A two-dimensional correction method for SAR image aiming at BP imaging

Abstract

The SAR image two-dimensional correction method for BP imaging of the application obtains the two-dimensional gain of the antenna directional diagram by calculating the distance and the azimuth angle of each pixel point in the distance and azimuth direction diagram at the corresponding pulse transmission moment. Furthermore, the antenna gain of the pixel in the SAR image after the time accumulation of the synthetic aperture is calculated, so that the pixel gain value under the modulation of the distance-azimuth coupling gain is obtained, and the gain correction of the entire image is completed through the image pixel traversal, thereby solving the problem that the distance-azimuth antenna directional diagram influence cannot be separated and effectively corrected under the complex flexible observation geometry, and the method is suitable for the radiation correction processing of the SAR image.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of signal processing, and particularly relates to a SAR image two-dimensional correction method for BP imaging. BACKGROUND

[0002] In order to meet the flexible and complex beam coverage observation requirements of a high-resolution spaceborne SAR load, the SAR load needs to realize earth imaging under a super-long synthetic aperture time and a beam non-along-track movement. The super-long synthetic aperture time and synthetic aperture length result in a large difference in Doppler frequency modulation history from traditional low-orbit observation. Therefore, the SAR imaging processing cannot use the traditional frequency domain imaging processing algorithm. In order to realize the imaging processing of the SAR under the high-resolution flexible observation task, a back-projection (BP) algorithm is used in the ground system to realize the signal coherence synthesis under the long synthetic aperture time.

[0003] Due to the special geometric relationship, the distance and azimuth of the scene are not consistent with the distance and azimuth pattern direction of the antenna, and the relationship changes with the change of the observation task geometry. At this time, the image is jointly modulated by the distance and the antenna pattern. Therefore, the traditional distance and azimuth antenna pattern independent correction method designed for the case that the scene distance and azimuth are consistent with the antenna pattern direction cannot be applied to the radiation correction processing of the SAR image. If a new method is not designed and corrected for the radiation characteristics under the complex observation geometry, the residual error of the image radiation correction will be increased, which seriously affects the inversion of the target backscattering coefficient and the target interpretation. SUMMARY

[0004] The purpose of the present application is to provide a SAR image two-dimensional correction method for BP imaging, so as to solve the problem that the method in the prior art is not suitable for correcting the SAR image.

[0005] In order to solve the above technical problems, the present application adopts the following technical solutions:

[0006] A SAR image two-dimensional correction method for BP imaging, comprising the following steps:

[0007] Step 1, determining the radar pulse transmission time, selecting a pixel point in the SAR image of BP imaging;

[0008] Step 2, calculating the distance angle of the selected pixel point in the target antenna distance pattern at the radar pulse transmission time;

[0009] Step 3, calculating the azimuth angle of the selected pixel point in the target antenna azimuth pattern at the radar pulse transmission time;

[0010] Step 4, according to the data obtained in steps 2 and 3, the two-dimensional gain value of the selected pixel point in the target antenna direction pattern at the radar pulse transmission time is calculated;

[0011] Step 5, according to the data obtained in step 4, the two-dimensional gain value of the selected pixel point in the target antenna direction pattern in the SAR image is calculated;

[0012] Step 6, the data obtained in step 5 is corrected;

[0013] Step 7, in the BP imaging SAR image, an uncorrected pixel point is selected, and steps 2-6 are repeated until all pixel points are traversed, and the two-dimensional correction of the BP imaging SAR image is completed.

[0014] The application also has the following characteristics:

[0015] Further, in step 1, after determining the radar pulse transmission time, the time is recorded as t.

[0016] Further, step 2 includes the following sub-steps:

[0017] Step 21, determine the three-dimensional position S and the corresponding antenna attitude A of the radar in the BP imaging SAR image at time t;

[0018] Wherein, the coordinates of the three-dimensional position S are represented as S(Sx, Sy, Sz);

[0019] The antenna attitude A is represented as A(y, p, r), y represents the yaw angle of the antenna attitude, p represents the pitch value of the antenna attitude, and r represents the roll value of the antenna attitude;

[0020] Step 22, determine the position P of the selected pixel point in the antenna range direction pattern;

[0021] Wherein, the coordinates of the position P of the selected pixel point are represented as P(l, b, h), l represents the longitude corresponding to the selected pixel point, b represents the latitude corresponding to the selected pixel point, and h represents the height corresponding to the selected pixel point;

[0022] Step 23, the distance angle value of the selected pixel point in the antenna range direction pattern at time t is calculated using the following formula:

[0023]

[0024] Wherein, The distance angle of the selected pixel point in the target antenna range direction pattern at time t is represented as

[0025] AntRg represents the distance angle calculation function.

[0026] Further, in step 3, the azimuth angle value of the selected pixel point in the target antenna azimuth pattern at the radar pulse emission moment is calculated using the following formula:

[0027] β p (t) = AntAz(S, P, A)

[0028] wherein β p (t) represents the azimuth angle of the selected pixel point in the target antenna azimuth pattern at time t;

[0029] AntAz represents the azimuth angle calculation function.

[0030] Further, in step 4, the target antenna two-dimensional gain value g ant2 (t) of the selected pixel point at time t is calculated using the following formula:

[0031]

[0032] wherein g r represents the target antenna range pattern one-way gain value;

[0033] g a represents the target antenna azimuth pattern one-way gain value.

[0034] Further, in step 5, the two-dimensional pattern gain value of the target antenna of the selected pixel point in the SAR image is calculated using the following formula:

[0035]

[0036] wherein t1 represents the starting pulse moment of the selected pixel point within the synthetic aperture time;

[0037] t2 represents the ending pulse moment of the selected pixel point within the synthetic aperture time.

[0038] Further, in step 6, the selected pixel point is corrected in the two-dimensional direction using the following formula:

[0039]

[0040] wherein I p_c (x rg ,y az ) represents the pixel value of the selected pixel point after the antenna two-dimensional pattern gain correction;

[0041] I p (x rg ,y az ) represents the pixel value of the selected pixel point;

[0042] xrg represents the distance position of the selected pixel point in the SAR image this time;

[0043] y az represents the azimuth position of the pixel point in the SAR image;

[0044] g r (0) represents the one-way gain of the range direction pattern when the target antenna distance angle is 0;

[0045] g a (0) represents the one-way gain of the azimuth direction pattern when the antenna azimuth angle is 0.

[0046] Compared with the prior art, the present application has the following technical effects:

[0047] (I) The SAR image two-dimensional correction method for BP imaging of the present application obtains the antenna direction gain by calculating the distance and azimuth angle of each pixel point in the corresponding pulse transmission moment under the distance and azimuth direction pattern, and further calculates the antenna gain of the pixel in the SAR image after the synthesis aperture time accumulation, so as to obtain the pixel gain value under the modulation of the range-azimuth coupling gain, and complete the gain correction of the entire image through image pixel traversal, thereby solving the problem that the range-azimuth antenna direction pattern cannot be separated and effectively corrected under complex flexible observation geometry, and the present application is suitable for correction processing of the SAR image. Figure Two

[0048] (II) The SAR image two-dimensional correction method for BP imaging of the present application gives a processing step flow for image correction. The flow is a general signal processing flow, which can be easily realized in various processors such as DSP, FPGA and ARM, and has good universality. The method of the present application is convenient to calculate, can effectively realize the SAR image two-dimensional correction of the BP imaging image under complex observation geometry, avoids complex transformation processing of the image, and can be realized only by adjusting the intensity value of the image, and the processing scheme is easy to implement, has very high parallelism and application efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure One is an antenna distance angle schematic diagram in an example of the present application;

[0050] Figure Two is an antenna azimuth angle schematic diagram in an example of the present application;

[0051] Figure Three is a schematic diagram of each transmission moment in the synthesis aperture time in an example of the present application;

[0052] Figure Four is a two-dimensional traversal schematic diagram of all image pixels in an example of the present application.​ DETAILED DESCRIPTION

[0053] It should be noted that all methods in the present application, if not specifically stated, all use the methods known in the prior art.

[0054] The following gives specific embodiments of the present application, it should be noted that the present application is not limited to the following specific embodiments, any equivalent transformation made on the basis of the technical solutions of the present application falls within the protection scope of the present application.

[0055] The embodiments of the present application mainly include the determination of the pixel point antenna distance angle at the radar pulse transmission time, the determination of the pixel point antenna azimuth angle, the calculation of the pixel point antenna two-dimensional gain value, the calculation of the SAR image pixel point antenna two-dimensional directional pattern gain value, the pixel point antenna two-dimensional directional pattern gain correction, and the two-dimensional traversal of each pixel in the image. The specific steps are as follows:

[0056] A SAR image two-dimensional correction method for BP imaging, comprising the following steps:

[0057] Step 1, determining the radar pulse transmission time, selecting a pixel point in the SAR image of BP imaging;

[0058] Step 2, calculating the distance angle value of the selected pixel point in the radar pulse transmission time in the antenna distance direction pattern;

[0059] Step 3, calculating the azimuth angle value of the selected pixel point in the radar pulse transmission time in the antenna azimuth direction pattern;

[0060] Step 4, according to the data obtained in steps 2 and 3, calculating the two-dimensional gain value of the target antenna of the selected pixel point at the radar pulse transmission time;

[0061] Step 5, according to the data obtained in step 4, calculating the two-dimensional directional pattern gain value of the target antenna of the selected pixel point in the SAR image;

[0062] Step 6, correcting the data obtained in step 5;

[0063] Step 7, selecting an uncorrected pixel point in the SAR image of BP imaging, repeating steps 2-6 until all pixel points are traversed, completing the two-dimensional correction of the SAR image of BP imaging.

[0064] Further, in step 1, after determining the radar pulse transmission time, the time is recorded as t.

[0065] For the SAR image data after BP imaging under irregular observation geometry, the directions of the range direction pattern and the azimuth direction pattern are no longer consistent with the two-dimensional distribution directions of the image. Especially when the observation beam is in the case of curved irregular ground scanning, the antenna illumination history of the target point presents irregular and complex curve patterns in the antenna beam.

[0066] Therefore, for each pulse emission moment, the state of the target in the antenna pattern needs to be calculated independently. In order to obtain the antenna pattern gain history of the target, the antenna observation angle of the target point under each pulse should be determined first.

[0067] The target antenna range angle diagram under complex observation geometry is shown in Figure One From the diagram, it can be seen that the antenna pattern direction is inconsistent with the image direction. In the target antenna range angle calculation method, the radar position, antenna attitude and target position need to be calculated jointly.

[0068] Specifically, step 2 includes the following sub-steps:

[0069] Step 21, determining the three-dimensional position S of the radar and the corresponding antenna attitude A(y, p, r) in the SAR image of the BP imaging at time t; wherein the coordinates of the three-dimensional position S are represented as S(Sx, Sy, Sz); y represents the antenna attitude yaw angle, p represents the antenna attitude pitch value, and r represents the antenna attitude roll value;

[0070] Step 22, determining the position P of the selected pixel point in the antenna range direction pattern;

[0071] Wherein, the coordinates of the position P are represented as P(l, b, h), l represents the longitude corresponding to the selected pixel point, b represents the latitude corresponding to the selected pixel point, and h represents the height corresponding to the selected pixel point;

[0072] Step 23, using the following formula to calculate the range angle value of the selected pixel point in the antenna range direction pattern at time t:

[0073]

[0074] Wherein, represents the range angle of the selected pixel point in the target antenna range direction pattern at time t;

[0075] AntRg represents the range angle calculation function.

[0076] For the pulse emission moment t, the azimuth angle value β of the selected pixel point in the antenna azimuth direction pattern pSimilarly, the position P of the selected pixel point, the antenna attitude A and the radar position S are used to calculate and determine;

[0077] Specifically, the target antenna azimuth angle diagram under complex observation geometry is as shown in Figure Three .

[0078] In step 3, the value of the azimuth angle of the selected pixel point in the radar pulse transmission moment in the antenna azimuth direction pattern is calculated using the following formula:

[0079] β p (t) = AntAz(S, P, A)

[0080] Wherein, β p (t) represents the azimuth angle of the selected pixel point in the target antenna azimuth direction pattern at time t;

[0081] AntAz represents the azimuth angle calculation function.

[0082] Further, in step 4, the target antenna two-way two-dimensional gain value g ant2 (t) corresponding to the selected pixel point at time t is calculated using the following formula:

[0083]

[0084] Wherein, g r represents the target antenna range direction pattern one-way gain value;

[0085] g a represents the target antenna azimuth direction pattern one-way gain value.

[0086] According to the SAR imaging principle, each pixel point in the image is obtained by coherent synthesis of each observation pulse in the azimuth synthesis aperture time, as shown in Figure Three . Therefore, according to the coherent synthesis characteristics, in step 5, the two-dimensional pattern gain value of the target antenna of the selected pixel point in the SAR image is calculated using the following formula in the synthesis aperture time:

[0087]

[0088] Wherein, t1 represents the starting pulse moment of the selected pixel point in the synthesis aperture time;

[0089] t2 represents the end pulse moment of the selected pixel point in the synthesis aperture time.

[0090] Further, in step 6, the selected pixel point is corrected in two-dimensional direction using the following formula:

[0091]

[0092] Among them, I p_c (x rg ,y az The selected pixels represent the pixel values ​​after the antenna's two-dimensional pattern gain has been corrected.

[0093] I p (x rg ,y az The pixel value of the selected pixel in this test;

[0094] x rg This indicates the distance and location of the selected pixel in the SAR image;

[0095] y az This indicates the azimuth position of the selected pixel in the SAR image;

[0096] g r (0) represents the single-pass gain of the range pattern when the target antenna range angle is 0;

[0097] g a (0) represents the single-pass gain of the azimuth pattern when the antenna azimuth angle is 0.

[0098] For a two-dimensional image, perform a pixel-by-pixel traversal in the distance and orientation dimensions. Assume the number of distance points in the image is N, and the number of orientation points is M. For example... Figure Four As shown, for all pixels in the image at positions 1 to N in distance and positions 1 to M in azimuth, the antenna two-dimensional pattern gain correction is traversed to complete the correction process for the entire image.

[0099] This embodiment addresses the characteristic of image being jointly modulated by the range and azimuth patterns when the scene's range and azimuth patterns do not align with the antenna's range and azimuth pattern under specific observation geometry. For the target image obtained through BP imaging, the range and azimuth angles of each pixel under each pulse transmission time are first calculated. Then, the antenna direction at the pulse transmission time is calculated based on the range and azimuth angles. Figure Two Two-dimensional gain correction is then performed on the pixel in the SAR image after time accumulation of the synthetic aperture. This is followed by antenna gain correction for the pixel value, and a two-dimensional pixel traversal is performed on the image to complete the gain correction for the entire image. This solves the problem of the inability to separate and effectively correct the range-azimuth antenna pattern influence under complex and flexible observation geometry. This antenna pattern correction method can simply and directly implement antenna gain correction for BP images under various observation modes, without involving complex image transformation processing. It only requires adjusting the intensity values ​​of the image, making the processing scheme easy to implement, the algorithm robust, and highly practical.

Claims

1. A method for 2D correction of SAR images for BP imaging, characterized in that, The method comprises the following steps: Step 1, determining the radar pulse emission time, an optional pixel point in the BP imaging SAR image; Step 2, calculating the range angle of the selected pixel point in the target antenna range direction pattern at the radar pulse emission time; Step 3, calculating the azimuth angle of the selected pixel point in the target antenna azimuth direction pattern at the radar pulse emission time; Step 4, according to the data obtained in steps 2 and 3, calculating the two-dimensional gain value of the selected pixel point corresponding to the antenna at the radar pulse emission time; Step 5, according to the data obtained in step 4, calculating the two-dimensional pattern gain value of the target antenna of the selected pixel point in the SAR image; Step 6, correcting the data obtained in step 5; In step 6, the selected pixel point is corrected in two dimensions using the following formula: wherein I p_c (x rg ,y az ) represents the pixel value of the selected pixel point in this time, which represents the pixel value of the antenna two-dimensional directional pattern gain after correction. I p (x rg ,y az ) represents the pixel value of the selected pixel point this time. x rg represents the distance position of the selected pixel point in the SAR image this time; y az represents the azimuth position of the pixel point in the SAR image; g r (0) represents the one-way gain of the range direction pattern at the target antenna distance angle of 0; g a (0) denotes the free-space gain of the azimuth pattern at an antenna azimuth angle of 0; Step 7, selecting an uncorrected pixel point in the BP imaging SAR image, repeating steps 2-6 until all pixel points are traversed, completing the two-dimensional correction of the BP imaging SAR image.

2. The SAR image two-dimensional correction method for BP imaging according to claim 1, wherein, In step 1, after determining the radar pulse emission time, the time is recorded as t.

3. The SAR image two-dimensional correction method for BP imaging according to claim 2, wherein, Step 2 comprises the following steps: Step 21, determining the three-dimensional position S and the corresponding antenna attitude A of the radar in the BP imaging SAR image at time t; Wherein, the coordinates of the three-dimensional position S are represented as S(Sz, Sy, Sz); The antenna attitude A is represented as A(y, p, r), y represents the yaw angle of the antenna attitude, p represents the pitch value of the antenna attitude, and r represents the roll value of the antenna attitude; Step 22, determining the position P of the selected pixel point in the antenna range direction pattern; Wherein, the coordinates of the position P of the selected pixel point are represented as P(l, b, h), l represents the longitude corresponding to the selected pixel point, b represents the latitude corresponding to the selected pixel point, and h represents the height corresponding to the selected pixel point; Step 23, using the following formula to calculate the value of the range angle of the selected pixel point in the antenna range direction pattern at time t: wherein, denotes the distance angle of the selected pixel point at time t in the target antenna distance direction pattern. AntRg represents the range angle calculation function.

4. The SAR image two-dimensional correction method for BP imaging according to claim 3, wherein, In step 3, the value of the azimuth angle of the selected pixel point in the target antenna azimuth direction pattern at the radar pulse emission time is calculated using the following formula: β p (t) = AntAz(S, P, A) wherein β p (t) denotes the azimuth angle of the selected pixel point at the target antenna azimuth pattern at time t. AntAz represents the azimuth angle calculation function.

5. The SAR image two-dimensional correction method for BP imaging according to claim 4, wherein, In step 4, the target antenna two-way two-dimensional gain value g corresponding to the selected pixel point at time t is calculated using the following formula ant2 (t): where g r represents the target antenna distance-to-direction pattern one-way gain value; g a represents the target antenna azimuth pattern one-way gain value.

6. The SAR image two-dimensional correction method for BP imaging according to claim 5, wherein, In step 5, the two-dimensional pattern gain value of the target antenna of the selected pixel point in the SAR image is calculated using the following formula: Wherein, t1 represents the starting pulse time of the selected pixel point within the synthetic aperture time; t2 represents the end pulse time of the selected pixel point within the synthetic aperture time.

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