Interferometric synthetic aperture radar image registration method based on phase diagram quality evaluation

Abstract

The invention discloses an interferometric synthetic aperture radar image registration method based on phase diagram quality evaluation, and the method comprises the following steps: S1, obtaining an interferometric synthetic aperture radar interferometric image pair, and selecting a plurality of registration control points which are uniformly distributed in the interferometric image pair; wherein the image pair comprises a main image and an auxiliary image; s2, respectively setting a matching window and a search window by taking each registration control point as a center; s3, performing quality evaluation on the interferometric phase diagram between the main image and the auxiliary image in the matching window to obtain position offsets, and performing quality control on each position offset and a corresponding registration control point; s4, fitting a function relationship between the position offset of each registration control point subjected to quality control and the image coordinate of the registration control point, and re-sampling the auxiliary image to realize accurate registration of the whole interference image pair; according to the method, the quality of the interferometric phase diagram is continuously evaluated in the registration process, and high-precision registration of the main image and the auxiliary image in the interferometric phase diagram is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of image registration, and in particular to an interferometric synthetic aperture radar image registration method based on phase image quality assessment. Background Art

[0002] Synthetic Aperture Radar Interferometry (InSAR) combines interferometry and synthetic aperture radar (SAR) techniques. Using at least two antennas, InSAR repeatedly images the same area of ​​the Earth's surface, generating two highly coherent images. It then extracts valuable terrain elevation, displacement, and other information from the interferometric phase. InSAR can be categorized as Cross-Track Interferometric Synthetic Aperture Radar (XTI-SAR) or Along-Track Interferometric Synthetic Aperture Radar (ATI-SAR). XTI-SAR effectively observes terrain elevation, while ATI-SAR effectively monitors ocean surface motion.

[0003] The primary and secondary images acquired by the two antennas of an InSAR system are often misaligned in both range and azimuth. This results in low interferometric phase quality, or even the inability to obtain a valid interferometric image. Therefore, image registration is essential before extracting the interferometric phase. Image registration is the first and most important step in interferometric processing. High-precision registration is a prerequisite for ensuring high coherence and, therefore, accurate elevation or velocity measurements. Common InSAR registration methods can be divided into two categories: those based on correlation functions and those based on interferogram features. The former, which primarily includes the correlation coefficient method (CCA) and the modified correlation coefficient method (MCCA), uses the complex correlation coefficient between the primary and secondary images as the registration metric. The latter, which primarily includes the average fluctuation function algorithm (AFFA) and the maximum spectrum algorithm (MSA), uses the signal-to-noise ratio of the interferometric phase spectrum as the registration metric for complex images.

[0004] The goal of InSAR registration is to obtain high-quality interferometric phase images. The interferometric phase is very sensitive to registration accuracy. The higher the registration accuracy, the better the interferometric phase image quality. Therefore, the quality of the interferometric phase image itself can represent the registration effect. The quality of the interferometric phase image can be assessed by both indirect and direct methods. The correlation coefficient can characterize the quality of the interferometric phase to a certain extent, but the correlation coefficient essentially represents the degree of correlation between the primary and auxiliary images, and its evaluation of the quality of the interferometric phase image is indirect. Currently, there are two main methods for directly evaluating the quality of interferometric phase images, namely the phase standard deviation (PSD) and the sum of phase differences (SPD). These two methods each evaluate the quality of the interferometric phase image by performing direct statistical calculations on the interferometric phase image.

[0005] When directly evaluating the quality of interferometric phase images, phase wrapping can significantly interfere with the evaluation results, necessitating the prior unwrapping of the interferometric phase. However, phase unwrapping is a subsequent step in interferometric registration, and the quality of interferometric phase images before and during registration is typically poor, making phase unwrapping difficult. Therefore, attempts to achieve high-precision registration of primary and secondary images by continuously evaluating the quality of interferometric phase images during the registration process are difficult to implement. Currently, there is no method for achieving primary and secondary image registration by continuously and directly evaluating the quality of interferometric phase images during the registration process.

[0006] This paper introduces a phase correction method when calculating the SPD of the interferometric phase image. This method avoids the interference caused by phase wrapping and enables the SPD to accurately assess the quality of the interferometric phase image during the registration process. Therefore, this paper uses the phase-corrected SPD as a measure of interferometric phase image quality as a registration evaluation criterion, achieving high-quality InSAR image registration. Summary of the Invention

[0007] The purpose of the present invention is to provide an interferometric synthetic aperture radar image registration method based on phase image quality evaluation, which continuously evaluates the quality of the interferometric phase image during the registration process to achieve high-precision registration of the main and auxiliary images in the interferometric phase image.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] An interferometric synthetic aperture radar image registration method based on phase image quality assessment comprises the following steps:

[0010] S1. Acquire an interferometric synthetic aperture radar interferometric image pair, and select a plurality of uniformly distributed registration control points in the interferometric image pair; wherein the interferometric image pair includes a primary image and an auxiliary image;

[0011] S2. Set the matching window and search window respectively with each registration control point as the center;

[0012] S3. Traverse all registration control points, perform quality assessment on the interference phase map between the primary image and the auxiliary image within the matching window of each registration control point, obtain the position offset of each registration control point, and perform quality control on each position offset and the corresponding registration control point;

[0013] S4. Fitting the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates, and resampling the auxiliary image to achieve accurate registration of the entire interference image pair.

[0014] Furthermore, in S3, the quality of the interference phase map between the primary image and the auxiliary image in the matching window of each registration control point is evaluated to obtain the position offset of each registration control point, specifically:

[0015] The matching window is moved within the search window of the auxiliary image. The interference phase of the main image and the auxiliary image within the matching window is calculated by conjugate complex multiplication operation. The sum of the local phase difference at a specific pixel point is calculated to evaluate the quality of the interference phase map between the main image and the auxiliary image within the matching window.

[0016] Among them, the interference phase of the main image and the auxiliary image in the matching window is expressed as:

[0017]

[0018] in, is the interference phase, c1 and c2 represent the complex data of the primary image and the auxiliary image respectively, i and j are the positions of the pixels in the azimuth and range directions respectively; * represents the complex conjugate operation, and arg represents the argument operation of the complex data;

[0019] The sum of the local phase differences between the main image and the auxiliary image at pixel (i, j) is S local , the expression is:

[0020]

[0021] Where l and k represent the coordinates of the nearest neighbor pixel of pixel (i, j) in the azimuth and distance directions with (i, j) as the center, respectively;

[0022] The sum of the overall phase difference S of the interference phase pattern between the main image and the auxiliary image in the entire matching window is obtained. whole , the expression is:

[0023]

[0024] Where m and n are the sizes of the interference pattern in azimuth and range, respectively.

[0025] Furthermore, in S3, the method for calculating the position offset is specifically as follows:

[0026] The matching window in the auxiliary image is moved in the azimuth and range directions within the search window to obtain a two-dimensional phase difference sum array. The minimum value of the phase difference sum array is found, and the difference between its position and the center position of the matching window in the main image is the coarse position offset of the registration control point.

[0027] A 5×5 window is selected with the minimum value of the phase difference sum array as the center, and two-dimensional cubic spline interpolation is performed within the window at intervals of 0.01 pixels. Then, the minimum phase difference sum value in the new two-dimensional array is found. The difference between its position and the position of the corresponding matching window center in the main image is the precise position offset of the registration control point.

[0028] Furthermore, in the process of evaluating the quality of the interference phase image between the main image and the auxiliary image in the matching window of each registration control point, the phase difference between the main image and the auxiliary image at the pixel point (i, j) is also included. Correction is performed, the expression is:

[0029]

[0030] Furthermore, in S2, the size of the matching window and the search window are determined by:

[0031] The size of the search window is capped at 1 / 4 of the length or width of the entire interferometric phase pattern;

[0032] The matching window is smaller than the search window and is not smaller than the matching window threshold.

[0033] Furthermore, the matching window threshold is determined by:

[0034] Select a certain registration control point, take matching windows of different sizes, calculate the relationship between the position offset of the registration control point and the matching window size, and take the minimum window size when the position offset change tends to be stable as the matching window threshold.

[0035] Furthermore, in S3, quality control is performed on each position offset and the corresponding registration control point, including:

[0036] Eliminate unqualified position offsets and corresponding registration control points, specifically:

[0037] Calculate the root mean square error of the position offset of all registration control points:

[0038]

[0039] Among them, l i is the position offset of the i-th registration control point, is the average value of the position offset of all registration control points;

[0040] Among them, the position offset that meets the following conditions is considered to be of unqualified quality:

[0041]

[0042] Furthermore, in S4, the expression for fitting the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates is:

[0043] Fit the cubic function relationship between the position offset of the control point and its image coordinates x and y:

[0044]

[0045] Among them, g and h are the position offsets of the control point on the x-axis and y-axis respectively, and a i and b i (i=1,…,9) are fitting parameters.

[0046] Furthermore, in S3, after quality control is performed on each position offset and the corresponding registration control point, the number of qualified registration control points is no less than 10.

[0047] Furthermore, in S4, the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates is fitted, and the auxiliary image is resampled to achieve accurate registration of the entire interference image pair, specifically:

[0048] The coordinates of each pixel in the auxiliary image are substituted into the cubic function of the position offset with respect to the image coordinates to calculate the position offset of each pixel; then the auxiliary image is resampled at the position offset using a cubic spline interpolation method.

[0049] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0050] (1) In existing synthetic aperture radar interferometry registration processing algorithms, there is no method for directly evaluating the quality of the interferometric phase pattern to achieve primary and secondary image registration. The present invention introduces a phase correction method to overcome the influence of phase wrapping. By continuously and directly evaluating the quality of the interferometric phase pattern, the primary and secondary images in synthetic aperture radar interferometry are registered. This fills the gap in the lack of registration methods based on direct evaluation of the interferometric phase pattern quality and provides an effective approach for subsequent image registration processing in synthetic aperture radar interferometry systems.

[0051] (2) The present invention also introduces a method that combines the matching window threshold with the position offset quality control, thereby improving the computational efficiency of the image registration algorithm of the synthetic aperture radar interferometer system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0053] The interferometric synthetic aperture radar image registration method based on phase image quality assessment of the present invention is further described below with reference to the accompanying drawings;

[0054] Figure 1 This is an overall flow chart of the registration method in Example 1 of the present invention.

[0055] Figure 2 : is a schematic diagram of the registration matching window and search window of the registration method in Example 1 of the present invention; wherein, Figure 2 (a) in the figure is a schematic diagram of the matching window in the main image. Figure 2 (b) is a schematic diagram of moving the matching window within the search window of the auxiliary image for registration;

[0056] Figure 3 is a graph showing the changing trend of position offset along with the matching window in Example 2 of the present invention;

[0057] Figure 4 is the interference phase image before registration in Example 2 of the present invention;

[0058] Figure 5 is the interference phase image after registration in Example 2 of the present invention;

[0059] Figure 6 is the interference phase image before registration in Example 3 of the present invention;

[0060] Figure 7 This is the interference phase image after registration in Example 3 of the present invention. DETAILED DESCRIPTION

[0061] The following embodiments of the present invention are described in further detail with reference to the accompanying drawings and examples. The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0062] In order to better understand the purpose, structure and function of the present invention, the present invention is further described in detail below with reference to the accompanying drawings.

[0063] Example 1

[0064] like Figure 1 As shown, the present invention provides an interferometric synthetic aperture radar image registration method based on phase image quality assessment. For the primary and secondary images in the complex interferometric synthetic aperture radar (InSAR) interferometric data, the registration control points, matching windows, and search windows are selected, and the matching window threshold is determined. Each control point is accurately registered by continuously and directly assessing the quality of the interferometric phase image, and its position offset is calculated. The quality of the control point position offset is assessed, and low-quality position offsets and corresponding control points are removed. The secondary image is interpolated and resampled to complete the registration of the primary and secondary synthetic aperture radar interferometric (InSAR) images. This method can be applied to both cross-track interferometric SAR (XTI-SAR) interferometric data and along-track interferometric SAR (ATI-SAR) interferometric data. The following provides an embodiment for each type of data.

[0065] Specifically, the process of the interferometric synthetic aperture radar image registration method based on phase image quality assessment is as follows: Figure 1 The specific implementation steps are as follows:

[0066] 1) Select the registration control points, matching window and search window, and determine the matching window threshold.

[0067] For the primary and secondary images in the InSAR interferometric complex data, multiple uniformly distributed registration control points are selected. After subsequent quality control, the total number of control points should be greater than 10; the primary and secondary images are both complex data; multiple uniformly distributed registration control points are selected in the image pair.

[0068] Centered on the registration control point, select the matching window and search window, such as Figure 2 As shown in the figure, an M*M matching window is selected in the primary and secondary images, and an N*N search window is selected in the secondary image centered at the same coordinate position. 1 / 4 of the length or 1 / 4 of the width of the entire image is used as the upper limit of the search window N. The matching window size M is smaller than the search window size N, but not smaller than the matching window threshold.

[0069] Determine the matching window threshold as the lower limit of the matching window selection. The method is as follows: randomly select a control point, take matching windows of different sizes from small to large, calculate the relationship between the position offset of the registration control point and the size of the matching window, and when the position offset changes tend to be stable, take the size of the smallest window as the matching window threshold.

[0070] 2) Accurately align individual control points by continuously and directly evaluating the quality of the interferometric phase image and calculating their position offset.

[0071] First, for a certain registration control point, the matching window is moved within the search window of the auxiliary image. The interferometric phase between the primary and auxiliary images within the matching window is calculated through conjugate complex multiplication operations, and the quality of the interferometric phase map between the primary and auxiliary images within the matching window is evaluated:

[0072]

[0073] in, is the interference phase, c1 and c2 represent the complex data of the primary and auxiliary images respectively, i and j are the positions of the pixels in the azimuth and range directions respectively; the asterisk “*” represents the complex conjugate operation, and “arg” represents the amplitude angle operation of the complex data.

[0074] The sum of the local phase differences at the pixel point (i, j), namely the local SPD, is calculated by the following formula:

[0075]

[0076] Where l and k represent the coordinates of the nearest neighbor pixel of pixel (i, j) in the azimuth and distance directions with (i, j) as the center, respectively;

[0077] In order to avoid the influence of phase winding on the SPD value, the phase difference in the above formula The following corrections are required:

[0078]

[0079] The overall SPD of the interferometric phase pattern within the entire matching window is calculated using the following formula:

[0080]

[0081] Where m and n are the sizes of the interference pattern in azimuth and range, respectively.

[0082] Then, the matching window in the auxiliary image is moved within the search window along the azimuth and range directions. The SPD of each matching window is calculated according to formulas (2)-(4), thereby obtaining a two-dimensional array of SPD values. The minimum value of the SPD array is found, and the difference between its position and the center position of the matching window in the main image is the coarse position offset of the registration control point.

[0083] Finally, a 5×5 window is selected with the minimum value of the SPD two-dimensional array as the center, and cubic spline interpolation is performed within the window at intervals of 0.01 pixels. Then, the minimum SPD value in the new two-dimensional array is found, and the difference between its position and the position of the corresponding matching window center in the main image is the precise position offset of the registration control point.

[0084] 3) Accurately align all registration control points, evaluate the quality of the control point position offsets, and remove low-quality position offsets and corresponding control points.

[0085] Traverse all control points and use the above single control point registration method to calculate the precise position offset of each control point, and eliminate the position offsets and corresponding control points with unqualified quality. The method for identifying whether the quality of the position offset is unqualified is as follows:

[0086] Calculate the root mean square error (RMSE) of the position offsets of all control points:

[0087]

[0088] Among them, l i is the position offset of the i-th control point, The average value of all position offsets of the points; the position offsets that meet the following conditions are of unqualified quality and need to be eliminated;

[0089]

[0090] 4) Interpolate and resample the auxiliary image to complete the registration.

[0091] Fit the position offset of the quality-controlled control points as a function of their image coordinates x and y:

[0092]

[0093] Among them, g and h are the position offsets of the control point on the x-axis and y-axis respectively, and a i and b i (i=1,…,9) are fitting parameters.

[0094] The image coordinates of each pixel in the auxiliary image are substituted into formula (7) to calculate the position offset of each pixel, and then the auxiliary image is resampled at the position offset using the cubic spline interpolation method to achieve accurate registration of the entire interference image pair.

[0095] Example 2

[0096] The present invention further provides an example of registering XTI-SAR experimental data using the interferometric synthetic aperture radar image registration method based on phase image quality assessment in Example 1:

[0097] This example uses the registration method in Example 1 to register XTI-SAR experimental data. The data comes from an airborne SIR-C / X-SAR system and was collected in 1994 near the Enta volcano in Italy. It is unregistered repeat-track XTI-SAR data. The method of the present invention is used to register this data. The curve of position offset versus matching window is shown in Figure 1. Figure 3 As shown in the figure, it can be found that the matching window threshold in this embodiment is about 65. The interference phase before registration is as follows Figure 4 As shown in the figure, the snowflake-like scattered noise is dominant and there are no regular stripes. The interference phase after registration is as follows Figure 5 As shown in the figure, periodic interference fringes appear, indicating that the interference registration is successfully achieved.

[0098] Example 3

[0099] The present invention further provides an example of registering ATI-SAR experimental data using the interferometric synthetic aperture radar image registration method based on phase image quality assessment in Example 1:

[0100] This embodiment uses the registration method in Example 1 to register ATI-SAR experimental data. The data comes from the TanDEM-X satellite formation and was collected in 2012 in the Orkney Islands in northern Scotland to measure ocean current velocity. The original data has been registered. To simulate unregistered data, the auxiliary image is artificially shifted by -5 pixels in azimuth and -3 pixels in range, respectively. The simulated data is registered using the method of the present invention. The interferometric phase before registration is as follows: Figure 6 As shown in the figure, the snowflake-like scattered noise is dominant and there are no regular stripes. The interference phase after registration is as follows Figure 5 As shown in the figure, periodic interference fringes appear, indicating that the interference registration is successfully achieved.

[0101] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An interferometric synthetic aperture radar image registration method based on phase image quality assessment, characterized in that: The following steps are involved: S1. Acquire an interferometric synthetic aperture radar interferometric image pair, and select a plurality of uniformly distributed registration control points in the interferometric image pair; wherein the interferometric image pair includes a primary image and an auxiliary image; S2. Set the matching window and search window respectively with each registration control point as the center; S3. Traverse all registration control points, perform quality assessment on the interference phase map between the primary image and the auxiliary image within the matching window of each registration control point, obtain the position offset of each registration control point, and perform quality control on each position offset and the corresponding registration control point; S4. Fitting the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates, and resampling the auxiliary image to achieve accurate registration of the entire interference image pair.

2. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S3, the quality of the interference phase map between the primary image and the auxiliary image in the matching window of each registration control point is evaluated to obtain the position offset of each registration control point, specifically: The matching window is moved within the search window of the auxiliary image. The interference phase of the main image and the auxiliary image within the matching window is calculated by conjugate complex multiplication operation. The sum of the local phase difference at a specific pixel point is calculated to evaluate the quality of the interference phase map between the main image and the auxiliary image within the matching window. Among them, the interference phase of the main image and the auxiliary image in the matching window is expressed as: in, is the interference phase, c1 and c2 represent the complex data of the primary image and the auxiliary image respectively, i and j are the positions of the pixels in the azimuth and range directions respectively; * represents the complex conjugate operation, and arg represents the argument operation of the complex data; The sum of the local phase differences between the main image and the auxiliary image at pixel (i, j) is S local , the expression is: Where l and k represent the coordinates of the nearest neighbor pixel of pixel (i, j) in the azimuth and distance directions with (i, j) as the center, respectively; The sum of the overall phase difference S of the interference phase pattern between the main image and the auxiliary image in the entire matching window is obtained. whole , the expression is: Where m and n are the sizes of the interference pattern in azimuth and range, respectively.

3. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 2, characterized in that: In S3, the method for calculating the position offset is specifically as follows: The matching window in the auxiliary image is moved in the azimuth and range directions within the search window to obtain a two-dimensional phase difference sum array. The minimum value of the phase difference sum array is found, and the difference between its position and the center position of the matching window in the main image is the coarse position offset of the registration control point. A 5×5 window is selected with the minimum value of the phase difference sum array as the center, and two-dimensional cubic spline interpolation is performed within the window at intervals of 0.01 pixels. Then, the minimum phase difference sum value in the new two-dimensional array is found. The difference between its position and the position of the corresponding matching window center in the main image is the precise position offset of the registration control point.

4. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 2, characterized in that: In the process of quality evaluation of the interference phase map between the main image and the auxiliary image in the matching window of each registration control point, it also includes: the phase difference between the main image and the auxiliary image at the pixel point (i, j) Correction is performed, the expression is:

5. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S2, the size of the matching window and the search window is determined as follows: The size of the search window is capped at 1 / 4 of the length or width of the entire interferometric phase pattern; The matching window is smaller than the search window and is not smaller than the matching window threshold.

6. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 5, characterized in that: The method for determining the matching window threshold is: Select a certain registration control point, take matching windows of different sizes, calculate the relationship between the position offset of the registration control point and the matching window size, and take the minimum window size when the position offset change tends to be stable as the matching window threshold.

7. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S3, quality control is performed on each position offset and the corresponding registration control point, including: Eliminate unqualified position offsets and corresponding registration control points, specifically: Calculate the root mean square error of the position offset of all registration control points: Among them, l i is the position offset of the i-th registration control point, is the average value of the position offset of all registration control points; Among them, the position offset that meets the following conditions is considered to be of unqualified quality:

8. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S4, the expression for fitting the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates is: Fit the cubic function relationship between the position offset of the control point and its image coordinates x and y: Among them, g and h are the position offsets of the control point on the x-axis and y-axis respectively, and a i and b i (i=1,…,9) are fitting parameters.

9. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S3, after quality control is performed on each position offset and the corresponding registration control point, the number of qualified registration control points is no less than 10.

10. The interferometric synthetic aperture radar image registration method based on phase image quality assessment according to claim 1, characterized in that: In S4, the functional relationship between the position offset of each quality-controlled registration control point and its image coordinates is fitted, and the auxiliary image is resampled to achieve accurate registration of the entire interference image pair, specifically: The coordinates of each pixel in the auxiliary image are substituted into the cubic function of the position offset with respect to the image coordinates to calculate the position offset of each pixel; then the auxiliary image is resampled at the position offset using a cubic spline interpolation method.

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