Circumferential synthetic aperture radar imaging method

Abstract

The invention provides a circumferential synthetic aperture radar imaging method, which comprises the following steps: acquiring a transmitting signal and an echo signal of radar circular motion, and preprocessing the transmitting signal and the echo signal to obtain a preprocessed echo signal; a compressed sensing method is adopted to convert a signal estimation and recovery problem into a norm minimization problem for signal filtering, and a target echo signal is obtained; dividing a plurality of sub-apertures by adopting a back projection algorithm, carrying out gridding processing on an imaging region, and calculating echo time delays of points in a grid and each position of the radar; and performing signal compensation on a target echo signal according to the echo time delay, performing azimuth focusing through coherent superposition processing to generate a plurality of corresponding sub-images, and performing step-by-step non-coherent fusion on the plurality of sub-images to obtain a circumferential SAR image. According to the invention, a small amount of effective data can be fully utilized for high-quality imaging, and high-resolution image quality is maintained while the imaging speed and the processing efficiency are improved.

Description

Technical Field

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

[0002] At present, the application of millimeter-wave radar technology in urban planning and traffic management has shown great potential. Millimeter-wave radar can transmit and receive millimeter-wave signals and determine the speed of the vehicle by measuring the Doppler shift of the signal. At the same time, by analyzing the intensity and time delay of the reflected signal, the radar can determine the location and number of vehicles, thereby evaluating the vehicle density. This method requires a large number of antennas for imaging, a large synthetic aperture, a long scanning time, a long imaging time, and its low carrier frequency and bandwidth result in insufficient imaging effect. In addition, the large amount of monitoring data generated by millimeter-wave radar needs to be processed and analyzed in real time, which puts higher requirements on data processing technology and algorithms. In order to extract useful information from massive data, efficient data processing capabilities and advanced algorithm support are required. If these problems are not solved, they may limit the widespread application and effect of millimeter-wave radar technology in urban traffic management.

[0003] Through the above analysis, the problems and defects of the existing technology are as follows: the existing near-field millimeter wave imaging system must increase the number of antennas in order to ensure high-resolution imaging, which not only increases the cost of the system but also brings a lot of additional calculations to the imaging. At the same time, due to the increase in the number of antennas, the synthetic aperture will also take a lot of time, making the system lose real-time performance.

[0004] Therefore, there is an urgent need for a circular synthetic aperture radar imaging method that can perform high-quality imaging based on a small amount of valid data, improve imaging speed and processing efficiency while maintaining high-resolution image quality. Summary of the invention

[0005] Based on this, it is necessary to provide a circular synthetic aperture radar imaging method to address the above technical problems.

[0006] A circular synthetic aperture radar imaging method comprises the following steps: acquiring a transmitting signal and an echo signal of a circular motion of a radar, and preprocessing the transmitting signal and the echo signal to obtain a preprocessed echo signal; converting a signal estimation and recovery problem into a norm minimization problem by using a compressed sensing method, performing signal filtering, and obtaining a target echo signal; dividing a plurality of sub-apertures by using a back-projection algorithm, performing grid processing on an imaging area, and calculating the echo time delay between a point in the grid and each radar position; performing signal compensation on the target echo signal according to the echo time delay, performing azimuth focusing by coherent superposition processing, generating a plurality of corresponding sub-images, and obtaining a circular SAR image by stepwise incoherent fusion of the plurality of sub-images.

[0007] In one embodiment, the step of acquiring a transmit signal and an echo signal of a circular motion of a radar, and preprocessing the transmit signal and the echo signal to obtain a preprocessed echo signal comprises: a millimeter wave radar sends a transmit signal along a circular path, and receives a corresponding echo signal; and the transmit signal and the echo signal are sequentially passed through a mixer and a low-pass filter to obtain the following signal:

[0008] s IF (t) = σexp(j2πτ(αt+f c ));

[0009] In the formula, σ represents the amplitude of the echo signal, τ represents the time delay of the signal for one round trip, α represents the modulation frequency of the frequency modulated continuous wave, and f c Indicates the carrier frequency of the frequency modulated continuous wave; at sampling time N s The sampling frequency F s Sampling is performed to obtain the echo signal, which is:

[0010]

[0011] Where n is the number of pulses, j is the imaginary unit, R is the distance from the target to the antenna, λ is the signal wavelength, and F s Indicates the signal sampling frequency, f IF represents the carrier frequency of the intermediate frequency signal; performing discrete Fourier transform on the echo signal, and expressing the echo signal as:

[0012]

[0013] Where k is the frequency number after period extension in the frequency domain. Where ω is the angular velocity, Distance resolution Where B represents the frequency modulation bandwidth and c is the speed of light. The radar transmission mode is converted to two-transmit and four-receive. According to the echo signal after discrete Fourier transform, the echo signal after different antenna preprocessing is obtained as follows:

[0014]

[0015] Where θ represents the horizontal angle between the target and the radar board plane, l represents the channel number of the antenna, and d represents the distance from the radar to the target.

[0016] In one embodiment, the method of converting the signal estimation and recovery method into a norm minimization problem by using a compressed sensing method, performing signal filtering, and obtaining a target echo signal includes: assuming that the radar system is within the observation interval [r 0 ,r 1], the resolution in the range direction is Δr, then the target scattering center in the range unit is represented by a one-dimensional vector α, which is:

[0017] α T =[α 1 α 2 …α 1 …α L-1 α L ] 1×L ;

[0018] where α l =σ l exp[-j4πf c r l (τ) / c],σ l For the l Backscatter coefficient of the scattering center within the distance unit, r l (τ) = r 0 +lΔr,l∈[1:L],L=1+(r 1 -r 0 ) / Δr; the echo signal s 0 (t) Delay, construct the sparse basis, which is:

[0019]

[0020] The echo signal is represented based on the sparse basis as follows:

[0021] x(t)=Ψα+n x (t);

[0022] Where n x (t) is the noise; using the compressed sensing method, the problem of estimating the target scattering center α based on the observed value of the echo signal x(t) is converted into a norm minimization problem, which is:

[0023]

[0024] Wherein, Φ is the observation matrix, and ε is the noise coefficient; the preprocessed echo signal is filtered through the norm minimization problem to obtain the target echo signal.

[0025] In one embodiment, the method of dividing the imaging area into multiple sub-apertures by using a back-projection algorithm, performing grid processing on the imaging area, and calculating the echo delay between the points in the grid and each radar position includes: dividing the imaging area into multiple sub-apertures by using a back-projection algorithm, and performing grid processing on the imaging area according to the multiple sub-apertures, selecting a point P in the grid, and calculating the echo delay between the point P and n positions, wherein the formula is:

[0026]

[0027] In the formula, r pn Represents the distance between point P and position n.

[0028] In one embodiment, the target echo signal is compensated according to the echo time delay, azimuth focusing is performed through coherent superposition processing to generate corresponding multiple sub-images, and a circular SAR image is obtained by step-by-step incoherent fusion of multiple sub-images, including: signal compensation for the target echo signal according to the echo time delay, coherent accumulation of all compensated echo signals in the time domain, and obtaining a sub-image corresponding to the sub-aperture, the formula is:

[0029]

[0030] In the formula, x p represents the sub-image corresponding to the sub-aperture P, and s(·) represents the compensated echo signal; a plurality of sub-images are generated according to the compensated echo signal; based on the plurality of sub-images, the BP algorithm is used for focusing imaging to obtain the two-dimensional imaging results of each range gate at different angles; the two-dimensional imaging results in all range gates are coherently superimposed to obtain a circular SAR image.

[0031] In one of the embodiments, the signal compensation of the target echo signal according to the echo delay includes: performing delay compensation on the target echo signal of each channel according to the echo delay, so that the echo signal is consistent with the position of the frequency modulated continuous wave transmitted when the slide rail moves; at the slow time t, the actual phase of the echo signal of the test area received by the receiving antenna n from the transmitting antenna is:

[0032]

[0033] In the formula, R n (t) is the distance from the receiving antenna n to the target, and R(t) is the distance from the transmitting antenna to the target. The antenna transmitting and receiving positions are virtually equivalent to a virtual phase center point m, and the virtual phase of the virtual phase center point is:

[0034]

[0035] In the formula, R m (t) is the distance from the virtual equivalent point m to the target; a phase deviation is calculated based on the virtual phase and the actual phase, and phase compensation is performed on the echo signal after delay compensation based on the phase deviation to obtain a compensated echo signal.

[0036] In one of the embodiments, the signal compensation of the target echo signal according to the echo delay also includes: performing delay compensation on the target echo signal of each channel according to the echo delay, so that the echo signal is consistent with the position of transmitting the frequency modulated continuous wave when the slide rail moves; according to the BP algorithm, taking the distance from the center of the rotation circle to the center of the area to be measured as the standard phase, performing phase compensation according to the difference between the point and the center distance, and obtaining the compensated echo signal, the BP algorithm formula is:

[0037]

[0038] In the formula, σ(x 0 ,y 0 ,z 0 ) represents the imaging result, (x 0 ,y 0 ,z 0 ) is a reference point in the imaging area, and s(x,y,z) represents the signal intensity at the spatial position (x,y,z).

[0039] In one embodiment, the step of obtaining a circular SAR image by incoherently fusing multiple sub-images comprises: i ,y i ) is Taylor expanded, and the corresponding spectral compression function is constructed for the quadratic term in the expansion term. The formula is:

[0040]

[0041] In the formula, the distance wave number K is split into K c +K r , Using spectrum compression function F c1 and F c2 The azimuth spectrum width after spectrum compression is:

[0042]

[0043] After compensation by the spectrum compression function, multiple sub-images are fused with the image in the polar coordinate system to obtain a circular SAR image.

[0044] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: by acquiring the transmitting signal and the echo signal of the circular motion of the radar, and preprocessing the transmitting signal and the echo signal to obtain the preprocessed echo signal, the compressed sensing method is used to convert the signal estimation and recovery problem into a norm minimization problem, and the signal is filtered to obtain the target echo signal; a back projection algorithm is used to divide a plurality of sub-apertures, the imaging area is gridded, and the echo delay between the points in the grid and each radar position is calculated; the target echo signal is compensated according to the echo delay, azimuth focusing is performed through coherent superposition processing, and a corresponding plurality of sub-images are generated, and a circular SAR image is obtained by step-by-step incoherent fusion of the plurality of sub-images, and the compressed sensing and back projection algorithms are combined to make full use of a small amount of effective data for high-quality imaging, improve the imaging speed and processing efficiency while maintaining the high-resolution image quality, and the high-resolution imaging can assist urban planning, monitor traffic flow, and facilitate planning and management. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 A schematic diagram of a flow chart of a circular synthetic aperture radar imaging method in an embodiment;

[0046] Figure 2 A schematic diagram of a circular synthetic aperture radar imaging method in one embodiment;

[0047] Figure 3 A schematic diagram of radar installation in a usage scenario in an embodiment;

[0048] Figure 4 is a schematic diagram of imaging of an experimental point in an embodiment;

[0049] Figure 5 It is an imaging diagram of a real object in an embodiment. DETAILED DESCRIPTION

[0050] Before describing the specific embodiments of the present invention, the overall concept of the present invention is described as follows:

[0051] The present invention is mainly developed based on the radar imaging process. Currently, near-field imaging of radar imaging requires a large number of antenna arrays, which is costly and takes a long time to synthesize the aperture.

[0052] Therefore, the present invention proposes a circular synthetic aperture radar imaging method, which obtains the transmitting signal and the echo signal of the circular motion of the radar, and pre-processes the transmitting signal and the echo signal to obtain the pre-processed echo signal, and uses the compressed sensing method to convert the signal estimation and recovery problem into a norm minimization problem to perform signal filtering to obtain the target echo signal; uses the back projection algorithm to divide multiple sub-apertures, grids the imaging area, and calculates the echo delay between the points in the grid and each radar position; performs signal compensation on the target echo signal according to the echo delay, performs azimuth focusing through coherent superposition processing, generates corresponding multiple sub-images, and obtains a circular SAR image through step-by-step incoherent fusion of multiple sub-images. Combining the compressed sensing and back projection algorithms, it is possible to fully utilize a small amount of effective data for high-quality imaging, improve the imaging speed and processing efficiency while maintaining the high-resolution image quality. The high-resolution imaging can assist urban planning, monitor traffic flow, and facilitate planning and management.

[0053] After introducing the overall concept of the present invention, in order to make the purpose, technical solution and advantages of the present invention more clear, the present invention is further described in detail by specific implementation methods in combination with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0054] In one embodiment, before radar imaging, various parameters of the acquisition system are initialized, including radar transmit pulse interval, transmit pulse starting frequency, motor rotation angular velocity, etc.; the radar adopts a two-transmit and four-receive transmission and reception method to store the data of each receiving array element, and performs circular motion driven by a precision motor to obtain 360° all-round information. The system receives all angle information according to the angle and stores the echo signal. After pre-processing, it is a three-dimensional discrete data set of the number of receiving array elements × the number of angle sampling points × the number of distance sampling points, which is convenient for subsequent processing.

[0055] In one embodiment, Figure 1 and Figure 2 As shown, a circular synthetic aperture radar imaging method is provided, comprising the following steps:

[0056] Step S110, acquiring the transmitting signal and the echo signal of the circular motion of the radar, and preprocessing the transmitting signal and the echo signal to obtain the preprocessed echo signal.

[0057] Specifically, the millimeter wave radar can use the IWR1642 product of Texas Instruments TI, which is placed facing the target to be measured. The radar layout method is as follows: Figure 3As shown, the radar moves along a circular path, the radar sends out a transmission signal and receives a corresponding echo signal, and the transmission signal and the echo signal are preprocessed. The preprocessing operation may include determining parameters such as radar coordinates and target area center coordinates, or calculating the distance and incident angle from the radar to each point in the target area, thereby obtaining the echo signals of different antennas after preprocessing.

[0058] Among them, step S110 includes: the millimeter wave radar moves along a circular path to send out a transmission signal and receive a corresponding echo signal; the transmission signal and the echo signal pass through a mixer and a low-pass filter in sequence to obtain a signal as follows:

[0059] s IF (t) = σexp(j2πτ(αt+f c ));

[0060] In the formula, σ represents the amplitude of the echo signal, τ represents the time delay of the signal for one round trip, α represents the modulation frequency of the frequency modulated continuous wave signal, and f c Indicates the carrier frequency of the frequency modulated continuous wave; at sampling time N s The sampling frequency f s Sampling is performed to obtain the echo signal, which is:

[0061]

[0062] Where n is the number of pulses, j is the imaginary unit, R is the distance from the target to the antenna, λ is the signal wavelength, and F s It means that f IF represents the carrier frequency of the intermediate frequency signal; the echo signal is subjected to discrete Fourier transform and is represented as:

[0063]

[0064] Where k is the frequency number after period extension in the frequency domain. Where ω is the angular velocity, Distance resolution Where B represents the frequency modulation bandwidth and c is the speed of light;

[0065] The radar transmission mode is converted to two-transmit and four-receive. According to the echo signal after discrete Fourier transform, the target echo signal after different antenna preprocessing is obtained as follows:

[0066]

[0067] Where θ represents the horizontal angle between the target and the radar board plane, l represents the channel number of the antenna, and d represents the distance from the radar to the target.

[0068] Specifically, after the radar transmits a signal and receives the corresponding echo signal, the transmit signal and the echo signal are preprocessed, including passing the transmit signal and the echo signal through a mixer and a low-pass filter in turn. Each antenna signal in each frame of data ultimately retains only one intermediate frequency signal. The intermediate frequency signal maintains the signal quality while limiting the frequency conversion amount of the electronic device. During the receiving process, the radio frequency signal is first converted into an intermediate frequency signal and finally converted into a baseband signal, thereby reducing the complexity of directly processing high-frequency signals and being able to improve signal quality through filtering and other technologies.

[0069] The sampling time and sampling frequency are set to obtain the corresponding echo signal, and the sampled echo signal is discrete Fourier transformed to obtain the transformed echo signal; the previous transmission mode of the radar can be three transmit and four receive or four transmit and 12 receive, etc., and the radar transmission mode is converted to two transmit and four receive. Under this configuration, the two transmitting antennas transmit signals alternately, and the four receiving antennas sample the transmitted signals at the same time, and eight echo signals are generated in each sampling period; the echo signals after preprocessing of different antennas are represented according to the transformed echo signals, so as to facilitate the subsequent processing of the echo signals.

[0070] Step S120: Using a compressed sensing method, the signal estimation and recovery problem is converted into a norm minimization problem, and signal filtering is performed to obtain a target echo signal.

[0071] Specifically, the compressed sensing method is used to replace the electrostatic SAR matched filter imaging technology, and the pre-processed target echo signal estimation and recovery problem is converted into a norm minimization problem. The pre-processed echo signal is filtered through the norm minimization problem to obtain the target echo signal. The measurement value is obtained through the analog information conversion measurement framework, and the coefficient is reconstructed using the sparse Bayesian algorithm to achieve distance compression based on compressed sensing. After the compressed sensing matched filtering, the phase information of the radar signal propagation and the backscattering coefficient of the target are reconstructed. After the echo signal is range compressed, the reconstructed phase information is used to use the circular sub-aperture synthetic aperture radar imaging algorithm in the azimuth direction to process the range compressed data, so that higher-precision two-dimensional radar imaging can be achieved.

[0072] Wherein, step S120 includes: assuming that the radar system is in the observation interval [r 0 ,r 1 ], the resolution in the range direction is Δr, then the target scattering center in the range unit is represented by a one-dimensional vector α, which is:

[0073] α T =[α 1 α 2 …α l …α L-1 α L ] 1×L ;

[0074] where α l =σ l exp[-j4πf c r l (τ) / c],σ l For the l Backscatter coefficient of the scattering center within the distance unit, r l (τ) = r 0 +lΔr,l∈[1:L],L=1+(r 1 -r 0 ) / Δr;

[0075] The echo signal s 0 (t) Delay, construct the sparse basis, which is:

[0076]

[0077] The echo signal is represented based on a sparse basis:

[0078] x(t)=Ψα+n x (t);

[0079] Where n x (t) is noise; using the compressed sensing method, the problem of estimating the target scattering center α based on the observed value of the echo signal x(t) is expressed as:

[0080]

[0081] In the formula, Φ is the observation matrix and ε is the noise coefficient. The preprocessed echo signal is filtered through the norm minimization problem to obtain the target echo signal.

[0082] Specifically, the radar signal is combined into a matrix according to the distance dimension and the angle dimension. Assuming that the radar system is [r 0 ,r 1 ], the resolution in the range direction is Δr, then the target scattering center in the range unit can be represented by a one-dimensional vector α. When there is no target in a certain range unit k, α k = 0. Since the target only occupies a small part of the observation area in SAR imaging, the number of non-zero elements K in α is much smaller than the number of range units L.

[0083] Radar signal processing based on compressed sensing converts the signal estimation and recovery problem into a norm minimization problem. An analog information conversion measurement frame is used to reconstruct the signal through a sparse Bayesian algorithm to achieve distance compression, thereby performing signal filtering. This can remove a large amount of noise signals and obtain a more accurate target echo signal.

[0084] Step S130, using a back-projection algorithm to divide the imaging area into multiple sub-apertures, performing grid processing on the imaging area, and calculating the echo delay between the points in the grid and each radar position.

[0085] Specifically, the synthetic aperture is divided into multiple sub-apertures using a back-projection algorithm, and a polar coordinate system is established for each sub-aperture with the vector pointing to the center of the scene as the polar axis. In each polar coordinate system, polar coordinate grid division is performed with the center of the scene as the center. The pulse pressure signal of the sampling point of the nth array element in each sub-aperture in the mth direction is back-projected to the polar coordinate grid corresponding to the sub-aperture to obtain a sub-aperture image. In order to accurately calculate the echo delay between each grid point and different positions of the radar, the imaging area is carefully gridded. The calculation of the echo delay directly affects the accuracy and resolution of the imaging.

[0086] When performing sub-aperture division, ideal sub-aperture division can be performed based on the target structural characteristics and the changes in scattering response with azimuth, to prevent spectrum blur, reduce the sampling rate, and improve computational efficiency. The full aperture can be divided into multiple sub-apertures, and each sub-aperture can generate a corresponding low-resolution sub-image.

[0087] The imaging area is divided into a grid according to multiple sub-apertures, and a reference point is set in the middle of the synthetic aperture. The residual values ​​of the distances from the remaining radar positions to the imaging grid and the reference point are traversed and calculated. After the distance differences are converted into phases, the electromagnetic waves emitted by radars at other positions are phase compensated in the time domain to ensure that the system maintains stable gain and phase characteristics within a specific frequency range.

[0088] Wherein, step S130 includes: dividing the aperture into a plurality of sub-apertures, and performing grid processing on the imaging area according to the plurality of sub-apertures, selecting a point P in the grid, and calculating the echo delay between the point P and n positions, the formula is:

[0089]

[0090] In the formula, r pn Represents the distance between point P and position n.

[0091] Specifically, the synthetic aperture is divided into multiple sub-apertures, and the imaging area is gridded according to the multiple sub-apertures. Then, a point is selected according to the gridded grid, and the echo delay between it and multiple positions is calculated. This allows subsequent phase correction to be performed based on the echo delay, ensuring that echo signals from different perspectives can be accurately aligned, so that the signals of the same target point are stacked together in the final imaging to form a target.

[0092] Step S140, performing signal compensation on the target echo signal according to the echo delay, performing azimuth focusing through coherent superposition processing, generating corresponding multiple sub-images, and obtaining a circular SAR image by step-by-step incoherent fusion of the multiple sub-images.

[0093] Specifically, the time and space signal compensation of the target echo signal is performed through the calculated echo delay, and the azimuth focusing is performed through the coherent superposition processing. The focusing method is the back projection algorithm to generate multiple sub-images corresponding to the sub-aperture. The low-resolution sub-images of the sub-aperture are coherently fused step by step to obtain the circular SAR image. The imaging results of point targets and physical targets are shown as follows: Figure 4 and Figure 5 As shown, in the process of improving the resolution, the amount of calculation is less than that of the image processing unit and high-resolution imaging is performed directly, thereby effectively maintaining the target detail features and ensuring the final imaging quality.

[0094] Wherein, step S140 includes: performing signal compensation on the target echo signal according to the echo delay, coherently accumulating all compensated echo signals in the time domain, and obtaining a sub-image corresponding to the sub-aperture, and the formula is:

[0095]

[0096] In the formula, x p represents the sub-image corresponding to the sub-aperture P, and s(·) represents the compensated echo signal; multiple sub-images are generated according to the compensated echo signal; based on the multiple sub-images, the BP algorithm is used to focus the imaging to obtain the two-dimensional imaging results of each range gate at different angles; the two-dimensional imaging results in all range gates are coherently superimposed to obtain the circular SAR image.

[0097] Specifically, according to the obtained echo time delay, the target echo signal is compensated in time and space to obtain the compensated echo signal, and coherent accumulation is performed in the time domain to obtain sub-images corresponding to multiple sub-apertures. The BP (Back Projection) algorithm is used to focus and image the multiple sub-images to obtain two-dimensional imaging results of each range gate at different angles. All the two-dimensional imaging results are coherently added to obtain a circular SAR image. The imaging algorithm has good robustness, and the obtained circular SAR image has high resolution, which is suitable for imaging of various trajectories.

[0098] The specific steps of compensating the target echo signal according to the echo delay are as follows: experimentally compensate the target echo signal of each channel according to the echo delay so that the echo signal is consistent with the position of the transmitted frequency modulated continuous wave when the slide rail moves; at the slow time t, the actual phase of the echo signal of the test area received by the receiving antenna n from the transmitting antenna is:

[0099]

[0100] In the formula, R n (t) is the distance from the receiving antenna n to the target, and R(t) is the distance from the transmitting antenna to the target. The antenna transmitting and receiving positions are virtually equivalent to a point m, and the virtual phase of the virtual phase center point is:

[0101]

[0102] In the formula, R m (t) is the distance from the virtual equivalent point m to the target; the phase deviation is calculated according to the virtual phase and the actual phase, and the phase compensation is performed on the echo signal after delay compensation according to the phase deviation to obtain the compensated echo signal.

[0103] In one embodiment, the signal compensation method further includes: performing delay compensation on the target echo signal of each channel according to the echo delay, so that the echo signal is consistent with the position of transmitting the frequency modulated continuous wave when the slide rail moves; according to the BP algorithm, taking the distance from the center of the rotating circle to the center of the area to be measured as the standard phase, performing phase compensation according to the difference between the point and the center distance, and obtaining the compensated echo signal, the BP algorithm formula is:

[0104]

[0105] In the formula, σ(x 0 ,y 0 ,z 0 ) represents the imaging result, (x 0 ,y 0 ,z 0 ) is a reference point in the imaging area, and s(x,y,z) represents the signal intensity at the spatial position (x,y,z).

[0106] Specifically, when transmitting a signal, there will be a time difference in the signal due to the difference in distance and time, so it is necessary to perform delay compensation on the received echo signal, so as to correct the arrival time of each signal, so that the echo signals received by radars at different locations arrive at the same time. When performing delay compensation, the timestamp of the transmitted signal is recorded, and the target echo signal is time-corrected according to the timestamp. The geometric delay is calculated according to the different distances, and the echo signal is further delayed compensated to ensure that the signals with different transmission times can be correctly aligned.

[0107] After adjusting the arrival time of the echo signal through delay compensation, the macroscopic difference of the phase can also be corrected, but the microscopic phase error still exists, such as the phase offset caused by the frequency bandwidth, the change of the transmission medium or the non-ideal propagation path. Therefore, it is also necessary to perform phase compensation on the echo signal after delay compensation. When performing phase compensation, the equivalent phase center principle can be used to perform phase compensation on the echo signal, or the BP algorithm can be used to obtain the standard phase for phase compensation to obtain the compensated echo signal, so as to facilitate high-quality imaging based on the compensated echo signal.

[0108] Wherein, step S140 includes: by i ,y i ) is Taylor expanded, and the corresponding spectral compression function is constructed for the quadratic term in the expansion term. The formula is:

[0109]

[0110] In the formula, the distance wave number K is split into K c +K r , Using spectrum compression function F c1 and F c2 The azimuth spectrum width after spectrum compression is:

[0111]

[0112] After compensation through the spectrum compression function, multiple sub-images are fused similarly with the image in the polar coordinate system to obtain a circular SAR image.

[0113] Specifically, since the quadratic term in the Taylor expansion term is the cause of spectrum broadening, a corresponding spectrum compression function is constructed to compress the spectrum of the echo signal, change the frequency domain characteristics of the signal, achieve the focusing effect of the signal, and improve the resolution and target detection capability of the radar system by compressing the received signal; after compensation through the spectrum compression function, multiple sub-images are fused with the image in the polar coordinate system to obtain a circular SAR image. Since the bandwidth is proportional to the aperture length after imaging in the rectangular coordinate system, by dividing the sub-aperture with a shorter length, the sub-aperture sampling rate can be greatly reduced while ensuring that the spectrum is unambiguous.

[0114] After compression using the constructed spectral compression function, a new azimuth spectrum width and the corresponding sub-image are obtained. The sub-image is fused with the image in the polar coordinate system. Since the bandwidth is proportional to the aperture length after imaging in the rectangular coordinate system, the spectrum can be ensured to be unambiguous while the sub-aperture sampling frequency is greatly reduced by dividing the image into sub-apertures with shorter lengths.

[0115] In the circular sub-aperture synthetic aperture radar imaging process, as the radar moves in a circle, the target structure characteristics and scattering response change with the azimuth. By reasonably dividing the sub-apertures and constructing the spectrum compression function through Taylor expansion slant range, the appropriate sub-aperture length is selected to prevent spectrum blur, reduce the sampling rate and improve the computational efficiency. Each sub-aperture generates a low-resolution sub-image, and then the resolution is gradually improved through the incoherent fusion of the sub-aperture images, and finally a full-aperture image is formed to complete the entire imaging process.

[0116] In this embodiment, the transmit signal and the echo signal of the circular motion of the radar are acquired, and the transmit signal and the echo signal are preprocessed to obtain the preprocessed echo signal, and the compressed sensing method is used to convert the signal estimation and recovery problem into a norm minimization problem for signal filtering to obtain the target echo signal; the back projection algorithm is used to divide the multiple sub-apertures, the imaging area is gridded, and the echo delay between the points in the grid and each radar position is calculated; the target echo signal is compensated according to the echo delay, azimuth focusing is performed through coherent superposition processing, and corresponding multiple sub-images are generated, and the multiple sub-images are fused step by step incoherently to obtain a circular SAR image. Combining the compressed sensing and back projection algorithms, a small amount of effective data can be fully utilized for high-quality imaging, and the imaging speed and processing efficiency can be improved while maintaining the high-resolution image quality. The high-resolution imaging can assist urban planning, monitor traffic flow, and facilitate planning and management.

[0117] In one embodiment, the above method can achieve all-weather monitoring without being affected by adverse weather conditions. Whether it is heavy rain, heavy fog, strong wind or night environment, the millimeter wave radar can work stably to ensure the continuity and accuracy of the monitoring data. This is especially important for vehicle volume detection and application scenarios that require long-term and continuous monitoring of urban planning. In addition, the above method adopts a non-contact monitoring method, which avoids the physical impact and interference of traditional contact sensors on the monitored object. It is particularly suitable for areas where direct contact is not suitable, such as vehicle flow detection, which improves the safety and applicability of detection.

[0118] Obviously, those skilled in the art should understand that the modules or steps of the present invention described above can be implemented by a general-purpose computing device, they can be concentrated on a single computing device, or distributed on a network composed of multiple computing devices, and optionally, they can be implemented by a program code executable by a computing device, so that they can be stored in a computer storage medium (ROM / RAM, magnetic disk, optical disk) and executed by the computing device, and in some cases, the steps shown or described can be executed in a different order than that here, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. Therefore, the present invention is not limited to any specific combination of hardware and software.

[0119] The above contents are further detailed descriptions of the present invention in combination with specific implementation methods, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as falling within the scope of protection of the present invention.

Claims

1. A circular synthetic aperture radar imaging method, characterized in that: The following steps are involved: Acquire a transmission signal and an echo signal of the circular motion of the radar, and preprocess the transmission signal and the echo signal to obtain a preprocessed echo signal; The compressed sensing method is used to convert the echo signal estimation and recovery problem into a norm minimization problem, and the signal is filtered to obtain the target echo signal. The back-projection algorithm is used to divide multiple sub-apertures, the imaging area is gridded, and the echo delay between the points in the grid and each radar position is calculated; Signal compensation is performed on the target echo signal according to the echo time delay, azimuth focusing is performed through coherent superposition processing to generate corresponding multiple sub-images, and a circular SAR image is obtained by step-by-step incoherent fusion of the multiple sub-images.

2. The circular synthetic aperture radar imaging method according to claim 1, characterized in that: The acquiring of the transmitting signal and the echo signal of the circular motion of the radar, and preprocessing the transmitting signal and the echo signal to obtain the preprocessed echo signal, includes: The millimeter wave radar moves along a circular path to send out a transmission signal and receive a corresponding echo signal; The transmission signal and the echo signal are passed through the mixer and the low-pass filter in turn to obtain the signal: S IF (t)=σexp(j2πτ(αt+f c )); In the formula, σ represents the amplitude of the echo signal, τ represents the time delay of the signal for one round trip, α represents the modulation frequency of the frequency modulated continuous wave, and f c Indicates the carrier frequency of the frequency modulated continuous wave; At sampling time N s The sampling frequency F s Sampling is performed to obtain the echo signal, which is: Where n is the number of pulses, j is the imaginary unit, R is the distance from the target to the antenna, λ is the signal wavelength, and F s Indicates the signal sampling frequency, f IF Indicates the carrier frequency of the intermediate frequency signal; The echo signal is subjected to discrete Fourier transform and expressed as: Where k is the frequency number after period extension in the frequency domain. Where ω is the angular velocity, Distance resolution Where B represents the frequency modulation bandwidth and c is the speed of light; The radar transmission mode is converted to two-transmit and four-receive. According to the echo signal after discrete Fourier transform, the echo signal after different antenna preprocessing is obtained: Where θ represents the horizontal angle between the target and the radar board plane, l represents the channel number of the antenna, and d represents the distance from the radar to the target.

3. The circular synthetic aperture radar imaging method according to claim 2, characterized in that: The method of converting the signal estimation and recovery method into a norm minimization problem by using a compressed sensing method, performing signal filtering, and obtaining a target echo signal includes: Assuming that the range resolution of the radar system in the observation interval [r0, r1] is Δr, the target scattering center in the range unit is represented by a one-dimensional vector α, which is: α T ＝[α1 α2 … α l … α L-1 α L ] 1×L ； where α l =σ l exp[-j4πf c r l (τ) / c],σ l For the l Backscatter coefficient of the scattering center within the distance unit, r l (r)=r0+lΔr, l∈[1:L], L=1+(r1-r0) / Δr; Delay the echo signal s0(t) and construct a sparse basis, which is: The echo signal is represented based on the sparse basis as follows: x(t)=Ψα+n x (t); Where n x (t) is noise; Using the compressed sensing method, the problem of estimating the target scattering center α based on the observed value of the echo signal x(t) is converted into a norm minimization problem, which is: Where Φ is the measurement matrix and ε is the noise coefficient; The pre-processed echo signal is filtered through the norm minimization problem to obtain a target echo signal.

4. The circular synthetic aperture radar imaging method according to claim 3, characterized in that: The method of using a back-projection algorithm to divide a plurality of sub-apertures, performing grid processing on the imaging area, and calculating the echo delay between points in the grid and each radar position includes: The back projection algorithm is used to divide multiple sub-apertures, and the imaging area is gridded according to the multiple sub-apertures. A point P in the grid is selected to calculate the echo delay between point P and n positions. The formula is: In the formula, r pn Represents the distance between point P and position n.

5. The circular synthetic aperture radar imaging method according to claim 4, characterized in that: The target echo signal is compensated according to the echo time delay, azimuth focusing is performed through coherent superposition processing to generate a corresponding plurality of sub-images, and a circular SAR image is obtained by stepwise incoherent fusion of the plurality of sub-images, including: The target echo signal is compensated according to the echo delay, and all compensated echo signals are coherently accumulated in the time domain to obtain a sub-image corresponding to the sub-aperture. The formula is: In the formula, x p represents the sub-image corresponding to the sub-aperture P, and s(·) represents the compensated echo signal; generating a plurality of sub-images according to the compensated echo signal; According to the multiple sub-images, the BP algorithm is used to focus the imaging to obtain the two-dimensional imaging results of each range gate at different angles; The two-dimensional imaging results within all range gates are coherently superimposed to obtain a circular SAR image.

6. The circular synthetic aperture radar imaging method according to claim 5, characterized in that: The performing signal compensation on the target echo signal according to the echo delay includes: Performing time delay compensation on the target echo signal of each channel according to the echo time delay, so that the target echo signal is consistent with the position of transmitting the frequency modulated continuous wave when the slide rail moves; At the slow time t, the actual phase of the echo signal of the area to be measured received by the receiving antenna n from the transmitting antenna is: In the formula, R n (t) is the distance from the receiving antenna n to the target, and R(t) is the distance from the transmitting antenna to the target; The antenna transmitting and receiving positions are virtually equivalent to a virtual phase center point m, and the virtual phase of the virtual phase center point is: In the formula, R m (t) is the distance from the virtual equivalent point m to the target; A phase deviation is calculated according to the virtual phase and the actual phase, and phase compensation is performed on the echo signal after delay compensation according to the phase deviation to obtain a compensated echo signal.

7. The circular synthetic aperture radar imaging method according to claim 5, characterized in that: The signal compensation of the target echo signal according to the echo delay further includes: Performing time delay compensation on the target echo signal of each channel according to the echo time delay, so that the target echo signal is consistent with the position of transmitting the frequency modulated continuous wave when the slide rail moves; According to the BP algorithm, the distance from the center of the rotating circle to the center of the area to be measured is taken as the standard phase, and phase compensation is performed according to the difference in the distance from the point to the center to obtain the compensated echo signal. The BP algorithm formula is: Where σ(x0,y0,z0) represents the imaging result, (x0,y0,z0) is a reference point in the imaging area, and s(x,y,z) represents the signal intensity at the spatial position (x,y,z).

8. The circular synthetic aperture radar imaging method according to claim 5, characterized in that: The step of obtaining a circular SAR image by incoherently fusing multiple sub-images step by step includes: By i ,y i ) is Taylor expanded, and the corresponding spectral compression function is constructed for the quadratic term in the expansion term. The formula is: In the formula, the distance wave number K is split into K c +K r , Using spectrum compression function F c1 and F c2 The azimuth spectrum width after spectrum compression is: After compensation by the spectrum compression function, multiple sub-images are fused with the image in the polar coordinate system to obtain a circular SAR image.

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