Synthetic aperture radar imaging method and device and imaging equipment

Abstract

The invention provides a synthetic aperture radar imaging method and device and imaging equipment, and relates to the technical field of remote sensing, and the specific scheme comprises the steps: transmitting a pulse signal which comprises at least one sub-pulse; and establishing a sub-pulse wide-range bunching imaging echo model, wherein the sub-pulse wide-range bunching imaging echo model is a theoretical model under ideal receiving conditions. And receiving at least one echo signal to obtain a sub-scene aliasing original echo. And generating at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide bunching imaging echo model. And carrying out image splicing to generate an azimuth wide-width bunching imaging image. Therefore, high-resolution images can be obtained in the synthetic aperture radar imaging process, the azimuth imaging width can be effectively widened, the operation complexity of an imaging algorithm can be reduced, and images can be collected in real time.

Description

Technical Field

[0001] The present invention relates to the field of remote sensing technology, and in particular to a synthetic aperture radar imaging method, device and imaging equipment. Background Art

[0002] Synthetic Aperture Radar (SAR) is a high-resolution imaging radar. SAR imaging technology is a high-resolution remote sensing technology that uses active microwave imaging. Specifically, the SAR system transmits a series of pulse signals, which are usually electromagnetic waves in the microwave or radar band. These pulse signals propagate to the surface and are reflected back. The SAR system receives these reflected signals and amplifies, filters, and phase corrects the received signals to generate high-resolution images. SAR imaging technology has the ability to obtain high-resolution images all day and all weather, and is therefore widely used in terrain mapping, disaster monitoring, military reconnaissance and other fields.

[0003] It is worth noting that the schemes for applying SAR imaging technology in reality still face some limitations. For example, in the strip imaging scheme, the beam of the fixed radar antenna is used to achieve wide-width imaging in azimuth. At the same time, due to the fixed radar antenna, its azimuth resolution is poor. For another example, in the traditional spotlight imaging scheme, the imaging resolution of the radar azimuth can be effectively improved by adjusting the antenna beam to point to a specific imaging area. However, due to the limited width of the azimuth imaging, this imaging method can only achieve single beam width imaging. For another example, in the sub-pulse diversity azimuth width time-domain spotlight imaging system, wide-width imaging in azimuth can be achieved, but its time-domain imaging algorithm requires the use of complex interpolation operations, and the corresponding computational complexity is high, the real-time performance is poor, and it is not conducive to real-time processing. Therefore, in the study of SAR imaging technology, when providing high-resolution images, the azimuth imaging width can be widened, and reducing the computational complexity of the imaging algorithm is the main research direction. Summary of the invention

[0004] The present invention provides a synthetic aperture radar imaging method, device and imaging equipment, which can obtain high-resolution images in the process of synthetic aperture radar imaging, effectively widen the azimuth imaging width, and reduce the computational complexity of the imaging algorithm, so that images can be collected in real time.

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

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

[0007] It should be understood that the echo signal received by SAR is the signal after the sub-pulse is reflected. Each sub-pulse corresponds to a sub-scene. Therefore, the echo signal of each sub-scene is processed according to the sub-pulse wide-band spotlight imaging echo model to generate the image of this sub-scene, that is, the original sub-scene image. Finally, these original sub-scene images are stitched to generate the azimuth wide-band spotlight imaging image.

[0008] Among them, the sub-pulses are modulated to different distance frequency regions, and different sub-pulses are independent of each other in time and distance frequency regions, so that the sub-pulses can acquire images at different distances, thereby effectively widening the imaging width of the image in the azimuth direction. Furthermore, after receiving the echo signal of the sub-pulse, the original sub-scene images are generated respectively, and the images are spliced ​​to obtain a wide-width beam image in the azimuth direction. The imaging operation complexity of this imaging method is low, which effectively shortens the imaging time, thereby facilitating the realization of real-time image acquisition.

[0009] In combination with the first aspect, in a possible implementation, generating at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide-band spotlight imaging echo model includes: performing bandpass filtering on the sub-scene aliasing original echo to obtain at least one sub-scene echo. Performing the following processing on each sub-scene echo respectively:

[0010] In the sub-pulse wide-band spotlight imaging echo model, the corresponding compensation function is constructed according to the sub-scene echo, and the linear frequency modulation is de-interpolated to obtain a two-dimensional pre-processed image. The two-dimensional pre-processed image is subjected to beam domain two-dimensional interpolation processing 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.

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

[0012] In combination with the first aspect, in another possible implementation, the method further includes: obtaining parameters of a desired imaging area. These area parameters may include: coordinates of at least one sub-imaging center, radar coordinates, center coordinates of a range-frequency area, and slant range history.

[0013] In combination with the first aspect, in another possible real-time manner, constructing a corresponding compensation function based on the sub-scene echo includes: determining the sub-scene center coordinates corresponding to the sub-scene echo, and constructing a compensation function of the sub-pulse based on the sub-scene center coordinates.

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

[0015] The synthetic aperture radar is used to: transmit a pulse signal, which includes at least one sub-pulse. When the SAR transmits a sub-pulse, the sub-pulse has been modulated to a frequency value in the corresponding range frequency region, that is, the frequency value of each sub-pulse is different, so that different sub-pulses point to imaging areas in different azimuths. Further, at least one echo signal is received to obtain a sub-scene aliased original echo. The echo signal is obtained after at least one sub-pulse signal is reflected, and the sub-scene aliased original echo is formed by the aliasing of the reflected echo signal of at least one sub-pulse.

[0016] The image signal processor is used to: establish a sub-pulse wide-band spotlight imaging echo model, which is a theoretical model under ideal reception conditions. Generate at least one original sub-scene image based on the sub-scene aliasing original echo and the sub-pulse wide-band spotlight imaging echo model. Perform image stitching on these original sub-scene images to generate an azimuth wide-band spotlight imaging image.

[0017] In combination with the second aspect, in a possible implementation, the signal processor is used to: generate at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide-band spotlight imaging echo model. The signal processor is specifically used to: perform bandpass filtering on the sub-scene aliasing original echo to obtain at least one sub-scene echo. Perform the following processing on each sub-scene echo respectively:

[0018] In the sub-pulse wide-band spotlight imaging echo model, the corresponding compensation function is constructed according to the sub-scene echo, and the linear frequency modulation is de-interpolated to obtain a two-dimensional pre-processed image. The two-dimensional pre-processed image is subjected to beam domain two-dimensional interpolation processing 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] In conjunction with the second aspect, in another possible implementation, the signal processor is further used to obtain parameters of the desired imaging area, which include coordinates of at least one sub-imaging center, radar coordinates, center coordinates of the range-frequency area, and slant range history.

[0020] In combination with the second aspect, in another possible implementation, the signal processor is further used to construct a corresponding compensation function based on the sub-scene echo, including: determining the sub-scene center coordinates corresponding to the sub-scene echo, and constructing a compensation function of the sub-pulse based on the sub-scene center coordinates.

[0021] In a third aspect, the present invention further provides an imaging device, comprising: a processor and a memory for storing processor executable instructions; wherein the processor is configured to execute instructions to implement the method in the first aspect and any possible implementation manner thereof.

[0022] In a fourth aspect, the present invention also provides a server, which may include 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 in the first aspect and any possible implementation manner thereof.

[0023] In a fifth aspect, the present invention further provides a computer-readable storage medium having computer instructions stored thereon, which, when executed on an imaging device, implement the method mentioned in the first aspect and any possible implementation manner thereof.

[0024] It can be understood that the beneficial effects that can be achieved by the device in the second aspect and any possible implementation manner provided above, the imaging device in the third aspect, the server in the fourth aspect and the computer-readable storage medium in the fifth aspect can be referred to as the beneficial effects in the first aspect and any possible design manner thereof, and will not be repeated here. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A flow chart of a synthetic aperture radar imaging method provided by an embodiment of the present invention;

[0026] Figure 2 A flow chart of another synthetic aperture radar imaging method provided by an embodiment of the present invention;

[0027] Figure 3A schematic diagram of an imaging coordinate system established with the center coordinates of the total imaging scene as the coordinate origin provided by an embodiment of the present invention;

[0028] Figure 4 A schematic diagram of establishing an imaging coordinate system with the sub-scene center coordinates as the coordinate origin provided by an embodiment of the present invention;

[0029] Figure 5 A wide-format imaging geometric diagram provided by an embodiment of the present invention;

[0030] Figure 6 A schematic diagram of scene layout in a simulation experiment provided by an embodiment of the present invention;

[0031] Fig. 7A A schematic diagram of a two-dimensional time domain aliasing echo in a simulation experiment provided by an embodiment of the present invention;

[0032] Figure 7B A schematic diagram of azimuth frequency domain aliasing echo in a simulation experiment provided by an embodiment of the present invention;

[0033] Figure 7C A schematic diagram of distance-frequency domain aliased echo data in a simulation experiment provided by an embodiment of the present invention;

[0034] Fig. 8A A schematic diagram of the distance frequency domain filtering result of the first sub-scene in a simulation experiment provided by an embodiment of the present invention;

[0035] Figure 8B A schematic diagram of the distance frequency domain filtering result of the second sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0036] Figure 8C A schematic diagram of the distance frequency domain filtering results of the third sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0037] Fig. 9A A schematic diagram of a two-dimensional time domain signal result of a first sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0038] Fig. 9B A schematic diagram of a two-dimensional time domain signal result of a second sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0039] Fig. 9C A schematic diagram of a two-dimensional time domain signal result of a third sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0040] Fig. 10A A schematic diagram of the dechirp processing result of the first sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0041] Fig. 10BA schematic diagram of the dechirp processing result of the second sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0042] Fig. 10C A schematic diagram of the dechirp processing result of the third sub-scenario in a simulation experiment provided by an embodiment of the present invention;

[0043] Fig.11 A schematic diagram of a three-dimensional imaging result of a first sub-scene in a simulation experiment provided by an embodiment of the present invention;

[0044] Fig.12 A top view of the frequency domain imaging result of the first sub-scene in the simulation experiment provided by the embodiment of the present invention;

[0045] Fig.13 A schematic diagram of three target points on the right side of the frequency domain of the first sub-scenario in the simulation experiment provided by an embodiment of the present invention;

[0046] Fig.14 A schematic diagram of geometric correction results of the first sub-scene imaging result in a simulation experiment provided by an embodiment of the present invention;

[0047] Fig.15 An enlarged view of three target points on the right side of the imaging result of the first sub-scene in the simulation experiment provided by the embodiment of the present invention after geometric correction;

[0048] Fig.16 A schematic diagram of imaging after image splicing in a simulation experiment provided by an embodiment of the present invention;

[0049] Fig.17A A schematic diagram of a conventional phased array original echo signal provided by an embodiment of the present invention;

[0050] Fig. 17B A schematic diagram of an image after two-dimensional dechirp processing provided by an embodiment of the present invention;

[0051] Fig. 17C A schematic diagram of traditional aliasing imaging provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0052] In the following, the terms "first" and "second" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this embodiment, unless otherwise specified, "plurality" means two or more.

[0053] Based on the important role of synthetic aperture radar, it has important applications in real-time radar image acquisition. In SAR imaging technology, microwave signals are transmitted by SAR, and the microwave signals propagate to the surface of the object and are reflected. SAR receives the echo signal and generates radar images based on the echo signal. This imaging method by transmitting microwave signals does not need to rely on objective environmental factors such as ambient light, and can achieve all-weather and all-time image acquisition. However, there are also some influencing factors in the actual application of SAR.

[0054] For example, in the process of SAR synthetic aperture, the deviation of the platform motion trajectory will lead to the introduction of phase error, resulting in blurred or artifact radar imaging. In addition, during the imaging process, such as between the emission of microwave signals and the reception of echo signals, the target and the radar move relative to each other (such as in the process of photographing high-speed moving objects). Because of the change in position, the reflected echo signal introduces Doppler frequency shift, which interferes with the imaging image. In addition, SAR images in the oblique direction, and changes in terrain height can also cause geometric distortion in the captured 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 a 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 a wide coverage range and continuous imaging. The imaging feature of this imaging mode is a wide coverage range. If a SAR is carried on an aircraft and this imaging mode is adopted, the aircraft can collect images in real time during a flight mission, which can ensure that the collected images cover a large area. Therefore, this method is suitable for wide-area remote sensing. On the other hand, this imaging method can keep the resolution of distance and azimuth fixed within the entire imaging range, so the images collected in this way are convenient for data analysis. Moreover, in this imaging mode, the collected images are also continuous, which makes it possible to achieve the stability of the imaging process and ensure the imaging efficiency without dynamic adjustment of the antenna. In summary, this strip imaging mode can achieve large-width imaging in azimuth by fixing the antenna beam, but the resolution of this imaging mode in azimuth needs to be improved.

[0056] In some other implementations, there is also a traditional beamforming imaging mode. It should be noted that the traditional beamforming imaging algorithm processes SAR echo data based on mathematical models and physical principles. By controlling the beam emitted by the antenna to constantly point to the desired detection area, it achieves a time that is superior to the synthetic aperture in the strip imaging mode, thereby increasing the target coherent accumulation angle. Furthermore, a corresponding imaging processing method is used to synthesize echo signals from multiple angles, 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, under this imaging method, 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 beam imaging mode. This imaging mode is an imaging mode between the strip imaging mode and the traditional beam imaging mode. Specifically, the sliding beam mode continuously adjusts the azimuth angle so that the center of the beam always points to the virtual rotation point far away from the inclined plane, and the scanning speed of the beam in the imaging area is between zero and the radar platform speed, and a scanning belt is formed in the scanning area. In this way, the captured image obtained in this imaging mode shows that the detection area is larger than the detection area of ​​the traditional beam imaging mode, but smaller than the detection area of ​​the strip imaging mode.

[0058] In addition, in the sliding beam imaging mode, for the detection targets at the same position and different azimuth positions, not only the azimuth frequency history has different starting times, but the Doppler center frequency also changes. In addition, as the aperture increases, the range of change of the Doppler center frequency will become larger. In this way, the entire azimuth bandwidth will increase, and it may exceed the pulse repetition frequency, resulting in the occurrence of azimuth spectrum aliasing. In addition, the azimuth resolution of the sliding beam imaging mode is related to the radar platform speed, antenna size, and antenna beam scanning speed on the ground, and the azimuth resolution becomes worse as the oblique angle increases. On the other hand, the resolution of the image obtained by the sliding beam imaging mode is between the traditional beam imaging mode and the strip imaging mode.

[0059] In some other implementations, there is also a sub-pulse graded azimuth width time-domain beamforming imaging system. This imaging mode is a SAR imaging system based on sub-pulse diversity and time-domain beamforming imaging technology. Specifically, in the imaging process, by using sub-pulse diversity technology in the azimuth direction and combining it with the time-domain beamforming method, 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 in the imaging process, the echo signal needs to be subjected to two-dimensional dechirp processing (dechirp processing) and then interpolation processing. This will lead to a higher computational complexity of the imaging, making the real-time performance of the imaging poorer, affecting the real-time performance of the imaging.

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

[0061] The embodiment of the present invention 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 calculations, and ensure the real-time imaging.

[0062] Please refer to Figure 1 A flow chart of a synthetic aperture radar imaging method provided by an embodiment of the present invention. Figure 1 As shown, the method includes steps 101 to 106.

[0063] It should be noted that the embodiments of the present invention are based on specific implementation scenarios and provide a comprehensive and complete implementation method. In the specific implementation, some steps can be added or deleted according to the needs, which is only an example description here.

[0064] Step 101: Obtaining parameters of a desired imaging area.

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

[0066] Step 102: Transmit a pulse signal, where the pulse signal includes at least one sub-pulse.

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

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

[0069]

[0070] in, represents 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 emission interval, T sp represents the sub-pulse duration, and μ represents the modulation frequency.

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

[0072]

[0073] In the embodiment of the present invention, each sub-pulse is modulated to a different range frequency region, Δf i is the frequency offset of the ith sub-pulse relative to the first sub-pulse, and its expression is:

[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 be effectively separated in the range-frequency domain, the frequency interval between two sub-pulses needs to satisfy Δf ≥ B sub The constraint condition, and then the corresponding radar transmission signal satisfies the following formula 4:

[0076]

[0077] In this way, there is no need to change the antenna aperture parameters, 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 is widened by the method provided in the embodiment of the present invention.

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

[0079] It is worth mentioning that in the pulse transmission method provided by the embodiment of the present invention, the azimuth imaging range corresponding to the sub-pulse diversity is N times the imaging range in the traditional beamforming imaging method. Moreover, there is no need to design the antenna as multiple-in multiple-out (MIMO), and there is no loss in the gain of the transmitting antenna. In addition, since the sub-pulses are independent pulse signals, the method provided by the embodiment of the present invention has a more flexible beam coverage area control capability.

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

[0081] The sub-pulse wide-band spotlight imaging echo model is a theoretical model under ideal reception conditions. Therefore, the echo signal that may appear under ideal conditions is taken as an example to illustrate the process of establishing the sub-pulse wide-band spotlight imaging echo model.

[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 region. Among them, it can be a discrete region or a continuous region. The embodiment of the present invention takes azimuth wide beam continuous imaging as an example. Exemplarily, it is assumed that N sub-pulses are set to point to N adjacent regions respectively, and the interval between two adjacent regions in these adjacent regions is the 3dB beam width of the main beam.

[0083] For example, the nth sub-pulse points to the nth imaging area, and the coordinates of the center of the nth imaging area are (x n ,y n ,0). When the azimuth time is zero, the inclination angle of the scene center relative to the radar is expressed as θ nc0 , the shortest slant distance from the scene center to the radar is expressed as R 0 Then, there is a coordinate (x np ,y np ,0). For this point target, its pitch angle, shortest slant range and distance between the radar and the target are θ np0 , R Bnp and Then, the coordinates of this point target in the imaging plane coordinate system can be expressed as (x np ,R Bnp ). Thus, the echo of the nth sub-scene can be expressed by formula 5 as follows:

[0084]

[0085] Among them, 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. np ( k ) represents the instantaneous slant range between the target and the radar, which is expressed by the following formula 6:

[0086]

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

[0088]

[0089] In addition, since the echo data of different sub-scenes cannot be separated in time, the signal received by the radar is the aliased signal of each sub-scene. The aliased signal of the sub-scene can be expressed by the following formula 8:

[0090]

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

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

[0093] Step 105: Generate at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide-band spotlight imaging echo model.

[0094] It is understandable that the echo signals of different sub-scenes may be aliased in the time domain. Therefore, if the aliased sub-scene echo signals are directly processed, the image may be aliased. Furthermore, after receiving these echo signals and obtaining the sub-scene aliased original echoes, the sub-scene aliased original echoes need 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 implementations, please refer to Figure 2 , is a flow chart of a synthetic aperture radar imaging method provided by an embodiment of the present invention, such as Figure 2 As shown, the method includes steps 101 to 106, wherein steps 51 to 54 illustrate specific implementation steps for generating an original sub-scene image. The sub-scene aliasing original echo includes multiple sub-scene echo signals, and only one original sub-scene image is taken as an example here, so steps 52 to 54 are operation steps that need to be repeatedly performed.

[0096] Step 51: Perform bandpass filtering on the sub-scene aliased original echo to obtain at least one sub-scene echo.

[0097] Step 52: In the sub-pulse wide-band spotlight imaging echo model, a corresponding compensation function is constructed according to the sub-scene echo, and the sub-pulse echo signal is de-linearly modulated according to the compensation function to obtain a two-dimensional pre-processed image.

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

[0099] Step 54: geometrically correct the sub-scene image in the original coordinate system to generate an original sub-scene image.

[0100] The following is a detailed description of how to generate an original sub-scene image.

[0101] First, before transmitting the sub-pulse signal, the sub-pulse has been modulated at different distance frequencies. Therefore, a bandpass filter is constructed, and the sub-scene aliased original echo is input into the bandpass filter to obtain the original sub-pulse echo. Exemplarily, a down-conversion function as described in Formula 9 is constructed:

[0102]

[0103] Among them, after conjugate multiplication After that, the signal of the corresponding nth sub-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 and comparing it with H fliter (f r ) and convert the result of the multiplication back to the time domain. Based on this, the echo data of the nth sub-scene area can be obtained, which can be expressed by the following formula 11:

[0106]

[0107] Similarly, by constructing different bandpass filters and processing the aliased signals accordingly, non-aliased signals in all areas can be obtained eventually.

[0108] It is worth noting that the received echo signals are from N sub-scenes, and the azimuth dimension of each sub-scene is roughly equal to the width of a single beam. Therefore, direct imaging of these multiple sub-scene echo signals that are aliased together will cause serious Doppler ambiguity in the imaging, seriously affecting the imaging quality. Therefore, in the implementation method provided by the embodiment of the present invention, the aliased original sub-scene echo signals are subjected to bandpass filtering, which can effectively separate the sub-pulse echo signals from different areas, thereby effectively solving the problem of Doppler ambiguity in imaging. Therefore, the method provided by the embodiment of the present invention can maintain the original pulse signal when there is a repetition frequency PRF, and still be able to effectively image.

[0109] Furthermore, there is a fixed time delay between different sub-pulses. And this time delay is coupled with the sub-scene. This will cause a fixed range deviation in the images of different regions, which will affect the subsequent imaging stitching. For this reason, it is also necessary to perform range registration processing on the separated echoes. Using the stationary phase principle, the echo data is transformed into the range Fourier transform as shown in the following formula 12:

[0110]

[0111] From the above formula 12, it can be determined that f r With Δτ n There is a certain coupling, which leads to envelope errors in echoes in different regions. Therefore, a correction function is constructed in the range-frequency domain, and its expression is formula 13:

[0112]

[0113] Y n (f r ,t k ) and H com (f r ) and converting it into distance time domain, we can get 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, the back-projection (BP) imaging method can achieve target imaging in any configuration. However, its computational complexity is relatively large. The polar coordinate format algorithm (PFA), as a commonly used frequency domain beamforming algorithm, has the advantages of high precision and fast computation. Exemplarily, in the process of processing the echo signal of each sub-scene, the coordinates of the regional center of the sub-scene are used as the coordinate origin to establish the corresponding imaging plane coordinates, and then the polar coordinate format algorithm is used for imaging processing.

[0117] Please refer to Figure 3 and Figure 4 , Figure 3 This is a schematic diagram of the imaging coordinate system established with the center coordinates of the total imaging scene as the coordinate origin. Figure 4 This is a schematic diagram of the imaging coordinate system established with the coordinates of the imaging center of the sub-scene as the origin. Figure 3 As shown, the coordinates of the sub-scene center origin can be expressed as o′ n , which is in x′ n -y′ n The coordinates in the coordinate system are (x n ,0). Figure 4As shown, in the sub-scene imaging coordinate system, the instantaneous position coordinate of the radar is (vt k -x n ,R 0 ), the target position is (x′ np ,y′ np ), where x′ np =x np -x n , y′ np =R Bnp -R 0 . And there is an instantaneous oblique angle θ(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 in FIG, 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 slope distance between the sub-scene center coordinate and the target coordinate can be expressed by the following formula 17:

[0122]

[0123] Further constructing the reference signal using the sub-scene center coordinates can be expressed by the following formula 18:

[0124]

[0125] Where T ref is the pulse width of the reference signal. Then, the reference function is conjugated and multiplied by the sub-scene data (for the sake of convenience, only a single target is considered at this time), and the following formula 19 is obtained:

[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 sub-scene center relative to the positive direction of the x′ axis, and the counterclockwise direction is positive. And

[0130] Exemplarily, the above equation can be converted to the range frequency domain using the stationary phase principle (POSP) and can be expressed as follows:

[0131]

[0132] 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. Both the second and third terms will cause the target's range migration. Since the range frequency satisfies , so the distance frequency domain signal can be expressed by formula 21 as follows:

[0133]

[0134] For this purpose, a residual video phase compensation function can be constructed, and the compensation function can be expressed by the following formula 22:

[0135]

[0136] After compensation, the corresponding distance frequency domain signal is as follows:

[0137]

[0138] Then converted to the distance time domain, it can be expressed by the following formula 24:

[0139]

[0140] make

[0141]

[0142] Due to the limitation of echo pulse width, K Rn The relationship shown in the following formula 25 is satisfied:

[0143]

[0144] Then, the two-dimensional time domain data after RVP compensation can be further expressed by the following formula 26:

[0145]

[0146] in Thus, we can get the expression of echo data in the two-dimensional beam domain. Next, we can calculate the slant range difference ΔR np (t k ) in r np=0, the second-order Taylor expansion is performed, and the following formula 27 is obtained:

[0147]

[0148] Where ΔR np_s ( k ) is the wavefront curvature influence term, which will cause the target to have defocus and position deviation.

[0149] After ignoring the wavefront curvature term, the above slant distance history is substituted into the equation, which can be expressed as follows:

[0150]

[0151] in Then the echo data is processed by two-dimensional Fourier transform to obtain the final imaging result:

[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 by performing the above processing on each sub-scene echo signal respectively, the corresponding sub-scene original image can be obtained.

[0155] Step 106: stitching these original sub-scene images to generate a wide-band focus imaging image in azimuth.

[0156] It can be understood that in the method provided by the embodiment of the present invention, wide beam coverage in azimuth is achieved through multi-frequency sub-pulse diversity processing. A multi-frequency sub-pulse azimuth wide-beam frequency domain imaging framework and related technologies have been established, and Doppler ambiguity-free wide-band imaging can be achieved without changing the system pulse repetition frequency. The specific implementation method provides the relevant processing flow of the frequency domain imaging algorithm under the sub-pulse diversity system. Compared with the time domain imaging algorithm, the frequency domain algorithm has low computational complexity and short processing time under the same imaging scenario. It is more suitable for wide-band imaging processing.

[0157] Please refer to Figure 5 , is a schematic diagram of wide-format imaging geometry provided by an embodiment of the present invention. Figure 5 The direction indicated by V is the direction of radar operation.

[0158] The following will be combined with simulation software to illustrate the simulation results of the method provided by the embodiment of the present invention. Here, three sub-scenarios are used as an example.

[0159] Please refer to Table 1 below for the simulation parameters used when running the simulation software.

[0160]

[0161] Table 1: Simulation parameters

[0162] Please refer to Figure 6 , is a schematic diagram of the layout provided by an embodiment of the present invention. Figure 6 As shown, 9 point targets are arranged in each sub-scene, and points of the same color are point targets of a sub-scene. For example, the blue point is the point target of the first sub-scene, the red point is the point target of the second sub-scene, and the green point is the point target of the third sub-scene.

[0163] Furthermore, through calculation, it can be obtained that the Doppler bandwidth of the corresponding sub-scene after azimuth dechirp is about 528Hz. Therefore, in order to ensure that the Doppler is alias-free, the repetition frequency of the system pulse signal can be set to 550Hz (at this time, the Doppler bandwidth of the entire scene is about 1584Hz). Then, the aliased echo data can be obtained as follows Figure 7A-7C Aliased echo data shown. Fig. 7A It is a schematic diagram of two-dimensional time domain aliasing echo. Figure 7B This is a schematic diagram of azimuth frequency domain aliasing echo. Figure 7C This is a schematic diagram of distance frequency domain aliasing echo data.

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

[0165] Based on the above Figure 7A-7C In the result display shown, if the method provided by the embodiment of the present invention is used to construct a corresponding bandpass filter for the echo data, the data of different sub-scenes can be separated. Figures 8A-9C The data of echo separation of three sub-scenes are shown respectively. Fig. 8A This is the distance frequency domain filtering result of the first sub-scene. Figure 8B This is the distance frequency domain filtering result of the second sub-scene. Figure 8C The distance frequency domain filtering result of the third sub-scenario is shown. Fig. 9A This is the two-dimensional time domain signal result display of the first sub-scenario. Fig. 9B This is the two-dimensional time domain signal result display of the second sub-scenario. Fig. 9C This is the two-dimensional time domain signal result display for the third sub-scenario.

[0166] Then, the corresponding two-dimensional dechirp function is constructed for each sub-scene echo data, and the dechirp processing results of each scene can be obtained, such as Figure 10A-10C As shown, the two-dimensional dechirp processing results of three sub-scenes are displayed. Fig. 10A This is the result of dechirp processing for the first sub-scenario. Fig. 10B This is the result of dechirp processing for the second sub-scenario. Fig. 10C This is the result of dechirp processing for the third sub-scenario.

[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 follows Figure 11-13 shown. Fig.11 is the 3D imaging result of the first sub-scene, Fig.12 is a top view of the frequency domain imaging result of the first sub-scene, Fig.13 This is an enlarged diagram of the three target points on the right side of the frequency domain of the first sub-scene.

[0168] like Fig.11 and Fig.12 As shown, after beam domain interpolation imaging processing is performed, it can be determined that the target point in the sub-scene is well focused. Fig.13 It can be seen that the corresponding target may have certain geometric distortion, which is caused by the wavefront bending. Therefore, before stitching multiple sub-scene original images, it is necessary to perform geometric distortion correction on the sub-scene original images.

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

[0170] Based on the same processing as the first sub-scene, the second and third sub-scenes are processed in the same way to obtain the imaging results of the second and third sub-scenes. The three sub-scene images are stitched together to obtain a wide-width aliasing-free imaging result. Fig.16 As shown, this is the imaging result after image stitching.

[0171] It can be seen from the above results that by using sub-pulse diversity, the frequency domain imaging algorithm of the present invention can achieve large azimuth width beamforming (width expansion of several times the number of sub-pulses) 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 was adjusted so that the radar single beam covered the same imaging width and the traditional phased array beamforming imaging system was adopted. The corresponding pulse repetition frequency remained at 550Hz (the echo azimuth bandwidth was 1584Hz at this time). At this time, the point target position information remained unchanged. Figure 17A-Figure 17C Phased array imaging results are shown.

[0173] like Fig.17A As shown, it is the original echo signal of the traditional phased array. Fig. 17B This is the image after two-dimensional dechirp processing. Fig. 17C This is traditional aliasing imaging.

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

[0175] In another simulation experiment, compared with the sub-pulse grouping system, the method provided by the embodiment of the present invention has a shorter operation time, which is helpful to achieve real-time detection. The following Table 2 is a time comparison table of the time domain imaging algorithm and the imaging algorithm provided by the embodiment of the present invention.

[0176]

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

[0178] An embodiment of the present invention further provides a synthetic aperture radar imaging device, which includes a SAR and an image processor.

[0179] SAR can transmit pulse signals and receive echo signals. The image processor can establish a sub-pulse wide-band spotlight imaging echo model, which is a theoretical model under ideal reception conditions. And according to the sub-pulse wide-band spotlight imaging echo model, the echo signal of each sub-scene is processed to generate an image of this sub-scene, that is, the original sub-scene image. Finally, these original sub-scene images are stitched to generate an azimuth wide-band spotlight imaging image.

[0180] Those skilled in the art will appreciate that the imaging device shown in the embodiment of the present invention does not constitute a limitation on the imaging device, and in actual applications, the SAR imaging device may include more or fewer components.

[0181] It is understandable that the above embodiments may be stored in a computer-readable storage medium in the form of code or computer instructions. When the imaging device is operated, the imaging device can implement the steps in the above method embodiments. For example, the computer-readable storage medium may 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, an optical data storage device, etc.

[0182] In the several embodiments provided by the present invention, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are only schematic. For example, the division of the modules or units is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another device, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or units, which can be electrical, mechanical or other forms.

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

[0184] In addition, each functional unit in each embodiment of the present invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0185] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a readable storage medium. Based on this understanding, the technical solution of the embodiment of the present invention is essentially or the part that contributes to the prior art or all or part of the technical solution can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a device (which can be a single-chip microcomputer, chip, etc.) or a processor (processor) to perform all or part of the steps of the method described in each embodiment of the present invention. The aforementioned storage medium includes: U disk, mobile hard disk, read only memory (ROM), random access memory (RAM), disk or optical disk and other media that can store program code.

[0186] The above contents are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A synthetic aperture radar imaging method, characterized in that: include: Transmitting a pulse signal, wherein the pulse signal includes at least one sub-pulse; wherein the sub-pulse is modulated to a frequency value in a corresponding range-frequency region, so that different sub-pulses point to imaging regions in different azimuths; Establishing a sub-pulse wide-amplitude spotlight imaging echo model, wherein the sub-pulse wide-amplitude spotlight imaging echo model is a theoretical model under an ideal receiving condition; Receive at least one echo signal to obtain a sub-scene aliased original echo; wherein the echo signal is obtained after the at least one sub-pulse signal is reflected, and the sub-scene aliased original echo is formed by aliasing the reflected echo signal of at least one sub-pulse; Generate at least one original sub-scene image according to the sub-scene aliased original echo and the sub-pulse wide-amplitude spotlight imaging echo model; Image stitching is performed on the at least one original sub-scene image to generate an azimuth wide-band focusing image.

2. The method according to claim 1, characterized in that Generating at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide-width spotlight imaging echo model comprises: Performing bandpass filtering on the sub-scene aliased original echo to obtain at least one sub-scene echo; The following processing is performed on each sub-scene echo: In the sub-pulse wide-band spotlight imaging echo model, a corresponding compensation function is constructed according to the sub-scene echo, and de-linear frequency modulation processing is performed to obtain a two-dimensional pre-processed image; Performing beam-domain two-dimensional interpolation processing on the two-dimensional pre-processed image 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.

3. The method according to claim 1 or 2, characterized in that: The method further comprises: Parameters of a desired imaging area are obtained, wherein the parameters include coordinates of at least one sub-imaging center, radar coordinates, center coordinates of the range-frequency area, and slant range history.

4. The method according to claim 3, characterized in that The constructing of a corresponding compensation function according to the sub-scene echo comprises: The sub-scene center coordinates corresponding to the sub-scene echo are determined, and a compensation function of the sub-pulse is constructed according to the sub-scene center coordinates.

5. A synthetic aperture radar imaging device, characterized in that: Includes synthetic aperture radar and image signal processor; The synthetic aperture radar is used to: transmit a pulse signal, wherein the pulse signal includes at least one sub-pulse; wherein the sub-pulse is modulated to a frequency value in a corresponding range frequency region, so that different sub-pulses point to imaging regions in different azimuths; Receive at least one echo signal, and perform range-frequency domain processing on the at least one echo signal to obtain an alias-free echo signal of at least one sub-scene; 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 can obtain an alias-free echo signal of a sub-scene after being processed; The image signal processor is used to: establish a sub-pulse wide-width bunched imaging echo model, which is a theoretical model under ideal receiving conditions; generate at least one original sub-scene image according to the sub-scene aliased original echo and the sub-pulse wide-width bunched imaging echo model; and perform image stitching on at least one of the original sub-scene images to generate an azimuth wide-width bunched image.

6. The device according to claim 5, characterized in that The image signal processor is used to generate at least one original sub-scene image according to the sub-scene aliasing original echo and the sub-pulse wide-band spotlight imaging echo model, The image signal processor is specifically used for: Performing bandpass filtering on the sub-scene aliased original echo to obtain at least one sub-scene echo; The following processing is performed on each sub-scene echo: In the sub-pulse wide-band bunching imaging echo model, a corresponding compensation function is constructed according to the sub-scene echo, and delinear frequency modulation processing is performed to obtain a two-dimensional pre-processed image; the two-dimensional pre-processed image is subjected to beam domain two-dimensional interpolation processing to obtain a sub-scene imaging image in the original coordinate system; and the sub-scene imaging image in the original coordinate system is subjected to geometric correction processing to generate an original sub-scene image.

7. The device according to claim 5 or 6, characterized in that The image signal processor is further used to obtain parameters of a desired imaging area, wherein the parameters include coordinates of at least one sub-imaging center, radar coordinates, center coordinates of the range-frequency area, and slant range history.

8. The device according to claim 7, characterized in that The image signal processor is further used to determine the sub-scene center coordinates corresponding to the sub-scene echo, and construct a compensation function of the sub-pulse according to the sub-scene center coordinates.

9. An imaging device, characterized in that: include: a processor and a memory for storing instructions executable by the processor; The processor is configured to execute the instructions to implement the method according to any one of claims 1 to 4.

Images