Fast decomposition projection imaging method for multi-base synthetic aperture radar

Abstract

The present invention discloses a method for rapid decomposition and projection imaging of a multi-base synthetic aperture radar. First, the echo received by each receiver is demodulated to the baseband; then each transceiver pair in the multi-base SAR is equivalent to a single-base SAR, and the echo of the overlapping part of the aperture is weighted and balanced; then the echo range is pulse compressed; then rapid decomposition and back-projection imaging are performed aperture by aperture; then the coarse resolution images of different apertures are layered and fused until they are fused into a complete aperture; finally, the slant range plane imaging result is projected onto the ground to obtain the imaging result. The method of the present invention is suitable for multi-base SAR imaging, overcomes the limitation of the frequency domain imaging algorithm on the configuration, and takes into account the efficiency at the same time, and can achieve efficient and high-resolution imaging of key areas.

Description

Technical Field

[0001] The present invention belongs to the field of radar imaging technology, and in particular relates to a multi-base synthetic aperture radar imaging technology in a one-transmit-multiple-receive mode. Background Art

[0002] Synthetic aperture radar (SAR) uses the relative motion between the radar and the target to synthesize a smaller real antenna aperture into a larger equivalent antenna aperture through data processing. It can penetrate obscured objects to obtain high-resolution images similar to optical photography, and has the characteristics of all-day, all-weather, and high-resolution. With the development of SAR, SAR has been widely used in underwater target detection, crop census, terrain mapping and other fields.

[0003] Multi-base SAR uses multiple platforms equipped with transmitters or receivers to move in space to form distributed virtual antennas for imaging. Compared with single-base SAR, it can observe the target area from more perspectives, obtain more information from echoes, and is easier to detect stealth targets. It can simultaneously obtain many distributed unit small apertures in a short period of time, and synthesize the equivalent spatial large aperture through aperture sequence connection to achieve high-resolution imaging of key areas.

[0004] Among them, efficient and high-precision imaging is a key to multi-base SAR technology. The literature "A Fast BP Algorithm With Wavenumber Spectrum Fusion for High-Resolution Spotlight SAR Imaging [J]. IEEE Geoscience and Remote Sensing Letters, 2014, 11 (9): 1460-1464." achieves fast SAR imaging by spectrum shifting and splicing in the wavenumber domain, but this method can only be used in linear aperture and monostatic mode, and cannot be extended to bistatic SAR and multi-base SAR; the literature "The Mathematics of Computerized Tomography. New York, NY, USA: Wiley, 1986" uses back-projection to image SAR. This method is applicable to any configuration, but the computational efficiency is very low. At present, there is little research on multi-base SAR imaging technology that takes into account both accuracy and efficiency in any configuration. Summary of the invention

[0005] In view of the above problems existing in the prior art, the present invention proposes a method for fast decomposition projection imaging of multi-base synthetic aperture radar.

[0006] The technical solution of the present invention is: a method for rapid decomposition projection imaging of a multi-base synthetic aperture radar, which specifically comprises the following steps:

[0007] S1. Initialization of multi-base synthetic aperture radar parameters, including: number of transceivers, transceiver motion parameters, transmission signal carrier frequency, transmission signal time width, bandwidth, system sampling rate, pulse repetition frequency;

[0008] S2. Echo acquisition and demodulation. Each receiver acquires the echo and demodulates it from the carrier frequency to the baseband. The echo signal of the i-th receiver is s i (τ, η), where τ is the fast time variable and η is the slow time variable;

[0009] S3. Echo equalization: Through single-base equivalence, the overlap between different apertures is obtained, and the echo of the overlapping part of the aperture is equalized. After equalization, the echo signal of the i-th receiver is

[0010] S4. Pulse compression is performed on the echo in the distance direction, that is, convolution is performed on the echo in the distance direction with the reference signal to obtain the result after pulse compression.

[0011] S5. Perform rapid decomposition and back-projection imaging on an aperture-by-aperture basis to obtain a coarse-resolution image in polar coordinate format for each aperture;

[0012] S6. The coarse-resolution images of different apertures are layered and fused until they are fused into a complete aperture;

[0013] S7. Project the slant range plane imaging result onto the ground to obtain a full-resolution image.

[0014] Furthermore, the specific steps of step S3 are as follows:

[0015] S31. For each transceiver pair in the multi-base synthetic aperture radar, the observation of the double-base pair of target points is equivalent to the observation of the single-base pair of target points on the target angle bisector, thereby equivalent each double-base pair to an aperture distributed in three-dimensional space;

[0016] S32. Analyze the overlap between apertures;

[0017] S33. Perform weighted equalization on the overlapping apertures. Assuming that there are A apertures overlapping at the azimuth time η0, the weighted equalization is expressed as:

[0018]

[0019] Among them, w a represents the weight of weighted equilibrium, s a (τ, η0) represents the ath aperture that needs to be balanced, Represents the aperture after weighted equalization.

[0020] Furthermore, the specific steps of step S5 are as follows:

[0021] S51. Perform sub-aperture division aperture by aperture, divide a complete aperture into K sub-apertures of length l, establish a local elliptical polar coordinate system at the center of each sub-aperture, with the focus being the transceiver position corresponding to the sub-aperture center, rotate the imaging grid point (r, θ) on the slant range plane (r and θ represent the polar diameter and polar angle on the local elliptical polar coordinate system, respectively) to (x, y) on the ground (x and y represent the horizontal and vertical coordinates in the rectangular coordinate system, respectively) with the line connecting the two foci of the local elliptical polar coordinate system as the axis, establish a one-to-one mapping relationship between the imaging grid on the slant range plane and the imaging grid on the ground, and represent this mapping relationship as f:

[0022] f(r,θ)=(x,y)

[0023] Then the imaging grid points (x, y) on the ground are projected;

[0024] S52. Back-projection to obtain K azimuthally low-resolution sub-aperture images:

[0025]

[0026] Where v is the speed of the receiver, η c is the sampling time of the azimuth center of the subaperture, s mc (τ, η) is the echo after pulse compression, f c is the carrier frequency, c is the speed of light, R(f(r, θ), η) is the sum of the distances from the imaging grid point f(r, θ) = (x, y) to the transceiver when the azimuth time is η;

[0027] S53. Sub-aperture hierarchical fusion. Assuming that n adjacent sub-apertures are fused each time, the transceiver position corresponding to the center of the n sub-apertures is taken as the focus, and a local elliptical polar coordinate system (r′, θ′) that can meet the resolution capability is constructed on the slant range plane and mapped to the ground (x′, y′). The sub-aperture image I p The value of the pixel closest to (x′, y′) on (x, y) (p=1, 2, ..., n) is assigned to (x′, y′), and finally the n sub-aperture image fusion result H(x′, y′) (i.e.: H(f(r′, θ′))) is obtained. This step is repeated hierarchically until all sub-aperture images are fused into a full-aperture image;

[0028] S54. Repeat steps S51 to S53 for all apertures, and then end step S5.

[0029] Furthermore, the specific steps of step S6 are as follows:

[0030] Aperture hierarchical fusion, assuming that j adjacent apertures are fused each time, the transceiver positions corresponding to the centers of these j apertures are taken as the focus, and a local elliptical polar coordinate system (r″, θ″) that can meet the resolution capability is constructed on the slant range plane, and mapped to the ground (x″, y″), and the value of the pixel point closest to (x″, y″) on the aperture image is assigned to (x″, y″), and finally the fusion result of j aperture images is obtained. This step is repeated hierarchically until all aperture images are fused, and step S6 is ended.

[0031] Beneficial effects of the present invention: The method of the present invention first demodulates the echo received by each receiver to the baseband; then each transceiver pair in the multi-base SAR is equivalent to a single-base SAR, and the echo of the overlapping part of the aperture is weighted and balanced; then the echo range is pulse compressed; then rapid decomposition and back-projection imaging are performed aperture by aperture; then the coarse-resolution images of different apertures are layered and fused until they are fused into a complete aperture; finally, the slant range plane imaging result is projected onto the ground to obtain the imaging result. The method of the present invention is suitable for multi-base SAR imaging, which overcomes the configuration restrictions of the frequency domain imaging algorithm, while taking into account efficiency, and can achieve efficient and high-resolution imaging of key areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the process of the present invention;

[0033] Figure 2 This is a schematic diagram of the multi-base synthetic aperture radar configuration;

[0034] Figure 3 It is a single-base equivalent schematic diagram;

[0035] Figure 4 It is a schematic diagram of the local elliptical polar coordinate system of the slope range surface;

[0036] Figure 5 This is a schematic diagram of the conversion of the local elliptical polar coordinates of the slope range surface into the ground rectangular coordinate system;

[0037] Figure 6 This is a schematic diagram of hierarchical fusion of sub-aperture images;

[0038] Figure 7 The simulation results of this method are as follows: (a) represents the imaging results of nine point targets, (b) represents the magnified result of the central point target, (c) represents the distance profile of the central point target, and (d) represents the azimuth profile of the central point target. DETAILED DESCRIPTION

[0039] The implementation of the present invention will be further described below in conjunction with the accompanying drawings.

[0040] The imaging method of the present invention converts a multi-base synthetic aperture radar in a one-transmit-many-receive mode into multiple bi-base synthetic aperture radars, converts each bi-base synthetic aperture radar into a single-base synthetic aperture, obtains the overlap between different synthetic apertures, and balances the echoes of the overlapping parts; then, each bi-base synthetic aperture radar echo is matched filtered; then, a fast decomposition back-projection algorithm is used to perform coarse imaging on each bi-base synthetic aperture radar; the coarse imaging results on different inclined planes are graded and fused, and finally, a high-resolution result of the entire multi-base synthetic aperture radar is obtained. The process is as follows: Figure 1 As shown in the figure, it mainly includes six parts: A. multi-base SAR parameter initialization; B. echo acquisition and demodulation; C. after each transceiver pair is equivalent to a single-base SAR, the echo of the overlapping part of the aperture is weighted and balanced; D. pulse compression of the echo range direction; E. rapid decomposition and back-projection imaging by aperture; F. layered fusion of coarse-resolution images of different apertures; G. projection of the slant range plane imaging results to the ground. Figure 2 It is a schematic diagram of the configuration of the multi-base SAR of this method.

[0041] A. Before implementing the method, it is necessary to determine: ① Imaging scene; ② Initial position and configuration of transceiver; ③ System parameters of multi-base SAR. The details are as follows:

[0042] A1. Assume that there are 9 point targets in the imaging scene, and their distribution positions are shown in Table 1 below, where point target 5 is the center of the imaging scene:

[0043] Table 1

[0044] Point number Point location Point target 1 (-40，40，0)m Point target 2 (-40，0，0)m Point target 3 (-40，-40，0)m Point target 4 (0，40，0)m Point target 5 (0，0，0)m Point target 6 (0，-40，0)m Point target 7 (40，40，0)m Point target 8 (40，0，0)m Point target 9 (40，-40，0)m

[0045] A2. In this embodiment, a multi-base SAR with one transmitter and eight receivers is used. The initial position and speed of each transmitter and receiver are shown in Table 2 below:

[0046] Table 2

[0047]

[0048]

[0049] A3. In this embodiment, the system parameters required are listed in Table 3:

[0050] Table 3

[0051] <![CDATA[Carrier frequency (f c )]]> 30GHz <![CDATA[Transmission signal bandwidth (B r )]]> 150MHz <![CDATA[Transmission signal pulse width (T r )]]> 5μs <![CDATA[Sampling frequency (f s )]]> 165MHz Pulse Repetition Frequency (PRF) 300Hz Number of receivers 8 RanNum: the number of sampling points in the range direction 2048 Azinum: Number of sampling points in azimuth 128

[0052] according to Figure 1 The specific process of this embodiment is as follows:

[0053] B. Echo collection and demodulation:

[0054] Each receiver samples the echo at a sampling rate f s After acquisition and demodulation to baseband, the echo is in the form of:

[0055]

[0056] Among them, τ is the slow time variable, and its value is:

[0057]

[0058] η is a fast time variable, and its value is:

[0059]

[0060] w r (), w a () is the envelope of the range and azimuth, using a rectangular window function, R i (η) is the sum of the distances from the transmitter and the i-th receiver to the target point, c represents the speed of light, K r Represents the frequency modulation, the value is

[0061] C. Each transceiver pair is equivalent to a monostatic SAR, and then the echoes of the overlapping apertures are weighted and equalized:

[0062] Each transceiver pair is equivalent to a monostatic SAR in the following way: the intersection of the angle bisector of the angle formed by the line connecting the transceiver and the center of the scene and the line connecting the transceivers is taken as the position of the equivalent monostatic SAR, such as Figure 3 As shown, θ1 = θ2. In this way, eight transmit-receive pairs can be equivalent to eight apertures, and then the echoes corresponding to the parts of the eight apertures that overlap in azimuth are weighted and equalized.

[0063] D. Pulse compression in echo distance direction:

[0064] After the previous step, the echo sampled at each azimuth moment is convolved with the reference signal in the time domain to achieve pulse compression. Here, the expression of the reference signal is:

[0065]

[0066] After pulse compression, the obtained

[0067]

[0068] in,

[0069] E. Rapid decomposition of backprojection imaging by aperture:

[0070] E1. Split a certain aperture into many short apertures. Here, the aperture (128 sampling points) is split into eight sub-apertures, and the number of sampling points in each sub-aperture is 16;

[0071] E2. With the transceiver position corresponding to each sub-aperture center as the focus, a local elliptical polar coordinate system (r, θ) is established on the slant range plane, as follows: Figure 4 As shown;

[0072] E3. Since the trajectories of the transceivers of the multi-base synthetic aperture radar may not be on the same straight line, the local elliptical polar coordinate systems of different sub-apertures will not be on the same plane, so the sub-aperture low-resolution images are located in different planes, which brings difficulties to the fusion of images. In order to ensure that the low-resolution images on different slant range planes are fused in one plane without error, it is necessary to rotate the imaging grid points on the slant plane to the ground with the local elliptical polar coordinate system and the line connecting the transceivers as the axis, and establish a one-to-one mapping relationship between the slant range plane imaging grid and the ground imaging grid (x, y), as shown in Figure 5 As shown;

[0073] E4. Back-projection to obtain the sub-aperture image:

[0074]

[0075] Where v is the speed of the receiver, η c is the sampling time of the azimuth center of the subaperture, s mc (τ, η) is the echo after pulse compression, m = 1, 2, ..., 8;

[0076] E5. Sub-aperture hierarchical fusion:

[0077] Assuming that every two adjacent sub-aperture images are fused into an image with a larger sub-aperture, there are three levels of fusion of the eight images, and the number of images in each level is 4, 2, and 1 respectively. Figure 6 As shown. The fusion process is as follows: each time two adjacent sub-apertures are fused, the transceiver positions corresponding to the centers of the two sub-apertures are used as the focus, a local elliptical polar coordinate system (r′, θ′) that can meet the resolution capability is constructed on the slant range plane, and mapped to the ground (x′, y′), the value of the pixel closest to (x′, y′) on the sub-aperture image I(x, y) is assigned to (x′, y′), and finally the fusion result of the two sub-aperture images H(x′, y′) (i.e.: H(f(r′, θ′))) is obtained, and this step is repeated hierarchically until all sub-aperture images are fused into a complete aperture image;

[0078] E6. Repeat steps E1 to E5 for each aperture.

[0079] F. Layered fusion of coarse resolution images with different apertures:

[0080] This step is similar to step D5: assuming that two adjacent apertures are fused each time, the transceiver positions corresponding to the centers of the two apertures are taken as the focus, and a local elliptical polar coordinate system (r″, θ″) that can meet the resolution capability is constructed on the slant range plane and mapped to the ground (x″, y″). The value of the pixel point closest to (x″, y″) on the two aperture images is assigned to (x″, y″), and finally the fusion result of the two aperture images is obtained. This step is repeated hierarchically until all aperture images are fused.

[0081] G. Map the slant range plane imaging results to the ground to obtain a high-resolution image.

[0082] The simulation results are as follows Figure 7 As shown in the figure, (a) represents the imaging results of nine point targets, (b) represents the magnified result of the central point target, (c) represents the distance profile of the central point target, and (d) represents the azimuth profile of the central point target. According to the simulation, the indicators shown in Table 4 can be obtained:

[0083] Table 4

[0084] distance position ISLR -12.09dB -10.29dB PSLR -14.99dB -13.55dB 3dB main lobe width 0.87m 0.88m

[0085] The resolution in range and azimuth is:

[0086]

[0087]

[0088] Among them, φ r represents the angle between the beam and the ground, θ syn It is the change of the target viewing angle during the period of beam coverage of eight apertures after single base equivalence.

[0089] According to the imaging result diagram and the indicators in Table 4, it can be concluded that this method has good focus on the target, which proves that this method can achieve efficient and high-precision imaging of multistatic SAR.

Claims

1. A method for rapid decomposition and projection imaging of a multi-base synthetic aperture radar, comprising the following steps: S1. Initialization of multi-base synthetic aperture radar parameters, the parameters include: Number of transceivers, transceiver motion parameters, transmission signal carrier frequency, transmission signal duration, bandwidth, system sampling rate, pulse repetition frequency; S2. Echo acquisition and demodulation. Each receiver acquires the echo and demodulates it from the carrier frequency to the baseband. The echo signal of the i-th receiver is s i (τ,η), where τ is the fast time variable and η is the slow time variable; S3. Echo equalization: Through single-base equivalence, the overlap between different apertures is obtained, and the echo of the overlapping part of the aperture is equalized. After equalization, the echo signal of the i-th receiver is The specific steps are as follows: S31. For each transceiver pair in the multi-base synthetic aperture radar, the observation of the double-base pair of target points is equivalent to the observation of the single-base pair of target points on the target angle bisector, thereby equivalent each double-base pair to an aperture distributed in three-dimensional space; S32. Analyze the overlap between apertures; S33. Perform weighted equalization on the overlapping apertures. Assuming that there are A apertures overlapping at the azimuth time η0, the weighted equalization is expressed as: Among them, w a represents the weight of weighted equilibrium, s a (τ,η0) represents the ath aperture that needs to be balanced, Represents the aperture after weighted equalization; S4. Pulse compression is performed on the echo in the distance direction, that is, convolution is performed on the echo in the distance direction with the reference signal to obtain the result after pulse compression. S5. Perform rapid decomposition and back-projection imaging on an aperture-by-aperture basis to obtain a coarse-resolution image in polar coordinate format for each aperture; The specific steps are as follows: S51. Perform sub-aperture division aperture by aperture, divide a complete aperture into K sub-apertures of length l, establish a local elliptical polar coordinate system at the center of each sub-aperture, with the focus being the transceiver position corresponding to the sub-aperture center, rotate the imaging grid point (r, θ) on the slant range plane, where r and θ represent the polar diameter and polar angle on the local elliptical polar coordinate system respectively, to (x, y) on the ground with the line connecting the two foci of the local elliptical polar coordinate system as the axis, where x and y represent the horizontal and vertical coordinates in the rectangular coordinate system respectively, and establish a one-to-one mapping relationship between the imaging grid on the slant range plane and the imaging grid on the ground, and represent this mapping relationship as f: f(r,θ)=(x,y) Then the imaging grid points (x, y) on the ground are projected; S52. Back-projection to obtain K azimuthally low-resolution sub-aperture images: Where v is the speed of the receiver, η c is the sampling time of the azimuth center of the subaperture, s mc (τ,η) is the echo after pulse compression, f c is the carrier frequency, c is the speed of light, and R(f(r,θ),η) is the sum of the distances from the imaging grid point f(r,θ) = (x,y) to the transceiver at the azimuth time η; S53. Sub-aperture hierarchical fusion. Assuming that n adjacent sub-apertures are fused each time, the transceiver position corresponding to the center of the n sub-apertures is taken as the focus, and a local elliptical polar coordinate system (r′, θ′) that can meet the resolution capability is constructed on the slant range plane and mapped to the ground (x′, y′). The sub-aperture image I p The value of the pixel closest to (x′, y′) on (x, y) is assigned to (x′, y′), p = 1, 2, …, n, and finally the fusion result of n sub-aperture images H(x′, y′) is obtained, that is: H(f(r′, θ′)). This step is repeated hierarchically until all sub-aperture images are fused into a full-aperture image; S54. Repeat steps S51 to S53 for all apertures, and then end step S5; S6. The coarse-resolution images of different apertures are layered and fused until they are fused into a complete aperture; The specific steps are as follows: Assuming that j adjacent apertures are fused each time, the transceiver position corresponding to the center of the j apertures is taken as the focus, and a local elliptical polar coordinate system (r″, θ″) that can meet the resolution capability is constructed on the slant range plane and mapped to the ground (x″, y″), and the value of the pixel point closest to (x″, y″) on the aperture image is assigned to (x″, y″), and finally the fusion result of j aperture images is obtained. This step is repeated hierarchically until the images of all apertures are fused, and step S6 ends; S7. Project the slant range plane imaging result onto the ground to obtain a full-resolution image.

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