Rapid registration method based on data bunching type video SAR (Synthetic Aperture Radar)

Abstract

The invention discloses a data bunching type video SAR (Synthetic Aperture Radar) fast registration method, and belongs to the technical field of radar imaging. According to the method, the affine transformation required by SAR image registration is disassembled into rotation transformation and translation transformation by utilizing SAR imaging characteristics. According to the method provided by the invention, SAR image calibration is carried out, the prior information of an inertial navigation system and echo data can be well utilized, and the calculation efficiency of SAR image registration is improved. In addition, the method only relates to simple arithmetic operation and does not relate to complex calculation operation such as a sliding window, and therefore the implementation mode of the method is very simple.

Description

Technical Field

[0001] The invention belongs to the technical field of radar imaging, and in particular relates to a fast registration method based on data spotlight video SAR. Background Art

[0002] Video synthetic aperture radar (Video SAR) is an advanced radar imaging technology that aims to generate a video-like sequence of continuous high-resolution radar images. It can capture continuous images in the time dimension, thereby achieving real-time monitoring of targets and observation of dynamic changes. However, due to platform motion, imaging angle changes and environmental factors, continuously acquired images may have spatial inconsistencies, which makes image registration play a key role in video SAR. Moreover, high frame rate, high resolution, and real-time performance have become a major trend in the development of video SAR. The realization of high frame rate, high resolution and real-time video SAR faces huge technical challenges, including huge data processing volume and high real-time requirements. Therefore, it is of great significance to develop a fast image registration method.

[0003] SAR image registration specifically refers to the process of geometrically correcting two or more images acquired under different conditions, so that the reference image and the image to be registered are in the same coordinate system. The video SAR system can obtain a series of SAR image sequences in time order, so a frame in the SAR image sequence can be selected as the reference frame, and the SAR image sequence of the remaining frames to be registered can be unified to the spatial coordinate system of the reference frame. The existing SAR image registration methods first extract the feature information of the reference image and the image to be registered, and then obtain the transformation matrix based on the feature information. From the perspective of feature detection, SAR image registration methods can be divided into two categories: registration methods based on regional information and registration methods based on feature matching. Registration methods based on regional information require intensive calculations in the entire image or selected areas, usually involving image correlation measurement, image transformation and interpolation, global search and optimization, with the characteristics of high registration accuracy and high computational cost, and it is difficult to meet the processing time requirements of video SAR. The registration method based on feature matching only needs to process the key areas in the image. While improving the operation speed, the registration accuracy is reduced. Especially when the radar viewing angle changes, the image resolution is low, and the texture information is blurred, the registration accuracy will be seriously reduced, and even mismatch will occur. Therefore, in the video SAR application scenario, there is an urgent need for a SAR image registration method that meets the registration accuracy while also having the characteristics of high processing speed and strong robustness. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a SAR image registration method in the video SAR application scenario, which not only meets the registration accuracy but also has the characteristics of high processing speed and strong robustness. Specifically, a fast registration method for video SAR based on data beamforming is provided, which utilizes the correlation of beamforming SAR scenes and the inertial navigation system to decompose the transformation matrix - rotation transformation and translation transformation in sequence, and is an efficient SAR image registration method.

[0005] The technical problem proposed by the present invention is solved in this way:

[0006] A fast registration method for data spotlight video SAR, comprising the following steps:

[0007] Step 1: Convert the SAR imaging radar position coordinates (lon, lat, alt) in the WGS-84 coordinate system to points (X, Y, Z) in the Earth-centered Earth-fixed rectangular coordinate system;

[0008] Step 2: Select the SAR original echo signal of any frame as the reference frame, and take the center position coordinates of the SAR imaging radar corresponding to the reference frame in the geocentric earth-fixed rectangular coordinate system as the imaging center of the reference frame; take the imaging center of the reference frame as the coordinate origin, respectively convert the SAR imaging radar position coordinates of all frames in the geocentric earth-fixed rectangular coordinate system and the position coordinates of the set scene center in the geocentric earth-fixed rectangular coordinate system to the north-east coordinate system;

[0009] Step 3: Generate a rotation matrix using the roll angle, pitch angle, and yaw angle corresponding to the imaging center of the reference frame, and transform the coordinates of the set scene center and the SAR imaging radar in the north-east coordinate system to the imaging coordinate system;

[0010] Step 4: The angle θ between the line of sight direction at any position of the radar during its motion and the line of sight direction of the imaging center of the reference frame is:

[0011]

[0012] in, is the displacement vector from the imaging center of the reference frame to the center of the set scene, is the displacement vector from the imaging center of any frame of the SAR imaging radar to the set scene center during radar motion, and || || represents the modulus.

[0013] Step 5: Correct the viewing angles of the SAR imaging radar in different azimuths to the azimuth viewing angles in the imaging coordinate system of the reference frame by two-dimensional interpolation. The SAR two-dimensional images of each frame are used as the imaging scene data V to be interpolated. Perform two-dimensional interpolation processing on the imaging scene data V to obtain the interpolated imaging scene data, so that the spatial frequency distribution of the imaging scene data is (X a , Ya ) to (X q , Y q ), (X a , Y a ) represent the spatial frequency distribution of the imaging scene before interpolation, (X q , Y q ) represent the spatial frequency distribution of the imaging scene after any frame perspective correction (rotation);

[0014] Step 6: Use the correlation of the spotlight SAR imaging echo data to perform offset correction in the range-frequency domain, azimuth-time domain and azimuth-frequency domain, range-time domain respectively;

[0015] When performing range offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to range Fourier transform to obtain the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame and the frame to be corrected; the range vector f in a certain azimuth direction in the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame is converted into τref The range vector f of each azimuth direction in the SAR signal data in the range, frequency, azimuth and time domain of the frame to be corrected τany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the distance offset τ , select the offset offfset corresponding to the maximum value in the correlation coefficient vector τ As the distance deviation correction;

[0016] Similarly, when performing azimuth offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to azimuth Fourier transform to obtain the SAR signal data of the reference frame and the frame to be corrected in the azimuth frequency domain, distance and time domain; the azimuth vector f of a certain distance direction in the SAR signal data of the reference frame in the azimuth frequency domain and distance and time domain is converted into tref The azimuth vector f of each range direction in the SAR signal data in the azimuth frequency domain and time domain of the frame to be corrected tany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the azimuth offset t , select the offset corresponding to the maximum value in the correlation coefficient vector t As the azimuth deviation correction;

[0017] Step 7: Circularly shift the interpolated imaging scene data by offset in the distance direction τ unit length to complete the distance offset correction; similarly, the interpolated imaging scene data is circularly shifted by offset in the azimuth direction t unit length to complete the azimuth offset correction.

[0018] Furthermore, in step 1, the conversion relationship between the WGS-84 coordinate system and the Earth-centered Earth-fixed rectangular coordinate system is as follows:

[0019]

[0020] Where lon, lat and alt represent the longitude, latitude and altitude of the SAR imaging radar respectively; X, Y and Z represent the x-axis, y-axis and z-axis coordinates of the SAR imaging radar in the ECEF coordinate system respectively; N is the curvature radius of the reference ellipsoid, e is the eccentricity of the ellipsoid in the WGS-84 coordinate system, and a is the major radius of the reference ellipsoid.

[0021] Furthermore, in step 2, the center position coordinates of the SAR imaging radar corresponding to the reference frame in the earth-centered earth-fixed rectangular coordinate system are marked as The position coordinates of the SAR imaging radar in the Earth-centered Earth-fixed rectangular coordinate system are (x ECEF ,y ECEF , z ECEF ), set the scene center position coordinates in the geocentric rectangular coordinate system as The position coordinates of the SAR imaging radar position in the north-east coordinate system and the position coordinates of the set scene center in the north-east coordinate system are expressed as follows:

[0022]

[0023]

[0024] Among them, N x 、E y and D z represents the position coordinates of the SAR imaging radar position coordinates in the N-axis, E-axis and D-axis in the north-east coordinate system; N a 、E a and D a Indicates the position coordinates of the scene center on the N axis, E axis and D axis in the north-east coordinate system respectively;

[0025] S is the coordinate transformation matrix from the Earth-centered Earth-fixed rectangular coordinate system to the North-East Earth coordinate system, expressed as:

[0026]

[0027] Among them, lon0, lat0 and alt0 represent the longitude, latitude and altitude of the imaging center of the reference frame respectively.

[0028] Furthermore, in step 3, the roll angle θ x , pitch angle θ y and the yaw angle θ z The corresponding rotation matrix Tx , T y and T z Respectively expressed as:

[0029]

[0030] The SAR imaging radar in the imaging coordinate system and the position coordinates (x, y, z) of the center of the scene are set and (x a ,y a , z a ) are:

[0031]

[0032]

[0033] Furthermore, in step 5, (X a , Y a ) and (X q , Y q ) have the following relationship:

[0034]

[0035] Where c is the speed of light, f c is the center frequency of the radar, f τ is the distance frequency, is the elevation angle of the imaging center of the current frame of the SAR imaging radar, and φ is the instantaneous azimuth angle of the phase center of the SAR imaging radar antenna.

[0036] The beneficial effects of the present invention are:

[0037] The method of the present invention splits the affine transformation into rotation transformation and translation transformation for image correction respectively according to the characteristics of radar imaging at different viewing angles, so that the implementation is relatively simple, the algorithm complexity is reduced, and the calculation efficiency is improved. While ensuring the accuracy of SAR image registration, the method of the present invention can also complete efficient image registration for SAR images with unclear features, and the implementation process does not involve complex calculation processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the process of the method of the present invention;

[0039] Figure 2 A SAR image of a reference frame in the method described in the embodiment;

[0040] Figure 3 is the 55th SAR image before correction in the method described in the embodiment;

[0041] Figure 4The 55th frame of the SAR image to be corrected after the rotation transformation in the method described in the embodiment;

[0042] Figure 5 It is the 55th frame of the SAR image to be corrected after the rotation and translation transformation in the method described in the embodiment. DETAILED DESCRIPTION

[0043] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0044] This embodiment provides a fast registration method for video SAR based on data spotlighting, and its flow chart is as follows: Figure 1 As shown, the input is the SAR original echo signal, and after imaging processing, a SAR two-dimensional image is obtained. The specific implementation includes the following steps:

[0045] Step 1: Convert the WGS-84 coordinate system (ALL) to the Earth-centered Earth-fixed rectangular coordinate system (ECEF). Convert the SAR imaging radar position coordinates (lon, lat, alt) in the LLA coordinate system to the points (X, Y, Z) in the ECEF coordinate system. The conversion relationship is as follows:

[0046]

[0047] Wherein, lon, lat and alt represent the longitude, latitude and altitude of the SAR imaging radar respectively; X, Y and Z represent the x-axis, y-axis and z-axis coordinates of the SAR imaging radar in the ECEF coordinate system respectively; the eccentricity of the ellipsoid in the WGS-84 coordinate system is e=8.1819190842622 -2 , the radius of curvature of the reference ellipsoid The major radius of the reference ellipsoid is a=6378137m.

[0048] Step 2: Select any frame of SAR original echo signal as the reference frame, and calculate the center position coordinates of the SAR imaging radar corresponding to the reference frame in the ECEF coordinate system. As the imaging center of the reference frame; taking the imaging center of the reference frame as the coordinate origin, the SAR imaging radar position coordinates (x ECEF ,y ECEF , z ECEF ) and set the position coordinates of the scene center in the ECEF coordinate system Convert to the North East Earth (NED) coordinate system:

[0049]

[0050] Among them, N x 、E y and D zrepresents the position coordinates of the SAR imaging radar position coordinates in the N-axis, E-axis and D-axis in the north-east coordinate system; N a 、E a and D a Represents the position coordinates of the N axis, E axis and D axis of the scene center in the north-east coordinate system; S is the transformation matrix from the ECEF coordinate system to the NED coordinate system, expressed as:

[0051]

[0052] Among them, lon0, lat0 and alt0 represent the longitude, latitude and altitude of the imaging center of the reference frame respectively.

[0053] Step 3: Use the roll angle θ corresponding to the imaging center of the reference frame recorded by the IMU x , pitch angle θ y and the yaw angle θ z Generate a rotation matrix to transform the coordinates of the scene center and the SAR imaging radar in the north-east coordinate system to the imaging coordinate system, expressed as:

[0054]

[0055] Among them, (x, y, z) and (x a ,y a , z a ) are the position coordinates of the SAR imaging radar and the set scene center in the imaging coordinate system; the rotation matrices corresponding to the roll angle, pitch angle and yaw angle are as follows:

[0056]

[0057] At this point, the position coordinates of the set scene center and the SAR imaging radar have been converted to an imaging coordinate system with the base frame imaging center as the origin.

[0058] Step 4: According to the angle formula of two vectors, the angle θ between the line of sight at any position of the radar during movement and the line of sight of the imaging center of the reference frame can be calculated:

[0059]

[0060] Among them, the displacement vector from the imaging center of the reference frame to the center of the set scene is The imaging center (x c ,y c ) to set the displacement vector of the scene center || || means modulo.

[0061] Step 5: The beam distribution of the SAR imaging radar in different azimuth directions can be regarded as the beam distribution obtained by rotating the reference coordinate system (the imaging coordinate system of the reference frame). Therefore, the viewing angle of the SAR imaging radar in different azimuth directions can be corrected to the azimuth viewing angle in the imaging coordinate system of the reference frame by two-dimensional interpolation.

[0062] The two-dimensional interpolation method in this embodiment adopts the two-dimensional interpolation function interp2(X) of Matlab. a , Y a , V, X q , Y q ) is implemented, each frame of the SAR two-dimensional image is used as the imaging scene data V to be interpolated, and the imaging scene data V is subjected to two-dimensional interpolation processing to obtain the interpolated imaging scene data, so that the spatial frequency distribution of the imaging scene data is (X a , Y a ) to (X q , Y q ), (X a , Y a ) represent the spatial frequency distribution of the imaging scene before interpolation, (X q , Y q ) represent the spatial frequency distribution of the imaging scene after any frame perspective correction (rotation), and the following relationship exists:

[0063]

[0064] The spatial frequency distribution of the imaging scene center before interpolation is:

[0065]

[0066] Where c is the speed of light, f c is the center frequency of the radar, f τ is the distance frequency, is the elevation angle of the imaging center of the current frame of the SAR imaging radar, and φ is the instantaneous azimuth angle of the phase center of the SAR imaging radar antenna.

[0067] Step 6: Since the beam center cannot always be focused on the center of the set scene during radar movement, position offsets will occur in the range and azimuth directions.

[0068] The correlation of spotlight SAR imaging echo data is used to perform offset correction in the range-frequency domain and azimuth-time domain and in the azimuth-frequency domain and range-time domain.

[0069] When performing range offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to range Fourier transform to obtain the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame and the frame to be corrected; the range vector f in a certain azimuth direction in the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame is converted into τref The range vector f of each azimuth direction in the SAR signal data in the range, frequency, azimuth and time domain of the frame to be corrected τany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the distance offset τ , select the offset corresponding to the maximum value in the correlation coefficient vector τ As the distance offset correction.

[0070] Similarly, when performing azimuth offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to azimuth Fourier transform to obtain the SAR signal data of the reference frame and the frame to be corrected in the azimuth frequency domain, distance and time domain; the azimuth vector f of a certain distance direction in the SAR signal data of the reference frame in the azimuth frequency domain and distance and time domain is converted into tref The azimuth vector f of each range direction in the SAR signal data in the azimuth frequency domain and time domain of the frame to be corrected tany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the azimuth offset t , select the offset corresponding to the maximum value in the correlation coefficient vector t As the azimuth offset correction.

[0071] corr τ and corr t Its expression is:

[0072] corr t =ifft(f tref ·conj(f tany ))

[0073] corr τ =ifft(f τref ·conj(f τany ))

[0074] Among them, ifft means inverse Fourier transform, and conj means taking the conjugate.

[0075] Step 7: Circularly shift the interpolated imaging scene data by offset in the distance direction τ Unit lengths can be used to complete the distance offset correction; similarly, the interpolated imaging scene data is circularly shifted in the azimuth by offset t The azimuth deviation correction can be completed with just one unit length.

[0076] The method described in this embodiment can obtain the final corrected image through rotation transformation (two-dimensional interpolation) and translation transformation (data movement).

[0077] In the simulation example described in this embodiment, Figure 2 A SAR image of a reference frame is given, and the coordinate system of the image frame will be used as the reference coordinate system to perform SAR image registration at different viewing angles.

[0078] Figure 3 The 55th SAR image before correction is given. It can be seen that due to the different viewing angles and the influence of beam jitter, the frame image to be corrected needs to be rotated and translated compared to the reference frame image.

[0079] Figure 4 The 55th frame of the SAR image to be corrected after the rotation transformation is given. It can be seen that after the rotation transformation, the viewing angle of the 55th frame has been calibrated to the viewing angle of the reference frame.

[0080] Figure 5 The 55th frame of the SAR image to be corrected after rotation and translation transformation is given. So far, all steps of the SAR registration method proposed in this embodiment have been completed. By comparing the reference frame image, it can be seen that the image to be registered has been calibrated to the same viewing angle and the same coordinate system. After selecting the same image window for display, the effectiveness of the method described in the present invention can be verified.

[0081] In summary, judging from the processing results, the method provided by the present invention has practical value.

Claims

1. A fast registration method for video SAR based on data spotlighting, characterized in that: The following steps are involved: Step 1: Convert the SAR imaging radar position coordinates (lon, lat, alt) in the WGS-84 coordinate system to points (X, Y, Z) in the Earth-centered Earth-fixed rectangular coordinate system; Step 2: Select the SAR original echo signal of any frame as the reference frame, and take the center position coordinates of the SAR imaging radar corresponding to the reference frame in the geocentric earth-fixed rectangular coordinate system as the imaging center of the reference frame; take the imaging center of the reference frame as the coordinate origin, respectively convert the SAR imaging radar position coordinates of all frames in the geocentric earth-fixed rectangular coordinate system and the position coordinates of the set scene center in the geocentric earth-fixed rectangular coordinate system to the north-east coordinate system; Step 3: Generate a rotation matrix using the roll angle, pitch angle, and yaw angle corresponding to the imaging center of the reference frame, and transform the coordinates of the set scene center and the SAR imaging radar in the north-east coordinate system to the imaging coordinate system; Step 4: The angle θ between the line of sight direction at any position of the radar during its motion and the line of sight direction of the imaging center of the reference frame is: in, is the displacement vector from the imaging center of the reference frame to the center of the set scene, is the displacement vector from the imaging center of any frame of the SAR imaging radar to the set scene center during radar motion, and ‖‖ represents the modulus. Step 5: Correct the viewing angles of the SAR imaging radar in different azimuths to the azimuth viewing angles in the imaging coordinate system of the reference frame by two-dimensional interpolation. The SAR two-dimensional images of each frame are used as the imaging scene data V to be interpolated. Perform two-dimensional interpolation processing on the imaging scene data V to obtain the interpolated imaging scene data, so that the spatial frequency distribution of the imaging scene data is (X a , Y a ) to (X q , Y q ), (X a , Y a ) represent the spatial frequency distribution of the imaging scene before interpolation, (X q , Y q ) represent the spatial frequency distribution of the imaging scene after any frame perspective correction (rotation); Step 6: Use the correlation of the spotlight SAR imaging echo data to perform offset correction in the range-frequency domain, azimuth-time domain and azimuth-frequency domain, range-time domain respectively; When performing range offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to range Fourier transform to obtain the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame and the frame to be corrected; the range vector f in a certain azimuth direction in the SAR signal data in the range frequency domain, azimuth and time domain of the reference frame is converted into τref The range vector f of each azimuth direction in the SAR signal data in the range, frequency, azimuth and time domain of the frame to be corrected τany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the distance offset τ , select the offset corresponding to the maximum value in the correlation coefficient vector τ As the distance deviation correction; Similarly, when performing azimuth offset correction, the SAR original echo signals of the reference frame and the frame to be corrected are subjected to azimuth Fourier transform to obtain the SAR signal data of the reference frame and the frame to be corrected in the azimuth frequency domain, distance and time domain; the azimuth vector f of a certain distance direction in the SAR signal data of the reference frame in the azimuth frequency domain and distance and time domain is converted into tref The azimuth vector f of each range direction in the SAR signal data in the azimuth frequency domain and time domain of the frame to be corrected tany Conjugate multiplication, and then inverse Fourier transform, to obtain the correlation coefficient vector corr of the azimuth offset t , select the offset corresponding to the maximum value in the correlation coefficient vector t As the azimuth deviation correction; Step 7: Circularly shift the interpolated imaging scene data by offset in the distance direction τ unit length to complete the distance offset correction; similarly, the interpolated imaging scene data is circularly shifted by offfset in the azimuth direction t unit length to complete the azimuth offset correction.

2. The method for fast registration of data spotlight video SAR according to claim 1, characterized in that: In step 1, the conversion relationship between the WGS-84 coordinate system and the Earth-centered, Earth-fixed rectangular coordinate system is as follows: Where lon, lat and alt represent the longitude, latitude and altitude of the SAR imaging radar respectively; X, Y and Z represent the x-axis, y-axis and z-axis coordinates of the SAR imaging radar in the ECEF coordinate system respectively; N is the curvature radius of the reference ellipsoid, e is the eccentricity of the ellipsoid in the WGS-84 coordinate system, and a is the major radius of the reference ellipsoid.

3. The method for fast registration of data spotlight video SAR according to claim 2, characterized in that: In step 2, the center position coordinates of the SAR imaging radar corresponding to the reference frame in the earth-centered rectangular coordinate system are marked as The position coordinates of the SAR imaging radar in the Earth-centered Earth-fixed rectangular coordinate system are (x ECEF ,y ECEF , z ECEF ), the position coordinates of the scene center in the geocentric rectangular coordinate system are set as The position coordinates of the SAR imaging radar position in the north-east coordinate system and the position coordinates of the set scene center in the north-east coordinate system are expressed as follows: Among them, N x 、E y and D z represents the position coordinates of the SAR imaging radar position coordinates in the N-axis, E-axis and D-axis in the north-east coordinate system; N a 、E a and D a Indicates the position coordinates of the scene center on the N axis, E axis and D axis in the north-east coordinate system respectively; S is the coordinate transformation matrix from the Earth-centered Earth-fixed rectangular coordinate system to the North-East Earth coordinate system, expressed as: Among them, lon0, lat0 and alt0 represent the longitude, latitude and altitude of the imaging center of the reference frame respectively.

4. The method for fast registration of data spotlight video SAR according to claim 1, characterized in that: In step 3, the roll angle θ x , pitch angle θ y and the yaw angle θ z The corresponding rotation matrix T x 、T y and T z Respectively expressed as: The SAR imaging radar in the imaging coordinate system and the position coordinates (x, y, z) of the center of the scene are set and (x a ,y a , z a ) are:

5. The method for fast registration of data spotlight video SAR according to claim 1, characterized in that: In step 5, (X a , Y a ) and (X q , Y q ) have the following relationship: Where c is the speed of light, f c is the center frequency of the radar, f τ is the distance frequency, is the elevation angle of the imaging center of the current frame of the SAR imaging radar, and φ is the instantaneous azimuth angle of the phase center of the SAR imaging radar antenna.

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