Terahertz video sar imaging method and device with quadratic phase error compensation

Abstract

The application relates to a kind of secondary phase error compensation terahertz video SAR imaging method and device, by the characteristics of terahertz video SAR imaging and the secondary phase error that appears in the imaging process, first by improved image displacement algorithm, obtain low resolution imaging, again with minimum image entropy criterion, construct the secondary phase error function related to the vertical distance of flight line, accurately compensate the residual secondary phase error of image, obtain fine high-resolution image.The secondary phase error function constructed by the application considers the influence of the vertical distance of different distance units and flight line on imaging, and can reduce the influence of the range dependence of distance unit on imaging.The method obtains more accurate Doppler frequency and secondary phase error function than image displacement algorithm by minimum image entropy processing, compensates the influence of secondary phase error on imaging, so as to obtain fine high-quality image.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of radar imaging and radar signal processing, in particular to a terahertz video SAR imaging method and device for secondary phase error compensation. BACKGROUND

[0002] Doppler frequency is an important parameter in SAR imaging, and the use of a secondary phase function related to the Doppler frequency to compensate for echo signals is a key step to achieve high-resolution radar imaging. The Doppler frequency parameter has a great influence on the imaging quality, and inaccurate Doppler frequency will cause the image to be out of focus, and even produce "ghosts", which can seriously affect the identification of the shape of the target.

[0003] From the derived expression, the Doppler frequency is subject to the motion state of the platform, such as the motion speed, acceleration, and yaw angle during the motion, which can be measured by the inertial navigation equipment of the flight platform and estimated jointly with the radar echo data. The Doppler frequency can also be estimated by the sub-aperture correlation method, reflectivity shift method, and frequency correlation method. The commonly used methods are image bias algorithm and minimum entropy method. However, when the residual distance migration is large, the effect of the Doppler frequency using the Map-drift algorithm is difficult to guarantee. The flight speed is used to calculate the Doppler frequency for 1 time coarse correction to eliminate most of the distance curvature, and then the Map-drift is used to estimate the accurate Doppler frequency. At the same time, a Doppler frequency estimation method based on image minimum entropy under narrowband conditions is used. This method searches for different Doppler frequencies using the minimum entropy criterion, and the Doppler frequency with the minimum image entropy is the correct Doppler frequency. The Map-drift algorithm relies on the correlation peak position to estimate the Doppler frequency error. Due to the discretization processing of the echo signal, when the azimuth position difference between the two views is lower than the minimum distance unit interval, the azimuth correlation will not be able to detect the error of the Doppler frequency. When the Doppler parameter is segmented and fitted, a large fluctuation error will be generated, which will affect the estimation accuracy of the Doppler frequency. To correct this error, the minimum entropy criterion of the image is introduced to correct the Doppler frequency of the Map-drift algorithm. Through the image displacement algorithm and the minimum entropy criterion, the Doppler frequency is estimated, which is the imaging method of many radars. In the terahertz video SAR, due to the low azimuth position difference between the images, the image displacement algorithm cannot accurately estimate the Doppler frequency value of the radar, and the estimated results are the same for different distance units.

[0004] However, in the terahertz video SAR, the Doppler frequency of different distance units is different, and the secondary phase error of the center position can be well compensated by using the same Doppler frequency. However, for the distance units far from the center position, there will be residual secondary phase error, and this distance-dependent variability will affect the imaging quality of the image. SUMMARY

[0005] Therefore, it is necessary to provide a terahertz video SAR imaging method and device with secondary phase error compensation to effectively improve imaging quality.

[0006] A terahertz video SAR imaging method with secondary phase error compensation, the method comprising:

[0007] Obtaining raw data, the raw data being radar echo signal data generated by continuous detection of a target region by a terahertz SAR radar, and pre-processing the raw data to obtain an echo signal;

[0008] Using a secondary phase function constructed by a preset initial Doppler frequency, performing preliminary secondary phase error compensation on the echo signal;

[0009] Dividing the full-aperture signal after preliminary compensation into two sub-aperture signals in the azimuth time domain, performing azimuth Fourier transform on the sub-aperture data to obtain corresponding image signals, finding a common feature point on the two image signals, recording the slant range of the feature point, selecting a distance unit where the feature point is located, performing cross-correlation operation on the selected distance unit to find a correlation peak position, obtaining a translation amount according to the correlation peak positions of the two image signals, and estimating a two-dimensional phase error frequency according to the translation amount;

[0010] If the translation amount obtained from the correlation peak positions of the two image signals is 0, the initial Doppler frequency is the frequency of the full-aperture signal, and if the translation amount obtained from the correlation peak positions of the two image signals is not 0, the initial Doppler frequency and the estimated two-dimensional phase error frequency are combined to obtain a new Doppler frequency;

[0011] Constructing a two-dimensional phase error function according to the new Doppler frequency, multiplying the two-dimensional phase error function by the echo signal after motion compensation and range compression to perform two-dimensional phase error compensation, saving the Doppler frequency as the initial Doppler frequency, dividing the full-aperture signal into two sub-aperture signals in the azimuth time domain, and performing image displacement processing until the translation amount obtained from the correlation peak positions of the two image signals is 0 to obtain the frequency of the full-aperture signal and a primary focus image signal;

[0012] Setting an initial value of image entropy, constructing a secondary phase error function using the frequency of the full-aperture signal and the vertical distance from the distance unit to the flight line, and compensating the primary focus image signal using the secondary phase error function;

[0013] The image entropy of the compensated image signal is compared with an initial value of the image entropy, if the obtained image entropy is not minimum, the current image entropy is set as the initial image entropy, the frequency modulation rate of the quadratic phase error function is changed according to a preset frequency modulation rate step, a new quadratic phase error function is constructed to compensate the initial focusing image signal, until the obtained image entropy is minimum, and the current compensated focusing image signal is the THz video SAR imaging result.

[0014] In one embodiment, when the echo signal is obtained by pre-processing the original data, the original data is subjected to motion compensation and distance compression to obtain a dechirp echo signal.

[0015] In one embodiment, the Doppler frequency modulation rate obtained according to the azimuth velocity measured by the inertial navigation system and the global positioning system is used as the initial Doppler frequency modulation rate.

[0016] In one embodiment, the two-dimensional phase error frequency modulation rate is estimated according to the translation amount, and the following formula is used:

[0017] ;

[0018] In the above formula, represents the azimuth repetition frequency, represents the number of azimuth signal acquisition points, represents the number of moving points between the front and rear sub-aperture spectra, i.e. the translation amount.

[0019] In one embodiment, if the translation amount obtained from the correlation peak position of the two image signals is not 0, the initial Doppler frequency modulation rate and the estimated two-dimensional phase error frequency modulation rate are combined to obtain a new Doppler frequency modulation rate, and the process is represented as:

[0020] ;

[0021] In the above formula, represents the initial Doppler frequency modulation rate, represents the estimated two-dimensional phase error frequency modulation, represents the slant range at which a feature point is recorded on the image signal, represents the vertical distance from the range unit to the flight line.

[0022] In one embodiment, the frequency modulation rate of the full-aperture signal and the vertical distance from the range unit to the flight line are used to construct a quadratic phase error function, and the quadratic phase error function is represented as:

[0023] ;

[0024] In the above formula, denotes the vertical distance from the range cell to the flight line, denotes the current Doppler frequency.

[0025] In one embodiment, the new quadratic phase error function is constructed to compensate for the initial focusing image signal, wherein the new quadratic phase error function is expressed as:

[0026] ;

[0027] In the above formula, denotes the vertical distance from the range cell to the flight line, denotes the current Doppler frequency, denotes the preset Doppler frequency step.

[0028] The application also provides a terahertz video SAR imaging device for quadratic phase error compensation, comprising:

[0029] An echo signal obtaining module is configured to obtain original data, which is radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar, and to pre-process the original data to obtain an echo signal.

[0030] An initial quadratic phase error compensation module is configured to perform initial quadratic phase error compensation on the echo signal by using a quadratic phase function constructed by a preset initial Doppler frequency.

[0031] A two-dimensional phase error Doppler frequency estimation module is configured to divide the full-aperture signal after initial compensation into two sub-aperture signals in the azimuth time domain, perform azimuth Fourier transform on the sub-aperture data respectively, obtain corresponding image signals, find a feature point that is common to the two image signals, record the slant range of the feature point, select a range cell where the feature point is located, perform cross-correlation operation on the selected range cell, find the correlation peak position, obtain a translation amount according to the correlation peak position of the two image signals, and estimate the two-dimensional phase error Doppler frequency according to the translation amount.

[0032] A Doppler frequency updating module is configured to, if the translation amount obtained from the correlation peak position of the two image signals is 0, take the initial Doppler frequency as the Doppler frequency of the full-aperture signal, and if the translation amount obtained from the correlation peak position of the two image signals is not 0, combine the initial Doppler frequency with the estimated two-dimensional phase error Doppler frequency to obtain a new Doppler frequency.

[0033] A two-dimensional phase error function construction module is configured to construct a two-dimensional phase error function according to the new Doppler frequency, multiply the two-dimensional phase error function with the echo signal after motion compensation and range compression, and perform two-dimensional phase error compensation, and save the Doppler frequency as an initial Doppler frequency, and divide the full-aperture signal in the azimuth time domain into two sub-aperture signals, and perform image shifting processing until the shift amount obtained from the correlation peak positions of the two image signals is 0, to obtain a frequency modulation of the full-aperture signal and a primary focus image signal.

[0034] A quadratic phase error function construction module is configured to set an initial value of image entropy, and construct a quadratic phase error function using the frequency modulation of the full-aperture signal and the vertical distance from the range unit to the flight line, and compensate the primary focus image signal using the quadratic phase error function.

[0035] A terahertz video SAR imaging obtaining module is configured to compare the image entropy of the compensated image signal with the initial value of the image entropy, set the current image entropy as the initial image entropy if the obtained image entropy is not the minimum, change the frequency modulation of the quadratic phase error function according to a preset frequency modulation step, construct a new quadratic phase error function to compensate the primary focus image signal, until the obtained image entropy is the minimum, and then the current compensated focus image signal is the terahertz video SAR imaging result.

[0036] The terahertz video SAR imaging method and device with quadratic phase error compensation described above, by the characteristics of terahertz video SAR imaging and the quadratic phase error occurring in the imaging process, first obtain a low-resolution image by the improved image shifting algorithm, then construct a quadratic phase error function related to the vertical distance from the range unit to the flight line according to the minimum image entropy criterion, accurately compensate the residual quadratic phase error of the image, and obtain a fine high-resolution image. The quadratic phase error function constructed by the method takes into account the influence of the vertical distance from different distance units to the flight line on imaging, which can reduce the influence of the range-dependent characteristic of the distance unit on imaging. The method obtains more accurate Doppler frequency modulation and quadratic phase error function than the image shifting algorithm by the minimum image entropy processing, compensates the influence of the quadratic phase error on imaging, and thus obtains a fine high-quality image. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 A schematic diagram of a planar model of radar collected signals in an embodiment;

[0038] Figure 2 A schematic diagram of the flow of the terahertz video SAR imaging method with quadratic phase error compensation in an embodiment;

[0039] Figure 3 A schematic diagram of point target imaging in an experiment, wherein, Figure 3(a) represents a schematic diagram of a point target position, Figure 3 (b) represents a schematic diagram of a point target imaging result;

[0040] Figure 4 is a schematic diagram of the quadratic phase error compensation of the intermediate point in an experiment, wherein, Figure 4 (a) represents a schematic diagram of the traditional quadratic phase error compensation, Figure 4 (b) represents a schematic diagram of the result of the quadratic phase error compensation using the method;

[0041] Figure 5 is a schematic diagram of the profile of the intermediate point of the point target obtained using the method in an experiment, wherein, Figure 5 (a) represents a schematic diagram of the profile in the range direction, Figure 5 (b) represents a schematic diagram of the profile in the azimuth direction;

[0042] Figure 6 is a structural block diagram of a terahertz video SAR imaging device for quadratic phase error compensation in an embodiment;

[0043] Figure 7 is an internal structure diagram of a computer device in an embodiment. DETAILED DESCRIPTION

[0044] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0045] First, the relevant theoretical part is introduced herein. As shown in Figure 1 is a planar model diagram of the signal collected by the terahertz radar, wherein the platform for carrying the radar, i.e., the carrier, flies along the axis, is the length of the collection interval, is a certain scattering point of the scene, is the synthetic aperture length of the radar. is the vertical distance of the scattering point to the flight line. represents the instantaneous distance of the antenna to the target point at time . Let represent the position of the radar antenna phase center (APC) at time , and let represent the instantaneous distance of the antenna to the target point, i.e.:

[0046] (1)

[0047] In formula (1), represents the instantaneous distance of the antenna to the target point at time The position of the radar APC at time 0.

[0048] Generally always , in the vicinity of the formula (1) Taylor expansion, keeping its quadratic, get:

[0049] (2)

[0050] Assuming the instantaneous speed of the carrier at each time represents the average speed and the disturbance speed , then:

[0051] (3)

[0052] Assuming the distance migration correction and distance compression have been completed, so you can not consider the specific form of the radar signal, the phase history of the echo is:

[0053] (4)

[0054] In formula (4), the indicates the wavelength of the terahertz radar.

[0055] The instantaneous Doppler frequency is obtained as:

[0056] (5)

[0057] In formula (5), the

[0058] ;

[0059] ;

[0060] .

[0061] The first term on the right side of formula (5) is the frequency generated by the average speed, the second term is the frequency caused by the change in speed, and the third term is the frequency caused by the acceleration of the speed change. In the periodic terahertz frequency band, the carrier speed changes slowly, and the synthetic aperture time is relatively short, so the acceleration is ignored, and the instantaneous frequency can be approximately expressed as:

[0062] (6)

[0063] Since , the frequency is independent of the position of the scattering point, and is applicable to all scattering points with a vertical distance of

[0064] Further, the estimation of Doppler frequency generally adopts the image bias algorithm (MDA). MDA divides the full aperture time into two non-overlapping sub-apertures, namely the front and rear sub-apertures, which can be decomposed into constants, first components and second components. The constants are the same as the second components, and the first components make the two sub-aperture images shift. By estimating the shift between the sub-apertures, the second order coefficient of the whole aperture is estimated.

[0065] Let the original data be a range cell azimuth signal , wherein, represents an echo sequence, represents the accumulation time of the echo signal. Since the frequency modulation rate is large, the initial frequency modulation rate is set as: wherein, the velocity and the squint angle are given by the inertial navigation, is the slant range from the scattering point to the radar. After compensating the azimuth signal with the initial frequency modulation rate, the range cell azimuth signal is obtained as:

[0066] (7)

[0067] In formula (7), represents the compensated frequency modulation rate.

[0068] Next, the MDA algorithm divides the azimuth signal into front and rear sub-apertures, i.e.:

[0069] (8)

[0070] wherein, ;

[0071] (9)

[0072] wherein, .

[0073] Further, the front and rear sub-apertures and are subjected to azimuth Fourier transform to obtain:

[0074] (10)

[0075] (11)

[0076] In formula (10) and (11), represents the frequency after the azimuth Fourier transform, , respectively represent:

[0077] (12)

[0078] (13)

[0079] From equation (10) to equation (13), it can be seen that the spectra of the front and rear sub-apertures are the same or similar. If the spectra of the front and rear sub-apertures are made to coincide by spectrum translation, the frequency modulation is 0. The frequency difference between the two spectra is , and the estimated frequency modulation is . The estimated value of the frequency modulation of the range cell azimuth signal is . The actual signal is discrete, and assuming that the azimuth repetition frequency is , the number of azimuth signal sampling points is , and the spectrum moves points between the front and rear sub-apertures, the estimated frequency modulation is:

[0080] (14)

[0081] The estimated value of the frequency modulation of all range cells is:

[0082] (15)

[0083] In equation (15), is the vertical distance of the range cell to the flight line.

[0084] In summary, for terahertz video SAR, the spectra of the front and rear sub-apertures coincide, and the estimation of the Doppler frequency modulation is also inaccurate. In the iterative process, the iteration often does not converge, and in severe cases, the image quality deteriorates. Taking a specific imaging scenario, the frequency modulation is kHz, 2048 points are taken, the spectrum distance is 1 frequency point, and the corresponding frequency modulation interval is 244 Hz / s. For the azimuth frequency modulation of terahertz video SAR, 244 Hz / s is enough to make the image from clear to blurred, and then to produce a serious "ghost" image. That is, it is difficult to obtain a fine image through the image displacement algorithm.

[0085] Then, it can be seen from equation (6) that the instantaneous frequency modulation is related to the vertical distance of the scattering point to the flight line , and the instantaneous frequency modulation of scattering points in different range cells is different, so when constructing the quadratic phase error function, the vertical distance of different range cells to the flight line needs to be considered.

[0086] Therefore, in the process of terahertz video SAR imaging, if the Doppler frequency of the full-aperture echo signal is regarded as a constant, the difference and the space variation of the Doppler frequency of different range cells are not considered, the compensation of the quadratic phase error is not complete, and even there is residual quadratic phase error at the center of the non-range cell. In the embodiment, reference Figure 2 is made to a terahertz video SAR imaging method for compensating quadratic phase error, which specifically comprises the following steps.

[0087] In step S100, original data is obtained, which is radar echo signal data generated by continuous detection of a target region by a terahertz SAR radar, and the original data is preprocessed to obtain echo signals.

[0088] In step S110, a quadratic phase function constructed by using a preset initial Doppler frequency is used to perform preliminary compensation of the quadratic phase error of the echo signals.

[0089] In step S120, the full-aperture signals after the preliminary compensation are divided into front and rear sub-apertures in the azimuth time domain, the sub-aperture data are subjected to azimuth Fourier transform respectively to obtain corresponding image signals, a common feature point is found on the two image signals, the slant range of the feature point is recorded, a range cell where the feature point is located is selected, cross-correlation operation is performed on the selected range cell, a correlation peak position is found, a translation amount is obtained according to the correlation peak positions of the two image signals, and a two-dimensional phase error frequency is estimated according to the translation amount.

[0090] In step S130, if the translation amount obtained from the correlation peak positions of the two image signals is 0, the initial Doppler frequency is the frequency of the full-aperture signals, and if the translation amount obtained from the correlation peak positions of the two image signals is not 0, the initial Doppler frequency and the estimated two-dimensional phase error frequency are combined to obtain a new Doppler frequency.

[0091] In step S140, a two-dimensional phase error function is constructed according to the new Doppler frequency, the two-dimensional phase error function is multiplied by the echo signals after motion compensation and range compression to perform two-dimensional phase error compensation, the Doppler frequency is saved as the initial Doppler frequency, the full-aperture signals are divided into front and rear sub-apertures in the azimuth time domain, and image displacement processing is performed until the translation amount obtained from the correlation peak positions of the two image signals is 0, so that the frequency of the full-aperture signals and the initial focused image signals are obtained.

[0092] In step S150, an initial value of image entropy is set, a quadratic phase error function is constructed by using the frequency of the full-aperture signals and the vertical distance from the range cell to the flight line, and the initial focused image signals are compensated by using the quadratic phase error function.

[0093] Step S160, compare the image entropy of the compensated image signal with the initial value of the image entropy, if the obtained image entropy is not the minimum, set the current image entropy as the initial image entropy, change the frequency modulation rate of the quadratic phase error function according to the preset frequency modulation rate step, construct a new quadratic phase error function to compensate the initial focusing image signal, until the obtained image entropy is the minimum, and then the current compensated focusing image signal is the terahertz video SAR imaging result.

[0094] In the embodiment, according to the imaging characteristics of the terahertz video SAR and the quadratic phase error occurring in the imaging process, the low-resolution image is obtained through the improved image displacement algorithm. Then, the quadratic phase error function related to the perpendicular distance of the flight line is constructed according to the minimum image entropy criterion, and the residual quadratic phase error of the image is compensated accurately to obtain the fine high-resolution image. The constructed quadratic phase error function considers the influence of the perpendicular distance of different distance units and the flight line on the imaging, which can reduce the influence of the range-dependent characteristic of the distance unit on the imaging. Through the minimum image entropy processing, more accurate Doppler frequency modulation and quadratic phase error function than the image displacement algorithm are obtained, the influence of the quadratic phase error on the imaging is compensated, and thus the fine high-quality image is obtained.

[0095] In step S100, based on the high-resolution and narrow-band detection characteristics of the terahertz frequency band, usually 0.3 THz-10 THz, the terahertz SAR radar system is used to carry out continuous coherent detection on a specified target area, such as a ground dynamic scene or a specific monitoring area, to generate and collect time-sequenced multi-frame radar echo signal data. The original data not only contains the scattering characteristic information of the target area (reflected as the signal amplitude change), but also carries the phase information generated by the relative motion between the radar and the target. Due to the high frame rate imaging requirement of the terahertz video SAR, the time interval between frames is short (usually milliseconds to seconds), which can reflect the dynamic change process of the target or scene. At the same time, the data needs to record the time stamp of each frame of detection, the radar working parameters (such as the transmission signal bandwidth, the pulse repetition frequency) and the initial platform position information, which provides basic parameter support for subsequent motion compensation, distance compression and other preprocessing steps, and is the core input data source to ensure the high-resolution and high-dynamic imaging quality of the terahertz video SAR.

[0096] In the embodiment, the original data is preprocessed to obtain the echo signal. After motion compensation and distance compression of the original data, the dechirp echo signal is obtained.

[0097] In step S110, the echo signal is preliminarily compensated for quadratic phase error, and the Doppler frequency modulation obtained from the azimuth velocity measured by the inertial navigation system and the global positioning system is used as the initial Doppler frequency modulation.

[0098] In the embodiment, the quadratic phase function constructed according to the initial Doppler frequency modulation is represented as:

[0099] (16)

[0100] In formula (16), f0represents the initial Doppler frequency modulation, represents the azimuth time.

[0101] Further, the echo signal is compensated for the first time by using the quadratic phase function represented by formula (16).

[0102] In step S120, the two-dimensional phase error frequency modulation is estimated according to the translation amount, and the following formula is used:

[0103] (17)

[0104] In formula (17), f0represents the initial Doppler frequency modulation, represents the azimuth repetition frequency, represents the number of azimuth signal sampling points, represents the number of moving points between the front and rear sub-aperture spectra, i.e., the translation amount.

[0105] In step S130, if the translation amount is not 0, the correlation peak position of the two image signals is obtained, the initial Doppler frequency modulation is combined with the estimated two-dimensional phase error frequency modulation, and a new Doppler frequency modulation is obtained, and the process is represented as:

[0106] (18)

[0107] In formula (18), f0represents the initial Doppler frequency modulation, represents the estimated two-dimensional phase error frequency modulation, represents the slant range at which the feature point is recorded on the image signal, represents the vertical distance from the range unit to the flight line.

[0108] In step S140, the two-dimensional phase error function is determined according to the updated Doppler frequency modulation . The two-dimensional phase error function is multiplied by the echo signal to compensate for the two-dimensional phase error, and the Doppler frequency modulation is saved as the initial Doppler frequency modulation .

[0109] In step S150, the frequency modulation of the full-aperture signal and the vertical distance from the range unit to the flight line are used to construct a quadratic phase error function, and the quadratic phase error function is represented as:

[0110] ​​(19)

[0111] In formula (19), d represents the vertical distance from the unit to the flight line, represents the current frequency.

[0112] In step S160, a new quadratic phase error function is constructed to compensate the primary focused image signal, and the new quadratic phase error function is expressed as:

[0113] (20)

[0114] In formula (20), d represents the vertical distance from the unit to the flight line, represents the current frequency, represents the preset frequency step.

[0115] Further, after the compensated focused image signal is smoothed by a 3*3 Gaussian window, an output image is obtained, and a final terahertz video SAR imaging result is obtained.

[0116] In this paper, the effectiveness of the method is also proved by experiments. In the experiment, the spotlight mode SAR image is obtained from the terahertz video SAR imaging, and the parameters of the simulation imaging scene are shown in Table 1.

[0117] Table 1 Parameters of simulation data imaging

[0118]

[0119] In the point target simulation, the integral side lobe ratio (ISLR) and the image sharpness are used for measurement. The integral side lobe ratio (ISLR) represents the ratio of the total energy in the main lobe region to the sum of the total energy in all side lobe regions. The image sharpness refers to the definition and contrast of the edges of objects in an image. It describes the definition of the detail part of an image, especially the “hardness” or “sharpness” of the edges between adjacent regions. The image entropy is the sum of the logarithm of the normalized energy and the normalized energy, which reflects the degree of image focusing.

[0120] After being processed by the improved image displacement algorithm in the method, the frequency point in the azimuth direction when the front and rear sub-aperture spectra coincide is 244, and after searching by the minimum entropy criterion, the frequency point in the azimuth direction when the front and rear sub-apertures reach the minimum image entropy is 27. Compared with the traditional quadratic phase error compensation, the contrast ratio is shown in Table 2.

[0121] Table 2 Comparison of point target simulation quality

[0122]

[0123] ​​In Table 2, mode 1 is the traditional quadratic phase error compensation, and mode 2 is the quadratic phase error compensation considering the distance perpendicular to the flight line proposed in the method.

[0124] As can be seen from Table 2, the quadratic phase error compensation considering the distance perpendicular to the flight line proposed in the method has smaller sidelobe ratio of the range and azimuth integral of the point target, similar image entropy value, and higher image sharpness, which indicates better image focusing effect.

[0125] Further, Figure 3 respectively are the positions of the point targets and the imaging results, and the positions of the imaging results are basically consistent with the actual positions. Figure 4 is the imaging result of the middle point target, and the traditional quadratic phase error compensation is used, and there is a cross in the longitudinal direction. In the method, the quadratic phase error compensation considering the distance perpendicular to the flight line is used, and the standard "star-shaped" image is obtained. Figure 5 is the range and azimuth profile of the middle point of the point target in the method, the profile is left-right symmetrical, and is the standard sinc profile.

[0126] In the above-mentioned terahertz video SAR imaging method of quadratic phase error compensation, according to the actual scene of the terahertz video SAR, the influence of different distance units on the image imaging quality is considered, the expression form of the quadratic phase error function is improved by improving the MDA algorithm, the accurate frequency modulation rate parameter and the quadratic phase error function are obtained by taking the image entropy as the measure, which is beneficial to obtain the fine image. At the same time, the low-resolution image is obtained by the improved MD algorithm in the method, and the high-resolution image is obtained by the minimum image entropy criterion, which has strong practicability.

[0127] It should be understood that, although Figure 1 the steps in the flowchart of the method are displayed in sequence according to the arrows, these steps are not necessarily executed in sequence according to the arrows. Unless otherwise specified in this article, the execution of these steps has no strict order limitation, 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 the sub-steps or stages is not necessarily sequential, but can be executed in rotation or alternation with other steps or sub-steps or stages of other steps.

[0128] In one embodiment, as Figure 6As shown, a terahertz video SAR imaging device with secondary phase error compensation is provided, comprising: an echo signal obtaining module 200, a preliminary secondary phase error compensation module 210, a two-dimensional phase error frequency modulation estimation module 220, a Doppler frequency modulation updating module 230, a two-dimensional phase error function constructing module 240, a secondary phase error function constructing module 250, and a terahertz video SAR imaging obtaining module 260, wherein:

[0129] The echo signal obtaining module 200 is configured to obtain raw data, which is radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar, and to pre-process the raw data to obtain an echo signal.

[0130] The preliminary secondary phase error compensation module 210 is configured to use a secondary phase function constructed by a preset initial Doppler frequency modulation to perform preliminary secondary phase error compensation on the echo signal.

[0131] The two-dimensional phase error frequency modulation estimation module 220 is configured to divide the full-aperture signal after preliminary compensation into two sub-aperture signals in the azimuth time domain, perform azimuth Fourier transform on the sub-aperture data respectively to obtain corresponding image signals, find a common feature point on the two image signals, record the slant range of the feature point, select a distance unit where the feature point is located, perform cross-correlation operation on the selected distance unit, find the correlation peak position, obtain a translation amount according to the correlation peak positions of the two image signals, and estimate the two-dimensional phase error frequency modulation according to the translation amount.

[0132] The Doppler frequency modulation updating module 230 is configured to, if the translation amount obtained from the correlation peak positions of the two image signals is 0, then the initial Doppler frequency modulation is the frequency modulation of the full-aperture signal, and if the translation amount obtained from the correlation peak positions of the two image signals is not 0, then the initial Doppler frequency modulation is combined with the estimated two-dimensional phase error frequency modulation to obtain a new Doppler frequency modulation.

[0133] The two-dimensional phase error function constructing module 240 is configured to construct a two-dimensional phase error function according to the new Doppler frequency modulation, multiply the two-dimensional phase error function with the echo signal after motion compensation and range compression to perform two-dimensional phase error compensation, save the Doppler frequency modulation as the initial Doppler frequency modulation, divide the full-aperture signal into two sub-aperture signals in the azimuth time domain, and perform image displacement processing until the translation amount obtained from the correlation peak positions of the two image signals is 0 to obtain the frequency modulation of the full-aperture signal and the primary focus image signal.

[0134] The quadratic phase error function construction module 250 is configured to set an initial value of image entropy, and construct a quadratic phase error function by using a frequency modulation rate of a full-aperture signal and a vertical distance from a range cell to a flight line, and compensate the initial focused image signal by using the quadratic phase error function.

[0135] The terahertz video SAR imaging obtaining module 260 is configured to compare the image entropy of the compensated image signal with the initial value of the image entropy, if the obtained image entropy is not the minimum, set the current image entropy as the initial image entropy, change the frequency modulation rate of the quadratic phase error function according to a preset frequency modulation rate step, construct a new quadratic phase error function to compensate the initial focused image signal, until the obtained image entropy is the minimum, and then the current compensated focused image signal is the terahertz video SAR imaging result.

[0136] The specific limitations of the terahertz video SAR imaging device with quadratic phase error compensation can refer to the limitations of the terahertz video SAR imaging method with quadratic phase error compensation, which will not be repeated here. Each module in the above terahertz video SAR imaging device with quadratic phase error compensation can be realized by software, hardware and combinations thereof. The above 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 that the processor can call and execute the operations corresponding to each module.

[0137] In one embodiment, a computer device is provided, which can be a terminal, and its internal structure diagram can be as shown in Figure 7 The computer device includes a processor, a memory, a network interface, a display screen and an input device connected by 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 the operating system and the computer program in the non-volatile storage medium to run. The network interface of the computer device is configured to communicate with external terminals through network connection. The computer program is executed by the processor to implement a terahertz video SAR imaging method with quadratic phase error compensation. 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 shell of the computer device, or an external keyboard, touchpad or mouse, etc.

[0138] Those skilled in the art can understand that Figure 7The 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 arrangement of components.

[0139] In one embodiment, a computer device is provided, including a memory and a processor, the memory stores a computer program, and the processor implements the specific steps in the above-mentioned secondary phase error compensation terahertz video SAR imaging method when executing the computer program.

[0140] In one embodiment, a computer readable storage medium is provided, which stores a computer program, and the processor implements the specific steps in the above-mentioned secondary phase error compensation terahertz video SAR imaging method when executing the computer program.

[0141] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiments can be completed by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer readable storage medium and can include the processes of the above-mentioned embodiments when executed. Any reference to memory, storage, database or other medium used in the embodiments provided by the present 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 various forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), Rambus direct RAM (RDRAM), direct RAM bus dynamic RAM (DRDRAM) and memory bus dynamic RAM (RDRAM) and the like.

[0142] Each technical feature of the above embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of each technical feature in the above embodiments are not described, however, as long as the combination of the technical features does not exist contradictory, it should be considered as the scope of the present application.

[0143] The above embodiments only express several implementation ways of the present application, and the description is more specific and detailed, but it should not be understood as a limitation to the scope of the application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, several modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. A method for secondary phase error compensation of terahertz video SAR imaging, 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, and pre-processing the original data to obtain an echo signal; using a preset initial Doppler frequency to construct a quadratic phase function, and performing preliminary quadratic phase error compensation on the echo signal; dividing the full-aperture signal after preliminary compensation into two sub-aperture signals in the azimuth time domain, performing azimuth Fourier transform on the sub-aperture signals respectively to obtain corresponding image signals, finding a common feature point on the two image signals, recording the slant range of the feature point, selecting a distance unit where the feature point is located, performing cross-correlation operation on the selected distance unit to find a correlation peak position, obtaining a translation amount according to the correlation peak positions of the two image signals, and estimating a two-dimensional phase error frequency according to the translation amount; if the translation amount obtained according to the correlation peak positions of the two image signals is 0, the initial Doppler frequency is the frequency of the full-aperture signal, and if the translation amount obtained according to the correlation peak positions of the two image signals is not 0, the initial Doppler frequency and the estimated two-dimensional phase error frequency are combined to obtain a new Doppler frequency; constructing a two-dimensional phase error function according to the new Doppler frequency, multiplying the two-dimensional phase error function by the echo signal after motion compensation and range compression to perform two-dimensional phase error compensation, saving the new Doppler frequency as the initial Doppler frequency, dividing the full-aperture signal into two sub-aperture signals in the azimuth time domain, and performing image displacement processing until the translation amount obtained according to the correlation peak positions of the two image signals is 0, thereby obtaining the frequency of the full-aperture signal and an initial focused image signal; setting an initial value of image entropy, constructing a quadratic phase error function using the frequency of the full-aperture signal and the vertical distance from the distance unit to the flight line, and compensating the initial focused image signal using the quadratic phase error function; comparing the image entropy of the compensated image signal with the initial value of the image entropy, setting the current image entropy as the initial image entropy if the obtained image entropy is not the minimum, changing the frequency of the quadratic phase error function according to a preset frequency step, constructing a new quadratic phase error function to compensate the initial focused image signal until the obtained image entropy is the minimum, and obtaining the compensated focused image signal as the terahertz video SAR imaging result.

2. The method of claim 1, wherein, When the original data is pre-processed to obtain the echo signal, the original data is subjected to motion compensation and range compression to obtain a dechirp echo signal.

3. The method of claim 2, wherein, The Doppler frequency obtained according to the azimuth velocity measured by the inertial navigation system and the global positioning system is used as the initial Doppler frequency.

4. The method of claim 3, wherein, The two-dimensional phase error frequency is estimated according to the translation amount, using the following formula: In the above formula, represents the azimuth repetition frequency, represents the azimuth signal collection point number, represents the front and rear sub-aperture spectrum moving point number, i.e. the translation amount.

5. The method of claim 4, wherein, if the translation amount obtained according to the correlation peak positions of the two image signals is not 0, the initial Doppler frequency and the estimated two-dimensional phase error frequency are combined to obtain a new Doppler frequency, and the process is represented as: In the above equation, represents the initial Doppler frequency, represents the estimated two-dimensional phase error frequency, represents the slant range of the feature point recorded on the image signal, represents the vertical distance from the range unit to the flight line.

6. The method of claim 5, wherein, A quadratic phase error function is constructed using the frequency modulation rate of the full-aperture signal and the vertical distance from the range cell to the flight line, wherein the quadratic phase error function is represented as: In the above formula, denotes the vertical distance of the unit to the route, denotes the current frequency, denotes the azimuth time.

7. The method of claim 6, wherein, The new quadratic phase error function is used to compensate the initial focused image signal, wherein the new quadratic phase error function is represented as: In the above formula, represents the vertical distance from the unit to the route, represents the current frequency adjustment, represents the preset frequency adjustment step.

8. A terahertz video SAR imaging device with quadratic phase error compensation, characterized by The device comprises: An echo signal obtaining module is configured to obtain original data, which is radar echo signal data generated by continuous detection of a target area by a terahertz SAR radar, and to obtain echo signals by preprocessing the original data; An initial quadratic phase error compensation module is configured to perform initial quadratic phase error compensation on the echo signals by using a quadratic phase function constructed by a preset initial Doppler frequency modulation rate; A two-dimensional phase error frequency modulation rate estimation module is configured to divide the full-aperture signal after initial compensation into two sub-apertures in the azimuth time domain, to perform azimuth Fourier transform on the sub-aperture data to obtain corresponding image signals, to find a common feature point in the two image signals, to record the slant range of the feature point, to select a range cell where the feature point is located, to perform cross-correlation operation on the selected range cell to find a correlation peak position, to obtain a translation amount according to the correlation peak position of the two image signals, and to estimate the two-dimensional phase error frequency modulation rate according to the translation amount; A Doppler frequency modulation rate updating module is configured to determine whether the translation amount obtained from the correlation peak position of the two image signals is 0, wherein if the translation amount is 0, the initial Doppler frequency modulation rate is the frequency modulation rate of the full-aperture signal, and if the translation amount is not 0, the initial Doppler frequency modulation rate is combined with the estimated two-dimensional phase error frequency modulation rate to obtain a new Doppler frequency modulation rate; A two-dimensional phase error function construction module is configured to construct a two-dimensional phase error function according to the new Doppler frequency modulation rate, to multiply the two-dimensional phase error function by the echo signal after motion compensation and range compression to perform two-dimensional phase error compensation, to save the new Doppler frequency modulation rate as the initial Doppler frequency modulation rate, to divide the full-aperture signal into two sub-apertures in the azimuth time domain, and to perform image displacement processing until the translation amount obtained from the correlation peak position of the two image signals is 0 to obtain the frequency modulation rate of the full-aperture signal and the initial focused image signal; A quadratic phase error function construction module is configured to set an initial value of image entropy, to construct a quadratic phase error function using the frequency modulation rate of the full-aperture signal and the vertical distance from the range cell to the flight line, and to compensate the initial focused image signal by using the quadratic phase error function; A terahertz video SAR imaging obtaining module is configured to compare the image entropy of the compensated image signal with the initial value of the image entropy, to set the current image entropy as the initial image entropy if the obtained image entropy is not the minimum, to change the frequency modulation rate of the quadratic phase error function according to a preset frequency modulation rate step, to construct a new quadratic phase error function to compensate the initial focused image signal until the obtained image entropy is the minimum, and to obtain the compensated focused image signal as the terahertz video SAR imaging result.

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