CN103969646A - Direction-unchanged target enhancing imaging method based on prior knowledge - Google Patents

Abstract

The invention belongs to the field of radar signal processing, and particularly relates to a direction-unchanged target enhancing imaging method based on prior knowledge. The method comprises the steps that firstly, radar echoes are divided into N subapertures according to the azimuth, so that subaperture images corresponding to directional visual angles are obtained, a region to be imaged is divided into even imaging grids, and subaperture imaging pictures are obtained; a direction-unchanged target is enhanced. The method aims at solving the technical problems that front end imaging and rear end detection are mutually independent and do not serve each other in minefield detection in the prior art, namely the prior knowledge of the unchanged direction of a mine target is introduced to the front end imaging process, the mine target is enhanced in advance, a protruding weak small mine target is finally obtained, and SAR images of most man-made and natural clutters are restrained.

Description

A kind of unchanged direction target Enhanced Imaging method based on priori

Technical field

The invention belongs to radar signal processing field, be specifically related to a kind of unchanged direction target Enhanced Imaging method based on priori.

Background technology

Ultra broadband SAR minefield detection technology is a kind of application of the Weak target in detection of complex environment in large area, and technical difficulty is large, is to detect a mine in recent years and the study hotspot in ULTRA-WIDEBAND RADAR field.Common way is that the scene areas first radar beam being irradiated carries out SAR imaging, then to the (area-of-interest: Region of Interesting) carry out feature extraction, finally carry out the discriminating of land mine target of detected ROI in SAR image.Typical application system have the dirigible of Chinese National University of Defense technology development carry GP-SAR (visit ground synthetic-aperture radar: Ground Penetrate Synthetic Aperture Radar), MirageGPR system, the Mineseeker based on airship platform of Britain and the APGPR system based on UH-60 helicopter etc. of the U.S. based on unmanned helicopter platform.In the processing of these systems, imaging, detects above, and discriminating is separately to consider.All kinds of targets when imaging in scene are all equal to and are considered as isotropy point target, during detection and identification, utilize as far as possible the time-frequency combination feature of land mine target to differentiate subsequently.For the robustness of raising system under low signal to noise ratio (SCR:Signal Clutter Ratio) condition, it is complicated that feature extraction, training and sorting algorithm are day by day tending towards.

2007 annuities add that (gold adds at its PhD dissertation, the theory and technology research > > of < < ultra broadband SAR shallow embedding target imaging and detection, National University of Defense Technology's doctorate paper, 2007) in the concept of " towards the imaging detecting " is proposed, point out in imaging, to introduce the prior imformation of target to be detected, be follow-up detection and discriminating service.Land mine is a kind of typical unchanged direction target (Christopher R.Ratto, Peter A.Torrione, and Leslie M.Collins, Exploiting Ground-Penetrating RadarPhenomenology in a Context-Dependent Framework for Landmine Detection andDiscrimination.IEEE Transactions on Geoscience and Remote Sensing, 2011, 49 (5): 1689-1699), there is axially symmetric structure, it is characterized as the unchanged direction of scattering properties the most significantly, and most of artificial and natural clutter scatterings have Broadside flash effect.If be not limited to traditional target imaging, detection and identification process, the priori of unchanged direction target to be detected is incorporated in imaging process, in front end imaging process, consider the unchanged direction of unchanged direction target, and in advance it is strengthened, be expected to outstanding small and weak land mine target in complicated imaging circumstances, suppress the artificial and natural clutter that great majority do not possess this characteristic, reduce the pressure of subsequent detection and improve subsequent detection effect.How the unchanged direction of land mine is incorporated in imaging process, in advance it is strengthened, realizing " towards the imaging of landmine detection " is a remote minefield detection key issue urgently to be resolved hurrily.

Summary of the invention

The technical problem to be solved in the present invention is to detect shortcoming separate, that do not serve mutually for the front end imaging existing in existing minefield detection and rear end, a kind of unchanged direction target Enhancement Method towards detecting is proposed, in front end imaging process, introduce this classification target unchanged direction priori of land mine, in advance it is strengthened, finally obtain outstanding small and weak land mine target, suppress the SAR image of most of artificial and natural clutters.

Technical scheme of the present invention is:

A unchanged direction target Enhanced Imaging method based on priori, is characterized in that comprising the following steps:

(s1) obtain sub-aperture imaging figure step as follows: radar return, along orientation to being divided into N piece aperture, is obtained to corresponding each orientation view angle theta _nsub-subaperture image In, region to be imaged is divided into even imaging grid (i, j), with p _i,jrepresent sub-subaperture image I ⁿpixel point value, sub-subaperture image is further expressed as wherein, n=1 ..., N; I=1 ..., M; J=1 ..., W; N, M, W is natural number;

(s2) unchanged direction target is strengthened, this step is divided into following 3 steps,

(s201) calculate sub-subaperture image I ⁿweight map

$I_p^n=p_{i,j}^n/\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}\left|p_{i,j}^n\right|---\left(1\right),$

(s202) the orientation scattering entropy of computed image (being abbreviated as ASE, Azimuth Scattering Entropy) value,

The orientation scattering properties tolerance E of computed image _i:

$E_I=-\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}_p^n\ln \left(I_p^n\right)---\left(2\right),$

The orientation scattering entropy H of computed image _i:

$H_I=\frac{1}{\ln \left(N\right)-E_I}---\left(3\right),$

(s203) the figure image intensifying based on orientation scattering entropy,

According to sub-subaperture image I ⁿ, calculate full aperture SAR image I:

$I=\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}^n---\left(4\right),$

Utilize H _ifull aperture SAR image I is strengthened to the image I after being enhanced ^h:

I ^H＝I·(H _I) ^λ(5)，

Wherein, λ is slack variable, and span is 0～2.

Preferably, described N value is the natural number between 8～16.

Preferably, described λ value is 1.

Adopt the present invention can reach following technique effect:

The one, effectively strengthen unchanged direction target, suppress each different in nature target of orientation, reduce false-alarm.The present invention is not limited to traditional target imaging, detection and identification process, the prioris such as invariable rotary structure of land mine target to be detected are incorporated in imaging process, realized " towards the imaging of land mine target detection ", artificial and the natural clutter that can suppress each opposite sex of most of orientation in complex background environment, outstanding small and weak land mine target, greatly lower the pressure of subsequent detection, be applicable to the minefield detection imaging of various platforms.The 2nd, effective clutter reduction and secondary lobe, improve picture contrast.When SAR carries at airborne platform, due to far away apart from land mine target, a little less than target scattering echo, signal to noise ratio is lower.Add because the inevitable target secondary lobe of introducing of formation method is made an uproar the end of raising image, cause picture contrast poor.Adopt image enchancing method of the present invention, not only can suppress the strong clutter such as stone, soil block and grass roots, can also effectively suppress image secondary lobe, improve picture contrast and interpretability, effectively improve subsequent detection effect.The 3rd, extra computing expense is little, be convenient to engineering applies.In SAR imaging processing, in order to reduce computing expense and cost, conventionally understand chooser aperture parallel imaging in real time, more last synthetic algorithm.The present invention only need be on sub-aperture imaging basis extra computation ASE, need the calculated amount that increases less, be convenient to engineering and apply.

Accompanying drawing explanation

Fig. 1 is the method flow diagram that the unchanged direction target based on ASE strengthens;

Fig. 2 is the orientation scattering signatures figure of Electromagnetic Simulation trihedral angle and unchanged direction target;

Fig. 3 is the SAR image of Electromagnetic Simulation trihedral angle and unchanged direction target;

Fig. 4 is Electromagnetic Simulation trihedral angle after the present invention strengthens and the SAR image of unchanged direction target;

Fig. 5 is measured data target arrangenent diagram;

Fig. 6 is the orientation scattering signatures figure of measured data target;

Fig. 7 is the SAR image in actual measurement scene;

Fig. 8 is the SAR image that adopts the actual measurement scene after the present invention strengthens.

Embodiment

Below in conjunction with the drawings and specific embodiments, the invention will be further described.

As shown in Figure 1, the method flow diagram that the unchanged direction target based on ASE strengthens, whole flow process is mainly divided into sub-subaperture image and forms, calculates the weight of sub-subaperture image, the ASE of computed image, generation full aperture image and unchanged direction target strengthen.For clear elaboration measuring principle of the present invention, in conjunction with embodiment, further illustrate.

The first step, obtains sub-aperture imaging.

Radar return, along orientation to being divided into N piece aperture, is obtained to corresponding each orientation view angle theta _n(n=1 ..., sub-subaperture image I N) ⁿ(n=1 ..., N).The impact of acceptor subaperture image azimuth resolution, the value of N is generally smaller; On the other hand, backend application requires the number of sub-subaperture image as far as possible many.Therefore, the value of N need be selected according to actual conditions compromise.In the present invention, N can be taken as 8-16.Region to be imaged is divided into even imaging grid (i, j), and i represents that orientation is to pixel number, and j represents that distance is to pixel number, the orientation that M and W are respectively region to be imaged to distance to total pixel number, with p _i,jrepresent sub-subaperture image I ⁿpixel point value, sub-subaperture image is further expressed as

Second step, the enhancing of unchanged direction target.

This step can be subdivided into three steps:

(1) calculate the weight map of sub-subaperture image.

Utilize following formula to calculate sub-subaperture image I ⁿweight map

$I_p^n=p_{i,j}^n/\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}\left|p_{i,j}^n\right|$

(2) ASE of computed image.

Utilize the orientation scattering properties tolerance E of following formula computed image _i:

$E_I=-\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}_p^n\ln \left(I_p^n\right)$

The ASE figure H of computed image _i:

$H_I=\frac{1}{\ln \left(N\right)-E_I}$

(3) the figure image intensifying based on ASE.

Can be by sub-subaperture image I ⁿ, obtain full aperture SAR image I:

$I=\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}^n$

Utilize H _ifull aperture SAR image I is strengthened to the image I after being enhanced ^h:

I ^H＝I·(H _I) ^λ

(5) in formula, λ is slack variable, and span 0～2 has determined the degree that unchanged direction target is strengthened.The signal to noise ratio (S/N ratio) of radar target (Signal-to-Noise ratio, SNR) has determined the value of λ.λ is taken as 1 can obtain good unchanged direction target enhancing effect when guaranteeing robustness.

Because land mine axially symmetric structure has unchanged direction, from above process, the present invention can carry out figure image intensifying to unchanged direction targets such as land mines, suppresses the artificial and natural clutter that great majority do not possess unchanged direction.This process can strengthen software by designed image and realize, and this software strengthens two modules by sub-aperture imaging, unchanged direction target and forms, respectively corresponding two steps of the present invention.

As shown in Figure 2, the orientation scattering signatures figure of Electromagnetic Simulation trihedral angle and unchanged direction target.Horizontal ordinate is visual angle, orientation, and ordinate is target azimuth scattering strength.Utilize XFDTD Electromagnetic Simulation software emulation to generate right cylinder and the trihedral echo of same characteristic features size; Then use Subaperture method to obtain respectively the sub-subaperture image of this two classifications target extract this two classifications target orientation scattering strength feature.As shown in Figure 2, right cylinder is owing to having rotational symmetry, and its orientation scattering strength is hardly with orientation visual angle change, and trihedral angle has obvious Broadside flash effect.

As shown in Figure 3, the SAR image of Electromagnetic Simulation trihedral angle, unchanged direction target.Horizontal ordinate be orientation to, ordinate be distance to.By XFDTD Electromagnetic Simulation software, obtain circle mark SAR echo data, it is carried out to conventional SAR imaging processing.Wherein, system bandwidth is 2GHz, depression angle is constant is 60 °, in echo, adding signal to noise ratio (S/N ratio) is the white Gaussian noise of 30dB, diameter be set be 0.3m in scene, highly for the right cylinder of 0.1m (is positioned at distance to-0.414m, orientation is to 0m place) and rib long be the trihedral angle of 0.3m (be positioned at apart to 1m, orientation is to 0m place), the distance at trihedral summit and right cylinder center is 1.414m.As shown in Figure 3, visible trihedral amplitude is far better than right cylinder target.Right cylinder upwards exists bimodal in distance, respectively the forward position of corresponding circle cylinder upper surface and the scattering on rear edge.

Fig. 4 is for the trihedral angle after the present invention's enhancing and the SAR image of unchanged direction target for Electromagnetic Simulation data acquisition.Horizontal ordinate be orientation to, ordinate be distance to.The data that circle mark SAR echo data is used with Fig. 3 are identical.Slack variable λ value is 1.Comparison diagram 3 and Fig. 4, the forward position of right cylinder upper surface and all obtained effective enhancing along scattering center afterwards in Fig. 4, trihedral angle and imaging secondary lobe have all obtained effective inhibition.Take trihedral amplitude as benchmark, and after Enhanced Imaging, cylindrical amplitude has improved about 33dB.

Fig. 5 is measured data target arrangenent diagram.Carrier aircraft heading is for from west to east, and radar illumination region is carrier aircraft right side.Wherein A district is smooth cement flooring, and B district is stone and the densely covered rough earth of weeds, in A district, along carrier aircraft heading interval 5m, places successively 4 targets. for antitank mine, being respectively 3 rib length is 0.3m, points to and to be respectively northwest to 45 °, direct north, northeastward 45 ^°trihedral angle, ' 1. ' and ' 2. ' is ground two circular pit.

Fig. 6 is the orientation scattering signatures figure of typical target in measured data.Horizontal ordinate is visual angle, orientation, and ordinate is target azimuth scattering strength.Due to the impact of position and attitude jitter, compare Fig. 2, the measured data two classification target orientation scattering strength features of obtaining for twice have fluctuation, however trihedral Broadside flash characteristic embodies still clearly.

Fig. 7 is the SAR image of actual measurement scene.Horizontal ordinate be orientation to, ordinate be distance to.Measured data is carried out to conventional SAR imaging processing.Therefore in Fig. 7-a, the amplitude of land mine is obviously weaker than trihedral angle 2, but is slightly better than trihedral angle 1, distinguishes land mine and trihedral angle 1 and 3 is more difficult from amplitude.For the ease of with imaging results comparison of the present invention, choose that to take centered by typical target the zone marker of 4 meters * 4 meters be subregion.Choose land mine and be labeled as A1 subregion, trihedral angle 2 is labeled as A2 subregion; In rough earth B district and A district, smooth cement flooring, also choosing respectively two zone markers is in addition A3 and A4 subregion.Fig. 7-b, 7-c, 7-d, 7-e are respectively the enlarged drawing of four sub regions.

Fig. 8 is the SAR image that adopts the actual measurement scene after the present invention strengthens.Horizontal ordinate be orientation to, ordinate be distance to.Slack variable λ value 1 (data of using are identical with the data of using in Fig. 7).From Fig. 8-a, visible land mine target has obtained effective enhancing, and trihedral angle and land clutter have all obtained effective inhibition, and wherein trihedral angle 1 and 3 inhibition are the most obvious.Corresponding diagram 7-a chooses land mine and is labeled as B1 subregion in Fig. 8-a; Trihedral angle 2 is labeled as B2 subregion; In rough earth B district and A district, smooth cement flooring, also choose two corresponding regions in addition and be labeled as B3 and B4 subregion.Fig. 8-b, 8-c, 8-d, 8-e are respectively the enlarged drawing of four sub regions.Compare Fig. 7-b, the letter miscellaneous noise ratio of the land mine target in Fig. 8-b significantly improves.Compare Fig. 7-c, the orientation of Fig. 8-c has lowered 7.55dB to integration secondary lobe ratio.B region is the rough earth that is covered with stone, soil block and grass roots, comprises a large amount of clutters in Fig. 7-d, and in Fig. 8-d, clutter inhibition is remarkable.A district is smooth cement flooring, is mainly noise component in Fig. 7-e, and Fig. 8-e also has certain inhibition.

The present invention has taken into full account the unchanged direction feature of this class unchanged direction target axially symmetric structure of land mine, make full use of ASE information, unchanged direction target in imaging scene is strengthened, finally obtain outstanding small and weak land mine target, suppress the SAR image of most of artificial and natural clutters.Adopt simulative data and flight-test data, the method for the invention is tested respectively, for unchanged direction target, strengthen effect remarkable.

Claims (3)

1. the unchanged direction target Enhanced Imaging method based on priori, is characterized in that comprising the following steps:

(s1) obtain sub-aperture imaging figure, step is as follows: radar return, along orientation to being divided into N piece aperture, is obtained to corresponding each orientation view angle theta _nsub-subaperture image I ⁿ, region to be imaged is divided into even imaging grid (i, j), with p _i,jrepresent sub-subaperture image I ⁿpixel point value, sub-subaperture image is further expressed as wherein, n=1 ..., N; I=1 ..., M; J=1 ..., W; N, M, W is natural number;

(s2) unchanged direction target is strengthened, this step is divided into following 3 steps,

(s201) calculate sub-subaperture image I ⁿweight map , $I_p^n=p_{i,j}^n/\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}\left|p_{i,j}^n\right|---\left(1\right),$

(s202) the orientation scattering entropy of computed image,

The orientation scattering properties tolerance E of computed image _i:

$E_I=-\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}_p^n\ln \left(I_p^n\right)---\left(2\right),$

The orientation scattering entropy H of computed image _i:

$H_I=\frac{1}{\ln \left(N\right)-E_I}---\left(3\right),$

(s203) the figure image intensifying based on orientation scattering entropy,

According to sub-subaperture image I ⁿ, calculate full aperture SAR image I:

$I=\underset{n=1}{\overset{N}{\mathrm{\&Sigma;}}}{I}^n---\left(4\right),$

Utilize H _ifull aperture SAR image I is strengthened to the image I H after being enhanced:

I ^H＝I·(H _I) ^λ(5)，

Wherein, λ is slack variable, and span is 0～2.

2. a kind of unchanged direction target Enhanced Imaging method according to claim 1, is characterized in that: described N value is the natural number between 8～16.

3. a kind of unchanged direction target Enhanced Imaging method according to claim 1, is characterized in that: described λ value is 1.