CN105699947A - SAR image sidelobe inhibition method - Google Patents

Abstract

The invention belongs to the technical field of radar imaging, and specifically relates to an SAR image sidelobe inhibition method. The method can well improve the recognition accuracy of a target through the comparison of an original image with a windowed image. Meanwhile, the method carries out the partitioning of an imaging region, and different areas employ different reference standards. The method also can correct the pulse response deformation caused by range migration and other factors. The method enables the main lobe width to remain unchanged while effectively inhibiting sidelobe. In addition, the whole implementation process just refers to some simple arithmetic operation, and does not refer to complex operation, such as inversion and characteristic decomposition. Therefore, the method is very simple in implementation.

Description

A kind of SAR image side lobe suppression method

Technical field

The present invention relates to radar imaging technology field, be specifically related to a kind of synthetic aperture radar SAR (syntheticapertureradar) image side lobe suppression method。

Background technology

Owing to the frequency domain support region of SAR imaging system two dimension is limited so that the impulse response function of synthetic aperture radar is in distance to being upwards sinc function with orientation, and in image, the main lobe of weak signal target is flooded in the too high meeting of secondary lobe of strong target echo。Particularly in SAR image interpretation and target interpretation process, the SAR image of high secondary lobe will be substantially reduced the system disposal ability to weak signal target, and the interpretation precision of strong target is as secondary lobe disturbs and is affected simultaneously。Therefore, reducing secondary lobe has highly important meaning for reducing false-alarm and raising Testing of Feeble Signals ability。

The more commonly used in Sidelobe Suppression is at the laggard row window adding in frequency domain of matched filtering, processes and with fixing weighting function, the data in whole aperture are carried out amplitude weighting。Pulse pressure waveform after windowing process has relatively low secondary lobe, can meet and be actually needed but simultaneously make again main lobe broadening, amplitude reduction, have a strong impact on the quality of image。Consequently, it is desirable to find better sidelobe level suppression technology。

1974,Propose a kind of quality observing into figure for improving synthetic aperture radio astronomy, it is possible to eliminate the method producing impact because space frequency cover is imperfect, by improving the Sidelobe Suppression that the method is used for SAR image。The main thought of the method is that the response of secondary lobe is replaced in the position the finding out target place impulse response blocking secondary lobe。It on the basis of complete suppressed sidelobes, can keep main lobe width constant。But its shortcoming is it is also obvious that under many circumstances, only can not judge whether this point exists target by the amplitude size of SAR echo。The judgement of mistake can make the appearance of false target and the missing inspection of weak signal target。

Summary of the invention

For above-mentioned existing problems or deficiency, the invention provides a kind of SAR image side lobe suppression method, effectively improve the appearance of false target and the missing inspection problem of weak signal target。

This SAR image side lobe suppression method, comprises the following steps:

Step 1, areas imaging is divided into K region, by adopting actual parameter and same imaging mode to emulate the response h obtaining each regional center place point target through normalized_k(m, n) (k=1,2..., K, m=1,2..., M, n=1,2..., N), wherein M be SAR image orientation to count, N be the distance of SAR image to counting, it is carried out windowing process and meets with a response

Step 2, directly generate according to actual parameter the point target echo response S after desirable normalized (m, n), and its secondary lobe is blocked leave behind main lobe be designated as h (m, n)；

The method intercepting main lobe is as follows:

Step 2-1: (m, n) amplitude maximum point place localizer unit, is designated as Sa (n), searches for first 0, each limit value point from its maximum to both sides, a little all the institute outside these 2 is set to 0, the signal after setting to 0 is designated as Sa to take S₁(n)；

Step 2-2: obtain distance to blocking the impulse response S after secondary lobe by following formula_rcut(m,n)

$S_{rcut}(m,n)=IFFT\left(\frac{FFT\left(S\right(m,n\left)\right)*FFT\left({\mathrm{Sa}}_1\right(n\left)\right)}{FFT\left(Sa\right(n\left)\right)}\right)---\left(1\right)$

Wherein, signal is done fast Fourier transform by FFT () expression, and signal is done inverse fast fourier transform by IFFT () expression；

Step 2-3: (m, n) amplitude maximum point place distance unit, is designated as Sr (m), searches for first 0, each limit value point from its maximum to both sides, a little all the institute outside these 2 is set to 0, the signal after setting to 0 is designated as Sr to take S₁(m)；

Step 2-4: by following formula obtain orientation to block the impulse response h after secondary lobe (m, n)

$h(m,n)=IFFT\left(\frac{FFT\left(S_{rcut}\right(m,n\left)\right)*FFT\left({\mathrm{Sr}}_1\right(m\left)\right)}{FFT\left(Sr\right(m\left)\right)}\right)---\left(2\right)$

Step 3: (m n) carries out windowing process to SAR image Sr

$\hat{S}r(m,n)=FFT\left(IFFT\right(Sr\left(m,n\right))*win(m,n\left)\right)---\left(3\right)$

Wherein,For the SAR image after windowing, (m n) is the two-dimentional window function to add to win；

Step 4: contrast two width image Sr_i(m, n) andFind out the point being maximum in two images andMiddle individualism and meetMaximum point (m₀,n₀) position at place, remember that it is (m_i,n_i)；At Sr_i(m deducts the echo response of this region and the product of circulation step-length γ in n)；If this point is simplyThe maximum point of middle individualism, then make γ=1, otherwise, makes γ=γ₀, γ₀For initial cycle step-length, 0 < γ₀< 1；

Sr_i+1(m, n)=Sr_i(m,n)-γ*Sr_i(m_i,n_i)*h_k(m-m_i,n-n_i)(4)

Srout_i+1(m, n)=Srout_i(m,n)+γ*Sr_i(m,n)*h(m-m_i,n-n_i)(5)

Wherein, h_k(m-m_i,n-n_i) represent h_k(m n) moves to (m_i,n_i) response, Sr_i(m n) represents remaining part after i-th (i=0,1,2 ...) secondary iteration and Sr₀(m, n)=Sr (m, n)。Srout_i(m n) represents the result after ith iteration Sidelobe Suppression；

Namely image after windowing is done identical operation:

$\hat{S}{r}_{i+1}(m,n)=\hat{S}{r}_i(m,n)-\mathrm{\&gamma;}*\hat{S}{r}_i(m_i,n_i)*{\hat{h}}_k(m-m_i,n-n_i)---\left(6\right)$

Wherein,Representing willMove toResponse,Represent the remainder after i iteration of the image after windowing；

Step 5: repeat the iterative operation in step 4, until residual signal reaches noise level；So far, it is possible to obtain the image after Sidelobe Suppression；Judge that the condition whether iteration terminates is as follows:

E_l<α*E₀(7)

Wherein, E_lFor Sr_l(m, energy n),The threshold coefficient that α (0 < α < 1) is decision condition。

The present invention utilizes by the comparison of image after original image and windowing, it is possible to well improve the accuracy rate identifying target。Meanwhile, imaging region carrying out piecemeal, different regions adopts different reference standards, additionally it is possible to the impulse response deformation that the factors such as good correction distance migration cause。This make effect suppressed sidelobes to be maintained with main lobe width constant。In addition, the whole implementation process in the present invention all only relates to some simple arithmetical operations, is not related to invert, the complex calculation such as feature decomposition, and therefore the present invention realizes also very simple。

In sum, the invention have the advantages that: effect suppressed sidelobes to be maintained with main lobe width constant；Identify that the accuracy rate of target is high；It is simple that implementation process does not relate to complex calculation process。

Accompanying drawing explanation

Fig. 1 is the flow chart of SAR image side lobe suppression method of the present invention；

Fig. 2 is the point target imaging results of regional after piecemeal；

Imaging results after the windowing of regional point target after Fig. 3 piecemeal；

Fig. 4 is desirable point target imaging results；

Fig. 5 is the desirable point target imaging results after blocking secondary lobe；

Fig. 6 is SAR image that will be pending；

Fig. 7 is the result to pending SAR image windowing；

Fig. 8 is the result after suppressed sidelobes。

Detailed description of the invention

Below in conjunction with accompanying drawing, the present invention is described further with embodiment。

Flow operations shown in preceding method and Fig. 1, the imaging pattern being specifically provided that employing circumference SAR of simulation parameter, the distance of distance by radar scene center is 10 meters, and the bandwidth launching signal is 30GHz, the center carrier frequence of signal is 300GHz, and the imaging algorithm of employing is RD algorithm。

Fig. 2 and Fig. 3 is the result that step 1 processes: imaging region divides the result after the point target echo response by emulating acquisition regional center and windowing。

Fig. 4 and Fig. 5 is the result that step 2 processes。Wherein, Fig. 4 is desirable point target imaging results, and Fig. 5 is the main lobe part after blocking secondary lobe。

Fig. 6 is SAR image to be processed, and as can be seen from the figure point target there occurs distortion due to the reason of range migration compared to desirable point target echo, can be seen that the two of the upper right corner point is too near and inseparable due to distance simultaneously。

Fig. 7 is Fig. 6 process by step 3 after result after windowing, as can be seen from the figure two points in the upper right corner can separate, but while it can also be seen that secondary lobe all broadenings of point target。

Fig. 8 is the result after suppressed sidelobes of the present invention, and the while that as can be seen from the figure secondary lobe being repressed, main lobe does not have broadening。And, the distortion that echo produces due to range migration have also been obtained good correction。

In sum, it can be seen that the effectiveness of SAR image side lobe suppression method provided by the invention and helpfulness the result processed。

Claims (1)

1. a SAR image side lobe suppression method, comprises the following steps:

Step 1, areas imaging is divided into K region, by adopting actual parameter and same imaging mode to emulate the response h obtaining each regional center place point target through normalized_k(m, n) (k=1,2..., K, m=1,2..., M, n=1,2..., N), wherein M be SAR image orientation to count, N be the distance of SAR image to counting, it is carried out windowing process and meets with a response

Step 2, directly generate according to actual parameter the point target echo response S after desirable normalized (m, n), and its secondary lobe is blocked leave behind main lobe be designated as h (m, n)；

The method intercepting main lobe is as follows:

Step 2-1: (m, n) amplitude maximum point place localizer unit, is designated as Sa (n), searches for first 0, each limit value point from its maximum to both sides, a little all the institute outside these 2 is set to 0, the signal after setting to 0 is designated as Sa to take S₁(n)；

Step 2-2: obtain distance to blocking the impulse response S after secondary lobe by following formula_rcut(m,n)

$S_{rcut}(m,n)=IFFT\left(\frac{FFT\left(S\right(m,n))*FFT\left({\mathrm{Sa}}_1\right(n))}{FFT\left(Sa\right(n))}\right)---\left(1\right)$

Wherein, signal is done fast Fourier transform by FFT () expression, and signal is done inverse fast fourier transform by IFFT () expression；

Step 2-3: (m, n) amplitude maximum point place distance unit, is designated as Sr (m), searches for first 0, each limit value point from its maximum to both sides, a little all the institute outside these 2 is set to 0, the signal after setting to 0 is designated as Sr to take S₁(m)；

Step 2-4: by following formula obtain orientation to block the impulse response h after secondary lobe (m, n)

$h(m,n)=IFFT\left(\frac{FFT\left(S_{rcut}\right(m,n))*FFT\left({\mathrm{Sr}}_1\right(m))}{FFT\left(Sr\right(m))}\right)---\left(2\right)$

Step 3: (m n) carries out windowing process to SAR image Sr

$\hat{S}r(m,n)=FFT\left(IFFT\right(Sr\left(m,n\right))*win(m,n\left)\right)---\left(3\right)$

Wherein,For the SAR image after windowing, (m n) is the two-dimentional window function to add to win；

Step 4: contrast two width image Sr_i(m, n) andFind out the point being maximum in two images andMiddle individualism and meetMaximum point (m₀,n₀) position at place, remember that it is (m_i,n_i)；At Sr_i(m deducts the echo response of this region and the product of circulation step-length γ in n)；If this point is simplyThe maximum point of middle individualism, then make γ=1, otherwise, makes γ=γ₀, γ₀For initial cycle step-length, 0 < γ₀< 1；

Sr_i+1(m, n)=Sr_i(m,n)-γ*Sr_i(m_i,n_i)*h_k(m-m_i,n-n_i)(4)

Srout_i+1(m, n)=Srout_i(m,n)+γ*Sr_i(m,n)*h(m-m_i,n-n_i)(5)

Wherein, h_k(m-m_i,n-n_i) represent h_k(m n) moves to (m_i,n_i) response, Sr_i(m n) represents remaining part after i-th (i=0,1,2...) secondary iteration and Sr₀(m, n)=Sr (m, n)。Srout_i(m n) represents the result after ith iteration Sidelobe Suppression；

Namely image after windowing is done identical operation:

$\hat{S}{r}_{i+1}(m,n)=\hat{S}{r}_i(m,n)-\mathrm{\&gamma;}*\hat{S}{r}_i(m_i,n_i)*{\hat{h}}_k(m-m_i,n-n_i)---\left(6\right)$

Wherein,Representing willMove to (m_i,n_i) response,Represent the remainder after i iteration of the image after windowing；

Step 5: repeat the iterative operation in step 4, until residual signal reaches noise level；So far, it is possible to obtain the image after Sidelobe Suppression；Judge that the condition whether iteration terminates is as follows:

E_l<α*E₀(7)

Wherein, E_lFor Sr_l(m, energy n),The threshold coefficient that α (0 < α < 1) is decision condition。