Three-dimensional wobbling ship target imaging reconstruction method under staggered sar system

Abstract

The application belongs to the technical field of microwave remote sensing, and relates to a three-dimensional swing ship target imaging reconstruction method under a staggered SAR system. The application solves the problems of serious imaging spectrum aliasing and serious defocusing of a three-dimensional swing ship target under the existing staggered SAR system. The application establishes a moving target signal echo model based on the staggered SAR system, detects echo signals of the moving target by using the moving target signal echo model, samples the detected echo signals of the moving target by using an optimal linear unbiased estimation interpolation method, obtains uniform target signals, converts the uniform target signals into a SAR image based on a two-dimensional spectrum by using a series inversion method, performs distance and azimuth inverse transformation on the SAR image to the echo domain to obtain equivalent ISAR echo signals, focuses the equivalent ISAR echo signals by using a distance-instantaneous Doppler time-frequency analysis method, and reconstructs three-dimensional swing ship target imaging. The application is suitable for three-dimensional swing ship target imaging.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of microwave remote sensing. BACKGROUND

[0002] SAR imaging of sea surface ship targets urgently requires high resolution and wide swath. High resolution SAR images provide valuable information about targets at sea, such as small ships that are difficult to detect in low resolution images, which helps to identify ship types and information related to maritime safety, monitoring and management. The wide coverage (i.e. swath) enables satellites to observe vast maritime areas more frequently in order to observe changes in these areas. Existing high resolution wide (HRWS) systems are mainly used for imaging stationary scenes, but are limited for imaging moving targets. A typical HRWS SAR system uses multiple receiving channels arranged along the azimuth direction, combined with a moving target indication (MTI) system to achieve clutter suppression and moving target imaging. However, low pulse repetition frequency (PRF) operation exacerbates azimuth ambiguities, and the long antenna size increases system complexity. Staggered SAR, which combines variable pulse repetition period (PRI) with elevation multi-beam technology, can achieve continuous high resolution wide swath imaging without blind areas, and this technology is proposed to be applied to Tandem-L and NISAR satellites. Due to the conflict between receiving and transmitting timing, there will be pulse missing and non-uniform sampling in the azimuth dimension, which needs to be reconstructed. However, existing reconstruction algorithms are designed for stationary scenes, and will produce obvious image defocus for complex moving targets. Compared with traditional uniform sampling, the non-uniform sampling inherent in the Staggered SAR system itself will cause spectral aliasing, and the complex motion of moving targets will exacerbate the degree of spectral aliasing, resulting in defocusing of ship target imaging and reducing the reconstruction effect. SUMMARY

[0003] The present application is to solve the problem of serious spectral aliasing and defocusing of three-dimensional oscillating ship target imaging under the existing Staggered SAR system, and provides a reconstruction method for three-dimensional oscillating ship target imaging under the Staggered SAR system.

[0004] The reconstruction method for three-dimensional oscillating ship target imaging under the Staggered SAR system, comprising:

[0005] Step one: establish a moving target signal echo model based on the Staggered SAR system, and use the moving target signal echo model to detect the echo signal of the moving target;

[0006] Step two: use the optimal linear unbiased estimation interpolation method to sample the detected echo signal of the moving target to obtain uniform target signals;

[0007] Step 3: Based on the two-dimensional spectrum, the uniform target signal is converted into a SAR image using the series inversion method;

[0008] Step 4: Perform inverse range-azimuth transformation on the SAR image to the echo domain to obtain the equivalent ISAR echo signal;

[0009] Step 5: Focus the equivalent ISAR echo signal using the range-instantaneous Doppler time-frequency analysis method to reconstruct the three-dimensional oscillating ship target image.

[0010] Furthermore, in this invention, in step one, the moving target signal echo model based on the Staggered SAR system is established as follows:

[0011]

[0012] in, This represents the echo signal of a moving target based on the Staggered SAR system. It is a fast time domain. For the slow time domain, Let be the scattering coefficient of the i-th scattering point. For wavelength, For frequency modulation slope, For the azimuth window function, when the pulse is lost, the blind zone matrix... A value of 0 indicates the blind zone matrix is ​​in the form of a fully received pulse. The value is 1. For satellite and the first The slant distance between each scattering point For distance-dimensional window functions, At the speed of light, The duration of the pulse.

[0013] Furthermore, in this invention, the satellite and the... Slope distance between scattering points for:

[0014] (4)

[0015] in, It is a constant term introduced by the motion of the radar platform. The coefficients are first-order terms introduced by the motion of the radar platform. These are the coefficients of the second-order term introduced by the motion of the radar platform. These are the coefficients of the third-order terms introduced by the motion of the radar platform; It is a constant term introduced by the target motion. The coefficients of the first-order term are introduced by the target motion. These are the coefficients of the second-order term introduced by the target motion. These are the coefficients of the third-order terms introduced by the target motion; When the hull experiences three-dimensional oscillation, the first The position of each scattering point in the target coordinate system This represents the satellite's position in the target coordinate system.

[0016] Furthermore, the satellite's position in the target coordinate system for:

[0017]

[0018] in, For coordinate rotation matrix, From point The vector pointing to the ship target, where point P is the intersection of the satellite sensor beam pointing direction and the Earth's surface. The axis is the direction of Earth's angular momentum; To take into account the Earth's angular velocity and curved orbit, the radar in Position coordinates in coordinate system.

[0019] Furthermore, in this invention, when the hull experiences three-dimensional oscillation, the first... The position of each scattering point in the target coordinate system for:

[0020]

[0021] in, , For the first target on the ship The initial time of each scattering point is at The position of the coordinate system This is the ship's translational speed;

[0022]

[0023]

[0024]

[0025] , , The sinusoidal rotation angle is the three-dimensional oscillation of a ship target under the influence of ocean waves, including roll, pitch, and yaw.

[0026] Furthermore, in this invention, , and The calculation method is the same, specifically:

[0027]

[0028] in, These represent the oscillation amplitude, oscillation period, and initial phase, respectively.

[0029] Furthermore, in this invention, in step two, obtaining a uniform target signal is as follows:

[0030]

[0031] in, Indicates a uniform target signal. , This represents the actual non-uniformly sampled signal in the azimuth direction, q=1,……Q, For a matrix with Q rows and Q columns, the first... Line 1 Column elements are represented as :

[0032] ;

[0033] express Related functions, For column vectors, column vectors The The column elements are:

[0034]

[0035] This represents the uniform sampling time to be reconstructed. This refers to the non-uniform sampling time caused by the variation in PRI.

[0036] Furthermore, Related functions :

[0037]

[0038]

[0039]

[0040]

[0041] SNR stands for Signal-to-Noise Ratio. For the Kronecker function, It is an azimuth echo signal. for conjugate, For satellite speed, The azimuth aperture length of the antenna. For uniform sampling time.

[0042] Furthermore, in step three of this invention, the uniform target signal is converted into a SAR image based on the two-dimensional spectrum using a series inversion method:

[0043]

[0044] in, , , , and They are represented as follows:

[0045]

[0046]

[0047]

[0048]

[0049] in, Represents SAR images, It is a two-dimensional frequency domain window function for range-azimuth. For the distance dimension frequency, For azimuth frequency, This represents the distance migration compensation function. This represents the remaining distance compensation function. Represents a higher-order phase compensation function; ,,, , At the speed of light, The center frequency of the carrier. It is a constant term introduced by the target motion. It is a constant term introduced by the motion of the radar platform.

[0050] Furthermore, in this invention, in step five, the reconstructed three-dimensional oscillating ship target image is as follows:

[0051] In step five, the reconstructed three-dimensional image of the oscillating ship target is as follows:

[0052]

[0053] in, To smooth the time-frequency analysis results of the pseudo-Wigner-Willy distribution, For azimuth signal, for conjugate, and Let M be the first and second window functions that slide over time, and M be the parameter for suppressing cross terms.

[0054] This invention explores the imaging of ship targets with complex three-dimensional oscillation characteristics under the Staggered SAR system, achieving accurate focusing imaging of moving targets in a high-resolution, wide-swath system. A signal model of a moving target under a curved orbit in Staggered SAR is established, considering the Earth's rotation and orbital curvature, as well as the target's translation and three-dimensional oscillation. A two-dimensional spectrum is derived using a series inversion method to achieve high-order phase compensation. Comparative experimental results show that the reconstruction algorithm proposed in this invention can accurately focus on complexly moving ship targets, improving imaging performance. Attached Figure Description

[0055] Figure 1 Flowchart of the Staggered SAR three-dimensional oscillating ship target imaging and reconstruction method;

[0056] Figure 2 For Staggered SAR imaging geometry of surface ships;

[0057] Figure 3 Flowchart of signal processing for three-dimensional oscillating ship target imaging using Staggered SAR;

[0058] Figure 4 This is a reconstruction result of Staggered SAR imaging.

[0059] Figure 5 A magnified view of the selected moving target area;

[0060] Figure 6 The image shows the processing result of the algorithm proposed in this invention. Detailed Implementation

[0061] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0062] Specific Implementation Method 1: Referring to the detailed description of this implementation method, the three-dimensional oscillating ship target imaging and reconstruction method under the StaggeredSAR system described in this implementation method includes:

[0063] Step 1: Establish a moving target signal echo model based on the Staggered SAR system, and use the moving target signal echo model to receive the moving target signal to be detected;

[0064] Step 2: Use the optimal linear unbiased estimation interpolation method to resample the received moving target signal to obtain a uniform target signal.

[0065] Step 3: Based on the two-dimensional spectrum, the uniform target signal is converted into a SAR image using the series inversion method;

[0066] Step 4: Perform an inverse range-azimuth transform on the SAR image to the echo domain to obtain the equivalent ISAR echo signal;

[0067] Step 5: Focus the equivalent ISAR echo signal using the range-instantaneous Doppler time-frequency analysis method to reconstruct the three-dimensional oscillating ship target image.

[0068] In this embodiment, a hybrid SAR and ISAR method is mainly used to refocus the moving target. Step 3 is SAR processing, which fills in the motion of the radar platform and obtains a defocused SAR image. At this time, there is still a defocus component caused by the target motion, so it is necessary to convert the SAR image into a processable equivalent ISAR echo.

[0069] Furthermore, in this embodiment, in step one, the moving target signal echo model based on the Staggered SAR system is established as follows:

[0070]

[0071] in, This represents the echo signal of a moving target based on the Staggered SAR system. It is a fast time domain. For the slow time domain, Let be the scattering coefficient of the i-th scattering point. For wavelength, For frequency modulation slope, For the azimuth window function, when the pulse is lost, the blind zone matrix... A value of 0 indicates the blind zone matrix is ​​in the form of a fully received pulse. The value is 1. For satellite and the first The slant distance between each scattering point For distance-dimensional window functions, At the speed of light, The duration of the pulse.

[0072] Furthermore, in this embodiment, the satellite and the... Slope distance between scattering points for:

[0073] (4)

[0074] in, It is a constant term introduced by the motion of the radar platform. The coefficients are first-order terms introduced by the motion of the radar platform. These are the coefficients of the second-order term introduced by the motion of the radar platform. These are the coefficients of the third-order terms introduced by the motion of the radar platform; It is a constant term introduced by the target motion. The coefficients of the first-order term are introduced by the target motion. These are the coefficients of the second-order term introduced by the target motion. These are the coefficients of the third-order terms introduced by the target motion; When the hull experiences three-dimensional oscillation, the first The position of each scattering point in the target coordinate system This represents the satellite's position in the target coordinate system.

[0075] Furthermore, in this embodiment, the satellite's position in the target coordinate system... for:

[0076]

[0077] in, For coordinate rotation matrix, From point The vector pointing to the ship target, point P, is the intersection of the satellite sensor beam pointing direction and the Earth's surface. The axis is the direction of Earth's angular momentum; To account for Earth's angular velocity and curved orbit, radar in Position coordinates in coordinate system.

[0078] In this embodiment, the The origin of the coordinate system is , The axis points to the Earth's center. The shaft is located at Inside, perpendicular to Axis, specifically as Figure 2 As shown.

[0079] Furthermore, in this invention, when the hull experiences three-dimensional oscillation, the first... The position of each scattering point in the target coordinate system for:

[0080]

[0081] in, , For the first target on the ship The initial time of each scattering point is at The position of the coordinate axes This represents the ship's translational speed.

[0082]

[0083]

[0084]

[0085] , , The sinusoidal rotation angle is the three-dimensional oscillation of a ship target under the influence of ocean waves, including roll, pitch, and yaw.

[0086] Furthermore, in this invention, , and The extreme methods are the same, specifically:

[0087]

[0088] in, These represent the oscillation amplitude, oscillation period, and initial phase in the three dimensions of roll, pitch, and yaw, respectively.

[0089] Furthermore, in this invention, in step two, obtaining a uniform target signal is as follows:

[0090]

[0091] in, Indicates a uniform target signal. , This represents the actual non-uniformly sampled signal in the azimuth direction, q=1,……Q, For a matrix with Q rows and Q columns, the first... Line 1 Column elements are represented as :

[0092] ;

[0093] express Related functions, For column vectors, column vectors The The column elements are:

[0094]

[0095] This represents the uniform sampling time to be reconstructed. This refers to the non-uniform sampling time caused by the variation in PRI.

[0096] In this embodiment, q and s are the indices of the non-uniform sampling time and the uniform sampling time, respectively. Therefore, the above... The index of the non-uniform sampling time and the uniform sampling time in matrix G is exactly the qth row and sth column.

[0097] Furthermore, Related functions :

[0098]

[0099]

[0100]

[0101]

[0102] SNR stands for Signal-to-Noise Ratio. For the Kronecker function, It is an azimuth echo signal. for conjugate, For satellite speed, The azimuth aperture length of the antenna. To ensure uniform sampling time, This is the azimuth aperture length of the antenna.

[0103] Furthermore, in step three of this invention, the uniform target signal is converted into a SAR image based on the two-dimensional spectrum using a series inversion method:

[0104]

[0105] in, , , , and They are represented as follows:

[0106]

[0107]

[0108]

[0109]

[0110] in, , , The compensation functions are the distance migration compensation functions. Remaining distance compensation function and higher-order phase compensation function ; For the distance dimension frequency, For azimuth frequency, At the speed of light, The center frequency of the carrier. It is a constant term introduced by the target motion. It is a constant term introduced by the motion of the radar platform. It is a two-dimensional frequency domain window function for range-azimuth. Represents SAR images, This is the phase compensation function.

[0111] In this embodiment, the received signals are multiplied using these compensation functions, and then an inverse Fourier transform is performed within a certain range. The inverse Fourier transform eliminates the two-dimensional phase coupling between range and azimuth, resulting in a decoupled two-dimensional frequency domain result. Azimuth compression is achieved through the interaction with the azimuth compensation function. This is achieved through multiplication, resulting in a SAR image that is defocused due to target motion.

[0112] Furthermore, in this invention, in step five, the reconstructed three-dimensional oscillating ship target image is as follows:

[0113]

[0114] in, To smooth the time-frequency analysis results of the pseudo-Wigner-Willy distribution, For azimuth signal, for conjugate, and Let M be the first and second window functions that slide over time, and M be the parameter for suppressing cross terms.

[0115] This invention utilizes fitted aperture radar imaging (ISAR) technology. In step three of the imaging process, the defocused moving target region is selected, and a two-dimensional inverse Fourier transform is used to perform an inverse range-azimuth transform, transforming the image domain to the echo domain to obtain equivalent original echo data. Range-instantaneous Doppler technology is then used to refocus complex moving targets on ships. After time-frequency analysis processing using range-instantaneous Doppler, the resulting focused image eliminates the blurring caused by complex target motion and uneven azimuth sampling in the Staggered SAR system.

[0116] To verify the beneficial effects of the present invention, the following simulation experiments were conducted:

[0117] The experiment used finely varied linear PRI sequences. The flowchart of the Staggered SAR three-dimensional oscillating ship target imaging signal processing is as follows: Figure 3 As shown, firstly, based on the changing PRI sequence, the blind zone distribution is calculated to obtain the SAR echo in staggered mode. To address the non-uniform sampling problem in the azimuth dimension and the echo missing problem caused by the changing PRI, the BLU algorithm is used for reconstruction. After reconstruction, range compression is achieved, and the two-dimensional spectrum of the reconstructed echo is obtained after azimuth FFT. Range-azimuth decoupling is completed using the range migration compensation function, the residual range compensation function, and the higher-order phase compensation function, respectively. The signal is transformed to the range-Doppler domain by range-dimensional IFFT. After azimuth compression, focusing is achieved for stationary scenes, but moving targets remain defocused. Based on this, ship targets on the sea surface are detected. When a target is detected, a sub-image is extracted, inversely transformed to the echo domain, and motion compensation and phase correction are completed using the hybrid SAR / ISAR concept. Time-frequency analysis is performed using SPWVD, and the final ship target focusing result is obtained after azimuth time-dimensional sampling.

[0118] In this invention, imaging processing is performed on ship targets exhibiting translational and three-dimensional oscillation on the sea surface. The ship moves at a uniform speed. The amplitudes of the movement and three-dimensional oscillation parameters are respectively , , The oscillation periods are respectively , , The reconstruction results after traditional Staggered SAR imaging processing are as follows: Figure 4 As shown, although a higher-order range-azimuth coupling term was added to mitigate the effects of the curved trajectory, the image still suffers from severe defocus and exhibits obvious artifacts in the azimuth distribution. Figure 5 This is a magnified view of the selected target area. Figure 6 The proposed algorithm can correctly reconstruct scattering points, significantly improve the imaging performance of three-dimensional swaying sea surface ship targets in Staggered SAR, and obtain better focused image quality.

[0119] While the invention has been described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the invention. Therefore, it should be understood that many modifications can be made to the exemplary embodiments, and other arrangements can be designed without departing from the spirit and scope of the invention as defined by the appended claims. It should be understood that different dependent claims and features described herein can be combined in ways different from those described in the original claims. It is also understood that features described in conjunction with individual embodiments can be used in other described embodiments.

Claims

1. A method for imaging and reconstructing three-dimensional oscillating ship targets under Staggered SAR system, characterized in that, include: Step 1: Establish a moving target signal echo model based on the Staggered SAR system, and use the moving target signal echo model to detect the echo signal of the moving target; Step 2: Use the optimal linear unbiased estimation interpolation method to sample the echo signal of the detected moving target to obtain a uniform target signal; Step 3: Based on the two-dimensional spectrum, the uniform target signal is converted into a SAR image using the series inversion method; Step 4: Perform inverse range-azimuth transformation on the SAR image to the echo domain to obtain the equivalent ISAR echo signal; Step 5: Focus the equivalent ISAR echo signal using the range-instantaneous Doppler time-frequency analysis method to reconstruct the three-dimensional oscillating ship target image.

2. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 1, characterized in that, In step one, the moving target signal echo model based on the Staggered SAR system is established as follows: in, This represents the echo signal of a moving target based on the Staggered SAR system. It is a fast time domain. For the slow time domain, Let be the scattering coefficient of the i-th scattering point. For wavelength, For frequency modulation slope, For the azimuth window function, when the pulse is lost, the blind zone matrix... A value of 0 indicates the blind zone matrix is ​​in the form of a fully received pulse. The value is 1. For satellite and the first Slope distance between scattering points For distance-dimensional window functions, At the speed of light, The duration of the pulse.

3. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 2, characterized in that, Satellite and the Slope distance between scattering points for: in, It is a constant term introduced by the motion of the radar platform. The coefficients are first-order terms introduced by the motion of the radar platform. These are the coefficients of the second-order term introduced by the motion of the radar platform. These are the coefficients of the third-order terms introduced by the motion of the radar platform; It is a constant term introduced by the target motion. The coefficients of the first-order term are introduced by the target motion. These are the coefficients of the second-order term introduced by the target motion. These are the coefficients of the third-order terms introduced by the target motion; When the hull experiences three-dimensional oscillation, the first The position of each scattering point in the target coordinate system This represents the satellite's position in the target coordinate system.

4. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 3, characterized in that, Satellite position in target coordinate system for: in, For coordinate rotation matrix, From point The vector pointing towards the ship target, where point P is the intersection of the satellite sensor beam pointing direction and the Earth's surface. To take into account the Earth's angular velocity and curved orbit, the radar in Position coordinates within the coordinate system.

5. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 4, characterized in that, When the hull is in three-dimensional oscillation, the first The position of each scattering point in the target coordinate system for: in, , For the first target on the ship The initial time of each scattering point is at The position of the coordinate system This is the ship's translational speed; , , The sinusoidal rotation angle is the angle by which a ship, under the influence of ocean waves, undergoes three-dimensional oscillations of roll, pitch, and yaw.

6. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 5, characterized in that, , and The calculation method is the same, specifically: in, These represent the oscillation amplitude, oscillation period, and initial phase, respectively.

7. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 6, characterized in that, In step two, a uniform target signal is obtained as follows: in, Indicates a uniform target signal. , This represents the actual non-uniformly sampled signal in the azimuth direction, q=1,……Q, For a matrix with Q rows and Q columns, the first... The element in row s and column s is represented as : express Related functions, For column vectors, column vectors The The column elements are: This represents the uniform sampling time to be reconstructed. This is due to the non-uniform sampling time caused by the variation in PRI. For uniform sampling time.

8. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 7, characterized in that, Related functions : SNR stands for Signal-to-Noise Ratio. For the Kronecker function, It is an azimuth echo signal. for conjugate, For satellite speed, This is the azimuth aperture length of the antenna.

9. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 8, characterized in that, In step three, the uniform target signal is converted into a SAR image using a series inversion method based on the two-dimensional spectrum: in, , , , and They are represented as follows: in, Represents SAR image, It is a two-dimensional frequency domain window function for range-azimuth. For the distance dimension frequency, For azimuth frequency, This represents the distance migration compensation function. This represents the remaining distance compensation function. Represents a higher-order phase compensation function; , , , At the speed of light, The center frequency of the carrier. It is a constant term introduced by the target motion. It is a constant term introduced by the motion of the radar platform.

10. The method for imaging and reconstructing three-dimensional oscillating ship targets under the Staggered SAR system according to claim 9, characterized in that, In step five, the reconstructed three-dimensional image of the oscillating ship target is as follows: in, To smooth the time-frequency analysis results of the pseudo-Wigner-Willy distribution, For azimuth signal, for conjugate, and Let M be the first and second window functions that slide over time, and M be the parameter for suppressing cross terms.

Images