SAR image two-dimensional correction method for BP imaging

Abstract

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

Description

Technical Field

[0001] The invention belongs to the field of signal processing, and in particular relates to a SAR image two-dimensional correction method for BP imaging. Background Art

[0002] In order to meet the flexible and complex beam coverage observation requirements of high-resolution spaceborne SAR payloads, SAR payloads need to achieve ultra-long synthetic aperture time and ground imaging under non-along-track beam movement. The ultra-long synthetic aperture time and synthetic aperture length cause its Doppler frequency modulation process to be significantly different from traditional low-orbit observations. Therefore, SAR imaging processing cannot use traditional frequency domain imaging processing algorithms. In order to realize SAR imaging processing under high-resolution flexible observation tasks, the back-projection (BP) algorithm is used in the ground system to achieve signal coherent synthesis under 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 antenna pattern directions of the antenna. Their relationship changes with the change of the observation task geometry. At this time, the image is jointly modulated by the distance and antenna pattern. Therefore, the traditional independent correction method for the distance and azimuth antenna pattern designed for the case where the distance and azimuth of the scene are consistent with the antenna pattern direction cannot be applied to the radiation correction processing of SAR images. If new methods are not designed and corrected for the radiation characteristics under complex observation geometry, the residual error of the radiation correction of the image will increase, which will seriously affect the inversion of the target backscatter coefficient and the target interpretation. Summary of the invention

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

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

[0006] A two-dimensional correction method for SAR images for BP imaging comprises the following steps:

[0007] Step 1, determine the radar pulse emission time, and select any pixel point in the SAR image of BP imaging;

[0008] Step 2, at the moment of radar pulse transmission, calculate the range angle of the pixel point selected this time under the target antenna range direction pattern;

[0009] Step 3, at the moment of radar pulse transmission, calculate the azimuth angle of the pixel point selected this time under the azimuth direction pattern of the target antenna;

[0010] Step 4, based on the data obtained in step 2 and step 3, at the time of radar pulse transmission, calculate the antenna two-way two-dimensional gain value corresponding to the pixel point selected this time;

[0011] Step 5, calculating the two-dimensional radiation pattern gain value of the target antenna at the pixel point selected this time in the SAR image according to the data obtained in step 4;

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

[0013] Step 7, randomly select a pixel point that has not been corrected in the BP imaging SAR image, and repeat steps 2-6 until all the pixel points are traversed, thereby completing the two-dimensional correction of the BP imaging SAR image.

[0014] The present invention also has the following features:

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

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

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

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

[0019] The antenna attitude A is expressed as A(y,p,r), where 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, determining the position P of the pixel point selected this time in the antenna range direction pattern;

[0021] The coordinates of the position P of the pixel point selected this time are expressed as P(l,b,h), where l represents the longitude corresponding to the pixel point selected this time, b represents the latitude corresponding to the pixel point selected this time, and h represents the height corresponding to the pixel point selected this time;

[0022] Step 23, using the following formula, calculate the value of the range angle of the selected pixel point under the antenna range pattern at time t:

[0023]

[0024] in, Indicates the distance angle of the pixel point selected this time under the target antenna range direction diagram at time t;

[0025] AntRg represents the distance angle calculation function.

[0026] Furthermore, in step 3, the azimuth angle of the pixel point selected this time under the azimuth direction pattern of the target antenna at the time of radar pulse transmission is calculated using the following formula:

[0027] β p (t) = AntAz(S,P,A)

[0028] Among them, β p (t) represents the azimuth angle of the selected pixel point under the azimuth direction pattern of the target antenna at time t;

[0029] AntAz represents the azimuth angle calculation function.

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

[0031]

[0032] Among them, g r Indicates the one-way gain value of the target antenna range pattern;

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

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

[0035]

[0036] Wherein, t1 represents the starting pulse time of the pixel point selected this time within the synthetic aperture time;

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

[0038] Furthermore, in step 6, the selected pixel point is corrected in two dimensions using the following formula:

[0039]

[0040] Among them, I p_c (x rg ,y az ) indicates that the pixel point selected this time represents the pixel value after gain correction of the antenna two-dimensional radiation pattern;

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

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

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

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

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

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

[0047] (I) The two-dimensional correction method of SAR image for BP imaging of the present invention obtains the antenna direction by calculating the distance and azimuth angle of each pixel point at the corresponding pulse emission time and the azimuth direction diagram. Figure 2 dimensional gain. Then, the antenna gain of the pixel in the SAR image after synthetic aperture time integration is calculated, so as to obtain the pixel gain value under range-azimuth coupled gain modulation, and the gain correction of the entire image is completed by traversing the image pixels, thus solving the problem that the range-azimuth antenna pattern influence cannot be separated and effectively corrected under complex and flexible observation geometry, which is suitable for the correction processing of SAR images.

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

[0049] Figure 1 is a schematic diagram of antenna distance angle in an embodiment of the present invention;

[0050] Figure 2 is a schematic diagram of antenna azimuth in an embodiment of the present invention;

[0051] Figure 3 is a schematic diagram of various emission moments within the synthetic aperture time in an example of the present invention;

[0052] Figure 4 The figure is a schematic diagram of two-dimensional traversal of pixels of the entire image in an example of the present invention. DETAILED DESCRIPTION

[0053] It should be noted that, unless otherwise specified, all methods in the present invention adopt methods known in the prior art.

[0054] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0055] The embodiment of the present invention mainly includes determining the distance angle of the pixel antenna at the moment of radar pulse transmission, determining the azimuth angle of the pixel antenna, calculating the two-dimensional gain value of the pixel antenna, calculating the two-dimensional directional pattern gain value of the pixel antenna of the SAR image, correcting the two-dimensional directional pattern gain of the pixel antenna, and two-dimensional traversal of each pixel in the image. The specific steps are as follows:

[0056] A two-dimensional correction method for SAR images for BP imaging comprises the following steps:

[0057] Step 1, determine the radar pulse emission time, and select any pixel point in the SAR image of BP imaging;

[0058] Step 2, calculating the value of the range angle of the pixel point selected this time under the antenna range direction pattern at the time of radar pulse transmission;

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

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

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

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

[0063] Step 7, randomly select a pixel point that has not been corrected in the BP imaging SAR image, and repeat steps 2-6 until all the pixel points are traversed, thereby completing the two-dimensional correction of the BP imaging SAR image.

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

[0065] For SAR image data after BP imaging under irregular observation geometry, unlike the observation image in the conventional positive side view mode, the direction of the range and azimuth patterns is no longer consistent with the two-dimensional distribution direction of the image. In particular, when the observation beam is in a curved and irregular scanning of the ground, the antenna irradiation history of the target point presents an irregular and complex curve shape in the antenna beam.

[0066] Therefore, for each pulse transmission moment, it is necessary to independently calculate the state of the target in the antenna pattern. In order to obtain the antenna pattern gain history received by the target, the antenna observation angle corresponding to the target point under each pulse should be determined first.

[0067] The schematic diagram of target antenna range angle under complex observation geometry is as follows: Figure 1 As shown in the figure, the direction of the antenna pattern is inconsistent with the image direction. The target antenna range angle calculation method needs to combine the radar position, antenna attitude and target position for calculation.

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

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

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

[0071] The coordinates of the position P are expressed as P(l,b,h), where l represents the longitude corresponding to the pixel point selected this time, b represents the latitude corresponding to the pixel point selected this time, and h represents the height corresponding to the pixel point selected this time;

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

[0073]

[0074] in, Indicates the distance angle of the pixel point selected this time under the target antenna range direction diagram at time t;

[0075] AntRg represents the distance angle calculation function.

[0076] For the pulse transmission time t, the value of the azimuth angle of the selected pixel point under the antenna azimuth pattern β pThe position P of the pixel point selected this time, the antenna posture A and the radar position S are also used to calculate and determine;

[0077] Specifically, the target antenna azimuth diagram under complex observation geometry is as follows: Figure 3 shown.

[0078] In step 3, the azimuth angle of the selected pixel under the antenna azimuth pattern at the time of radar pulse transmission is calculated using the following formula:

[0079] β p (t) = AntAz(S,P,A)

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

[0081] AntAz represents the azimuth angle calculation function.

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

[0083]

[0084] Among them, g r Indicates the one-way gain value of the target antenna range pattern;

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

[0086] According to the SAR imaging principle, each pixel in the image is obtained by coherent synthesis of each observation pulse within the azimuth synthetic aperture time, such as Figure 3 Therefore, according to the coherent synthesis characteristics, within the synthetic aperture time, in step 5, the two-dimensional radiation pattern gain value of the target antenna at the pixel point selected in the SAR image is calculated using the following formula:

[0087]

[0088] Wherein, t1 represents the starting pulse time of the pixel point selected this time within the synthetic aperture time;

[0089] t2 represents the end pulse moment of the pixel point selected this time within the synthetic aperture time.

[0090] Furthermore, in step 6, the selected pixel point is corrected in two dimensions using the following formula:

[0091]

[0092] Among them, I p_c (x rg ,y az ) The pixel point selected this time represents the pixel value after gain correction of the antenna two-dimensional radiation pattern;

[0093] I p (x rg ,y az ) The pixel value of the pixel point selected this time;

[0094] x rg Indicates the distance position of the pixel point selected this time in the SAR image;

[0095] y az Indicates the azimuth position of the pixel point selected this time in the SAR image;

[0096] g r (0) indicates the one-way gain of the range pattern when the target antenna range angle is 0;

[0097] g a (0) indicates the one-way gain of the azimuth pattern when the antenna azimuth angle is 0.

[0098] For a two-dimensional image, perform pixel-by-pixel traversal in the distance and azimuth dimensions. Assume that the number of image distance points is N and the number of azimuth points is M. Figure 4 As shown, for all pixel points in each row from 1 to N in distance and each column from 1 to M in azimuth in the image, the antenna two-dimensional pattern gain correction is traversed to complete the correction processing of the entire image.

[0099] This embodiment is aimed at the special observation geometry. When the distance and azimuth of the scene are inconsistent with the distance and azimuth antenna pattern of the antenna, the image is modulated by the joint distance and antenna pattern. For the target image obtained by BP imaging, the distance and azimuth of each pixel point under the distance and azimuth pattern at each pulse emission time are first calculated. Then, the antenna direction at the pulse emission time is calculated based on the distance and azimuth. Figure 2 dimensional gain. On this basis, the antenna gain of the pixel in the SAR image after synthetic aperture time integration is calculated. Then, the pixel value is corrected for antenna gain, and the image is traversed in two dimensions to complete the gain correction of the entire image, which solves the problem that the influence of the distance and azimuth antenna pattern cannot be separated and effectively corrected under complex and flexible observation geometry. This antenna pattern correction method can simply and directly realize the antenna gain correction of BP images under various observation modes. It does not involve complex transformation processing of the image, but can be achieved only by adjusting the intensity value of the image. The processing scheme is easy to implement, the algorithm is robust, and it is practical.

Claims

1. A two-dimensional correction method for SAR images for BP imaging, characterized in that: The following steps are involved: Step 1, determine the radar pulse emission time, and select any pixel point in the SAR image of BP imaging; Step 2, at the moment of radar pulse transmission, calculate the range angle of the pixel point selected this time under the target antenna range direction pattern; Step 3, at the moment of radar pulse transmission, calculate the azimuth angle of the pixel point selected this time under the azimuth direction pattern of the target antenna; Step 4, based on the data obtained in step 2 and step 3, at the time of radar pulse transmission, calculate the antenna two-way two-dimensional gain value corresponding to the pixel point selected this time; Step 5, calculating the two-dimensional radiation pattern gain value of the target antenna at the pixel point selected this time in the SAR image according to the data obtained in step 4; Step 6, correcting the data obtained in step 5; Step 7, randomly select a pixel point that has not been corrected in the BP imaging SAR image, and repeat steps 2-6 until all the pixel points are traversed, thereby 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, characterized in that: In step 1, after the radar pulse transmission time is determined, this time is recorded as t.

3. The two-dimensional correction method for SAR images for BP imaging according to claim 2, characterized in that: Step 2 includes the following sub-steps: Step 21, determining the three-dimensional position S of the radar and the corresponding antenna posture A in the SAR image of BP imaging at time t; Wherein, the coordinates of the three-dimensional position S are expressed as S(Sx, Sy, Sz); The antenna attitude A is expressed as A(y,p,r), where 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 pixel point selected this time in the antenna range direction pattern; The coordinates of the position P of the pixel point selected this time are expressed as P(l,b,h), where l represents the longitude corresponding to the pixel point selected this time, b represents the latitude corresponding to the pixel point selected this time, and h represents the height corresponding to the pixel point selected this time; Step 23, using the following formula, calculate the value of the range angle of the selected pixel point under the antenna range direction pattern at time t: in, Indicates the distance angle of the pixel point selected this time under the target antenna range direction diagram at time t; AntRg represents the distance angle calculation function.

4. The two-dimensional correction method for SAR images for BP imaging according to claim 3, characterized in that: In step 3, the azimuth angle of the pixel point selected this time under the target antenna azimuth pattern at the time of radar pulse transmission is calculated using the following formula: β p (t)0AntAz(S,P,A) Among them, β p (t) represents the azimuth angle of the selected pixel point under the azimuth direction pattern of the target antenna at time t; AntAz represents the azimuth angle calculation function.

5. The two-dimensional correction method for SAR images for BP imaging according to claim 4, characterized in that: In step 4, the two-dimensional gain value g of the target antenna corresponding to the pixel point selected at time t is calculated using the following formula: ant2 (t): Among them, g r Indicates the one-way gain value of the target antenna range pattern; g a Indicates the one-way gain value of the target antenna azimuth pattern.

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

7. The two-dimensional correction method for SAR images for BP imaging according to claim 6, characterized in that: In step 6, the following formula is used to correct the selected pixel point in the two-dimensional direction: Among them, I p_c (x rg ,y az ) indicates that the pixel point selected this time represents the pixel value after gain correction of the antenna two-dimensional radiation pattern; I p (x rg ,y az ) represents the pixel value of the pixel point selected this time; x rg Indicates the distance position of the pixel point selected this time in the SAR image; y az Indicates the azimuth position of the pixel in the SAR image; g r (0) represents the one-way gain of the range pattern when the target antenna range angle is 0; g a (0) indicates the one-way gain of the azimuth pattern when the antenna azimuth angle is 0.

Images