A high frame rate distributed SAR moving target imaging method

Abstract

The present invention discloses a high frame rate distributed SAR moving target imaging method, comprising the following steps: using each dual station in the high frame rate distributed SAR radar to perform equivalent phase center processing on the imaged moving target to obtain the moving target equivalent sub-aperture echo; estimating the two-dimensional motion parameters of the moving target; constructing phase compensation functions for the static and moving components respectively to eliminate or reduce the influence of the moving target motion component; estimating the moving target complete echo under the high frame rate distributed configuration; using the complete echo estimation value to perform imaging focusing on the moving target and output the image. The present invention verifies the possibility of high frame rate distributed SAR configuration for moving target imaging, and provides a new means for moving target observation and imaging.

Description

Technical Field

[0001] The invention belongs to the field of radar technology, and in particular relates to a high frame rate distributed SAR moving target imaging method. Background Art

[0002] Synthetic aperture radar (SAR) is a microwave remote sensing imaging system. Compared with optical imaging systems, it is not restricted by climate and lighting conditions and can perform high-resolution imaging of the observed scene at any time and weather conditions. As a result, SAR technology is favored in the defense and civilian fields. In recent years, with the rapid development of technologies such as digital signal processing and large-scale integrated circuits, traditional single-station SAR imaging algorithms and practical application systems have moved from theory to practice, and the imaging speed and image clarity have been continuously improved. However, since single-station SAR is a technical implementation method that replaces spatial sampling with time sampling, it always has the inherent limitation of long data acquisition time, resulting in low data acquisition efficiency in the case of long aperture.

[0003] In order to effectively shorten the radar data acquisition time under long aperture conditions and improve the real-time performance of SAR imaging, the concept of high frame rate distributed SAR is introduced. The schematic diagram of the high frame rate distributed SAR system configuration can be found in Figure 1 .

[0004] exist Figure 1 In the figure, the high frame rate distributed SAR system is composed of N SAR platforms. Position ① in the figure indicates the starting position of platform 1, position ② indicates the ending position of platform 1 and the starting position of platform 2, position ③ indicates the ending position of platform n-1 and the starting position of platform n, position ④ indicates the ending position of platform N-1 and the starting position of platform N, and position ⑤ indicates the ending position of platform N. All SAR platforms move simultaneously along the track. Assuming that the intervals between the SAR platforms are equal under ideal conditions, and they will complete the data acquisition of their respective sub-apertures at the same time and at the same speed, it is natural that such a high frame rate SAR system can form a complete long synthetic aperture.

[0005] However, in actual work, due to factors such as inaccurate initial positions of each platform, inconsistent movement speeds, and high complexity of the signal receiving and transmitting system in the high frame rate SAR system, the long aperture data obtained after the distributed SAR system actually records data will be incomplete, and data at some locations may be missing, forming a sparse aperture. At this time, the sparse aperture diagram of the high frame rate SAR system is as follows Figure 2 shown.

[0006] exist Figure 2In the figure, position ① indicates the starting position of platform 1, position ② indicates the actual end position of platform 1, position ③ indicates the ideal end position of platform 1 and the starting position of platform 2, position ④ indicates the end position of platform n-1 and the ideal start position of platform n, position ⑤ indicates the actual start position of platform n, position ⑥ indicates the actual end position of platform N-1, position ⑦ indicates the ideal end position of platform N-1 and the starting position of platform N, and position ⑧ indicates the end position of platform N. It can be found that the aperture data between positions ② and ③, between positions ④ and ⑤, and between positions ⑥ and ⑦ are missing. Therefore, for a long full aperture, there will be a lack of some aperture data in the combined aperture obtained by high frame rate distributed SAR, which is regarded as a sparse aperture.

[0007] Under the high frame rate distributed SAR system configuration, each distributed platform observes the imaging scene at different locations at the same time. Therefore, the high frame rate distributed SAR moving target echo model is significantly different from the existing single-station moving target echo model, and the complexity is greatly improved. Its particularity is mainly manifested in that the high frame rate distributed SAR moving target echo is actually composed of multiple moving target sub-aperture echoes, and is constrained by the system configuration. These moving target sub-aperture echoes are not strictly connected end to end, showing sparse aperture characteristics. The sparse aperture will cause the performance of the moving target imaging results to be greatly reduced. Therefore, the main technical problem to be solved by the present invention is how to construct a high frame rate distributed SAR moving target echo model under the high frame rate distributed SAR system configuration, and use the sparse aperture of the moving target to obtain high-resolution imaging results.

[0008] At present, there is no public sparse aperture imaging method for moving targets based on high frame rate distributed SAR. The technologies closest to the present invention are as follows: the Chinese invention application "A Non-uniform Missing Aperture Imaging Method for Distributed Along-track Formation SAR System" published in 2022 and the journal article "Along-track Swarm SAR Echo Modeling and Sub-aperture Collaboration Imaging based on Sparse Constraints" published in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing in 2023.

[0009] The above prior art is mainly a method for imaging stationary targets under a high frame rate distributed SAR configuration. Due to the difference in the objects to be imaged, the proposed method cannot be directly applied to the field of moving target imaging. The disadvantage of the prior art is that it can only image stationary targets under a high frame rate distributed SAR configuration. Once the target becomes a non-cooperative moving target, the prior art cannot accurately focus on the moving target. The reason is: in the imaging of stationary targets, the target and the platform are cooperative. With the movement of the platform, the phase relationship between the stationary target and the moving platform at different azimuth positions is determined. In the imaging of moving targets, the target is non-cooperative, and only the platform is cooperative. This leads to the unknown phase relationship between the non-cooperative moving target and the moving platform at different azimuth positions. At the same time, the moving target echo model under the high frame rate distributed SAR configuration is also significantly different from the existing various types of moving target echo models. Therefore, the prior art cannot image moving targets under a high frame rate distributed SAR configuration. Summary of the invention

[0010] There are significant advantages in using high frame rate distributed SAR to observe and image moving targets. In a shorter data acquisition time, the motion state of the moving target can be simplified to the greatest extent, which is conducive to the coherent accumulation of moving target echoes. In view of this, the present invention applies the high frame rate distributed SAR system to the field of moving target imaging for the first time to improve the coherent accumulation of moving target echoes.

[0011] To achieve the above-mentioned purpose, the high frame rate distributed SAR moving target imaging method disclosed in the present application comprises the following steps:

[0012] Use the dual stations in the high frame rate distributed SAR radar to perform equivalent phase center processing on the imaged moving target to obtain the equivalent sub-aperture echo of the moving target;

[0013] Estimate the two-dimensional motion parameters of the moving target;

[0014] Construct phase compensation functions for static and moving components respectively to eliminate or reduce the influence of moving target motion components;

[0015] Estimation of complete echoes of moving targets in high frame rate distributed configuration;

[0016] The complete echo estimation value is used to image and focus the moving target and output the image.

[0017] Preferably, the high frame rate distributed SAR equivalent sub-aperture echo corresponding to the moving target It is expressed as:

[0018]

[0019] Where t, t sa ,c,j,π,f c , Kr Respectively represent fast time, slow time, speed of light, imaginary unit, pi, center frequency, modulation frequency and noise, w r and w a Represent the range and azimuth window functions respectively, Indicates the slope distance relationship.

[0020] Preferably, the radial velocity of the non-cooperative moving target during the high frame rate distributed SAR observation time is v r , azimuth velocity is v y For a two-dimensional uniform motion, the slope distance relationship is It is expressed as:

[0021]

[0022] Among them, R 0 , Y 0 , Y virn With V virn They represent the target distance position and target azimuth position respectively.

[0023] Preferably, in order to eliminate the phase error between the stationary component and the moving component of the moving target in batches, the stationary component phase compensation function θ in the high frame rate distributed SAR stationary target imaging algorithm is first used. ypc (f r ,η) Perform phase compensation on the equivalent echo of high frame rate distributed SAR moving target to obtain the phase compensated signal have

[0024]

[0025] Among them, f r and η represent the slow time corresponding to the distance frequency and the long aperture, respectively. represents the high frame rate distributed SAR moving target equivalent echo of a small dimension in the range frequency domain, θ ypc (f r ,η) represents the phase compensation function in the high frame rate distributed SAR stationary target imaging algorithm;

[0026] The phase term of the moving target motion component is compensated, and the phase error function of the motion component is θ mt-pc (f r ,η) is expressed as:

[0027]

[0028] Signal after secondary phase compensation It is expressed as:

[0029]

[0030] At this point, the phase error of the moving target motion component has been eliminated, so It is the high frame rate distributed SAR echo with small dimension in range frequency domain corresponding to the stationary target;

[0031] Using the two-dimensional time domain signal after quadratic phase compensation Reconstruct the complete sparse representation signal sS without the influence of the moving target phase term pc (t,f η ), where f η Represents the azimuth frequency.

[0032] Preferably, a high frame rate distributed SAR moving target equivalent echo in a small dimension in the range frequency domain It is expressed as:

[0033]

[0034] ΔR mt (η) represents the slant range vector ΔR corresponding to the moving target motion component mt The specific value corresponding to the ηth moment in , mt represents the moving target.

[0035] Preferably, the moving target equivalent sub-aperture echo Equivalent echo of moving target Transform to range frequency domain get:

[0036]

[0037] in Represented by The slope range term expression formed by the combination;

[0038] Using data to remove the matrix Will The zero-value azimuth sampling points in the image are removed to obtain the small-dimensional high-frame-rate distributed SAR moving target equivalent echo. have:

[0039]

[0040] Preferably, the complete aperture reconstruction and imaging method in the high frame rate distributed SAR stationary target imaging algorithm is used to obtain high-quality moving target imaging results under the high frame rate distributed SAR configuration.

[0041] The beneficial effects of the present invention are as follows:

[0042] For the first time, a moving target imaging method based on high frame rate distributed SAR was proposed, which verified the feasibility of high frame rate distributed SAR configuration for moving target imaging and provided a new means for moving target observation and imaging. Experimental results showed that the two-dimensional imaging results of moving targets in this application are better than the existing algorithms. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagram of high frame rate distributed SAR system configuration;

[0044] Figure 2 Schematic diagram of sparse aperture of high frame rate distributed SAR system;

[0045] Figure 3 Flow chart of the method of this application;

[0046] Figure 4 Imaging results of the classic range Doppler imaging algorithm;

[0047] Figure 5 The imaging results of existing stationary target imaging algorithms based on high frame rate distributed SAR;

[0048] Figure 6 The existing imaging results of the missing aperture SAR moving target imaging algorithm;

[0049] Figure 7 Imaging results of this application. DETAILED DESCRIPTION

[0050] The present invention is further described below in conjunction with the accompanying drawings, but the present invention is not limited in any way. Any changes or substitutions made based on the teachings of the present invention belong to the protection scope of the present invention.

[0051] The implementation flow chart of the technical solution of this application is as follows Figure 3 As shown in the figure, when there is no moving target in the imaging scene, the nth equivalent sub-aperture echo can be obtained by performing equivalent phase center processing on each pair of dual stations in the high frame rate distributed SAR. It can be expressed as

[0052]

[0053] Where t, t sa ,c,j,π,f c , K r With n 0 They represent fast time, slow time, speed of light, imaginary unit, pi, center frequency, modulation frequency and noise respectively. r and w a Represent the range and azimuth window functions respectively.

[0054] Slope range term can be expressed as

[0055]

[0056] Where R 0 , Y 0 , Y virn With V virn They represent the target range position, target azimuth position, platform initial motion position and platform motion speed respectively. When the target in the imaging scene is a moving target, the slant range between the target and the high frame rate distributed SAR system will also be related to the motion state of the moving target. Assume that the non-cooperative moving target has a radial velocity of v during the high frame rate distributed SAR observation time. r , azimuth velocity is v y For a two-dimensional uniform motion, the slope distance relationship is can be expressed as

[0057]

[0058] Therefore, the high frame rate distributed SAR equivalent sub-aperture echo corresponding to the moving target The actual need is expressed as

[0059]

[0060] Moving target equivalent sub-aperture echo The configuration is the same as that of a single-station SAR moving target, so the motion parameters of the moving target can be directly estimated by referring to the conventional moving target motion parameter estimation method.

[0061] After obtaining the two-dimensional motion parameter estimates of the moving target, it is also necessary to consider the problem of sparse and incomplete moving target echoes under the high frame rate distributed SAR configuration. Therefore, this application needs to design a phase compensation function that can enhance the sparsity of complete echoes based on the high frame rate distributed SAR system configuration and the motion parameter estimates, and eliminate or greatly reduce the influence of the moving target motion component. If this component is not eliminated, a large aperture estimation error will be generated during the aperture reconstruction process. The reconstructed part will also contain space-time inequivalent phase errors, which cannot be further eliminated in the subsequent imaging process.

[0062] Will Expanded, it can be expressed as

[0063]

[0064] By doing Taylor approximation expansion on the above formula, we can find that

[0065]

[0066] The slope distance term caused by the motion component is represented as

[0067]

[0068] Observing the above formula, we can find that since the starting position and speed of different equivalent platforms may be different, at the same slow time t sa , the instantaneous slant distances between different equivalent platforms and moving targets may be different. Therefore, consider the equivalent sub-aperture echo of the moving target. Equivalent echo of moving target Transform to range frequency domain Can get

[0069]

[0070] where f r and η represent the slow time corresponding to the range frequency and the long aperture, respectively. Represented by Then, use the data removal matrix Will By removing the zero-value azimuth sampling points in the image, a small-dimensional high-frame-rate distributed SAR moving target equivalent echo can be obtained. have,

[0071]

[0072] At this time, the high frame rate distributed SAR moving target equivalent echo in the small dimension of the range frequency domain can be expressed as

[0073]

[0074] Obviously, ΔR mt (η) represents the vector ΔR mt The specific value corresponding to the nth moment in , and the specific value is related to the specific configuration of the high frame rate distributed SAR.

[0075] In order to eliminate the phase error between the static component and the moving component of the moving target in batches, this application first uses the static component phase compensation function θ in the high frame rate distributed SAR static target imaging algorithm. ypc (f r ,η) Perform phase compensation on the equivalent echo of high frame rate distributed SAR moving target to obtain the phase compensated signal have

[0076]

[0077] Since the phase compensation function θ in the high frame rate distributed SAR stationary target imaging algorithm ypc (f r,η) is only designed for stationary targets, and the phase term corresponding to the motion component of the moving target cannot be eliminated. This will cause the sparse representation signal of the high frame rate distributed SAR moving target echo to be defocused, and the sparsity is insufficient, which is not conducive to the use of compressed sensing methods. Therefore, it is also necessary to compensate for the phase term of the moving target motion component. The phase error function θ mt-pc (f r ,η) can be expressed as

[0078]

[0079] Signal after secondary phase compensation can be expressed as

[0080]

[0081] At this point, the phase error of the moving target motion component has been eliminated, so It can actually be regarded as a small-dimensional high-frame-rate distributed SAR echo in the range-frequency domain corresponding to a stationary target. The key step of this application is to use the two-dimensional time domain signal after quadratic phase compensation Reconstruct the complete sparse representation signal sS without the influence of the moving target phase term pc (t,f η ), where f η represents the azimuth frequency. Specifically, the two-dimensional time domain signal after quadratic phase compensation can be regarded as the compressed signal x in the compressed sensing method, the complete sparse representation signal without the influence of the moving target phase term can be regarded as the complete signal y in the compressed sensing method, and the azimuth inverse Fourier transform matrix is ​​the compressed sensing matrix phi in the compressed sensing method. The specific reconstruction process can be implemented by using compressed sensing algorithms such as various convex optimization algorithms, non-convex optimization algorithms, greedy algorithms, sparse Bayesian algorithms, etc.

[0082] Finally, by using the full aperture reconstruction and imaging method in the high frame rate distributed SAR stationary target imaging algorithm, high-quality moving target imaging results can be obtained under the high frame rate distributed SAR configuration. This application does not specifically limit the specific full aperture reconstruction and imaging method, as long as it is a related algorithm that conforms to the compressed sensing framework, such as various convex optimization algorithms, non-convex optimization algorithms, greedy algorithms, sparse Bayesian algorithms, etc.

[0083] The core idea of ​​the technical solution proposed in the present invention is to first model the high frame rate distributed SAR moving target echo, then estimate the motion parameters of the moving target under a special echo model, and then use the motion parameters of the moving target to determine the phase relationship between the target and the moving platform, so as to achieve the purpose of improving the existing imaging algorithm. Therefore, how to estimate the motion parameters is not the focus of this application; which method is used to estimate and reconstruct the complete aperture echo is not the focus of this application. The most important thing about this application is that it proposes a moving target imaging method based on high frame rate distributed SAR for the first time, verifies the possibility of high frame rate distributed SAR configuration for moving target imaging, and provides a new means for moving target observation and imaging. Specifically, it has nothing to do with the specific speed change of the moving target (uniform motion, uniformly accelerated motion, variable speed motion, etc.); it has nothing to do with the specific motion state of the moving target (two-dimensional motion along the ground, three-dimensional motion, etc.); it has nothing to do with the specific working mode of the SAR system (strip mode, beam mode, etc.); it has nothing to do with the specific motion mode of the SAR system (linear motion, curvilinear motion, circular motion, etc.); it has nothing to do with the platform type of the SAR system (airborne SAR system, unmanned aerial vehicle SAR system, vehicle-mounted SAR system, satellite-mounted SAR system, etc.); it has nothing to do with the signal waveform of the SAR system (stepped frequency signal, random frequency signal, linear frequency modulation signal, continuous wave signal, etc.), etc. In the part of complete aperture echo reconstruction, there is no special limitation on the specific reconstruction algorithm, as long as it is a related algorithm that conforms to the compressed sensing framework, such as various convex optimization algorithms, non-convex optimization algorithms, greedy algorithms, sparse Bayesian algorithms, etc.

[0084] This application uses three different existing imaging methods to compare with the method proposed in this application. The comparison results are as follows: Figure 4-Figure 7 Specifically, the existing method 1 refers to the existing classic range Doppler imaging algorithm; the existing method 2 refers to the existing static target imaging algorithm based on high frame rate distributed SAR; the existing method 3 refers to the existing missing aperture SAR moving target imaging algorithm. By comparing the imaging results, it can be found that the present application obtains the optimal moving target two-dimensional imaging result under the high frame rate distributed SAR configuration.

[0085] The beneficial effects of the present invention are as follows:

[0086] For the first time, a moving target imaging method based on high frame rate distributed SAR was proposed, which verified the feasibility of high frame rate distributed SAR configuration for moving target imaging and provided a new means for moving target observation and imaging. Experimental results showed that the two-dimensional imaging results of moving targets in this application are better than the existing algorithms.

[0087] As used herein, the word "preferred" is intended to be used as an example, instance, or illustration. Any aspect or design described herein as "preferred" is not necessarily to be construed as being more advantageous than other aspects or designs. On the contrary, the use of the word "preferred" is intended to present concepts in a specific way. The term "or" as used in this application is intended to mean an inclusive "or" rather than an exclusive "or". That is, unless otherwise specified or clear from the context, "X uses A or B" means any one of the naturally included permutations. That is, if X uses A; X uses B; or X uses both A and B, then "X uses A or B" is satisfied in any of the foregoing examples.

[0088] Moreover, although the present disclosure has been shown and described with respect to one or implementations, those skilled in the art will think of equivalent variations and modifications based on the reading and understanding of this specification and the accompanying drawings. The present disclosure includes all such modifications and variations, and is limited only by the scope of the appended claims. In particular, with respect to the various functions performed by the above-mentioned components (such as elements, etc.), the terms used to describe such components are intended to correspond to any component (unless otherwise indicated) that performs the specified function of the component (such as it is functionally equivalent), even if the structure is not equivalent to the disclosed structure of the function in the exemplary implementation of the present disclosure shown herein. In addition, although the specific features of the present disclosure have been disclosed with respect to only one of several implementations, such features can be combined with one or other features of other implementations that may be desired and advantageous for a given or specific application. Moreover, insofar as the terms "including", "having", "containing" or their variations are used in specific embodiments or claims, such terms are intended to be included in a manner similar to the term "comprising".

[0089] The functional units in the embodiments of the present invention may be integrated into a processing module, or each unit may exist physically separately, or multiple or more units may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium. The above-mentioned storage medium may be a read-only memory, a disk or an optical disk, etc. The above-mentioned devices or systems may execute the storage method in the corresponding method embodiment.

[0090] To sum up, the above embodiment is an implementation mode of the present invention, but the implementation mode of the present invention is not limited by the embodiment. Any other changes, modifications, substitutions, combinations, and simplifications that deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A high frame rate distributed SAR moving target imaging method, characterized in that: The following steps are involved: Use the dual stations in the high frame rate distributed SAR radar to perform equivalent phase center processing on the imaged moving target to obtain the equivalent sub-aperture echo of the moving target; Estimate the two-dimensional motion parameters of the moving target; Construct phase compensation functions for static and moving components respectively to eliminate or reduce the influence of moving target motion components; Estimation of complete echoes of moving targets in high frame rate distributed configuration; Use the complete echo estimation value to focus the moving target and output the image; High frame rate distributed SAR equivalent sub-aperture echo corresponding to moving targets It is expressed as: ； in t , t sa , c , j , π , f c , K r They represent fast time, slow time, speed of light, imaginary unit, pi, center frequency, and modulation frequency. w r and w a Represent the range and azimuth window functions respectively, Indicates the slope distance relationship; The radial velocity of the non-cooperative moving target within the high frame rate distributed SAR observation time is: v r The azimuth velocity is v y For a two-dimensional uniform motion, the slope distance relationship is It is expressed as: ； in, R 0 , Y 0 , Y virn and V virn They represent the target distance position, target azimuth position, platform initial motion position and platform motion speed respectively; In order to eliminate the phase error between the stationary component and the moving component of the moving target in batches, the stationary component phase compensation function in the high frame rate distributed SAR stationary target imaging algorithm is first used. Perform phase compensation on the equivalent echo of high frame rate distributed SAR moving target to obtain the phase compensated signal ,have ； in, f r and η Represent the slow time corresponding to the distance frequency and the long aperture, Represents the high frame rate distributed SAR moving target equivalent echo in a small dimension in the range frequency domain, represents the phase compensation function in the high frame rate distributed SAR stationary target imaging algorithm; Compensate the phase term of the moving target motion component, the phase error function of the motion component It is expressed as: ； Represents the slant range vector corresponding to the moving target motion component Middle The specific value corresponding to the moment, mt represents a moving target; Signal after secondary phase compensation It is expressed as: ； At this point, the phase error of the moving target motion component has been eliminated, so It is the high frame rate distributed SAR echo with small dimension in range frequency domain corresponding to the stationary target; Using the two-dimensional time domain signal after quadratic phase compensation Reconstructing the complete sparse representation signal without the influence of the moving target phase term ,in Represents the azimuth frequency.

2. The high frame rate distributed SAR moving target imaging method according to claim 1, characterized in that: High frame rate distributed SAR moving target equivalent echo in small dimension in range frequency domain It is expressed as: ; Represents the slant range between the target and the radar corresponding to the slow time η.

3. The high frame rate distributed SAR moving target imaging method according to claim 2, characterized in that: The equivalent sub-aperture echo of the moving target Equivalent echo of moving target Transform to range frequency domain ,get: ; in Represented by The slope range term expression formed by the combination; Using data to remove the matrix Will The zero-value azimuth sampling points in the image are removed to obtain the small-dimensional high-frame-rate distributed SAR moving target equivalent echo. ,have: 。 4. The high frame rate distributed SAR moving target imaging method according to claim 1, characterized in that: By utilizing the complete aperture reconstruction and imaging method in the high frame rate distributed SAR stationary target imaging algorithm, high-quality moving target imaging results are obtained under the high frame rate distributed SAR configuration.

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