CN108062745B - Method for enhancing spatial resolution capability of large forward-leaning SAR (synthetic aperture radar) image of aircraft platform - Google Patents
Method for enhancing spatial resolution capability of large forward-leaning SAR (synthetic aperture radar) image of aircraft platform Download PDFInfo
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Abstract
The invention provides a method for enhancing the spatial resolution capability of a large forward-oblique SAR image of an aircraft platform. The method adopts the weight factors to carry out noise suppression processing on the SAR image, applies large weight factors in an image smooth area to suppress noise, and reduces the noise level of the radar image; in the edge area, a small weight factor is applied to keep the image edge and detail information, so that the detail is enhanced, the image edge contour is more obvious, and the noise suppression and the detail maintenance are considered.
Description
Technical Field
The invention relates to a method for enhancing the spatial resolution capability of a large forward-leaning SAR image of an aircraft platform, and belongs to the technical field of accurate guidance.
Background
SAR imaging has the advantages of all-time and all-weather imaging which is incomparable with optics (infrared and visible light), and is widely applied to platforms such as airborne platforms, satellite-borne platforms, aircraft platforms and the like. Due to the space and loads that are limited to certain aircraft platforms, the antenna size is typically small, making its full aperture azimuth resolution much higher than the matching reference map. In practice, this requires that the radar be imaged in a large forward pitch mode when the target is located directly in front of the aircraft's direction of flight. Because the clutter of the sea background or the land background is complex, the clutter is mixed with the target echo, so that the imaging result of the target is fuzzy. In the radar imaging post-processing process, when target features are further extracted, image restoration and super-resolution reconstruction are difficult points in signal processing. How to further improve the spatial resolution of the SAR image under the limitation of hardware conditions attracts the attention of more researchers. The common SAR image enhancement algorithm has too many iteration times and high calculation complexity, and cannot realize image enhancement quickly.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for simply and quickly enhancing the SAR image spatial resolution capability of an aircraft platform.
The technical solution of the invention is as follows: a method for enhancing the spatial resolution capability of a large forward-leaning SAR image of an aircraft platform is realized by the following steps:
firstly, setting a Gaussian kernel, determining a Gaussian filter based on a Hessian matrix, and filtering an SAR original image;
the method comprises the steps of the prior art, Gaussian kernel setting is detailed in Zhang Xiaoyun and Liu allowed treatise on 'Performance analysis of Gaussian kernel support vector machine' (computer engineering 29(8):2003), and Gaussian filter determination is detailed in Euroman, Chenhong bin and Bajie 'Gaussian filter characteristic analysis and application research'.
Secondly, determining weight factors w (u (x, y)) of detail enhancement and noise suppression of each pixel variable u in the SAR image after the filtering processing in the first step,
a2.1, obtaining a gray level change value zeta (x, y) of each pixel variable u and coordinates u (x, y) thereof in the SAR image by using the formula (1),
wherein k isi×kjIs a region, k, centered on the coordinate u (x, y)i、kjThe value is determined according to the precision requirement of the image, the higher the precision requirement is, the larger the value is, the calculated amount is correspondingly increased, and the field can select according to the requirements of precision and calculation speed.
u (x + i, y + j) is the coordinates of the pixels around the pixel variable u;
a2.2, obtaining a weight factor w (u (x, y)) of detail enhancement and noise suppression of the pixel variable u by using the formula (2),
where min (ζ) and max (ζ) are the minimum and maximum values of the gray level change values ζ (x, y) of all pixel variables in the SAR image;
thirdly, according to the weight factors of detail enhancement and noise suppression of each pixel variable u in the SAR image obtained in the second step, the difference characteristic value D (x, y) of the pixel variable u is obtained by using a formula (3),
D(x,y)＝(λ1-λ2)λ1w(u(x,y)) (3)
wherein λ1And λ2The maximum value and the minimum value of the pixel corresponding to the pixel variable u in the SAR image after filtering processing;
fourthly, processing the SAR image after the first filtering by using a total variation model of a formula (4) to obtain an SAR image with enhanced resolution,
wherein theta is a contrast factor, where theta is,
representing the gradient of the coordinates u (x, y) of each pixel variable.
The value range of the contrast factor theta is 0-1, the influence of the value on the SAR image resolution is small, the contrast degree of the SAR image is reflected, the larger the value of theta is, the larger the contrast between different things in the processed image is, but the detail display of a single object is influenced; a contrast factor theta is selected by a person skilled in the art according to requirements of the processed SAR image, and the optimal value range is 0.4-0.7.
For the pixels of the smooth region, since D (x, y) is close to zero,
approaching 1, which means that a large total variation regularization is strengthened and noise is suppressed; for edges and detail pixels, since D (x, y) is large
This, being small, will weaken the strength of the total variation regularization term and texture detail will be preserved.
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the method, noise suppression and edge maintenance are considered, the flat area and edge structure information in the image are partitioned by using the weight factors of detail enhancement and noise suppression, and the resolution of the radar image is improved;
(2) the method adopts the weight factors to carry out noise suppression processing on the SAR image, applies large weight factors in an image smooth area to suppress noise, and reduces the noise level of the radar image; in the edge area, a small weight factor is applied to keep the image edge and detail information, so that the detail is enhanced, the image edge outline is more obvious, and the noise suppression and the detail maintenance are considered;
(3) the invention has simple and quick calculation mode and can quickly realize resolution enhancement.
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FIG. 1 is an original image according to an embodiment of the present invention;
FIG. 2 is an image processed by the method of the present invention;
FIG. 3 is a flow chart of the present invention.
Detailed Description
The present invention will be described in detail with reference to the following examples and accompanying drawings.
The invention is realized by the following steps as shown in figure 3:
1. and setting a Gaussian kernel, determining a Gaussian filter based on a Hessian matrix, and filtering the SAR original image.
And setting the size of the Gaussian kernel to be 5 x 5 and the variable to be 8.0, and combining the Gaussian kernel with the pixel point coordinates to obtain the Gaussian filter.
2. The weighting factors w (u (x, y)) for detail enhancement and noise suppression for each pixel u in the filtered SAR image are determined.
1) To balance accuracy and computation speed, ki×kjTaking a range of 3 x 3, rewriting the formula (1) into a formula (1-1) to obtain a gray level change value zeta (x, y) of each pixel u in the SAR image,
2) obtaining weight factors w (u (x, y)) of detail enhancement and noise suppression of the pixel u by using the formula (2),
where min (ζ) and max (ζ) are the minimum and maximum values of the gray level change value ζ (x, y) corresponding to the pixel u.
3. Obtaining a difference characteristic value D (x, y) of the pixel u by using a formula (3),
D(x,y)＝(λ1-λ2)λ1w(u(x,y)) (3)
4. the filtered SAR image is processed by utilizing the total variation model of the formula (4) to obtain the SAR image with enhanced resolution,
wherein theta is a contrast factor, the value in the embodiment is 0.7,
represents the gradient of u (x, y).
The original image of fig. 1 is processed according to the above steps to obtain the result shown in fig. 2. As is apparent from fig. 2, image noise is suppressed, detail contours are more apparent, and resolution is improved.
The invention has not been described in detail and is in part known to those of skill in the art.
Claims (4)
1. A method for enhancing the spatial resolution capability of a large forward-leaning SAR image of an aircraft platform is characterized by comprising the following steps:
firstly, setting a Gaussian kernel, determining a Gaussian filter based on a Hessian matrix, and filtering an SAR original image;
secondly, determining weight factors w (u (x, y)) of detail enhancement and noise suppression of each pixel variable u in the SAR image after the filtering processing in the first step,
a2.1, obtaining a gray level change value zeta (x, y) of each pixel variable u and coordinates u (x, y) thereof in the SAR image by using the formula (1),
wherein k isi×kjIs a region, k, centered on the coordinate u (x, y)i、kjIs a natural number greater than 2, and u (x + i, y + j) is the coordinate of the pixel around the pixel variable u;
a2.2, obtaining a weight factor w (u (x, y)) of detail enhancement and noise suppression of the pixel variable u by using the formula (2),
where min (ζ) and max (ζ) are the minimum and maximum values of the gray level change values ζ (x, y) of all pixel variables in the SAR image;
thirdly, according to the weight factors of detail enhancement and noise suppression of each pixel variable u in the SAR image obtained in the second step, the difference characteristic value D (x, y) of the pixel variable u is obtained by using a formula (3),
D(x,y)＝(λ1-λ2)λ1w(u(x,y)) (3)
wherein λ1And λ2The maximum value and the minimum value of the pixel corresponding to the pixel variable u in the SAR image after filtering processing;
fourthly, processing the SAR image after the first filtering by using a total variation model of a formula (4) to obtain an SAR image with enhanced resolution capability,
wherein theta is a contrast factor, where theta is,
representing the gradient of the coordinates u (x, y) of each pixel variable.
2. The method for enhancing the spatial resolution of the SAR image of the aircraft platform in the forward inclination direction according to claim 1, wherein the method comprises the following steps: and in the fourth step, the value range of the contrast factor theta is 0-1.
3. The method for enhancing the spatial resolution of the SAR image of the aircraft platform in the forward inclination direction according to claim 1, wherein the method comprises the following steps: and in the fourth step, the value range of the contrast factor theta is 0.4-0.7.
4. The method for enhancing the spatial resolution of the SAR image of the aircraft platform in the forward inclination direction according to claim 1, wherein the method comprises the following steps: in said step A2.1 ki×kjIs 3 × 3.
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