A synthetic aperture radar imaging method, apparatus and imaging device

Abstract

The application provides a synthetic aperture radar imaging method, device and imaging equipment, and relates to the technical field of remote sensing, and a specific scheme comprises the following steps: transmitting a pulse signal, wherein the pulse signal comprises at least one sub-pulse; establishing a sub-pulse wide-beam imaging echo model, wherein the sub-pulse wide-beam imaging echo model is a theoretical model under ideal receiving conditions; receiving at least one echo signal to obtain a sub-scene mixed original echo; generating at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model; and performing image splicing to generate an azimuth wide-beam imaging image. In the process of synthetic aperture radar imaging, a high-resolution image can be obtained, the azimuth imaging wide-beam can be effectively widened, and the operation complexity of the imaging algorithm can be reduced, so that the image can be collected in real time.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of remote sensing, and in particular to a synthetic aperture radar imaging method, device and imaging equipment. BACKGROUND

[0002] Synthetic aperture radar (SAR) is a kind of high-resolution imaging radar, and SAR imaging technology is a kind of high-resolution remote sensing technology using active microwave imaging. Specifically, a SAR system transmits a series of pulse signals, which are usually electromagnetic waves in the microwave or radar band. After these pulse signals propagate to the ground and are reflected back, the SAR system receives these reflected signals and processes the received signals through amplification, filtering and phase correction, etc. to generate a high-resolution image. SAR imaging technology has the ability to obtain high-resolution images all day long, and is widely used in topographic mapping, disaster monitoring and military reconnaissance, etc.

[0003] It is worth noting that the application of SAR imaging technology in reality also faces some limitations. For example, in the strip imaging scheme, the azimuth direction wide-range imaging is realized by fixing the radar antenna beam, and at the same time, due to the fixed radar antenna, the azimuth direction resolution is poor. For another example, in the traditional spotlight imaging scheme, the imaging resolution in the radar direction can be effectively improved by adjusting the antenna beam to constantly point to a specific imaging area, but due to the limitation of the imaging width in the azimuth direction, this imaging method can only realize single-beam width imaging. For another example, in the sub-pulse diversity azimuth width time-domain spotlight imaging system, wide-range imaging in the azimuth direction can be realized, but the time-domain imaging algorithm needs to use complex interpolation operations, and the corresponding operation complexity is high, which is not conducive to real-time processing. Therefore, in the research of SAR imaging technology, when providing high-resolution images, widening the imaging width in the azimuth direction and reducing the operation complexity of the imaging algorithm are the main research directions. SUMMARY

[0004] The present application provides a synthetic aperture radar imaging method, device and imaging equipment, which can obtain high-resolution images while effectively widening the imaging width in the azimuth direction and reducing the operation complexity of the imaging algorithm during the process of synthetic aperture radar imaging, so as to realize real-time image acquisition.

[0005] To achieve the above technical purposes, the embodiments of the present application adopt the following technical solutions:

[0006] In a first aspect, the present application provides a synthetic aperture radar imaging method, which can include: transmitting a pulse signal, the pulse signal including at least one sub-pulse. Wherein, when the SAR transmits a sub-pulse, the sub-pulse has been modulated to a frequency value in a corresponding range frequency area, that is, the frequency value of each sub-pulse is different, so that different sub-pulses point to different azimuth imaging areas. A sub-pulse wide-beam imaging echo model is established, which is a theoretical model under ideal conditions. Further, at least one echo signal is received to obtain a sub-scene mixed original echo. Wherein, the echo signal is obtained after at least one sub-pulse signal is reflected, and the sub-scene mixed original echo is formed by mixing at least one reflected sub-pulse echo signal. According to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model, at least one original sub-scene image is generated.

[0007] It should be understood that the echo signal received by the SAR is the signal reflected by the sub-pulse, and each sub-pulse corresponds to a sub-scene. Therefore, the echo signal of each sub-scene can be processed according to the sub-pulse wide-beam imaging echo model to generate an image of the sub-scene, i.e., an original sub-scene image. Finally, image stitching is performed on the original sub-scene images to generate an azimuth wide-beam imaging image.

[0008] Wherein, the sub-pulse is modulated to different range frequency areas, and different sub-pulses are independent of each other in time and range frequency areas, so that the sub-pulse can collect images at different distances, thereby effectively widening the imaging width of the image in the azimuth direction. Further, after receiving the echo signal of the sub-pulse, an original sub-scene image is generated respectively, and image stitching can obtain an azimuth wide-beam image. The imaging operation complexity of this imaging method is low, which effectively shortens the imaging time, thereby facilitating real-time image acquisition.

[0009] In combination with the first aspect, in a possible implementation, according to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model, generating at least one original sub-scene image includes: performing band-pass filtering processing on the sub-scene mixed original echo to obtain at least one sub-scene echo. Each sub-scene echo is processed as follows:

[0010] In the sub-pulse wide-beam imaging echo model, a corresponding compensation function is constructed according to the sub-scene echo, and de-chirp processing is performed to obtain a two-dimensional pre-processing image. The two-dimensional pre-processing image is subjected to beam domain two-dimensional interpolation processing to obtain a sub-scene imaging image in an original coordinate system. The sub-scene imaging image in the original coordinate system is subjected to geometric correction processing to generate an original sub-scene image.

[0011] It can be understood that, compared with the interpolation processing of the time domain image, the beam domain two-dimensional interpolation processing is performed on the two-dimensional preprocessed image after being converted into a two-dimensional image, which effectively reduces the imaging algorithm of the sub-pulse echo signal and helps to realize real-time image acquisition.

[0012] With reference to the first aspect, in another possible implementation of the first aspect, the method further includes: obtaining expected imaging area parameters. The area parameters can include: coordinates of at least one sub-imaging center, radar coordinates, center coordinates of a range-frequency area, and a slant range history.

[0013] With reference to the first aspect, in another possible implementation of the first aspect, constructing the corresponding compensation function according to the sub-scene echo includes: determining sub-scene center coordinates corresponding to the sub-scene echo, and constructing the compensation function of the sub-pulse according to the sub-scene center coordinates.

[0014] In a second aspect, the present application further provides a synthetic aperture radar imaging device, including a synthetic aperture radar and an image signal processor.

[0015] The synthetic aperture radar is configured to: transmit a pulse signal, the pulse signal including at least one sub-pulse. Wherein, when the SAR transmits the sub-pulse, the sub-pulse has been modulated to a frequency value in a corresponding range-frequency area, that is, the frequency value of each sub-pulse is different, so that different sub-pulses point to different azimuth imaging areas. Further, at least one echo signal is received to obtain a sub-scene aliasing raw echo. Wherein, the echo signal is obtained by reflecting at least one sub-pulse signal, and the sub-scene aliasing raw echo is formed by aliasing at least one reflected sub-pulse echo signal.

[0016] The image signal processor is configured to: establish a sub-pulse wide swath spotlight imaging echo model, the sub-pulse wide swath spotlight imaging echo model being a theoretical model under ideal conditions. According to the sub-scene aliasing raw echo and the sub-pulse wide swath spotlight imaging echo model, at least one original sub-scene image is generated. The original sub-scene images are spliced to generate an azimuth wide swath spotlight imaging image.

[0017] With reference to the second aspect, in a possible implementation of the second aspect, the signal processor is configured to: generate at least one original sub-scene image according to the sub-scene aliasing raw echo and the sub-pulse wide swath spotlight imaging echo model. The signal processor is specifically configured to: perform band-pass filtering processing on the sub-scene aliasing raw echo to obtain at least one sub-scene echo. Each sub-scene echo is processed as follows:

[0018] In the sub-pulse wide beam imaging echo model, a corresponding compensation function is constructed according to the sub-scene echo, linear frequency modulation processing is performed to obtain a two-dimensional pre-processing image. The two-dimensional pre-processing image is subjected to two-dimensional interpolation processing in the beam domain to obtain a sub-scene imaging image in the original coordinate system. The sub-scene imaging image in the original coordinate system is subjected to geometric correction processing to generate an original sub-scene image.

[0019] With reference to the second aspect, in a possible implementation of the second aspect, the signal processor is further configured to obtain expected imaging region parameters. The region parameters include: coordinates of at least one sub-imaging center, radar coordinates, center coordinates of a range-frequency region, and slant range history.

[0020] With reference to the second aspect, in a possible implementation of the second aspect, the signal processor is further configured to construct a corresponding compensation function according to the sub-scene echo, including: determining sub-scene center coordinates corresponding to the sub-scene echo, and constructing a compensation function of the sub-pulse according to the sub-scene center coordinates.

[0021] The third aspect provides an imaging device, including: a processor and a memory for storing processor-executable instructions; wherein the processor is configured to execute the instructions to implement the method in the first aspect and any possible implementation of the first aspect.

[0022] The fourth aspect provides a server, which can include a processor and a memory for storing processor-executable instructions; wherein the processor is configured to execute the instructions to implement the method in the first aspect and any possible implementation of the first aspect.

[0023] The fifth aspect provides a computer-readable storage medium, which stores computer instructions, and when the computer instructions are executed on an imaging device, the method in the first aspect and any possible implementation of the first aspect is implemented.

[0024] It can be understood that the beneficial effects achieved by the device in the second aspect and any possible implementation of the second aspect, the imaging device in the third aspect, the server in the fourth aspect, and the computer-readable storage medium in the fifth aspect can refer to the beneficial effects of the first aspect and any possible implementation of the first aspect, which will not be described here. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A synthetic aperture radar imaging method flowchart provided by an embodiment of the present application;

[0026] Figure 2 Another synthetic aperture radar imaging method flowchart provided by an embodiment of the present application;

[0027] Figure 3An imaging coordinate system established with a total imaging scene center coordinate as a coordinate origin is provided for the embodiment of the present application;

[0028] Figure 4 An imaging coordinate system established with a sub-scene center coordinate as a coordinate origin is provided for the embodiment of the present application;

[0029] Figure 5 A wide-aperture imaging geometry is provided for the embodiment of the present application;

[0030] Figure 6 A scene point distribution in a simulation experiment is provided for the embodiment of the present application;

[0031] Figure 7A A two-dimensional time-domain aliasing echo in a simulation experiment is provided for the embodiment of the present application;

[0032] Figure 7B An azimuth frequency-domain aliasing echo in a simulation experiment is provided for the embodiment of the present application;

[0033] Figure 7C A range frequency-domain aliasing echo data in a simulation experiment is provided for the embodiment of the present application;

[0034] Figure 8A A range frequency-domain filtering result of a first sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0035] Figure 8B A range frequency-domain filtering result of a second sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0036] Figure 8C A range frequency-domain filtering result of a third sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0037] Figure 9A A two-dimensional time-domain signal result of the first sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0038] Figure 9B A two-dimensional time-domain signal result of the second sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0039] Figure 9C A two-dimensional time-domain signal result of the third sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0040] Figure 10A A dechirp processing result of the first sub-scene in a simulation experiment is provided for the embodiment of the present application;

[0041] Figure 10BA second sub-scene dechirp processing result schematic diagram in a simulation experiment provided by the embodiment of the present application is shown in FIG. 6.

[0042] Figure 10C A third sub-scene dechirp processing result schematic diagram in a simulation experiment provided by the embodiment of the present application is shown in FIG. 7.

[0043] Figure 11 A first sub-scene three-dimensional imaging result schematic diagram in a simulation experiment provided by the embodiment of the present application is shown in FIG. 8.

[0044] Figure 12 A first sub-scene frequency domain imaging result top view provided by the embodiment of the present application is shown in FIG. 9.

[0045] Figure 13 A first sub-scene frequency domain right side three target point enlarged schematic diagram provided by the embodiment of the present application is shown in FIG. 10.

[0046] Figure 14 A first sub-scene imaging result geometric correction result schematic diagram provided by the embodiment of the present application is shown in FIG. 11.

[0047] Figure 15 A first sub-scene imaging result geometric correction result right side three target point enlarged diagram provided by the embodiment of the present application is shown in FIG. 12.

[0048] Figure 16 An imaging schematic diagram after image splicing provided by the embodiment of the present application is shown in FIG. 13.

[0049] Figure 17A A traditional phased array original echo signal schematic diagram provided by the embodiment of the present application is shown in FIG. 14.

[0050] Figure 17B An imaging schematic diagram after two-dimensional dechirp processing provided by the embodiment of the present application is shown in FIG. 15.

[0051] Figure 17C A traditional aliasing imaging schematic diagram provided by the embodiment of the present application is shown in FIG. 16. DETAILED DESCRIPTION

[0052] Hereinafter, the terms "first" and "second" are only used for description purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined with "first" and "second" can explicitly or implicitly include one or more of the features. In the description of the embodiment, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0053] Based on the important role of synthetic aperture radar, there is an important application in real-time acquisition of radar images. In the SAR imaging technology, the SAR transmits microwave signals, the microwave signals propagate to the surface of the object and are reflected, and the SAR receives the echo signals and generates radar images according to the echo signals. This imaging method by transmitting microwave signals does not need to rely on environmental factors such as environmental light, and can realize all-weather and all-period image acquisition. However, in the practical application of SAR, there are also some influencing factors.

[0054] For example, in the process of synthetic aperture, the deviation of the platform trajectory of the SAR will cause phase error, resulting in imaging blur or artifacts in radar imaging. In addition, during imaging, such as between transmitting microwave signals and receiving echo signals, the target and the radar have relative motion (such as during imaging of high-speed moving objects), and because of the change in position, the reflected echo signal introduces Doppler shift, which interferes with the imaging image. In addition, the SAR images in the direction of oblique view, and the change of terrain height will also cause the phenomenon of geometric distortion in the imaging of the radar image.

[0055] In some implementations, a strip imaging mode is set. This imaging mode is the most basic working mode of SAR. The core feature of this imaging mode is that the azimuth angle of the antenna remains fixed. The launch platform moves continuously along the preset trajectory and periodically transmits pulse signals while moving. In this way, it can be ensured that the radar obtains a long strip image with wide coverage and continuous imaging. The imaging feature of this imaging mode is wide coverage. If a SAR is carried on an airplane and this imaging mode is used, the airplane can acquire images in real time in one flight mission, and can ensure that the acquired images cover a large area, so this way is suitable for wide-area remote sensing. On the other hand, this imaging method can keep the range and azimuth resolution fixed in the entire imaging range, so the image acquired by this method is convenient for data analysis. Moreover, the image acquired in this imaging mode also has continuity, which makes the antenna also does not need to be dynamically adjusted to realize the stability of the imaging process and ensure the imaging efficiency. In summary, this strip imaging mode can realize large-width azimuth imaging by fixing the antenna beam, but the azimuth resolution of this imaging mode needs to be improved.

[0056] In some other implementations, there is also a traditional spotlight imaging mode. It should be noted that the traditional spotlight imaging algorithm is based on mathematical models and physical principles to process SAR echo data, and by controlling the constant pointing direction of the antenna beam to the desired detection area, the time of the synthetic aperture in the stripmap imaging mode is optimized, and the target coherent accumulation angle is increased. Further, by using the corresponding imaging processing method, the echo signals from multiple angles are combined, and the time domain or frequency domain characteristics of these echo signals are used to reconstruct and enhance the images obtained from these signals. This imaging method can ultimately obtain a two-dimensional high-resolution image of the target. However, in this imaging mode, the imaging width in the azimuth direction is limited, and only single-beam width imaging can be achieved.

[0057] In some other implementations, there is also a sliding spotlight imaging mode. This imaging mode is a mode between the stripmap imaging mode and the traditional spotlight imaging mode. Specifically, the sliding spotlight mode continuously adjusts the azimuth angle so that the beam center always points to a virtual rotating point far away from the slant plane. The scanning speed of the beam in the imaging area is between zero and the radar platform speed, and a scanning strip is formed in the scanning area. In this way, the imaging area obtained by this imaging mode is larger than that of the traditional spotlight imaging mode, but smaller than that of the stripmap imaging mode.

[0058] In addition, in the sliding spotlight imaging mode, the detection targets at the same position and different azimuth positions not only have different starting times in the azimuth frequency history, but also have varying Doppler center frequencies. In addition, as the aperture increases, the range of the Doppler center frequency will increase. In this way, the entire azimuth bandwidth will increase, and it is possible that the azimuth spectrum will be aliased, causing the azimuth spectrum to overlap. In addition, the azimuth resolution of the sliding spotlight imaging mode is related to the radar platform speed, the antenna size, and the scanning speed of the antenna beam on the ground, and the azimuth resolution becomes worse as the slant angle increases. In another aspect, the resolution of the image obtained by the sliding spotlight imaging mode is between that of the traditional spotlight imaging mode and that of the stripmap imaging mode.

[0059] In some other implementations, there is also a sub-pulse fractional azimuth width time-domain spotlight imaging system. This imaging mode is a SAR imaging system based on sub-pulse diversity and time-domain spotlight imaging technology. Specifically, by using sub-pulse diversity technology in the azimuth direction during imaging, combined with time-domain spotlight methods, the azimuth resolution can be optimized and the image quality can be improved. The image quality obtained by using this imaging system is good, but after two-dimensional dechirp processing of the echo signal during imaging, interpolation processing is also required. In this way, the imaging operation complexity is high, which makes the imaging real-time poor and affects the imaging real-time.

[0060] The scheme provided by the embodiment of the present application will be described in detail below with reference to the drawings.

[0061] The embodiment of the present application provides a synthetic aperture radar imaging method, which can expand the azimuth imaging width on the basis of ensuring the azimuth imaging resolution, further reduce the complexity of imaging operation, and ensure the imaging real-time performance.

[0062] Please refer to Figure 1 A flowchart of a synthetic aperture radar imaging method provided by the embodiment of the present application is shown in FIG. 1. Figure 1 As shown in the figure, the method comprises steps 101-106.

[0063] It should be noted that the embodiment of the present application is based on a specific implementation scenario, and provides a comprehensive and complete implementation manner. In a specific implementation, some steps can be added or deleted according to requirements, which are only used for example description.

[0064] Step 101: Obtain the expected imaging area parameters.

[0065] The area parameters include the coordinates of at least one sub-imaging center, the radar coordinates, the center coordinates of the range-frequency area, and the slant range history.

[0066] Step 102: Transmit a pulse signal, which comprises at least one sub-pulse.

[0067] When the SAR transmits a sub-pulse, the sub-pulse has been modulated to a frequency value in the corresponding range-frequency area, that is, the frequency value of each sub-pulse is different, so that different sub-pulses point to different azimuth imaging areas.

[0068] In some implementations, it is assumed that the radar pulse repetition frequency (PRF) is a preset frequency, and each pulse signal comprises N sub-pulses. N is a natural number greater than 1. Then, the corresponding baseband sub-pulse signal can be represented by the following formula 1:

[0069]

[0070] Wherein, represents the fast time, rect() is the envelope of the signal, Δτ n = (n-1)T Δ , = 1, 2, …, is the transmission delay of the nth sub-pulse relative to the first sub-pulse, s Δ represents the sub-pulse transmission interval, T sp represents the sub-pulse duration, and μ represents the frequency modulation rate.

[0071] Specifically, the envelope rect() of the signal can be expressed by the envelope function formula 2 as follows:

[0072]

[0073] Each sub-pulse in the embodiment of the present application is modulated to a different range frequency area, Δf i is the frequency offset of the i-th sub-pulse relative to the first sub-pulse, and the expression is as follows:

[0074]

[0075] where Δf represents the frequency increment between two adjacent transmitted sub-pulses. In some implementations, in order to ensure that different sub-pulses can exist in the range frequency domain, the frequency interval between two sub-pulses needs to satisfy the constraint condition of Δf≥B sub , and the corresponding radar transmitted signal satisfies formula 4 as follows:

[0076]

[0077] In this way, the antenna aperture parameters do not need to be changed, and the imaging range can be widened. In this implementation, the coverage area corresponding to a single sub-pulse is equivalent to the imaging range of the pulse signal in the traditional beamforming imaging mode, so the imaging range widened by the method provided in the embodiment of the present application is widened.

[0078] In some implementations, in a phased array radar imaging device, different sub-pulses can be directed to different azimuth imaging areas by controlling the phases between the array elements in the phased array.

[0079] It is worth mentioning that in the pulse transmission method provided in the embodiment of the present application, the azimuth imaging range corresponding to the sub-pulse diversity is N times the imaging range in the traditional beamforming imaging mode. Moreover, the antenna does not need to be designed as multiple-in multiple-out (MIMO), and the gain of the transmitting antenna is not lost. In addition, since the sub-pulses are mutually independent pulse signals, the method provided in the embodiment of the present application has more flexible beam coverage area control capability.

[0080] Step 103: Establish a sub-pulse wide-beam imaging echo model.

[0081] The sub-pulse wide-beam imaging echo model is a theoretical model under ideal conditions. Therefore, the process of establishing the sub-pulse wide-beam imaging echo model is described by taking the echo signal that can occur under ideal conditions as an example.

[0082] It should be noted that since each sub-pulse signal is an independent pulse signal, and each sub-pulse is directed to a different azimuth direction area. The area can be discrete or continuous. The embodiment of the present application takes the continuous imaging of the azimuth wide beam as an example. For example, it is assumed that N sub-pulses are arranged to be directed to N adjacent areas respectively, and the interval between two adjacent areas in the adjacent areas is 3dB beam width of the main beam.

[0083] For example, the nthsub-pulse is directed to the nthimaging area, and the coordinates of the center of the nthimaging area are (x n ,y n ,0). At the azimuth time of zero, the tilt angle of the scene center relative to the radar is denoted as θ nc0 , and the shortest slant range of the scene center relative to the radar is denoted as R0. Then, there is a point target with coordinates (x np ,y np ,0) in the sub-scene. For this point target, the elevation angle, the shortest slant range and the distance between the radar and the target are θ np0 , R Bnp and Then, the coordinates of the point target in the imaging plane coordinate system can be represented as (x np ,R Bnp ). Thus, the echo of the nthsub-scene can be represented by formula 5 as follows:

[0084]

[0085] where σ np is the complex scattering coefficient of the target, t k is the azimuth slow time, w a ( k ) is the azimuth window function of the target. In the focused imaging mode, c can be considered as a constant, so its influence can be ignored in the subsequent modeling process. R np ( k ) represents the instantaneous slant range history between the target and the radar, and is represented by formula 6 as follows:

[0086]

[0087] where x′ np =x np -x n represents the azimuth distance difference between the target and the center coordinates of the sub-scene. Assuming that each sub-scene contains P scattering units, the echo data corresponding to a single sub-scene can be represented by formula 7 as follows:

[0088]

[0089] In addition, since the echo data of different sub-scenarios cannot be separated in time, the signal received by the radar is a mixed signal of each sub-scenario, which can be expressed by formula 8 as follows:

[0090]

[0091] It is worth mentioning that in the embodiment of the present application, a single pulse signal is divided into multiple sub-pulses, each sub-pulse is adjusted to correspond to a different area for beamforming imaging, and the energy of the system can be uniformly distributed in a wider area. In this way, the azimuth imaging bandwidth can be expanded, and the imaging range can be obtained far beyond other imaging methods. For example, compared with the traditional beamforming imaging mode, the imaging range is expanded by N times.

[0092] Step 104: receiving at least one echo signal to obtain a sub-scene mixed original echo.

[0093] Step 105: generating at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide beamforming imaging echo model.

[0094] It can be understood that the echo signals of different sub-scenarios can be mixed in the time domain, therefore, if the mixed sub-scene echo signals are directly processed, the images will be mixed with each other. Further, after receiving the echo signals to obtain the sub-scene mixed original echo, the sub-scene mixed original echo needs to be separated, and each sub-scene echo signal is further processed to obtain the original sub-scene image corresponding to each sub-pulse.

[0095] In some embodiments, please refer to Figure 2 A synthetic aperture radar imaging method flowchart provided by the embodiment of the present application is shown in Figure 2 The method includes steps 101-106, wherein steps 51-54 explain the specific implementation steps of generating an original sub-scene image. The sub-scene mixed original echo includes multiple sub-scene echo signals, and only one original sub-scene image is taken as an example here, therefore, steps 52-54 are operation steps that need to be repeatedly executed.

[0096] Step 51: performing band-pass filtering processing on the sub-scene mixed original echo to obtain at least one sub-scene echo.

[0097] Step 52: constructing a corresponding compensation function according to the sub-scene echo in the sub-pulse wide beamforming imaging echo model, and performing linear frequency modulation processing on the sub-pulse echo signal according to the compensation function to obtain a two-dimensional pre-processing image.

[0098] Step 53: performing beam domain two-dimensional interpolation processing on the two-dimensional pre-processing image to obtain a sub-scene imaging image in an original coordinate system.

[0099] Step 54: Geometric correction is performed on the sub-scene imaging graph in the original coordinate system to generate an original sub-scene image.

[0100] The following will be described in detail for generating an original sub-scene image.

[0101] First, before transmitting the sub-pulse signal, the sub-pulse has been modulated at different range frequencies. Therefore, a bandpass filter is constructed, and the sub-scene aliasing original echo input into the bandpass filter can obtain the original sub-pulse echo. For example, a down conversion function as described in formula 9 is constructed:

[0102]

[0103] Where the conjugate multiplication is performed After that, the signal of the corresponding nthsub-scene can be down-converted to the baseband frequency. Then, a bandpass filter is constructed, and the expression is formula 10:

[0104]

[0105] By converting the down-converted signal to the range frequency domain, multiplying it by H fliter (f r ), and converting the multiplied result back to the time domain, based on this, the echo data of the nthsub-scene region can be obtained, which can be expressed as formula 11:

[0106]

[0107] Similarly, by constructing different bandpass filters and processing the aliasing signals accordingly, the non-aliasing signals of all regions can be finally obtained.

[0108] It is worth noting that the received echo signal is from N sub-scenes, and the azimuth dimension of each sub-scene is approximately equal to the width of a single beam. Therefore, directly imaging these aliasing multiple sub-scene echo signals will cause serious Doppler ambiguity in imaging, which seriously affects the imaging quality. Therefore, in the embodiments provided by the present application, the aliasing original sub-scene echo signals are filtered by a bandpass filter, which can effectively separate the sub-pulse echo signals from different regions, thereby effectively solving the problem of Doppler ambiguity in imaging. Therefore, the method provided by the present application can maintain the original pulse signal to exist when the repetition frequency PRF is effective for imaging.

[0109] Further, since there is a fixed time delay between different sub-pulses. And this time delay is coupled with the sub-scene. This will cause the images of different regions to exist a fixed distance deviation, and then affect the subsequent imaging stitching. Therefore, the separated echo also needs to be processed for distance registration. Using the principle of stationary phase, the echo data is subjected to distance Fourier transform as shown in the following formula 12:

[0110]

[0111] From the above formula 12, it can be determined that f r and Δτ n There is a certain coupling, which causes the envelope error of the echo of different regions. Therefore, a correction function is constructed in the distance frequency domain, and its expression is formula 13:

[0112]

[0113] Y n (f r ,t k ) is multiplied by H com (f r ), and converted to distance time domain to obtain the following formula 14:

[0114]

[0115] After the above processing, the echo data of each sub-scene has been separated and compensated.

[0116] In actual beamforming imaging, the back projection (BP) imaging method can realize target imaging under any configuration. However, its operation amount is large. As a kind of common frequency domain beamforming imaging algorithm, the polar format algorithm (PFA) has the advantages of high precision and fast operation. For example, in the process of processing each sub-scene echo signal, the center coordinates of the region of the sub-scene are taken as the coordinate origin to establish the corresponding imaging plane coordinates, and then the polar format algorithm is used for imaging processing.

[0117] Please refer to Figure 3 and Figure 4 , Figure 3 is a schematic diagram of an imaging coordinate system established with the center coordinates of the total imaging scene as the coordinate origin, Figure 4 is a schematic diagram of an imaging coordinate system established with the center coordinates of the sub-scene as the coordinate origin. As Figure 3 shown, the center origin coordinates of the sub-scene can be represented as o′ n , whose coordinates in the x′ n -y′ n coordinate system are (x n , 0). As Figure 4As shown, in the sub-scene imaging coordinate system, the instantaneous position coordinate of the radar is (vt k -x n ,R0), the target position is (x′ np ,y′ np ), where x′ np =x np -x n , y′ np =R Bnp -R0. And there is an instantaneous oblique angle θ between the target and the radar (t k ,x′ np ,y′ np ) can be expressed as follows:

[0118]

[0119] In such Figure 4 In the coordinate system established by the center coordinates of the sub-scene shown, the one-way slant range history between the radar and the target coordinates can be expressed by the following formula 16:

[0120]

[0121] The one-way slant distance between the sub-scene center coordinates and the target coordinates can be expressed by the following formula 17:

[0122]

[0123] The reference signal constructed using the sub-scene center coordinates can be expressed as follows:

[0124]

[0125] Where T ref is the pulse width of the reference signal. Then, by conjugating and multiplying the reference function with the sub-scene data (for ease of explanation, only a single target is considered at this time), we can obtain the following formula 19:

[0126]

[0127] in:

[0128]

[0129] A is σ np w a (t k ), since the subsequent derivation process focuses on the phase information, the influence of amplitude is ignored in the subsequent derivation. is the angle between the line connecting the radar and the center of the sub-scene relative to the positive direction of the x′ axis, is the angle between the target and the center of the sub-scene relative to the positive direction of the x′ axis, and is positive in the counterclockwise direction.

[0130] For example, the conversion of the above equation to the range frequency domain using the principle of stationary phase (POSP) can be expressed as equation 20 as follows:

[0131]

[0132] where the first term is the azimuth phase modulation term, the second term is the residual video phase (RVP) term, and the third term is the envelope tilt term. The second and third terms both cause range migration of the target. Since the range frequency satisfies the relationship , the range frequency domain signal can also be expressed as equation 21 as follows:

[0133]

[0134] To this end, a residual video phase compensation function can be constructed. The compensation function can be expressed as equation 22 as follows:

[0135]

[0136] After compensation, the corresponding range frequency domain signal is equation 23 as follows:

[0137]

[0138] Further converted to the range time domain, it can be expressed as equation 24 as follows:

[0139]

[0140] Let

[0141]

[0142] Due to the limitation of the echo pulse width, K Rn satisfies the relationship shown in equation 25 as follows:

[0143]

[0144] Further, the two-dimensional time domain data after RVP compensation can be further expressed as equation 26 as follows:

[0145]

[0146] where Thus, the expression of the echo data in the two-dimensional beam domain is obtained. Next, the second-order Taylor expansion of the slant range history difference ΔR np (t k ) at r np = 0 is performed, and equation 27 is obtained as follows:

[0147]

[0148] where ΔR np_s ( k ) is the wavefront curvature impact term, which will cause defocus and position deviation of the target.

[0149] After ignoring the wavefront curvature term, the slant range history is brought in, which can be expressed by the following formula 28:

[0150]

[0151] where Then the echo data is processed by two-dimensional Fourier transform, and the final imaging result is obtained as:

[0152] Y nRVP (x′ np ,y′ np )=FFT(y nRVP (x′ np ,y′ np ))

[0153] =sinc(x′-x′ np )sinc(y′-y′ np )

[0154] It is worth mentioning that the above processing is performed on each sub-scene echo signal respectively, and the corresponding sub-scene original image can be obtained.

[0155] Step 106: Image stitching is performed on the original sub-scene images to generate an azimuth wide-beam imaging image.

[0156] It can be understood that in the method provided by the embodiment of the application, the azimuth wide-beam coverage is realized through the multi-frequency sub-pulse diversity processing. The multi-frequency sub-pulse azimuth wide-beam frequency domain imaging framework and related technologies are established, and the wide-beam imaging without Doppler ambiguity can be realized on the basis of not changing the system pulse repetition frequency. In the specific implementation manner, the related processing flow of the frequency domain imaging algorithm suitable for the sub-pulse diversity system is given. Compared with the time domain imaging algorithm, the frequency domain algorithm has low operation complexity, and the processing time is short under the same imaging scene. The frequency domain algorithm is more suitable for wide-beam imaging processing.

[0157] Please refer to Figure 5 , which is a wide-beam imaging geometry diagram provided by the embodiment of the application. As Figure 5 indicated, the direction of V is the radar running direction.

[0158] In the following, the simulation result diagram of the method provided by the embodiment of the application will be described by combining with the simulation software. Herein, three sub-scenes are taken as an example.

[0159] The simulation parameters used when running the simulation software are shown in Table 1 below.

[0160]

[0161] Table 1: Simulation parameter table

[0162] Reference is made to Figure 6 , a point distribution schematic provided for the embodiment of the present application. As shown in Figure 6 , 9 point targets are arranged in each sub-scene, and point targets of the same color belong to one sub-scene. For example, blue points are point targets of the first sub-scene, red points are point targets of the second sub-scene, and green points are point targets of the third sub-scene.

[0163] Further, through calculation, it can be obtained that the dechirped Doppler bandwidth of the corresponding sub-scene azimuth is about 528 Hz. Therefore, to ensure no aliasing in Doppler, the repetition frequency of the system pulse signal can be set to 550 Hz (at this time, the full-scene Doppler bandwidth is about 1584 Hz). Further, the aliasing echo data can be obtained as shown in Figure 7A - Figure 7C . Figure 7A is a two-dimensional time-domain aliasing echo schematic, Figure 7B is an azimuth frequency-domain aliasing echo schematic, Figure 7C is a distance frequency-domain aliasing echo data schematic.

[0164] As can be seen from the above Figure 7A and Figure 7B , severe azimuth aliasing occurs at this time, which is caused by the long synthetic aperture time and the large corresponding imaging width. Figure 7C is a corresponding distance frequency-domain echo data profile obtained after the original echo is converted to the distance frequency domain. As can be seen from the figure, at this time, the echo data of different sub-scenes is separated from each other in the distance frequency domain.

[0165] Based on the results shown in the above Figure 7A - Figure 7C , if the method provided by the embodiment of the present application is used to construct a corresponding bandpass filter for echo data, different sub-scene data is separated. Reference is made to Figure 8A - Figure 9C , which respectively shows the data of three sub-scene echo separations. As shown in Figure 8A , is a distance frequency-domain filtering result display of the first sub-scene, Figure 8B is a distance frequency-domain filtering result display of the second sub-scene, Figure 8C is a distance frequency-domain filtering result display of the third sub-scene. As shown in Figure 9A , is a two-dimensional time-domain signal result display of the first sub-scene, Figure 9B is a two-dimensional time-domain signal result display of the second sub-scene, Figure 9C is a two-dimensional time-domain signal result display of the third sub-scene.

[0166] Further, a corresponding two-dimensional dechirp function is constructed for each sub-scene echo data, and the dechirp processing result of each scene can be obtained, as shown in Figure 10A - Figure 10C , which is a two-dimensional dechirp processing result display of three sub-scenes. Figure 10A , which is a dechirp processing result display of the first sub-scene, Figure 10B , which is a dechirp processing result display of the second sub-scene, Figure 10C , which is a dechirp processing result display of the third sub-scene,

[0167] Taking the first sub-scene as an example, the corresponding beam domain interpolation imaging processing is performed on the first sub-scene data. The processing result is as shown in Figure 11 - Figure 13 . Figure 11 , which is a three-dimensional imaging result of the first sub-scene, Figure 12 , which is a top view of the frequency domain imaging result of the first sub-scene, Figure 13 , which is an enlarged view of the three target points on the right side of the first sub-scene.

[0168] As shown in Figure 11 and Figure 12 , after the beam domain interpolation imaging processing, it can be determined that the target points in the sub-scene are well focused. Figure 13 It can be seen that the corresponding target may have certain geometric distortion, which is caused by wavefront bending. Therefore, before splicing the multiple sub-scene original images, the geometric distortion correction of the sub-scene original image is also needed.

[0169] Please refer to Figure 14 and Figure 15 , Figure 14 , which is a result display after the geometric correction of the imaging result of the first sub-scene, Figure 15 , which is an enlarged view of the three target points on the right side of the first sub-scene after the geometric correction of the imaging result.

[0170] Based on the same processing as the first sub-scene, the same processing is performed on the second sub-scene and the third sub-scene, and the imaging results of the second sub-scene and the third sub-scene can be obtained. After image splicing of the three sub-scene images, the large azimuth wide non-aliasing imaging result can be obtained. As shown in Figure 16 , which is the imaging result after image splicing.

[0171] From the above results, it can be seen that by using sub-pulse diversity, the frequency domain imaging algorithm of the present application can realize large azimuth wide beamforming imaging (azimuth wide expansion by sub-pulse number times) without losing resolution and changing the pulse repetition frequency compared with the traditional phased array radar.

[0172] In another simulation experiment, the radar array aperture is adjusted so that the radar single-beam covers the same imaging width and adopts the traditional phased array beamforming imaging system, and the corresponding pulse repetition frequency is still maintained at 550Hz (at this time, the echo azimuth bandwidth is 1584Hz). At this time, the point target position information is still unchanged. The phased array imaging result as shown in Figure 17A - Figure 17C

[0173] As shown in Figure 17A , the traditional phased array raw echo signal is Figure 17B , the imaging after two-dimensional dechirp processing is Figure 17C , and the traditional aliasing imaging is

[0174] As can be seen from Figure 17A - Figure 17C , after two-dimensional dechirp processing using the scene center, the imaging as shown in Figure 17B , due to the low system pulse repetition frequency, serious azimuth Doppler aliasing occurs. After the line-of-sight interpolation frequency domain imaging, as shown in Figure 17C , at this time, the azimuth wide beam beamforming imaging cannot be effectively realized, and the Doppler aliasing energy will affect the imaging quality of the imaging area. The method in the embodiment of the present application can realize the azimuth wide beam beamforming imaging under a low system pulse repetition frequency.

[0175] In another simulation experiment, compared with the sub-pulse diversity system, the operation time of the method provided in the embodiment of the present application is shorter, which is helpful to realize real-time detection. Table 2 below is a time comparison table of the time domain imaging algorithm and the imaging algorithm provided in the embodiment of the present application.

[0176]

[0177] Table 2: Time comparison table of time domain imaging algorithm and imaging algorithm of the present application

[0178] The embodiment of the present application also provides a synthetic aperture radar imaging device, which comprises a SAR and an image processor.

[0179] The SAR can transmit pulse signals and receive echo signals. The image processor can establish a sub-pulse wide beamforming imaging echo model, which is a theoretical model under ideal conditions. And according to the sub-pulse wide beamforming imaging echo model, the echo signal of each sub-scene is processed, and the image of this sub-scene, i.e. the original sub-scene image, can be generated. Finally, the original sub-scene images are spliced to generate a wide beamforming imaging image in the azimuth direction.

[0180] Those skilled in the art can understand that the imaging device shown in the embodiment of the present application does not constitute a limitation on the imaging device, and in actual application, the SAR imaging device can include more or fewer components.​

[0181] It can be understood that the above-mentioned embodiments can be stored in a computer readable storage medium in the form of code or computer instructions. When the imaging device is run, the imaging device can implement the steps in the above-mentioned method embodiments. For example, the computer readable storage medium can be a read-only memory (ROM), a random access memory (RAM), a compact disc read-only memory (CD-ROM), a magnetic tape, a floppy disk, and an optical data storage device, etc.

[0182] In several embodiments provided in the present application, it should be understood that the disclosed apparatus and method can be implemented by other means. For example, the above-described apparatus embodiments are only illustrative, for example, the division of the modules or units is only a logical function division, and actual implementation can have another division manner, for example, a plurality of units or components can be combined or integrated into another apparatus, or some features can be ignored or not executed. In addition, the coupling or direct coupling or communication connection between the units or components shown or discussed can be indirect coupling or communication connection through some interfaces, apparatuses or units, and can be electrical, mechanical or other forms.

[0183] The units described as separate components can or can not be physically separated, and the components shown as units can be one physical unit or multiple physical units, that is, can be located in one place or can be distributed to multiple different places. Part or all of the units can be selected according to actual needs to achieve the purpose of the embodiment scheme.

[0184] In addition, each functional unit in each embodiment of the present application can be integrated in one processing unit, or each unit can be physically present separately, or two or more units can be integrated in one unit. The integrated unit can be realized in the form of hardware or in the form of a software functional unit.

[0185] The integrated unit, if implemented in the form of a software function unit and sold or used as an independent product, can be stored in a readable storage medium. Based on such understanding, the technical solutions of the embodiments of the present application essentially or the parts that make contributions to the prior art or the whole or part of the technical solutions can be embodied in the form of a software product. The software product is stored in a storage medium, includes a plurality of instructions for making a device (which can be a single-chip microcomputer, a chip, etc.) or a processor execute all or part of the steps of the method described in the embodiments of the present application. The foregoing storage medium includes a U disk, a mobile hard disk, a read-only memory (ROM), a random access memory (RAM), a magnetic disk or an optical disk, and various media that can store program codes.

[0186] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any change or replacement within the technical scope disclosed by the present application should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A synthetic aperture radar imaging method, characterized by, The method comprises: transmitting a pulse signal, the pulse signal comprising at least one sub-pulse; wherein the sub-pulses are modulated to one frequency value in a corresponding range of distance frequencies, so that different sub-pulses are directed to different azimuth imaging areas; establishing a sub-pulse wide-beam imaging echo model, the sub-pulse wide-beam imaging echo model being a theoretical model under ideal reception conditions; receiving at least one echo signal to obtain a sub-scene mixed original echo; wherein the echo signal is obtained after the at least one sub-pulse signal is reflected, and the sub-scene mixed original echo is formed by mixing the reflected echo signals of at least one sub-pulse; generating at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model; image stitching is performed on the at least one original sub-scene image to generate an azimuth wide-beam image; the generation of the at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model comprises: band-pass filtering processing is performed on the sub-scene mixed original echo to obtain at least one sub-scene echo; each sub-scene echo is processed as follows: in the sub-pulse wide-beam imaging echo model, a corresponding reference function is constructed according to the sub-scene echo, and de-chirp processing is performed to obtain a two-dimensional pre-processing image; the two-dimensional pre-processing image is sequentially subjected to residual video phase compensation, beam domain conversion processing, Taylor expansion, and two-dimensional Fourier transform to obtain a sub-scene imaging image in an original coordinate system; geometric correction processing is performed on the sub-scene imaging image in the original coordinate system to generate an original sub-scene image; the method further comprises: obtaining expected imaging area parameters, the parameters comprising coordinates of at least one sub-imaging center, radar coordinates, center coordinates of the range of distance frequencies, and slant range history; the construction of the corresponding reference function according to the sub-scene echo comprises: determining sub-scene center coordinates corresponding to the sub-scene echo, and constructing a reference function of the sub-pulse according to the sub-scene center coordinates.

2. A synthetic aperture radar imaging apparatus, characterized by comprising: The method comprises: the synthetic aperture radar is configured to transmit a pulse signal, the pulse signal comprising at least one sub-pulse; wherein the sub-pulses are modulated to one frequency value in a corresponding range of distance frequencies, so that different sub-pulses are directed to different azimuth imaging areas; at least one echo signal is received, and distance frequency domain processing is performed on the at least one echo signal to obtain at least one sub-scene un-mixed echo signal; wherein each of the at least one echo signal is obtained after the at least one sub-pulse signal is reflected, and each echo signal is processed to obtain a sub-scene un-mixed echo signal; the image signal processor is configured to establish a sub-pulse wide-beam imaging echo model, the sub-pulse wide-beam imaging echo model being a theoretical model under ideal reception conditions; and generate at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide-beam imaging echo model. image stitching is performed on the at least one original sub-scene image to generate an azimuth wide swath spotlight image; when the image signal processor is used to generate at least one original sub-scene image according to the sub-scene mixed original echo and the sub-pulse wide swath spotlight imaging echo model, the image signal processor is specifically configured to: band-pass filtering processing is performed on the sub-scene mixed original echo to obtain at least one sub-scene echo; the following processing is performed on each sub-scene echo respectively: in the sub-pulse wide swath spotlight imaging echo model, a corresponding reference function is constructed according to the sub-scene echo, linear frequency modulation processing is performed to obtain a two-dimensional pre-processing image, the two-dimensional pre-processing image is sequentially subjected to residual video phase compensation, beam domain conversion processing, Taylor expansion and two-dimensional Fourier transform to obtain a sub-scene imaging image in an original coordinate system, and geometric correction processing is performed on the sub-scene imaging image in the original coordinate system to generate an original sub-scene image; the image signal processor is further configured to acquire expected imaging region parameters, the parameters including coordinates of at least one sub-imaging center, radar coordinates, center coordinates of the range frequency region and slant range history; the image signal processor is further configured to determine sub-scene center coordinates corresponding to the sub-scene echo, and construct a reference function of the sub-pulse according to the sub-scene center coordinates.

3. An image forming apparatus characterized by comprising: comprise: a processor and a memory for storing instructions executable by the processor; wherein the processor is configured to execute the instructions to implement the method of claim 1.

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