Laser synthetic aperture sparse imaging method for moving targets based on coherent array detector

Abstract

The present disclosure provides a kind of based on coherent array detector moving target laser synthetic aperture sparse imaging method, comprising: using narrowband laser emission signal to irradiate the target to be detected, and the target to be detected exists rotation and translation;The multiple frames of laser complex images of the target to be detected are collected by laser local oscillator coherent array detector;On the basis of the motion parameter estimation of the target to be detected, the multiple frames of laser complex images are carried out Fourier inverse transform in digital domain, and the multiple frames of spatial sampling signals are obtained, and the multiple frames of spatial sampling signals are carried out phase compensation and motion compensation;The spatial sampling center is calculated, and the multiple frames of spatial sampling signals are spliced, and large range space sparse sampling signal is formed;By lengthening observation time, the sparsity of the large range space sparse sampling signal is reduced, and by compressed sensing processing, the high-resolution image reconstruction and image sidelobe suppression of large range space sparse sampling signal are realized.

Description

Technical Field

[0001] This disclosure relates to the field of laser synthetic aperture imaging technology, and in particular to a laser synthetic aperture sparse imaging method for moving targets based on a coherent array detector. Background Technology

[0002] Large-aperture telescopes can achieve high-resolution imaging, but they are difficult to manufacture and support on various platforms. Currently, inverse synthetic aperture radar (ISAL) is generally used for high-resolution imaging. ISAL acquires echo signals in a time-division manner by measuring the relative motion between the laser radar and the target, effectively increasing the telescope aperture. When the laser is self-emitting and self-receiving with a center wavelength of 1.55 μm, a tiny rotation of 15.5 μrad (3 arcseconds) of the target can achieve imaging with a resolution of 5 cm, regardless of the detection distance. When the moving target rotates slowly and at a small angle, the target's rotation and translation are equivalent in ISAL imaging.

[0003] ISAL (Intense Optical Array) systems mostly use single-element detectors and achieve high-resolution range-azimuth imaging by emitting and processing broadband laser signals, which suffers from problems such as system complexity and small field of view. Array detectors have the advantage of a large field of view, but currently commonly used array detectors employ incoherent detection systems. The loss of image phase information means that the acquired images cannot reflect target motion information under far-field conditions, and the imaging resolution is limited by factors such as telescope aperture and detector pixel size. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] To address the existing technical problems, this disclosure provides a method for laser synthetic aperture sparse imaging of moving targets based on a coherent array detector, which at least partially solves the above-mentioned technical problems.

[0006] (II) Technical Solution

[0007] This disclosure provides a method for laser synthetic aperture sparse imaging of moving targets based on a coherent array detector, comprising: illuminating the target with a laser emission signal to form a target laser echo signal, wherein the target is moving based on a motion direction, including the x′ and y′ directions, and the motion includes rotation and / or translation; xyz is a telescope coordinate system, with the x′ axis parallel to the x-axis and the y′ axis parallel to the y-axis; the laser emission signal is a narrowband laser signal; performing a Fourier transform on the target laser echo signal through a telescope to form a telescope received signal; and sampling the telescope received signal through a laser local oscillator coherent array detector to form a multi-frame laser complex image, the laser complex image including... Amplitude and phase information are obtained; motion parameters of the target are estimated; inverse Fourier transform is performed on multiple frames of laser complex images in the digital domain to obtain multiple frames of spatial sampling signals; phase compensation and motion compensation are performed on the multiple frames of spatial sampling signals according to the motion parameters, and the center of the compensated multiple frames of spatial sampling signals is calculated; the compensated multiple frames of spatial sampling signals are stitched together according to the center of the multiple frames of spatial sampling signals to form a large-scale spatial sparse sampling signal; by extending the observation time, the sparsity of the large-scale spatial sparse sampling signal is reduced, and high-resolution image reconstruction and image sidelobe suppression are performed on the large-scale spatial sparse sampling signal through compressed sensing processing to obtain the laser synthetic aperture image of the target.

[0008] Optionally, the laser synthetic aperture sparse imaging method for moving targets based on a coherent array detector further includes: modulating a seed source signal with a reference intermediate frequency signal to obtain a modulated signal; amplifying the modulated signal to obtain a laser emission signal; wherein the laser emission signal adopts a transmit-receive separation method and a wide beam emission form, the laser local oscillator coherent array detector adopts a wide field of view reception form, and the wide beam and wide field of view cover the target to be detected; the laser emission signal is a narrowband pulsed laser signal or a narrowband continuous wave laser signal; and the reference intermediate frequency signal is used to realize the sampling time and phase synchronization between the laser emission signal and the target laser echo signal, and is used to perform intermediate frequency sampling and low-pass filtering on the telescope received signal in the electronics.

[0009] Optionally, sampling the telescope-received signal using a laser local oscillator coherent array detector to form a multi-frame laser complex image includes: coherently detecting the telescope-received signal and the laser local oscillator signal to form an intermediate frequency (IF) signal, wherein the laser local oscillator signal and the seed source signal are from the same source; performing IF sampling on the IF signal and then performing analog-to-digital conversion to form a three-dimensional digital signal, wherein the three dimensions include elevation, azimuth, and fast time direction; and performing low-pass filtering and integration processing on the three-dimensional digital signal in the fast time direction to obtain a two-dimensional multi-frame laser complex image, wherein the two dimensions include elevation and azimuth.

[0010] Optionally, phase compensation and motion compensation of multi-frame spatial sampling signals include: performing single-frame second-order phase compensation and single-frame motion phase compensation on each frame of spatial sampling signal to obtain multi-frame compensated spatial sampling signals, wherein the motion phase includes rotational phase and / or translational phase; and performing slow-time vibration phase compensation on the multi-frame compensated spatial sampling signals to obtain multi-frame vibration compensated spatial sampling signals, which are used for splicing multi-frame compensated spatial sampling signals under target vibration conditions.

[0011] Optionally, slow-time vibration phase compensation of multi-frame compensated spatial sampling signals includes: calculating the mean of each frame compensated spatial sampling signal to form a one-dimensional slow-time signal; using a spatial correlation algorithm to calculate the one-dimensional slow-time signal to obtain the vibration phase; and performing vibration phase compensation on the multi-frame compensated spatial sampling signals based on the vibration phase to obtain multi-frame vibration compensated spatial sampling signals.

[0012] Optionally, estimating the motion parameters of the target to be detected includes: determining multiple prominent points at different locations on the target; performing interferometric phase integration on multiple prominent points in adjacent sampled multi-frame laser complex images to obtain a phase curve; fitting the phase curve to obtain phase change information; and obtaining the motion parameters based on the phase change information.

[0013] Optionally, performing an inverse Fourier transform on the multi-frame laser complex image in the digital domain to obtain multi-frame spatial sampling signals includes: increasing the overlap rate of the multi-frame spatial sampling signals and increasing the number of frames of the multi-frame laser complex image to suppress the influence of noise on the imaging results; and performing low-pass filtering on the multi-frame laser complex image to improve the signal-to-noise ratio of the multi-frame laser complex image.

[0014] Optionally, the laser local oscillator coherent array detector is positioned on the focal plane of the telescope, and the telescope employs a thin-film diffraction mirror.

[0015] (III) Beneficial Effects

[0016] Compared with existing technologies, the laser synthetic aperture sparse imaging method for moving targets based on coherent array detectors provided in this disclosure has at least the following advantages:

[0017] (1) The target to be detected in this disclosure moves in two directions. Multiple low-resolution complex images are acquired by an array detector. The phase information in the complex images can represent the motion information of the target to be detected. The motion of the target to be detected can be used to form a virtual large-aperture telescope. Based on the phase-compensated complex images, high-resolution laser synthetic aperture imaging can be obtained.

[0018] (2) This disclosure uses narrowband laser signals, eliminating the need for lidar to transmit and process broadband signals, resulting in a simple system with fewer devices. The imaging system based on a laser local oscillator coherent array detector and a diffractive thin-film mirror is characterized by its simplicity and ease of engineering implementation.

[0019] (3) This disclosure does not involve range resolution. Laser synthetic aperture imaging in the elevation and azimuth directions has strict mathematical relationships and clear physical meaning, which is consistent with the traditional concept of matched filter coherent imaging.

[0020] (4) Under the condition of a fixed resolution increase factor, the increase in the overlap rate of the spatial sampling signals of the low-resolution complex image requires an increase in the number of frames of the low-resolution complex image used in laser synthetic aperture imaging, which helps to suppress the influence of noise on the high-resolution imaging results. Attached Figure Description

[0021] The above and other objects, features and advantages of this disclosure will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0022] Figure 1 A flowchart illustrating a method for laser synthetic aperture sparse imaging of moving targets based on a coherent array detector according to an embodiment of the present disclosure is shown.

[0023] Figure 2 A schematic diagram illustrating the three-dimensional geometry of a laser synthetic aperture imaging system according to an embodiment of the present disclosure is shown.

[0024] Figure 3 A schematic diagram of a receiving system for laser synthetic aperture imaging according to an embodiment of the present disclosure is shown.

[0025] Figure 4 A schematic diagram illustrating the shape of a moving target according to an embodiment of the present disclosure is shown.

[0026] Figure 5A This schematically illustrates a three-dimensional matrix diagram composed of spatially sampled signals corresponding to multiple frames of low-resolution complex images according to an embodiment of the present disclosure;

[0027] Figure 5B A schematic diagram illustrating a comparison between the estimated vibration phase and the actual vibration phase according to an embodiment of the present disclosure is shown.

[0028] Figure 5C A schematic diagram illustrates the laser synthetic aperture imaging result before vibration phase compensation according to an embodiment of the present disclosure;

[0029] Figure 5D A schematic diagram illustrates the laser synthetic aperture imaging result after vibration phase compensation according to an embodiment of the present disclosure;

[0030] Figure 6A An amplitude diagram of a low-resolution complex image according to an embodiment of the present disclosure is illustrated schematically;

[0031] Figure 6BA phase map of a low-resolution complex image according to an embodiment of the present disclosure is schematically shown;

[0032] Figure 6C A schematic diagram of a raster target slice of a low-resolution complex image according to an embodiment of the present disclosure is shown.

[0033] Figure 6D A schematic diagram illustrating the corresponding spatial sampling signal of a low-resolution complex image according to an embodiment of the present disclosure is shown.

[0034] Figure 7A An amplitude diagram of laser synthetic aperture imaging results of a micro-rotating target with a spatial sampling signal overlap rate of 0, according to an embodiment of the present disclosure, is illustrated.

[0035] Figure 7B The phase diagram of the laser synthetic aperture imaging result of a micro-rotating target with a spatial sampling signal overlap rate of 0, according to an embodiment of the present disclosure, is schematically shown.

[0036] Figure 7C A lattice target slice diagram illustrating the laser synthetic aperture imaging result of a micro-rotating target with a spatial sampling signal overlap rate of 0 according to an embodiment of the present disclosure is shown schematically.

[0037] Figure 7D The diagram schematically illustrates the corresponding spatial sampling signal of the laser synthetic aperture imaging result of a micro-rotating target when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure.

[0038] Figure 8A An amplitude diagram of the laser synthetic aperture imaging result of a translational target with a spatial sampling signal overlap rate of 0, according to an embodiment of the present disclosure, is illustrated.

[0039] Figure 8B The phase diagram of the laser synthetic aperture imaging result of a translational target with a spatial sampling signal overlap rate of 0, according to an embodiment of the present disclosure, is schematically shown.

[0040] Figure 8C A schematic diagram of a lattice target slice of a translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure;

[0041] Figure 8D The diagram schematically illustrates the corresponding spatial sampling signal diagram of the translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0 according to an embodiment of the present disclosure;

[0042] Figure 9A An amplitude diagram of laser synthetic aperture imaging results is schematically shown when the spatial sampling signal overlap rate is 75% according to an embodiment of the present disclosure;

[0043] Figure 9BA phase diagram of laser synthetic aperture imaging results with a spatial sampling signal overlap rate of 75% according to an embodiment of the present disclosure is illustrated schematically.

[0044] Figure 9C A schematic diagram of a lattice target slice of laser synthetic aperture imaging results with a spatial sampling signal overlap rate of 75% according to an embodiment of the present disclosure is shown.

[0045] Figure 9D The diagram illustrates the corresponding spatial sampling signal of a laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 75% according to an embodiment of the present disclosure.

[0046] Figure 10 The illustration schematically shows a spatial sampling signal map corresponding to the laser synthetic aperture imaging result formed by multiple observations when the spatial sampling signal overlap rate is 75%, the target motion is a mixture of translation and rotation, and the rotation speed and direction of motion are different. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0048] It should be noted that similar or identical parts are referred to by the same reference numerals in the accompanying drawings or description. The technical features of the various embodiments exemplified in the specification can be freely combined to form new solutions without conflict. Furthermore, each claim can stand alone as an embodiment, or the technical features in the various claims can be combined to form new embodiments. In the drawings, the shape or thickness of the embodiments may be enlarged and indicated in a simplified or convenient manner. Moreover, elements or implementations not shown or described in the drawings are those known to those skilled in the art. Additionally, although this document provides examples of parameters containing specific values, it should be understood that the parameters need not be exactly equal to the corresponding values, but can approximate the corresponding values ​​within acceptable error tolerances or design constraints.

[0049] Unless there are technical obstacles or contradictions, the various embodiments described above in this disclosure can be freely combined to form other embodiments, all of which are within the protection scope of this disclosure.

[0050] Although this disclosure has been described in conjunction with the accompanying drawings, the embodiments disclosed in the drawings are intended to illustrate preferred embodiments of this disclosure and should not be construed as limiting the disclosure. The dimensions in the drawings are merely illustrative and should not be construed as limiting the disclosure.

[0051] While some embodiments of the general concept of this disclosure have been shown and described, those skilled in the art will understand that changes may be made to these embodiments without departing from the principles and spirit of the general concept of this disclosure, the scope of which is defined by the claims and their equivalents.

[0052] According to embodiments of this disclosure, Figure 1 A flowchart illustrating a method for laser synthetic aperture sparse imaging of moving targets based on a coherent array detector according to an embodiment of the present disclosure is shown. Figure 1 As shown, a laser synthetic aperture sparse imaging method for moving targets based on a coherent array detector includes, for example:

[0053] S110 uses a laser emission signal to illuminate the target to be detected, forming a target laser echo signal. The target to be detected moves based on the direction of motion, which includes the x′ and y′ directions. The motion includes rotation and / or translation. xyz is the telescope coordinate system, with the x′ axis parallel to the x-axis and the y′ axis parallel to the y-axis. The laser emission signal is a narrowband laser signal.

[0054] For example, the motion of the target to be detected includes translation, rotation, and a mixture of translation and rotation.

[0055] According to embodiments of this disclosure, Figure 2 A schematic diagram illustrating the three-dimensional geometry of a laser synthetic aperture imaging system according to an embodiment of the present disclosure is shown. Figure 2 As shown, the target to be detected is, for example, a moving target such as a satellite in the far field. The satellite rotates about axes in the x′ and y′ directions and translates at a velocity v in the x′-y′ plane.

[0056] For example, narrowband laser signals are transmitted and received separately using a wide beam transmission method. A wide field of view is received using a laser local oscillator coherent array detector. Pitch-azimuth imaging is achieved by utilizing the two-dimensional motion of the target to be detected. Both the transmitted signal beam and the received field of view can cover the target to be detected.

[0057] It is understood that narrowband laser signal refers to a narrowband laser signal relative to the signal emitted by the inverse synthetic aperture lidar (ISAL). The azimuth resolution of ISAL imaging is related to its bandwidth, which can reach hundreds of MHz or more. The laser signal bandwidth used in the simulation of the method proposed in this disclosure is, for example, 1 MHz, and the resolution of each frame of complex image is related to parameters such as telescope aperture, focal length, and array detector pixel size.

[0058] For example, a laser seed source generates a narrowband pulsed or continuous wave laser signal. This seed source signal is modulated using an electronic reference intermediate frequency (IF) signal, amplified, and then emitted by the laser. The reference IF signal is used to synchronize the time and phase of the emitted laser signal with the target laser echo signal during electronic IF sampling. Simultaneously, it is used in the electronics to perform IF sampling and low-pass filtering on the telescope's received signal.

[0059] S120 uses a telescope to perform a Fourier transform on the target laser echo signal to generate the telescope's received signal.

[0060] S130 samples the telescope-received signal through a laser local oscillator coherent array detector to form multiple frames of laser complex images, which include amplitude and phase information.

[0061] According to embodiments of this disclosure, for example, multiple laser composite images are formed through steps S131 to S133.

[0062] Step S131: Coherently detect the telescope-received signal and the laser local oscillator signal to form an intermediate frequency signal. The laser local oscillator signal and the seed source signal are from the same source.

[0063] Step S132: After intermediate frequency sampling of the intermediate frequency signal, it is converted into a three-dimensional digital signal through analog-to-digital conversion. The three dimensions include pitch, azimuth and fast time directions.

[0064] Step S133: Low-pass filtering and integration are performed on the three-dimensional digital signal in the fast time direction to obtain a two-dimensional multi-frame laser complex image, which includes the pitch and azimuth directions.

[0065] For example, a laser local oscillator coherent array detector is placed on the focal plane of the telescope, and the telescope uses a thin-film diffraction mirror.

[0066] According to embodiments of this disclosure, Figure 3 A schematic diagram of a receiving system for laser synthetic aperture imaging according to an embodiment of the present disclosure is shown. Figure 3 As shown, the timer simultaneously generates a reference intermediate frequency (IF) signal and a sampling clock for the IF sampling analog-to-digital converter (ADC). The reference IF signal is used to synchronize the time and phase of the laser emission signal with the electronic ADC signal sampling, and is also used in the electronics for IF sampling and low-pass filtering. The acousto-optic modulator uses the electronic reference IF signal to modulate the seed source signal. The modulated signal is amplified by the laser to form a narrowband pulsed or continuous wave laser emission signal. This laser seed source simultaneously generates the laser local oscillator signal.

[0067] For example, the laser complex image and the laser local oscillator signal are coherently detected (heterodyne detection) on a laser local oscillator coherent array detector. The detection result is an elevation-azimuth-fast time three-dimensional signal, which is converted into a digital signal by an intermediate frequency sampling analog-to-digital converter. The fast time signal acquired by each pixel is gating, filtering, and integrating, and the three-dimensional data is converted into elevation-azimuth two-dimensional complex image data, making laser synthetic aperture imaging possible.

[0068] For example, laser synthetic aperture imaging systems provide two-dimensional imaging in the elevation and azimuth directions, without involving range resolution, and their system structure and imaging effects are similar to those of traditional optical imaging systems.

[0069] For example, compared with laser synthetic aperture imaging, the complex images acquired by laser local oscillator coherent array detectors are low-resolution images, and their resolution is related to parameters such as telescope aperture, focal length, and array detector pixel size.

[0070] According to embodiments of this disclosure, Figure 4 A schematic diagram illustrating the shape of a moving target according to an embodiment of the present disclosure is shown. Figure 4 As shown, the micro-rotating target consists of, for example, a three-dimensional satellite target and a 3×3 dot matrix target, and the distance between the center of the micro-rotating target and the center of the telescope is, for example, 20 km.

[0071] S140, Estimate the motion parameters of the target to be detected.

[0072] According to embodiments of this disclosure, for example, the motion parameters of the target to be detected are estimated through steps S141 to S144.

[0073] Step S141: Determine the distinctive points at multiple different locations on the target to be detected.

[0074] Step S142: Perform interferometric phase integration on multiple prominent points in adjacent sampled multi-frame laser complex images to obtain the phase curve.

[0075] Step S143: Fit the phase curve to obtain phase change information.

[0076] Step S144: Obtain motion parameters based on phase change information.

[0077] For example, by selecting distinctive points at different locations on the target, interferometric phase integration is performed on the distinctive points in adjacent images. The influence of noise is reduced by fitting the data, and the rotational angular velocity and translational velocity parameters are solved based on the phase change information of multiple complex images, thereby estimating the target motion parameters.

[0078] S150 performs an inverse Fourier transform on multiple frames of laser complex images in the digital domain to obtain multiple frames of spatial sampling signals.

[0079] According to embodiments of this disclosure, for example, multiple frames of spatial sampling signals are obtained through steps S151 to S152.

[0080] Step S151: Increase the overlap rate of the multi-frame spatial sampling signals and increase the number of frames of the multi-frame laser complex image to suppress the influence of noise on the imaging results.

[0081] Step S152: Low-pass filtering is performed on the multi-frame laser complex image to improve the signal-to-noise ratio of the multi-frame laser complex image.

[0082] For example, given a fixed detector pixel size and telescope focal length, the signal-to-noise ratio of complex images acquired by a coherent array detector can be improved by low-pass filtering the laser complex image.

[0083] For example, let the effective aperture size of the telescope be D. This effective aperture is determined by the telescope aperture and the pixel size of the array detector. When the telescope's plane coordinates (x, y) satisfy x 2 +y 2 ≤(D / 2) 2 When the angular resolution corresponding to the low-resolution complex image is lower than the diffraction-limited resolution of the telescope, the effective aperture size D of the telescope is smaller than the actual aperture of the telescope. At this time, the imaging resolution is limited by the pixel size of the array detector. Increasing the telescope aperture cannot improve the imaging resolution, but it can improve the signal-to-noise ratio of the laser complex image.

[0084] S160, based on motion parameters, performs phase compensation and motion compensation on the multi-frame spatial sampling signals, and calculates the center of the compensated multi-frame spatial sampling signals.

[0085] According to embodiments of this disclosure, for example, multiple frames of compensated images are obtained through steps S161 to S162.

[0086] Step S161: Perform single-frame second-order phase compensation and single-frame motion phase compensation on each frame of spatial sampling signal to obtain multi-frame compensated spatial sampling signals. The motion phase includes rotational phase and / or translational phase.

[0087] For example, suppose the target rotates around the x′ and y′ axes and translates in three-dimensional space, with the x′ and y′ axes parallel to the x and y axes, respectively. The telescope is located in the xy plane, and its center coincides with the origin of the coordinate axes. The distance between the center of the moving target and the center of the telescope is R0. The moving target can be considered as consisting of multiple point targets with minimal spacing. Then, the coordinate change of the i-th point in the moving target is:

[0088]

[0089] Where, ω x (t) and ω y(t) represents the angular velocities of the target's rotation around the x′ and y′ axes, respectively, t is the slow time for the coherent array detector to acquire a single frame of complex images, and f s This is the slow-time sampling frequency of the coherent array detector. and Let x be the rotation axis vector of the x′ and y′ axes, W be the coordinate transformation matrix, and Δx be the rotation axis vector of the x′ and y′ axes. i (t), Δy i (t) and Δz i (t) represents the translational distance of the target in three-dimensional space.

[0090] For example, single-frame second-order phase and single-frame motion phase are generated by the changes in distance between the telescope and the target caused by the telescope's plane range and the target's motion, respectively. Single-frame second-order phase Motion phase The expressions for the spatial sampling centers corresponding to the low-resolution complex image are as follows:

[0091]

[0092]

[0093] (2Δθ x R0, 2Δθ y R0) (4)

[0094] Where (x, y) are the coordinates of the telescope plane, λ is the center wavelength of the laser, R0 is the distance between the center of the moving target and the center of the telescope, t is the slow time for the coherent array detector to acquire multiple complex images, T is the duration of the slow time for the coherent array detector to acquire multiple complex images, and Δθ x =arctan(Δx / R0) and Δθ y =arctan(Δy / R0) represents the angles projected onto the target in the x and y directions during its motion, respectively, where Δx and Δy are the projected distances of the target in the x and y directions, respectively. mid (t), y mid (t), z mid (t) represents the coordinates of a reference point during the target's motion. For a slightly rotating target, the distance between this reference point and multiple (e.g., two) intersection points of the rotation axis is the average of the distances between all points on the target and the intersection points of the rotation axis. For a translational target, this reference point is a prominent point in the laser composite image.

[0095] For example, a laser synthetic aperture imaging system employs a separate transmit and receive pulse system, with a wide-beam laser signal transmission and a wide-field-of-view reception by a coherent array detector. Therefore, when the coherent array detector acquires a low-resolution image, the laser echo signal of a moving target on the telescope plane and its phase compensation expression are as follows:

[0096]

[0097] R i (x, y, t) = R t (t)+R r (x, y, t) (6)

[0098]

[0099]

[0100]

[0101] Where (x, y) are the coordinates of the telescope plane, s(x, y, t) is the laser echo signal of the moving target on the telescope plane, s′(x, y, t) is the phase-compensated laser echo signal, and N t For the number of point targets, (x i (t), y i (t), z i (t) represents the coordinates of the i-th target point, (x) t y t , z t () represents the laser coordinates, t is the slow time for the coherent array detector to acquire multiple frames of complex images, τ is the laser emission signal pulse width, λ is the laser center wavelength, c is the speed of light, and R... t (t) represents the distance the laser signal travels to the i-th target point, R. r (x, y, t) represents the distance the laser echo signal travels from the i-th point target to the telescope plane, R i (x, y, t) represents the total transmission and reception distance of the target signal at the i-th point.

[0102] Step S162: Perform slow-time vibration phase compensation on the multi-frame compensation spatial sampling signal to obtain a multi-frame vibration compensation spatial sampling signal, which is used for splicing the multi-frame compensation spatial sampling signal under the target vibration condition.

[0103] According to embodiments of this disclosure, slow-time vibration phase compensation is performed on multi-frame phase-compensated images through steps S1621 to S1623, for example.

[0104] Step S1621: Calculate the mean of the compensated spatial sampling signal for each frame to form a one-dimensional slow-time signal.

[0105] Step S1622: The spatial correlation algorithm is used to calculate the vibration phase of the one-dimensional slow-time signal.

[0106] Step S1623: Based on the vibration phase, perform vibration phase compensation on the multi-frame compensation spatial sampling signal to obtain the multi-frame vibration compensation spatial sampling signal.

[0107] According to embodiments of this disclosure, Figure 5A The diagram schematically illustrates a three-dimensional matrix composed of spatially sampled signals corresponding to multiple frames of low-resolution complex images according to an embodiment of the present disclosure. Figure 5B A schematic diagram illustrating a comparison between the estimated vibration phase and the actual vibration phase according to an embodiment of the present disclosure is provided. Figure 5C The diagram schematically illustrates the laser synthetic aperture imaging result before vibration phase compensation according to an embodiment of the present disclosure. Figure 5D The diagram schematically illustrates the laser synthetic aperture imaging result after vibration phase compensation according to an embodiment of the present disclosure.

[0108] like Figure 5A As shown, a rigid moving target vibrates sinusoidally along the z-axis. Vibration phase compensation simulation analysis is performed based on the spatial sampling signals corresponding to multiple frames of low-resolution complex images acquired by a coherent array detector. The spatial sampling signals corresponding to the multiple frames of low-resolution complex images constitute a three-dimensional matrix xyt, where (x, y) are the coordinates of the telescope plane, and t is the slow time of the coherent array detector acquiring the multiple frames of complex images. Vibration phase compensation can be achieved by processing the slow-time dimension of this three-dimensional matrix.

[0109] For example, since the target vibration phases in the xy plane of a three-dimensional matrix are similar, the three-dimensional matrix can be averaged in the xy plane to form a one-dimensional echo signal. Given the estimated parameters of the micro-rotating target, the vibration phase of the one-dimensional echo signal is calculated using the Space Correlation Algorithm (SCA), and slow-time vibration phase compensation is performed on the three-dimensional matrix.

[0110] According to embodiments of this disclosure, such as Figure 5B As shown, the estimated vibration phase is close to the actual vibration phase. Figure 5C and Figure 5D As shown, vibration phase compensation reduces the entropy of the imaging result from 11.3286 to 10.3554. Vibration phase compensation can effectively solve the blurring problem of laser synthetic aperture imaging results.

[0111] S170, based on the center of the multi-frame spatial sampling signal, splices the compensated multi-frame spatial sampling signal to form a large-scale spatially sparse sampling signal.

[0112] According to embodiments of this disclosure, such as Figure 5A As shown, the spatial sampling signal corresponding to the compensated image is a two-dimensional matrix in the xy-plane, formed by inverse Fourier transform of complex images acquired by a coherent array detector. The spatial sampling centers corresponding to multiple frames of compensated images are determined, and the spatial sampling signals of multiple frames are stitched together to obtain a spatially sparse sampling signal with a large effective interval. This spatially sparse sampling signal is then subjected to Fourier transform to generate a high-resolution image, as shown below. Figure 7AAs shown, the compensation process involves phase processing of the spatial sampling signal. Multiple frames of spatial sampling signals with different centers and the same size cover a wider area than a single frame. By performing phase compensation on the spatial sampling signal, the effective range of the spatial sampling signal is increased, thereby improving image resolution. This is of great significance for high-resolution imaging of targets in far-field conditions.

[0113] For example, the micro-target consists of a 50m×50m three-dimensional satellite and a 3×3 dot matrix with a spacing of 0.3m. The image is unaffected by noise. The simulation parameters are shown in Table 1.

[0114] Table 1 Parameters of the Laser Synthetic Aperture Imaging System for Moving Targets

[0115]

[0116] According to embodiments of this disclosure, Figure 6A An amplitude diagram of a low-resolution complex image according to an embodiment of the present disclosure is illustrated schematically. Figure 6B The phase map of a low-resolution complex image according to an embodiment of the present disclosure is illustrated schematically. Figure 6C A schematic diagram of a raster target slice of a low-resolution complex image according to an embodiment of the present disclosure is shown. Figure 6D The diagram schematically illustrates a corresponding spatial sampling signal of a low-resolution complex image according to an embodiment of the present disclosure.

[0117] like Figure 6C As shown, low-resolution complex images cannot distinguish point targets with a spacing of 0.3m, such as... Figure 6D As shown, since the pixel resolution of the coherent array detector is lower than the image resolution corresponding to the diffraction limit of the telescope, the spatial sampling signal range corresponding to the low-resolution complex image acquired by the coherent array detector is smaller than the telescope size.

[0118] According to embodiments of this disclosure, Figure 7A An amplitude diagram of a micro-rotating target laser synthetic aperture imaging result is schematically shown when the spatial sampling signal overlap rate is 0 according to an embodiment of the present disclosure. Figure 7B The diagram illustrates a phase map of a micro-rotating target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure. Figure 7C The illustration shows a lattice target slice of the laser synthetic aperture imaging result of a micro-rotating target when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure. Figure 7D The diagram schematically illustrates the corresponding spatial sampling signal of the laser synthetic aperture imaging result of a micro-rotating target when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure.

[0119] According to embodiments of this disclosure, Figure 8AThe diagram schematically illustrates the amplitude of a translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure. Figure 8B The diagram illustrates a phase map of a translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure. Figure 8C The illustration shows a lattice target slice of the translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure. Figure 8D The diagram schematically illustrates the corresponding spatial sampling signal of a translational target laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 0, according to an embodiment of the present disclosure.

[0120] like Figure 7C and Figure 8C As shown, coherent array detector synthetic aperture imaging improves resolution by 9 times, and laser synthetic aperture imaging can resolve lattice targets with a spacing of 0.3m. Figure 7D and Figure 8D As shown, laser synthetic aperture imaging with a spatial sampling signal overlap rate of 0 uses 13 frames of low-resolution complex images. The rotation angle of the micro-rotating target in the x and y directions between two adjacent complex image acquisitions is 0.53 μrad, and the total rotation angle of the micro-rotating target in the x and y directions is 6.36 μrad. The theoretical values ​​of the spatial sampling signal range and resolution in the x and y directions of the imaging results are 0.28 m and 0.11 m, respectively.

[0121] For example, by increasing the overlap rate of the spatial sampling signals and increasing the number of laser complex image frames, the resolution of laser synthetic aperture imaging can be improved while suppressing the influence of noise on the imaging results.

[0122] According to embodiments of this disclosure, Figure 9A An amplitude diagram of laser synthetic aperture imaging results with a spatial sampling signal overlap rate of 75% according to an embodiment of the present disclosure is illustrated. Figure 9B The diagram illustrates a phase map of laser synthetic aperture imaging results when the spatial sampling signal overlap rate is 75% according to an embodiment of the present disclosure. Figure 9C The illustration shows a lattice target slice of laser synthetic aperture imaging results with a spatial sampling signal overlap rate of 75% according to an embodiment of the present disclosure. Figure 9D The diagram illustrates the corresponding spatial sampling signal of a laser synthetic aperture imaging result when the spatial sampling signal overlap rate is 75% according to an embodiment of the present disclosure.

[0123] like Figure 9CAs shown, coherent array detector synthetic aperture imaging improves resolution by 9 times, while laser synthetic aperture imaging can resolve lattice targets with a spacing of 0.3m. Furthermore, improving the overlap rate of spatial sampling signals corresponding to low-resolution complex images requires increasing the number of low-resolution complex image frames to achieve the resolution improvement, while simultaneously suppressing the impact of noise on high-resolution imaging results. Figure 9D As shown, laser synthetic aperture imaging with a spatial sampling signal overlap rate of 75% uses 49 frames of low-resolution complex images. The angle of rotation of the target in the x and y directions between two adjacent complex image acquisitions is 0.13 μrad, and the total angle of rotation of the target in the x and y directions is 6.36 μrad. The theoretical values ​​of the spatial sampling signal range and resolution in the x and y directions of the imaging results are 0.28 m and 0.11 m, respectively.

[0124] S180 reduces the sparsity of large-scale spatially sparse sampling signals by extending the observation time, and performs high-resolution image reconstruction and image sidelobe suppression on the large-scale spatially sparse sampling signals through compressed sensing processing to obtain the laser synthetic aperture image of the target to be detected.

[0125] According to embodiments of this disclosure, Figure 10 The illustration schematically shows a spatial sampling signal map corresponding to the laser synthetic aperture imaging result formed by multiple observations when the spatial sampling signal overlap rate is 75%, the target motion is a mixture of translation and rotation, and the rotation speed and direction of motion are different.

[0126] For example, such as Figure 7D , Figure 8D and Figure 9D As shown, the spatial sampling signals formed by the two-axis rotation and two-directional translation of the target to be detected are highly sparse. For example... Figure 10 As shown, by extending the observation time and utilizing the non-uniformity and randomness of the target motion, the sparsity of the spatial sampling signal is reduced. The sparsely sampled image is reconstructed through compressed sensing, thereby reducing the influence of image sidelobes caused by sparse sampling.

[0127] In summary, this disclosure proposes a laser synthetic aperture imaging method for moving targets based on a coherent array detector. By acquiring multiple frames of low-resolution complex images of a far-field micro-rotating target using a laser local oscillator coherent array detector, and based on target parameter estimation, performing multiple processing steps such as vibration phase compensation of the complex images and phase compensation of the spatial sampling signals corresponding to each frame of the complex images, a large-scale spatial sampling signal interval is constructed based on the spatial sampling center. This achieves laser synthetic aperture imaging with strict mathematical relationships and clear physical meaning, effectively improving image resolution under far-field conditions.

[0128] It should be understood that the specific order or hierarchy of steps in the disclosed process is an example of an exemplary method. Based on design preferences, it should be understood that the specific order or hierarchy of steps in the process may be rearranged without departing from the scope of this disclosure. The appended method claims provide elements of various steps in an exemplary order and are not intended to limit the scope to a specific order or hierarchy.

[0129] It should also be noted that the directional terms mentioned in the embodiments, such as "up," "down," "front," "back," "left," and "right," are only for reference to the directions in the accompanying drawings and are not intended to limit the scope of protection of this disclosure. Throughout the drawings, the same elements are represented by the same or similar reference numerals. Conventional structures or constructions will be omitted when they may cause confusion in understanding this disclosure. Furthermore, the shapes, sizes, and positional relationships of the components in the drawings do not reflect their actual size, scale, or actual positional relationships.

[0130] In the detailed description above, various features are combined together in a single embodiment to simplify this disclosure. This approach to disclosure should not be construed as reflecting an intention that embodiments of the claimed subject matter require more features than are explicitly stated in each claim. Rather, as reflected in the appended claims, this disclosure is in a state of having fewer features than all of the features of the single disclosed embodiment. Therefore, the appended claims are hereby explicitly incorporated into the detailed description, with each claim representing a separate preferred embodiment of this disclosure.

[0131] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this disclosure, "a plurality of" means at least two, such as two, three, etc., unless otherwise expressly specified. The term "comprising" as used in the specification or claims is interpreted in a manner similar to the term "including," as "including" is used as a conjunction in the claims. The use of any term "or" in the specification or claims is intended to mean "non-exclusive or."

[0132] The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of this disclosure. It should be understood that the above descriptions are merely specific embodiments of this disclosure and are not intended to limit this disclosure. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the protection scope of this disclosure.

Claims

1. A method for laser synthetic aperture sparse imaging of moving targets based on a coherent array detector, characterized in that, include: A laser emission signal is used to illuminate the target to be detected, forming a target laser echo signal. The target to be detected moves based on a direction of motion, which includes... direction and Direction, the motion includes rotation and translation, For the telescope coordinate system, shaft and The axes are parallel. shaft and The axes are parallel, and the laser emission signal is a narrowband laser signal; The target laser echo signal is subjected to Fourier transform by a telescope to form the telescope received signal; The laser local oscillator coherent array detector samples the signal received by the telescope to form multiple frames of laser complex images, which include amplitude information and phase information. Estimating the motion parameters of the target to be detected includes: determining multiple prominent points at different locations on the target to be detected; performing interferometric phase integration on the multiple prominent points in the adjacent sampled multi-frame laser complex images to obtain a phase curve; fitting the phase curve to obtain phase change information; and obtaining the motion parameters based on the phase change information. Perform an inverse Fourier transform on the multi-frame laser complex image in the digital domain to obtain multi-frame spatial sampling signals; Based on the motion parameters, phase compensation and motion compensation are performed on the multi-frame spatial sampling signals, and the center of the compensated multi-frame spatial sampling signals is calculated; wherein, the motion compensation includes compensation for rotational phase and translational phase; the translational phase is caused by the translational distance of the target to be detected in three-dimensional space; Based on the center of the multi-frame spatial sampling signal, the compensated multi-frame spatial sampling signal is spliced ​​together to form a large-scale spatially sparse sampling signal. By extending the observation time to take advantage of the non-uniformity and randomness of the target's motion, the sparsity of the large-scale spatial sparse sampling signal is reduced. Then, through compressed sensing processing, high-resolution image reconstruction and image sidelobe suppression are performed on the large-scale spatial sparse sampling signal to obtain the laser synthetic aperture image of the target.

2. The method according to claim 1, characterized in that, Also includes: The seed source signal is modulated using a reference intermediate frequency signal to obtain a modulated signal; The modulation signal is amplified to obtain the laser emission signal; The laser emission signal adopts a separate transmit and receive method and a wide beam emission form, the laser local oscillator coherent array detector adopts a wide field of view reception form, and the wide beam and the wide field of view cover the target to be detected; The laser emission signal is a narrowband pulsed laser signal or a narrowband continuous wave laser signal; as well as The reference intermediate frequency signal is used to synchronize the sampling time and phase of the laser emission signal and the target laser echo signal, and to perform intermediate frequency sampling and low-pass filtering on the telescope received signal in the electronics.

3. The method according to claim 2, characterized in that, The step of sampling the telescope-received signal through a laser local oscillator coherent array detector to form multiple laser complex images includes: The telescope receives a signal and the laser local oscillator signal, and coherently detects them to form an intermediate frequency signal. The laser local oscillator signal and the seed source signal are from the same source. After the intermediate frequency signal is sampled at intermediate frequency, it is converted into a three-dimensional digital signal by analog-to-digital conversion. The three dimensions include pitch, azimuth and fast time directions. The three-dimensional digital signal in the fast time direction is subjected to low-pass filtering and integration to obtain the two-dimensional multi-frame laser complex image, wherein the two dimensions include the pitch direction and the azimuth direction.

4. The method according to claim 1, characterized in that, The phase compensation and motion compensation of the multi-frame spatial sampling signals include: Perform single-frame second-order phase compensation and single-frame motion phase compensation on the spatial sampling signal of each frame to obtain multi-frame compensated spatial sampling signals. The motion phase includes rotational phase and translational phase. Slow-time vibration phase compensation is performed on the multi-frame compensated spatial sampling signal to obtain a multi-frame vibration compensated spatial sampling signal, which is used to splice the multi-frame compensated spatial sampling signal under the target vibration condition.

5. The method according to claim 4, characterized in that, The slow-time vibration phase compensation of the multi-frame compensated spatial sampling signal includes: Calculate the mean of the compensated spatial sampling signal for each frame to form a one-dimensional slow-time signal; The vibration phase is obtained by calculating the one-dimensional slow-time signal using a spatial correlation algorithm; Based on the vibration phase, vibration phase compensation is performed on the multi-frame compensation spatial sampling signal to obtain the multi-frame vibration compensation spatial sampling signal.

6. The method according to claim 1, characterized in that, The step of performing an inverse Fourier transform on the multi-frame laser complex image in the digital domain to obtain multi-frame spatial sampling signals includes: To improve the overlap rate of the multi-frame spatial sampling signals and increase the number of frames in the multi-frame laser composite image, the influence of noise on the imaging results is suppressed; and Low-pass filtering is applied to the multi-frame laser composite image to improve the signal-to-noise ratio of the multi-frame laser composite image.

7. The method according to claim 1, characterized in that, The laser local oscillator coherent array detector is positioned on the focal plane of the telescope, which employs a thin-film diffraction mirror.

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