CN102788972A - Self-focusing method suitable for ultra-high-resolution SAR (synthetic aperture radar) imaging - Google Patents

Abstract

The invention relates to a self-focusing of method suitable for the ultra-high-resolution SAR (synthetic aperture radar) imaging, comprising the following steps: (1) subjecting the two-dimensional echo data to polar coordinate format conversion; (2) reducing the resolution of the distance vector; (3) carrying out coarse-resolution imaging to the distance vector; (4) estimating the azimuth phase error; (5) calculating the residual range migration; (6) compensating the residual error; and (7) carrying out two-dimensional Fourier transform for imaging.

Description

A kind of auto-focus method that is applicable to ultrahigh resolution SAR imaging

Technical field

The present invention relates to a kind of synthetic-aperture radar (synthetic aperture radar is called for short SAR) imaging signal processing method, particularly relate to a kind of synthetic-aperture radar autofocus algorithm.

Background technology

Synthetic-aperture radar (Synthetic Aperture Radar is called for short SAR) is a kind of New System radar that ground scenery is carried out to picture through signal processing technology.SAR realizes through distance and bearing bidimensional high-resolution the imaging of target; Wherein distance obtains through broadband signal is carried out process of pulse-compression to high resolving power; The orientation high resolving power is then handled through the synthetic aperture data are concerned with and is realized that this relevant processing depends on the instantaneous relative position information of accurately knowing between radar and the target.In the practical application, receive factor affecting such as radar site disturbance and electromagnetic wave propagation medium be inhomogeneous, this coherence often is difficult to directly be guaranteed.The major measure of taking at present is to increase auxiliary motion measurement unit (typically like Inertial Measurement Unit and GPS) to measure and obtain radar site information, and ignores the uneven influence of propagation medium.Yet; Raising along with imaging resolution; The positional information precision that motion measurement unit provides possibly still can't satisfy coherence's requirement; And the inhomogeneous radar return delay error effect that causes of propagation medium also becomes and can not ignore, with the critical limitation factor that becomes the vernier focusing imaging of following ultrahigh resolution SAR system gradually.Therefore be necessary to study the way from radar return extracting data and compensating error, i.e. auto-focus method.

The delay error of echo forms images to SAR has the influence of two aspects: the one, can produce extra range migration, and in the SAR imaging process, can't be compensated, the 2nd, can cause image orientation to defocus in the orientation to introducing a phase error.When delay error less; The additional distance migration that produces is less than one during apart from resolution element; This residual range migration effect can be ignored fully; The phase error of orientation one dimension only need estimated and compensate to self-focusing this moment, and this also is the prerequisite that present nearly all autofocus algorithm (typical algorithm such as sub-aperture algorithm MD, phase difference algorithm PD, phase gradient autofocus algorithm PGA, eigenvalue method and based on the algorithm of image criterion etc.) is supposed, like document 1 (Mancill; C.E., and J.M.Swiger.A Map DriftAutofocus Technique for Correcting High Order SAR Phase Errors.27 ^ThAnnual Tri-Service Radar Symposium.Record, Monterey, CA; 1981, pp.391-400.), document 2 (G.N.Yoji.Phase Difference Auto Focusing for SyntheticAperture Radar Imaging.United States Patent No.4999635,1991.), document 3 (Wahl; D.E., P.H.Eichel, D.C.Ghiglia; And C.V.Jakowatz; Jr.Phase Gradient Autofocus-A Robust Tool for High Resolution SAR PhaseCorrection.IEEE Transaction on Aerospace and Electronic Systems, 30 (3), 1994; Pp.827-834.) and document 4 (C.V.Jakowatz; Jr., D.E.Wahl.Eigenvector Method for Maximum-likelihood Estimation of Phase Errors in SyntheticAperture Radar Imagery.J.Opt.Soc.Am.A., 10 (12); 1993, pp.2539-2546.) in disclosed technology.Yet; Along with the increase of error, when especially imaging resolution is high especially, residual range migration span will become inevitable from the unit; Compensation when therefore, effectively autofocus algorithm must be considered residual range migration and orientation phase error with this understanding.Document 5 (D.W.Warner, D.C.Ghiglia, A.FitzGerrel; J.Beaver.Two-dimensional Phase Gradient Autofocus.Proceedings of SPIE; Vol.4123,2000, disclose in pp.162-173.) traditional one dimension phase gradient autofocus algorithm (PGA) has been expanded to bidimensional; Having proposed bidimensional phase gradient autofocus algorithm (2-D PGA) attempts to address this problem; But said in conclusion as the article author, this method will reach practical as one dimension PGA, still exists many problems to need to solve.

Proofread and correct problem when at present, also not having proven technique can solve residual range migration and orientation phase error in the prior art.

Summary of the invention

The objective of the invention is to solve correction problem when residual range migration is with the orientation phase error in the ultrahigh resolution SAR imaging.

To achieve these goals, the present invention provides a kind of auto-focus method that is applicable to ultrahigh resolution SAR imaging, comprises the steps:

(1) the bidimensional echo data is carried out the polar format conversion;

(2) reduce distance to resolution;

(3) distance forms images to coarse resolution;

(4) estimate the orientation phase error;

(5) calculate residual range migration;

(6) residual error compensation;

(7) bidimensional Fourier transform imaging.

Wherein, step (1) is through two one dimension interpolation of distance and bearing, realizes the conversion of data layout from polar coordinates to the rectangular coordinates.

Step (2) be data after step (1) is handled in distance to carrying out the frequency spectrum intercepting; Reduce distance to resolution; It is in order to prevent that step (1) from handling the residual target span in back and separating and distinguish the unit follow-up orientation phase error estimation and phase error of influence of walking about to the purpose of resolution that the frequency spectrum intercepting reduces distance; Therefore the ratio of frequency spectrum intercepting depends on that residual range migration strides the number of resolution element, and for example residual range migration is crossed over N resolution element, and then intercepting ratio should be greater than 1/N; After guaranteeing that resolution reduces, residual range migration can be ignored.

Step (3) is that the data after step (2) is handled are done bidimensional fast Flourier (Fourier) conversion (FFT), is embodied as picture.

Step (4) is step (3) to be handled the conventional autofocus algorithm of imagery exploitation (like the phase gradient autofocus algorithm) that obtains handle, and estimates to obtain the orientation phase error.

Step (5) is according to the analytic relationship between residual range migration and the orientation phase error, utilizes step (4) to estimate that the orientation phase error that obtains directly calculates the estimated value of residual range migration.

Step (6) is residual range migration and an orientation phase error of utilizing estimation to obtain, and the data after step (1) is handled are carried out the residual error compensation.

Step (7) is that data were made bidimensional FFT after step (6) was handled the compensation that obtains, and realizes the vernier focusing imaging.

Compared with prior art, the invention has the beneficial effects as follows:

(1) only to compensate the orientation phase error different with conventional autofocus algorithm, compensation when the present invention realizes residual range migration and orientation phase error, and compensation precision is higher.

(2) the residual range migration method of relevant estimation with sub-aperture (or pulse) is compared, and the present invention directly calculates residual range migration, and counting yield is higher, and the estimated value of residual range migration derives from the orientation phase error estimation and phase error simultaneously, so estimated accuracy is also higher.

Description of drawings

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

Fig. 2 is signal processing flow figure of the present invention.

Fig. 3 after to be data through PFA handle apart from compressed image (part).

Fig. 4 be adopt after the inventive method compensation apart from compressed image (part).

Fig. 5 is measured data image after polar format algorithm (PFA) is handled.

Fig. 6 is the result after the PFA image is handled through conventional self-focusing (adopting the phase gradient autofocus algorithm).

Fig. 7 adopts the inventive method to handle the final imaging results that obtains.

Embodiment

Below in conjunction with accompanying drawing and embodiment the present invention is done further detailed explanation.

Spotlight SAR Imaging imaging geometry model is as shown in Figure 1, is without loss of generality, and supposes that radar is operated in strabismus mode, and the angle of squint is θ ₀, the corresponding θ of positive side-looking ₀=0.Texas tower is with the horizontal unaccelerated flight of speed v, and antenna phase center (APC) instantaneous position coordinate is (x ₀+ vt, y ₀, z ₀), wherein t represents the orientation time, at aperture center moment t=0, θ ₀+ θ with

Be respectively the instantaneous azimuth and the angle of pitch of antenna phase center, equal θ constantly respectively in aperture center ₀With Supposing again has a point target in the scene, its position is (x _p, y _p).Antenna phase center is designated as R respectively to the instantaneous distance of this target and scene center _tAnd R _a

Radar return, can be expressed as after matched filtering and orientation motion compensation through demodulation, distance

$S(t,f_{\mathrm{\τ}})=\exp \left\{j\frac{4\mathrm{\π}}{c}(f_0+f_{\mathrm{\τ}})R_{\mathrm{\Δ}}\right\}---\left(1\right)$

Wherein c is a radio wave propagation speed, f ₀Be the carrier frequency that transmits, f _τFor the distance to frequency, its effective range does

B is a transmitted signal bandwidth, and respective distances is to resolution

R _Δ=R _a-R _tBe the difference distance of APC to scene center and target.

Method provided by the invention is handled to echo data shown in the formula (1), and final goal is the vernier focusing image that obtains point target, and its treatment scheme is as shown in Figure 2, and concrete performing step comprises:

(1) distance of polar format conversion is to interpolation

Polar format algorithm (PFA) distance is equivalent to the frequency of adjusting the distance to interpolation and makes a following change of scale on mathematics

$f_{\mathrm{\τ}}=\mathrm{\δ}{\stackrel{\·}{f}}_{\mathrm{\τ}}+f_0(\mathrm{\δ}-1)---\left(2\right)$

Wherein

is the frequency of distance after the conversion, and

is scale factor.Therefore, after interpolation, signal can be expressed as through distance

Wherein

is the function of orientation time t, can do following Taylor expansion about t to it

r(t)＝y′ _p+x′ _pt+ε(t) (4)

Y ' wherein _p+ x ' _pT is a constant term and once, and the reflection target is the position in the image in the end, receives visual angle conversion, inclination of wave front and other error effect, x ' _p, y ' _pMight not equal x _p, y _p, but this does not influence image focusing, can be to its correction through the resampling realization of image area, and the present invention does not discuss; ε (t) is secondary and the above higher order term of secondary, is the key factor that causes image defocus.

(2) orientation of polar format conversion is to interpolation

The orientation interpolation come down to adjust the distance data after the interpolation in the orientation time do nonlinear transformation

wherein

$\mathrm{\θ}(\stackrel{\·}{t},{\stackrel{\·}{f}}_{\mathrm{\τ}})=\frac{f_0\stackrel{\·}{t}}{(f_0+{\stackrel{\·}{f}}_{\mathrm{\τ}})-0.5\mathrm{\Ω}\sin \left(2{\mathrm{\θ}}_0\right)f_0\stackrel{\·}{t}}---\left(5\right)$

T is the orientation time variable after the conversion in the formula, Ω=v/y ₀

The signal that therefore, can obtain after the orientation interpolation does

Wherein

is constant term, and

is the bidimensional phase error.

Be the analytical error effect, can error term be made following Taylor expansion about frequency of distance

$g(\stackrel{\·}{t},{\stackrel{\·}{f}}_{\mathrm{\τ}})={\mathrm{\φ}}_0\left(\stackrel{\·}{t}\right)+{\mathrm{\φ}}_1\left(\stackrel{\·}{t}\right){\stackrel{\·}{f}}_{\mathrm{\τ}}+{\mathrm{\φ}}_2\left(\stackrel{\·}{t}\right){\stackrel{\·}{f}}_{\mathrm{\τ}}^2+\·\·\·---\left(7\right)$

Can solve

Wherein is residual orientation phase error,

corresponding residual range migration (residual range migration amount

).

According to formula (8) and formula (9), can obtain residual range migration item

and have relation as follows with orientation phase error

${\mathrm{\φ}}_1\left(\stackrel{\·}{t}\right)=\frac{1}{f_0}\{{\mathrm{\φ}}_0\left(\stackrel{\·}{t}\right)-\frac{\stackrel{\·}{t}}{{[1-0.5\mathrm{\Ω}\sin \left(2{\mathrm{\θ}}_0\right)\stackrel{\·}{t}]}^2}\frac{d{\mathrm{\φ}}_0\left[{\mathrm{\θ}}^{-1}\left(t\right)\right]}{\mathrm{dt}}{|}_{t=\mathrm{\θ}(\stackrel{\·}{t},0)}\}---\left(10\right)$

Wherein

is the inverse mapping of

, and its expression formula can obtain as follows according to formula (5):

${\mathrm{\θ}}^{-1}\left(t\right)=\frac{t}{1+0.5\mathrm{\Ω}\sin \left(2{\mathrm{\θ}}_0\right)t}---\left(11\right)$

(3) reduce distance to resolution apart from the frequency spectrum intercepting

With the data after the polar format conversion in distance to carrying out the frequency spectrum intercepting; Reduce range resolution, the purpose that the frequency spectrum intercepting reduces range resolution is in order to prevent that residual target from striding the resolution element follow-up orientation phase error estimation and phase error of influence of walking about, so the ratio of frequency spectrum intercepting depends on that residual range migration strides the number of resolution element; For example residual range migration is crossed over N resolution element; Then intercepting ratio should be greater than 1/N, and after guaranteeing that resolution reduces, residual range migration can be ignored.Keep centre frequency constant during intercepting; Like the intercepting ratio is 1/N, and then the scope of frequency of distance is after the intercepting

(4) bidimensional FFT imaging

After the polar format conversion through step (1) and step (2); Obtained the quadrature bidimensional sampling of object scene frequency spectrum; Step (3) reduce resolution apart from the frequency spectrum intercepting after, again through behind distance and bearing bidimensional FFT, just obtaining the orientation full resolution distance coarse resolution image of object scene.

(5) estimate the orientation phase error

The image that step (4) obtains, range resolution is lower, and residual range migration is no longer striden resolution element, therefore can ignore.Can utilize this moment conventional autofocus algorithm (like the phase gradient autofocus algorithm) to carry out the orientation phase error estimation and phase error, the phase error of supposing to estimate to obtain is for

(6) calculate residual range migration

According to relation shown in the formula (10), can utilize step (5) to estimate that the orientation phase error that obtains directly calculates residual range migration item

${\widehat{\mathrm{\φ}}}_1\left(\stackrel{\·}{t}\right)=\frac{1}{f_0}\{{\widehat{\mathrm{\φ}}}_0\left(\stackrel{\·}{t}\right)-\frac{\stackrel{\·}{t}}{{[1-0.5\mathrm{\Ω}\sin \left(2{\mathrm{\θ}}_0\right)\stackrel{\·}{t}]}^2}\frac{d{\widehat{\mathrm{\φ}}}_0\left[{\mathrm{\θ}}^{-1}\left(t\right)\right]}{\mathrm{dt}}{|}_{t=\mathrm{\θ}(\stackrel{\·}{t},0)}\}---\left(12\right)$

(7) residual error compensation

Data after utilizing step (5) and step (6) to estimate to obtain orientation phase error and residual range migration step (2) is handled compensate, and promptly formula (6) multiply by following compensation rate

$S_E(\stackrel{\·}{t},{\stackrel{\·}{f}}_{\mathrm{\τ}})=\exp \{-j[{\widehat{\mathrm{\φ}}}_0\left(\stackrel{\·}{t}\right)+{\widehat{\mathrm{\φ}}}_1\left(\stackrel{\·}{t}\right){\stackrel{\·}{f}}_{\mathrm{\τ}}\left]\right\}---\left(13\right)$

Therefore the signal after can being compensated does

(8) bidimensional FFT imaging

If orientation phase error and the residual range migration estimated are enough accurate; Then residual phase error term

can be ignored usually, so formula (14) can be approximated to be

At this moment, the signal after the compensation is a bidimensional FFT, just can realizes vernier focusing imaging target

F representes bidimensional Fourier conversion, sinc in the formula _a(u)=sin (π B _dU)/(π B _dU) be that the orientation is to Sinc function, sinc _r(x)=sin (π B _τX)/(π B _τX) be that distance is to Sinc function, B _dAnd B _τBe respectively azran descriscent signal bandwidth.Following formula also can be write as the form of spatial domain coordinate

Where

and

denote the direction and distance of airspace coordinates.

Utilize measured data to handle the autofocus algorithm that the present invention is proposed and verify that experimental result has fully proved validity of the present invention.

Measured data is certain high-resolution air-borne test SAR admission.This experimental radar works in X-band, and transmitted signal bandwidth is 1.18GHz, and respective distances is superior to 0.15m to theoretical resolution, adopts beam bunching mode admission data, and effective length of synthetic aperture is 2300m during processing, and counterparty's potential theory resolution is about 0.067m.Because this radar instantaneous position information of not having the motion measurement unit to provide, thus PFA when handling the hypothesis radar do linear uniform motion.Fig. 3 has provided data after handling through PFA apart from compressed image (part), can see clearly that from figure though passed through the range migration correction of PFA, target is still residual to have very big span to walk about from the unit, must consider it is compensated.Fig. 4 be adopt after the inventive method compensation apart from compressed image, can see that the range migration of object residue is all effectively compensated.Further; Fig. 7 has provided and has adopted the inventive method to handle the final imaging results that obtains; In order to contrast, provided the result after PFA image and PFA image are handled through conventional self-focusing (adopting the phase gradient autofocus algorithm here) simultaneously, respectively like Fig. 5 and shown in Figure 6.Because residual range migration and phase error are all very big; Therefore the PFA image two dimension clearly occurred and has defocused; Conventional autofocus algorithm only is applicable to the phase error compensation under the negligible situation of residual range migration; Therefore also unsatisfactory to the result of these data, and after adopting the inventive method to handle, imaging effect obtains very significantly improving.

Claims (8)

1. an auto-focus method that is applicable to ultrahigh resolution SAR imaging comprises the steps:

(1) the bidimensional echo data is carried out the polar format conversion;

(2) reduce distance to resolution;

(3) distance forms images to coarse resolution;

(4) estimate the orientation phase error;

(5) calculate residual range migration;

(6) residual error compensation;

(7) bidimensional Fourier transform imaging.

2. auto-focus method according to claim 1, wherein step (1) is through two one dimension interpolation of distance and bearing, realizes the conversion of data layout from polar coordinates to the rectangular coordinates.

3. auto-focus method according to claim 1, wherein step (2) be data after step (1) is handled in distance to carrying out the frequency spectrum intercepting, reduce distance to resolution.

4. auto-focus method according to claim 1, wherein step (3) is that the data after step (2) processing are done the bidimensional FFT, is embodied as picture.

5. auto-focus method according to claim 1, wherein step (4) is step (3) to be handled the conventional autofocus algorithm of the imagery exploitation that obtains handle, and estimates to obtain the orientation phase error.

6. auto-focus method according to claim 1, wherein step (5) is according to the analytic relationship between residual range migration and the orientation phase error, utilizes step (4) to estimate that the orientation phase error that obtains directly calculates the estimated value of residual range migration.

7. auto-focus method according to claim 1, wherein step (6) is residual range migration and an orientation phase error of utilizing estimation to obtain, the data after step (1) is handled are carried out the residual error compensation.

8. auto-focus method according to claim 1, wherein step (7) is that data were made the bidimensional Fourier transform after step (6) was handled the compensation that obtains, and realizes the vernier focusing imaging.

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