CN109782277B - Pri-variable strabismus bunching SAR imaging method, device and equipment and storage medium - Google Patents
Pri-variable strabismus bunching SAR imaging method, device and equipment and storage medium Download PDFInfo
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Abstract
The application discloses a method, a device and equipment for imaging a strabismus bunching SAR with variable PRI, and a storage medium. Relates to the technical field of synthetic aperture radars. The technical scheme comprises the following steps: restoring the range migration form of the radar echo data in the variable PRI mode; removing azimuth spectrum aliasing and distance direction time domain aliasing of radar echo data; reducing the sampling uniformity of azimuth signals of radar echo data; carrying out phase compensation on the radar echo data subjected to the reduction processing; and imaging the radar echo data after phase compensation. According to the technical scheme of the embodiment of the application, under-sampling under the condition that the azimuth signal of the radar echo data in the variable PRI mode is non-uniform can be restored under the condition that the calculation efficiency is guaranteed, and therefore the processing efficiency of the frequency domain imaging algorithm is improved.
Description
Technical Field
The present disclosure relates generally to the field of synthetic aperture radar technology, and more particularly to a PRI-varying squint bunching SAR imaging method, apparatus, device, and storage medium.
Background
For Synthetic Aperture Radar (SAR), the imaging mode has front, side and oblique view modes. Compared with the front side view and the side view, the strabismus mode increases the maneuverability and the flexibility of the satellite-borne synthetic aperture radar system, and can realize short-time revisit of the region of interest by matching with beam pointing adjustment; the method is particularly significant for detecting military targets with radar scattering cross sections depending on observation angles.
Under the squint bunching condition, if the SAR system adopts a fixed Pulse Repetition Interval (PRI), since a distance walk item exists in a distance process from a target scene to the SAR, echo data of the target scene is gradually removed from a data receiving window, thereby reducing an effective imaging range. To overcome this problem, it is ensured that the fixed PRI is large enough to accommodate the time shift of the receive window, so as to increase the ratio of valid data by controlling the start time of the receive window in a distance walking manner. However, to ensure that the fixed PRI is sufficiently large, adjustments to the PRI are required.
However, if the SAR system adopts a variable PRI strategy, due to the change of the PRI, the ramp distance history of the echo data of the SAR system changes, and non-uniform sampling also exists in the azimuth signal; in a beamforming mode, an undersampling problem and the like still exist in the azimuth signal of the echo data of the SAR system, and the problems cause that the existing imaging algorithm cannot effectively solve the problem that the squint beamforming SAR system images in a variable PRI mode.
Therefore, it is desirable to propose an imaging method based on strabismus bunching SAR with varying PRI mode to solve the above problems.
Disclosure of Invention
In view of the above-mentioned defects or shortcomings in the prior art, it is desirable to provide an imaging method, apparatus, device and storage medium for a PRI-variable squint beamforming synthetic aperture radar, which can implement the reduction of undersampling under the condition of non-uniformity of its azimuth signal under the condition of ensuring the computational efficiency, and ensure the validity of the imaging result.
In a first aspect, an embodiment of the present application provides an imaging method for strabismus bunching SAR in a variable PRI mode, where the method includes:
and recovering the range migration form of the radar echo data in the variable PRI mode.
And removing azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data.
And restoring the sampling uniformity of the azimuth signals of the radar echo data.
And performing phase compensation on the radar echo data subjected to the reduction processing.
And imaging the radar echo data after phase compensation.
In one or more embodiments of the first aspect, recovering range-migrated forms of radar echo data in a varying PRI mode comprises:
and removing the time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode and the change of the slant range course of the radar echo data relative to the slant range course in the fixed PRI mode by using a first filter so as to restore the range migration form of the radar echo data in the variable PRI mode to the range migration form in the fixed PRI mode.
The first filter is represented as
Δτ ni The time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode is obtained; Δ R ni The change of the slope distance course of the radar echo data relative to the slope distance course in the fixed PRI mode is obtained; f. of τ Is the range frequency in the fixed PRI mode; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the first aspect, removing azimuth spectral aliasing and range-to-time domain aliasing in radar echo data comprises:
the second filter is used for carrying out frequency spectrum shifting, distance walk removing and deskewing on the radar echo data so as to remove azimuth spectrum aliasing and distance time domain aliasing in the radar echo data, and the second filter is used for:
wherein t is ni The azimuth pulse transmitting time of radar echo data is; v. of r The equivalent speed when the radar echo data is imaged is obtained; theta.theta. c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance direction frequency of radar echo data; f. of 0 Carrier frequency of radar echo data; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the first aspect, restoring sampling uniformity of azimuth signals of radar return data comprises:
and processing azimuth signals of the radar echo data by using non-uniform Fourier transform (NUFFT).
In one or more embodiments of the first aspect, performing phase compensation on the radar echo data subjected to the restoring process includes:
and restoring the distance walking generated by the second filter by using a third filter, wherein the third filter is as follows:
wherein t is ui Sampling time for uniform azimuth of radar echo data; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance frequency of radar echo data; v. of r Equivalent speed when imaging for radar echo data; theta.theta. c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the first aspect, imaging the phase compensated radar echo data includes:
and imaging the radar echo data subjected to phase compensation by adopting a nonlinear frequency modulation scaling NCS algorithm or a range-Doppler RD algorithm or a omega K algorithm.
In one or more embodiments of the first aspect, the imaging the radar echo data after the phase compensation by using a nonlinear frequency modulation and scaling NCS algorithm includes:
and imaging the radar echo data after phase compensation through improved cubic phase filtering and improved linear scaling operation of a range-Doppler domain.
In one or more embodiments of the first aspect, improved cubic phase filtering, comprises:
performing cubic phase filtering by using a fourth filter;
the fourth filter is
Wherein, Y (f) a ) For terms of cubic phase perturbation in cubic phase filteringA coefficient; f. of a Is the Doppler frequency; f. of τ The range direction frequency of radar echo data in a fixed PRI mode; theta (f) a ) Squint angle for radar echo data expressed in doppler frequency; r is a radical of hydrogen 0ref The nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta.theta. c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the first aspect, the linear scaling operation of the improved range-doppler domain comprises:
performing linear scaling operation of a range-Doppler domain by using a fifth filter;
the fifth filter is
Where τ is the range time relative to the radar echo data in the fixed PRI mode; f. of a Is the Doppler frequency; q. q.s 2 (f a ) Linearly scaling the operational factor for the radar echo data; q. q.s 3 (f a ) Introducing an additional scaling operation factor for a phase disturbance term of radar echo data; tau is ref Time delay of radar echo data corresponding to a reference slope distance; r is 0ref The nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta.theta. c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In a second aspect, an embodiment of the present application further provides an imaging apparatus for strabismus bunching SAR in a variable PRI mode, where the apparatus includes:
and the range migration form recovery module is configured for recovering the range migration form of the radar echo data in the variable PRI mode.
And the de-aliasing module is configured to remove azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data, restore the sampling uniformity of azimuth signals of the radar echo data, and perform phase compensation on the restored radar echo data.
And the imaging module is configured to image the radar echo data after the phase compensation.
In one or more embodiments of the second aspect, the range migration form recovery module is a first filter, configured to remove a time delay of a transmission time of the radar echo data with respect to a transmission time in the fixed PRI mode, and remove a change of a range-slope course of the radar echo data with respect to a range-slope course in the fixed PRI mode, so as to recover the range migration form of the radar echo data in the varying PRI mode to the range migration form in the fixed PRI mode;
the first filter is:
where Δ τ is ni The time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode is obtained; Δ R ni The change of the slope distance course of the radar echo data relative to the slope distance course in the fixed PRI mode is obtained; f. of τ The range direction frequency of radar echo data in a fixed PRI mode; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the second aspect, the antialiasing module configuration comprises a second filter, an azimuthal processing module, and a third filter,
a second filter configured to remove azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data;
the direction processing module is configured for restoring the sampling uniformity of the azimuth signal of the radar echo data;
and the third filter is configured to perform phase compensation on the radar echo data subjected to the recovery processing.
In one or more embodiments of the second aspect, the second filter is:
wherein t is ni Transmitting the azimuth pulse of the radar echo data to the pulse transmitting time; v. of r Equivalent speed when imaging for radar echo data; theta c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance frequency of radar echo data; f. of 0 Is the carrier frequency of the radar echo data.
In one or more embodiments of the second aspect, the orientation processing module is configured to:
and processing azimuth signals of the radar echo data by utilizing non-uniform Fourier transform (NUFFT).
In one or more embodiments of the second aspect, the third filter is
Wherein t is ui Sampling time for uniform azimuth of radar echo data; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance direction frequency of radar echo data; v. of r The equivalent speed when the radar echo data is imaged is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the second aspect, the imaging module includes a cubic phase filtering sub-module and a linear scaling sub-module.
And the cubic phase filtering submodule is configured to perform cubic phase filtering on the radar echo data subjected to the phase compensation by adopting improved cubic phase filtering.
And the linear scaling submodule is configured to perform linear scaling operation on the radar echo data subjected to the phase compensation by adopting the linear scaling operation of the improved range-Doppler domain.
In one or more embodiments of the second aspect, the cubic phase filtering sub-module is a fourth filter,
wherein Y (f) a ) The coefficient is a cubic phase perturbation term in cubic phase filtering; f. of a Is the Doppler frequency; f. of τ Distance direction frequency of radar echo data in a fixed PRI mode; theta (f) a ) Squint angle for radar echo data expressed in doppler frequency; r is 0ref The nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta c The equivalent squint angle of the center moment of the radar echo data is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
In one or more embodiments of the second aspect, the linear scaling sub-module is a fifth filter,
wherein the sum is relative to the distance in the fixed PRI mode; f. of a Is the Doppler frequency; q. q.s 2 (f a ) Linearly scaling the operation factor for the radar echo data; q. q of 3 (f a ) An additional scaling operation factor is introduced for a phase disturbance item of radar echo data; tau is ref Time delay of radar echo data corresponding to the reference slope distance; pi is the circumference ratio; j is an imaginary unit.
In a third aspect, the present application provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor, where the processor executes the computer program to implement the method provided in any embodiment of the first aspect.
In a fourth aspect, the present application further provides a computer-readable storage medium, on which a computer program is stored, where the computer program, when executed by a processor, implements the method provided in any embodiment of the first aspect.
According to the technical scheme provided by the embodiment of the application, the range migration form of the obtained radar echo data in the variable PRI mode is restored to the range migration form in the fixed PRI mode, so that the radar echo data can be imaged by using a modified conventional imaging algorithm, and the calculation efficiency is improved. By removing azimuth spectrum aliasing and distance direction time domain aliasing of radar echo data, sampling uniformity of azimuth direction signals of the radar echo data is restored, and phase compensation is performed on the restored radar echo data, so that undersampled restoration of the radar echo data under the condition of non-uniform azimuth signals is realized; by imaging the radar echo data after phase compensation, the effectiveness of the final imaging result is ensured.
Drawings
Other features, objects and advantages of the present application will become more apparent upon reading of the detailed description of non-limiting embodiments made with reference to the following drawings:
fig. 1 shows a schematic flowchart of an imaging method for squint bunching SAR in a PRI-varying mode according to an embodiment of the present application;
FIG. 2 shows a schematic diagram of a further embodiment of the present application with respect to S1;
FIG. 3 shows a timing diagram of the transmit times of radar echo data in varying PRI and fixed PRI modes;
FIG. 4 shows a schematic view of a further embodiment of the present application with respect to S2;
FIG. 5 is a diagram showing a result of simulation of an imaging method of a strabismus bunching SAR in a variable PRI mode according to an embodiment of the present application;
fig. 6 shows a block diagram of an imaging device of a variable PRI squint beamforming SAR provided by an embodiment of the present application;
fig. 7 is a block diagram illustrating a component structure of an anti-aliasing module 602 in an imaging apparatus for a variable PRI strabismus bunching SAR provided in an embodiment of the present application;
FIG. 8 illustrates a schematic block diagram of a computer system 800 suitable for use in implementing embodiments of the present application.
Detailed Description
The present application will be described in further detail with reference to the following drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the relevant invention and not restrictive of the invention. It should be noted that, for convenience of description, only the portions related to the present invention are shown in the drawings.
It should be noted that the embodiments and features of the embodiments in the present application may be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments with reference to the attached drawings.
Referring to fig. 1, fig. 1 shows a schematic flowchart of an imaging method for a PRI-varying strabismus bunching SAR provided in an embodiment of the present application. As shown in fig. 1, the method comprises the following specific steps:
s1, recovering a range migration form of radar echo data in a variable PRI mode.
According to the method and the device, the radar echo data in the variable PRI mode are obtained by aiming at the synthetic aperture radar SAR in the variable PRI squint bunching mode, the range migration form of the radar echo data is the range migration form in the variable PRI mode, and when the radar echo data in the range migration form in the variable PRI mode is used for imaging, a conventional imaging algorithm cannot be adopted. The imaging processing can be performed after a large-range correction is performed on a conventional imaging algorithm, which has an image effect on both the calculation efficiency and the imaging effect. Therefore, the range migration form of the obtained radar echo data in the variable PRI mode is restored to the range migration form in the fixed PRI mode, and the range migration form of the radar echo data in the variable PRI mode can be restored by multiplying the radar echo data in the variable PRI mode by the corresponding time delay filter in the range frequency domain.
S2, removing azimuth spectrum aliasing and distance direction time domain aliasing of the radar echo data, restoring the sampling uniformity of azimuth direction signals of the radar echo data, and performing phase compensation on the restored radar echo data.
In the embodiment of the application, to perform undersampling reduction on the azimuth signal of the radar echo data in the variable PRI mode, two-step de-aliasing needs to be performed under the non-uniform condition, that is, azimuth spectrum de-aliasing and range-to-time domain de-aliasing, the azimuth spectrum aliasing of the radar echo data can be removed by adopting an azimuth de-aliasing method such as de-skew operation under the non-uniform condition, and the range-to-time domain aliasing of the radar echo data can also be removed by adopting a range-walk-off mode or other range-to-time domain de-aliasing methods. After two-step de-aliasing, the sampling uniformity of the azimuth signal is restored for the radar echo data, and phase compensation is performed for phase change possibly generated in the restoring process.
By removing azimuth spectrum aliasing and distance direction time domain aliasing of radar echo data, sampling uniformity of azimuth direction signals of the radar echo data is restored, phase compensation is carried out on the restored radar echo data, and undersampled restoration of the radar echo data azimuth signals under the non-uniform condition is achieved.
And S3, imaging the radar echo data after phase compensation.
The radar echo data processed by the S1 and the S2 can be subjected to imaging processing by adopting a conventional imaging algorithm.
Referring to fig. 2, fig. 2 shows a schematic diagram of S1 according to an embodiment of the present disclosure. As shown in fig. 2, S1 includes:
s101, radar echo data received by an SAR in a variable PRI mode are obtained;
in this embodiment of the application, in consideration of the situation that intensity weighting of a distance direction signal envelope and an azimuth direction can be generally ignored, radar echo data of a single-point target received by an SAR in a PRI-varying squint beamforming mode may be expressed as:
wherein t is ni The azimuth pulse transmitting time of the radar echo data in the variable PRI mode; τ' ni Is relative to t ni Distance to time; k r The frequency modulation rate of the distance direction linear frequency modulation signal of the radar echo data; r (t) ni ) Is a time t ni Time-of-day single-point target-to-SAR systemThe skew distance of (a); λ is the carrier wavelength of the SAR signal; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
S102, determining the time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode and the change of the slope distance process relative to the slope distance process in the fixed PRI mode.
Changes in the form of range migration that result in radar echo data may consist in: firstly, the starting time of a receiving window changes along with PRI, so that the relative time delay of radar echo data changes, namely the time delay of the transmitting time of the radar echo data relative to the transmitting time in a fixed PRI mode; secondly, the real slope distance process of the radar echo data in the variable PRI mode is different from the slope distance process in the fixed PRI mode, namely the change of the slope distance process of the radar echo data relative to the slope distance process in the fixed PRI mode.
Fig. 3 shows timing charts of the transmission timings of the radar echo data in the variable PRI mode and the fixed PRI mode to illustrate an influence caused by a delay of the transmission timing of the radar echo data in the variable PRI mode with respect to the transmission timing in the fixed PRI mode. As shown in fig. 3, the transmission time in the fixed mode is t ui The time delay of the data receiving window relative to each pulse is set to a constant value tau d And τ is relative to t ui Distance to time, τ' ni Is relative to t ni Is τ 'to time' ni τ 'is' ni ＝τ+Δτ ni In which Δ τ ni ＝t ui -t ni . Wherein, Δ τ ni I.e. the time delay of the transmission moment of the radar echo data with respect to the transmission moment in the fixed PRI mode, results in a change of range migration.
The change of the slope course of the radar echo data in the variable PRI mode relative to the slope course in the fixed PRI mode corresponds to the time t ni And t ui The difference between the slope distances:
R(t ni ) To correspond to the time t ni The skew distance of (a); r (t) ui ) To correspond to the time t ui The skew distance of (a); v. of r Imaging equivalent speed for SAR; r 0 The nearest slope distance of the scene center of the SAR is obtained; theta c The equivalent squint angle of the SAR at the central moment is obtained; theta i Corresponding to time t for SAR ui The equivalent squint angle of (c).
In summary, Δ R is set in advance under the condition that the scene center of SAR is set in advance ni Expressed as the change in ramp history in the varying PRI mode relative to the ramp history in the fixed PRI mode, Δ τ ni Expressed as the time delay of the transmission moment in the variable PRI mode relative to the transmission moment in the fixed PRI mode, can be determined by the above-mentioned DeltaR ni And Δ τ ni To express changes in range migration. Therefore, the radar echo data in the varying PRI mode can be further expressed as:
s103, establishing a first filter.
And transforming the radar echo data in the variable PRI mode to a range frequency domain to obtain a first filter required for recovering range migration:
Δτ ni the time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode is obtained; Δ R ni The change of the slope distance course of the radar echo data relative to the slope distance course in the fixed PRI mode is obtained; f. of τ Is the range direction frequency in the fixed PRI mode.
S104, removing the time delay of the transmitting time of the radar echo data in the variable PRI mode relative to the transmitting time in the fixed PRI mode and the change of the slant range course of the radar echo data in the variable PRI mode relative to the slant range course in the fixed PRI mode by using a first filter, so as to restore the range migration form of the radar echo data in the variable PRI mode to the range migration form in the fixed PRI mode.
Referring to fig. 4, fig. 4 shows a schematic diagram of another embodiment of the present application about S2, and as shown in fig. 4, S2 includes the following specific steps:
s201, removing azimuth spectrum aliasing and distance-to-time domain aliasing in the radar echo data in the variable PRI mode.
Namely, a second filter is used for carrying out spectrum shifting, distance walk removing and deskewing on radar echo data in a variable PRI mode so as to remove azimuth spectrum aliasing and distance time domain aliasing in the radar echo data in the variable PRI mode, and the second filter is represented as follows:
wherein t is ni Transmitting the azimuth pulse of the radar echo data to the pulse transmitting time; v. of r The equivalent speed when the radar echo data is imaged is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance direction frequency of radar echo data; f. of 0 Is the carrier frequency of the radar echo data.
And S202, restoring the sampling uniformity of the azimuth signal of the radar echo data.
In the embodiment of the application, the non-uniform Fourier transform NUFFT can be used for processing the azimuth signal of the radar echo data in the variable PRI mode, so that the sampling of the azimuth signal of the radar echo data in the variable PRI mode is uniform.
And S203, performing phase compensation on the radar echo data subjected to the reduction processing.
In the embodiment of the present application, the distance walking generated by restoring the second filter with the third filter is as follows:
wherein t is ui Sampling time for uniform azimuth of radar echo data; k is a radical of formula a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance direction frequency of radar echo data; v. of r The equivalent speed when the radar echo data is imaged is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained.
And S3, imaging the radar echo data after phase compensation. In the embodiment of the application, a nonlinear frequency modulation and scaling NCS algorithm, a range-doppler RD algorithm or an ω K algorithm may be adopted to image the radar echo data after phase compensation.
The conventional NCS imaging algorithm includes the following steps: the method comprises the steps of carrying out azimuth direction Fourier transform (FFT) processing on radar echo data to be imaged, carrying out three times of phase filtering, carrying out range direction inverse Fourier transform (IFFT), carrying out linear scaling operation of a range-Doppler domain, carrying out range direction Fourier transform (FFT), carrying out consistent matched filtering and two-dimensional inverse Fourier transform (IFFT), and finally obtaining an imaging result of the radar echo data.
In the embodiment of the application, after the range migration recovery and the azimuth unmixing, the conventional NCS imaging algorithm should be adjusted correspondingly due to the range time domain convolution: firstly, performing consistent distance migration correction in a two-dimensional frequency domain, and combining the step with cubic phase filtering; correspondingly, the linear scaling operation of the subsequent range-doppler domain is correspondingly adjusted.
And imaging the radar echo data after phase compensation through improved cubic phase filtering and improved linear scaling operation of a range-Doppler domain.
Wherein, the improved cubic phase filtering increases the correction of the consistent range migration of the two-dimensional frequency domain, for example, the cubic phase filtering is performed by using a fourth filter.
In the embodiment of the present application, the fourth filter is:
wherein，Y(f a ) The coefficient is a cubic phase perturbation term in cubic phase filtering; f. of a Is the Doppler frequency; f. of τ The range direction frequency of radar echo data in a fixed PRI mode; theta (f) a ) Squint angle for radar echo data expressed in doppler frequency; r is 0ref The nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained.
Wherein, the fifth filter is used for linear scaling operation of the range-Doppler domain.
The fifth filter is:
where τ is the range time relative to the radar echo data in the fixed PRI mode; f. of a Is the Doppler frequency; q. q of 2 (f a ) Scaling the operational factors for the conventional linearity of the radar echo data; q. q.s 3 (f a ) An additional scaling operation factor is introduced for a phase disturbance item of radar echo data; tau. ref Is the time delay of the corresponding radar echo data at the reference slope.
The time delay of the radar echo data corresponding to the reference slope distance can be calculated by the following formula:
r 0ref the nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta.theta. c The equivalent squint angle of the radar echo data center moment is obtained.
By now the effect of azimuth-varying PRI can be considered eliminated, and the remaining operations are consistent with the conventional NCS algorithm.
It should be noted that while the operations of the methods of the present application are depicted in the drawings in a particular order, this does not require or imply that these operations must be performed in this particular order, or that all of the illustrated operations must be performed, to achieve desirable results. Rather, the steps depicted in the flowcharts may change order of execution. Additionally or alternatively, certain steps may be omitted, multiple steps combined into one step execution, and/or one step broken down into multiple step executions.
The following simulation was performed according to the imaging method of the embodiment of the present application:
assuming that 5 point targets are distributed within a range of 6km × 4km (distance × azimuth), simulation parameters are shown in table 1. The final simulation result is shown in fig. 5, and the performance index of the point target is shown in table 2. As can be seen from the table, the target performance indexes of all points are close to theoretical values.
TABLE 1 parameters used for simulation
| (symbol) | Means of | Value taking |
| R 0 | Nearest slope distance of scene center | 750km |
| v r | Imaging equivalent velocity | 7200m/s |
| θ c | Central oblique angle | 20deg |
| θ s | Maximum squint angle | 20.48deg |
| PRI M | Maximum PRI | 0.25ms |
| T a | Time of data acquisition | 2s |
| f 0 | Carrier frequency | 9.6GHz |
| B r | Bandwidth of pulse | 100MHz |
| τ p | Pulse time width | 10us |
TABLE 2 Point target Performance analysis
Wherein: PSLR: peak sidelobe ratio, theoretical-13.26 dB. ISLR: the side lobe ratio is integrated, and the theoretical value is-10.00 dB. IRW: main lobe aspect ratio, theoretical value 1.
Referring to fig. 5, fig. 5 shows simulation results of the azimuth signal using the simulation parameters shown in table 1 and table 2. If no processing measure is taken on the azimuth non-uniformity, the non-uniform sampling will cause azimuth defocusing; if non-uniform interpolation is adopted, the area near the main lobe can be focused, but the side lobe area is raised; by adopting the imaging method provided by the embodiment of the application, the good low-level characteristic of a side lobe area can be ensured, and the phenomenon that a weak target in a final SAR image is shielded by a side lobe of a strong target can be inhibited.
Referring to fig. 6, fig. 6 is a block diagram illustrating a structure of an imaging apparatus for a variable PRI strabismus bunching SAR provided by an embodiment of the present application, as shown in fig. 6, the apparatus includes:
and the range migration form recovery module 601 is configured to recover the range migration form of the radar echo data in the variable PRI mode to the range migration form in the fixed PRI mode.
According to the method and the device, the radar echo data in the variable PRI mode are obtained aiming at the synthetic aperture radar SAR in the variable PRI squint bunching mode, the range migration form of the radar echo data is the range migration form in the variable PRI mode, and when the radar echo data in the range migration form in the variable PRI mode is used for imaging, a conventional imaging algorithm cannot be adopted. The imaging processing can be performed after a large-range correction is performed on a conventional imaging algorithm, which has an image effect on both the calculation efficiency and the imaging effect. Therefore, the module restores the range migration form of the obtained radar echo data in the variable PRI mode to the range migration form in the fixed PRI mode, and the restoration of the range migration form of the radar echo data can be realized by multiplying the radar echo data in a range frequency domain by a corresponding time delay filter.
And the de-aliasing module 602 is configured to remove the azimuth spectrum aliasing and the distance-to-time domain aliasing of the radar echo data, restore the sampling uniformity of the azimuth signal of the radar echo data, and perform phase compensation on the restored radar echo data.
In the embodiment of the application, to perform undersampling reduction on the azimuth signal of the radar echo data in the variable PRI mode, two-step de-aliasing needs to be performed under the non-uniform condition, that is, azimuth spectrum de-aliasing and range-to-time domain de-aliasing, an azimuth de-aliasing method such as a deskewing operation under the non-uniform condition can be adopted to remove the azimuth spectrum aliasing of the radar echo data, and a range-to-time domain aliasing method or other range-to-time domain de-aliasing methods can also be adopted to remove the range-to-time domain aliasing of the radar echo data. After two-step de-aliasing, the sampling uniformity of the azimuth signal is restored for the radar echo data, and phase compensation is performed for phase change possibly generated in the restoring process.
By removing azimuth spectrum aliasing and distance direction time domain aliasing of radar echo data, sampling uniformity of azimuth direction signals of the radar echo data is restored, phase compensation is carried out on the restored radar echo data, and undersampled restoration of the radar echo data azimuth signals under the non-uniform condition is achieved.
And an imaging module 603 configured to image the radar echo data after the phase compensation. The radar echo data processed by the range migration form recovery module 601 and the de-aliasing module 602 can be subjected to imaging processing by adopting a conventional imaging algorithm.
In this embodiment of the application, the range migration form recovery module 601 is a first filter, and is configured to remove a time delay of a transmission time of the radar echo data in the varying PRI mode relative to a transmission time in the fixed PRI mode, and remove a change of a range skew of the radar echo data in the varying PRI mode relative to a range skew in the fixed PRI mode, so as to recover the range migration form of the radar echo data in the varying PRI mode to the range migration form in the fixed PRI mode.
The first filter is:
where Δ τ is ni The time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode is obtained; Δ R ni The change of the slope distance course of the radar echo data relative to the slope distance course in the fixed PRI mode is obtained; f. of τ Distance direction frequency of radar echo data in a fixed PRI mode; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
Referring to fig. 7, fig. 7 shows a block diagram of the aliasing resolution module 602, which includes a second filter 701, an azimuth processing module 702, and a third filter 703.
The second filter 701 is configured to remove azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data in the variable PRI mode.
And the direction processing module 702 is configured to restore the sampling uniformity of the azimuth signal of the radar echo data in the changed PRI mode.
And a third filter 703 configured to perform phase compensation on the radar echo data subjected to the restoration processing.
In this embodiment, the second filter 701 is:
wherein t is ni The azimuth pulse transmitting time of the radar echo data; v. of r The equivalent speed when the radar echo data is imaged is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained; c is the speed of light; k is a radical of formula a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance frequency of radar echo data; f. of 0 Is the carrier frequency of the radar echo data.
In this embodiment of the present application, the azimuth processing module 702 is configured to process an azimuth signal of radar echo data by using non-uniform fourier transform NUFFT.
In the embodiment of the present application, the third filter 703 is represented as
Wherein t is ui Sampling time for uniform azimuth of radar echo data; k is a radical of a The azimuth frequency modulation rate of the radar echo data is obtained; f. of r Distance frequency of radar echo data; v. of r Equivalent speed when imaging for radar echo data; theta.theta. c Equivalent skew for radar echo data center timeViewing angle.
In the embodiment of the present application, the imaging module 603 includes a cubic phase filtering sub-module and a linear scaling sub-module.
And the third-time phase filtering submodule is configured for performing third-time phase filtering on the radar echo data subjected to the phase compensation by adopting improved third-time phase filtering.
The improved triple phase filtering adds the consistent distance migration correction in a two-dimensional frequency domain, and the triple phase filtering submodule is a fourth filter:
wherein Y (f) u ) The coefficient of a cubic phase perturbation term in cubic phase filtering is obtained; f. of a Is the Doppler frequency; f. of τ The range direction frequency of radar echo data in a fixed PRI mode; theta (f) a ) Squint angle for radar echo data expressed in doppler frequency; r is a radical of hydrogen 0ref The nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta.theta. c The equivalent squint angle of the radar echo data at the central moment is obtained; c is the speed of light; pi is the circumference ratio; j is an imaginary unit.
And the linear scaling submodule is configured to perform linear scaling operation on the radar echo data subjected to the phase compensation by adopting the linear scaling operation of the improved range-Doppler domain.
The linear scaling submodule is a fifth filter:
wherein in time relative to distance in the fixed PRI mode; f. of a Is the Doppler frequency; q. q of 2 (f a ) Linearly scaling the operational factor for the radar echo data; q. q.s 3 (f a ) Introducing an additional scaling operation factor for a phase disturbance term of radar echo data; tau. ref Is the time delay of the corresponding radar echo data at the reference slope.
The time delay of the radar echo data corresponding to the reference slope distance can be calculated by the following formula:
r 0ref the nearest slope distance of the central point of the imaging area of the radar echo data is obtained; theta c The equivalent squint angle of the radar echo data center moment is obtained.
Referring now to FIG. 8, shown is a schematic diagram of a computer device 800 suitable for use in implementing embodiments of the present application.
As shown in fig. 8, the computer apparatus 800 includes a Central Processing Unit (CPU) 801 which can perform various appropriate actions and processes in accordance with a program stored in a Read Only Memory (ROM) 802 or a program loaded from a storage section 508 into a Random Access Memory (RAM) 803. In the RAM 803, various programs and data necessary for the operation of the system 800 are also stored. The CPU 801, ROM 802, and RAM 803 are connected to each other via a bus 804. An input/output (I/O) interface 805 is also connected to bus 804.
The following components are connected to the I/O interface 805: an input portion 806 including a keyboard, a mouse, and the like; an output section 807 including components such as a Cathode Ray Tube (CRT), a Liquid Crystal Display (LCD), and a speaker; a storage section 808 including a hard disk and the like; and a communication section 809 including a network interface card such as a LAN card, a modem, or the like. The communication section 809 performs communication processing via a network such as the internet. A drive 810 is also connected to the I/O interface 805 as necessary. A removable medium 811 such as a magnetic disk, an optical disk, a magneto-optical disk, a semiconductor memory, or the like is mounted on the drive 810 as necessary, so that the computer program read out therefrom is mounted on the storage section 808 as necessary.
In particular, the processes described above with reference to fig. 1-2 may be implemented as computer software programs, according to embodiments of the present disclosure. For example, embodiments of the present disclosure include a computer program product comprising a computer program tangibly embodied on a machine-readable medium, the computer program containing program code for performing the method of fig. 1-2. In such an embodiment, the computer program may be downloaded and installed from a network through the communication section 809 and/or installed from the removable medium 811.
The flowchart and block diagrams in the figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present application. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems which perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.
The units or modules described in the embodiments of the present application may be implemented by software or hardware. The described units or modules may also be provided in a processor, and may be described as: a processor includes a range migration form recovery unit, an antialiasing unit, and an imaging unit. Where the names of the units or modules do not in some cases constitute a limitation of the unit or module itself, for example, a range migration form restoring unit may also be described as a "unit for range migration form restoring".
As another aspect, the present application also provides a computer-readable storage medium, which may be the computer-readable storage medium included in the apparatus in the above-described embodiments; or it may be a separate computer readable storage medium not incorporated into the device. The computer-readable storage medium stores one or more programs for use by one or more processors in performing the imaging methods described in the embodiments of the present application.
The above description is only a preferred embodiment of the application and is illustrative of the principles of the technology employed. It will be appreciated by those skilled in the art that the scope of the invention herein disclosed is not limited to the particular combination of features described above, but also encompasses other arrangements formed by any combination of the above features or their equivalents without departing from the inventive concept. For example, the above features may be replaced with (but not limited to) features having similar functions disclosed in the present application.
Claims (16)
1. A method for imaging strabismus bunching SAR in a varying PRI mode, the method comprising:
recovering range migration forms of radar echo data in a varying PRI mode, comprising:
removing the time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode and the change of the slant range process of the radar echo data relative to the slant range process in the fixed PRI mode by using a first filter so as to restore the range migration form of the radar echo data to the range migration form in the fixed PRI mode;
the first filter is represented as
Dt ni The time delay of the transmitting time of the radar echo data relative to the transmitting time in a fixed PRI mode is obtained;
DR ni is the change of the slope course of the radar echo data relative to the slope course in the fixed PRI mode;
f t for a fixed PRI modeDistance to frequency;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit;
removing azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data;
restoring the sampling uniformity of the azimuth signals of the radar echo data;
performing phase compensation on the radar echo data subjected to the reduction processing;
and imaging the radar echo data subjected to the phase compensation.
2. The method of claim 1, wherein the removing azimuth spectral aliasing and range-to-time domain aliasing in the radar echo data comprises:
carrying out frequency spectrum shifting, distance walk removal and deskew on the radar echo data by utilizing a second filter so as to remove azimuth spectrum aliasing and distance time domain aliasing in the radar echo data, wherein the second filter is as follows:
wherein t is ni Transmitting the azimuth pulse of the radar echo data to a pulse transmitting moment;
v r the equivalent speed when the radar echo data is imaged is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
k a the azimuth frequency modulation rate of the radar echo data is obtained;
f r distance direction frequency of the radar echo data;
f 0 the carrier frequency of the radar echo data;
pi is the circumference ratio;
j is an imaginary unit.
3. The method of claim 2, wherein the restoring the sampling uniformity of the azimuth signals of the radar return data comprises:
and processing azimuth signals of the radar echo data by using non-uniform Fourier transform (NUFFT).
4. The method of claim 3, wherein the phase compensating the radar echo data that has undergone the restoring process comprises:
restoring the distance walk generated by the second filter by using a third filter, wherein the third filter is as follows:
wherein t is ui Sampling time for uniform azimuth of radar echo data;
k a the azimuth frequency modulation rate of the radar echo data is obtained;
f r distance direction frequency of the radar echo data;
v r the equivalent speed when the radar echo data is imaged is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit.
5. The method of claim 4, wherein said imaging the phase compensated radar echo data comprises:
and imaging the radar echo data subjected to the phase compensation by adopting a nonlinear frequency modulation and scaling NCS algorithm or a range-Doppler RD algorithm or an omega K algorithm.
6. The method of claim 5, wherein the imaging the phase compensated radar echo data using a non-linear frequency modulation and scaling (NCS) algorithm comprises:
and imaging the radar echo data subjected to the phase compensation through improved cubic phase filtering and improved linear scaling operation of a range-Doppler domain.
7. The method of claim 6, wherein the improved cubic phase filtering comprises:
performing cubic phase filtering by using a fourth filter;
the fourth filter is
Wherein, Y (f) a ) The coefficient of a cubic phase perturbation term in the cubic phase filtering is obtained;
f a is the Doppler frequency;
f t distance direction frequency of the radar echo data in a fixed PRI mode;
q(f a ) A squint angle for the radar echo data expressed in doppler frequency;
r 0ref the nearest slope distance of the central point of the imaging area of the radar echo data is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit.
8. The method of claim 6, wherein said linear scaling operation of the modified range-doppler domain comprises:
performing linear scaling operation of the range-Doppler domain by using a fifth filter;
the fifth filter is
Wherein t is the range time relative to the radar echo data in a fixed PRI mode;
f a is the Doppler frequency;
q 2 (f a ) Scaling the operating factor for the radar echo data;
q 3 (f a ) Introducing an additional scaling operation factor for a phase disturbance term of the radar echo data;
t ref time delay of radar echo data corresponding to the reference slope distance;
r 0ref the nearest slope distance of the central point of the imaging area of the radar echo data is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit.
9. An imaging apparatus for strabismus bunching SAR in a varying PRI mode, the apparatus comprising:
the range migration form recovery module is configured to recover a range migration form of the radar echo data in the varying PRI mode, and the range migration form recovery module is a first filter and is used for removing time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode and removing change of a range skew of the radar echo data relative to the range skew in the fixed PRI mode so as to recover the range migration form of the radar echo data in the varying PRI mode into the range migration form in the fixed PRI mode;
the first filter is:
wherein Dt ni The time delay of the transmitting time of the radar echo data relative to the transmitting time in the fixed PRI mode is obtained;
DR ni the change of the slope distance course of the radar echo data relative to the slope distance course in a fixed PRI mode is obtained;
f t is the range-wise frequency of the radar echo data in a fixed PRI mode;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit;
the de-aliasing module is configured to remove azimuth spectrum aliasing and distance direction time domain aliasing of the radar echo data, restore the sampling uniformity of azimuth direction signals of the radar echo data, and perform phase compensation on the radar echo data subjected to the restoration processing;
and the imaging module is configured to image the radar echo data after the phase compensation.
10. The apparatus of claim 9, wherein the de-aliasing module configuration comprises a second filter, an azimuth processing module, and a third filter,
the second filter is configured to remove azimuth spectrum aliasing and distance-to-time domain aliasing of the radar echo data;
the azimuth processing module is configured to restore the sampling uniformity of the azimuth signal of the radar echo data;
and the third filter is configured to perform phase compensation on the radar echo data subjected to the restoration processing.
11. The apparatus of claim 10, wherein the second filter is:
wherein t is ni Transmitting the azimuth pulse of the radar echo data to a pulse transmitting moment;
v r the equivalent speed when the radar echo data is imaged is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
k a the azimuth frequency modulation rate of the radar echo data is obtained;
f r distance direction frequency of the radar echo data;
f 0 the carrier frequency of the radar echo data.
12. The apparatus of claim 10, wherein the orientation processing module is configured to:
and processing the azimuth signals of the radar echo data by utilizing non-uniform Fourier transform (NUFFT).
13. The apparatus of claim 10, wherein the third filter is
Wherein t is ui Sampling time for uniform azimuth of the radar echo data;
k a the azimuth frequency modulation rate of the radar echo data is obtained;
f r distance direction frequency of the radar echo data;
v r the equivalent speed when the radar echo data is imaged is obtained;
q c the equivalent squint angle of the radar echo data center moment is obtained;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit.
14. The apparatus of claim 10, wherein the imaging module comprises a cubic phase filtering sub-module and a linear scaling sub-module;
the third-time phase filtering submodule is configured to perform third-time phase filtering on the radar echo data subjected to the phase compensation by adopting improved third-time phase filtering;
and the linear scaling submodule is configured to perform linear scaling operation on the radar echo data subjected to the phase compensation by adopting linear scaling operation of an improved range-Doppler domain.
15. The apparatus of claim 14, wherein the cubic phase filtering sub-module is a fourth filter;
the fourth filter is:
wherein Y (f) a ) The coefficient of a cubic phase perturbation term in the cubic phase filtering is obtained;
f a is the Doppler frequency;
f t is the range-wise frequency of the radar echo data in a fixed PRI mode;
q(f a ) A squint angle for the radar echo data expressed in doppler frequency;
r 0ref the nearest slope distance of the central point of the imaging area of the radar echo data is obtained;
q c the equivalent squint angle of the center moment of the radar echo data is obtained;
c is the speed of light;
pi is the circumference ratio;
j is an imaginary unit.
16. The apparatus of claim 14, wherein the linear scaling sub-module is a fifth filter,
where t is the distance versus time in the fixed PRI mode;
f a is the Doppler frequency;
q 2 (f a ) Scaling the operating factor for the radar echo data;
q 3 (f a ) Introducing an additional scaling operation factor for a phase disturbance term of the radar echo data;
t ref time delay of radar echo data corresponding to the reference slope distance;
pi is the circumference ratio;
j is an imaginary unit.
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