Terahertz high-squint SAR (Synthetic Aperture Radar) imaging method, device and equipment based on frequency scaling
By performing residual video phase correction, movement correction, frequency scaling, distance compression and azimuth third-order phase filtering in terahertz large strabismus SAR imaging, the problem of poor imaging quality under large terahertz bandwidth and large strabismus angle is solved, and accurate focus and high-quality imaging are achieved.
Patent Information
- Application Number
- CN202510548075.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-28
- Publication Date
- 2025-05-30
AI Technical Summary
Under large bandwidth and large slant angles in the terahertz band, existing imaging algorithms are difficult to achieve precise focus, resulting in poor imaging quality.
By performing residual video phase and movement correction processing in the distance frequency domain-azimuth time domain, combining frequency scaling, distance compression, azimuth direction third-order phase filtering and nonlinear scaling operations, geometric correction is finally performed to achieve accurate focus.
The focus accuracy and imaging quality of terahertz large strabismus SAR imaging is improved, and the azimuth frequency space change problem under large bandwidth and large strabismus angle is solved, ensuring that the image correctly reflects the target position relationship.
Smart Images
Figure CN120065246A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the technical field of terahertz synthetic aperture radar imaging, and particularly to a terahertz large squint SAR imaging method, device and equipment based on frequency scaling. Background Art
[0002] Synthetic Aperture Radar (SAR) virtualizes a larger aperture through the movement of the radar, facilitating two-dimensional high-resolution imaging. Terahertz waves, due to their short wavelengths, are easy to implement large-bandwidth signals and high-gain narrow beams, and are easy to achieve high resolution in both the range and azimuth directions. Moreover, the synthetic aperture time is short, facilitating fast imaging. Traditional forward-looking imaging technology has some limitations, requiring the radar trajectory and beam to be perpendicular to each other, which results in the inability to use forward-looking imaging in some scenarios. Squint SAR, on the other hand, has almost no specific requirements for the beam pointing and can image targets in front of the platform, thus providing richer target information and angular views. However, at large bandwidths and large squint angles, the range migration increases significantly and the two-dimensional coupling effect is enhanced, making it difficult to apply many existing imaging algorithms.
[0003] Although the frequency scaling algorithm is convenient for processing terahertz echo data and takes into account the range curvature, it does not consider the spatial variation of the azimuth chirp rate in the azimuth direction and is difficult to achieve precise focusing under the conditions of large bandwidth and large squint angle in the terahertz band, resulting in poor imaging quality. Summary of the Invention
[0004] Based on this, it is necessary to provide a terahertz large squint SAR imaging method, device and equipment based on frequency scaling that can achieve precise focusing for the above technical problems.
[0005] A terahertz large squint SAR imaging method based on frequency scaling, the method comprising: Based on the terahertz large squint strip SAR imaging geometric model, obtain the de-chirped terahertz radar echo signal; Perform residual video phase and walk correction processing on the de-chirped terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal; After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain the echo signal after range processing; After performing azimuth third-order phase filtering on the echo signal after range processing, perform non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain the echo signal after azimuth processing; Perform geometric correction on the azimuth-processed echo signal to obtain the terahertz large squint SAR imaging result.
[0006] In one embodiment, on the range frequency domain - azimuth time domain, performing residual video phase and motion correction processing on the deramped terahertz radar echo signal, the obtained preliminarily corrected echo signal includes: Perform range FFT on the deramped terahertz radar echo signal to obtain the target echo signal in the range frequency domain - azimuth time domain; Multiply the target echo signal in the range frequency domain - azimuth time domain by a first reference function, remove the residual video phase, and then perform inverse range FFT transformation back to the two-dimensional time domain to obtain the target echo signal after removing the residual video phase; Multiply the target echo signal after removing the residual video phase by the time domain correction range migration function to obtain the preliminarily corrected echo signal after removing the first-order motion term.
[0007] In one embodiment, the step of transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling processing includes: Perform azimuth FFT and then range FFT on the preliminarily corrected echo signal in sequence, select the scene center as the reference point for frequency scaling operation, multiply the scaling function by the echo signal to obtain the first intermediate echo signal after removing range curvature; Among them, the scaling function is expressed as: ; In the above formula, is the range frequency, is the azimuth frequency, is the quadratic phase correction coefficient, is the position of the reference target in the range frequency domain, is the approximation of the chirp rate at the reference range.
[0008] In one embodiment, performing range compression processing in the range time domain - azimuth frequency domain, and then performing residual phase compensation in the two-dimensional frequency domain to obtain the range-processed echo signal, includes: Perform inverse range FFT transformation on the first intermediate echo signal to the range time domain - azimuth frequency domain, and then multiply it by the range compression function to obtain the second intermediate echo signal; Perform range FFT transformation on the second intermediate echo signal to the two-dimensional frequency domain, and then multiply it by the residual phase compensation function to complete the residual phase compensation, and obtain the range-processed echo signal.
[0009] In one embodiment, when performing azimuth third-order phase filtering on the range-processed echo signal, the three-time phase filtering function used is expressed as: ; In the above formula, represents the slant range, represents the carrier wavelength, represents the squint angle, represents the radar platform motion speed, represents the approximation coefficient of.
[0010] In one embodiment, in the range frequency domain - azimuth time domain, after performing azimuth third-order phase filtering on the range-processed echo signal and then performing a non-linear scaling operation, the non-linear frequency modulation scaling reference function used is expressed as: ; In the above formula, is the azimuth frequency modulation rate, is a parameter related to the azimuth frequency modulation frequency, is the azimuth slow time.
[0011] In one embodiment, when performing geometric correction on the azimuth-processed echo signal: performing geometric correction processing at the point target position of the azimuth-processed echo signal, first performing the first geometric correction using a first correction reference function in the two-dimensional time domain, and then performing the second geometric correction using a second correction reference function in the range time domain - azimuth frequency domain.
[0012] The present application also provides a terahertz large squint SAR imaging device based on frequency scaling, and the device includes: A signal acquisition module, configured to acquire a de-skewed terahertz radar echo signal based on the terahertz large squint strip SAR imaging geometric model; A preliminary correction module, configured to perform residual video phase and walk correction processing on the de-skewed terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal; A range processing module, configured to transform the preliminarily corrected echo signal to the two-dimensional frequency domain to complete frequency scaling processing, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain a range-processed echo signal; An azimuth processing module, configured to perform azimuth third-order phase filtering on the range-processed echo signal, perform a non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain an azimuth-processed echo signal; A terahertz large squint SAR imaging module is used to perform geometric correction on the echo signal processed in the azimuth direction to obtain a terahertz large squint SAR imaging result.
[0013] A computer device includes a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the following steps are implemented: Based on the terahertz large squint stripmap SAR imaging geometric model, obtain the de-skewed terahertz radar echo signal; Perform residual video phase and motion correction processing on the de-skewed terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal; After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain a range-processed echo signal; After performing azimuth third-order phase filtering on the range-processed echo signal, perform a non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain an azimuth-processed echo signal; Perform geometric correction on the azimuth-processed echo signal to obtain a terahertz large squint SAR imaging result.
[0014] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the following steps are implemented: Based on the terahertz large squint stripmap SAR imaging geometric model, obtain the de-skewed terahertz radar echo signal; Perform residual video phase and motion correction processing on the de-skewed terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal; After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain a range-processed echo signal; After performing azimuth third-order phase filtering on the range-processed echo signal, perform a non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain an azimuth-processed echo signal; Perform geometric correction on the azimuth-processed echo signal to obtain a terahertz large squint SAR imaging result.
[0015] The above terahertz large squint SAR imaging method, device and equipment based on frequency scaling perform residual video phase and motion correction processing on the de-chipped target echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal. After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling processing, range compression processing is performed in the range time domain - azimuth frequency domain, and then residual phase compensation is performed in the two-dimensional frequency domain to obtain the echo signal after range processing. After performing azimuth third-order phase filtering on the echo signal after range processing, a non-linear scaling operation is performed in the range frequency domain - azimuth time domain, and then azimuth compression is completed in the two-dimensional frequency domain. The signal after azimuth compression is converted to the range frequency domain - azimuth time domain to obtain the echo signal after azimuth processing. Finally, geometric correction is performed on the echo signal after azimuth processing to obtain a high-quality terahertz large squint SAR imaging result with precise focusing. Description of the Drawings
[0016] Figure 1 It is an application environment diagram of the terahertz large squint SAR imaging method based on frequency scaling in an embodiment; Figure 2 It is a schematic diagram of the geometric model of squint SAR imaging in an embodiment; Figure 3 It is an echo signal diagram after de-chirping in an embodiment, Figure 3 (a) is the echo diagram in two-dimensional time domain, Figure 3 (b) is the echo diagram in range frequency domain - azimuth time domain; Figure 4 It is a schematic diagram for comparing the imaging results before and after geometric correction in an embodiment, Figure 4 (a) is the imaging result diagram before geometric correction; Figure 4 (b) is the imaging result diagram after geometric correction; Figure 5 It is a block diagram of the specific implementation steps of this method in an embodiment; Figure 6 It is a schematic diagram of the point target simulation scenario in a simulation experiment; Figure 7 It is the imaging result of each target point in the imaging plane in a simulation experiment using this method at an oblique angle of 15°, Figure 7 (a) is the imaging result diagram, Figure 7 (b) is the azimuth cross-section diagram of the center point, Figure 7 (c) is the range cross-section diagram of the center point; Figure 8 It is the imaging result of each target point in the imaging plane in a simulation experiment using this method at an oblique angle of 30°, Figure 8 (a) is the imaging result diagram, Figure 8 (b) is the azimuth cross-section diagram of the center point,Figure 8 (c) Cross-sectional view of the center point distance Figure 9 The imaging results of each target point in the imaging plane using this method in a simulation experiment at an oblique angle of 45° Figure 9 (a) Imaging result diagram Figure 9 (b) Cross-sectional view of the center point in the azimuth direction Figure 9 (c) Cross-sectional view of the center point distance Figure 10 The imaging results of each target point in the imaging plane using this method in a simulation experiment at an oblique angle of 60° Figure 10 (a) Imaging result diagram Figure 10 (b) Cross-sectional view of the center point in the azimuth direction Figure 10 (c) Cross-sectional view of the center point distance Figure 11 Structure block diagram of a terahertz large squint SAR imaging device based on frequency scaling in an embodiment Figure 12 Internal structure diagram of a computer device in an embodiment Specific implementation manner
[0017] In order to make the purpose, technical solutions and advantages of this application clearer, the following further details this application in combination with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain this application and are not used to limit this application.
[0018] In the prior art, when processing terahertz echo data using the frequency scaling algorithm, range curvature is considered, but the spatial variation of the azimuth modulation frequency is not considered in the azimuth direction. This leads to difficulties in achieving precise focusing when imaging under the conditions of large bandwidth and large squint angle in the terahertz band, resulting in poor imaging quality. As Figure 1 shown, a terahertz large squint SAR imaging method based on frequency scaling is provided, including the following steps: Step S100, based on the terahertz large squint strip SAR imaging geometric model, obtain the deramped terahertz radar echo signal.
[0019] Step S110, perform residual video phase and motion correction processing on the deramped terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain the preliminarily corrected echo signal.
[0020] Step S120, transform the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain the range-processed echo signal.
[0021] In step S130, after performing azimuth third-order phase filtering on the range-processed echo signal, a non-linear scaling operation is carried out in the range frequency domain - azimuth time domain, and then azimuth compression is completed in the two-dimensional frequency domain. The azimuth-compressed signal is then converted to the range frequency domain - azimuth time domain to obtain the azimuth-processed echo signal.
[0022] In step S140, geometric correction is performed on the azimuth-processed echo signal to obtain the terahertz large squint SAR imaging result.
[0023] In this application, considering that when the squint angle increases, the range migration amount increases significantly and is much larger than the range curvature, resulting in severe coupling between the azimuth and range directions, a terahertz large squint SAR imaging method based on frequency scaling is proposed. This method first performs time-domain migration correction to facilitate subsequent two-dimensional separation processing, then corrects the range curvature through frequency scaling operations, and then uses a second-order range compression function and phase compensation to complete range compression. To eliminate azimuth coupling, a non-linear frequency modulation scaling method also needs to be used for azimuth processing. After compensating for the third-order phase term, a non-linear frequency modulation scaling operation is carried out, and azimuth compression is completed by multiplying with a reference function. Finally, geometric correction is performed. Compared with traditional processing methods, this method can solve the problem of spatial variation of azimuth chirp rate under terahertz large bandwidth and large squint angle, ensure the focusing accuracy in the azimuth direction, and at the same time correct the geometric relationship, improving the focusing effect of the final image.
[0024] In step S100, first, a geometric model for terahertz large squint stripmap SAR imaging is constructed, and then the target echo signal is obtained based on this model.
[0025] In this embodiment, the radar for obtaining the target echo signal operates in stripmap mode. Based on the terahertz large squint stripmap SAR imaging geometric model, as Figure 2 shown, where the radar platform flies at a constant speed along a straight trajectory parallel to the X-axis, and the flight altitude is , the squint angle of the radar beam pointing is fixed at . The moment when the platform is at point A is the starting point of azimuth slow time. The distance from the point target to the aircraft flight path is the closest distance . When the beam center scans through the target , the slant range is . The instantaneous squint angle of the target is . On the line parallel to the carrier aircraft flight path passing through the target , the starting illumination position of the beam center is point . After passing through the slow time , the carrier aircraft moves to point, and at this time its lateral distance is , assume a point target and the point The distance between them is , then from the geometric relationship, the instantaneous slant range between the point target and the radar is : (1) Usually, the radar transmitted signal is a linear frequency modulated (LFM) signal, expressed as: (2) In formula (2), is the total time, represents the fast time in the range direction, is the slow time in the azimuth direction, is the signal pulse width, is the linear frequency modulation rate of the pulse signal, is the center frequency.
[0026] Furthermore, take the distance at the center of the scene as the reference distance, and the corresponding echo signal at this distance is the reference signal, expressed as: (3) For a point target with an instantaneous slant range of , the corresponding echo signal can be expressed as: (4) After performing coherent demodulation of dechirping on the echo signal and the reference signal, the output signal is the required de-slanted echo signal: (5) In formula (5), , Figure 3 is the carrier wavelength. The second term in the exponential term is called the Residual Video Phase (RVP) term, which is introduced by the de-slanting system and has no effect on imaging. Then in a certain embodiment, the de-slanted target echo signal, as shown in Figure 3 , where Figure 3 (a) is the two-dimensional time-domain echo diagram, (b) is the range-frequency domain azimuth-time domain echo diagram.
[0027] In this embodiment, after the radar platform detects the target area, under the above-mentioned geometric model of terahertz large squint strip SAR imaging, the de-slanted target echo signal can be expressed in the form shown in formula (5).
[0028]
[0028] Next, enter step S110 to perform preliminary phase and motion correction on the target echo signal. The process of obtaining the preliminarily corrected echo signal includes: performing range FFT on the target echo signal to obtain the target echo signal in the range frequency domain - azimuth time domain, multiplying the target echo signal in the range frequency domain - azimuth time domain by the first reference function, removing the residual video phase, and then performing inverse range FFT transformation back to the two-dimensional time domain to obtain the target echo signal after removing the residual video phase. Multiply the target echo signal after removing the residual video phase by the time domain correction range migration function to obtain the preliminarily corrected echo signal after removing the first motion term.
[0029] In this embodiment, the first reference function is expressed as: (6) Further, after removing the residual video phase, performing inverse range FFT transformation back to the two-dimensional time domain, the target echo signal after removing the residual video phase is expressed as: (7) In this embodiment, the time domain correction range migration function is expressed as: (8) Further, multiplying the time domain correction range migration function expressed by formula (8) by the echo signal can remove the first motion term, and the preliminarily corrected echo signal is expressed as: (9) In step S120, transform the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, including: performing azimuth FFT and range FFT on the preliminarily corrected echo signal in sequence, selecting the scene center as the reference point for frequency scaling operation, and multiplying the scaling function by the echo signal to obtain the first intermediate echo signal after removing the range curvature.
[0030] Specifically, after performing two-dimensional FFT on the preliminarily corrected echo signal to convert it to the two-dimensional frequency domain and ignoring the signal envelope, the echo signal is transformed into: (10) In formula (10), is the range frequency, is the azimuth frequency, is the new range chirp rate after range migration correction, is the original scaling factor, is the quadratic phase correction coefficient. Among them, these three parameters are respectively expressed as: ; , where ; ; Further, select the scene center as the reference point for the frequency scaling operation, multiply the scaling function by the echo signal to obtain the first intermediate echo signal after removing the range curvature, where the scaling function is expressed as: (11) In formula (11), , is the position of the reference target in the range frequency domain, is the approximate chirp rate at the reference range, expressed as .
[0031] In this embodiment, range compression processing is performed in the range time domain - azimuth frequency domain, and then residual phase compensation is performed in the two-dimensional frequency domain to obtain the echo signal after range direction processing, including: performing range IFFT on the first intermediate echo signal to transform it to the range time domain - azimuth frequency domain, multiplying it by the range compression function to obtain the second intermediate echo signal, and then performing range FFT on the second intermediate echo signal to transform it to the two-dimensional frequency domain, multiplying it by the residual phase compensation function to complete the residual phase compensation and obtain the echo signal after range direction processing.
[0032] Specifically, after performing range IFFT on the first intermediate echo signal to transform it to the range time domain - azimuth frequency domain, the echo signal is transformed into: (12) Further, multiplying the compression function by the echo signal shown in formula (12) can obtain the signal after range compression, where the range compression function is: (13) Next, perform range FFT on the compressed echo signal, that is, the second intermediate echo signal, to transform the echo signal to the two-dimensional frequency domain, expressed as: (14) Further, multiply the residual phase compensation function by the echo signal shown in formula (14) to remove the residual phase. The residual phase compensation function used is expressed as: (15) In formula (15), is the target range position, expressed as: (16) Then, the echo signal after the range direction processing in step S120 is expressed as: (17) As can be seen from Equation (17), the point target is located at the slant range at this time. After range processing, ignoring the range envelope and expanding it into the Taylor series, the signal can be expressed as: (18) In Equation (18), is the azimuth modulation frequency, with specific spatial variability, expressed as: ; As can be seen from Equation (18), the azimuth modulation frequency has spatial variability, and a non-linear chirp scaling operation is required to correct the modulation frequency.
[0033] In step 130, when performing azimuth third-order phase filtering on the echo signal after range processing, the cubic phase filtering function used is expressed as: (19) In Equation (19), represents the slant range, represents the carrier wavelength, represents the approximation coefficient of, and in one embodiment, it takes the value of 1.1.
[0034] In this embodiment, multiplying the cubic phase filtering function by the echo signal after range processing can filter out the cubic phase. After filtering out the high-order phase, the non-linear chirp scaling operation can be performed. This operation is carried out in the range frequency domain - azimuth time domain. After azimuth IFFT, the echo signal is: (20) In this embodiment, when performing the non-linear scaling operation, the non-linear chirp scaling reference function used is expressed as: (21) Furthermore, multiplying the non-linear chirp scaling reference function by the echo signal shown in Equation (20) can obtain the azimuth-scaled echo. After the azimuth non-linear chirp scaling operation, azimuth FFT is performed to transform it into the two-dimensional frequency domain, and the echo signal is: (22) Next, for azimuth compression of Equation (22), the azimuth compression reference function used is expressed as: (23) Multiplying Equation (23) by Equation (22) can obtain the azimuth-compressed echo signal. After azimuth IFFT, converting the echo signal to the range frequency domain - azimuth time domain can obtain the final signal, that is, the echo signal after range processing, expressed as: (24) As can be seen from formula (24), at this time, the point target is located at , and geometric correction processing needs to be performed.
[0035] In step S140, when performing geometric correction on the azimuth-processed echo signal: geometric correction processing is performed on the point target position in the azimuth-processed echo signal. First, the first geometric correction is performed using a first correction reference function in the two-dimensional time domain, and then the second geometric correction is performed using a second correction reference function in the range time domain - azimuth frequency domain.
[0036] In this embodiment, the data range term of formula (24) is IFFT-transformed to the two-dimensional time domain for geometric correction, which is equivalent to the inverse transform of the range migration processing. The first correction reference function is: (25) After that, the signal is FFT-transformed in the azimuth direction to the range time domain - azimuth frequency domain for further correction. The second correction reference function is: (26) After multiplying the signal after the first correction by the above-mentioned second correction reference function, and then using the azimuth IFFT to transform it back to the range frequency domain - azimuth time domain, the final imaging result can be obtained. In one of the embodiments, the images before and after geometric correction are as Figure 4 shown.
[0037] As Figure 5 shown, it is a flow block diagram of imaging using this method.
[0038] In this article, echoes are also generated according to the parameters shown in Table 1, and the method is simulated and verified to prove its effectiveness.
[0039] Table 1 SAR imaging parameters
[0040] In the simulation experiment, the point target simulation scenario is as Figure 6 shown. Imaging is performed respectively at the oblique angles of , and . Three point targets with a spacing of 5 m are arranged along the radar movement direction in the imaging plane, and two point targets with a spacing of 5 m are arranged along the direction perpendicular to the radar movement direction for the middle point. The imaging results at different oblique angles are as Figure 7 , Figure 8 , Figure 9 , Figure 10 shown.
[0041] As Figure 7 shown, the following are the imaging results of each target point in the imaging plane at an oblique angle of 15° using this method, Figure 7 (a) is the imaging result diagram, where the horizontal axis is the azimuth length and the vertical axis is the range length; Figure 7 (b) is the azimuth cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude; Figure 7 (c) is the range cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude.
[0042] As Figure 8 shown, the following are the imaging results of each target point in the imaging plane at an oblique angle of 30° using this method, Figure 8 (a) is the imaging result diagram, where the horizontal axis is the azimuth length and the vertical axis is the range length; Figure 8 (b) is the azimuth cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude; Figure 8 (c) is the range cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude.
[0043] As Figure 9 shown, the following are the imaging results of each target point in the imaging plane at an oblique angle of 45° using this method, Figure 9 (a) is the imaging result diagram, where the horizontal axis is the azimuth length and the vertical axis is the range length; Figure 9 (b) is the azimuth cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude; Figure 9 (c) is the range cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude.
[0044] As Figure 10 shown, the following are the imaging results of each target point in the imaging plane at an oblique angle of 60° using this method, Figure 10 (a) is the imaging result diagram, where the horizontal axis is the azimuth length and the vertical axis is the range length; Figure 10 (b) is the azimuth cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude; Figure 10 (c) is the range cross-section diagram of the center point, where the horizontal axis is the azimuth sampling point and the vertical axis is the normalized amplitude.
[0045] The peak sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR) of the range and azimuth of the center point target at different oblique angles are calculated to quantitatively evaluate the focusing quality of the image. The focusing performance parameters of the point target impulse response function (IRF) are shown in Table 2.
[0046] Table 2 Analysis of Point Target Focusing Performance at Different Oblique Angles
[0047] As can be seen from Table 2, the two-dimensional PSLR of the point target at different squint angles is close to the theoretical value of -13.26 dB, and the ISLR is close to -10 dB. Therefore, it can be proved that the proposed method has a good focusing effect under the condition of large squint angle in terahertz SAR imaging.
[0048] In the above terahertz large squint SAR imaging method based on frequency scaling, considering that when the squint angle increases, the range migration amount increases significantly and is much larger than the range curvature, resulting in severe coupling between the azimuth and range directions, a terahertz large squint SAR imaging method based on frequency scaling is proposed. This method first performs time-domain migration correction to facilitate subsequent two-dimensional separation processing, then corrects the range curvature through frequency scaling operation, and then uses the second-order range compression function and phase compensation to complete the range compression. To eliminate the azimuth coupling, a non-linear chirp scaling method is also required for azimuth processing. After compensating the cubic phase term, a non-linear chirp scaling operation is performed, and the azimuth compression is completed by multiplying with the reference function. Finally, geometric correction is carried out. Compared with the traditional processing method, this method can solve the problem of spatial variation of azimuth chirp rate under the conditions of large terahertz bandwidth and large squint angle, ensure the azimuth focusing accuracy, and correct the geometric relationship at the same time, thus improving the focusing effect of the final image. Compared with the existing technical solutions, the advantage of this method lies in combining the frequency scaling operation in the range direction, removing the high-order phase in the azimuth direction, and compressing with reference to the ANCS method, effectively solving the problem of azimuth defocusing under large bandwidth and large squint angle, ensuring the imaging quality, and the image after geometric correction can correctly reflect the position relationship between point targets.
[0049] It should be understood that although Figure 1 the steps in the flowchart of Figure 1 are shown in sequence according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise clearly stated in this article, there is no strict order limit for the execution of these steps, and these steps can be executed in other orders. Moreover,
[0050] In one embodiment, as Figure 11 shown, a terahertz large squint SAR imaging device based on frequency scaling is provided, including: a signal acquisition module 200, a preliminary correction module 210, a range direction processing module 220, an azimuth direction processing module 230, and a terahertz large squint SAR imaging module 240, where: The signal acquisition module 200 is configured to acquire the de - slanted terahertz radar echo signal based on the terahertz large - squint strip SAR imaging geometric model; The preliminary correction module 210 is configured to perform residual video phase and walk correction processing on the de - slanted terahertz radar echo signal in the range frequency domain - azimuth time domain to obtain a preliminarily corrected echo signal; The range - direction processing module 220 is configured to transform the preliminarily corrected echo signal to the two - dimensional frequency domain to complete the frequency scaling processing, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two - dimensional frequency domain to obtain a range - direction processed echo signal; The azimuth - direction processing module 230 is configured to perform azimuth - direction third - order phase filtering on the range - direction processed echo signal, perform a non - linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two - dimensional frequency domain, and convert the azimuth - compressed signal to the range frequency domain - azimuth time domain to obtain an azimuth - direction processed echo signal; The terahertz large - squint SAR imaging module 240 is configured to perform geometric correction on the azimuth - direction processed echo signal to obtain a terahertz large - squint SAR imaging result.
[0051] For the specific limitations of the terahertz large - squint SAR imaging device based on frequency scaling, reference can be made to the limitations of the terahertz large - squint SAR imaging method based on frequency scaling in the above text, which will not be elaborated here. Each module in the above - mentioned terahertz large - squint SAR imaging device based on frequency scaling can be implemented in whole or in part by software, hardware, and their combinations. The above - mentioned modules can be embedded in the processor of the computer device in hardware form or be independent of it, or can be stored in the memory of the computer device in software form, so as to facilitate the processor to call and execute the operations corresponding to the above - mentioned modules.
[0052] In one embodiment, a computer device is provided. The computer device can be a terminal, and its internal structure diagram can be as Figure 12As shown in the figure. The computer device includes a processor, a memory, a network interface, a display screen, and an input device connected through a system bus. Among them, the processor of the computer device is used to provide computing and control capabilities. The memory of the computer device includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs in the non-volatile storage medium. The network interface of the computer device is used to communicate with an external terminal through a network connection. When the computer program is executed by the processor, it implements a terahertz large squint SAR imaging method based on frequency scaling. The display screen of the computer device can be a liquid crystal display screen or an electronic ink display screen. The input device of the computer device can be a touch layer covering the display screen, or a button, a trackball, or a touchpad provided on the housing of the computer device, or an external keyboard, a touchpad, or a mouse, etc.
[0053] Those skilled in the art can understand that Figure 12 the structure shown in the figure is only a block diagram of some structures related to the solution of this application, and does not constitute a limitation on the computer device to which the solution of this application is applied. The specific computer device may include more or fewer components than those shown in the figure, or combine some components, or have different component arrangements.
[0054] In one embodiment, a computer device is provided, including a memory and a processor. A computer program is stored in the memory. When the processor executes the computer program, the following steps are implemented: Based on the terahertz large squint strip SAR imaging geometric model, obtain the dechirped terahertz radar echo signal; In the range frequency domain - azimuth time domain, perform residual video phase and motion correction processing on the dechirped terahertz radar echo signal to obtain a preliminarily corrected echo signal; After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling process, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain a range-processed echo signal; After performing azimuth third-order phase filtering on the range-processed echo signal, perform a non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain an azimuth-processed echo signal; Perform geometric correction on the azimuth-processed echo signal to obtain the terahertz large squint SAR imaging result.
[0055] In one embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the following steps are implemented: Based on the geometric model of terahertz large squint stripmap SAR imaging, obtain the dechirped terahertz radar echo signal; In the range frequency domain - azimuth time domain, perform residual video phase and motion correction processing on the dechirped terahertz radar echo signal to obtain a preliminarily corrected echo signal; After transforming the preliminarily corrected echo signal to the two-dimensional frequency domain to complete the frequency scaling processing, perform range compression processing in the range time domain - azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain the echo signal after range processing; After performing azimuth third-order phase filtering on the echo signal after range processing, perform a non-linear scaling operation in the range frequency domain - azimuth time domain, then complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth-compressed signal to the range frequency domain - azimuth time domain to obtain the echo signal after azimuth processing; Perform geometric correction on the echo signal after azimuth processing to obtain the terahertz large squint SAR imaging result.
[0056] Those of ordinary skill in the art can understand that all or part of the processes in the above-described embodiment methods can be completed by instructing relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the above-described method embodiments. Among them, any reference to a memory, storage, database, or other medium used in the various embodiments provided in the present application can include non-volatile and / or volatile memories. Non-volatile memories can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memories can include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.
[0057] The technical features of the above embodiments can be combined arbitrarily. For the sake of concise description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope recorded in this specification.
[0058] The above-described embodiments merely represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended claims.
Claims
1. A terahertz high squint SAR imaging method based on frequency scaling, characterized in that: The method comprises: Based on the terahertz high squint strip SAR imaging geometric model, the de-squinted terahertz radar echo signal is obtained; In the range frequency domain-azimuth time domain, the de-skewed terahertz radar echo signal is subjected to residual video phase and movement correction processing to obtain a preliminarily corrected echo signal; After the echo signal after preliminary correction is transformed into the two-dimensional frequency domain to complete frequency scaling processing, range compression processing is performed in the range time domain-azimuth frequency domain, and then residual phase compensation is performed in the two-dimensional frequency domain to obtain the echo signal after range processing; After performing a third-order phase filter on the echo signal after range processing, a nonlinear scaling operation is performed in the range frequency domain-azimuth time domain, and then azimuth compression is completed in the two-dimensional frequency domain, and the azimuth compressed signal is converted to the range frequency domain-azimuth time domain to obtain an echo signal after azimuth processing; The azimuthally processed echo signal is geometrically corrected to obtain a terahertz high-squint SAR imaging result.
2. The terahertz high squint SAR imaging method based on frequency scaling according to claim 1, characterized in that: The residual video phase and movement correction processing is performed on the de-skewing terahertz radar echo signal in the range frequency domain-azimuth time domain to obtain a preliminary corrected echo signal, which includes: Performing range FFT on the de-slanted terahertz radar echo signal to obtain a target echo signal in the range frequency domain-azimuth time domain; The target echo signal in the range frequency domain-azimuth time domain is multiplied by the first reference function, and after removing the residual video phase, the range IFFT transform is performed back to the two-dimensional time domain to obtain the target echo signal after the residual video phase is removed; The target echo signal after the residual video phase is removed is multiplied by the time domain corrected range walk function to obtain the echo signal after the preliminary correction with the one-time walk term removed.
3. The terahertz high squint SAR imaging method based on frequency scaling according to claim 2, characterized in that: The step of transforming the echo signal after preliminary correction into a two-dimensional frequency domain to complete frequency scaling processing includes: After performing azimuth FFT and range FFT on the echo signal after preliminary correction, the center of the scene is selected as a reference point for frequency scaling, and the scaling function is multiplied by the echo signal to obtain a first intermediate echo signal after removing the range curvature; Wherein, the variable scaling function is expressed as: ； In the above formula, is the distance frequency, is the azimuth frequency, is the quadratic phase correction coefficient, is the position of the reference target in the range frequency domain, is the approximate frequency modulation rate at the reference distance.
4. The terahertz high squint SAR imaging method based on frequency scaling according to claim 3, characterized in that: The distance compression processing is performed in the distance time domain-azimuth frequency domain, and then the residual phase compensation is performed in the two-dimensional frequency domain to obtain the echo signal after the range processing, including: After performing range IFFT transformation on the first intermediate echo signal to the range time domain-azimuth frequency domain, the signal is multiplied by the range compression function to obtain a second intermediate echo signal; After performing range-direction FFT transformation on the second intermediate echo signal into a two-dimensional frequency domain, the second intermediate echo signal is multiplied by a residual phase compensation function to complete residual phase compensation, thereby obtaining the echo signal after range-direction processing.
5. The terahertz high squint SAR imaging method based on frequency scaling according to claim 4, characterized in that: When performing a third-order phase filter in azimuth on the echo signal after the range processing, a cubic phase filter function is used, which is expressed as: ； In the above formula, represents the slope distance, represents the carrier wavelength, Indicates oblique angle, Indicates the speed of the radar platform. express The approximate coefficient of .
6. The method for terahertz high squint SAR imaging based on frequency scaling according to claim 5, characterized in that: In the range frequency domain-azimuth time domain, after performing a third-order phase filter in the azimuth direction on the echo signal processed in the range direction, a nonlinear frequency modulation scaling reference function is used to express it as follows: ； In the above formula, is the azimuth frequency modulation, is a parameter related to the azimuth frequency modulation, It is the slow time in azimuth.
7. The terahertz high squint SAR imaging method based on frequency scaling according to claim 6, characterized in that: When performing geometric correction on the echo signal after azimuth processing: perform geometric correction on the midpoint target position of the echo signal after azimuth processing, first perform the first geometric correction in the two-dimensional time domain using a primary correction reference function, and then perform the second geometric correction in the distance time domain-azimuth frequency domain using a secondary correction reference function.
8. A terahertz high squint SAR imaging device based on frequency scaling, characterized in that: The device comprises: A signal acquisition module, used to acquire the de-slanted terahertz radar echo signal based on the terahertz high-squint strip SAR imaging geometric model; A preliminary correction module is used to perform residual video phase and movement correction processing on the de-skewed terahertz radar echo signal in the range frequency domain-azimuth time domain to obtain a preliminary corrected echo signal; A range processing module is used to transform the echo signal after preliminary correction into the two-dimensional frequency domain to complete frequency scaling processing, perform range compression processing in the range time domain-azimuth frequency domain, and then perform residual phase compensation in the two-dimensional frequency domain to obtain the echo signal after range processing; An azimuth processing module is used to perform azimuth third-order phase filtering on the echo signal after range processing, perform nonlinear scaling in the range frequency domain-azimuth time domain, complete azimuth compression in the two-dimensional frequency domain, and convert the azimuth compressed signal into the range frequency domain-azimuth time domain to obtain the echo signal after azimuth processing; The terahertz high squint SAR imaging module is used to perform geometric correction on the echo signal after azimuth processing to obtain a terahertz high squint SAR imaging result.
9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, wherein: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 7 are implemented.