CN106646471B - Airborne High Resolution SAR imaging method based on orientation space-variant error compensation - Google Patents
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Abstract
The invention belongs to Radar Technology fields, disclose a kind of airborne High Resolution SAR imaging method based on orientation space-variant error compensation；Include: SAR radar receives echo-signal, echo-signal is successively carried out to obtain orientation wave-number domain signal apart from pulse pressure, range migration correction, azimuth Fourier transform；Orientation piecemeal is carried out in each range gate to the orientation wave-number domain signal, each side's seat block signal after obtaining orientation piecemeal；In each side's seat block, its orientation matched filtering function of node-by-node algorithm carries out coarse resolution imaging；It calculates the coarse resolution image and carries out azimuth Fourier transform, the fusion of orientation wave-number spectrum is realized by cyclic shift and splicing, obtains the complete wave-number spectrum of the range gate, then carries out orientation inverse Fourier transform；Each range gate is repeated in and executes orientation piecemeal, spectrum fusion, until obtaining the corresponding processing result of all range gates, to obtain full resolution imaging results.
Description
Technical Field
The invention belongs to the technical field of radars, and particularly relates to an airborne high-resolution SAR imaging method based on azimuth space-variant error compensation, which can be used for airborne high-resolution SAR imaging.
Background
Motion compensation is an important signal processing link for airborne Synthetic Aperture Radar (SAR) imaging. Under the condition of long synthetic aperture high-resolution imaging, the accuracy of motion compensation is crucial to the low-altitude small unmanned aerial vehicle-mounted SAR imaging result, and the influence of the motion compensation is represented as space variability in the distance direction and the azimuth direction. The airborne SAR distance space-variant motion error compensation mainly adopts a two-step compensation method. However, the method has the limitation that when the motion fluctuation of the aircraft is large or the radar works in a high wave band, the influence of the residual azimuth space-variant error on the azimuth focusing is not negligible.
At present, the compensation methods for the azimuth space-variant motion error mainly include a sub-aperture terrain and aperture dependent algorithm (SATA) and a precise terrain and aperture dependent motion compensation method (PTA). SATA is efficient but its accuracy is affected by introducing the assumption that the sub-aperture motion error is constant. The PTA can compensate the azimuth space-variant error more accurately, but the azimuth wave number spectrum process does not consider the residual phase influence, so that the accuracy is still limited.
Disclosure of Invention
The invention provides an airborne high-resolution SAR imaging method based on azimuth space-variant error compensation, which can accurately compensate high-order azimuth space-variant motion errors.
The technical idea of the invention is as follows: and (3) dividing sub-blocks in an azimuth wave number domain after distance pulse pressure and RCMC (range migration correction) are carried out on the original echo by adopting a traditional RD (range Doppler) algorithm, establishing a rough imaging network to calculate an accurate azimuth matching filter function point by point, and carrying out rough resolution imaging. And then, converting the coarse imaging result of each sub-block into an azimuth wave number domain by using azimuth Fourier transform, circularly shifting and sequentially splicing to obtain a complete azimuth wave number spectrum, and finally obtaining a compensated full-resolution SAR image by inverse Fourier transform.
In order to achieve the purpose, the invention is realized by adopting the following technical scheme.
An airborne high-resolution SAR imaging method based on orientation space-variant error compensation comprises the following steps:
step 1, acquiring an echo signal of an airborne high-resolution SAR, and sequentially performing range pulse pressure, range migration correction and azimuth Fourier transform on the echo signal to obtain an azimuth beam domain signal, wherein the azimuth beam domain signal is contained in P range gates; p is a positive integer greater than zero;
step 2, performing azimuth blocking on azimuth beam domain signals in the p-th range gate to obtain Q azimuth sub-block signals in the p-th range gate; wherein, the initial value of P is 1, and P ═ 1., P ], Q is a positive integer greater than zero;
step 3, calculating an azimuth matched filter function corresponding to each data point in the qth azimuth sub-block signal for the qth azimuth sub-block signal in the pth range gate, thereby obtaining azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate; wherein the initial value of Q is 1, and Q ═ 1,. Q ];
step 4, respectively forming the azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate into an azimuth matched filter bank of the qth azimuth sub-block signal in the pth range gate; all data points in the q azimuth sub-block signal in the p-th range gate pass through the azimuth matching filter bank respectively to obtain a time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate, and the time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate is used as a coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate; performing azimuth Fourier transform on the coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate to obtain a beam spectrum of the q azimuth sub-block signal in the p-th range gate;
step 5, adding 1 to the value of Q, and sequentially and repeatedly executing the steps 3-4 until the beam spectrums of the Q azimuth sub-block signals in the p-th range gate are obtained, and shifting and splicing the beam spectrums of the Q azimuth sub-block signals in the p-th range gate to obtain a complete beam spectrum in the p-th range gate;
and 6, adding 1 to the value of P, and sequentially and repeatedly executing the steps 2 to 5 until the complete beam spectrums in the P range gates are obtained, and taking the complete beam spectrums in the P range gates as the airborne high-resolution SAR imaging result.
The technical scheme of the invention has the characteristics and further improvements that:
(1) in the step 1, obtaining the azimuth beam domain signal specifically includes: azimuth wave number domain signal S (K)x，x，r)：
S(Kx，x，r)＝∫exp{-jKrc[Rn(X，x，r)+Δrε(X，x，r)]-jKxX}dX
Wherein x is the azimuth position variable of the data point relative to the beam center, x is the azimuth coordinate of the carrier, r is the pitch variable of the wave number center under the current range gate, and KxAs a function of the azimuthal wavenumber, KrcFor the transform coefficients of the beam domain, Krc4 pi/λ, λ being the wavelength, Rn(X, X, r) is the target slope distance under the current range gate, for the oblique view angle of the carrier, the abscissa of the projection of the beam center on the ground isΔrεIs the residual orientation space-variant error.
(2) In step 2, if the length of the azimuth beam domain signal in the p-th range gate is set to be N, the length N of the azimuth sub-block signal is used during azimuth blockingaThe following conditions are satisfied:
wherein PRF is the pulse repetition frequency, and M is the coarse resolution grid length.
(3) In step 3, calculating an azimuth matching filter function corresponding to each data point, specifically: for data point (x)pR) corresponding to the azimuth-matched filter function Φ (K)x，xpAnd r) is:
Φ(Kx，xp，r)＝Krc[Rn(X*，xp，r)+Δrε(X*)]+KxX*
wherein x is*To settle the phase point, xpR is a constant value under the current range gate for the azimuthal position of the data point relative to the beam center.
(4) In step 4, all data points in the qth azimuth sub-block signal in the pth range gate are respectively passed through the azimuth matched filter bank to obtain a time domain signal after frequency domain filtering of the qth azimuth sub-block signal in the pth range gate, which specifically includes:
for a data point (x) in the qth azimuth sub-block signal within the pth range gatepR) of the frequency domain filtered time domain signal S obtained after passing through the bank of azimuth matched filtersu(x, r) is:
wherein, Kx∈[-ΔKa/2，ΔKa/2]As a function of azimuthal wavenumber, Δ KaWidth of azimuthal wavenumber spectrum, KuIs the center of the azimuthal wavenumber spectrum.
(5) In step 5, shifting and splicing the beam spectrums of the Q azimuth sub-block signals in the p-th range gate to obtain a complete beam spectrum in the p-th range gate, which specifically includes:
after the beam spectrums of Q azimuth sub-block signals in the p-th range gate are obtained, the beam spectrums of the Q azimuth sub-block signals are respectively symmetrical about an origin;
and shifting each beam spectrum in the beam spectrums of the Q azimuth sub-block signals to a position of the wave number spectrum center relative to the whole spectrum width according to the azimuth sequence, and splicing the beam spectrums of the Q azimuth sub-block signals to ensure that the beam spectrums of each azimuth sub-block signal are continuous and are not overlapped with each other.
Compared with the prior art, the invention has the advantages that:
(1) the invention adopts the backward projection processing idea, and compared with the traditional orientation space-variant motion compensation algorithm, the accurate focusing under the condition of high wave band and large motion error can be realized; (2) the invention adopts the idea of block processing, and effectively reduces the operation amount compared with the traditional back projection algorithm.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic flow chart of an airborne high-resolution SAR imaging method based on azimuth space-variant error compensation according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of wave number spectrum shift and splicing adopted in simulation provided by the embodiment of the invention;
FIG. 3 is a schematic diagram of a point target motion error in a first simulation provided by an embodiment of the present invention;
FIG. 4 is a schematic diagram illustrating comparison between an FFBP algorithm and an orientation impulse response curve of each algorithm in a simulation I according to an embodiment of the present invention;
fig. 5 is a schematic diagram of a result of FFBP processing measured data in the second simulation provided in the embodiment of the present invention;
FIG. 6 is a schematic diagram illustrating a comparison between FFBP and various algorithm processing local amplification results in a second simulation provided in the embodiment of the present invention;
FIG. 7 is a schematic diagram comparing the reference scattering point azimuth impulse response curves in the second simulation according to the embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment of the invention provides an airborne high-resolution SAR imaging method based on azimuth space-variant error compensation, which comprises the following steps of:
step 1, acquiring an echo signal of an airborne high-resolution SAR, and sequentially performing range pulse pressure, range migration correction and azimuth Fourier transform on the echo signal to obtain an azimuth beam domain signal, wherein the azimuth beam domain signal is contained in P range gates; p is a positive integer greater than zero.
In the step 1, obtaining the azimuth beam domain signal specifically includes: azimuth wave number domain signal S (K)x，x，r)：S(Kx，x，r)＝∫exp{-jKrc[Rn(X，x，r)+Δrε(X，x，r)]-jKxX}dX
Wherein x is the azimuth position variable of the data point relative to the beam center, x is the azimuth coordinate of the carrier, r is the pitch variable of the wave number center under the current range gate, and KxAs a function of the azimuthal wavenumber, KrcFor the transform coefficients of the beam domain, Krc4 pi/λ, λ being the wavelength, Rn(X, X, r) is the target slope distance under the current range gate, for the oblique view angle of the carrier, the abscissa of the projection of the beam center on the ground isΔrεIs the residual orientation space-variant error.
Step 2, performing azimuth blocking on azimuth beam domain signals in the p-th range gate to obtain Q azimuth sub-block signals in the p-th range gate; wherein P has an initial value of 1, and P ═ 1., P ], Q is a positive integer greater than zero.
In step 2, if the length of the azimuth beam domain signal in the p-th range gate is set to be N, the length N of the azimuth sub-block signal is used during azimuth blockingaThe following conditions are satisfied:
wherein PRF is the pulse repetition frequency, and M is the coarse resolution grid length.
The above-mentioned basis makes the frequency spectrum contained in the azimuth sub-block not larger than the frequency spectrum of the coarse resolution grid, that is, the frequency spectrum of the azimuth sub-block cannot be aliased in the coarse resolution grid.
Step 3, calculating an azimuth matched filter function corresponding to each data point in the qth azimuth sub-block signal for the qth azimuth sub-block signal in the pth range gate, thereby obtaining azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate; wherein the initial value of Q is 1, and Q ═ 1.
In step 3, calculating an azimuth matching filter function corresponding to each data point, specifically: for data point (x)p，rp) Its corresponding orientation matched filter function phi (K)x，xp，rp) Comprises the following steps:
Φ(Kx，xp，rp)＝Krc[Rn(X*，xp，rp)+Δrε(X*)]+KxX*
wherein x is*To settle the phase point, xpR is a constant value under the current range gate for the azimuthal position of the data point relative to the beam center.
In particular, x*To stationary phase points:
X*＝p1y+p2y2+p3y3+x
wherein, a0-a4to be RnThe polynomial fit coefficients of Taylor expansion around X-X ═ 0 are:
Δrε(X)≈a0+a1(X-x)+a2(X-x)2+a3(X-x)3+a4(X-x)4
step 4, respectively forming the azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate into an azimuth matched filter bank of the qth azimuth sub-block signal in the pth range gate; all data points in the q azimuth sub-block signal in the p-th range gate pass through the azimuth matching filter bank respectively to obtain a time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate, and the time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate is used as a coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate; and carrying out azimuth Fourier transform on the coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate to obtain a beam spectrum of the q azimuth sub-block signal in the p-th range gate.
In step 4, all data points in the qth azimuth sub-block signal in the pth range gate are respectively passed through the azimuth matched filter bank to obtain a time domain signal after frequency domain filtering of the qth azimuth sub-block signal in the pth range gate, which specifically includes:
for a data point (x) in the qth azimuth sub-block signal within the pth range gatepR) of the frequency domain filtered time domain signal S obtained after passing through the bank of azimuth matched filtersu(x, r) is:
wherein, Kx∈[-ΔKa/2，ΔKa/2]As a function of azimuthal wavenumber, Δ KaWidth of azimuthal wavenumber spectrum, KuIs the center of the azimuthal wavenumber spectrum.
And 5, adding 1 to the value of Q, and sequentially and repeatedly executing the steps 3-4 until the beam spectrums of the Q azimuth sub-block signals in the p-th range gate are obtained, and shifting and splicing the beam spectrums of the Q azimuth sub-block signals in the p-th range gate to obtain a complete beam spectrum in the p-th range gate.
In step 5, shifting and splicing the beam spectrums of the Q azimuth sub-block signals in the p-th range gate to obtain a complete beam spectrum in the p-th range gate, as shown in fig. 2, specifically including:
after the beam spectrums of Q azimuth sub-block signals in the p-th range gate are obtained, the beam spectrums of the Q azimuth sub-block signals are respectively symmetrical about an origin;
and shifting each beam spectrum in the beam spectrums of the Q azimuth sub-block signals to a position of the wave number spectrum center relative to the whole spectrum width according to the azimuth sequence, and splicing the beam spectrums of the Q azimuth sub-block signals to ensure that the beam spectrums of each azimuth sub-block signal are continuous and are not overlapped with each other.
And 6, adding 1 to the value of P, and sequentially and repeatedly executing the steps 2 to 5 until the complete beam spectrums in the P range gates are obtained, and taking the complete beam spectrums in the P range gates as the airborne high-resolution SAR imaging result.
The effect of the present invention can be further illustrated by the following simulation experiments:
1) simulation conditions are as follows:
the point target simulation parameters of the invention are shown in table 1:
TABLE 1 Point target simulation parameters
The motion parameters are obtained by calculation according to the measured aircraft inertial navigation record, as shown in fig. 3.
2. Simulation content and result analysis:
simulation 1: the wave number center point is subjected to one-dimensional imaging under oblique angles of 0 degree and 5 degrees by using the method of the invention and compared with the processing results of TWO-STEP, PTA and SATA algorithms, as shown in FIG. 4, the Peak Side Lobe Ratio (PSLR), the Integrated Side Lobe Ratio (PSLR) and the Impulse Response Width (IRW) are respectively used as the evaluation standard quantization comparison processing results as shown in tables 2 and 3.
Table 2 simulation-quantitative analysis results at 0 degree oblique view
TABLE 3 simulation of quantitative analysis results at 5 degree oblique view
Simulation 2: the method of the invention is used for imaging in a front side view stripe mode and comparing with Two-step, PTA and SATA algorithm results. The simulation parameters are the same as simulation one, the data size is 8192 x 16384, and the intercepted part of the processing result is shown in fig. 5. Fig. 6 shows a selected image of scene 1, scene 2 and other algorithm processing results. Two scattering points A, B are selected and compared with their orientation impulse response function as shown in FIG. 7, and their quantitative statistics are shown in tables 4 and 5.
TABLE 4 simulation two scattering point A quantitative analysis results
TABLE 5 simulation two scattering point B quantitative analysis results
3. And (3) simulation result analysis:
as can be seen from tables 2 and 3, the PLSR, ISLR and IRW values of the method provided by the invention are all smaller than those of other algorithms, so that the effect is best.
From fig. 6, it can be found that the processing effect of the "Two-Step" motion compensation is the worst, and the processing results of PTA and SATA have different defocus phenomena, because the residual uncompensated motion error exists at the edge point of the block, while the method provided by the present invention has the best effect, and the processing result of the isolated point A, B also verifies the performance of the method provided by the present invention.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the appended claims.
Claims (6)
1. An airborne high-resolution SAR imaging method based on orientation space-variant error compensation is characterized by comprising the following steps:
step 1, acquiring an echo signal of an airborne high-resolution SAR, and sequentially performing range pulse pressure, range migration correction and azimuth Fourier transform on the echo signal to obtain an azimuth beam domain signal, wherein the azimuth beam domain signal is contained in P range gates; p is a positive integer greater than zero;
step 2, performing azimuth blocking on azimuth beam domain signals in the p-th range gate to obtain Q azimuth sub-block signals in the p-th range gate; wherein, the initial value of P is 1, and P ═ 1., P ], Q is a positive integer greater than zero;
step 3, calculating an azimuth matched filter function corresponding to each data point in the qth azimuth sub-block signal for the qth azimuth sub-block signal in the pth range gate, thereby obtaining azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate; wherein the initial value of Q is 1, and Q ═ 1,. Q ];
step 4, respectively forming the azimuth matched filter functions corresponding to all data points in the qth azimuth sub-block signal in the pth range gate into an azimuth matched filter bank of the qth azimuth sub-block signal in the pth range gate; all data points in the q azimuth sub-block signal in the p-th range gate pass through the azimuth matching filter bank respectively to obtain a time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate, and the time domain signal subjected to frequency domain filtering of the q azimuth sub-block signal in the p-th range gate is used as a coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate; performing azimuth Fourier transform on the coarse resolution imaging result of the q azimuth sub-block signal in the p-th range gate to obtain a beam spectrum of the q azimuth sub-block signal in the p-th range gate;
step 5, adding 1 to the value of Q, and sequentially and repeatedly executing the steps 3 to 4 until the beam spectrums of the Q azimuth sub-block signals in the p-th range gate are obtained, and shifting and splicing the beam spectrums of the Q azimuth sub-block signals in the p-th range gate to obtain a complete beam spectrum in the p-th range gate;
and 6, adding 1 to the value of P, and sequentially and repeatedly executing the steps 2 to 5 until the complete beam spectrums in the P range gates are obtained, and taking the complete beam spectrums in the P range gates as the airborne high-resolution SAR imaging result.
2. The method as claimed in claim 1, wherein the step 1 is performed by using an airborne high-resolution SAR imaging method based on the compensation of the orientation space-variant errorThe obtaining of the azimuth beam domain signal specifically includes: azimuth beam domain signal S (K)x,x,r)：
S(Kx,x,r)＝∫exp{-jKrc[Rn(X,x,r)+△rε(X,x,r)]-jKxX}dX
Wherein, X is the azimuth position variable of the data point relative to the beam center, X is the azimuth coordinate of the carrier, r is the beam center slant distance variable under the p-th range gate, and KxFor azimuthal beam variation, KrcFor the transform coefficients of the beam domain, Krc4 pi/λ, λ being the wavelength, Rn(X, X, r) is the target slope distance under the p-th range gate, for the oblique view angle of the carrier, the abscissa of the projection of the beam center on the ground is△rεAnd (X, X, r) is residual azimuth space-variant error.
3. The method as claimed in claim 1, wherein in step 2, if the length of the azimuth beam domain signal in the p-th range gate is N, the length N of each azimuth sub-block signal obtained when the azimuth beam domain signal in the p-th range gate is subjected to azimuth blocking is set as NaThe following conditions are satisfied:
wherein PRF is the pulse repetition frequency, and M is the coarse resolution grid length.
4. The airborne height based on the compensation of the orientation space-variant error according to claim 2The SAR resolution imaging method is characterized in that in the step 3, an orientation matching filter function corresponding to each data point is calculated, and the method specifically comprises the following steps: for a data point (x) in the qth azimuth sub-block signal within the pth range gatepR) corresponding to the azimuth-matched filter function Φ (K)x,xpAnd r) is:
Φ(Kx,xp,r)＝Krc[Rn(X*,xp,r)+△rε(X*,xp,r)]+KxX*
wherein X is a stationary phase point, XpAnd r is the azimuth position of the data point relative to the beam center, and is a beam center slope distance variable under the p-th range gate, and is a constant value under the p-th range gate.
5. The method according to claim 4, wherein in step 4, all data points in the q-th azimuth sub-block signal in the p-th range gate are respectively passed through the azimuth matched filter bank to obtain a time domain signal after frequency domain filtering of the q-th azimuth sub-block signal in the p-th range gate, specifically:
for a data point (x) in the qth azimuth sub-block signal within the pth range gatepR) of the frequency domain filtered time domain signal S obtained after passing through the bank of azimuth matched filtersu(x, r) is:
wherein, KxFor azimuthal beam variation, Kx∈[-△Ka/2,△Ka/2]，△KaFor the spectral width of the azimuth beam, KuFor the azimuth beam spectrum center, e represents the belongings.
6. The method according to claim 5, wherein in step 5, the beam spectrums of the Q azimuth sub-block signals within the p-th range gate are shifted and spliced to obtain a complete beam spectrum within the p-th range gate, and specifically includes:
after the beam spectrums of Q azimuth sub-block signals in the p-th range gate are obtained, the beam spectrums of the Q azimuth sub-block signals are respectively symmetrical about an origin;
and shifting each beam spectrum in the beam spectrums of the Q azimuth sub-block signals to a position of the center of the beam spectrum relative to the whole spectrum width according to the azimuth sequence, and splicing the beam spectrums of the Q azimuth sub-block signals to ensure that the beam spectrums of each azimuth sub-block signal are continuous and are not overlapped.
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