An OAM forward-looking imaging method based on time-varying pattern

Abstract

The application discloses an OAM forward-looking imaging method based on time-varying modes, which firstly realizes imaging of a vortex wave front-view radar in a time-varying mode, and the implementation scheme is as follows: on the basis of three-dimensional echo, distance compression is firstly realized by a matched filtering method; under the system, the radar transmits N pulse periods in one mode, and switches to the next mode after a certain pulse period; therefore, synthetic aperture echo is decomposed into multiple sub-aperture echoes, and the mode of the echo in each sub-aperture remains unchanged; then, a target image of different modes is obtained in the range-pitch dimension by using a BP algorithm. Finally, the azimuth compression is performed in the mode dimension by using an FFT method, and finally, three-dimensional imaging of the target is realized. The application can not only improve the resolution, but also solve the left-right ambiguity problem in the forward-looking radar imaging.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of radar imaging, and particularly relates to an OAM forward-looking imaging method based on time-varying modes. BACKGROUND

[0002] Vortex electromagnetic waves are one of the hotspots in the field of electromagnetism in recent years, especially in the field of radar imaging. Electromagnetic waves not only have spin angular momentum, but also can carry orbital angular momentum. As a new multiplexing dimension, orbital angular momentum has been widely used in communication, radar, sensing and other systems. In the field of 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 using the movement of the carrier platform in the azimuth direction, the synthesis of virtual aperture can achieve higher azimuth resolution than the real aperture. In addition, vortex electromagnetic wave technology is combined with new radar imaging systems such as inverse synthetic aperture radar, interferometric synthetic aperture radar and multiple-input multiple-output synthetic aperture radar, and significant imaging results have been achieved.

[0003] Although vortex electromagnetic wave technology has shown great application potential in radar imaging, the existing research methods still face some challenges. In forward-looking radar imaging, especially in forward-looking three-dimensional imaging algorithms under large viewing angles, there is still no mature method. The current research usually transmits multiple modes within one pulse repetition period, 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 to be solved in this field. SUMMARY

[0004] The purpose of the present application is to provide an OAM forward-looking imaging method based on time-varying modes, which solves the problems of left-right blurring and low resolution of targets close to the track direction in forward-looking imaging.

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

[0006] Step 1: Obtain three-dimensional echo data by beam scanning under the time-varying mode system;

[0007] Step 2: Realize pulse compression in the range direction by a matched filter method;

[0008] Step 3: In the backward projection process, convert the imaging of the time-varying mode into an imaging process with invariant mode, and divide the entire synthetic aperture into N sub-apertures, and the mode of each sub-aperture remains unchanged;

[0009] Step 4: Generate a range-pitch dimension image in each sub-aperture by using a BP algorithm.

[0010] Step 5, after the pitch compression, the echo is compressed in azimuth direction using FFT method, and the target three-dimensional imaging is obtained.

[0011] Further, the step 1 is to obtain three-dimensional echo data by beam scanning under the time-varying mode system, the radar transceiver antenna adopts a uniform circular array, the imaging mode adopts a multi-transmission and multi-reception mode, the center frequency of the transmitted signal is f c , the bandwidth is B, and the mode range is [l min , l max ].

[0012] Further, the step 2 is to realize the pulse compression in the range direction by the matched filter method; a Gaussian-shaped filter is used to eliminate the effect of frequency modulation in the range direction; the signal is up-sampled by inserting zero values in the middle of the signal to expand the signal and improve the image resolution.

[0013] Further, the step 3 is to convert the time-varying mode imaging into a mode-invariant imaging process in the back-projection process, and the entire synthetic aperture is divided into N sub-apertures, the mode of each sub-aperture remains unchanged; the mode of the radar transmitted vortex electromagnetic wave changes with the slow time, the mode is switched once every L pulse periods, and a total of N×L pulse repetition intervals PRI are passed; the initial mode is l0, and the mode at any time can be represented as

[0014]

[0015] Where |·| represents the modulo operation, i represents the number of pulse periods passed, t s represents the slow time.

[0016] Further, the step 4 is to generate the range-pitch dimension image in each sub-aperture using the BP algorithm; 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 changing Doppler phase difference in the echo; the changing Doppler phase in the echo is:

[0017]

[0018] The total Doppler frequency f d can be obtained by derivation of formula (2) as follows:

[0019]

[0020] Where r P is the instantaneous slant range of the target and the radar, l represents the mode number, the target pitch angle θ0 is the included angle between the slant range r0 and the positive z-axis direction, represents the target azimuth angle, λ is the wavelength, and va The radar operates at a speed.

[0021] An electronic device comprising a memory, a processor and a computer program stored on the memory and executable on the processor, the processor implementing the steps of the method when executing the program.

[0022] A computer-readable storage medium having stored thereon a computer program, the program implementing the steps of the method when executed by a processor.

[0023] A computer program product comprising a computer program, the computer program implementing the steps of the method when executed by a processor.

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

[0025] (1) By improving the traditional forward-looking OAM imaging method, the left-right blurring phenomenon in forward-looking SAR imaging is eliminated;

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

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

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

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

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

[0031] The present application will be further described in detail below with reference to the accompanying drawings, and it should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

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

[0033] Step 1, obtain three-dimensional echo data by beam scanning under the time-varying mode system.

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

[0035]

[0036] Where t f represents fast time, t s represents slow time. σ0 is the target scattering coefficient, ω tf represents the distance pulse envelope. r(t s ) is the distance history between the synthetic aperture center and the target point in the imaging scene. l represents the mode number, J l (·) is the first-order l-th Bessel function, k is the wave number, a is the radius of the circular array, and the target elevation angle θ is the angle between the slant range r and the z positive half-axis direction. The target azimuth angle is the angle between the projection of the target slant range on the xoy plane and the x positive half-axis (the half-axis on which the real number is positive). K represents the frequency modulation slope, c represents the speed of light, and f c represents the carrier frequency.

[0037] Step 2, realize pulse compression in the range direction by matched filtering method; the specific implementation method is as follows:

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

[0039]

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

[0041] Step 3, to realize target focusing, the imaging of time-varying mode should be converted into the imaging process of mode-invariant in the backward projection process. The entire synthetic aperture is divided into N sub-apertures, as shown in Figure 2 , the mode of each sub-aperture remains unchanged; the specific implementation method is as follows:

[0042] The mode of the vortex electromagnetic wave transmitted by the radar changes with time, and the mode is switched every L pulse period. A total of N×L pulse repetition intervals PRI are passed, and the starting mode is l0. The above relationship can be represented as:

[0043]

[0044] Where |·| represents the modulo operation, and i represents the number of pulse periods passed.

[0045] The whole synthetic aperture is cut into multiple sub-apertures, the mode keeps unchanged in single aperture, each sub-aperture can generate an image, and the sub-aperture division image in xoz plane Figure 2 is shown.

[0046] Taking the front, middle and rear sub-aperture as examples, the target P(x0, y0, z0) has the pitch angles θ0, θ1 and θ2 at the centers of the three sub-apertures, and satisfies the relationship θ0< θ1< θ2. The instantaneous slant range r P of the target to the radar is a The length L a of the synthetic aperture satisfies the relationship L P <<r sub . The view angle of each sub-aperture to the target changes little, and the above relationship can be approximated as θ0≈θ1≈θ2. The radar transmits N modes of vortex electromagnetic waves in total, and the length of each sub-aperture is L a = L a / N, the running speed of the radar is v sub , and the time of each sub-aperture is T sub = L a / v .

[0047] Step 4, the backward projection algorithm is used in each sub-aperture to generate the image in the range-pitch dimension; the specific implementation method is as follows:

[0048] The echo of each sub-aperture is compressed in range and then imaged by the backward projection. Assuming that the radar is located at z=0 in the scene at the initial time, the echo signal of a single aperture of mode l can be expressed as:

[0049]

[0050] wherein the range history under each sub-aperture is as follows L start represents the starting point of each sub-aperture.

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

[0052] Let z' = L start +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 tis the slant range of single grid unit Q to radar t is the elevation angle of single grid unit Q to radar is the azimuth angle of single grid unit Q to radar. The appropriate compensation function is selected in the phase compensation of back-projection as follows:

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

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

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

[0058]

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

[0060] Step 5, after range-elevation compression, the echoes are compressed in the azimuth direction by using the FFT method, and three-dimensional imaging of the target is obtained; the specific implementation method is as follows:

[0061] Since the modes between the sub-apertures maintain independence, after correcting the range-elevation imaging results of each sub-aperture, the azimuth compression is performed. Since the azimuth angle of the target does not change, the echoes between each sub-aperture, i.e., between each mode, are directly processed after rearranging the echoes, so as to determine the azimuth angle of the target and realize three-dimensional imaging. Without considering the modulation effect of the Bessel function for the time being, the final three-dimensional compressed echo result is as follows:

[0062]

[0063] wherein l max represents the maximum mode. Since the time-varying SAR cuts the synthetic aperture into multiple small sub-apertures, the elevation resolution brought by the sub-aperture is lower than that of the single mode, but the single mode SAR cannot realize azimuth angle imaging. In order to realize three-dimensional imaging, multiple groups of single mode SAR results must be obtained, so the sub-aperture forward-looking three-dimensional imaging is realized by sacrificing the elevation resolution in exchange for the azimuth resolution.

[0064] Embodiment 1

[0065] In order to verify the anti-aliasing ability of the method proposed in the application, an x-z 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 3To achieve two-dimensional imaging results in this invention, a synthetic aperture length of L is used. a Under the conditions of a target elevation angle of θ0 = 0.68°, a radar depth of 10m, a mode range of -32 to 32m, and a target elevation angle of θ0 = 0.68°, imaging of a target P (x = 18m, y = 30m, z = 999.3m) located to the right of the track was performed using both single-mode and time-varying mode methods. The radar parameters are shown in Table 1. Figure 3 As shown in (a), single-mode imaging suffers from symmetrical artifacts and poor resolution, making it difficult to determine the true target in the image. Figure 3 (b) It can be seen that, compared with single-mode imaging results, time-varying mode imaging not only eliminates left-right blurring but also improves resolution. When the synthetic aperture length is increased to 50m, as... Figure 3 As shown in (c), the real target and artifacts are separated in the single-mode imaging results, but... Figure 3 (b) The resolution performance is still relatively poor.

[0066] Table 1 Forward-looking SAR imaging parameters

[0067]

[0068] Example 2

[0069] To verify the effectiveness of the proposed method for 3D imaging, the total synthetic aperture length was set to 99m, the mode range to -16 to 16, and the mode was switched every 160 pulses within the pulse repetition interval. The distance grid range was 3950m to 4050m, and the grid length was set to 0.1m. The elevation range was 0° to 40°, and the grid length was set to 0.5°. Other parameters were consistent with Table 1. Simulations were performed on 27 point targets in space. The target at the center was 5m away from its adjacent targets in the distance direction, 20° away in the azimuth direction, and 5° away in the elevation direction. The imaging results are as follows: Figure 4 As shown. Figure 4 (a) shows the three-dimensional imaging results in space. Figure 4 (b)~ Figure 4 (d) Corresponding to Figure 4 (a) shows cross-sectional images from different dimensions. Using the method of this invention, the positions of 27 point targets can be accurately obtained, thus proving the effectiveness of this invention.

Claims

1. A time-varying pattern based OAM forward looking imaging method, characterized in that, It comprises the following steps: Step 1, obtaining three-dimensional echo data by beam scanning under time-varying mode system; Step 2, realizing pulse compression in the range direction by matching filter method; Step 3, converting the imaging of time-varying mode into the imaging process of mode-invariant in the process of back projection, dividing the whole synthetic aperture into N sub-apertures, and keeping the mode of each sub-aperture unchanged; The mode of vortex electromagnetic wave emitted by the radar varies 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 l0, and the mode at any time is represented as where | • | represents a modulo operation, i represents the number of elapsed pulse periods, t s represents the slow time; Step 4, generating the image in the range-elevation dimension by BP algorithm in each sub-aperture; Step 5, after range-elevation compression, the echo is compressed in the azimuth direction by FFT method, and three-dimensional imaging of the target is obtained.

2. The time-varying pattern based OAM forward looking imaging method according to claim 1, wherein, The three-dimensional echo data is obtained by beam scanning under the time-varying mode system, the radar transmitting and receiving antenna adopts a uniform circular array, the imaging mode adopts a multi-transmitting and multi-receiving mode, the center frequency of the transmitting signal is f c , the bandwidth is B, and the mode range is [l min ,l max ].

3. The time-varying pattern based OAM forward looking imaging method of claim 1, wherein, Step 2, the pulse compression in the range direction is realized by matching filter method; a filter with Gaussian shape is used to eliminate the effect of frequency modulation in the range direction; the signal is up-sampled, and the signal is expanded by inserting zero values in the middle of the signal.

4. The time-varying pattern based OAM forward looking imaging method of claim 1, wherein, Step 4, the image in the range-elevation dimension is generated by BP algorithm in each sub-aperture; when the back projection algorithm is performed under the time-varying mode, the radial and rotational Doppler phases are compensated at the same time, so as to eliminate the varying Doppler phase difference in the echo; the varying Doppler phase in the echo is: Differentiating equation (2) gives the total Doppler frequency f d As follows: where r P is the instantaneous slant range of the target from the radar, / denotes the mode number, the target elevation angle θ0is the angle between the slant range r0direction and the positive z-axis direction, denotes the target azimuth angle, λ is the wavelength, and v a is the radar running speed.

5. An electronic device comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, The processor executes the program to realize the steps of the method in any one of claims 1-4.

6. A computer-readable storage medium having stored thereon a computer program, characterized in that, The program is executed by the processor to realize the steps of the method in any one of claims 1-4.

7. A computer program product comprising a computer program, characterized in that, The computer program is executed by the processor to realize the steps of the method in any one of claims 1-4.

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