Terahertz video sar fine imaging method, device, equipment and medium

Abstract

The application relates to a terahertz video SAR fine imaging method, device, equipment and medium. The method comprises the following steps: approximating a distance difference as a linear function related to a platform speed, correcting a distance walk through a keystone method, considering the influence of the vertical distance of scattering points in different distance units to a flight line on imaging quality in secondary phase error compensation, constructing a corresponding secondary phase error function, and accurately compensating secondary phase errors in radar signals under a minimum entropy criterion. After phase gradient autofocus, the image is still affected by a coherent spot. At this time, an anisotropic expansion filtering method is used to suppress the influence of the coherent spot. The method can obtain a fine terahertz video SAR image.

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 superimposes each distance unit, that is, the BP algorithm compensates the phase of 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:

[0007] obtaining original data, which is radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar in an off-nadir mode;

[0008] performing distance Fourier transform on the original data to obtain a distance frequency domain phase compensated radar signal by eliminating residual video phase through a compensation function in a distance frequency domain azimuth time domain;

[0009] deriving 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, determining that the distance migration is caused by a heading speed of a moving platform and an azimuth time, and performing radar signal distance migration correction through a keystone transform to obtain a distance migration corrected radar signal;

[0010] performing secondary phase error compensation on the distance migration corrected radar signal based on a minimum entropy criterion to obtain a secondary phase error compensated radar signal, wherein a preliminary secondary phase function is constructed according to a preset Doppler frequency and a vertical distance of a scattering point to an air route, an initial image entropy is saved as a current image entropy value when an image entropy of the phase compensated signal is not the minimum, the preset Doppler frequency is updated according to a preset step, and the secondary phase function is reconstructed using the updated Doppler frequency;

[0011] performing azimuth Fourier transform on the secondary phase error compensated radar signal to obtain a radar image, compensating for high-order phase errors existing in the radar image through gradient phase autofocusing compensation, and obtaining a terahertz video SAR fine imaging result using an anisotropic extension filter.

[0012] In one embodiment, residual video phase of the radar signal is eliminated in a distance frequency domain azimuth time domain through a compensation function, wherein the compensation function is expressed as:

[0013] .

[0014] In the above formula, denotes 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.

[0015] In one embodiment, an approximate expression of a difference between an actual distance of a target to a radar and a reference distance is derived according to an imaging geometric model, and it is determined that the distance migration is caused by a heading speed of a moving platform and an azimuth time, and is expressed as:

[0016] .

[0017] In the above formula, represents the difference between the distance of the target to the radar and the distance of the reference position to the radar, represents the distance of the target to the radar, represents the distance of the reference position to the radar, represents the difference between the distance of the target to the radar and the distance of the reference position to the radar in an ideal state without considering the azimuth velocity, represents the flight velocity.

[0018] 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 adjust the slow time axis of each frequency by linear interpolation, which is represented as:

[0019] .

[0020] In the above formula, represents the coherent difference frequency, represents the center frequency.

[0021] In one embodiment, the preliminary quadratic phase function is represented as:

[0022] .

[0023] In the above formula, represents the preset Doppler frequency, represents the vertical distance of the scattering point to the flight line.

[0024] 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 includes:

[0025] Gaussian smoothing filtering is performed on the image after the high-order phase error compensation according to the width of the preset Gaussian window;

[0026] The first-order gradients in the vertical and horizontal directions of the image after the Gaussian smoothing filtering are solved, and the nonlinear expansion coefficient is obtained according to the first-order gradient obtained by the solving, the preset contrast factor, and the nonlinear expansion transmission function;

[0027] According to the nonlinear expansion coefficient and the step factor of the preset contrast factor, the additive operator splitting algorithm is used to filter the image after the Gaussian smoothing filtering again to obtain the terahertz video SAR fine imaging result.

[0028] In one embodiment, the nonlinear expansion transmission function is represented as:

[0029] .

[0030] In the above formula, denotes a Gaussian smoothing image, denotes a gradient of a contrast factor.

[0031] The application also provides a terahertz video SAR fine imaging device, the device comprising:

[0032] A radar data acquisition module is configured to acquire original data, which is radar echo signal data generated by continuous detection of a target region by a terahertz SAR radar under a desloping system;

[0033] A range frequency domain phase compensation module is configured to eliminate residual video phase by a compensation function in a range frequency domain azimuth time domain after a Fourier transform of the original data to obtain a range frequency domain phase compensated radar signal;

[0034] A range walk correction module is configured to derive an approximate expression of a difference between an actual range of a target to a radar and a reference range according to an imaging geometric model to explicitly indicate that the range walk is caused by a heading speed of a moving platform and an azimuth time, and to correct the range walk of the radar signal by a keystone transform to obtain a range walk corrected radar signal;

[0035] 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 the 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;

[0036] An anisotropic extension filtering module is configured to perform an azimuth Fourier transform on the quadratic phase error compensated radar signal to obtain a radar image, compensate high-order phase errors existing in the radar image by a gradient phase autofocusing compensation, and obtain a terahertz video SAR fine imaging result by an anisotropic extension filtering.

[0037] 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.

[0038] A computer readable storage medium having stored thereon a computer program which, when executed by a processor, implements the specific steps of the following terahertz video SAR fine imaging method.

[0039] The terahertz video SAR fine imaging method, device, equipment and medium described above, by performing distance Fourier transform on the time-sequenced multi-frame radar echo signal data, i.e. original data, generated by continuous detection of a target area by a terahertz SAR radar under a desloping system, the radar signal in the distance frequency domain azimuth time domain is obtained, the residual video phase of the radar signal is eliminated in the distance frequency domain azimuth time domain by a compensation function, the distance frequency domain phase compensated radar signal is obtained, at the same time, according to an imaging geometric model, the difference expression between the actual distance of the target to the radar and the reference distance is derived, the distance migration caused by the distance difference in the distance frequency domain phase compensated radar signal is corrected by a keystone transform, the distance migration corrected radar signal is obtained, the secondary phase error compensation is performed on the distance migration corrected radar signal based on the minimum entropy criterion, the secondary phase error compensated radar signal is obtained, the azimuth Fourier transform is performed on the secondary phase error compensated radar signal, the radar image is obtained, the high-order phase error existing in the radar image is compensated by a gradient phase autofocusing, and then an anisotropic extension filtering is adopted to obtain the terahertz video SAR fine imaging result. The method can be used for fine imaging of terahertz video SAR. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 It is a flowchart of the terahertz video SAR fine imaging method in one embodiment;

[0041] Figure 2 It is a THz-ViSAR imaging geometric model diagram extracted from a terahertz video SAR imaging scene in one embodiment;

[0042] Figure 3 It is a diagram of the position of a point target and the imaging result in one experiment, wherein, Figure 3 (a) is the position of the point target, Figure 3 (b) is the image formed by the point target;

[0043] Figure 4 It is a diagram of the imaging result of a middle point target in one experiment, wherein, Figure 4 (a) indicates a diagram of the result after the secondary phase error compensation of the traditional Doppler frequency only, Figure 4 (b) indicates a diagram of the result after the compensation of the secondary phase error function related to the perpendicular distance of the flight line constructed in the method;

[0044] Figure 5 The images are generated from the shoreline of a lake using the distance-Doppler algorithm and the method described in this paper, respectively, in an experiment. Figure 5 (a) is an image of a lake shore obtained using the distance-Doppler algorithm. Figure 5 (b) Images of a lake shore obtained using this method;

[0045] Figure 6 This is a structural block diagram of a terahertz video SAR fine imaging device in one embodiment;

[0046] Figure 7 This is an internal structural diagram of a computer device in one embodiment. Detailed Implementation

[0047] 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.

[0048] 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.

[0049] 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.

[0050] Specifically, such as Figure 1 As shown, this application proposes a terahertz video SAR fine imaging method, which specifically includes the following steps:

[0051] 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.

[0052] Step S110, after the original data is subjected to the range direction Fourier transform, the residual video phase is eliminated in the range frequency direction azimuth time domain by a compensation function to obtain a range frequency domain phase compensated radar signal.

[0053] Step S120, according to an imaging geometric model, a difference between an actual distance of a target to a radar and a reference distance is derived, the range migration is caused by a heading speed of a moving platform and an azimuth time, and a radar signal range migration is corrected through a keystone transform to obtain a range migration corrected radar signal.

[0054] Step S130, based on a minimum entropy criterion, a secondary phase error compensation is performed on the range 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 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.

[0055] Step S140, an azimuth direction Fourier transform is performed on the secondary phase error compensated radar signal to obtain a radar image, a high-order phase error existing in the radar image is compensated through a gradient phase autofocusing, and a terahertz video SAR fine imaging result is obtained by using an anisotropic extension filter.

[0056] In step S100, the original data is in a strip mode under a terahertz frequency band video synthetic aperture radar (THz-ViSAR) imaging geometric model as shown in Figure 2 . A flight route of an aircraft is in an XOZ plane, parallel to the X axis direction, a flight speed is , a point target P is in a line-of-sight direction-flight route direction coordinate system, an initial position is (x0, y0, z0) , a beam line-of-sight is along an AA1 direction, an oblique viewing angle is ; the radar moves along the azimuth direction from the A point to the B point, , when the radar reaches the B point, the beam line-of-sight is changed to the BB1 direction, at this time, an instantaneous slant range is expressed as:

[0057] (1)

[0058] In formula (1), when the positive side view .

[0059] In the terahertz frequency band, the working frequency of the video SAR is high, generally, a dechirp pulse compression mode is used in the range direction. Supposing that a linear frequency modulation signal transmitted by the radar is:

[0060] (2)

[0061] where, .

[0062] In formula (2), is the center frequency, is the range pulse width, is the frequency modulation rate, is the fast time, is the slow time, is the total time. Let the reference range be , the reference signal is:

[0063] (3)

[0064] In formula (3), is the pulse width of the reference signal, is the speed of light.

[0065] Let the range of a point target to the radar be , the signal received by the radar is:

[0066] (4)

[0067] If , the difference frequency signal is:

[0068] (5)

[0069] In formula (5), the exponential term is the range term, the exponential term is the Doppler phase shift term, and the exponential term is the residual video phase shift term (RVP). The range relative to the reference point is a fixed value within a period of fast time , point target simulation shows that the point target is accurately imaged in the range direction. For the slow time , it is changing, the change of will change the frequency in the corresponding range term, and will also change the azimuth term and the residual video phase shift term, thus presenting the spatial variation.

[0070] The Fourier transform of the difference frequency signal of formula (5) with respect to the fast time is obtained, and the expression in the difference frequency domain is:

[0071] (6)

[0072] In formula (6), , For coherent difference frequency.

[0073] In step S110, the residual video phase of the radar signal is eliminated by a compensation function in the range frequency domain azimuth time domain, to obtain a range frequency domain phase compensated radar signal. Wherein, the residual video phase term is removed by the compensation function to obtain:

[0074] (7)

[0075] That is, formula (7) is the expression of the range frequency domain phase compensated radar signal.

[0076] Further, in step S120, according to the imaging geometric model, the approximate expression of the difference between the actual distance of the target to the radar and the reference distance is derived, and the distance walk is caused by the moving platform heading speed and the slow time, so that a fine approximation is made to obtain:

[0077] (8)

[0078] In formula (8), represents the difference between the distance of the target to the radar and the distance of the reference position to the radar, represents the distance of the target to the radar, represents the distance of the reference position to the radar, represents the difference between the distance of the target to the radar and the distance of the reference position to the radar in an ideal state without considering the azimuth velocity, represents the flight speed, represents the slow time. Then formula (7) becomes:

[0079] (9)

[0080] Further, the distance walk of the radar signal caused by the distance difference in the range frequency domain phase compensated radar signal is corrected by using the Keystone transformation, wherein the Keystone transformation adjusts the slow time axis of each frequency by linear interpolation:

[0081] (10)

[0082] Then formula (9) is converted to:

[0083] (11)

[0084] After the azimuth Fourier transform of formula (11), the following is obtained:

[0085] (12)

[0086] In formula (12), is the azimuth pulse width, is the azimuth frequency. At this time, formula (12) is the radar signal after range walk correction.

[0087] Considering that the quadratic phase error cannot be ignored in the echo signal of the terahertz video SAR, in particular, the conventional image displacement algorithm cannot obtain an accurate Doppler frequency, that is, after the full aperture is divided into front and rear sub-apertures, the image domain sub-aperture is shifted to coincide, and the compensated image is still in a defocused state. Therefore, in step S130, the method of minimum image entropy is used to compensate the quadratic phase error of the radar signal after range walk correction.

[0088] In this embodiment, the radar signal after range walk correction is compensated for quadratic phase error based on the minimum entropy 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 the radar signal after range walk correction for quadratic phase 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 using the updated Doppler frequency and re-compensating the radar signal after range walk correction for quadratic phase until the image entropy is less than the preset image entropy.

[0089] Specifically, the preliminary quadratic phase function is represented as:

[0090] (13)

[0091] In formula (13), represents the preset Doppler frequency, represents the vertical distance of the scattering point to the flight line, represents the slow time.

[0092] In this embodiment, the Doppler frequency obtained from the azimuth velocity measured by the inertial navigation system (INS) and the global positioning system (GPS) is taken as the preset Doppler frequency .

[0093] Specifically, the updated Doppler frequency is represented as:

[0094] (14)

[0095] In formula (14), This indicates the step size for Doppler frequency modulation. This indicates the preset Doppler modulation frequency or the Doppler modulation frequency used in the current construction of the quadratic phase function.

[0096] Furthermore, the quadratic phase error function reconstructed based on the updated Doppler modulation frequency is expressed as:

[0097] .

[0098] Through the processing in step S130, a more accurate Doppler modulation frequency can be obtained, and the secondary phase error of different distance cells is finely compensated, resulting in a high-quality image. However, after phase gradient autofocus, although the image resolution is high, speckle still exists in the image. Due to the influence of speckle, small targets in the image are submerged in noise. Therefore, anisotropic extended filtering is used to suppress speckle.

[0099] In this embodiment, after compensating for high-order phase errors in the radar image through gradient phase autofocus, the anisotropic extended filtering is then applied, including: performing Gaussian smoothing filtering on the image after high-order phase error compensation according to the width of a preset Gaussian window; solving for the first-order gradients in the vertical and horizontal directions of the Gaussian smoothed image; obtaining the nonlinear extension coefficients based on the solved first-order gradients, a preset contrast factor, and the nonlinear extended transfer function; and using an additive operator splitting algorithm based on the step size factor of the nonlinear extension coefficients and the preset contrast factor to filter the Gaussian smoothed image again, thereby obtaining the terahertz video SAR fine imaging result.

[0100] Specifically, the nonlinear extended transfer function is expressed as:

[0101] (15)

[0102] In formula (15), For Gaussian smoothed images, yes gradient, This is the contrast factor.

[0103] In this paper, the effectiveness of the proposed method is also demonstrated through simulation experiments. The parameters of the simulated imaging scene are shown in Table 1.

[0104] Table 1. Parameters for Simulation Data Imaging

[0105]

[0106] Figure 3 These are the location of the point target and the imaging result, respectively. Figure 3 (a) represents the location of the point target.Figure 3 (b) the image of the point target, comparison Figure 3 (a) and Figure 3 (b), it can be seen that the formed position is consistent with the actual position.

[0107] Figure 4 is the imaging result of the intermediate point target, Figure 4 (a) adopts the traditional quadratic phase error compensation only with Doppler frequency, the point target exists cross in the longitudinal direction, and the focusing is not complete. Figure 4 (b) adopts the quadratic phase error function constructed according to the present application and related to the perpendicular distance of the flight line for compensation, the point target shape is good, which indicates that the focusing effect is good.

[0108] Figure 5 are the images of a certain lake bank formed by using the distance Doppler algorithm and the algorithm of the present application respectively, Figure 5 (a) is the image of a certain lake bank formed by using the distance Doppler algorithm, Figure 5 (b) is the image of a certain lake bank formed by using the algorithm of the present application. Figure 5 (a) and Figure 5 (b) are compared, through the human eye observation, the image boundary formed by the algorithm of the present application is wider, and the details are more abundant.

[0109] In the above-mentioned terahertz video SAR fine imaging method, based on the terahertz video SAR imaging scene, the influence of the motion 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 motion speed and time, and the keystone transformation is adopted to correct the distance walk. In the present 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, thereby reducing the influence of the distance space variation on the imaging. Further, the anisotropic expansion filtering method is adopted in the image with residual speckle, the image details are preserved, and the speckle is removed in a targeted manner. Through the above processing, the terahertz video SAR target fine imaging is realized.

[0110] It should be understood that, although Figure 1 the steps in the flowchart of the present application are shown in sequence according to the arrows, these steps are not necessarily executed in sequence according to 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 sequences. Moreover, Figure 1At least one of the steps in the method can comprise a plurality of sub-steps or stages which are not necessarily performed at the same time but can be performed at different times, and the order of the sub-steps or stages is not necessarily sequential but can be performed in rotation or alternation with other steps or sub-steps or stages of other steps.

[0111] In one embodiment, as shown in FIG. 1, a THz video SAR fine imaging device is provided, comprising: a radar data acquisition module 200, a range frequency domain phase compensation module 210, a range walk correction module 220, a quadratic phase error compensation module 230, and an anisotropic extension filtering module 240, wherein: Figure 6

[0112] The radar data acquisition module 200 is configured to acquire raw data, which is radar echo signal data generated by a THz SAR radar in a desloping system for continuous detection of a target region;

[0113] The range frequency domain phase compensation module 210 is configured to, after performing a range direction Fourier transform on the raw data, eliminate residual video phase in the range frequency domain azimuth time domain by a compensation function to obtain a range frequency domain phase compensated radar signal;

[0114] The range walk correction module 220 is configured to, according to an imaging geometric model, derive an approximate expression of a difference between an actual distance of a target to a radar and a reference distance, explicitly correct the range walk caused by a heading speed of a moving platform and an azimuth direction time, and perform a keystone transform to correct the range walk of the radar signal, to obtain a range walk corrected radar signal;

[0115] The 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 the minimum, the preset Doppler frequency is updated according to a preset step, and the quadratic phase function is reconstructed using the updated Doppler frequency;

[0116] The anisotropic extension filtering module 240 is configured to perform an azimuth direction Fourier transform on the quadratic phase error compensated radar signal to obtain a radar image, compensate high-order phase errors existing in the radar image by gradient phase autofocusing, and perform anisotropic extension filtering to obtain a THz video SAR fine imaging result.

[0117] ​The specific definitions of the terahertz video SAR fine imaging device can refer to the definitions 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, and a combination thereof, in whole or in part. 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 of the above modules.

[0118] In an embodiment, a computer device, which can be a terminal, has an internal structure diagram as shown in Figure 7 The computer device includes a processor, a memory, a network interface, a display screen, and an input device connected through a system bus. The processor of the computer device is configured 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 running the operating system and the computer program in the non-volatile storage medium. The network interface of the computer device is configured to communicate with external terminals through a 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. 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 housing of the computer device, or an external keyboard, touchpad, or mouse, etc.

[0119] Those skilled in the art can understand that Figure 7 The structure shown in the foregoing 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 diagram, or combine certain components, or have a different arrangement of components.

[0120] In an embodiment, a computer device is provided, which includes a memory and a processor. The memory stores a computer program. The processor executes the computer program to implement the specific steps in the terahertz video SAR fine imaging method.

[0121] In an embodiment, a computer readable storage medium is provided, which stores a computer program. The computer program is executed by a processor to implement the specific steps in the terahertz video SAR fine imaging method.

[0122] 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.

[0123] 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.

[0124] 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 protection of the present application should be subject to the appended claims.

Claims

1. A terahertz video SAR fine imaging method, characterized in that, The method includes: Acquire raw data, which is radar echo signal data generated by terahertz SAR radar continuously detecting the target area under deskewing mode; After performing a 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. 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. It is clear that the range travel is caused by the heading speed and azimuth time of the moving platform. Then, the radar signal range travel is corrected by the Keystone transformation to obtain the radar signal after range travel correction. Based on the minimum entropy criterion, the radar signal after range movement correction is subjected to secondary phase error compensation to obtain the radar signal after secondary phase error compensation. In this process, 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 phase-compensated signal is not at its 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. After the secondary phase error is compensated, the radar signal is subjected to azimuth Fourier transform to obtain a radar image. Gradient phase autofocus is used to compensate for the high-order phase error in the radar image, and then anisotropic extended filtering is used to obtain the 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 the range-frequency domain and azimuth-time domain using a compensation function, wherein the compensation function is expressed as: In the above formula, Indicates slow time. Indicates the coherent difference frequency. This represents the difference between the distance from the target to the radar and the distance from the reference position to the radar. Represents the speed of light. Indicates frequency modulation. Represents an imaginary number.

3. The terahertz video SAR fine imaging method according to claim 2, characterized in that, 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. It is clear that the range travel is caused by the yaw speed and azimuth time of the moving platform, expressed as: In the above formula, 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.

4. The terahertz video SAR fine imaging method according to claim 3, characterized in that, When correcting radar signal range travel using keystone transformation: The range drift caused by range difference in the radar signal after range-frequency domain phase compensation is corrected by using the Keystone transform. The slow time axis of each frequency is readjusted using linear interpolation via the Keystone transform, as shown below: In the above formula, Indicates the coherent difference frequency. Indicates the center frequency.

5. The terahertz video SAR fine imaging method according to claim 4, characterized in that, The preliminary quadratic phase function is expressed as: In the above formula, This indicates the preset Doppler modulation frequency. This represents the vertical distance from the scattering point to the flight path.

6. The terahertz video SAR fine imaging method according to any one of claims 1-5, characterized in that, When compensating for higher-order phase errors in the radar image through gradient phase self-focusing, and then applying anisotropic extended filtering, the process includes: Gaussian smoothing filtering is applied to the image after high-order phase error compensation according to the preset width of the Gaussian window. The first-order gradients in the vertical and horizontal directions of the image after Gaussian smoothing are solved, and the nonlinear expansion coefficients are obtained based on the solved first-order gradients, the preset contrast factor, and the nonlinear expansion transfer function. Based on the step size factor of the nonlinear spread coefficient and the preset contrast factor, the additive operator splitting algorithm is used to filter the Gaussian smoothed image again to obtain the terahertz video SAR fine imaging result.

7. The terahertz video SAR fine imaging method according to claim 6, characterized in that, The nonlinear extended transmission function is expressed as: In the above formula, Represents a Gaussian smoothed image. express gradient, This represents the contrast factor.

8. A terahertz video SAR fine imaging device, characterized in that, The device includes: The radar data acquisition module is used to acquire raw data, which is radar echo signal data generated by the terahertz SAR radar continuously detecting the target area under the deskewing mode; The range-frequency-domain phase compensation module is used to perform range-to-Fourier transform on the original data, and then eliminate the remaining video phase in the range-frequency-domain azimuth-time domain through a compensation function to obtain the range-frequency-domain phase-compensated radar signal. The range travel correction module is used to derive an approximate expression for the difference between the actual distance from the target to the radar and the reference distance based on the imaging geometry model. It clarifies that the range travel is caused by the heading speed and azimuth time of the moving platform. Then, it corrects the range travel of the radar signal through Keystone transformation to obtain the radar signal after range travel correction. The secondary phase error compensation module is used to perform secondary phase error compensation on the radar signal after range movement correction based on the minimum entropy criterion, to obtain a radar signal with secondary phase error compensation. In this module, a preliminary secondary phase function is constructed based on a preset Doppler modulation frequency and the vertical distance from the scattering point to the flight path. When the image entropy of the phase-compensated signal is not at its minimum, the current image entropy value is saved as the initial image entropy. The preset Doppler modulation frequency is updated according to a preset step size, and the secondary phase function is reconstructed using the updated Doppler modulation frequency. An anisotropic extended filtering module is used to perform azimuth-directed Fourier transform on the radar signal after secondary phase error compensation to obtain a radar image. Gradient phase autofocus is used to compensate for high-order phase errors in the radar image, and then anisotropic extended filtering is used to obtain terahertz video SAR fine imaging results.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method according to any one of claims 1 to 7.

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

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