CN113671503B - Imaging method suitable for satellite-borne variable PRF SAR system - Google Patents
Imaging method suitable for satellite-borne variable PRF SAR 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
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- 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
- G01S13/9021—SAR image post-processing techniques
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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
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- 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
- G01S13/9004—SAR image acquisition techniques
- G01S13/9011—SAR image acquisition techniques with frequency domain processing of the SAR signals in azimuth
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Abstract
The invention relates to an imaging method suitable for a satellite-borne variable PRF SAR system, and belongs to the field of radar imaging processing. And then, aiming at the residual correction error items remained after correction by the method, an error response factor is analyzed, an image is reconstructed by using a backward projection algorithm, and the blur-free imaging under a PRFSAR system is realized while the error is eliminated.
Description
Technical Field
The invention belongs to the field of radar imaging processing, and mainly constructs a satellite-borne SAR imaging processing algorithm to solve the problem of radar imaging azimuth ambiguity. The method is particularly suitable for the problem of azimuth Doppler ambiguity caused by uneven sampling of azimuth dimension under a satellite-borne variable pulse repetition frequency (Pulse Repetition Frequency, PRF) SAR system, and realizes the imaging processing of a variable PRF-SAR system on a ground scene.
Background
The spaceborne synthetic aperture radar (Synthetic Aperture Radar, SAR) has long acting distance and wide coverage range, and has all-day and all-weather earth observation capability. However, the conventional spaceborne SAR has an inherent contradiction that high azimuth resolution and wide swath cannot be considered. To address this contradiction, researchers have proposed an on-board SAR regime that employs a variable pulse repetition frequency (Pulse Repetition Frequency, PRF). Under the system, the radar acquires SAR images of high-resolution wide swaths by using a reflective antenna and Digital Beam Forming (DBF). More importantly, by means of the variable PRF, the radar of the system can solve the problem of blind areas which cannot be overcome by the conventional spaceborne SAR based on the DBF system. In addition, the SAR of the system has the advantage of low cost, so that the SAR has important significance in research. However, the echo of the variable PRF-SAR system has the characteristics of non-uniform sampling in azimuth and data loss, and the processing of data by using the traditional SAR imaging algorithm can cause serious degradation of imaging quality and even complete failure of imaging. Therefore, research on effective imaging focusing of ground scenes under a variable PRF system is of great significance.
Disclosure of Invention
Technical problem to be solved
In order to solve the problem of non-uniform sampling in azimuth, the invention provides an imaging method suitable for a satellite-borne variable PRF SAR system.
Technical proposal
An imaging method suitable for a satellite-borne variable PRF SAR system is characterized by comprising the following steps:
step 1: the radar system echo signal after clutter suppression is obtained as a two-dimensional matrix S (t n ,t m )，S(t n ,t m ) The method is a nrn multiplied by nan dimensional matrix, wherein each row of data of the matrix is a result obtained by non-uniformly sampling the echo azimuth dimension; pair matrixPerforming FFT processing on the columns of the radar echo signal, namely realizing distance Fourier transform, and respectively storing the results in a matrix S (f n ,t m ) In (a) and (b);
wherein t is n For a fast distance, t m Slow time for non-uniform sampling azimuth, f n Represented as distance-to-frequency domain coordinates,b is the bandwidth of the transmitted signal, Δf is the distance from the frequency domain interval,/->n=0, 1.. nrn-1, nrn represents distance-direction points, nan represents azimuth-direction points;
step 2: constructing a reference signal vector s_ref (i, 1) according to known radar parameters, i=1, 2.. nrn, s_ref (i, 1) being a nrn ×1 vector;
step 3: taking out S (f) obtained in step 1 n ,t m ) The conjugate of the reference signal vector S_ref (i, 1) is multiplied by a point to obtain a data matrix S (f) after the distance pulse pressure n ,t m )；
Step 4: constructing a nonlinear scaling correction matrix H_CS (i, j) according to known radar parameters, wherein the H_CS (i, j) is a nrn multiplied by nan matrix;
step 5: taking out S (f) obtained in step 3 n ,t m ) The conjugate of the dot product correction matrix H_CS (i, j) yields a data matrix S' (f) after nonlinear scaling correction n ,t m ' s); wherein t is m ' representation squareA bit uniform sampling time sequence;
step 6: from known radar parameters, a distance warping correction matrix self_RCMC (f n ,f m )，Self_RCMC(f n ,f m ) A nrn × nan matrix;
step 7: s' (f) obtained in step 5 is taken out n ,t m ') performing Fourier transform of azimuth dimension, and multiplying reference signal vector Self_RCMC (f) by point in two-dimensional frequency domain n ,f m ) And performing two-dimensional inverse Fourier transform of the distance and azimuth to obtain a data matrix S "(t) after the curvature correction n ,t m ')；
Step 8: taking out S "(t) obtained in step 7 n ,t m ') performing delay compensation and coherent accumulation on the data, and storing the result in a matrix S sum (t n ,t m ') to obtain a focused ground target imaging result graph.
The invention further adopts the technical scheme that: the step 2 is specifically as follows: reference signal vector based on known radar parametersWhere γ represents the tone frequency, γ=b/Tp, B represents the transmit signal bandwidth, tp represents the transmit pulse width, f r Represented as distance to the frequency domain.
The invention further adopts the technical scheme that: the step 4 is specifically as follows:
according to known radar parameters, the pulse repetition time sequence is:
setting the azimuth slow time to correspond to the uniform sampling time:
t m '＝[PRI 0 ,2PRI 0 ,3PRI 0 ,…,i·PRI 0 ,…I·PRI 0 ]
obtaining a relation between the non-uniform sampling time sequence and the uniform sampling time sequence:
wherein delta is the difference between adjacent sampling intervals, PRI 0 For pulse repetition rate, t m ' is a uniform sampling time sequence;
substituting the above relation into the oblique distance expression, the following can be obtained:
constructing a nonlinear scaling correction matrix according to the oblique distance expression as follows:
wherein h is cs Is a phase compensation factor:
f c representing the carrier frequency of a radar transmitting signal, c is the propagation speed of electromagnetic waves, v is the movement speed of a radar platform, R b Is the closest distance of the radar to the scene center point.
The invention further adopts the technical scheme that: the step 6 is specifically as follows: constructing a distance curvature correction matrix according to known radar parameters:
wherein the method comprises the steps ofPRF is pulse repetition frequency, < >>
The invention further adopts the technical scheme that: the step 8 is specifically as follows: dividing an imaging scene into grid points to obtain coordinates of all grid points, taking the nearest distance as a reference point, calculating the distance between a radar and all the grid points in the current azimuth direction from an azimuth starting point, calculating delay time delta t of all the grid points relative to the nearest distance reference point, and performing phase compensation exp (j 2 pi f) on echo data by utilizing the delay of each grid point c Δt), S "(t) for each radar position n ,t m ') performing coherent accumulation to obtain an accumulated sum S sum (t n ,t m ')，S sum (t n ,t m ') is the obtained focusing target imaging result diagram.
Advantageous effects
The invention provides a new azimuth non-uniform sampling reconstruction method, which is to introduce a nonlinear scaling technology into an azimuth Doppler time domain to process echo, so that echo signals after processing are equivalent to time uniform sampling, then residual correction error items remained after correction of the method are analyzed, error response factors are analyzed, an image is reconstructed by using a backward projection algorithm, and the blur-free imaging under a variable PRF SAR system is realized while errors are eliminated.
The invention solves the problem that the existing SAR imaging technology cannot carry out imaging processing on the SAR system with the variable pulse repetition frequency system, aims at the problem of azimuth Doppler blurring caused by uneven sampling of azimuth dimension, corrects the uneven sampling data by using a nonlinear scaling method, and solves the residual error items after correction by using a back projection algorithm so as to achieve the effect of carrying out good focusing on the target.
Drawings
The drawings are only for purposes of illustrating particular embodiments and are not to be construed as limiting the invention, like reference numerals being used to refer to like parts throughout the several views.
FIG. 1 azimuth non-uniformly sampled echo signals;
FIG. 2 is a distance pulse compression process of echoes;
fig. 3 performs a warp correction process;
FIG. 4 shows the imaging result after the BP algorithm is used for carrying out azimuth matching pulse pressure;
figure 5 is a view of the imaging result in azimuth cut.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention. In addition, technical features of the embodiments of the present invention described below may be combined with each other as long as they do not collide with each other.
The method comprises the following steps:
step 1, the radar system echo signal after clutter suppression is a two-dimensional matrix S (t n ,t m )，S(t n ,t m ) A nrn x nan dimensional matrix, wherein each row of data of the matrix is the result of non-uniform sampling of the echo azimuth dimension. Pair matrixPerforming FFT processing on the columns of the radar echo signal, namely realizing distance Fourier transform, and respectively storing the results in a matrix S (f n ,t m ) In (a) and (b);
wherein t is n For a fast distance, t m Slow time for non-uniform sampling azimuth, f n Represented as distance-to-frequency domain coordinates,b is the bandwidth of the transmitted signal, Δf is the distance from the frequency domain interval,/->n=0, 1.. nrn-1, nrn represents distance-direction points and nan represents azimuth-direction points.
Step 2, constructing a reference signal vector s_ref (i, 1) according to known radar parameters, i=1, 2.. nrn, s_ref (i, 1) being a nrn ×1 vector; wherein nrn represents distance to points;
step 3, taking out S (f) obtained in step 1 n ,t m ) The conjugate of the reference signal vector S_ref (i, 1) is multiplied by a point to obtain a data matrix S (f) after the distance pulse pressure n ,t m )；
Step 4, constructing a nonlinear scaling correction matrix H_CS (i, j) according to known radar parameters, wherein the H_CS (i, j) is a nrn multiplied by nan matrix;
wherein nrn represents distance direction points and nan represents azimuth direction points;
step 5, taking out S (f) obtained in step 3 n ,t m ) The conjugate of the dot product correction matrix H_CS (i, j) yields a data matrix S' (f) after nonlinear scaling correction n ,t m ')；
Wherein t is m ' represents a time series of azimuthally uniform sampling.
Step 6, constructing a distance bending correction matrix self_RCMC (f) according to the known radar parameters n ,f m )，Self_RCMC(f n ,f m ) A nrn × nan matrix;
step 7, taking out S' (f) obtained in step 5 n ,t m ') performing Fourier transform of azimuth dimension, and multiplying reference signal vector Self_RCMC (f) by point in two-dimensional frequency domain n ,f m ) And performing two-dimensional inverse Fourier transform of the distance and azimuth to obtain a data matrix S "(t) after the curvature correction n ,t m ')；
Step 8, taking out S "(t) obtained in the step 7 n ,t m ') performing delay compensation and coherent accumulation on the data, and storing the result in a matrix S sum (t n ,t m ') to obtain a focused ground target imaging result graph.
The technical scheme is characterized in that:
the specific operation of the step 2 is as follows:
reference signal vector based on known radar parameters
Wherein gamma represents the toneFrequency, γ=b/Tp, B represents transmit signal bandwidth, tp represents transmit pulse width, f r Expressed as distance to the frequency domain
The specific operation of the step 4 is as follows:
according to known radar parameters, the pulse repetition time sequence is:
setting the azimuth slow time to correspond to the uniform sampling time:
t m '＝[PRI 0 ,2PRI 0 ,3PRI 0 ,…,i·PRI 0 ,…I·PRI 0 ]
obtaining a relation between the non-uniform sampling time sequence and the uniform sampling time sequence:
wherein delta is the difference between adjacent sampling intervals, PRI 0 For pulse repetition rate, t m ' is a uniformly sampled time series.
Substituting the above relation into the oblique distance expression, the following can be obtained:
constructing a nonlinear scaling correction matrix according to the oblique distance expression as follows:
wherein h is cs Is a phase compensation factor:
f c representing the carrier frequency of a radar transmitting signal, c is the propagation speed of electromagnetic waves, v is the movement speed of a radar platform, R b Is the closest distance of the radar to the scene center point.
The specific operation of the step 6 is as follows:
constructing a distance curvature correction matrix according to known radar parameters:
wherein the method comprises the steps ofPRF is pulse repetition frequency, < >>m＝0,1，......,nan-1。
The specific operation of the step 8 is as follows:
dividing an imaging scene into grid points to obtain coordinates of all grid points, taking the nearest distance as a reference point, calculating the distance between a radar and all the grid points in the current azimuth direction from an azimuth starting point, calculating delay time delta t of all the grid points relative to the nearest distance reference point, and performing phase compensation exp (j 2 pi f) on echo data by utilizing the delay of each grid point c Δt), S "(t) for each radar position n ,t m ') performing coherent accumulation to obtain an accumulated sum S sum (t n ,t m ')，S sum (t n ,t m ') is the obtained focusing target imaging result diagram.
Thus, the synthetic radar target imaging algorithm under the variable pulse repetition frequency system is basically completed.
The effectiveness of the present invention is further verified by simulation experimental data as follows.
Simulation experiment (one)
1. Simulation parameters
To verify the effectiveness of the method of the present invention, the simulation parameters in Table 1 are presented herein.
TABLE 1 partial simulation data parameters
| Carrier frequency | 9.6GHz | Pulse repetition frequency | 5000Hz |
| Radar platform speed | 7600m/s | Azimuth dimension antenna aperture | 6m |
| Radar platform height | 780km | Pitch dimensional antenna aperture | 1.44m |
| Frequency modulation bandwidth of transmitted signal | 200MHz | Pulse width of transmitted signal | 2*10^-7s |
| Complex sampling frequency | 280MHz | Non-uniform spacing difference | 10^-8s |
2. Emulation content
Fig. 1-5 illustrate the processing of radar azimuth non-uniform sampled data to obtain a blur-free imaging processing result by using the nonlinear scaling correction proposed by the present invention. The target imaging focusing effect of the method can be seen from the figure, and the target imaging problem under the variable pulse repetition frequency SAR system can be effectively solved by adopting the method.
In conclusion, the simulation experiment verifies the correctness, the effectiveness and the reliability of the invention.
While the invention has been described with reference to certain preferred embodiments, it will be understood by those skilled in the art that various changes and substitutions of equivalents may be made without departing from the spirit and scope of the invention.
Claims (5)
1. An imaging method suitable for a satellite-borne variable PRF SAR system is characterized by comprising the following steps:
step 1: the radar system echo signal after clutter suppression is obtained as a two-dimensional matrix S (t n ,t m )，S(t n ,t m ) The method is a nrn multiplied by nan dimensional matrix, wherein each row of data of the matrix is a result obtained by non-uniformly sampling the echo azimuth dimension; pair matrixPerforming FFT processing on the columns of the radar echo signal, namely realizing distance Fourier transform, and respectively storing the results in a matrix S (f n ,t m ) In (a) and (b);
wherein t is n For a fast distance, t m Slow time for non-uniform sampling azimuth, f n Represented as distance-to-frequency domain coordinates,b is the bandwidth of the transmitted signal, Δf is the distance from the frequency domain interval,/->n=0, 1.. nrn-1, nrn represents distance directionPoints nan represents azimuth points;
step 2: constructing a reference signal vector s_ref (i, 1) according to known radar parameters, i=1, 2.. nrn, s_ref (i, 1) being a nrn ×1 vector;
step 3: taking out S (f) obtained in step 1 n ,t m ) The conjugate of the reference signal vector S_ref (i, 1) is multiplied by a point to obtain a data matrix S (f) after the distance pulse pressure n ,t m )；
Step 4: constructing a nonlinear scaling correction matrix H_CS (i, j) according to known radar parameters, wherein the H_CS (i, j) is a nrn multiplied by nan matrix;
step 5: taking out S (f) obtained in step 3 n ,t m ) The conjugate of the dot product correction matrix H_CS (i, j) yields a data matrix S' (f) after nonlinear scaling correction n ,t m ' s); wherein t is m ' represents an azimuthal uniform sampling time sequence;
step 6: from known radar parameters, a distance warping correction matrix self_RCMC (f n ,f m )，Self_RCMC(f n ,f m ) A nrn × nan matrix;
step 7: s' (f) obtained in step 5 is taken out n ,t m ') performing Fourier transform of azimuth dimension, and multiplying reference signal vector Self_RCMC (f) by point in two-dimensional frequency domain n ,f m ) And performing two-dimensional inverse Fourier transform of the distance and azimuth to obtain a data matrix S "(t) after the curvature correction n ,t m ')；
Step 8: taking out S "(t) obtained in step 7 n ,t m ') performing delay compensation and coherent accumulation on the data, and storing the result in a matrix S sum (t n ,t m ') to obtain a focused ground target imaging result graph.
2. The imaging method suitable for a space-borne variable PRF SAR system according to claim 1, wherein step 2 comprises the following steps: reference signal vector based on known radar parametersWhere γ represents the tone frequency, γ=b/Tp, B represents the transmit signal bandwidth, tp represents the transmit pulse width, f r Represented as distance to the frequency domain.
3. The imaging method suitable for a space-borne variable PRF SAR system according to claim 1, wherein step 4 is specifically as follows:
according to known radar parameters, the pulse repetition time sequence is:
setting the azimuth slow time to correspond to the uniform sampling time:
t m '＝[PRI 0 ,2PRI 0 ,3PRI 0 ,…,i·PRI 0 ,…I·PRI 0 ]
obtaining a relation between the non-uniform sampling time sequence and the uniform sampling time sequence:
wherein delta is the difference between adjacent sampling intervals, PRI 0 For pulse repetition rate, t m ' is a uniform sampling time sequence;
substituting the above relation into the oblique distance expression, the following can be obtained:
constructing a nonlinear scaling correction matrix according to the oblique distance expression as follows:
wherein h is cs Is a phase compensation factor:
f c representing the carrier frequency of a radar transmitting signal, c is the propagation speed of electromagnetic waves, v is the movement speed of a radar platform, R b Is the closest distance of the radar to the scene center point.
4. The imaging method suitable for use in an on-board variable PRF SAR system according to claim 1, wherein step 6 is specifically as follows: constructing a distance curvature correction matrix according to known radar parameters:
wherein the method comprises the steps ofPRF is pulse repetition frequency, < >>m＝0,1,......,nan-1。
5. The imaging method suitable for use in an on-board variable PRF SAR system according to claim 1, wherein step 8 is specifically as follows: dividing an imaging scene into grid points to obtain coordinates of all grid points, taking the nearest distance as a reference point, calculating the distance between a radar and all the grid points in the current azimuth direction from an azimuth starting point, calculating delay time delta t of all the grid points relative to the nearest distance reference point, and performing phase compensation exp (j 2 pi f) on echo data by utilizing the delay of each grid point c Δt), S "(t) for each radar position n ,t m ') performing coherent accumulation to obtain an accumulated sum S sum (t n ,t m ')，S sum (t n ,t m ') is the obtained focusing target imaging result diagram.
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| EP3144702A1 (en) * | 2015-09-17 | 2017-03-22 | Institute of Electronics, Chinese Academy of Sciences | Method and device for synthethic aperture radar imaging based on non-linear frequency modulation signal |
| CN110376564A (en) * | 2019-07-30 | 2019-10-25 | 西北工业大学 | The biradical configuration of GEO and LEO synthesizes radar ground motion imaging method |
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| EP3144702A1 (en) * | 2015-09-17 | 2017-03-22 | Institute of Electronics, Chinese Academy of Sciences | Method and device for synthethic aperture radar imaging based on non-linear frequency modulation signal |
| CN110376564A (en) * | 2019-07-30 | 2019-10-25 | 西北工业大学 | The biradical configuration of GEO and LEO synthesizes radar ground motion imaging method |
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| 机载TOPS模式两步成像算法研究;关一夫;杨源;贾文通;李健;邱峰;;空军预警学院学报(第03期);全文 * |
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