A method for eliminating false PS points in navigation satellite interferometric SAR

Abstract

The present invention relates to a method for eliminating false PS points for navigation satellite interferometric SAR (Interferometric SAR), and belongs to the technical field of bistatic synthetic aperture radar. First, based on the system configuration of the bistatic SAR, the fuzzy function of the direct wave is used to adaptively extract the area where the direct wave response is located; then, based on the principle of minimizing regional energy, the amplitude and phase of the direct wave response in the area are estimated; finally, the direct wave response is weighted using a set of estimated amplitudes and phases, and the direct wave interference in the echo image is canceled to eliminate the false PS points in the SAR echo image caused by the direct wave sidelobe interference. This invention solves the problem that false PS points in SAR images affect the image quality, improves the accuracy of PS point selection, provides a prerequisite for the subsequent acquisition of high-precision deformation variables, and plays an important role in the practical application of GNSS-InBSAR systems.

Description

Technical Field

[0001] The present invention relates to a method for eliminating false PS points for navigation satellite interferometry SAR, belongs to the technical field of bistatic synthetic aperture radar, and particularly relates to a method for eliminating false PS points for navigation satellite interferometry SAR, where PS is a permanent scatterer point. Background Art

[0002] The interferometric bistatic synthetic aperture radar based on the navigation satellite system (GNSS-InBSAR) is a bistatic SAR system that uses navigation satellites as external radiation sources and arranges receivers on the ground or near the ground to receive the echoes of the target scene. It has outstanding advantages such as short re-orbit time, wide coverage, continuous monitoring in time and space, and low cost.

[0003] However, because this system uses a horn antenna to receive echo signals, the direct wave signal also enters the backlobe of the echo antenna. Furthermore, this signal is not reflected by the scene and is stronger than the scene echo. Furthermore, the direct wave signal is a C / A code, and its pulse compression results in a triangular wave with no sidelobe roll-off. Consequently, false PS points, caused by interference from the direct wave's sidelobes, appear in the echo imaging results, affecting PS point selection and the accuracy of subsequent deformation inversion. Summary of the Invention

[0004] In view of this, the present invention provides a method for eliminating false PS points for navigation satellite interferometric SAR. The method mainly constructs a direct wave interference extraction model and estimates the amplitude and phase of the direct wave interference in the image by parameter estimation, thereby achieving the elimination of false PS points. The method first extracts the direct wave response area according to the system configuration, then estimates the amplitude and phase of the direct wave response, and finally uses the estimated parameters to perform weighted cancellation on the interference area in the direct wave and echo image. When the energy of the area is minimized, it indicates that the false PS points in the image have been eliminated.

[0005] The present invention is achieved through the following technical solutions.

[0006] A method for eliminating false PS points in navigation satellite interferometric SAR, the method comprising the following steps:

[0007] Step 1: Based on the system configuration of the bistatic SAR, adaptively extract the direct wave response area in the echo image and the direct wave image to obtain the direct wave response of the echo image and the direct wave response of the direct wave image;

[0008] Step 2: estimating the amplitude value of the direct wave response of the echo image extracted in step 1 based on the amplitude value of the direct wave response of the direct wave image extracted in step 1, and estimating the phase value of the direct wave response of the echo image extracted in step 1 based on the phase value of the direct wave response of the direct wave image extracted in step 1;

[0009] Step 3: Use the amplitude and phase values of the direct wave response of the echo image estimated in step 2 to cancel the false PS points in the echo image caused by direct wave interference, thereby completing the elimination of false PS points for navigation satellite interferometric SAR.

[0010] In the step 1, the direct wave response is extracted according to the position of the direct wave using the system configuration of the bistatic SAR;

[0011] In the step 2, for the extracted direct wave response area, the amplitude and phase of the direct wave interference are estimated using the criterion of regional energy minimization;

[0012] In the step three, the direct wave response is weighted using the estimated amplitude value and phase value, and the weighted direct wave response is canceled with the direct wave interference in the echo image to remove the false PS points in the image.

[0013] Beneficial effects

[0014] (1) The present invention solves the problem of false PS point interference caused by direct wave side lobes in the GNSS-InBSAR system, improves the accuracy of PS point selection, and provides a prerequisite for the subsequent acquisition of high-precision deformation variables, which plays an important role in the practical application of the GNSS-InBSAR system.

[0015] (2) The present invention discloses a method for removing false PS points for navigation satellite interferometric SAR. First, based on the system configuration of a bistatic SAR, the ambiguity function of the direct wave is used to adaptively extract the region where the direct wave response is located. Then, based on the principle of minimizing regional energy, the amplitude and phase of the direct wave response in the region are estimated. Finally, the direct wave response is weighted using the estimated set of amplitudes and phases, and the direct wave interference in the echo image is canceled to eliminate the false PS points in the SAR echo image caused by the direct wave sidelobe interference.

[0016] (3) The present invention solves the problem of false PS points in SAR images affecting image quality, improves the accuracy of PS point selection, provides a prerequisite for the subsequent acquisition of high-precision deformation variables, and plays an important role in the practical application of GNSS-InBSAR systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1This is a system block diagram of the navigation satellite interferometric SAR used in the present invention;

[0018] Figure 2 This is a comparison chart of the SAR image and the scene optical image.

[0019] Figure 3 Flowchart for false PS point elimination;

[0020] Figure 4 Extract images for the direct wave region;

[0021] Figure 5 It is the SAR image after the false PS points are eliminated;

[0022] Figure 6 Energy comparison diagram before and after the false PS points are eliminated. DETAILED DESCRIPTION

[0023] The present invention is described in further detail below.

[0024] Figure 1 The configuration diagram of the navigation satellite interferometric SAR system used in this specific embodiment is as follows: the navigation satellite interferometric SAR system adopts the northeast sky coordinate system, the transmitter is Beidou IGSO and MEO satellite, and its pitch angle It is defined as the angle with the horizontal plane, which is positive when it is upward. The azimuth angle θ is defined as the angle with the due east direction, which is positive when it is counterclockwise. The receiver is stationary and placed at the origin of the system. The receiver has two antennas, which receive echo signals and direct wave signals respectively.

[0025] In actual imaging, there are sometimes strip-shaped false PS points in the image, such as Figure 2 The rectangular area is shown. Further analysis revealed that the target's appearance is due to the direct wave signal entering the echo signal from the backlobe pattern of the echo antenna. Since the direct wave's far-end sidelobe intensity is roughly the same and higher than the target's scattered echo intensity, the interference of the direct wave's sidelobe is clearly visible in the image, forming a series of false PS points, which degrade image quality and seriously interfere with PS point selection, requiring removal.

[0026] Based on this, the present invention aims at the problem of false PS points existing in the imaging results of the GNSS-InBSAR system and proposes a method for eliminating false PS points in the image: adaptively extracting the direct wave interference area, and performing parameter estimation on its amplitude and phase, and performing cancellation processing based on the minimum regional energy criterion to achieve the elimination of false PS points in the image.

[0027] like Figure 3 As shown in FIG, a method for eliminating false PS points in an image comprises the following steps:

[0028] Step 1: Extract the area where the direct wave response is located based on the bistatic SAR configuration.

[0029] The GNSS-InBSAR system adopts a dual-base configuration. Its resolution is somewhat different from that of a single-base SAR system. The echo antenna is placed at the origin, so the direct wave response position is known, which is P(x D ,y D ). Therefore, the ambiguity function of the direct wave is expressed as:

[0030]

[0031] Where Q is the target vector near the target point P. λ represents the carrier wavelength and c is the speed of light. Φ TP , Φ RP are the unit vectors from the transmitter and receiver to the target point P. β is the bistatic angle, i.e. Φ TP and Φ RP The angle between them, Θ is the unit vector in the direction of the β angle bisector. ω E is the transmitter’s equivalent angular velocity, and Ξ is its equivalent direction of motion. p(~) and m(~) are the autocorrelation functions in the range and azimuth directions, respectively.

[0032] According to formula (1), the ambiguity function of the direct wave can be calculated, and then the direct wave response area S D Perform adaptive extraction:

[0033] S D =χ(P D (x D ,y D ),Q) (2)

[0034] Among them, P D (x D ,y D ) represents the position of the direct wave response in the direct wave image.

[0035] This embodiment takes the imaging results of the Beidou2_IGSO5 satellite on July 2, 2021 as an example. According to formula (2), the direct wave response area in the direct wave imaging is adaptively extracted. The result is as follows: Figure 4 shown.

[0036] Using the above method, the direct wave interference area S in the echo image is E To perform the extraction:

[0037] S E =χ(P E (x D ,y D ),Q) (3)

[0038] Among them, PE (x D ,y D ) represents the position of the direct wave response in the echo image.

[0039] Step 2: Estimate the amplitude and phase of the direct wave interference in the echo image.

[0040] In echo imaging, the direct wave interference and the ambiguity function of the direct wave have the following relationship:

[0041]

[0042] Among them, A D represents the amplitude of direct wave interference, represents its phase. Both parameters are unknown and are affected by various errors and cannot be accurately obtained through calculation. Therefore, the estimation of the two parameters can be obtained by minimizing the residual energy of the image, namely:

[0043]

[0044] in, and denote the magnitude and phase estimates, respectively.

[0045] Step 3: Cancel the false PS points.

[0046] Using the direct wave interference amplitude and phase estimates obtained in step 2, the direct wave response S D Weighted, and then combined with the direct wave interference S E To cancel, that is:

[0047]

[0048] Finally, for S CLEAN Perform radiation correction and obtain At this point, the bistatic SAR image is obtained after removing the false PS points.

[0049] In this embodiment, Figure 4 The amplitude and phase of the direct wave response are estimated, and after weighted cancellation, the image with false PS points removed is obtained as follows: Figure 5 After statistics, the number of false PS points in the image caused by direct wave sidelobe interference was reduced from 9 to 0, and the valid PS points in the scene were not affected.

[0050] Figure 6 is the energy change of the region before and after the cancellation of the false PS point, from Figure 6It can be seen intuitively that the influence of the direct wave sidelobe is significantly suppressed. The energy in this area is reduced by an average of about 10dB after the false PS points are eliminated, reaching the average level of scene noise. This shows that in the bistatic SAR echo image, the false PS points caused by the interference of the direct wave sidelobe are completely removed, and the image quality is effectively improved. This plays a vital role in the accurate selection of PS points and the acquisition of high-precision deformation variables.

[0051] In summary, the present invention proposes a method for eliminating false PS points in SAR images to remove the false PS points generated by the backlobe of the echo antenna receiving the direct wave signal, providing a guarantee for the accurate selection of PS points and laying the foundation for achieving high-precision deformation inversion.

[0052] The above specific embodiments merely illustrate the design principles of the present invention. The shapes and names of the components described herein may vary and are not limiting. Therefore, those skilled in the art may modify or substitute equivalents for the technical solutions described in the above embodiments. Such modifications and substitutions, without departing from the inventive spirit and technical solutions of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A method for eliminating false PS points in navigation satellite interferometric SAR, characterized in that The steps of the method include: Navigation satellite interferometric SAR uses the northeast sky coordinate system, and the transmitter is BeiDou IGSO and MEO satellites, with an elevation angle of The azimuth angle θ is defined as the angle with the horizontal plane, which is positive when it is upward. The azimuth angle θ is defined as the angle with the east direction, which is positive when it is counterclockwise. The receiver is stationary and placed at the origin. The receiver has two antennas, which receive the echo signal and the direct wave signal respectively. Step 1: Extract the area where the direct wave response is located based on the bistatic SAR configuration; The ambiguity function of the direct wave is expressed as: Where Q is the target vector near the target point P, λ represents the carrier wavelength, c is the speed of light, Φ TP , Φ RP are the unit vectors from the transmitter and receiver to the target point P, respectively, and β is the bistatic angle, i.e. Φ TP and Φ RP The angle between them, Θ is the unit vector in the direction of the β angle bisector, ω E is the transmitter’s equivalent angular velocity, Ξ is its equivalent motion direction, p(~) and m(~) are the autocorrelation functions in the range and azimuth directions, respectively; Direct wave response area S D Perform adaptive extraction: S D =χ(P D (x D ,y D ),Q) Among them, P D (x D ,y D ) represents the position of the direct wave response in the direct wave image; The direct wave interference area S in the echo image E To perform the extraction: S E =χ(P E (x D ,y D ),Q) Among them, P E (x D ,y D ) represents the position of the direct wave response in the echo image; Step 2: Estimate the amplitude and phase of the direct wave interference in the echo image; In echo imaging, the direct wave interference and the ambiguity function of the direct wave have the following relationship: Among them, A D represents the amplitude of direct wave interference, Represents its phase, and by minimizing the residual energy of the image, we get the estimates of the two parameters, namely: in, and denote the magnitude and phase estimates respectively; Step 3: cancel out the false PS points; Using the direct wave interference amplitude and phase estimates obtained in step 2, the direct wave response S D Weighted, and then combined with the direct wave interference S E To cancel, that is: Finally, for S CLEAN Perform radiation correction and obtain At this point, the bistatic SAR image is obtained after removing the false PS points.

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