Single-antenna bistatic forward-looking SAR ground target series inversion imaging method and equipment

Abstract

The present invention discloses a single-antenna bistatic forward-looking SAR ground target series inversion imaging method and device, which is applied to a single-antenna receiver, and includes: receiving a radar echo signal containing ground clutter and moving target information from an imaging scene; performing range pulse compression, azimuth de-skew and azimuth spectrum correction on the received echo signal to obtain a filtered signal; performing parity extraction on the filtered signal along the azimuth direction, and performing wedge transformation on the extracted signals of the two channels to obtain a transformed first channel signal and a transformed second channel signal; performing a ground clutter cancellation operation based on series inversion based on the transformed first channel signal and the transformed second channel signal to obtain a ground clutter cancellation signal containing moving target information; and performing imaging result detection of the moving target based on the ground clutter cancellation signal. The present invention can suppress ground clutter in a complex imaging scene containing moving targets and retain the moving targets.

Description

Technical Field

[0001] The invention belongs to the field of radar technology, and in particular relates to a single-antenna bistatic forward-looking SAR ground target series inversion imaging method and equipment. Background Art

[0002] Bistatic forward-looking synthetic aperture radar (SAR) can break through the limitation of single-static SAR that cannot perform forward-looking high-resolution imaging, and can achieve high-precision SAR focused imaging in the area directly in front of its receiving platform. Using bistatic forward-looking SAR to image ground moving targets can detect ground moving targets in front (i.e., moving targets on the ground), and can be applied in the fields of traffic monitoring and vehicle search. However, due to the non-cooperative motion of ground moving targets, the echo signal has a large linear range migration and azimuth spectrum ambiguity, and due to the clutter of the ground static scene, the ground moving target signal is masked by the strong ground clutter under the single-antenna configuration, making it difficult to detect ground moving targets. In response to the above problems, there is still no algorithm for suppressing ground clutter in the imaging scene under the single-antenna bistatic forward-looking SAR configuration as a reference.

[0003] In order to solve the problem of moving target imaging in bistatic forward-looking SAR, moving target imaging and detection can be performed through amplitude and phase information; multi-frequency SAR, multi-antenna SAR and dual-speed SAR are proposed for unambiguous motion parameter estimation under the premise of completely removing range migration; target motion parameters can also be obtained through range movement in the signal. These methods all require the detection of the target before estimating the target parameters, but there is serious ground clutter interference in the actual echo signal, and due to cost and space limitations, only a single antenna configuration can be used, resulting in these methods being ineffective and inapplicable.

[0004] The imaging algorithm based on wedge transform can correct the linear range migration of moving and static targets when the target speed cannot be obtained, but the aliasing of the azimuth spectrum will make the wedge transform invalid. In the monostatic SAR imaging system, the imaging algorithm based on azimuth de-skew and wedge transform method can solve this problem to a certain extent. However, in the bistatic forward-looking SAR, due to the more complex configuration, the two-dimensional coupling of the echo signal is more serious, and the two-dimensional spectrum has a serious tilt, this method is no longer applicable.

[0005] That is to say, the prior art has the following disadvantages:

[0006] 1) Existing SAR imaging methods cannot achieve fast focusing imaging of moving targets with unknown motion parameters in the imaging scene under the bistatic forward-looking SAR configuration.

[0007] 2) The existing ground moving target imaging detection method cannot suppress the ground clutter in the imaging scene under the single-antenna bistatic forward-looking SAR configuration, which seriously affects the target detection effect.

[0008] 3) The existing multi-channel clutter suppression method based on multiple antennas along the track has complex signal processing. Summary of the invention

[0009] In order to solve the above problems existing in the prior art, the present invention provides a single-antenna bistatic forward-looking SAR ground target series inversion imaging method and device.

[0010] The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0011] The present invention provides a single-antenna bistatic forward-looking SAR ground target series inversion imaging method, characterized in that it is applied to a single-antenna receiver, and the method comprises:

[0012] Receive radar echo signals from the imaging scene, which contain ground clutter and moving target information;

[0013] Performing range pulse compression, azimuth de-skewing and azimuth spectrum correction on the received echo signal to obtain a filtered signal;

[0014] Performing parity extraction on the filtered signal along the azimuth direction, and performing wedge transformation on the extracted signals of the two channels respectively, to obtain a transformed first channel signal and a transformed second channel signal;

[0015] Based on the transformed first channel signal and the transformed second channel signal, ground clutter cancellation is performed to obtain a ground clutter cancelled signal containing the moving target information;

[0016] The imaging result of the moving target is detected according to the signal after the ground clutter is cancelled.

[0017] The present invention also provides a single-antenna bistatic forward-looking SAR ground target level inversion imaging device, comprising a processor, a communication interface, a memory and a communication bus, wherein the processor, the communication interface and the memory communicate with each other through the communication bus;

[0018] The memory is used to store computer programs;

[0019] The processor is used to implement the steps of the above-mentioned single-antenna bistatic forward-looking SAR ground target series inversion imaging method when executing the program stored in the memory.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The present invention uses azimuth de-skewing and azimuth spectrum correction filters to compress and correct the spectrum, and then performs signal extraction and wedge transformation, so that the ground clutter and the echo signal of the moving target under the bistatic forward-looking SAR configuration can be quickly imaged and focused; then, the present invention cancels the ground clutter based on the transformed first channel signal and the transformed second channel signal, so that the moving target in the image can be detected. The main operation of the present invention is completed in the range frequency domain azimuth time domain of the signal, and all signal processing operations only require fast Fourier transform (Fast Fourier Transform, FFT) and complex multiplication, with low computational complexity and strong robustness. The method proposed in the present invention is based on a single antenna configuration, using a set of transceiver antenna equipment, in the echo signal of the bistatic forward-looking SAR, it can suppress ground clutter in a complex imaging scene containing moving targets, retain moving targets, and realize moving target detection in the presence of ground clutter, which is conducive to the extraction, identification and positioning of moving targets in the monitoring area. Therefore, the present invention can image and detect moving targets in the forward-looking area of ​​the aircraft, and can be used in the fields of traffic monitoring, environmental reconnaissance, etc. The method is not limited to the application of bistatic forward-looking SAR imaging based on airborne platforms. In the application of bistatic forward-looking SAR imaging based on moving platforms such as unmanned aerial vehicle platforms, missile-borne platforms and satellite-borne platforms, the method proposed in the present invention can also image and detect moving targets in the forward-looking imaging scene area on the basis of imaging focusing and ground clutter suppression, and has wide applications in civil and remote sensing fields.

[0022] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 is a spatial geometric configuration diagram of a transmitter and receiver provided by an embodiment of the present invention;

[0024] Figure 2 It is a flow chart of a single-antenna bistatic forward-looking SAR ground target series inversion imaging method provided by an embodiment of the present invention;

[0025] Figure 3 is a two-dimensional spectrum diagram of an original echo signal provided by an embodiment of the present invention;

[0026] Figure 4 yes Figure 3 The two-dimensional spectrum diagram of the original echo signal after azimuth de-slant filtering is shown;

[0027] Figure 5 yes Figure 4 The two-dimensional spectrum diagram of the signal obtained after the azimuth spectrum correction is shown;

[0028] Figure 6It is a schematic diagram of the geometric relationship between any ground stationary clutter target point and the sampling centers of the first channel and the second channel of the transceiver in the imaging scene provided by the embodiment of the present invention;

[0029] Figure 7 is a schematic diagram of the distribution of the phase error of the channel compensation filter constructed by the series inversion method provided by an embodiment of the present invention;

[0030] Figure 8 and Fig. 9 It is a schematic diagram comparing the ground imaging result and the clutter cancellation result provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below with reference to specific embodiments, but the embodiments of the present invention are not limited thereto.

[0032] The single-antenna bistatic forward-looking SAR ground target level inversion imaging method provided by the present invention is applied to a single-antenna receiver in a bistatic forward-looking SAR configuration. For example, Figure 1 is a spatial geometric configuration diagram of a transmitter and receiver. Figure 1 As shown, the transmitter illuminates the target area and the forward-looking receiver records the radar echo scattered by the target scene. Figure 1 In the figure, the points on the flight trajectory of the transmitter and the points on the flight trajectory of the receiver represent the sampling positions of the transmitter and the receiver respectively. Establish a Cartesian right-hand coordinate system for the origin, that is , and Respectively The X and Y coordinates of the transmitter. , similarly, and Respectively The X and Y coordinates of the receiver. The flight altitude of the receiver is , the flight altitude of the transmitter is Taking the ground as the XY plane, define the transmitter's velocity as , the receiver speed is , and the direction of the velocity is parallel to the Y axis. The center of the scene is point, express The Y coordinate of the point. is a moving target point at any position in the scene, and at the center of the synthetic aperture, Point is located , and Respectively The X and Y coordinates of the point, the speed of the moving target point is , the velocity direction is along any direction in the XY plane. Under this configuration, the slant range model of the moving target can be expressed as:

[0033] (1);

[0034] in, The target point of the movement (Also called sports target ) of the slope range, and They are The projection of the X-axis and Y-axis in the Cartesian right-hand coordinate system, For slow time, and when and When it is taken as 0, the slant range history of a stationary target at any point can be obtained.

[0035] Figure 2 FIG. 1 is a flow chart of a single-antenna bistatic forward-looking SAR ground target level inversion imaging method provided by an embodiment of the present invention, such as Figure 2 As shown, the method includes:

[0036] S101, receiving a radar echo signal containing ground clutter and moving target information from an imaging scene.

[0037] S102, performing range pulse compression, azimuth de-skew and azimuth spectrum correction on the received echo signal to obtain a filtered signal.

[0038] S103 , performing parity extraction on the filtered signal along the azimuth direction, and performing wedge transformation on the extracted signals of the two channels respectively, to obtain a transformed first channel signal and a transformed second channel signal.

[0039] Here, the wedge transform may be implemented in a variety of ways. For example, the wedge transform may be implemented by using a sinc interpolation method.

[0040] S104 , performing a ground clutter cancellation operation based on series inversion based on the transformed first channel signal and the transformed second channel signal to obtain a ground clutter cancelled signal containing moving target information.

[0041] S105: Detect the imaging result of the moving target according to the signal after the ground clutter is cancelled.

[0042] In some embodiments, the above S102 can be implemented by the following steps:

[0043] S1021. Demodulate the received echo signal to obtain a demodulated echo signal.

[0044] Exemplarily, the demodulated echo signal The expression is as follows:

[0045] (2);

[0046] in, , For quick time, is the modulation frequency of the transmitter's transmission signal, is the speed of light, is the carrier wavelength of the transmitted signal, is the carrier frequency of the radar transmitting signal, and are respectively the envelope window function and azimuth rectangular window function of the radar signal transmitted by the transmitter. The width of the azimuth rectangular window function is the synthetic aperture time. Is an imaginary unit.

[0047] S1022. Perform range-domain Fourier transform on the demodulated echo signal to obtain a frequency-domain signal.

[0048] For example, the frequency domain signal can be obtained by using the stationary phase method The expression is as follows:

[0049] (3);

[0050] in, is the distance frequency variable, is the distance envelope function of the distance spectrum, the first exponential term is the distance linear frequency modulation term, and the second exponential term is the azimuth modulation term.

[0051] S1023, using a two-dimensional filter including a range pulse compression filter, an azimuth de-skew filter and an azimuth spectrum correction filter to simultaneously perform range pulse compression, azimuth de-skew and azimuth spectrum correction on the frequency domain signal to obtain a filtered signal.

[0052] Here, in the range frequency domain, the range pulse pressure filter It can be written as:

[0053] (4);

[0054] Here, after the frequency domain signal described in the above formula (3) is subjected to the range pulse compression, due to the existence of range migration in the signal after the range pulse compression, the target echo signal will present an inclined curve in the two-dimensional time domain. exist The Taylor series expansion is performed at , and its fourth-order Taylor approximation is:

[0055] (5);

[0056] (6);

[0057] in, The values ​​of are 1, 2, 3, 4, express of Derivatives, Indicates slow time The value of the slant range history of the moving target when is 0. Therefore, the phase of the frequency domain signal after the range pulse pressure It can be written as formula (7):

[0058] (7);

[0059] Exemplary, azimuth de-skew filter The expression can be written as:

[0060] (8);

[0061] in, , , Respectively represent the center point of the scene of , , The value of , The spatial variation of can usually be ignored, that is, , When performing the Taylor expansion operation, the present invention does not limit the order of the Taylor expansion and can be set according to actual needs.

[0062] After the frequency domain signal after range pulse compression is de-skewed by azimuth, the phase of the signal It can be written as:

[0063] (9);

[0064] (10);

[0065] (11);

[0066] (12);

[0067] (13);

[0068] Azimuth de-skewing reduces the probability of aliasing of the signal in the azimuth frequency domain, but in bistatic forward-looking SAR, the two-dimensional spectrum of the signal has serious aliasing, tilt and offset. For example, Figure 3 is the two-dimensional spectrum of the original echo signal, Figure 4 yes Figure 3 The two-dimensional spectrum of the original echo signal after azimuth de-slant filtering is shown. It can be seen that the two-dimensional spectrum of the signal is compressed, which reduces the aliasing of the two-dimensional spectrum. Figure 4 The two-dimensional spectrum of the signal shown still has a certain tilt and offset.

[0069] Exemplary, azimuth spectrum correction filter It is expressed as:

[0070] (14);

[0071] in, Represents the center point of the scene Doppler Center .

[0072] For example, Figure 5 yes Figure 4 The two-dimensional spectrum of the signal shown is obtained after the azimuth spectrum correction processing. Obviously, the energy of the echo is greatly compressed in the azimuth frequency domain, and the tilt of the spectrum is corrected, and the azimuth spectrum will not be truncated.

[0073] Since the above filtering operations are all performed in the range frequency domain of the signal, a two-dimensional filter including the above three filters is constructed in this case, through which the signal can be simultaneously corrected for range pulse pressure, azimuth de-skew and azimuth spectrum. Exemplarily, the process of filtering the frequency domain signal described in the above formula (3) using a two-dimensional filter is expressed as:

[0074] (15);

[0075] in, represents the filtered signal, is the expression of the two-dimensional filter, and .

[0076] It can be seen that the present invention only needs to perform one filtering operation on the echo signal to simultaneously complete the range pulse compression, spectrum compression and spectrum correction of the signal, which greatly reduces the complexity of signal processing. In addition, since the radial velocity of the receiver relative to the target in the bistatic forward-looking SAR is relatively large, the Doppler center of the echo signal is It is often greater than the acceptable pulse repetition frequency of the system, and the received echo signal will be blurred in the azimuth direction. Sometimes the signal is even truncated into two parts in the azimuth frequency domain, resulting in spectrum aliasing, as mentioned above. Figure 3In addition, due to the randomness of the position and speed of the moving target, the moving target echo signal will have additional unknown ambiguity and aliasing in the azimuth direction, making the wedge transform directly applied to the above Figure 3 When the ground moving target signal is shown, the imaging will fail, which is mainly manifested in the failure of linear range migration correction and the appearance of false targets. The present invention uses azimuth de-skew and azimuth spectrum correction to perform signal preprocessing before wedge transformation, which can greatly shorten the azimuth bandwidth of the signal, thereby significantly avoiding the phenomenon of azimuth spectrum aliasing of the ground moving target signal.

[0077] In some embodiments, the above S103 can be implemented by the following steps:

[0078] S1031. Perform odd-even extraction on the filtered signal along the azimuth direction to obtain an odd-numbered first channel signal and an even-numbered second channel signal.

[0079] For example, when a signal has 1024 pulses, it can be sampled along slow time to extract pulses corresponding to the odd sequence as the first channel signal, and extract pulses corresponding to the even sequence as the second channel signal, so that the first channel signal and the second channel signal are both signals composed of 512 pulses.

[0080] Exemplarily, the first channel signal and the second channel signal and The discrete form of :

[0081] (16);

[0082] in, The value of is 1, 2, represents the slow time sampling interval, Indicates Channel signal, for The number of points of represents the integer domain, Indicates the azimuth focus position of the moving target. It can be written as: (17).

[0083] S1032: Perform line frequency modulation conversion on the first channel signal to focus the first channel signal in the range Doppler domain to obtain a converted first channel signal.

[0084] S1033: Perform line frequency modulation conversion on the second channel signal to focus the second channel signal in the range Doppler domain to obtain a converted second channel signal.

[0085] Exemplarily, when performing wedge transformation, the constructed wedge transformation factor is: (18); among them, For the new slow time.

[0086] In the present invention, the wedge transformation can be efficiently realized by utilizing the line frequency modulation transformation, and the signal imaging can be directly focused in the range Doppler domain.

[0087] For example, The line frequency modulation transformation can be written as formula (19);

[0088] (19);

[0089] in, represents the independent variable after line frequency modulation transformation, represents the signal after line frequency modulation transformation; if the wedge transformation shown in the above formula (18) is to be established, the formula (19) can be set to , , that is, the azimuth spectrum of the signal after the wedge transform can be obtained by resampling in the azimuth frequency domain using the line frequency modulation transform. and After the line frequency modulation, the signal is transformed into a two-dimensional spectrum. At this time, the first channel signal obtained after the transformation is and the second channel signal They can be written as:

[0090] (20);

[0091] (twenty one);

[0092] in, is the synthetic aperture time, express The frequency domain variable, , and They are the first channel signal corresponding to The second channel signal corresponds to At this point, the remaining linear range migration of the signal has been corrected, and the signal has completed imaging focusing in the azimuth direction.

[0093] In some embodiments, the above S104 can be implemented by the following steps:

[0094] S1041 . Determine a first channel complex image and a second channel complex image according to the transformed first channel signal and the transformed second channel signal.

[0095] Specifically, the transformed first channel signal is subjected to a range inverse FFT to obtain a first channel complex image; the transformed second channel signal is subjected to a range inverse FFT to obtain a second channel complex image. , the second channel complex image The expressions are as follows:

[0096] (twenty two);

[0097] (twenty three);

[0098] in, Indicates the range bandwidth of the signal. From then on, unified imaging and focusing of the echo signals of ground clutter and moving targets is completed.

[0099] S1042: Perform ground clutter cancellation based on the first channel complex image and the second channel complex image to obtain a ground clutter cancelled signal containing moving target information.

[0100] After S1041, the echo signal is processed into a two-channel SAR image, such as formula (22) and (23), then the focus position of any point target including ground clutter and moving target is , the phase at the focus position is , The values ​​of are 1 and 2. Since the two channels obtained are not sampled successively at the same position, . It is less than half of the distance unit, so the deviation of the focus position can be ignored, but the phase difference at the focus position between the two images cannot be ignored. Due to the existence of the phase difference between the two channels, the clutter cannot be directly eliminated. Therefore, the present invention adopts a channel phase compensation method based on series inversion to compensate the phase difference between the two channels by polynomial fitting phase. Specifically, a channel compensation filter constructed using the series inversion method is used to compensate the phase of the second channel complex image to obtain a compensated second channel complex image. After that, the first channel complex image and the compensated second channel complex image are canceled to obtain a signal after ground clutter cancellation containing moving target information.

[0101] Exemplarily, the process of ground clutter cancellation can be expressed as:

[0102] (twenty four);

[0103] in, represents the signal after ground clutter cancellation, represents the channel compensation filter, Represents the residual phase of the moving target after channel compensation. For ground stationary clutter, , so the ground clutter will be cancelled. As for the ground moving target, due to the offset of its azimuth focus position, the phase of the ground moving target at the focus position will not be compensated by the channel compensation function. is fully compensated, so there is a residual phase, that is, , so that the moving target signal will not be cancelled.

[0104] Exemplary channel compensation filter The expression is as follows:

[0105] (25);

[0106] in, Indicates that the independent variable and Indicated , Represents any stationary target point in the imaging scene The center slant distance difference between the first channel and the second channel.

[0107] Here, the principle of constructing the channel compensation filter using the series inversion method is as follows:

[0108] In order to distinguish it from the moving target, any ground stationary clutter target point in the imaging scene (i.e. any stationary target point in the imaging scene) is set as , express The X coordinate of the point, express The Y coordinate of the point, and The geometric relationship between the point and the sampling center of the first channel and the second channel of the transmitter and receiver is as follows: Figure 6 shown. Figure 6 middle, and Respectively represent the transmitter positions at the slow time sampling center of the first channel and the second channel; similarly, and represent the receiver positions at the slow time sampling center of the first channel and the second channel respectively, and Respectively and The X-direction spacing and Y-direction spacing between and Respectively and The Y-direction spacing and Z-direction spacing between. The difference in center slope distance between the first and second channels of a point is: (26). However, the signal focus plane is in the range Doppler domain. To construct the channel compensation function (i.e., channel compensation filter), it needs to be projected to the focus position. Assume that the center point of the scene is The focus position is the image center, and the above The focus position of the point in the SAR image is ,but It can be written as: (27), among which, and About the center point of the scene of and Known and All are about the ground plane coordinates function, so it can be solved by formula (27) arrive The mapping of and However, in formula (27) It is expressed in the form of double root, as shown in the above formulas (1) and (6), which makes it difficult to obtain an analytical expression for the inverse function of formula (27). The present invention proposes to use the binary series inversion method to solve the binary inverse function of formula (27) and obtain arrive The mapping relationship of the channel compensation filter is obtained. An expression in the signal domain.

[0109] First, formula (27) is used at the center of the scene The two-variable Taylor series expansion at can be written as:

[0110] (28);

[0111] (29);

[0112] (30);

[0113] (31);

[0114] (32);

[0115] (33);

[0116] (34);

[0117] (35);

[0118] (36);

[0119] (37);

[0120] (38);

[0121] (39);

[0122] (40);

[0123] (41);

[0124] in, and Represents the speed of the transmitter The components on the X and Y axes, and Respectively represent the speed of the receiver Components on the Z and Y axes.

[0125] The series expansion of the inverse function of formula (27) can be set as:

[0126] (42);

[0127] in, and All by and Substituting formula (42) into formula (28) yields:

[0128] (43);

[0129] To make the left and right sides of formula (43) equal, the system of equations can be solved as follows:

[0130] (44);

[0131] (45);

[0132] (46);

[0133] (47);

[0134] (48);

[0135] (49);

[0136] (50);

[0137] (51);

[0138] (52);

[0139] (53);

[0140] Combining equations (26) and (42) yields arrive The mapping relationship is based on Can be constructed To compensate the second channel complex image. That is to say, the series inversion method is used to construct The steps can be summarized as follows:

[0141] S1. Construct any stationary target point in the imaging scene The expression for the center slant distance difference between the first channel and the second channel is the above formula (26);

[0142] S2, with the center of the beam pointing to a point on the ground When the focus position is the image center, construct Point at the focus position of the SAR image The expression of is formula (27);

[0143] S3. At the center of the scene The binary Taylor series expansion at , we get the expansion, which is the above formula (28);

[0144] S4. Construction The two-variable Taylor series expansion of the inverse function of the expression of is the above formula (42);

[0145] S5. Substitute the series expansion into the above expansion to solve it, and according to the solution, The expression and series expansion of the central slant distance difference between the first and second channels of a point gives The difference in center slope distance between the first and second channels of a point The value of

[0146] S6. According to The difference in center slope distance between the first and second channels of a point The value of expression.

[0147] The error of the channel compensation filter is analyzed as follows:

[0148] Consider a square imaging scene (i.e., imaging area) with a side length of 2 km. Arrange a matrix of points in the scene and calculate the corresponding and focus position , the true value can be obtained . According to the focus position of each point , the fitting value is calculated by series inversion method Therefore, the phase error of the channel compensation filter (i.e., channel compensation function) is It can be written as: (54), among which, The distribution in the scene is as follows Figure 7 As shown, obviously, The maximum value of In addition, when When it is small, it can be approximated as the clutter suppression ratio. The channel compensation function obtained by the series inversion method can make the clutter suppression ratio reach 24.2dB, which can achieve a good clutter suppression effect. For example, by Figure 8 and Fig. 9 Describe the effect. Figure 8 The first channel complex image and the second channel complex image before clutter suppression. Obviously, due to the strong ground clutter, the moving target cannot be detected; Fig. 9 This is a SAR image after ground clutter suppression and moving target detection. Obviously, after clutter suppression, the ground moving target forms an isolated strong point in the SAR image, and the moving target can be detected using the traditional target detection method; specifically, Fig. 9 The small white dots circled by the two small rectangles in the figure are two different ground moving targets, and the two large rectangles are magnified images of the two different circled ground moving targets.

[0149] The method provided by the present invention adopts azimuth de-skewing and wedge transformation based on line frequency modulation transformation, which can achieve fast imaging while maintaining accuracy. In order to better apply it in engineering, the computational complexity of the method provided by the present invention is analyzed as follows:

[0150] Assume that the echo signal distance points are , the number of azimuth points is , the number of complex multiplications of a distance fast Fourier (inverse) transform is , the number of complex multiplications of an azimuth fast Fourier (inverse) transform is , the number of complex multiplications in a filter complex multiplication is The wedge transform based on line frequency modulation does not require interpolation, which can greatly improve the computational efficiency. After the signal is extracted by odd and even numbers, assuming that the number of azimuth sampling points after azimuth zero filling is , after the wedge transformation, the number of azimuth points is restored to Therefore, the number of complex multiplications required to calculate the line frequency modulation transform of the extracted two-channel signal is: (55), among which, To satisfy The smallest positive integer that is an integer power of 2. Assume that the number of complex multiplications for complex image cancellation is , then the number of all complex multiplications of the method provided by the present invention is: (56), the number of zeros in the orientation is generally small, so we set , , then formula (56) can be simplified to: (57). Therefore, the total amount of computation of the method provided by the present invention is .

[0151] The method provided by the present invention has the following advantages:

[0152] 1. In the imaging process, the method provided by the present invention first performs azimuth de-skewing on the demodulated echo signal to compress the azimuth spectrum; secondly, an azimuth spectrum correction filter is constructed to perform consistent range migration correction and azimuth spectrum shifting with the parameters of the scene center point, thereby shortening the signal azimuth spectrum and moving the signal to the baseband, avoiding spectrum aliasing and blurring; finally, a wedge transform based on line frequency modulation azimuth spectrum resampling is performed on the echo signal, so that the signal is directly focused in the azimuth frequency domain without the need for azimuth inverse Fourier transform, thereby efficiently and uniformly processing the residual linear range migration of targets with unknown motion parameters. Then, a range inverse Fourier transform is performed to focus the signal upward in the range, completing the focusing of the energy of ground clutter and moving targets in the imaging scene. Compared with the existing dual-base forward-looking SAR imaging method, which uses the parameters of the known target or estimates the motion parameters of the target in a clutter-free background to perform imaging, the method proposed by the present invention can be applied to the imaging of moving targets with unknown motion parameters, is not affected by the clutter environment, and has lower computational complexity.

[0153] 2. The present invention extracts the signal while imaging and focusing the echo signal obtained by a single antenna, and divides it into a first channel complex image and a second channel complex image. Then, based on the analysis of the phase of the first channel complex image and the second channel complex image, a phase compensation filter is constructed by using the series inversion method to phase compensate the two channel images and then cancel the ground clutter. Therefore, the method proposed by the present invention can suppress clutter in the signal with a strong ground clutter background, so that the moving target in the image can be detected. However, the existing single-antenna moving target detection method based on parameter estimation needs to detect and estimate the moving target signal in the absence of clutter, and the method will fail in an environment where clutter actually exists. In addition, the existing ground clutter suppression method based on multiple antennas and multiple channels requires multiple antennas to be arranged along the heading. For the bistatic forward-looking SAR, the hardware cost is large and the space occupied is large. It cannot be applied to platforms such as small aircraft, missiles, fighters and drones, and the scope of application is limited. The method proposed by the present invention can overcome the defects of the above existing methods, and the existing single-antenna bistatic SAR system can be used to complete ground clutter suppression and moving target imaging detection.

[0154] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0155] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0156] In the specification, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude a plurality of components. Certain measures are recorded in different embodiments, but this does not mean that these measures cannot be combined to produce good effects.

[0157] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, 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 protection scope of the present invention.

Claims

1. A single-antenna bistatic forward-looking SAR ground target series inversion imaging method, characterized in that: Applied to a single-antenna receiver, the method comprises: Receive radar echo signals from the imaging scene, which contain ground clutter and moving target information; Performing range pulse compression, azimuth de-skewing and azimuth spectrum correction on the received echo signal to obtain a filtered signal; Performing odd-even extraction on the filtered signal along the azimuth direction to obtain an odd-numbered first channel signal and an even-numbered second channel signal; Performing line frequency modulation conversion on the first channel signal to focus the first channel signal in a range Doppler domain, thereby obtaining a converted first channel signal; Performing line frequency modulation conversion on the second channel signal to focus the second channel signal in the range Doppler domain to obtain a converted second channel signal; Determine a first channel complex image and a second channel complex image according to the transformed first channel signal and the transformed second channel signal; performing ground clutter cancellation based on the first channel complex image and the second channel complex image to obtain a ground clutter cancelled signal containing the moving target information; The imaging result of the moving target is detected according to the signal after the ground clutter is cancelled.

2. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 1, characterized in that: The determining of the first channel complex image and the second channel complex image according to the transformed first channel signal and the transformed second channel signal comprises: Performing range inverse FFT on the transformed first channel signal to obtain a first channel complex image; Performing range inverse FFT on the transformed second channel signal to obtain a second channel complex image.

3. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 1, characterized in that: The performing ground clutter cancellation based on the first channel complex image and the second channel complex image to obtain a ground clutter cancelled signal containing the moving target information includes: Using a channel compensation filter constructed using a series inversion method to compensate the phase of the second channel complex image to obtain a compensated second channel complex image; The first channel complex image and the compensated second channel complex image are cancelled to obtain a ground clutter cancelled signal containing the moving target information.

4. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 1, characterized in that: The step of performing range pulse compression, azimuth de-skewing and azimuth spectrum correction on the received echo signal to obtain a filtered signal includes: Demodulating the received echo signal to obtain a demodulated echo signal; Performing range-domain Fourier transform on the demodulated echo signal to obtain a frequency domain signal; A two-dimensional filter including a range pulse compression filter, an azimuth de-skew filter and an azimuth spectrum correction filter is used to simultaneously perform range pulse compression, azimuth de-skew and azimuth spectrum correction on the frequency domain signal to obtain the filtered signal.

5. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 3, characterized in that: The expression of the signal after ground clutter cancellation is as follows: ； ； ； ； ； ； in, represents the signal after the ground clutter is cancelled, represents the first channel complex image, represents the second channel complex image, represents the channel compensation filter, Indicates fast time, Indicates the azimuth focus position, represents the synthetic aperture time, Indicates slow time The frequency domain variable, The value of is 1~4. express of Derivatives, Represents the center point of the scene The value of represents the carrier wavelength, represents the distance bandwidth of the signal, express When the slope distance of the moving target is 0, represents the residual phase of the moving target after channel compensation, represents the speed of light, Represents an imaginary unit.

6. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 3 or 5, characterized in that: The expression of the channel compensation filter is as follows: ； in, represents the channel compensation filter, Indicates that the independent variable and Indicated , Represents any stationary target point in the imaging scene The center slant distance difference between the first channel and the second channel, Indicates fast time, Indicates slow time The frequency domain variable, Indicates the carrier wavelength.

7. The single-antenna bistatic forward-looking SAR ground target series inversion imaging method according to claim 6, characterized in that: Said The expression is as follows: ； ； ； ； ； ； ； ； ； ； ； ； ； ； ； in, Indicates the first channel signal corresponds to , Indicates the second channel signal corresponding to , express When the slope distance of the moving target is 0, and All by and At least two different values ​​of and They respectively represent the point under the transmitter for transmitting the radar echo signal in a Cartesian right-hand coordinate system established with the point under the receiver as the origin. The X and Y coordinates of represents the Y coordinate of the point on the ground where the beam center points in the Cartesian right-hand coordinate system, represents the flight altitude of the receiver, represents the flight altitude of the transmitter, represents the Y coordinate of the origin of the Cartesian right-hand coordinate system, and Respectively and The X-direction spacing and Y-direction spacing between and Respectively and The Y-direction spacing and Z-direction spacing between and represent the transmitter positions at the slow time sampling center of the first and second channels respectively, and represent the receiver positions at the slow time sampling center moments of the first channel and the second channel respectively, and Respectively represent any stationary target point in the imaging scene The X and Y coordinates of and Respectively represent the speed of the transmitter The components on the X and Y axes, and Respectively represent the speed of the receiver The components on the Z and Y axes, represents the carrier wavelength, Represents the stationary target point At the focus position of the SAR image.

8. A single-antenna bistatic forward-looking SAR ground target level inversion imaging device, comprising a processor, a communication interface, a memory and a communication bus, characterized in that: The processor, the communication interface and the memory communicate with each other via the communication bus; The memory is used to store computer programs; The processor is used to implement the method steps described in any one of claims 1-7 when executing the program stored in the memory.

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