sequence 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 self-focusing method to obtain a final imaging result.

[0070] In the present application, consideration of variable oblique viewing angle is added in imaging processing and motion error. In imaging processing, the CS scaling function is first used to correct the distance envelope spatial variation. Compared with the traditional imaging method, this method performs compensation before low-resolution wide-band imaging to avoid 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, making it possible to achieve subsequent real-time imaging, and improving the distance spatial variation line of sight motion error factor in motion compensation, thereby improving the accuracy of motion compensation to obtain a well-focused SAR image.

[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 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 motion error along the track to uniformly correct the distance envelope error spatial variation, and the instantaneous slant range R 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 is further rewritten as:

[0077]

[0078] In formula (4), ΔR c (t a ; R o ), ΔR v (t a ; R) respectively represent the distance envelope movement with the center point as the reference and the residual distance envelope space-variable error of any point target relative to the center point, Δr(t a ; R0) represents the distance variable 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 variable 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 fast 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 slope distance, Δr(t a ; R s ) represents the distance variable 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 following can be obtained by simplification:

[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 distance envelope non-empty variables of the motion error caused by the distance movement and the actual flight trajectory remain, 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 a 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 a 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 that has completed the uniform compensation of the range envelope error, the 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 the 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 the 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, at which time the range envelope error is accurately compensated. 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 completed in the two-dimensional time domain, including Doppler Center Correction (DCC), range space-variant phase error compensation and azimuth de-skew. Specifically, the data to be subjected to azimuth phase error compensation is subjected to range space-variant phase error compensation and Doppler center space-variant correction to obtain space-variant corrected data, and the space-variant corrected data is subjected to azimuth de-skew and residual video phase compensation through an azimuth matched filter, and then azimuth fast Fourier transform is 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-variant phase error compensation and Doppler center space-variant correction, using the following formula:

[0100]

[0101] Furthermore, the azimuth matched 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 distance when the center of the beam illuminates the target.

[0104] In this embodiment, if the inertial navigation accuracy is sufficient, there is no need for data-based motion compensation, and an image with good focusing performance can be obtained after performing step S120. However, in actual flight conditions, the output complex image is further estimated using a Phase Gradient Autofocus (PGA) algorithm to obtain a precisely focused image, and considering the space-varying characteristics of the error, the entire image is usually divided into small blocks. Since the phase error we are discussing is only space-varying in distance, the output complex image is subjected to a PGA algorithm with distance blocks for precise motion compensation.

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

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

[0107] Table 1 Simulation parameters

[0108]

[0109] Through point target simulation experiments, the effectiveness of the improved algorithm is analyzed and evaluated, 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, three point targets P0, P1, and P2 are set, arranged along the same beam center at the same time (with the same orientation in the imaging coordinate system), and have different distance coordinates (0m±1000m from the scene center), as shown in Figure 6 As shown in the figure, P0 is the center point, P1 and P2 are the near point and 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, 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 and analyzing the focusing effect of the proposed method with the traditional configuration SAR imaging algorithm in detail, 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 two algorithms after RCMC for the three point targets are shown in Table 1. 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 by the CS function using 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 a range gate, and the RCM trajectory is straightened.

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

[0115] The above simulation verifies that under ideal error-free conditions, the range travel 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: Fig. 9 The motion error caused by the platform is enough 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 distance envelope error and phase error respectively) can be used to obtain a well-focused point target.

[0117] First, the effectiveness of the motion compensation method for RCMC is verified. Fig.10 The correction results and partial enlargement after the space-variable range movement correction and range migration operations are shown in the figure. It can be seen from the figure that the migration error caused by the actual movement makes it impossible to accurately correct the RCM trajectory, and the echo energy spans multiple range gates.

[0118] Therefore, envelope correction is required. Fig.11 This is the correction result after motion compensation. Fig.11 (a) is the correction result after unified 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 space-variant envelope correction method in this paper, the correction results are as follows: Fig.11 As shown in (b), the echo energies of the two edge points are basically corrected to the same range gate, which effectively solves the influence of range space variation on the accuracy of RCMC 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. Fig.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 spatial variation of distance), formula B (considering only the spatial variation of pitch angle with distance) and formula A (considering both the spatial variation of pitch angle and squint angle with distance). Table 2 compares the imaging results of motion compensation using formula A (considering only the spatial variation of pitch angle and squint angle with distance). Fig.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, the two point targets on the edge are severely defocused in azimuth when using formula C, which is a unified compensation method for the scene center motion error. Formula B takes into account the influence of the pitch angle distance variation, so after compensation, the focusing effect of the two point targets is significantly improved, but the close-range points are still severely defocused. After compensation using formula A, the close-range points are successfully focused, and the side lobes are suppressed to a certain extent. The azimuth performance indicators are significantly improved compared to formula B.

[0121] Table 2 Comparison of target azimuth performance indicators after compensation of each method

[0122]

[0123] In the above-mentioned low-altitude, close-range, wide-swath SAR squint imaging method, in order to solve the problem of range space-variation existing in the case of low-altitude, close-range, wide-swath SAR squint imaging, the geometric configuration of close-range, wide-swath SAR imaging and the slant range equivalent model are first established based on the speed and position parameters obtained by INS. On the basis of theoretical analysis, compared with the traditional algorithm, the consideration of the variable slant angle is added in the imaging processing and motion error. In 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 to avoid RCM azimuth space-variation and subsequent compensation operations, and compensates the range migration correction together with the motion error, without the need for interpolation operation, reducing the number of FFT operations, and providing the possibility for the realization of subsequent real-time imaging. In the motion compensation, the range space-variation line of sight motion error factor is improved to improve the accuracy of motion compensation to obtain a well-focused SAR image.

[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. Moreover, 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, Fig.13 As shown, a low-altitude, short-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 construct a CS quadratic phase function in the two-dimensional time domain based on the idea of ​​combining the range movement with the along-track motion error to uniformly correct the range envelope error space-variation, and use the CS quadratic phase function to process the target echo signal to obtain the echo signal after the range envelope space-variation correction;

[0128] A complex image obtaining module 220 is used to perform range envelope correction and compensation and azimuth phase error compensation on the echo signal after the range envelope space-variation correction in sequence 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 self-focusing method to obtain a final imaging result.

[0130] For 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, which will not be repeated here. Each module in the above-mentioned low-altitude, close-range, wide-band SAR squint imaging device can be implemented in whole or in part by software, hardware, and a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in the form of hardware, or can be stored in the memory of the computer device in the form of software, so that the processor can call and execute the operations corresponding to the above 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: Fig.14 As shown. The computer device includes a processor, a memory, a network interface, a display screen and an input device connected through a system bus. Among them, 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 through 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 screen or an electronic ink display screen, and the input device of the computer device can be a touch layer covered on the display screen, or a button, trackball or touchpad set on the computer device housing, or an external keyboard, touchpad or mouse, etc.

[0132] Those skilled in the art will understand that Fig.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 those shown in the figure, or combine certain components, or have a different arrangement of components.

[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, wherein the target echo signals are detected by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario;

[0135] Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction.

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

[0137] The complex image is processed by 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, and when the computer program is executed by a processor, the following steps are implemented:

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

[0140] Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction.

[0141] According to the INS measurement data, the echo signal after the range envelope space-variation correction is sequentially corrected and compensated for the range envelope and the azimuth phase error is compensated, 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 can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program, and 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. As an illustration and not limitation, RAM is available in many 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 may be arbitrarily combined. 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-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached 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, wherein the target echo signals are detected by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario; Based on the idea of ​​combining the distance movement with the motion error along the track and uniformly correcting the empty variable of the distance 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 the echo signal after the distance envelope empty variable correction. According to the INS measurement data, the echo signal after the range envelope space-variation correction is sequentially corrected and compensated for the range envelope and the azimuth phase error is compensated, and the range and azimuth directions are respectively focused to obtain a complex image; The complex image is processed by 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: The CS quadratic phase function is expressed as: In the above formula, t a Indicates the azimuth slow time, t r Indicates the distance to the fast 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 slope distance, Δr(t a ; R s ) represents the distance variable factor, and c represents the speed of light.

3. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 2, characterized in that: When performing distance envelope correction and compensation: Performing range-direction fast Fourier transform on the echo signal after the range envelope space-variation correction to obtain the range frequency domain echo signal; The distance frequency domain echo signal is subjected to distance movement correction and distance space invariant envelope error compensation by using a distance envelope movement compensation function to obtain a distance frequency domain echo signal that has completed distance envelope correction and compensation; After obtaining the range frequency domain echo signal with fully compensated range envelope, a range matching filter is performed on it, and the processed data is subjected to a range inverse Fourier transform to obtain data that needs to be subjected to azimuth phase compensation after range processing, wherein when performing range matching filtering, a range matching 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, f r Indicates the distance frequency.

5. The low-altitude, close-range, wide-band SAR squint imaging method according to claim 4, characterized in that: When performing phase error compensation: Performing distance 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, an 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 azimuth matched filter and the residual video phase compensation function are expressed as: In the above formula, the azimuth modulation frequency R varies with distance and slant angle, and represents the instantaneous slant distance 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 by adopting a distance-blocked phase gradient self-focusing method.

8. A low-altitude, short-range, wide-band SAR oblique imaging device, characterized in that: The device comprises: A target echo signal acquisition module is used to acquire a target echo signal and related INS measurement data. The target echo signal is obtained by detecting the target by the SAR radar system in a low-altitude, close-range, wide-width, and large-squint imaging scenario; The distance envelope space-variation correction module is used to unify the idea of ​​correcting the distance envelope error space-variation based on the combination of distance movement and 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 the echo signal after the distance envelope space-variation correction; A complex image obtaining module is used to perform range envelope correction and compensation and azimuth phase error compensation on the echo signal after the range envelope space-variation correction in sequence according to the INS measurement data, and to perform focusing processing on 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 self-focusing 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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