Inverse synthetic aperture radar (ISAR) imaging method based on nonlinear frequency modulation signal, readable medium and device

Abstract

The invention discloses an ISAR imaging method based on a nonlinear frequency modulation signal, a readable medium and a device, and the method comprises the steps: transmitting a nonlinear frequency modulation signal to a target on a rotary table, and obtaining the echo signal data of the current angle of the target; performing pulse compression on the echo data to obtain pulse compression data of the target; performing FFT (Fast Fourier Transform) on the pulse compression data in the distance direction to a frequency domain, performing zero padding on the frequency domain, performing windowing processing on the frequency domain data, and performing IFFT (Inverse Fast Fourier Transform) to a time domain; rotating the rotary table and transmitting signals at the same time to obtain echo signals of the target at different angles, and performing pulse compression and frequency domain preprocessing to obtain a pulse compression data matrix of the target; and dividing an imaging area grid, projecting data in the pulse compression signal matrix to the imaging area grid, performing phase compensation, and converting a data unit into dB to obtain ISAR image data. According to the invention, ISAR imaging can be carried out on a nonlinear frequency modulation signal, side lobe interference can be effectively reduced, and a good focusing effect is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of near-field synthetic aperture technology, and in particular relates to an ISAR imaging method, a readable medium and a device based on a nonlinear frequency modulation signal. Background Art

[0002] Inverse Synthetic Aperture Radar (ISAR) imaging is a technology that produces high-resolution images of observed targets. It is widely used in RCS (Radar Cross Section) inspection of large-scale targets such as ships, vehicles, and aircraft. It is often used for stealth testing, surface coating repair and maintenance, and target structure design. Compared to SAR imaging, ISAR imaging technology easily achieves higher azimuth resolution, improving image quality.

[0003] A common radar imaging transmission signal is a linear frequency modulation signal. Since the sidelobes of linear frequency modulation signals are relatively high during the pulse compression process, other methods are usually introduced to suppress sidelobe interference, which increases the complexity of the imaging algorithm. The signal characteristics of nonlinear frequency modulation signals determine that they can obtain lower sidelobes during pulse compression without losing the signal-to-noise ratio of the output signal. They can effectively reduce sidelobe interference in imaging methods. Most of the common imaging algorithms currently used are targeted at linear frequency modulation signals and SAR imaging modes. These algorithms cannot usually be directly copied to ISAR data processing of nonlinear frequency modulation signals. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide an ISAR imaging method, a readable medium and a device based on nonlinear frequency modulation signals to achieve higher azimuth resolution and reduce sidelobe interference.

[0005] The object of the present invention is achieved through the following technical solution: an ISAR imaging method based on nonlinear frequency modulation signal, comprising the following steps: S1. Transmitting and receiving nonlinear frequency modulation signals: The radar transmits nonlinear frequency modulation signals to the target on the turntable, samples the echo data at the current angle of the target, and obtains the echo signal data at the target angle; S2. Perform pulse compression processing on the echo data: perform pulse compression processing on the echo data according to the nonlinear frequency modulation signal of the transmitting end; S3. Windowing and interpolation processing is performed on the data: the echo data is transformed into the frequency domain by performing FFT in the range direction to obtain range-frequency domain data, a Hamming window is added to the frequency domain data and zeros are added to the end of the data, and then the data is transformed into the time domain by performing IFFT.

[0006] S4. Obtaining a pulse compression data matrix of the echo signal: rotating the turntable at a set angular velocity. Each time the turntable rotates a specified angle, processing is performed according to steps S1 to S3 to obtain a pulse compression data matrix of the target echo signal. S5. Divide the imaging area into a grid: establish a coordinate system with the turntable as the center of the detection area and divide the imaging area into a grid; S6. Project the data onto the imaging grid and perform compensation: Calculate the distance from each grid point to the radar, project the pulse compression data matrix onto the imaging area grid, and perform phase compensation.

[0007] A readable medium stores a computer program, which executes the steps of an ISAR imaging method based on nonlinear frequency modulation signals when the computer program is run.

[0008] A device comprising a memory and one or more processors, wherein the memory stores a computer program running on the one or more processors, and when the one or more processors run the computer program, the steps of the ISAR imaging method based on nonlinear frequency modulation signals according to any one of claims 1 to 6 are performed.

[0009] The beneficial effects of the present invention are as follows: the present invention samples an ISAR imaging model, transmits a nonlinear frequency modulation signal at different angles to a target on a turntable, obtains echo signal data of the target at that angle, performs pulse compression on the echo data based on the nonlinear frequency modulation signal at the transmitting end, then performs windowing and interpolation processing on the echo data, simultaneously divides the detection area into an imaging area grid centered on the turntable, and finally performs phase compensation on the processed echo data before projecting it onto the imaging grid to obtain a two-dimensional image of the target. By sampling the ISAR imaging model and the characteristics of the nonlinear frequency modulation signal, the obtained two-dimensional image can reduce sidelobe interference and achieve higher azimuth resolution, effectively improving imaging quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a flow chart of the method of the present invention; Figure 2 The waveform diagram of the non-signal FM signal is transmitted; Figure 3 The waveform diagram of receiving non-signal FM signal; Figure 4 The signal after pulse compression Figure 5 This is a two-dimensional image of the downsampled nonlinear frequency modulation signal of the ISAR imaging model. DETAILED DESCRIPTION

[0011] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings, but the protection scope of the present invention is not limited to the following.

[0012] like Figure 1 As shown, an ISAR imaging method based on nonlinear frequency modulation signal includes the following steps: S1. Transmit and receive nonlinear frequency modulation signals: transmit nonlinear frequency modulation signals to different angles of the target on the turntable, and obtain the echo signal data of the target at different angles after sampling the echo data of the target at different angles; Figures 2 and 3 As shown, there are waveform diagrams of transmitting non-signal FM signals and receiving non-signal FM signals.

[0013] The step S1 includes the following process: S11. For nonlinear FM signals, the mathematical expression is: ; in, , is the pulse width, is the amplitude of the nonlinear FM signal; is the distance envelope, is the carrier frequency; is the modulation frequency; is the modulation index; S12. After the nonlinear frequency modulation signal is reflected by the target at the current angle, its echo signal is expressed as: ; in, is the target’s bearing time, i.e. is the instantaneous slant distance from the target to the receiver, is the speed of light, is the amplitude of the echo signal at the current angle of the target; S13. After down-conversion, the echo signal of the target at the current angle is: .

[0014] S2. Perform pulse compression processing on the echo data: perform pulse compression processing on the echo data according to the nonlinear frequency modulation signal of the transmitting end; The step S2 includes the following process: S21. Obtain the conjugate inversion signal of the transmitted signal: ; in To take the conjugate symbol; S22, convolve the conjugate inversion signal of the transmitted signal with the echo signal after down-conversion to obtain the pulse compressed signal ,in, Represents the signal after convolution and pulse compression Expressed as: ; Among them, the pulse compression envelope is the Fourier transform of the window function, is the sinc function. The image after pulse compression is as follows Figure 4 As shown; S3. Windowing and interpolation processing is performed on the data: the echo data is FFTed in the range direction to obtain the range-frequency domain data, a Hamming window is added to the frequency domain data and zeros are added to the end of the data, and then IFFT is performed to convert it to the time domain.

[0015] S31, yes Perform Fourier transform to frequency domain and multiply the frequency domain data by Hamming window; S32, the frequency domain data is padded with zeros to increase the data length. The length of the zero padding is generally not less than 8 times the current data length. Then the inverse Fourier transform is performed to the time domain to obtain the windowed and interpolated data. .

[0016] S4. Obtaining a pulse compression data matrix of the echo signal: rotating the turntable at a set angular velocity. Each time the turntable rotates a specified angle, processing is performed according to steps S1 to S3 to obtain a pulse compression data matrix of the target echo signal. The step S4 includes the following process: The turntable is rotated at a set angular velocity. Every time the target on the turntable rotates a specified angle, the radar emits a nonlinear frequency modulation signal. The echo signals at different target angles are acquired through data acquisition. The echo signals are processed according to steps S1 to S3 to obtain the pulse compression signal matrix of the echo signals: ; in, is the number of pulse repetitions.

[0017] S5. Divide the imaging area into a grid: establish a coordinate system with the turntable as the center of the detection area and divide the imaging area into a grid; Computational imaging range resolution , and azimuth resolution : ; ; in, B is the bandwidth of the transmitted signal, is the wavelength of the center frequency of the transmitted signal, is the total rotation angle of the target during the imaging process; According to the imaging distance resolution , and azimuth resolution , the desired detection area is divided into At the same time, a coordinate system is established with the center of the turntable to obtain the grid matrix of the desired detection area , that is, the imaging area grid: in, , , 、 are the length and width of the detection area, is the pixel value of the current grid point, are the coordinates of the grid points, and They are: , .

[0018] S6. Project the data onto the imaging grid and perform compensation: Calculate the distance from each grid point to the radar, project the pulse compression data matrix onto the imaging area grid, and perform phase compensation, such as Figure 5 As shown, it is a two-dimensional imaging diagram of the down-sampled nonlinear frequency modulation signal of the ISAR imaging model; S61. For any grid point , calculate the echo delay based on the distance between the radar and the grid point : ; in, is the distance from the grid point to the radar, , are the coordinates of the grid points, is the rotation speed of the turntable, is the distance from the radar to the center of the turntable; S62, according to the echo delay and direction time , in the pulse compression signal matrix In the example, the data at different positions and times are projected onto the matrix Grid points The pixel value obtained for: ; in, , and perform phase compensation on the pixel values at the grid points. The phase compensation value for: ; S63, root pulse compression signal matrix Repeat S61 and S62 for the data on the image to complete the projection and phase compensation of all grid points in the imaging area.

[0019] S64. After completing the above steps, the matrix Convert to The two-dimensional imaging matrix of unit : ; in, The grid points are The pixel value is in units of To take the absolute value sign.

[0020] A readable medium having a computer program stored thereon, wherein when the computer program is run, the steps of the ISAR imaging method based on nonlinear frequency modulation signal are executed. Figure 1 The program code of the method shown.

[0021] A device includes a memory and one or more processors, wherein the memory stores a computer program that runs on the one or more processors. When the one or more processors run the computer program, the steps of the ISAR imaging method based on nonlinear frequency modulation signals described in any one of claims 1 to 6 are performed. The device in this embodiment may include, but is not limited to, mobile terminals such as mobile phones, laptop computers, digital broadcast receivers, PDAs (personal digital assistants), PADs (tablet computers), PMPs (portable multimedia players), and in-vehicle terminals (e.g., in-vehicle navigation terminals), as well as fixed terminals such as digital TVs and desktop computers.

[0022] It should be noted that the computer-readable medium of the present invention may be a computer-readable signal medium or a computer-readable storage medium, or any combination thereof. Computer signal storage media may be, for example, but not limited to, electrical, magnetic, optical, electromagnetic, infrared, or semiconductor systems, devices, or components, or any combination thereof. More specific examples of computer-readable storage media may include, but are not limited to, an electrical connection having one or more wires, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination thereof.

[0023] In the embodiments of the application, a computer-readable storage medium may be any tangible medium containing or storing a program for use by or in conjunction with an instruction execution system, apparatus, or device. In this application, a computer-readable signal medium may include a data signal propagated in baseband or as part of a carrier wave, carrying computer-readable program code. Such a propagated data signal may take a variety of forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. A computer-readable signal medium may also be any computer-readable medium other than a computer-readable storage medium that can transmit, propagate, or transport a program for use by or in conjunction with an instruction execution system, apparatus, or device. The program code embodied on a computer-readable medium may be transmitted using any suitable medium, including but not limited to wire, optical cable, RF (radio frequency), or any suitable combination thereof. The computer-readable medium may be included in the device or may exist separately and not incorporated into the device.

[0024] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various deformations, modifications, and substitutions made to the technical solutions of the present invention by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present invention.

Claims

1. An ISAR imaging method based on nonlinear frequency modulation signals, characterized by: The following steps are involved: S1. Transmitting and receiving nonlinear frequency modulation signals: The radar transmits nonlinear frequency modulation signals to the target on the turntable, samples the echo data at the current angle of the target, and obtains the echo signal data at the target angle; S2. Perform pulse compression processing on the echo data: perform pulse compression processing on the echo data according to the nonlinear frequency modulation signal of the transmitting end; S3. Windowing and interpolation processing is performed on the data: the echo data is transformed into the frequency domain by performing FFT in the range direction to obtain range-frequency domain data, a Hamming window is added to the frequency domain data and zeros are added to the end of the data, and then the data is transformed into the time domain by performing IFFT; S4. Obtaining a pulse compression data matrix of the echo signal: rotating the turntable at a set angular velocity. Each time the turntable rotates a specified angle, processing is performed according to steps S1 to S3 to obtain a pulse compression data matrix of the target echo signal. S5. Divide the imaging area into a grid: establish a coordinate system with the turntable as the center of the detection area and divide the imaging area into a grid; S6. Project the data onto the imaging grid and perform compensation: Calculate the distance from each grid point to the radar, project the pulse compression data matrix onto the imaging area grid, and perform phase compensation.

2. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 1, wherein: The step S1 includes the following process: S11. For nonlinear FM signals, the mathematical expression is: ； in, , is the pulse width, is the amplitude of the nonlinear FM signal; is the distance envelope, is the carrier frequency; is the modulation frequency; is the modulation index; S12. After the nonlinear frequency modulation signal is reflected by the target at the current angle, its echo signal is expressed as: ； in, is the target’s bearing time, i.e. is the instantaneous slant distance from the target to the receiver, is the speed of light, is the amplitude of the echo signal at the current angle of the target; S13. After down-conversion, the echo signal of the target at the current angle is: 。 3. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 2, characterized in that: The step S2 includes the following process: S21. Obtain the conjugate inversion signal of the transmitted signal: ； in To take the conjugate symbol; S22, convolve the conjugate inversion signal of the transmitted signal with the echo signal after down-conversion to obtain the pulse compressed signal ,in, Represents the signal after convolution and pulse compression Expressed as: ； Among them, the pulse compression envelope is the Fourier transform of the window function, that is is the sinc function.

4. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 1, wherein: The step S3 includes the following process: S31, yes Perform Fourier transform to frequency domain and multiply the frequency domain data by Hamming window; S32, the frequency domain data is padded with zeros to increase the data length. The length of the zero padding is not less than 8 times the current data length. Then the inverse Fourier transform is performed to the time domain to obtain the windowed and interpolated data. .

5. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 4, characterized in that: The step S4 includes the following process: The turntable is rotated at a set angular velocity. Every time the target on the turntable rotates a specified angle, the radar emits a nonlinear frequency modulation signal. The echo signals at different target angles are acquired through data acquisition. The echo signals are processed according to steps S1 to S3 to obtain the pulse compression signal matrix of the echo signals: ； in, is the number of pulse repetitions.

6. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 1, characterized in that: The step S5 includes the following process: Computational imaging range resolution , and azimuth resolution : ； ； in, B is the bandwidth of the transmitted signal, is the wavelength of the center frequency of the transmitted signal, is the total rotation angle of the target during the imaging process; According to the imaging distance resolution , and azimuth resolution , the desired detection area is divided into At the same time, a coordinate system is established with the center of the turntable to obtain the grid matrix of the desired detection area , that is, the imaging area grid: in, , , 、 are the length and width of the detection area, is the pixel value of the current grid point, are the coordinates of the grid points, and They are: ， 。 7. The ISAR imaging method based on nonlinear frequency modulation signals according to claim 1, characterized in that: The step S6 includes the following process: S61. For any grid point , calculate the echo delay based on the distance between the radar and the grid point : ； in, is the distance from the grid point to the radar, , are the coordinates of the grid points, is the rotation speed of the turntable, is the distance from the radar to the center of the turntable; S62, according to the echo delay and direction time , in the pulse compression signal matrix In the example, the data at different positions and times are projected onto the matrix Grid points The pixel value obtained for: ； in, , and perform phase compensation on the pixel values at the grid points. The phase compensation value for: ； S63, root pulse compression signal matrix Repeat S61 and S62 to complete the projection and phase compensation of all grid points in the imaging area; S64. After completing the above steps, the matrix Convert to The two-dimensional imaging matrix of unit : ； in, The grid points are The pixel value is in units of To take the absolute value sign.

8. A readable medium having a computer program stored thereon, characterized in that: When the computer program is run, the steps of the ISAR imaging method based on nonlinear frequency modulation signals according to any one of claims 1 to 6 are executed.

9. A device comprising a memory and one or more processors, characterized in that: The memory stores a computer program running on the one or more processors, and when the one or more processors run the computer program, the steps of the ISAR imaging method based on nonlinear frequency modulation signals according to any one of claims 1 to 6 are executed.

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