OAM foresight imaging method based on time-varying mode

Abstract

The invention discloses an OAM foresight imaging method based on a time-varying mode, the method is used for imaging a vortex wave foresight radar in the time-varying mode for the first time, and the implementation scheme is as follows: on the basis of three-dimensional echoes, firstly, distance compression is realized by a matched filtering method; under the system, the radar emits N pulse periods in one mode, and after a certain pulse period, the radar is switched to the next mode; therefore, the synthetic aperture echo is decomposed into a plurality of sub-aperture echoes, and the mode of the echoes in each sub-aperture is kept unchanged; and then target images in different modes are obtained in a distance-pitch dimension by using a BP algorithm. And finally, performing azimuth angle compression on the modal dimension by using an FFT (Fast Fourier Transform) method, and finally realizing three-dimensional imaging of the target. According to the invention, the resolution can be improved, and the problem of left-right blurring in foresight radar imaging can be solved.

Description

Technical Field

[0001] The invention belongs to the technical field of radar imaging, and in particular relates to an OAM forward-looking imaging method based on a time-varying mode. Background Art

[0002] Vortex electromagnetic waves are one of the hot topics in electromagnetic research in recent years, especially in the field of radar imaging. Electromagnetic waves not only have spin angular momentum, but can also carry orbital angular momentum. As a new multiplexing dimension, orbital angular momentum has been widely used in communication, radar, perception and other systems. In radar imaging technology, vortex electromagnetic waves are combined with synthetic aperture radar technology to solve the problem of antenna aperture limitation in traditional radar imaging. By utilizing the movement of the carrier platform in the azimuth direction, the synthesis of the virtual aperture can achieve a higher azimuth resolution than the real aperture. In addition, the combination of vortex electromagnetic wave technology with new radar imaging systems such as inverse synthetic aperture radar, interferometric synthetic aperture radar and multi-input multi-output synthetic aperture radar has achieved remarkable imaging effects.

[0003] Although vortex electromagnetic wave technology shows great application potential in radar imaging, existing research methods still face some challenges. In forward-looking radar imaging, especially forward-looking three-dimensional imaging algorithms under large viewing angles are not yet mature. Current research usually transmits multiple modes within a pulse repetition period, regardless of whether these modes are transmitted simultaneously or sequentially, but this method not only increases the demand for computing memory, but also is difficult to implement in actual engineering due to the complexity of mode switching. Therefore, how to effectively optimize the application of vortex electromagnetic waves in radar imaging is still a technical problem that needs to be solved urgently in this field. Summary of the invention

[0004] The purpose of the present invention is to provide an OAM forward-looking imaging method based on a time-varying mode, so as to solve the problem of left-right blurring in forward-looking imaging and low resolution in the direction of a target approaching a track.

[0005] The technical solution to achieve the purpose of the present invention is: a method for performing OAM forward-looking imaging in a time-varying mode, comprising the following steps:

[0006] Step 1: Perform beam scanning in a time-varying mode to obtain three-dimensional echo data;

[0007] Step 2: Realize distance-up pulse compression by matched filtering method;

[0008] Step 3: In the back-projection process, the imaging of the time-varying mode is converted into the imaging process of the mode-invariant mode, and the entire synthetic aperture is divided into N sub-apertures, and the mode of each sub-aperture remains unchanged;

[0009] Step 4: Generate an image in the range-elevation dimension using the BP algorithm in each sub-aperture;

[0010] Step 5: After the range and elevation compression, use the FFT method to compress the echo in azimuth to obtain a three-dimensional image of the target.

[0011] Furthermore, in step 1, beam scanning is performed in a time-varying mode to obtain three-dimensional echo data. The radar transceiver antenna adopts a uniform circular array, the imaging mode adopts a multi-transmit and multi-receive mode, and the center frequency of the transmitted signal is f c , bandwidth is B, mode range is [l min ,l max ].

[0012] Furthermore, the step 2 implements range-up pulse compression by a matched filtering method; uses a Gaussian filter to eliminate the effect of range-up frequency modulation; and upsamples the signal to expand the signal by inserting zero values ​​in the middle of the signal to improve image resolution.

[0013] Furthermore, in step 3, the imaging of the time-varying pattern is converted into the imaging process of the pattern invariant during the back-projection process, and the entire synthetic aperture is divided into N sub-apertures, and the mode of each sub-aperture remains unchanged; the vortex electromagnetic wave pattern emitted by the radar changes with slow time, and the mode is switched once every L pulse periods, and a total of N×L pulse repetition intervals PRI are passed; the starting mode is l 0 , then the mode at any moment can be expressed as

[0014]

[0015] where |·| represents the modulo operation, i represents the number of pulse cycles, and t s Represents slow time.

[0016] Further, in step 4, the BP algorithm is used in each sub-aperture to generate an image in the range-elevation dimension; when the back-projection algorithm is performed in the time-varying mode, the radial and rotational Doppler phases are compensated at the same time to eliminate the Doppler phase difference that varies in the echo; the Doppler phase that varies in the echo is:

[0017]

[0018] Taking the derivative of equation (2) we can get the total Doppler frequency f d As shown below:

[0019]

[0020] where r P is the instantaneous slant range between the target and the radar, l represents the mode number, and the target pitch angle θ 0 is the slope distance r 0 The angle between the direction and the positive z-axis direction, represents the target azimuth, λ is the wavelength, v a is the radar operating speed.

[0021] An electronic device comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the steps of the above method are implemented when the processor executes the program.

[0022] A computer-readable storage medium stores a computer program, which implements the steps of the above method when executed by a processor.

[0023] A computer program product comprises a computer program, which implements the steps of the above method when executed by a processor.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) Eliminate the left-right blur phenomenon in forward-looking SAR imaging by improving the traditional forward-looking OAM imaging method;

[0026] (2) The SAR imaging resolution capability is improved by improving the traditional forward-looking OAM imaging method. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is the time-varying mode imaging scene used in the present invention.

[0028] Figure 2 It is a schematic diagram of the sub-aperture segmentation method used in the present invention.

[0029] Figure 3 It is a comparison between single mode and time-varying mode imaging simulated by the method of the present invention.

[0030] Figure 4 This is three-dimensional SAR imaging in a multi-target time-varying mode simulated by the method of the present invention. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention but not to limit the present invention.

[0032] The present invention proposes an OAM forward-looking imaging method based on a time-varying mode, the steps of which are as follows:

[0033] Step 1: Perform beam scanning in a time-varying mode to obtain three-dimensional echo data. The specific implementation method is as follows:

[0034] Time-varying pattern imaging scenarios such as Figure 1As shown in the figure, compared with traditional forward-looking SAR imaging, the radar does not transmit and receive echoes of multiple modes in the same pulse period, but transmits vortex electromagnetic waves of multiple pulse periods in a single mode, and switches the mode after a certain number of cycles. The echo signal can be expressed as:

[0035]

[0036] Among them, t f represents fast time, t s Represents slow time. 0 is the target scattering coefficient, ω tf Represents the range pulse envelope. r(t s ) is the distance from the center of the synthetic aperture to the target point in the imaging scene. l represents the number of modes, J l (·) is the first-order Bessel function of the first kind, k is the wave number, a is the radius of the circular array, and the target pitch angle θ is the angle between the slant range r direction and the z positive semi-axis direction. Target azimuth is the angle between the projection of the target slant range on the xoy plane and the direction of the positive x-axis (real numbers on this semi-axis are positive numbers). K represents the frequency modulation slope, c represents the speed of light, and f c Represents the carrier frequency.

[0037] Step 2: Realize distance-up pulse compression by matched filtering method; the specific implementation method is as follows:

[0038] The echo signal after distance compression is in the form of:

[0039]

[0040] Among them, p r (t) = |K|Tsinc(|K|T·t) is the range envelope, and T is the pulse width.

[0041] Step 3: To achieve target focusing, the imaging of the time-varying pattern should be transformed into the imaging process of the pattern invariant during the back-projection process, and the entire synthetic aperture is divided into N sub-apertures, such as Figure 2 As shown, the mode of each sub-aperture remains unchanged; the specific implementation method is as follows:

[0042] The vortex electromagnetic wave pattern emitted by the radar changes with time. The mode is switched every L pulse cycles. A total of N×L pulse repetition intervals PRI are passed. The starting mode is l 0 , then the above relationship can be expressed as:

[0043]

[0044] Here, |·| represents a modulo operation, and i represents the number of pulse cycles that have passed.

[0045] Now the entire synthetic aperture is cut into multiple sub-apertures. The pattern remains unchanged within a single aperture, and each sub-aperture can produce an image. The xoz face sub-aperture divides the image Figure 2 shown.

[0046] Take the front, middle and back sub-aperture examples respectively, and the target P(x 0 ,y 0 ,z 0 ) and the elevation angles at the aperture centers of the three ends are θ 0 ,θ 1 ,θ 2 And satisfy the relationship θ 0 <θ 1 <θ 2 The instantaneous slant range between the target and the radar is r P and synthetic aperture length L a Satisfies the following relationship L a <<r P The viewing angle of each sub-aperture relative to the target changes slightly, and the above relationship can be approximated as θ 0 ≈θ 1 ≈θ 2 A total of N modes of vortex electromagnetic waves are emitted, and the length of each sub-aperture is L sub =L a / N, radar operating speed v a , subaperture time T sub =L sub / v a .

[0047] Step 4: Generate an image in the range-elevation dimension using a back-projection algorithm in each sub-aperture; the specific implementation method is as follows:

[0048] After each sub-aperture echo is compressed, back-projection imaging is performed. Assuming that the radar is located at z = 0 in the scene at the start time, the single aperture echo signal of the transmission mode l can be expressed as:

[0049]

[0050] The distance history under each sub-aperture is as follows L start Represents the starting point of each sub-aperture.

[0051] Different sub-aperture starting points will cause migration in the range direction, and the migration distance will cause the pitch angle to change, so it is necessary to eliminate the migration phenomenon in the imaging results of different apertures. Under the premise that the synthetic aperture trajectory is known, adding the starting points of different sub-apertures to the phase compensation process of the back-projection algorithm can eliminate the migration phenomenon of the imaging results.

[0052] Let z' = Lstart +v a t s , formula (4) can be rewritten as:

[0053]

[0054] The coordinates of a single grid unit Q in the image domain in the scene coordinate system are r t is the slant range from a single grid cell Q to the radar, θ t is the elevation angle from a single grid unit Q to the radar, is the azimuth from a single grid unit Q to the radar. The appropriate compensation function is selected during the phase compensation of the back projection as follows:

[0055] H(t s )=exp(j4πr Q (t s ) / λ) (6)

[0056] in λ is the wavelength. The image generated by the accumulation after phase compensation is consistent with the preset coordinates, and the imaging results of each sub-aperture are the same.

[0057] Now the imaging results of different sub-apertures are arranged in order, which is the order of mode switching. The echoes of different modes after range and elevation compression are as follows:

[0058]

[0059] where ω l (·) represents a rectangular window function.

[0060] Step 5: After the range and elevation compression, the echo is compressed in azimuth using the FFT method to obtain a three-dimensional image of the target. The specific implementation method is as follows:

[0061] Since the modes between each sub-aperture remain independent, the azimuth compression is performed after the pitch imaging results of each sub-aperture are corrected. Since the target azimuth of each sub-aperture remains unchanged, the echoes between each sub-aperture, i.e., between each mode, are directly processed after the echoes are rearranged to determine the target azimuth and achieve three-dimensional imaging. Temporarily ignoring the modulation effect of the Bessel function, the final three-dimensional compressed echo result is:

[0062]

[0063] Among them l maxIndicates the maximum mode. Since the time-varying mode SAR cuts the synthetic aperture into multiple small sub-apertures, the sub-aperture brings lower elevation resolution than the single-mode, but the single-mode SAR cannot achieve azimuth imaging. In order to achieve three-dimensional imaging, it is necessary to obtain multiple sets of single-mode SAR results. Therefore, sub-aperture forward-looking three-dimensional imaging sacrifices elevation resolution in exchange for azimuth resolution.

[0064] Example 1

[0065] In order to verify the anti-blurring capability of the method proposed in the present invention, the xz two-dimensional plane with y=30m is selected as the imaging area. The z direction is defined as the track direction, and the x direction is defined as the cross-track direction. Figure 3 To achieve the two-dimensional imaging result of the present invention, when the synthetic aperture length is L a =10m, mode range is -32~32, target pitch angle θ 0 =0.68°, the target P (x=18m, y=30m, z=999.3m) located on the right side of the track is imaged using single mode and time-varying mode. The radar parameters are shown in Table 1. Figure 3 As shown in (a), single-mode imaging will produce symmetrical artifacts and poor resolution performance, making it difficult to determine the real target in the image. Figure 3 (b) It can be seen that compared with the single-mode imaging results, the time-varying mode imaging not only eliminates the left-right blur phenomenon but also improves the resolution performance. When the synthetic aperture length is increased to 50m, Figure 3 (c) shows that the real target is separated from the artifact in the single-mode imaging result, but Figure 3 (b) The resolution performance is still poor.

[0066] Table 1 Forward-looking SAR imaging parameters

[0067]

[0068] Example 2

[0069] In order to verify the effectiveness of the three-dimensional imaging method proposed in the present invention, the total synthetic aperture length is set to 99m, the mode range is -16 to 16, and the mode is switched every 160 pulses in the pulse repetition interval. The distance grid range is 3950m to 4050m, and the grid length is set to 0.1m. The pitch range is 0° to 40°, the grid length is set to 0.5°, and the other parameters are consistent with Table 1. 27 point targets in space are simulated. The target at the center position and its adjacent target are 5m apart in distance, 20° apart in azimuth, and 5° apart in pitch. The imaging results are shown in the figure. Figure 4 shown. Figure 4 (a) is the three-dimensional imaging result in space, Figure 4 (b)~ Figure 4 (d) Correspondence Figure 4 (a) The cross-sectional images of different dimensions. The method of the present invention can correctly obtain the positions of 27 point targets, thus proving the effectiveness of the present invention.

Claims

1. A forward-looking OAM imaging method based on a time-varying pattern, characterized in that: The steps include: Step 1: Perform beam scanning in a time-varying mode to obtain three-dimensional echo data; Step 2: Realize distance-up pulse compression by matched filtering method; Step 3: In the back-projection process, the imaging of the time-varying mode is converted into the imaging process of the mode-invariant mode, and the entire synthetic aperture is divided into N sub-apertures, and the mode of each sub-aperture remains unchanged; Step 4: Generate an image in the range-elevation dimension using the BP algorithm in each sub-aperture; Step 5: After the range and elevation compression, use the FFT method to compress the echo in azimuth to obtain a three-dimensional image of the target.

2. The OAM forward-looking imaging method based on a time-varying mode according to claim 1, characterized in that: In step 1, beam scanning is performed in a time-varying mode to obtain three-dimensional echo data. The radar transceiver antenna adopts a uniform circular array, the imaging mode adopts a multi-transmit and multi-receive mode, and the center frequency of the transmitted signal is f c , bandwidth is B, mode range is [l min ,l max ].

3. The OAM forward-looking imaging method based on a time-varying mode according to claim 1, characterized in that: The step 2 is to realize the range-up pulse compression by the matched filtering method; adopt the Gaussian filter to eliminate the effect of range-up frequency modulation; upsample the signal and expand the signal by inserting zero value in the middle of the signal.

4. The OAM forward-looking imaging method based on a time-varying mode according to claim 1, characterized in that: The process of converting the imaging of the time-varying mode into the imaging process of the mode-invariant mode in the back-projection process described in step 3 divides the entire synthetic aperture into N sub-apertures, and the mode of each sub-aperture remains unchanged; The vortex electromagnetic wave pattern emitted by the radar changes with slow time, and the mode is switched every L pulse cycles, and a total of N×L pulse repetition intervals PRI are passed; the starting mode is l0, so the mode at any moment is expressed as where |·| represents the modulo operation, i represents the number of pulse cycles, and t s Represents slow time.

5. The OAM forward-looking imaging method based on time-varying mode according to claim 1, characterized in that: In step 4, the BP algorithm is used in each sub-aperture to generate an image in the range-elevation dimension; when the back-projection algorithm is performed in the time-varying mode, the radial and rotational Doppler phases are compensated at the same time to eliminate the Doppler phase difference that varies in the echo; the Doppler phase that varies in the echo is: Taking the derivative of equation (2) we can get the total Doppler frequency f d As shown below: where r P is the instantaneous slant range between the target and the radar, l represents the mode number, and the target elevation angle θ0 is the angle between the slant range r0 and the z positive semi-axis direction. represents the target azimuth, λ is the wavelength, v a is the radar operating speed.

6. An electronic device comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the program, the steps of the method according to any one of claims 1 to 5 are implemented.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

8. A computer program product, comprising a computer program, characterized in that When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 5 are implemented.

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