Terahertz video SAR (Synthetic Aperture Radar) fine imaging method, device, equipment and medium

Abstract

The invention relates to a terahertz video SAR (Synthetic Aperture Radar) fine imaging method, device and equipment and a medium. The method comprises the following steps of: approximating a distance difference into a linear function related to the speed of a platform, correcting range walk through a keystone method, considering the influence of the vertical distance from scattering points of different distance units to an air route on imaging quality in secondary phase error compensation, constructing a corresponding secondary phase error function, and meanwhile, calculating the imaging quality under the minimum entropy criterion. According to the method, secondary phase errors in radar signals are accurately compensated, after phase gradient self-focusing, the image is still affected by speckles, at the moment, an anisotropic extended filtering method is adopted, the influence of the speckles is restrained, and a fine terahertz video SAR image can be obtained through the method.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radar imaging and signal processing, and particularly relates to a terahertz video SAR fine imaging method, device, equipment and medium. BACKGROUND

[0002] A terahertz video synthetic aperture radar THz-ViSAR system has the characteristics of all-weather, all-day, high-resolution real-time imaging of a general synthetic aperture radar, and can continuously perceive a moving target. In combination with an infrared, photoelectric or other sensing device, the THz-ViSAR can be widely applied to ground moving target detection and tracking occasions such as unmanned aerial vehicle field perception.

[0003] The THz-ViSAR has a high working frequency and is greatly affected by atmospheric attenuation. The imaging scene has particularity, data acquisition and real-time requirements, which bring challenges to imaging processing. At present, most video SAR imaging algorithms adopt a BP (Back-projection algorithm) algorithm in a time domain algorithm and a fast algorithm thereof. The BP algorithm establishes an imaging network and coherently stacks each distance unit, that is, the BP algorithm performs phase compensation on each distance unit. Obviously, the BP algorithm needs to traverse all distance units. Even if the fast BP algorithm is used, the imaging calculation amount is large, and the accuracy of the imaging model is required to be high. In the THz-ViSAR, it is difficult to realize real-time imaging of a moving target. There are also frequency domain imaging algorithms for video SAR imaging algorithms, mainly including a range migration algorithm (RMA), a chirp scaling algorithm (CSA) and a frequency scaling algorithm (FSA).

[0004] However, as a new imaging mode, the THz-ViSAR cannot obtain a high-quality SAR image by simply using the above frequency domain imaging algorithm. In the terahertz video SAR, because the radar has a high working frequency, an imaging range is small, an action distance is short, and a coherent spot has a great influence, it is difficult to obtain a fine SAR image by using a traditional simple approximate method. SUMMARY

[0005] Therefore, it is necessary to provide a terahertz video SAR fine imaging method, device, equipment and medium capable of obtaining a fine video SAR image in view of the above technical problems.

[0006] A terahertz video SAR fine imaging method, the method comprising: obtaining original data, the original data being radar echo signal data generated by a terahertz SAR radar in a desloping system for continuous detection of a target region; After distance Fourier transform on the original data, a residual video phase of a radar signal in a distance frequency domain azimuth time domain is eliminated by a compensation function to obtain a distance frequency domain phase compensation radar signal; According to an imaging geometric model, an approximate expression of a difference between an actual distance of a target to a radar and a reference distance is derived to determine that the distance migration is caused by a heading speed of a moving platform and an azimuth time, and a radar signal distance migration is corrected by a keystone transform to obtain a distance migration corrected radar signal; Based on a minimum entropy criterion, a secondary phase error compensation is performed on the distance migration corrected radar signal to obtain a secondary phase error compensated radar signal, wherein a preliminary secondary phase function is constructed according to a preset Doppler frequency modulation and a vertical distance of a scattering point to an air route, when an image entropy of the phase compensated signal is not the minimum, a current image entropy value is saved as an initial image entropy, the preset Doppler frequency modulation is updated according to a preset step, and the secondary phase function is reconstructed by using the updated Doppler frequency modulation; A radar image is obtained by performing an azimuth Fourier transform on the secondary phase error compensated radar signal, a high-order phase error existing in the radar image is compensated by a gradient phase self-focusing compensation, and a terahertz video SAR fine imaging result is obtained by using an anisotropic extension filter.

[0007] In one embodiment, a residual video phase of the radar signal in a distance frequency domain azimuth time domain is eliminated by a compensation function, wherein the compensation function is expressed as: .

[0008] In the above formula, denotes a slow time, denotes a coherent difference frequency, denotes a difference between a distance of a target to a radar and a distance of a reference position to the radar, denotes a speed of light, denotes a frequency modulation, denotes an imaginary number.

[0009] In one embodiment, according to an imaging geometric model, an approximate expression of a difference between an actual distance of a target to a radar and a reference distance is derived to determine that the distance migration is caused by a heading speed of a moving platform and an azimuth time, and is expressed as: .

[0010] In the above formula, denotes a difference between a distance of a target to a radar and a distance of a reference position to the radar, denotes the distance of the target to the radar, denotes the distance of the reference position to the radar, denotes the difference between the distance of the target to the radar and the distance of the reference position to the radar in the ideal state without considering the azimuth velocity, denotes the flight velocity.

[0011] In one embodiment, when the radar signal distance migration caused by the distance difference is corrected by the Keystone transformation after the distance frequency domain phase compensation of the radar signal, the Keystone transformation is used to readjust the slow time axis of each frequency by linear interpolation, which is expressed as: .

[0012] In the above formula, denotes the coherent difference frequency, denotes the center frequency.

[0013] In one embodiment, the preliminary quadratic phase function is expressed as: .

[0014] In the above formula, denotes the preset Doppler frequency, denotes the vertical distance of the scattering point to the flight line.

[0015] In one embodiment, when the anisotropic expansion filter is used after the high-order phase error existing in the radar image is compensated by the gradient phase self-focusing, it comprises: performing Gaussian smoothing filtering on the image after the high-order phase error compensation according to the width of the preset Gaussian window; solving the first-order gradient of the vertical and horizontal directions of the image after the Gaussian smoothing filtering, and obtaining a nonlinear expansion coefficient according to the solved first-order gradient, a preset contrast factor and a nonlinear expansion transfer function; according to the nonlinear expansion coefficient and a step factor of the preset contrast factor, using an additive operator splitting algorithm to filter the image after the Gaussian smoothing filtering again to obtain the terahertz video SAR fine imaging result.

[0016] In one embodiment, the nonlinear expansion transfer function is expressed as: .

[0017] In the above formula, denotes the Gaussian smoothing image, denotes the gradient of , and denotes the contrast factor.

[0018] The application also provides a terahertz video SAR fine imaging device, which comprises: A radar data acquisition module is configured to acquire original data, which is radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar under a desloping system; A distance frequency domain phase compensation module is configured to eliminate residual video phase by a compensation function to obtain a distance frequency domain phase compensated radar signal after distance Fourier transform of the original data; A range walk correction module is configured to derive an approximate expression of a difference between an actual distance of a target to a radar and a reference distance according to an imaging geometric model, to explicitly correct the range walk caused by a heading speed of a moving platform and a range time, and to correct the range walk of the radar signal by a keystone transform to obtain a range walk corrected radar signal; A quadratic phase error compensation module is configured to compensate a quadratic phase error of the range walk corrected radar signal based on a minimum entropy criterion to obtain a quadratic phase error compensated radar signal, wherein a preliminary quadratic phase function is constructed according to a preset Doppler frequency and a vertical distance of a scattering point to a flight line, an initial image entropy is saved when an image entropy of the phase compensated signal is not minimum, the preset Doppler frequency is updated according to a preset step, and the quadratic phase function is reconstructed by using the updated Doppler frequency; An anisotropic extension filtering module is configured to perform a range Fourier transform on the quadratic phase error compensated radar signal to obtain a radar image, to compensate high-order phase errors existing in the radar image by gradient phase autofocusing, and to obtain a terahertz video SAR fine imaging result by anisotropic extension filtering.

[0019] A computer device comprises a memory and a processor, the memory stores a computer program, and the processor implements specific steps in the above terahertz video SAR fine imaging method when executing the computer program.

[0020] A computer readable storage medium stores a computer program, and the computer program implements specific steps in the following terahertz video SAR fine imaging method when executed by a processor.

[0021] The terahertz video SAR fine imaging method, device, equipment and medium, through distance Fourier transform on time-sequenced multi-frame radar echo signal data, i.e. original data, generated by continuous detection of a target region by a terahertz SAR radar in a desloping system, distance frequency domain azimuth time domain radar signals are obtained, residual video phases of the radar signals are eliminated by a compensation function in the distance frequency domain azimuth time domain, distance frequency domain phase compensation radar signals are obtained, the difference between the actual distance of the target to the radar and the reference distance is derived according to an imaging geometric model, the distance travel caused by the distance difference in the distance frequency domain phase compensation radar signals is corrected by a keystone transform, distance travel correction radar signals are obtained, secondary phase error compensation is performed on the distance travel correction radar signals based on the minimum entropy criterion, secondary phase error compensation radar signals are obtained, azimuth Fourier transform is performed on the secondary phase error compensation radar signals, radar images are obtained, high-order phase errors existing in the radar images are compensated by gradient phase autofocusing, and terahertz video SAR fine imaging results are obtained by anisotropic extension filtering. The method can be used for fine imaging of terahertz video SAR. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 It is a flowchart of a terahertz video SAR fine imaging method in an embodiment; Figure 2 It is a THz-ViSAR imaging geometric model diagram extracted from a terahertz video SAR imaging scene in an embodiment; Figure 3 It is a diagram of the position of a point target and imaging results in an experiment, wherein, Figure 3 (a) is the position of the point target, Figure 3 (b) is an image formed by the point target; Figure 4 It is a diagram of the imaging results of a middle point target in an experiment, wherein, Figure 4 (a) indicates a diagram of results after secondary phase error compensation by a traditional method only with Doppler frequency modulation, Figure 4 (b) indicates a diagram of results after compensation by a secondary phase error function related to the distance perpendicular to the flight line in the method; Figure 5 It is an image of a lake bank formed by a distance Doppler algorithm and the method, respectively, in an experiment, wherein, Figure 5 (a) is an image of a lake bank formed by the distance Doppler algorithm, Figure 5 (b) is an image of a lake bank formed by the method; Figure 6This is a structural block diagram of a terahertz video SAR fine imaging device in one embodiment; Figure 7 This is an internal structural diagram of a computer device in one embodiment. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0024] In the terahertz band, due to the high operating frequency of radar, data acquisition typically employs a de-skewing method, resulting in de-skewing radar echo signals. In the low-frequency band, the difference between the target-to-radar distance and the reference-to-radar distance is generally very small over a very short time. This difference often results in a phase change in imaging that is less than π / 4 and is frequently ignored; in fact, imaging in most of the terahertz band is neglected. However, for terahertz video SAR, the difference between the target-to-radar distance and the reference-to-radar distance affects the quality of image focusing. In this application, the difference between the target-to-radar distance and the reference-to-radar distance is considered and expressed as a time function of horizontal velocity for a precise approximation.

[0025] Furthermore, the constructed quadratic phase error function typically uses the Doppler modulation frequency as a coefficient, which can yield good imaging results under less demanding conditions, such as low-frequency radar imaging. However, in the terahertz band, the Doppler modulation frequency varies between different range cells, exhibiting spatial variability. This spatial variability can lead to residual quadratic phase errors, affecting imaging quality. To address this issue, this application constructs a quadratic phase error function related to the vertical distance of the flight path and obtains a more accurate quadratic phase error function under the criterion of minimum image entropy.

[0026] Specifically, such as Figure 1 As shown, this application proposes a terahertz video SAR fine imaging method, which specifically includes the following steps: Step S100: Obtain raw data. The raw data is radar echo signal data generated by the terahertz SAR radar continuously detecting the target area under the deskewing mode.

[0027] Step S110: After performing range-to-Fourier transform on the original data, the remaining video phase is eliminated in the range-frequency domain and azimuth-time domain by a compensation function to obtain the range-frequency domain phase-compensated radar signal.

[0028] Step S120: Based on the imaging geometry model, derive an approximate expression for the difference between the actual distance from the target to the radar and the reference distance. Clarify that the range movement is caused by the heading speed and azimuth time of the moving platform. Then, correct the radar signal range movement through Keystone transformation to obtain the radar signal after range movement correction.

[0029] Step S130: Based on the minimum entropy criterion, perform secondary phase error compensation on the radar signal after range movement correction to obtain the radar signal after secondary phase error compensation. In this step, a preliminary secondary phase function is constructed based on the preset Doppler modulation frequency and the vertical distance from the scattering point to the flight path. When the image entropy of the signal after phase compensation is not the minimum, the current image entropy value is saved as the initial image entropy. The preset Doppler modulation frequency is updated according to the preset step size, and the secondary phase function is reconstructed using the updated Doppler modulation frequency.

[0030] Step S140: After secondary phase error compensation, the radar signal is subjected to azimuth Fourier transform to obtain the radar image. The high-order phase error in the radar image is compensated by gradient phase autofocus, and then anisotropic extended filtering is used to obtain the terahertz video SAR fine imaging result.

[0031] In step S100, the raw data is the imaging geometry model of the terahertz band apparent synthetic aperture radar (THz-ViSAR) in strip mode, such as... Figure 2 As shown. The aircraft's flight path is in the XOZ plane, parallel to the X-axis, and its flight speed is... The initial position of point target P in the line-of-sight-flight coordinate system is ( The beam line of sight is along the AA1 direction, and the oblique angle is... The radar moves along the azimuth direction from point A to point B. When the radar reaches point B, the beam line of sight changes to the BB1 ​​direction, at which point the instantaneous slant range... Represented as: (1) In formula (1), when looking straight ahead... .

[0032] In the terahertz band, video SAR operates at high frequencies, typically employing linear frequency modulation (LFM) in the range direction. Let the LFM signal transmitted by the radar be: (2) in, .

[0033] In formula (2), For the center frequency, For distance pulse width, To adjust the frequency, To save time, For slow time, For the entire time period. Let the reference distance be... The reference signal is: (3) In formula (3), The pulse width of the reference signal, It is the speed of light.

[0034] Let the distance from a point target to the radar be... The radar received the following signal from the target: (4) like The difference frequency signal is: (5) In formula (5), the exponent term For distance term and exponent term For the Doppler phase shift term, the exponential term Remaining video phase shift term (RVP). Distance relative to the reference point. In a fast time Within one cycle, it can be considered a fixed value. Point target simulation shows that the point target is accurately positioned in the range direction of the imaging. For slow time It is subject to change. The change will cause the frequency in the corresponding distance term to change, and will also cause the azimuth term and the remaining video phase shift term to no longer be fixed, but will change, thus exhibiting spatial variability.

[0035] Taking a fast-time Fourier transform of the difference frequency signal in formula (5) yields the expression in the difference frequency domain: (6) In formula (6), , It is the coherent difference frequency.

[0036] In step S110, the residual video phase of the radar signal is eliminated in the range-frequency domain and azimuth-time domain using a compensation function, resulting in a range-frequency domain phase-compensated radar signal. The compensation function... After removing the remaining video phase terms, we get: (7) That is, formula (7) is the expression of the radar signal after range-frequency domain phase compensation.

[0037] Furthermore, in step S120, based on the imaging geometry model, an approximate expression for the difference between the actual distance from the target to the radar and the reference distance is derived, clarifying that the distance travel is caused by the heading speed of the moving platform and the slow time. By making a fine approximation, we obtain: (8) In formula (8), This represents the difference between the distance from the target to the radar and the distance from the reference position to the radar. Indicates the distance from the target to the radar. Indicates the distance from the reference position to the radar. This represents the difference between the distance from the target to the radar under ideal conditions, without considering azimuth velocity, and the distance from the reference position to the radar. Indicates flight speed. This represents slow time. Therefore, formula (7) will become: (9) Furthermore, the range drift of the radar signal caused by range difference in the range-frequency domain phase-compensated radar signal is corrected using the Keystone transform. The Keystone transform readjusts the slow time axis of each frequency through linear interpolation. (10) Then formula (9) is transformed into: (11) After performing an azimuth-to-Fourier transform on formula (11), we obtain: (12) In formula (12), The width of the pulse is oriented in terms of direction. Let be the azimuth frequency. Then, formula (12) is the radar signal after range travel correction.

[0038] Considering that the secondary phase error in the echo signal of terahertz video SAR is not negligible, especially since conventional image shifting algorithms cannot obtain accurate Doppler frequency modulation (FCM), i.e., after the full aperture is divided into front and rear sub-apertures, the image domain sub-apertures are shifted to coincide, and compensation using the secondary phase error function still results in a defocused image, step S130 employs the minimum image entropy method to perform secondary phase error compensation on the radar signal after range travel correction.

[0039] In the embodiment, the radar signal after the range walk correction is compensated for the quadratic phase error based on the minimum entropy as the criterion, including: setting an initial image entropy, constructing a preliminary quadratic phase function according to a preset Doppler frequency and a vertical distance of a scattering point to a flight line, compensating for the quadratic phase of the radar signal after the range walk correction by using the preliminary quadratic phase function to obtain a preliminary quadratic phase compensated signal, calculating an image entropy of the preliminary quadratic phase compensated signal, comparing the image entropy with a preset image entropy, if the image entropy of the preliminary quadratic phase compensated signal is not the minimum, taking the current obtained image entropy as the initial image entropy, updating the preset Doppler frequency according to a step of the preset Doppler frequency, and reconstructing the quadratic phase function by using the updated Doppler frequency and re-compensating for the quadratic phase of the radar signal after the range walk correction until the image entropy is less than the preset image entropy.

[0040] Specifically, the preliminary quadratic phase function is represented as: (13) In the formula (13), represents the preset Doppler frequency, represents the vertical distance of the scattering point to the flight line, represents a slow time.

[0041] In the embodiment, the Doppler frequency obtained by the azimuth velocity measured by the inertial navigation system (INS) and the global positioning system (GPS) is taken as the preset Doppler frequency .

[0042] Specifically, the updated Doppler frequency is represented as: (14) In the formula (14), represents a step of the Doppler frequency, represents the preset Doppler frequency or the Doppler frequency used for constructing the quadratic phase function in the current time.

[0043] Further, the quadratic phase error function reconstructed according to the updated Doppler frequency is represented as: .

[0044] Through the processing of the step S130, the more accurate Doppler frequency can be obtained, and the quadratic phase error of different distance units is finely compensated to obtain the high-quality image. However, after the phase gradient autofocus, the resolution of the image is high, but the coherent speckle still exists in the image. Affected by the coherent speckle, the small targets in the image are submerged in the noise, therefore, the method of anisotropic diffusion filtering is adopted to suppress the coherent speckle.

[0045] In the embodiment, after the high-order phase error existing in the radar image is compensated by gradient phase self-focusing, the anisotropic expansion filtering is adopted, including: performing Gaussian smoothing filtering on the image after the high-order phase error compensation according to a preset width of Gaussian window, solving the first-order gradient of the vertical and horizontal directions of the image after the Gaussian smoothing filtering, obtaining a nonlinear expansion coefficient according to the first-order gradient, a preset contrast factor and a nonlinear expansion transfer function, performing filtering on the image after the Gaussian smoothing filtering again by using an additive operator splitting algorithm according to the nonlinear expansion coefficient and a step factor of the preset contrast factor, and obtaining a terahertz video SAR fine imaging result.

[0046] Specifically, the nonlinear expansion transfer function is represented as: (15) In the formula (15), is a Gaussian smoothing image, is a gradient of , and is a contrast factor.

[0047] In this paper, the effectiveness of the method is also proved by simulation experiments. The parameters of the simulation imaging scene are shown in Table 1.

[0048] Table 1 Parameters of simulation data imaging

[0049] Figure 3 are the positions of the point targets and the imaging results, respectively, Figure 3 (a) is the position of the point target, Figure 3 (b) is the image of the point target, and Figure 3 (a) and Figure 3 (b) can be seen: the formed position is consistent with the actual position.

[0050] Figure 4 is the imaging result of the middle point target, Figure 4 (a) adopts the traditional quadratic phase error compensation only with Doppler frequency, and the point target has a cross in the vertical direction and the focusing is not complete. Figure 4 (b) adopts the quadratic phase error function related to the perpendicular distance of the flight line constructed by the application for compensation, and the point target has a good shape, indicating that the focusing effect is good.

[0051] Figure 5 are images of a lake shore formed by using the range Doppler algorithm and the algorithm of the application, respectively, Figure 5 (a) is an image of a lake shore formed by using the range Doppler algorithm, Figure 5 (b) is an image of a lake shore formed by using the algorithm of the application.Figure 5 (a) and Figure 5 (b) compared, the image formed by the algorithm of the method has wider boundaries and more details in the human eye perception.

[0052] In the above-mentioned terahertz video SAR fine imaging method, based on the terahertz video SAR imaging scene, the influence of the moving platform speed on the relative distance is considered on the imaging geometry model, the relative distance is expressed as the sum of the ideal relative distance and the product of the moving speed and time, and the keystone transformation is used to correct the distance walk. In the method, a quadratic phase error function related to the perpendicular distance of the flight line is constructed when compensating the quadratic phase error, and the accurate Doppler frequency is obtained under the minimum entropy criterion, reducing the influence of distance space variability on imaging. Further, the anisotropic expansion filtering method is used in the image with residual speckle to retain image details and remove speckle in a targeted manner. Through the above processing, the terahertz video SAR target fine imaging is realized.

[0053] It should be understood that, although Figure 1 The steps in the flowchart of the method are displayed in sequence according to the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified herein, the execution of these steps is not strictly limited in sequence, and these steps can be executed in other orders. Moreover, Figure 1 At least part of the steps in the method can include multiple sub-steps or multiple stages, which are not necessarily executed at the same time, but can be executed at different times, and the execution order of these sub-steps or stages is not necessarily sequential, but can be executed in rotation or alternation with at least part of other steps or sub-steps or stages of other steps.

[0054] In one embodiment, as shown in Figure 6 A terahertz video SAR fine imaging device is provided, comprising: a radar data acquisition module 200, a distance frequency domain phase compensation module 210, a distance walk correction module 220, a quadratic phase error compensation module 230, and an anisotropic expansion filtering module 240, wherein: The radar data acquisition module 200 is configured to acquire raw data, wherein the raw data is radar echo signal data generated by a terahertz SAR radar under a desloping system for continuous detection of a target region; The distance frequency domain phase compensation module 210 is configured to eliminate residual video phase by a compensation function in the distance frequency domain azimuth time domain after distance Fourier transform of the raw data to obtain a distance frequency domain phase compensated radar signal; The distance walk correction module 220 is configured to derive an approximate expression of a difference between an actual distance of a target to the radar and a reference distance according to an imaging geometric model, and correct the distance walk of the radar signal by a keystone transform to obtain a radar signal after distance walk correction, wherein the distance walk is caused by a heading speed of the moving platform and a time in the azimuth direction. The secondary phase error compensation module is configured to perform secondary phase error compensation on the radar signal after distance walk correction based on a minimum entropy criterion to obtain a radar signal after secondary phase error compensation, wherein a preliminary secondary phase function is constructed according to a preset Doppler frequency and a vertical distance of a scattering point to the flight line, when the image entropy of the signal after phase compensation is not the minimum, the current image entropy value is saved as an initial image entropy, the preset Doppler frequency is updated according to a preset step, and the secondary phase function is reconstructed by using the updated Doppler frequency. The anisotropic expansion filtering module 240 is configured to perform an azimuth Fourier transform on the radar signal after secondary phase error compensation to obtain a radar image, compensate for high-order phase errors existing in the radar image by gradient phase autofocusing, and obtain a fine imaging result of the terahertz video SAR by anisotropic expansion filtering.

[0055] The specific limitations of the terahertz video SAR fine imaging device can refer to the limitations of the terahertz video SAR fine imaging method in the foregoing, and will not be described here. Each module in the terahertz video SAR fine imaging device described above can be realized by software, hardware, or a combination thereof. The above-mentioned modules can be embedded in or independent of the processor in the computer device in hardware form, or can be stored in the memory in the computer device in software form, so as to be called and executed by the processor to perform the operations corresponding to each module.

[0056] In one embodiment, a computer device is provided, which can be a terminal, and an internal structure diagram of the computer device can be as shown in Figure 7As 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 a computer program. The internal memory provides an environment for the operating system and the computer program in the non-volatile storage medium to run. The network interface of the computer device is used to communicate with the external terminal through the network connection. The computer program is executed by the processor to implement a terahertz video SAR fine imaging method. The display screen of the computer device can be a liquid crystal display screen or an electronic ink display screen, and the input device of the computer device can be a touch layer overlaid on the display screen, or a key, trackball or touchpad arranged on the shell of the computer device, or an external keyboard, touchpad or mouse, etc.

[0057] Those skilled in the art can understand that, Figure 7 The structure shown in the figure is only a block diagram of part of the structure related to the scheme of the present application, and does not constitute a limitation on the computer device to which the scheme of the present application is applied. The specific computer device can include more or fewer components than those shown in the figure, or combine certain components, or have a different component arrangement.

[0058] In one embodiment, a computer device is provided, including a memory and a processor, the memory stores a computer program, and the processor executes the computer program to implement the specific steps in the above-mentioned terahertz video SAR fine imaging method.

[0059] In one embodiment, a computer readable storage medium is provided, which stores a computer program, and the computer program is executed by the processor to implement the specific steps in the above-mentioned terahertz video SAR fine imaging method.

[0060] Those skilled in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a non-volatile computer readable storage medium, and when the computer program is executed, the processes of the above-mentioned embodiments of the methods can be included. Any reference to memory, storage, databases, or other media in this application can include non-volatile and / or volatile memory. Non-volatile memory can include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM), or flash memory. Volatile memory can include random access memory (RAM) or external cache memory. As an illustration but not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM (SLDRAM), Rambus direct RAM (RDRAM), direct RAMbus dynamic RAM (DRDRAM), and Rambus dynamic RAM (RDRAM), etc.

[0061] The technical features of the above embodiments can be combined in any way. In order to make the description simple, all possible combinations of the technical features in the above embodiments are not described, but as long as the combination of the technical features does not exist, it should be considered as the scope of the present application.

[0062] The above embodiments only express several implementation manners of the present application, and the description is more specific and detailed, but it should not be understood as a limitation on the scope of the patent. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, some modifications and improvements can be made, which are all within the scope of the present application. Therefore, the scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for fine imaging of terahertz video SAR, characterized in that, The method comprises: acquiring original data, the original data being radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar under a desloping system; after distance-direction Fourier transform of the original data, residual video phase of the radar signal is eliminated by a compensation function in a distance-frequency domain azimuth-time domain to obtain a distance-frequency domain phase compensation radar signal; according to an imaging geometric model, an approximate expression of a difference between an actual distance of a target to a radar and a reference distance is derived to determine that the distance migration is caused by a heading speed of a moving platform and an azimuth-time, and keystone transform is performed to correct the distance migration of the radar signal to obtain a distance migration corrected radar signal; based on a minimum entropy criterion, secondary phase error compensation is performed on the distance migration corrected radar signal to obtain a secondary phase error compensated radar signal, wherein a preliminary secondary phase function is constructed according to a preset Doppler frequency modulation and a vertical distance of a scattering point to a flight line, when the image entropy of the phase compensated signal is not the minimum, the current image entropy value is saved as an initial image entropy, the preset Doppler frequency modulation is updated according to a preset step, and the secondary phase function is reconstructed by using the updated Doppler frequency modulation; azimuth-direction Fourier transform is performed on the secondary phase error compensated radar signal to obtain a radar image, high-order phase errors existing in the radar image are compensated by gradient phase autofocusing, and anisotropic extension filtering is adopted to obtain a terahertz video SAR fine imaging result.

2. The terahertz video SAR fine imaging method according to claim 1, characterized in that, The residual video phase of the radar signal is eliminated in a distance-frequency domain azimuth-time domain by a compensation function, wherein the compensation function is expressed as: In the above formulae, denotes the slow time, denotes the coherent difference frequency, denotes the difference between the distance of the target to the radar and the distance of the reference position to the radar, denotes the speed of light, denotes the frequency modulation, denotes the imaginary number.

3. The terahertz video SAR fine imaging method according to claim 2, characterized in that, According to an imaging geometric model, an approximate expression of a difference between an actual distance of a target to a radar and a reference distance is derived to determine that the distance migration is caused by a heading speed of a moving platform and an azimuth-time, and is expressed as: In the above formulae, denotes the difference between the target-to-radar distance and the reference position-to-radar distance, denotes the target-to-radar distance, denotes the reference position-to-radar distance, denotes the difference between the target-to-radar distance and the reference position-to-radar distance in the ideal state without considering the azimuth velocity, denotes the flight velocity.

4. The terahertz video SAR fine imaging method according to claim 3, characterized in that, When the distance migration of the radar signal caused by the distance difference in the distance-frequency domain phase compensated radar signal is corrected by keystone transform, the slow time axis of each frequency is readjusted by linear interpolation by Keystone transform, and is expressed as: In the above formula, denotes the coherent difference frequency, denotes the center frequency.

5. The terahertz video SAR fine imaging method according to claim 4, characterized in that, The preliminary secondary phase function is expressed as: In the above formula, denotes a preset Doppler frequency, denotes the vertical distance of the scattering point to the flight line.

6. The terahertz video SAR fine imaging method according to any one of claims 1-5, characterized in that, After the high-order phase errors existing in the radar image are compensated by gradient phase autofocusing, when anisotropic extension filtering is adopted, the following steps are included: Gaussian smoothing filtering is performed on the image after high-order phase error compensation according to a width of a preset Gaussian window; a first-order gradient in a vertical and horizontal direction of the image after Gaussian smoothing filtering is solved, a nonlinear extension coefficient is obtained according to the solved first-order gradient, a preset contrast factor and a nonlinear extension transfer function; by using an additive operator splitting algorithm, the image after Gaussian smoothing filtering is filtered again according to the nonlinear extension coefficient and a step factor of the preset contrast factor to obtain a terahertz video SAR fine imaging result.

7. The terahertz video SAR fine imaging method according to claim 6, characterized in that, The nonlinear extension transfer function is expressed as: In the above formula, denotes a Gaussian smoothed image, denotes a gradient of a contrast factor.

8. A terahertz video SAR fine imaging device, characterized in that, The device comprises: a radar data acquisition module configured to acquire original data, the original data being radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar under a desloping system; A distance frequency domain phase compensation module is configured to eliminate residual video phase by a compensation function to obtain a distance frequency domain phase compensated radar signal after distance Fourier transform of the original data. A distance migration correction module is configured to derive an approximate expression of a difference between an actual distance of a target to a radar and a reference distance according to an imaging geometry model, to correct the distance migration of the radar signal by a keystone transform, and to obtain a distance migration corrected radar signal, where the distance migration is caused by a heading speed of a moving platform and a range time. A secondary phase error compensation module is configured to compensate a secondary phase error of the distance migration corrected radar signal based on a minimum entropy criterion, to obtain a secondary phase error compensated radar signal, and to save an initial image entropy as a current image entropy value when an image entropy of the phase compensated signal is not the minimum, to update a preset Doppler frequency according to a preset step, and to reconstruct a secondary phase function by using the updated Doppler frequency. An anisotropic expansion filter module is configured to perform a range Fourier transform on the secondary phase error compensated radar signal to obtain a radar image, to compensate a high order phase error in the radar image by a gradient phase self-focusing compensation, and to obtain a terahertz video SAR fine imaging result by an anisotropic expansion filter. 9.A computer device, comprising a memory and a processor, wherein the memory stores a computer program, and the computer device is configured to perform the method according to any one of claims 1-8 when the computer program is executed by the processor. The computer program is executed by the processor to implement the steps of the method of any one of claims 1 to 7.

10. A computer-readable storage medium having stored thereon a computer program, characterized in that, The computer program is executed by the processor to implement the steps of the method of any one of claims 1 to 7.

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