CN113341413A - High-resolution squint SAR echo Doppler spectrum recovery method and system - Google Patents
High-resolution squint SAR echo Doppler spectrum recovery method and system Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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- G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
- G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
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- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
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Abstract
The invention discloses a high-resolution squint SAR echo Doppler spectrum recovery method and a high-resolution squint SAR echo Doppler spectrum recovery system. Firstly, acquiring original echo data, and performing range-wise fast Fourier transform on the original echo data to obtain range-wise FFT data; then, performing range-wise pulse compression on the range-wise FFT data to obtain pulse compressed data; then, Doppler frequency shift elimination is carried out on the pulse compression data to obtain Doppler frequency shift elimination data; performing azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data; carrying out azimuth-direction linear frequency modulation Z conversion on the deramp processing data to obtain CZT data; and performing Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum, and subsequently obtaining a high-resolution SAR image by using a frequency domain imaging algorithm. The method avoids interpolation operation, simplifies operation, ensures imaging quality and has universality.
Description
Technical Field
The invention relates to the technical field of SAR imaging, in particular to a high-resolution squint SAR echo Doppler spectrum recovery method and system.
Background
Synthetic Aperture Radar (SAR) is a microwave remote sensing device with two-dimensional high resolution. By controlling the direction of the antenna beam, the SAR can effectively break through the limitation of the size of the radar antenna on the azimuth resolution. However, in practical applications, the Pulse Repetition Frequency (PRF) of the system is generally limited, and the total doppler bandwidth of the signal is much larger than the PRF of the system during the processing of the full-aperture high-resolution SAR signal. Meanwhile, errors of attitude control and beam pointing of the satellite-borne SAR can introduce a small squint angle, and the existence of the squint angle can aggravate distance and azimuth coupling of signals to cause displacement and distortion of a frequency spectrum. As known from nyquist sampling theorem, the doppler spectrum of the full aperture signal is aliased, and in order to apply an efficient and accurate frequency domain imaging algorithm, the two-dimensional spectrum without aliasing must be restored to obtain a high-resolution SAR image.
The methods widely used today for dealing with doppler spectrum aliasing include a sub-aperture method and a two-step method. The sub-aperture method avoids spectrum aliasing by dividing radar echo data into a plurality of sub-apertures in the azimuth direction and then processing the sub-apertures separately, but the division and recombination of the data increases algorithm complexity and reduces focused image quality. The two-step method mainly aims at full-aperture SAR data processing, and increases Doppler domain sampling interval by distance space invariant azimuth deramp and oversampling signals, so as to recover a spectrum without aliasing. An improved two-step method adopts a distance space-variant azimuth deramp to make up for the defects of the traditional two-step method, but does not consider the condition of oblique view angle, and causes non-uniform sampling of a Doppler domain, so that a frequency domain algorithm cannot be applied subsequently to obtain a high-quality image.
Disclosure of Invention
The invention provides a high-resolution squint SAR echo Doppler frequency spectrum recovery method and a high-resolution squint SAR echo Doppler frequency spectrum recovery system, which avoid interpolation operation, are simple in operation and ensure imaging quality.
In order to achieve the purpose, the invention provides the following scheme:
a high resolution squint SAR echo doppler spectrum recovery method, the method comprising:
acquiring original echo data;
performing range-wise fast Fourier transform on the original echo data to obtain range-wise FFT data;
performing range direction pulse compression on the range direction FFT data to obtain pulse compression data;
performing Doppler frequency shift elimination on the pulse compression data to obtain Doppler frequency shift elimination data;
performing azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data;
carrying out azimuth linear frequency modulation Z conversion on the deramp processing data to obtain CZT data;
and performing Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
Preferably, the doppler shift cancellation is performed on the pulse compression data to obtain doppler shift cancelled data, and the specific formula is as follows:
α1(ta,fr)＝α0(ta,fr)exp[-j2πfdc(fr)ta]；
wherein alpha is1(ta,fr) For removing data by Doppler shift, alpha0(ta,fr) In order to pulse-compress the data,
tais azimuth time, frIs the range frequency, fdc(fr) Is the doppler center frequency as a function of instantaneous range frequency introduced by the squint angle.
Preferably, the performing a deramp process on the doppler shift cancellation data to obtain a deramp process data specifically includes:
multiplying the Doppler frequency shift elimination data by a quadratic phase function of distance space-variant in the azimuth direction to obtain quadratic phase multiplication data;
carrying out azimuth zero padding on the secondary phase multiplication data, and then carrying out azimuth fast Fourier transform to obtain azimuth FFT data;
and performing residual secondary phase multiplication on the azimuth FFT data to obtain a deramp processing data.
Preferably, the doppler shift cancellation data is multiplied by a quadratic phase function of the distance space-variant in the azimuth direction, and the specific formula is as follows:
α2(ta,fr)＝α1(ta,fr)exp[-jπK(fr)ta 2]；
wherein alpha is2(ta,fr) For multiplying data by secondary phase, alpha1(ta,fr) In order to remove the data for the doppler shift,
tais azimuth time, frIs the range frequency, K (f)r) Is the doppler shift frequency that varies with the instantaneous range frequency.
Preferably, after performing azimuth zero padding on the secondary phase multiplied data, performing azimuth fast fourier transform to obtain azimuth FFT data, where the specific formula is:
α0(t′a,fr)＝FFT{zero-padding[α2(ta,fr)]}；
wherein alpha is0(t′a,fr) For the azimuthal FFT data, α2(ta,fr) Is the secondary phase multiplied data, t'aFor the zero-filled azimuth variable, taIs azimuth time, frIs the range frequency.
Preferably, the residual quadratic phase multiplication is performed on the azimuth FFT data to obtain a deramp processing data, and the specific formula is:
α1(t′a,fr)＝α0(t′a,fr)exp[-jπK(fr)(t′a)2]；
wherein alpha is1(t′a,fr) Processing data for deramp, α0(t′a,fr) In order to be the azimuth-direction FFT data,
t′afor the zero-filled azimuth variable, K (f)r) For Doppler frequency modulation, f, as a function of instantaneous range frequencyrIs the range frequency.
Preferably, the azimuth chirp Z transform CZT is performed on the deramp processing data to obtain CZT data, and the specific formula is as follows:
wherein alpha is0(fa,fr) As CZT data, α1(t′a,fr) The data is processed for the deramp,
n' is the length of the zero-padded azimuth data, faIs the azimuth frequency, ArFor the sampling start of the Z transformation on the unit circle, WrFor the sampling interval of the Z transformation on the unit circle, frIs distance to frequency, t'aIs the azimuth variable after zero padding.
Preferably, the CZT data is subjected to doppler modulation to obtain an aliasing-free two-dimensional frequency spectrum, and the specific formula is as follows:
wherein alpha is1(fa,fr) For aliasing-free two-dimensional spectra, α0(fa,fr) In order to be the CZT data,
fais the azimuth frequency, fdc0Doppler center frequency, K (f), invariant to instantaneous range frequencyr) For Doppler frequency modulation, f, as a function of instantaneous range frequencydc(fr) For Doppler centre frequency, f, varying with instantaneous range frequency introduced by squint anglerIs the range frequency.
The invention also provides a high-resolution squint SAR echo Doppler spectrum recovery system, which comprises:
the original echo data acquisition module is used for acquiring original echo data;
the range direction fast Fourier transform module is used for carrying out range direction fast Fourier transform on the original echo data to obtain range direction FFT data;
the pulse compression module is used for performing distance direction pulse compression on the distance direction FFT data to obtain pulse compression data;
the Doppler frequency shift elimination module is used for eliminating Doppler frequency shift of the pulse compression data to obtain Doppler frequency shift elimination data;
the azimuth deramp processing module is used for carrying out azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data;
the azimuth direction linear frequency modulation Z conversion module is used for carrying out azimuth direction linear frequency modulation Z conversion on the deramp processing data to obtain CZT data;
and the Doppler modulation recovery module is used for carrying out Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
Preferably, the azimuth deramp processing module specifically includes:
the second phase multiplying unit is used for multiplying the Doppler frequency shift elimination data by a second phase function of the distance space-variant in the azimuth direction to obtain second phase multiplication data;
the azimuth zero padding unit is used for carrying out azimuth zero padding on the secondary phase multiplication data to obtain zero padding data;
the azimuth fast Fourier transform unit is used for carrying out azimuth fast Fourier transform on the zero padding data to obtain azimuth FFT data;
and the residual secondary phase multiplying unit is used for performing residual secondary phase multiplication on the azimuth FFT data to obtain deramp processing data.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention discloses a high-resolution squint SAR echo Doppler frequency spectrum recovery method and system based on CZT (ChirpZ-Transform). And aiming at the data of Doppler frequency spectrum aliasing caused by insufficient system sampling rate and the existence of an oblique angle, sequentially performing range Fourier transform, pulse compression, Doppler frequency shift elimination and azimuth deramp, simultaneously realizing azimuth Fourier transform and resampling by using CZT, and recovering Doppler modulation of the data by phase multiplication, thereby recovering the aliasing-free two-dimensional frequency spectrum to obtain a high-resolution SAR image by using a frequency domain imaging algorithm. The method avoids interpolation operation, has simple operation and ensures the imaging quality.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.
Fig. 1 is a flowchart of a high-resolution squint SAR echo doppler spectrum recovery method of embodiment 1;
fig. 2 is a block diagram of a high-resolution squint SAR echo doppler spectrum recovery system according to embodiment 2;
FIG. 3 is a operational geometric model of the beamforming SAR of example 3;
FIG. 4 is a frequency spectrum of Doppler aliasing caused by insufficient system PRF in example 3;
FIG. 5 is the deramp data and two-dimensional spectrum after distance space invariant azimuth deramp and Doppler frequency shift cancellation method in example 3;
FIG. 6 is the two-dimensional spectrum obtained by applying CZT to the deramp data and the deramp data after the range-space-variant azimuth deramp and Doppler frequency shift cancellation method in example 3;
FIG. 7 is an imaging result of point targets 1, 2, 3 obtained from the two-dimensional spectrum aliased in FIG. 5(b) in example 3;
fig. 8 shows the imaging results of the two-dimensional spectrum without aliasing obtained in example 3, corresponding to the point targets 1, 2, and 3.
Description of the symbols: the method comprises the steps of 100-an original echo data acquisition module, 200-a distance direction fast Fourier transform module, 300-a pulse compression module, 400-a Doppler frequency shift elimination module, 500-an azimuth deramp processing module, 600-an azimuth direction linear frequency modulation Z transform module, 700-a Doppler modulation recovery module, 501-a secondary phase multiplication unit, 502-an azimuth direction zero padding unit, 503-an azimuth direction fast Fourier transform unit and 504-a residual secondary phase multiplication unit.
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 invention aims to provide a high-resolution squint SAR echo Doppler frequency spectrum recovery method which can avoid using interpolation operation, is simple in operation and has high imaging quality.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Example 1
As shown in fig. 1, the present embodiment discloses a high-resolution squint SAR echo doppler spectrum recovery method, which includes:
s1: raw echo data is acquired.
S2: and performing Fast Fourier Transform (FFT) on the original echo data to obtain distance FFT data.
S3: and performing distance direction pulse compression on the distance direction FFT data to obtain pulse compression data.
S4: and performing Doppler frequency shift elimination on the pulse compression data to obtain Doppler frequency shift elimination data.
S5: and performing azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data.
S6: and carrying out azimuth-direction linear frequency modulation Z (Chirp Z-Transform, CZT) on the deramp processing data to obtain CZT data.
S7: and performing Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
Specifically, the performing doppler shift cancellation on the pulse compression data to obtain doppler shift cancelled data specifically includes: multiplying the pulse compression data after the pulse compression in the distance direction by a linear phase function of distance space variation, wherein the specific formula is as follows:
α1(ta,fr)＝α0(ta,fr)exp[-j2πfdc(fr)ta]
wherein alpha is1(ta,fr) For removing data by Doppler shift, alpha0(ta,fr) In order to pulse-compress the data,
tais azimuth time, frIs the range frequency, fdc(fr) Is the doppler center frequency as a function of instantaneous range frequency introduced by the squint angle.
Specifically, the calculation formula of the doppler center frequency introduced by the squint angle and varying with the instantaneous distance frequency is as follows:
where c is the speed of light, fcIs the carrier frequency, vaAnd theta is the moving speed of the radar platform, and theta is the oblique view angle of the aperture center.
Specifically, the performing a deramp process on the doppler shift cancellation data to obtain a deramp process data specifically includes:
and S5.1, multiplying the Doppler frequency shift elimination data by a quadratic phase function of the distance space-variant in the azimuth direction to obtain quadratic phase multiplication data.
And S5.2, performing azimuth zero filling on the secondary phase multiplication data, and performing azimuth fast Fourier transform to obtain azimuth FFT data.
And S5.3, performing residual secondary phase multiplication on the azimuth FFT data to obtain a deramp processing data.
Specifically, the doppler shift cancellation data is multiplied by a quadratic phase function of the distance space-variant in the azimuth direction, and the specific formula is as follows:
α2(ta,fr)＝α1(ta,fr)exp[-jπK(fr)ta 2]
wherein alpha is2(ta,fr) For multiplying data by secondary phase, alpha1(ta,fr) In order to remove the data for the doppler shift,
tais azimuth time, frIs the range frequency, K (f)r) Is the doppler shift frequency that varies with the instantaneous range frequency.
Specifically, the calculation formula of the doppler shift frequency varying with the instantaneous range frequency is:
wherein，R0As the distance of action at the center of the scene, c is the speed of light, fcIs the carrier frequency, frIs the range frequency, vaAnd theta is the moving speed of the radar platform, and theta is the oblique view angle of the aperture center.
Specifically, the azimuth zero padding data is subjected to azimuth fast fourier transform to obtain azimuth FFT data, and the specific formula is as follows:
α0(t′a,fr)＝FFT{zero-padding[α2(ta,fr)]}
wherein alpha is0(t′a,fr) For the azimuthal FFT data, α2(ta,fr) Is the secondary phase multiplied data, t'aFor the zero-filled azimuth variable, taIs azimuth time, frIs the range frequency.
Specifically, the residual secondary phase multiplication is performed on the azimuth-direction FFT data to obtain a deramp processing data, and the specific formula is as follows:
α1(t′a,fr)＝α0(t′a,fr)exp[-jπK(fr)(t′a)2]
wherein alpha is1(t′a,fr) Processing data for deramp, α0(t′a,fr) In order to be the azimuth-direction FFT data,
t′afor the zero-filled azimuth variable, K (f)r) For Doppler frequency modulation, f, as a function of instantaneous range frequencyrIs the range frequency.
Specifically, the deramp processing data is subjected to azimuth chirp Z conversion CZT to obtain CZT data, and a specific formula is as follows:
wherein alpha is0(fa,fr) As CZT data, α1(t′a,fr) The data is processed for the deramp,
n' is the length of the zero-padded azimuth data, faIs the azimuth frequency, ArFor the sampling start of the Z transformation on the unit circle, WrFor the sampling interval of the Z transformation on the unit circle, frIs distance to frequency, t'aIs the azimuth variable after zero padding.
Specifically, the calculation formula of the sampling start point of the Z transform on the unit circle is:
wherein A isrFor the sampling start of the Z transformation on the unit circle, fa(0) Is the starting value of the azimuth frequency,
frfor range-wise frequency, PRF' is the system equivalent pulse repetition frequency after zero-filling oversampling of the data, fdc0Is the doppler center frequency that does not vary with the instantaneous range frequency.
Specifically, the calculation formula of the doppler center frequency that does not change with the instantaneous distance frequency is:
wherein f isdc0Is the Doppler center frequency which does not vary with the instantaneous range frequency, c is the speed of light, fcIs the carrier frequency, vaAnd theta is the moving speed of the radar platform, and theta is the oblique view angle of the aperture center.
Specifically, the calculation formula of the sampling interval of the Z transform on the unit circle is:
wherein, WrFor the sampling interval of the Z transformation on the unit circle, frIs the range frequency, fcIs the carrier frequency, and is,
and N' is the length of the azimuth data after zero padding.
Specifically, the CZT data is subjected to doppler modulation to obtain an aliasing-free two-dimensional frequency spectrum, and the specific formula is as follows:
wherein alpha is1(fa,fr) For aliasing-free two-dimensional spectra, α0(fa,fr) In order to be the CZT data,
fais the azimuth frequency, fdc0Doppler center frequency, K (f), invariant to instantaneous range frequencyr) For Doppler frequency modulation, f, as a function of instantaneous range frequencydc(fr) For Doppler centre frequency, f, varying with instantaneous range frequency introduced by squint anglerIs the range frequency.
Example 2
As shown in fig. 2, the present embodiment discloses a high-resolution squint SAR echo doppler spectrum recovery system, which includes:
a raw echo data acquiring module 100, configured to acquire raw echo data.
And a distance direction fast fourier transform module 200, configured to perform distance direction fast fourier transform on the original echo data to obtain distance direction FFT data.
And the pulse compression module 300 is configured to perform distance direction pulse compression on the distance direction FFT data to obtain pulse compressed data.
A doppler shift elimination module 400, configured to perform doppler shift elimination on the pulse compression data to obtain doppler shift eliminated data.
And the direction deramp processing module 500 is configured to perform direction deramp processing on the doppler frequency shift cancellation data to obtain deramp processing data.
And the azimuth direction linear frequency modulation Z conversion module 600 is used for carrying out azimuth direction linear frequency modulation Z conversion on the deramp processing data to obtain CZT data.
A doppler modulation recovery module 700, configured to perform doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
Specifically, the azimuth deramp processing module specifically includes:
a quadratic phase multiplying unit 501, configured to multiply the doppler shift cancellation data by a quadratic phase function of the distance space-variant in the azimuth direction, so as to obtain quadratic phase multiplication data.
An azimuth zero padding unit 502, configured to perform azimuth zero padding on the secondary phase multiplication data to obtain zero padding data.
An azimuth fast fourier transform unit 503, configured to perform azimuth fast fourier transform on the zero padding data to obtain azimuth FFT data.
And a residual secondary phase multiplying unit 504, configured to perform residual secondary phase multiplication on the azimuth FFT data to obtain a deramp processing data.
The specific formula involved in embodiment 2 is detailed in embodiment 1, and is not discussed one by one here.
Example 3
And performing a simulation experiment by using IDL software, wherein the subsequent imaging algorithm is a range migration algorithm. In the simulation experiment, when the SAR is operated in the bunching mode, the distribution diagram of the operating geometry and the point target is shown in fig. 3, and the operating parameters are shown in table 1:
fig. 4 is a spectrum of doppler aliasing due to system PRF insufficiency. In fig. 5, (a) and (b) are respectively the deramp data and the two-dimensional spectrum after distance invariant azimuth deramp and doppler frequency shift cancellation, and it can be seen that aliasing exists in the data after deramp and the corresponding two-dimensional spectrum in the azimuth direction. Fig. 6 (a) and (b) show the deramp data after applying the range-space-variant azimuth deramp and doppler frequency shift cancellation method of the present invention, and the two-dimensional spectrum obtained by performing CZT on the deramp data, respectively. Point objects 1, 2 and 3 in the second graph are selected, and a contour graph is drawn. Fig. 7 (a), (b), and (c) correspond to the imaging results of the point targets 1, 2, and 3, respectively, obtained by the two-dimensional spectrum aliased in fig. 5 (b). In fig. 8 (a), (b) and (c) correspond to the imaging results of the objects 1, 2, 3 obtained by the method of the present invention. The method provided by the invention can completely recover the aliasing-free two-dimensional frequency spectrum in the high-resolution full-aperture SAR processing and obtain a good imaging result.
The embodiment discloses a high-resolution squint SAR echo Doppler spectrum recovery method and system based on CZT (Chirp Z-Transform). Aiming at data with Doppler frequency spectrum aliasing caused by insufficient system sampling rate and the existence of an oblique angle, distance Fourier transform, pulse compression, Doppler frequency shift elimination and azimuth deramp are sequentially performed, CZT is used for simultaneously realizing azimuth Fourier transform and resampling, and then Doppler modulation of the data is recovered through phase multiplication, so that aliasing-free two-dimensional frequency spectrum is recovered to obtain a high-resolution SAR image by using a frequency domain imaging algorithm, operation is simplified, and imaging quality is guaranteed.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to assist in understanding the core concepts of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.
Claims (10)
1. A high-resolution squint SAR echo Doppler spectrum recovery method is characterized by comprising the following steps:
acquiring original echo data;
performing range-wise fast Fourier transform on the original echo data to obtain range-wise FFT data;
performing range direction pulse compression on the range direction FFT data to obtain pulse compression data;
performing Doppler frequency shift elimination on the pulse compression data to obtain Doppler frequency shift elimination data;
performing azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data;
carrying out azimuth linear frequency modulation Z conversion on the deramp processing data to obtain CZT data;
and performing Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
2. The method for restoring the Doppler spectrum of the high-resolution squint SAR echo according to claim 1, wherein the Doppler shift elimination is performed on the pulse compression data to obtain Doppler shift eliminated data, and the specific formula is as follows:
α1(ta,fr)＝α0(ta,fr)exp[-j2πfdc(fr)ta]；
wherein alpha is1(ta,fr) For removing data by Doppler shift, alpha0(ta,fr) In order to pulse-compress the data,
tais azimuth time, frIs the range frequency, fdc(fr) Is the doppler center frequency as a function of instantaneous range frequency introduced by the squint angle.
3. The method for restoring the doppler spectrum of the high-resolution squint SAR echo according to claim 1, wherein the performing a deramp process on the doppler shift cancellation data to obtain a deramp process data specifically comprises:
multiplying the Doppler frequency shift elimination data by a quadratic phase function of distance space-variant in the azimuth direction to obtain quadratic phase multiplication data;
carrying out azimuth zero padding on the secondary phase multiplication data, and then carrying out azimuth fast Fourier transform to obtain azimuth FFT data;
and performing residual secondary phase multiplication on the azimuth FFT data to obtain a deramp processing data.
4. The method for restoring the Doppler spectrum of the high-resolution squint SAR echo according to claim 3, wherein the Doppler frequency shift elimination data is multiplied by a quadratic phase function of the distance space-variant in the azimuth direction by the following specific formula:
α2(ta,fr)＝α1(ta,fr)exp[-jπK(fr)ta 2]；
wherein alpha is2(ta,fr) For multiplying data by secondary phase, alpha1(ta,fr) In order to remove the data for the doppler shift,
tais azimuth time, frIs the range frequency, K (f)r) Is the doppler shift frequency that varies with the instantaneous range frequency.
5. The high-resolution squint SAR echo Doppler spectrum recovery method according to claim 3, wherein the azimuth FFT data is obtained by performing azimuth fast Fourier transform after performing azimuth zero padding on the secondary phase multiplied data, and the specific formula is as follows:
α0(t′a,fr)＝FFT{zero-padding[α2(ta,fr)]}；
wherein alpha is0(t′a,fr) For the azimuthal FFT data, α2(ta,fr) Is the secondary phase multiplied data, t'aFor the zero-filled azimuth variable, taIs azimuth time, frIs the range frequency.
6. The high-resolution squint SAR echo Doppler spectrum recovery method according to claim 3, wherein the residual quadratic phase multiplication is performed on the azimuth FFT data to obtain a deramp processing data, and the specific formula is as follows:
α1(t′a,fr)＝α0(t′a,fr)exp[-jπK(fr)(t′a)2]；
wherein alpha is1(t′a,fr) Processing data for deramp, α0(t′a,fr) In order to be the azimuth-direction FFT data,
t′afor the zero-filled azimuth variable, K (f)r) For Doppler frequency modulation, f, as a function of instantaneous range frequencyrIs the range frequency.
7. The high-resolution squint SAR echo Doppler spectrum recovery method according to claim 1, wherein the azimuth chirp Z conversion CZT is performed on the deramp processed data to obtain CZT data, and the specific formula is as follows:
wherein alpha is0(fa,fr) As CZT data, α1(t′a,fr) The data is processed for the deramp,
n' is the length of the zero-padded azimuth data, faIn the form of an azimuth frequency, the azimuth frequency,Arfor the sampling start of the Z transformation on the unit circle, WrFor the sampling interval of the Z transformation on the unit circle, frIs distance to frequency, t'aIs the azimuth variable after zero padding.
8. The high-resolution squint SAR echo Doppler spectrum recovery method according to claim 1, wherein the Doppler modulation is recovered on the CZT data to obtain an aliasing-free two-dimensional spectrum, and the specific formula is as follows:
wherein alpha is1(fa,fr) For aliasing-free two-dimensional spectra, α0(fa,fr) In order to be the CZT data,
fais the azimuth frequency, fdc0Doppler center frequency, K (f), invariant to instantaneous range frequencyr) For Doppler frequency modulation, f, as a function of instantaneous range frequencydc(fr) For Doppler centre frequency, f, varying with instantaneous range frequency introduced by squint anglerIs the range frequency.
9. A high resolution squint SAR echo doppler spectral recovery system, the system comprising:
the original echo data acquisition module is used for acquiring original echo data;
the range direction fast Fourier transform module is used for carrying out range direction fast Fourier transform on the original echo data to obtain range direction FFT data;
the pulse compression module is used for performing distance direction pulse compression on the distance direction FFT data to obtain pulse compression data;
the Doppler frequency shift elimination module is used for eliminating Doppler frequency shift of the pulse compression data to obtain Doppler frequency shift elimination data;
the azimuth deramp processing module is used for carrying out azimuth deramp processing on the Doppler frequency shift elimination data to obtain deramp processing data;
the azimuth direction linear frequency modulation Z conversion module is used for carrying out azimuth direction linear frequency modulation Z conversion on the deramp processing data to obtain CZT data;
and the Doppler modulation recovery module is used for carrying out Doppler modulation recovery on the CZT data to obtain an aliasing-free two-dimensional frequency spectrum.
10. The high-resolution squint SAR echo doppler spectrum recovery system according to claim 9, wherein the azimuth deramp processing module specifically comprises:
the second phase multiplying unit is used for multiplying the Doppler frequency shift elimination data by a second phase function of the distance space-variant in the azimuth direction to obtain second phase multiplication data;
the azimuth zero padding unit is used for carrying out azimuth zero padding on the secondary phase multiplication data to obtain zero padding data;
the azimuth fast Fourier transform unit is used for carrying out azimuth fast Fourier transform on the zero padding data to obtain azimuth FFT data;
and the residual secondary phase multiplying unit is used for performing residual secondary phase multiplication on the azimuth FFT data to obtain deramp processing data.
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