CN115469310A - Airborne inverse synthetic aperture radar ship target focusing method and system based on long coherent processing interval, storage medium and electronic equipment - Google Patents
Airborne inverse synthetic aperture radar ship target focusing method and system based on long coherent processing interval, storage medium and electronic equipment Download PDFInfo
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Abstract
The invention discloses a long coherence processing interval-based ship target modeling and fine focusing method with a track deviation inverse synthetic aperture radar, which comprises the steps of firstly modeling the motion track of an airborne radar, including the track deviation of a radar platform, modeling the motion track of a ship target, and including the motion of six degrees of freedom of a ship; then, performing distance compression operation on the original radar echo data, and performing primary motion compensation on the relative translational motion of the radar and the ship target; then further compensating the residual distance migration, selecting an imaging time period and an imaging moment by using an automatic time window selection algorithm based on the maximum contrast, and selecting a time period with stable rotary motion; and finally, according to the obtained imaging time period and imaging time, performing imaging by using the azimuth Fourier transform to finish fine focusing. By adopting the method, the ship target image with better focusing effect can be obtained, and a better image basis is provided for subsequent detection, identification and other processing.
Description
Technical Field
The application relates to the technical field of signal processing, in particular to a method and a system for focusing an airborne inverse synthetic aperture radar ship target based on a long coherent processing interval, a storage medium and electronic equipment.
Background
An Inverse Synthetic Aperture Radar (ISAR) is a Radar imaging system for obtaining high-resolution moving target images, and obtains high-resolution images through relative rotation of a Radar and a target. Meanwhile, imaging, detection and identification of marine ship targets have great significance in the national defense and civil fields, so that imaging of marine ship targets becomes one of main research directions of ISAR imaging.
In actual ocean observation, the relative motion of the radar platform and the ship target seriously affects the quality of ISAR imaging. Therefore, accurate Motion Compensation (MOCO) is required to eliminate the relative translational Motion between them, making the echoes a classical turntable model. Generally, ISAR imaging systems for observing ship targets are installed on an airplane or on land. For the aircraft platform, in addition to the above-mentioned relative motion between the aircraft platform and the ship target, the phenomenon of trajectory deviation of the aircraft platform becomes serious with the increase of Coherent Processing Interval (CPI). Severe trajectory deviation causes more severe motion errors, and even if a robust MOCO algorithm is used, residual distance Migration (RCM) still occurs. For ship targets, there are 6 degrees of freedom of motion at sea, including roll, pitch, and yaw, referred to as rotational motion, and surge, sway, and heave, referred to as translational motion. The rotation motion causes the effective rotation vector of the ship target and the image projection plane to change constantly, and the image quality is influenced. It is noted that the translational motion also has a large impact on the picture quality under a long CPI, which is often ignored in previous motion models. Chinese patent application publication No. CN113589284A discloses an imaging method of an inverse synthetic aperture radar on a ship target, by which a high-resolution ship target image can be obtained, but the method focuses on optimal imaging time selection and ignores the influence of a motion model on imaging, and does not consider the influence of residual range migration, and the phase error is amplified in a long CPI, so that the finally obtained ship target image is not accurate enough.
Therefore, in order to obtain a high-quality ship target image, it is important to design a more accurate motion model under the condition of a long CPI and adopt a more effective method to compensate residual RCM, so as to focus the ship target image more finely.
Disclosure of Invention
The application aims to provide an airborne inverse synthetic aperture radar ship target focusing method, system, storage medium and electronic equipment based on long coherent processing interval, so that ship target images can be focused more finely, and the ISAR imaging quality is greatly improved.
The technical scheme of the invention is as follows:
the invention discloses a long-coherence-processing-interval-based airborne inverse synthetic aperture radar ship target modeling and fine focusing method, which comprises the following steps of:
s101, including the track deviation of the radar platform, and modeling the motion track of the airborne radar;
s102, including the six-degree-of-freedom motion of the ship, and modeling a ship target motion track;
s103, performing distance compression operation on the radar original echo data;
s104, performing primary motion compensation on relative translational motion of the radar and the ship target according to the airborne radar motion trail model obtained in the step S101 and the ship target motion trail model obtained in the step S102 by using a motion compensation algorithm based on joint distance alignment and phase compensation of image contrast maximization to obtain an image after coarse focusing;
s105, further compensating the residual range migration in the image obtained in the step S104 by using a dual-waveband phase gradient-based self-focusing algorithm;
and S106, selecting an imaging time period and an imaging moment according to the image data obtained after compensation in the step S105 by using an automatic time window selection algorithm based on the maximum contrast, and selecting the time period with stable rotary motion.
And S107, according to the imaging time period and the imaging time obtained in the step S106, performing imaging by using the azimuth Fourier transform, and finishing fine focusing.
Further, the trajectory deviation calculation formula for the radar platform in step S101 is as follows:
δr＝a(r)sin(ξx′)
ξ＝4π/λ
where δ r is the trajectory offset; a (r) is the sinusoidal deviation amplitude in the skew plane; λ is the wavelength, x 'is the sensor position, and ξ is the coefficient of x'.
Further, in step S102, the six-degree-of-freedom motion of the ship is included, and the specific method for modeling the ship target motion trajectory is as follows:
according to the hydrodynamic theory of ships, as an ideal rigid target, the displacement caused by three degrees of freedom included in the translational motion of ships can be expressed as:
wherein, Δ x, Δ Y, Δ Z respectively represent displacements caused by ship surging, swaying and heaving; a. The x，i 、A y，i 、A z，i ，ω x，i 、ω y，i 、ω z，i ，
The amplitude and angle of the i-th surging, swaying and heaving respectivelyFrequency and initial phase; n is a radical of x 、N y 、N z Representing the number of surging, swaying and heaving related frequency components;
the angles caused by the three degrees of freedom involved in the rotational motion of the ship can be expressed as:
wherein, theta x 、θ y 、θ z Respectively representing the rotation angles of the rolling, pitching and yawing of the ship; b is x，i 、B y，i 、B z，i ，Ω x，i 、Ω y，i 、Ω z，i ，ψ x，i 、ψ y，i 、ψ z，i The amplitude, angular frequency and initial phase that make up the ith roll, pitch and yaw, respectively; m x 、M y 、M z Representing the number of roll, pitch and yaw related frequency components.
Further, in step S103, a specific method for performing a distance compression operation on the radar original echo data is as follows: firstly, distance fast Fourier transform is carried out on radar original echo data, then distance direction matched filtering is carried out in a distance frequency domain, and finally, distance compression is completed by utilizing inverse distance fast Fourier transform.
Further, in step S104, a specific method for performing preliminary motion compensation on the relative translational motion of the radar and the ship target by using a motion compensation algorithm based on joint distance alignment and phase compensation for image contrast maximization is as follows:
firstly, modeling a translation component into a third-order polynomial; then, utilizing Radon transformation to estimate the initial value of the polynomial coefficient; finally, using the image contrast as an objective function for estimating the coefficients of the above polynomial, wherein the image contrast calculation formula is:
wherein A (-) represents the image space average over coordinates (x, y); i represents the image intensity.
Further, the method for further compensating for residual range migration by using the dual-band-based phase gradient self-focusing algorithm in step S105 includes:
firstly, respectively carrying out self-focusing estimation on two sub-band images taking offset frequency as a center to obtain phase errors of an upper sub-band image and a lower sub-band image;
then, the estimated phase error is used for deriving accurate residual range migration for compensation, and the formula is as follows:
wherein, the first and the second end of the pipe are connected with each other,
representing a residual range migration quantity;
and
an upper subband phase value and a lower subband phase value of the nth pulse are respectively obtained, and M is the number of distance units; m is a group of sub The number of subband distance units.
Finally, by mixing
The kth distance unit applied to the nth pulse can complete the residual range migration correction, and the specific formula is as follows:
wherein x is the distance walk corrected image, x 0 And in step S104, M represents the number of distance units, and j is an imaginary unit, in the image obtained by performing the distance compression operation on the radar original echo data.
Further, in step S106, an automatic time window selection algorithm based on the maximum contrast is used to select an imaging time period and an imaging time, and a time period with stable rotational motion is selected, which specifically includes:
and performing segmented RD imaging on the echo data with the long coherent processing interval, and searching for the optimal imaging time and the optimal imaging duration by using the image contrast as an evaluation function of the imaging quality. The optimal imaging moment can be estimated by the instantaneous doppler frequency, and the formula is as follows:
wherein x is 01 、x 02 Is the coordinate on a Cartesian coordinate system; Ω = Ω eff Is the effective rotation vector, and t is the azimuth time.
The invention also discloses a ship target focusing system containing the trajectory deviation inverse synthetic aperture radar based on the long coherent processing interval, which comprises the following components:
the data inversion unit is used for extracting original echo data from the measured data of the synthetic aperture radar ship target of the input system;
the motion compensation unit is used for carrying out motion compensation of the inverse synthetic aperture radar based on an image contrast maximization method on the original echo data to obtain a primary focusing image;
the residual range migration compensation unit is used for compensating the participating range migration by using a two-waveband phase gradient self-focusing method based on the preliminary focusing image to obtain a translation compensation completion image;
the automatic time window selection unit is used for completing the image based on translational compensation and selecting the optimal imaging time and the optimal imaging time period by utilizing the automatic time window selection method with the maximum image contrast;
and the imaging unit is used for obtaining the fine focusing result of the ship target containing the trajectory deviation inverse synthetic aperture radar by utilizing azimuth Fourier transform according to the selected imaging time and time period.
The invention also discloses a computer readable storage medium, wherein a computer program is stored in the computer readable storage medium, and when the computer program is executed by a processor, the computer readable storage medium is positioned in equipment to execute the method for finely focusing the ship target containing the trajectory deviation inverse synthetic aperture radar based on the long coherent processing interval.
The invention also discloses an electronic device, comprising a computer readable storage medium and a processor; the computer readable storage medium has stored therein a computer program which, when executed by the processor, causes an apparatus on the computer readable storage medium to perform the long coherence processing interval-based inverse synthetic aperture radar vessel target fine focusing method with trajectory deviation according to the present invention.
The invention has the following beneficial effects:
by adopting the method for establishing the ship target motion model containing the track deviation inverse synthetic aperture radar and finely focusing based on the long coherent processing interval, a more accurate motion model can be provided for an algorithm, a ship target image with a better focusing effect can be obtained by the method, and a better image basis is provided for subsequent detection, identification and other processing.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and not to limit the application, in which:
FIG. 1 is a schematic flow diagram of a method for fine focusing of a ship target including an inverse synthetic aperture radar with trajectory deviation based on a long coherence processing interval;
FIG. 2 is a schematic view of a ship target motion model;
FIG. 3 is a ship target simulation image obtained by processing signal data acquired by an inverse synthetic aperture radar through conventional two-step motion compensation;
FIG. 4 is a ship target simulation image obtained by processing signal data acquired by an inverse synthetic aperture radar using the method disclosed in the present invention;
FIG. 5 is a schematic diagram of a long coherence processing interval-based fine focusing system for a ship target with an inverse synthetic aperture radar with track deviation.
Detailed Description
As shown in fig. 1, the method for finely focusing an airborne inverse synthetic aperture radar ship target based on a long coherent processing interval comprises the following steps:
s101, modeling a motion track of an airborne radar, and including the track deviation of a radar platform;
specifically, according to the geometric information provided by the motion sensor in the inertial measurement unit, the approximate flight path of the aircraft can be known, and the trajectory deviation of the radar platform can be calculated by a sine function, so that the trajectory deviation can be expressed as:
δr＝a(r)sin(ξx′) (1)
ξ＝4π/λ (2)
where δ r is the trajectory offset; a (r) is the sinusoidal deviation amplitude in the skew plane; λ is the wavelength, x 'is the sensor position, and ξ is the coefficient of x'.
Generally, ISAR imaging systems for observing ship targets are installed on an airplane or on land. For an aircraft platform, besides that the relative motion between the aircraft platform and a ship target can cause the image to generate directional defocusing, along with the increase of the CPI, the track deviation phenomenon can also cause the image quality to be degraded, the motion track of the aircraft cannot be simply approximated to ideal linear motion, and the track deviation phenomenon needs to be considered. Therefore, the trajectory deviation is added into the motion model, a more accurate model basis can be provided for a subsequent algorithm, and the imaging resolution is effectively improved.
S102, modeling a ship target motion track, and including six-degree-of-freedom motion of a ship;
specifically, according to the hydrodynamic theory of ships, as an ideal rigid target, the displacement caused by three degrees of freedom included in the translational motion of ships can be expressed as:
wherein, deltaX, deltaY and DeltaZ respectively represent displacement caused by surging, swaying and heaving of the ship; a. The x，i 、A y，i 、A z，i ，ω x，i 、ω y，i 、ω z，i ，
The amplitude, angular frequency and initial phase constituting the ith surging, swaying and heaving, respectively; n is a radical of x 、N y 、N z Representing the number of surge, sway and heave related frequency components;
the angle caused by the three degrees of freedom involved in the vessel's rotational motion can be expressed as:
wherein, theta x 、θ y 、θ z Respectively representing rotation angles of rolling, pitching and yawing of the ship; b x，i 、B y，i 、B z，i ，Ω x，i 、Ω y，i 、Ω z，i ，ψ x，i 、ψ y，i 、ψ z，i The amplitude, angular frequency and initial phase that make up the ith roll, pitch and yaw, respectively; m is a group of x 、M y 、M z Representing the number of roll, pitch and yaw related frequency components.
Due to the fact that the effective rotation vector and the image projection plane of the ship target are constantly changed due to the rotation motion, the image quality is affected. While translational motion also has a large impact on picture quality under a long CPI, which is often ignored in previous motion models. Therefore, in order to obtain a high-quality ship target image, it is important to include the six-degree-of-freedom motion of the ship in the motion model in the case of a long CPI.
S103, performing distance compression operation on the radar original echo data;
firstly, distance fast Fourier transform is carried out on radar original echo data, then distance direction matched filtering is carried out in a distance frequency domain, and finally, distance compression is completed by utilizing inverse distance fast Fourier transform.
Step S104, performing preliminary motion compensation on relative translational motion of the radar and the ship target by using a motion compensation algorithm based on image contrast maximization and combined distance alignment and phase compensation;
specifically, the method comprises the following steps: modeling the translational component as a third order polynomial with an instantaneous slope distance R 0 It can be approximated that the third order taylor formula expands at the central instant t =0, i.e.:
wherein α represents the zero-order coefficient in the taylor polynomial, β and γ represent the radial velocity and acceleration of the target, respectively, t represents the azimuthal time, and δ is the numerator of the cubic coefficient in the taylor polynomial. Wherein the coefficient alpha is represented in the image as a target offset, without affecting the focus quality, and therefore the focus problem of the image can be converted into an estimate of the three coefficients.
Then, utilizing Radon transformation to carry out initial value estimation on polynomial coefficients, wherein the Radon transformation is mainly used for detecting straight lines or line segments in the image, the technology can obtain rough estimation of the coefficient beta, and particularly, the rough estimation is carried out on the image S after distance compression R And (3) carrying out Radon transformation to obtain an included angle between the trajectory of the scattering body and the abscissa axis, wherein the formula is as follows:
wherein phi is the included angle between the trajectory of the scattering body and the abscissa axis,
is the maximum value of the angle between the trajectory of the scatterer and the axis of abscissa, max φ Is the maximum value of the angle between the trajectory of the scatterer and the axis of abscissa, S R And beta is an estimation coefficient, wherein beta is an amplitude matrix of each pixel point of the image acquired by the radar.
And finally, using the image contrast as an objective function, and iteratively searching and finely estimating coefficients of the polynomial through maximization of the image contrast. The image contrast is a very accurate indicator for the quality of the image focus, with sufficient confidence as an evaluation of the degree of image focus, defined as:
wherein A (-) represents the image spatial average over coordinates (x, y); i represents image intensity and IC represents image contrast.
S105, further compensating the residual range migration by using a dual-waveband-based phase gradient self-focusing algorithm;
specifically, the method comprises the following steps: dividing two sub-band images by taking offset frequency as a center, paying attention to the fact that the two sub-bands need to have some overlap, otherwise the two sub-band images become completely irrelevant, and experiments show that the overlap ratio epsilon is larger than the maximum value overlap =0.7 is effective, and the number of distance units of a subband is obtained from the overlap ratio, and the calculation formula is:
wherein M is the number of distance units; m is a group of sub The number of distance units for a subband; e is the same as overlap Is the overlap ratio. Then, phase errors of the upper sub-band image and the lower sub-band image, which are estimated by a phase gradient self-focusing algorithm, are respectively used for the upper sub-band image and the lower sub-band image, and accurate residual distance migration is calculated by using the estimated phase errors, wherein a residual distance migration amount calculation formula is as follows:
wherein the content of the first and second substances,
representing a residual range migration quantity;
and
an upper sub-band phase value and a lower sub-band phase value of the nth pulse are respectively, and M is the number of distance units; m is a group of s u b The number of subband distance units. By mixing
The kth distance unit applied to the nth pulse can complete the residual range migration correction, and the specific formula is as follows:
wherein x is the distance walk corrected image, x 0 For the image after the preliminary motion compensation in step S104, M represents the number of distance units, and j is an imaginary unit.
S106, selecting an imaging time period and an imaging moment by using an automatic time window selection algorithm based on the maximum contrast, and selecting the time period with stable rotary motion to finish focusing imaging on the ship target;
specifically, the method comprises the following steps: and performing segmented RD imaging on the echo data with the long coherent processing interval, and searching for the optimal imaging time and the optimal imaging duration by using the image contrast as an evaluation function of the imaging quality. The optimal imaging moment can be estimated by the instantaneous doppler frequency, and the formula is as follows:
wherein x is 01 、x 02 Is the coordinate on the Cartesian coordinate system; Ω = Ω eff Is an effective rotation vector, t is an azimuth time, f d For optimal imaging moments, λ is the wavelength of the radar signal.
S107, according to the imaging time period and the imaging moment obtained in the step S106, performing imaging by using azimuth Fourier transform to finish fine focusing;
specifically, the method comprises the following steps: different motion states of the ship target, such as a side view or a top view of the ship target and the like, can be obtained by selecting different imaging moments, so that more ship target information can be obtained, and the method has high practicability.
The test was carried out using the ship model shown in fig. 2, which contains 301 scattering points in total, the coordinate spacing of each scattering point was 5 meters, and the reflection coefficients of the scattering points were set to unity values. Wherein the length of the vessel is 150 meters, the height of the vessel is 50 meters, the width of the vessel is 30 meters, and the initial bow orientation is the x-axis. Processing signal data acquired by the inverse synthetic aperture radar by using the traditional two-step motion compensation method to obtain a ship target simulation image as shown in figure 3; the distance alignment uses a global minimum entropy method, the phase compensation uses a phase gradient self-focusing method, ghost images appear on two sides of the treated ship body, and the main part of the ship body is defocused. The method of the invention is utilized to process the signal data acquired by the inverse synthetic aperture radar, and the obtained ship target simulation image is shown in figure 4, so that ghost can be seen to disappear, the main part of the ship body can be seen to be clear, and finally, a clearer ship target simulation image can be obtained.
The invention relates to a ship target focusing system containing a track deviation inverse synthetic aperture radar based on a long coherent processing interval, which comprises:
the data inversion unit is used for extracting original echo data from the measured data of the synthetic aperture radar ship target of the input system;
the motion compensation unit is used for carrying out motion compensation of the inverse synthetic aperture radar based on an image contrast maximization method on the original echo data to obtain a primary focusing image;
the residual range migration compensation unit is used for compensating the residual range migration by using a two-waveband phase gradient self-focusing method based on the preliminary focusing image to obtain a translation compensation completion image;
the automatic time window selection unit is used for completing the image based on translational compensation and selecting the optimal imaging time and the optimal imaging time period by utilizing the automatic time window selection method with the maximum image contrast;
and the imaging unit is used for obtaining the fine focusing result of the ship target containing the trajectory deviation inverse synthetic aperture radar by utilizing azimuth Fourier transform according to the selected imaging time and time period.
The invention relates to a computer readable storage medium, wherein a computer program is stored in the computer readable storage medium, and when the computer program is executed by a processor, the computer readable storage medium is used for equipment to execute the method for modeling and fine focusing of a ship containing the trajectory deviation inverse synthetic aperture radar based on a long coherent processing interval.
An electronic device of the present invention includes a computer-readable storage medium and a processor; the computer readable storage medium has stored therein a computer program which, when executed by the processor, causes an apparatus on the computer readable storage medium to perform the long coherence processing interval-based inverse synthetic aperture radar vessel target fine focusing method with trajectory deviation according to the present invention.
The Processor includes a Central Processing Unit (CPU), a Network Processor (NP), etc., and may also be a digital signal Processor, an application specific integrated circuit, an off-the-shelf programmable gate array or other programmable logic device, a discrete gate or transistor logic device, or a discrete hardware component.
The processor may be specifically configured to: modeling the motion track of the airborne radar, and including the track deviation of the radar platform; modeling a ship target motion track, and including six-degree-of-freedom motion of a ship; performing motion compensation on the relative translational motion of the radar and the ship target by utilizing a motion compensation algorithm based on the joint distance alignment and phase compensation of image contrast maximization; further compensating the residual range migration by using a dual-waveband-based phase gradient self-focusing algorithm; selecting an imaging time period and an imaging moment by using an automatic time window selection algorithm based on the maximum contrast, and selecting a time period with stable rotary motion; and according to the obtained imaging time period and imaging time, performing imaging by using the azimuth Fourier transform to complete fine focusing.
The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made to the present application by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims (10)
1. A method for focusing an airborne inverse synthetic aperture radar ship target based on a long coherent processing interval is characterized by comprising the following steps:
s101, including the track deviation of the radar platform, and modeling the motion track of the airborne radar;
s102, modeling a ship target motion track by including six-degree-of-freedom motion of a ship;
s103, performing distance compression operation on the radar original echo data;
s104, performing primary motion compensation on relative translational motion of the radar and the ship target according to the airborne radar motion track model obtained in the step S101 and the ship target motion track model obtained in the step S102 by using a motion compensation algorithm based on joint distance alignment and phase compensation of image contrast maximization to obtain an image after coarse focusing;
s105, further compensating residual range migration of the image after coarse focusing by using a dual-waveband-based phase gradient self-focusing algorithm;
s106, selecting an imaging time period and an imaging moment according to the image data obtained after compensation in the step S105 by utilizing an automatic time window selection algorithm based on the maximum contrast, and selecting a time period with stable rotary motion;
and S107, imaging by utilizing the azimuth Fourier transform according to the imaging time period and the imaging moment obtained in the step S106, and finishing fine focusing.
2. The method for focusing on a ship target by using an airborne inverse synthetic aperture radar based on a long coherent processing interval according to claim 1, wherein the trajectory deviation calculation formula for the radar platform in step S101 is as follows:
δr＝a(r)sin(ξx′)
ξ＝4π/λ
where δ r is the trajectory offset; a (r) is the sinusoidal deviation amplitude in the skew plane; λ is the wavelength, x 'is the sensor position, and ξ is the coefficient of x'.
3. The long coherence processing interval-based airborne inverse synthetic aperture radar ship target focusing method of claim 1, wherein in step S102, six-degree-of-freedom ship motion is included, and a specific method for modeling ship target motion trajectory is as follows:
according to the ship hydrodynamic theory, as an ideal rigid target, the displacement caused by three degrees of freedom included in the translational motion of a ship can be expressed as:
wherein, Δ X, Δ Y, Δ Z represent displacement caused by ship surging, swaying and heaving respectively; a. The x,i 、A y,i 、A z,i ，ω x,i 、ω y,i 、ω z,i ，
The amplitude, angular frequency and initial phase constituting the ith surging, swaying and heaving, respectively; n is a radical of x 、N y 、N z Representing the number of surging, swaying and heaving related frequency components;
the angle caused by the three degrees of freedom involved in the vessel's rotational motion can be expressed as:
wherein, theta x 、θ y 、θ z Respectively representing rotation angles of rolling, pitching and yawing of the ship; b x,i 、B y,i 、B z,i ，Ω x,i 、Ω y,i 、Ω z,i ，ψ x,i 、ψ y,i 、ψ z,i The amplitude, angular frequency and initial phase that make up the ith roll, pitch and yaw, respectively; m is a group of x 、M y 、M z Representing the number of roll, pitch and yaw related frequency components.
4. The method for focusing on the ship target of the airborne inverse synthetic aperture radar based on the long coherent processing interval according to claim 1, wherein in step S103, a distance compression operation is performed on radar original echo data, and the specific method is as follows: firstly, performing range fast Fourier transform on radar original echo data, then performing range direction matched filtering in a range frequency domain, and finally completing range compression by using range fast Fourier inverse transform.
5. The method for focusing on an airborne inverse synthetic aperture radar ship target based on a long coherent processing interval according to claim 1, wherein in step S104, a method for performing preliminary motion compensation on the relative translational motion of the radar and the ship target by using a motion compensation algorithm based on the joint distance alignment and phase compensation for maximizing image contrast is as follows:
firstly, modeling a translation component into a third-order polynomial; then, utilizing Radon transformation to estimate the initial value of the polynomial coefficient; finally, using the image contrast as an objective function for estimating the coefficients of the above polynomial, wherein the image contrast calculation formula is:
wherein A (-) represents the image space average over coordinates (x, y); i represents image intensity and IC represents image contrast.
6. The long coherence processing interval-based airborne inverse synthetic aperture radar vessel target focusing method of claim 1, wherein the step S105 of further compensating for residual range migration using a dual-band phase gradient-based self-focusing algorithm comprises:
firstly, respectively carrying out self-focusing estimation on two sub-band images taking offset frequency as a center to obtain phase errors of an upper sub-band image and a lower sub-band image, and then deriving accurate residual distance migration by using the estimated phase errors for compensation, wherein the formula is as follows:
wherein the content of the first and second substances,
representing a residual range migration quantity;
and
an upper subband phase value and a lower subband phase value of the nth pulse are respectively obtained, and M is the number of distance units; m sub The number of sub-band distance units;
then, by mixing
The kth distance unit applied to the nth pulse can complete the residual range migration correction, and the specific formula is as follows:
wherein x is the distance walk corrected image, x 0 For the image after the preliminary motion compensation in step S104, M represents the number of distance units, and j is an imaginary unit.
7. The method for focusing on an airborne inverse synthetic aperture radar ship target based on a long coherent processing interval according to claim 1, wherein in step S106, an automatic time window selection algorithm based on maximum contrast is used to select an imaging time period and an imaging time, and a time period with stable rotation motion is selected, and the specific method is as follows:
and performing segmented RD imaging on the echo data with the long coherent processing interval, and searching for the optimal imaging time and the optimal imaging duration by using the image contrast as an evaluation function of the imaging quality. The optimal imaging time can be estimated by the instantaneous doppler frequency, and the formula is as follows:
wherein x is 01 、x 02 Is the coordinate on a Cartesian coordinate system; Ω = Ω eff Is an effective rotation vector, t is azimuth time, f d Is the optimal imaging moment.
8. An airborne inverse synthetic aperture radar vessel target focusing system based on a long coherent processing interval, comprising:
the data inversion unit is used for extracting original echo data from the measured data of the synthetic aperture radar ship target of the input system;
the motion compensation unit is used for carrying out motion compensation of the inverse synthetic aperture radar based on an image contrast maximization method on the original echo data to obtain a primary focusing image;
the residual range migration compensation unit is used for compensating the participating range migration by using a two-waveband phase gradient self-focusing method based on the preliminary focusing image to obtain a translation compensation completion image;
the automatic time window selection unit is used for completing the image based on translational compensation and selecting the optimal imaging time and the optimal imaging time period by utilizing the automatic time window selection method with the maximum image contrast;
and the imaging unit is used for obtaining the fine focusing result of the ship target containing the trajectory deviation inverse synthetic aperture radar by utilizing azimuth Fourier transform according to the selected imaging time and time period.
9. A computer-readable storage medium, in which a computer program is stored, which computer program, when being executed by a processor, causes an apparatus on which the computer-readable storage medium is located to carry out the method of any one of claims 1-7.
10. An electronic device comprising a computer-readable storage medium and a processor; computer program stored on a computer-readable storage medium, which, when executed by a processor, causes an apparatus on the computer-readable storage medium to perform the method of any one of claims 1-7.
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