CN104391297A - Sub-aperture partition PFA (Polar Format Algorithm) radar imaging method - Google Patents

Abstract

The invention discloses a sub-aperture partition PFA (Polar Format Algorithm) radar imaging method, and belongs to the technical field of radar imaging. All apertures are partitioned into a plurality of sub-apertures; the sub-apertures are processed by using PFA to obtain azimuthal low-resolution images; movement error of the sub-aperture images is eliminated by using PGA (Phase Gradient Autofocus) processing; geometric distortion is eliminated through azimuthal up-sampling, space varying filter and geometric correction; partitioned sub-blocks with overlapped images of all the sub-aperture images obtained by the geometric correction are subjected to image registration; finally all the registered sub-aperture images are fused to obtain a final SAR image. The full-aperture data of a large block is partitioned into sub-aperture data of small blocks for processing, so that the imaging processing efficiency is improved; the partitioned sub-blocks with overlapped images are registered during implementation of the sub-aperture image registration, so that the registration of the entire images can be realized better and seamless splicing of the sub-blocks can also be realized after registration.

Description

A kind of division sub-aperture PFA radar imaging method

Technical field

The present invention relates to a kind of synthetic-aperture radar (synthetic aperture radar is called for short SAR) image-processing algorithms, specifically relate to a kind of division sub-aperture PFA radar imaging method, belong to radar imaging technology field.

Background technology

Synthetic-aperture radar (SAR) is a kind of round-the-clock, round-the-clock microwave imaging radar, and high-resolution feature makes it have irreplaceable effect in dual-use field.Imaging algorithm is that SAR signal transacting realizes high-resolution core, in numerous SAR imaging algorithms, polar format algorithm (PFA) because of its at imaging efficiency, compensate excellent properties in the flight of Texas tower non-co-planar and the linear range walk of correction of movement target etc. and attention.But, classical PFA process is all the monoblock data of full aperture processed, and there is inclination of wave front and be similar to, imaging results geometric fidelity and effective scene size are all subject to a definite limitation, in addition, the process of full aperture monoblock data adds consumption and the processing time of resource.Therefore, process large aperture data are needed to improve its algorithm.

Summary of the invention

Goal of the invention: for above-mentioned prior art, proposes a kind of division sub-aperture PFA radar imaging method, improves imaging results geometric fidelity and effective scene size.

Technical scheme: a kind of division sub-aperture PFA imaging radar method, comprise the steps: first full aperture to be divided into some sub-aperture, with PFA, sub-aperture process is shown that orientation is to low-resolution image, then sub-aperture image motion error is eliminated with PGA Autofocus processing, by orientation to a liter sampling, geometric distortion is eliminated again through space-variant filtering and geometry correction process, all sub-aperture images geometry correction obtained subsequently have overlapping division sub-block to carry out image registration, are finally concerned with by all sub-aperture images after registration to merge to obtain final SAR image.

Further, divide sub-aperture PFA imaging radar method to comprise the steps:

Step 1, by N _a× N _rfull aperture radar return Data Placement be N number of sub-aperture, then each sub-aperture echo data is N _ai× N _r, wherein N _afor full aperture radar echo pulse number, N _aifor each sub-aperture radar echo pulse number, n _rfor distance is to sampling number;

Step 2, carries out pulse compression to i-th sub-aperture echo data, obtains orientation time domain and distance frequency domain data;

Step 3, PFA imaging is carried out to i-th sub-aperture echo data, comprises following concrete steps:

Step 3-1, calculates metric space frequency domain interpolation point position K _y:

Wherein, c is the light velocity, f _cfor the carrier frequency that transmits, f _τfor distance is to frequency domain, θ and be respectively instantaneous azimuth and the angle of pitch of antenna phase center; According to described metric space frequency domain interpolation point position K _y, interpolation is carried out to the metric space frequency domain interpolation point position of each wave impulse of i-th sub-aperture echo data;

Step 3-2, computer azimuth spatial frequency domain interpolation point position K _x:

According to described director space frequency domain interpolation point position K _x, interpolation is carried out to each range unit director space frequency domain interpolation point position of i-th sub-aperture echo data;

Step 3-3, carries out two-dimentional inverse Fourier transform to the data behind orientation and metric space frequency domain interpolation, and obtaining pixel is N _ai× N _rsub-aperture SAR image;

Step 4, carries out PGA Autofocus processing to sub-aperture SAR image, comprises the steps:

Step 4-1,5% range unit selecting energy maximum in sub-aperture SAR image, if the data obtained are N _ai× N _r', N _r'=5%N _r, by selecting the described range unit obtained to carry out ring shift, make the maximum modulus value position of each range unit be positioned at the center of range unit;

Step 4-2, to the range unit through ring shift in the windowing of image area center;

Step 4-3, the complex pattern range unit after ring shift windowing is: G _k(x _a) (3)

Wherein, x _a=0,1 ..., N _ai-1, k=1 ..., N _r';

To described G _k(x _a) obtain g as IFFT _k(x _a), calculate described g according to (4) formula _k(x _a) the phase error gradient r of range unit _k(x _a):

$\left\{\begin{array}{cc}{r}_k\left(x_a\right)=g_k(x_a-1)\×{\left[g_k\left(x_a\right)\right]}^{*}& x_a=\mathrm{1,2},...,N_{\mathrm{ai}}-1\\ r_k\left(0\right)=0& x_a=0\end{array}\right.---\left(4\right)$

Wherein, k=1 ..., N _r'; r _k(0)=1, k=1 ..., N _r'; [] * represents complex conjugate, adopts (5) formula estimating phase error Δ φ (x _a):

$\mathrm{\Δ\φ}\left(x_a\right)=\arg \left[\frac{1}{{N_r}^{\′}}\underset{k=1}{\overset{{N_r}^{\′}}{\mathrm{\Σ}}}{r}_k\left(x_a\right)\right]---\left(5\right);$

Step 4-4, carries out phase error compensation according to the phase error that step 4-3 estimates to each range unit processing the SAR image obtained through step 3, obtains new sub-aperture SAR image;

Step 4-5, repeats step 4-1 to step 4-4 to the sub-aperture SAR image newly obtained and carries out 4-6 iteration correction; Wherein, window function width used for windowing in step 4-2 is once pressed 1/2 times of reduction by every iteration;

Step 5, doubly rising sampling to the sub-aperture image orientation after PGA self-focusing to N, to obtain pixel be N _a× N _rsub-aperture SAR image, its concrete steps are: the sub-aperture SAR image after self-focusing to orientation to Fourier transform, then thereafter mend (N-1) N _aiindividual zero, then carry out orientation to inverse Fourier transform;

Step 6, space-variant filtering is carried out to the sub-aperture SAR image after liter sampling, comprises following concrete steps:

Step 6-1, sub-aperture SAR image step 5 obtained is divided into N _x× N _yindividual sub-image, each sub-image size is L _x× L _y, between sub-block, be spaced apart L _sx× L _sy; Wherein N _x, N _ybe respectively azran descriscent sub-block number, L _x, L _ybe respectively azran ion block size, L _sx, L _sybe respectively Azimuth & Range to sub-block interval;

Step 6-2, to each sub-image, by the real space position (x of geometric distortion mapping relations computing center pixel cell _t, y _t):

x _t＝a ₀₁-f(x _t，y _t)

(6)

y _t＝a ₁₀-g(x _t，y _t)

Wherein:

(a in formula ₀₁, a ₁₀) be point target geometric position coordinate in the picture, r _cofor the distance of scene center and aperture center, r _ctfor the distance of impact point and aperture center, y _cfor the distance of scene center and aperture center subpoint in Y-axis, for the angle on scene center and aperture center line and ground, θ _sfor angle of squint;

Step 6-3, utilizes the real space position (x of described center pixel unit _t, y _t), structure filter function: $H_{(x_t,y_t)}\left(K_x\right)=\exp (-j{a}_{20}{K_x}^2),$ Wherein:

Orientation is carried out to described sub-image and obtains spatial frequency domain to FFT conversion, be multiplied by after do orientation again and switch back to spatial domain against FFT, obtain filtered sub-image;

Step 6-4, chooses the L at sub-image center after filtering _sx× L _sythe Output rusults of each sub-block filtering, as the output of this sub-block filtering, carries out splicing the image obtaining refocusing by size area;

Step 7, carries out geometry correction process to the filtered sub-aperture image of space-variant, comprises the steps:

Step 7-1, if imaging pattern exists angle of squint, first by the check point coordinate (x in scene stable coordinates system _m, y _m) be the coordinate (x of coordinate system that direction of visual lines is set up by following Rotating Transition of Coordinate _t, y _t):

$\left[\begin{array}{c}{x}_t\\ y_t\end{array}\right]=\left[\begin{array}{cc}\cos {\mathrm{\θ}}_s& -\sin {\mathrm{\θ}}_s\\ \sin {\mathrm{\θ}}_s& \cos {\mathrm{\θ}}_s\end{array}\right]\left[\begin{array}{c}{x}_M\\ y_M\end{array}\right]---\left(10\right);$

Step 7-2, by (x _t, y _t) change PFA image coordinate (a into ₀₁, a ₁₀);

Step 7-3, finds coordinate (a by interpolation in PFA image ₀₁, a ₁₀) complex pattern value, and put back to check point (x _m, y _m), thus sub-aperture SAR image after obtaining geometry correction;

Step 8, repeats step 2 to step 7, travels through N number of sub-aperture echo data, obtain N number of sub-aperture SAR image;

Step 9, image registration is carried out to N number of sub-aperture SAR image that step 8 obtains, comprises following concrete steps:

Step 9-1, is undertaken having overlapped partitioning to obtain N by each sub-aperture SAR image obtained _x' × N _y' individual sub-block, sub-block pixel size is L _x' × L _y', Azimuth & Range is respectively Δ L to overlaid pixel unit _x, Δ L _y, N _x' for image in orientation to divide block number, N _y' for image is at the block number of distance to division;

Step 9-2, takes out m sub-image of each sub-aperture SAR image, carries out image registration to this N number of sub-image;

Step 9-3, with the sub-block in first sub-aperture SAR image for benchmark, does two-dimensional correlation, i '=2,3 with the sub-block in the i-th ' individual sub-aperture SAR image ..., N;

Step 9-4, find out relevant after peak, peak cycle is displaced to center, record orientation to the unit number Δ x of distance to movement respectively _i' and Δ y _i';

Step 9-5, by m sub-block orientation in the i-th ' individual sub-aperture SAR image to distance to respectively loopy moving Δ x _{i '}with Δ y _{i '}unit number;

Step 9-6, repeats step 9-3 to step 9-5, travels through N number of sub-aperture SAR image;

Step 9-7, takes out the core (L of each sub-aperture SAR image m sub-block _x'-Δ L _x) × (L _y'-Δ L _y), image mosaic is carried out in the relevant position putting back to atom subaperture image;

Step 9-8, repetitive operation step 9-2, to step 9-7, travel through N _x' × N _y' individual sub-block, until the whole registration of all sub-blocks is complete;

Step 10, to the N number of sub-aperture SAR image coherence stack obtained, obtains final SAR image.

Further, the described (x in described step 6-2 _t, y _t) value-acquiring method comprises the steps:

Step 6-2-1, order and the azimuth-range error threshold Δ a of estimation is set, Δ r;

Step 6-2-2, utilizes (6) formula (7) to calculate with

Step 6-2-3, calculates $x_t^{n+1}=a_{01}-f(x_t^n,y_t^n)$ With $y_t^{n+1}=a_{10}-g(x_t^n,y_t^n);$

Step 6-2-4, judges whether to meet with do not meet, then get back to step 6-2-2 and perform, if met, iteration ends.

Beneficial effect: of the present invention full aperture is divided into some sub-aperture, with PFA, sub-aperture process is shown that orientation is to low-resolution image, then sub-aperture image motion error is eliminated with PGA Autofocus processing, by orientation to a liter sampling, geometric distortion is eliminated again through space-variant filtering and geometry correction process, all sub-aperture images geometry correction obtained subsequently have overlapping division sub-block to carry out image registration, are finally concerned with by all sub-aperture images after registration to merge to obtain final SAR image.The present invention will divide sub-aperture, PFA imaging, PGA self-focusing, space-variant filtering, geometry correction, and orientation is to a liter sampling, and sub-aperture image registration, the steps such as the relevant fusion of sub-aperture image effectively combine.The sub-aperture data processing being fritter by the full aperture Data Placement of bulk improves imaging processing efficiency; There is overlapping division sub-block to carry out registration image when carrying out sub-aperture image registration, better can realize the registration of whole image like this, the seamless splicing of sub-block can be realized again after registration.

Accompanying drawing explanation

Fig. 1 is Spotlight SAR Imaging data acquisition geometric model figure;

Wherein, scene center O is defined as true origin, and direction of visual lines floor projection is defined as Y-axis, θ _sfor angle of squint, arbitrfary point P coordinate (x in scene _t, y _t, 0);

Fig. 2 method flow diagram;

The measured data SAR image Local map of Fig. 3 the inventive method process.

Embodiment

The present invention institute in steps, conclusion all uses measured data to verify correctly on IDL, to do further explain below in conjunction with accompanying drawing to the present invention.

The present embodiment utilizes SAR measured data to divide sub-aperture PFA radar imaging method to this and makes checking and analyze.Measured data be by the airborne X-band pulsed radar of certain type over the ground bunching type detect the echo obtained, radar bandwidth is 1.16GHz, carrier frequency is 10GHz, chirp rate is 78956GHz/s, impulse sampling frequency is 1.5GHz, pulse repetition rate PRF is 2200Hz, and the carrier aircraft speed of a ship or plane is 154m/s, carrier aircraft height 5700m.SAR measured data orientation is 16384 to pore size Na, and distance is 32768 to size, and final imaging sizing grid is 16384 × 32768.

Divide sub-aperture PFA imaging radar method to comprise the steps:

Step 1, by N _a× N _rfull aperture radar return Data Placement be N=4 sub-aperture, then each sub-aperture echo data is N _ai× N _r, wherein N _a=16384 is full aperture radar echo pulse number, N _aifor each sub-aperture radar echo pulse number, n _r=32768 is that distance is to sampling number;

Step 2, carries out pulse compression to i-th sub-aperture echo data, obtains orientation time domain and distance frequency domain data;

Step 3, PFA imaging is carried out to i-th sub-aperture echo data, comprises following concrete steps:

Step 3-1, calculates metric space frequency domain interpolation point position K _y:

Wherein, c is the light velocity, f _cfor the carrier frequency that transmits, f _τfor distance is to frequency domain, θ and be respectively instantaneous azimuth and the angle of pitch of antenna phase center; According to described metric space frequency domain interpolation point position K _y, interpolation is carried out to the metric space frequency domain interpolation point position of each wave impulse of i-th sub-aperture echo data;

Step 3-2, computer azimuth spatial frequency domain interpolation point position K _x:

According to described director space frequency domain interpolation point position K _x, interpolation is carried out to each range unit director space frequency domain interpolation point position of i-th sub-aperture echo data;

Step 3-3, carries out two-dimentional inverse Fourier transform to the data behind orientation and metric space frequency domain interpolation, and obtaining pixel is N _ai× N _rsub-aperture SAR image;

Step 4, carries out PGA Autofocus processing to sub-aperture SAR image, comprises following concrete steps:

Step 4-1,5% range unit selecting energy maximum in sub-aperture SAR image, if the data obtained are N _ai× N _r', N _r'=5%N _r, by selecting the described range unit obtained to carry out ring shift, make the maximum modulus value position of each range unit be positioned at the center of range unit;

Step 4-2, to the range unit through ring shift in the windowing of image area center, window function have rectangular window, quarter window, Hamming window, Hanning window etc.;

Step 4-3, the complex pattern range unit after ring shift windowing is: G _k(x _a) (3)

Wherein, x _a=0,1 ..., N _ai-1, k=1 ..., N _r';

To described G _k(x _a) obtain g as IFFT _k(x _a), calculate described g according to (4) formula _k(x _a) the phase error gradient r of range unit _k(x _a):

$\left\{\begin{array}{cc}{r}_k\left(x_a\right)=g_k(x_a-1)\×{\left[g_k\left(x_a\right)\right]}^{*}& x_a=\mathrm{1,2},...,N_{\mathrm{ai}}-1\\ r_k\left(0\right)=0& x_a=0\end{array}\right.---\left(4\right)$

Wherein, k=1 ..., N _r'; r _k(0)=1, k=1 ..., N _r'; [] * represents complex conjugate, adopts (5) formula estimating phase error Δ φ (x _a):

$\mathrm{\Δ\φ}\left(x_a\right)=\arg \left[\frac{1}{{N_r}^{\′}}\underset{k=1}{\overset{{N_r}^{\′}}{\mathrm{\Σ}}}{r}_k\left(x_a\right)\right]---\left(5\right);$

Step 4-4, carries out phase error compensation according to the phase error that step 4-3 estimates to each range unit processing the SAR image obtained through step 3, obtains new sub-aperture SAR image;

Step 4-5, repeats step 4-1 to step 4-4 to the sub-aperture SAR image newly obtained and carries out 4-6 iteration correction; Wherein, window function width used for windowing in step 4-2 is once pressed 1/2 times of reduction by every iteration;

Step 5, doubly rising sampling to the sub-aperture image orientation after PGA self-focusing to N, to obtain pixel be N _a× N _rsub-aperture SAR image, its concrete steps are: the sub-aperture SAR image after self-focusing to orientation to Fourier transform, then thereafter mend (N-1) N _aiindividual zero, then carry out orientation to inverse Fourier transform;

Step 6, space-variant filtering is carried out to the sub-aperture SAR image after liter sampling, comprises following concrete steps:

Step 6-1, sub-aperture SAR image step 5 obtained is divided into N _x× N _yindividual sub-image, each sub-image size is L _x× L _y, between sub-block, be spaced apart L _sx× L _sy; Wherein N _x=31, N _y=64 are respectively azran descriscent sub-block number, L _x=1024, L _y=512 are respectively azran ion block size, L _sx=(N _ai-L _x)/(N _x-1), L _sy=512 are respectively Azimuth & Range to sub-block interval;

Step 6-2, to each sub-image, by the real space position (x of geometric distortion mapping relations computing center pixel cell _t, y _t):

x _t＝a ₀₁-f(x _t，y _t)

(6)

y _t＝a ₁₀-g(x _t，y _t)

Wherein:

(a in formula ₀₁, a ₁₀) be point target geometric position coordinate in the picture, r _cofor the distance of scene center and aperture center, r _ctfor the distance of impact point and aperture center, y _cfor the distance of scene center and aperture center subpoint in Y-axis, for the angle on scene center and aperture center line and ground, θ _sfor angle of squint; Wherein, (x _t, y _t) value-acquiring method comprises the steps:

Step 6-2-1, order and the azimuth-range error threshold Δ a of estimation is set, Δ r;

Step 6-2-2, utilizes (6) formula (7) to calculate with

Step 6-2-3, calculates $x_t^{n+1}=a_{01}-f(x_t^n,y_t^n)$ With $y_t^{n+1}=a_{10}-g(x_t^n,y_t^n);$

Step 6-2-4, judges whether to meet with do not meet, then get back to step 6-2-2 and perform, if met, iteration ends.

Step 6-3, utilizes the real space position (x of described center pixel unit _t, y _t), structure filter function: $H_{(x_t,y_t)}\left(K_x\right)=\exp (-j{a}_{20}{K_x}^2),$ Wherein:

Orientation is carried out to described sub-image and obtains spatial frequency domain to FFT conversion, be multiplied by after do orientation again and switch back to spatial domain against FFT, obtain filtered sub-image;

Step 6-4, chooses the L at sub-image center after filtering _sx× L _sythe Output rusults of each sub-block filtering, as the output of this sub-block filtering, carries out splicing the image obtaining refocusing by size area;

Step 7, carries out geometry correction process to the filtered sub-aperture image of space-variant, comprises the steps:

Step 7-1, if imaging pattern exists angle of squint, first by the check point coordinate (x in scene stable coordinates system _m, y _m) be the coordinate (x of coordinate system that direction of visual lines is set up by following Rotating Transition of Coordinate _t, y _t):

$\left[\begin{array}{c}{x}_t\\ y_t\end{array}\right]=\left[\begin{array}{cc}\cos {\mathrm{\θ}}_s& -\sin {\mathrm{\θ}}_s\\ \sin {\mathrm{\θ}}_s& \cos {\mathrm{\θ}}_s\end{array}\right]\left[\begin{array}{c}{x}_M\\ y_M\end{array}\right]---\left(10\right);$

Step 7-2, by (x _t, y _t) change PFA image coordinate (a into ₀₁, a ₁₀);

Step 7-3, finds coordinate (a by interpolation in PFA image ₀₁, a ₁₀) complex pattern value, and put back to check point (x _m, y _m), thus sub-aperture SAR image after obtaining geometry correction;

Step 8, repeats step 2 to step 7, travels through N number of sub-aperture echo data, obtain N number of sub-aperture SAR image;

Step 9, image registration is carried out to N number of sub-aperture SAR image that step 8 obtains, comprises following concrete steps:

Step 9-1, is undertaken having overlapped partitioning to obtain N by each sub-aperture SAR image obtained _x' × N _y' individual sub-block, sub-block pixel size is L _x' × L _y', L _x'=1024, L _y'=2048, Azimuth & Range is respectively Δ L to overlaid pixel unit _x=512, Δ L _y=1024, N _x' for image in orientation to divide block number, N _y' for image is at the block number of distance to division;

Step 9-2, takes out m sub-image of each sub-aperture SAR image, carries out image registration to this N number of sub-image;

Step 9-3, with the sub-block in first sub-aperture SAR image for benchmark, does two-dimensional correlation, i '=2,3 with the sub-block in the i-th ' individual sub-aperture SAR image ..., N;

Step 9-4, find out relevant after peak, peak cycle is displaced to center, record orientation to the unit number Δ x of distance to movement respectively _i' and Δ y _i';

Step 9-5, by m sub-block orientation in the i-th ' individual sub-aperture SAR image to distance to respectively loopy moving Δ x _{i '}with Δ y _{i '}unit number;

Step 9-6, repeats step 9-3 to step 9-5, travels through N number of sub-aperture SAR image;

Step 9-7, takes out the core (L of each sub-aperture SAR image m sub-block _x'-Δ L _x) × (L _y'-Δ L _y), image mosaic is carried out in the relevant position putting back to atom subaperture image;

Step 9-8, repetitive operation step 9-2, to step 9-7, travel through N _x' × N _y' individual sub-block, until the whole registration of all sub-blocks is complete;

Step 10, to the N number of sub-aperture SAR image coherence stack obtained, obtains final SAR image.

As can be seen from the present embodiment, the present invention will be divided into sub-aperture, PFA imaging, PGA self-focusing, space-variant filtering, geometry correction, and orientation is to a liter sampling, and sub-aperture image registration, the steps such as the relevant fusion of sub-aperture image effectively combine.The sub-aperture data processing being fritter by the full aperture Data Placement of bulk improves imaging processing efficiency, reduces resource consumption; There is overlapping division sub-block to carry out registration image when carrying out sub-aperture image registration, both better realizing the registration of whole image, realizing again the seamless splicing of sub-block after registration

The above is only the preferred embodiment of the present invention; it should be pointed out that for those skilled in the art, under the premise without departing from the principles of the invention; can also make some improvements and modifications, these improvements and modifications also should be considered as protection scope of the present invention.

Claims (3)

1. one kind divides sub-aperture PFA imaging radar method, it is characterized in that, comprise the steps: first full aperture to be divided into some sub-aperture, with PFA, sub-aperture process is shown that orientation is to low-resolution image, then sub-aperture image motion error is eliminated with PGA Autofocus processing, by orientation to a liter sampling, geometric distortion is eliminated again through space-variant filtering and geometry correction process, all sub-aperture images geometry correction obtained subsequently have overlapping division sub-block to carry out image registration, finally all sub-aperture images after registration are concerned with to merge and obtain final SAR image.

2. one according to claim 1 divides sub-aperture PFA imaging radar method, it is characterized in that, comprises the steps:

Step 1, by N _a× N _rfull aperture radar return Data Placement be N number of sub-aperture, then each sub-aperture echo data is N _ai× N _r, wherein N _afor full aperture radar echo pulse number, N _aifor each sub-aperture radar echo pulse number, n _rfor distance is to sampling number;

Step 2, carries out pulse compression to i-th sub-aperture echo data, obtains orientation time domain and distance frequency domain data;

Step 3, PFA imaging is carried out to i-th sub-aperture echo data, comprises following concrete steps:

Step 3-1, calculates metric space frequency domain interpolation point position K _y:

Wherein, c is the light velocity, f _cfor the carrier frequency that transmits, f _τfor distance is to frequency domain, θ and be respectively instantaneous azimuth and the angle of pitch of antenna phase center; According to described metric space frequency domain interpolation point position K _y, interpolation is carried out to the metric space frequency domain interpolation point position of each wave impulse of i-th sub-aperture echo data;

Step 3-2, computer azimuth spatial frequency domain interpolation point position K _x:

According to described director space frequency domain interpolation point position K _x, interpolation is carried out to each range unit director space frequency domain interpolation point position of i-th sub-aperture echo data;

Step 3-3, carries out two-dimentional inverse Fourier transform to the data behind orientation and metric space frequency domain interpolation, and obtaining pixel is N _ai× N _rsub-aperture SAR image;

Step 4, carries out PGA Autofocus processing to sub-aperture SAR image, comprises the steps:

Step 4-1,5% range unit selecting energy maximum in sub-aperture SAR image, if the data obtained are N _ai× N _r', N _r'=5%N _r, by selecting the described range unit obtained to carry out ring shift, make the maximum modulus value position of each range unit be positioned at the center of range unit;

Step 4-2, to the range unit through ring shift in the windowing of image area center;

Step 4-3, the complex pattern range unit after ring shift windowing is: G _k(x _a) (3)

Wherein, x _a=0,1 ..., N _ai-1, k=1 ..., N _r';

To described G _k(x _a) obtain g as IFFT _k(x _a), calculate described g according to (4) formula _k(x _a) the phase error gradient r of range unit _k(x _a):

$\left\{\begin{array}{cc}{r}_k\left(x_a\right)=g_k(x_a-1)\×{\left[g_k\left(x_a\right)\right]}^{*}& x_a=\mathrm{1,2},...,N_{\mathrm{ai}}-1\\ r_k\left(0\right)=0& x_a=0\end{array}\right.---\left(4\right)$

Wherein, k=1 ..., N _r'; r _k(0)=1, k=1 ..., N _r'; [] * represents complex conjugate, adopts (5) formula estimating phase error Δ φ (x _a):

$\mathrm{\Δ\φ}\left(x_a\right)=\arg \left[\frac{1}{{N_r}^{\′}}\underset{k=1}{\overset{{N_r}^{\′}}{\mathrm{\Σ}}}{r}_k\left(x_a\right)\right]---\left(5\right);$

Step 4-4, carries out phase error compensation according to the phase error that step 4-3 estimates to each range unit processing the SAR image obtained through step 3, obtains new sub-aperture SAR image;

Step 4-5, repeats step 4-1 to step 4-4 to the sub-aperture SAR image newly obtained and carries out 4-6 iteration correction; Wherein, window function width used for windowing in step 4-2 is once pressed 1/2 times of reduction by every iteration;

Step 5, doubly rising sampling to the sub-aperture image orientation after PGA self-focusing to N, to obtain pixel be N _a× N _rsub-aperture SAR image, its concrete steps are: the sub-aperture SAR image after self-focusing to orientation to Fourier transform, then thereafter mend (N-1) N _aiindividual zero, then carry out orientation to inverse Fourier transform;

Step 6, space-variant filtering is carried out to the sub-aperture SAR image after liter sampling, comprises following concrete steps:

Step 6-1, sub-aperture SAR image step 5 obtained is divided into N _x× N _yindividual sub-image, each sub-image size is L _x× L _y, between sub-block, be spaced apart L _sx× L _sy; Wherein N _x, N _ybe respectively azran descriscent sub-block number, L _x, L _ybe respectively azran ion block size, L _sx, L _sybe respectively Azimuth & Range to sub-block interval;

Step 6-2, to each sub-image, by the real space position (x of geometric distortion mapping relations computing center pixel cell _t, y _t):

x _t＝a ₀₁-f(x _t，y _t) (6)

y _t＝a ₁₀-g(x _t，y _t)

Wherein:

(a in formula ₀₁, a ₁₀) be point target geometric position coordinate in the picture, r _cofor the distance of scene center and aperture center, r _ctfor the distance of impact point and aperture center, y _cfor the distance of scene center and aperture center subpoint in Y-axis, for the angle on scene center and aperture center line and ground, θ _sfor angle of squint;

Step 6-3, utilizes the real space position (x of described center pixel unit _t, y _t), structure filter function: $H_{(x_t,y_t)}\left(K_x\right)=\exp (-j{a}_{20}{K_x}^2),$ Wherein:

Orientation is carried out to described sub-image and obtains spatial frequency domain to FFT conversion, be multiplied by after do orientation again and switch back to spatial domain against FFT, obtain filtered sub-image;

Step 6-4, chooses the L at sub-image center after filtering _sx× L _sythe Output rusults of each sub-block filtering, as the output of this sub-block filtering, carries out splicing the image obtaining refocusing by size area;

Step 7, carries out geometry correction process to the filtered sub-aperture image of space-variant, comprises the steps:

Step 7-1, if imaging pattern exists angle of squint, first by the check point coordinate (x in scene stable coordinates system _m, y _m) be the coordinate (x of coordinate system that direction of visual lines is set up by following Rotating Transition of Coordinate _t, y _t):

$\left[\begin{array}{c}{x}_t\\ y_t\end{array}\right]=\left[\begin{array}{cc}\cos {\mathrm{\θ}}_s& -\sin {\mathrm{\θ}}_s\\ \sin {\mathrm{\θ}}_s& \cos {\mathrm{\θ}}_s\end{array}\right]\left[\begin{array}{c}{x}_M\\ y_M\end{array}\right]---\left(10\right);$

Step 7-2, by (x _t, y _t) change PFA image coordinate (a into ₀₁, a ₁₀);

Step 7-3, finds coordinate (a by interpolation in PFA image ₀₁, a ₁₀) complex pattern value, and put back to check point (x _m, y _m), thus sub-aperture SAR image after obtaining geometry correction;

Step 8, repeats step 2 to step 7, travels through N number of sub-aperture echo data, obtain N number of sub-aperture SAR image;

Step 9, image registration is carried out to N number of sub-aperture SAR image that step 8 obtains, comprises following concrete steps:

Step 9-1, is undertaken having overlapped partitioning to obtain N by each sub-aperture SAR image obtained _x' × N _y' individual sub-block, sub-block pixel size is L _x' × L _y', Azimuth & Range is respectively Δ L to overlaid pixel unit _x, Δ L _y, N _x' for image in orientation to divide block number, N _y' for image is at the block number of distance to division;

Step 9-2, takes out m sub-image of each sub-aperture SAR image, carries out image registration to this N number of sub-image;

Step 9-3, with the sub-block in first sub-aperture SAR image for benchmark, does two-dimensional correlation, i '=2,3 with the sub-block in the i-th ' individual sub-aperture SAR image ..., N;

Step 9-4, find out relevant after peak, peak cycle is displaced to center, record orientation to the unit number Δ x of distance to movement respectively _{i '}with Δ y _{i '};

Step 9-5, by m sub-block orientation in the i-th ' individual sub-aperture SAR image to distance to respectively loopy moving Δ x _{i '}with Δ y _{i '}unit number;

Step 9-6, repeats step 9-3 to step 9-5, travels through N number of sub-aperture SAR image;

Step 9-7, takes out the core (L of each sub-aperture SAR image m sub-block _x'-Δ L _x) × (L _y'-Δ L _y), image mosaic is carried out in the relevant position putting back to atom subaperture image;

Step 9-8, repetitive operation step 9-2, to step 9-7, travel through N _x' × N _y' individual sub-block, until the whole registration of all sub-blocks is complete;

Step 10, to the N number of sub-aperture SAR image coherence stack obtained, obtains final SAR image.

3. one according to claim 2 divides sub-aperture PFA imaging radar method, it is characterized in that, the described (x in described step 6-2 _t, y _t) value-acquiring method comprises the steps:

Step 6-2-1, order and the azimuth-range error threshold Δ a of estimation is set, Δ r;

Step 6-2-2, utilizes (6) formula (7) to calculate with

Step 6-2-3, calculates $x_t^{n+1}=a_{01}-f(x_t^n,y_t^n)$ With $y_t^{n+1}=a_{01}-g(x_t^n,y_t^n);$

Step 6-2-4, judges whether to meet with do not meet, then get back to step 6-2-2 and perform, if met, iteration ends.