CN113376632B - Large strabismus airborne SAR imaging method based on pretreatment and improved PFA - Google Patents
Large strabismus airborne SAR imaging method based on pretreatment and improved PFA Download PDFInfo
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- 230000005012 migration Effects 0.000 claims abstract description 6
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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
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Abstract
The invention discloses a large strabismus airborne SAR imaging method based on pretreatment and improved PFA, which comprises the following steps: step 1: carrying out Doppler center frequency shift compensation on the original echo data and passing through a two-stage cascade azimuth smoothing filter; step 2: azimuth extraction is carried out on the filtered data, and Doppler frequency shift characteristics are recovered; step 3: estimating the number of shift points for roughly correcting the range migration track according to the parameters; step 4: embedding RCM rough correction operation in PCS-based distance direction processing to obtain maximum time domain interception point number, thereby reducing the operand of subsequent processing; step 5: performing azimuth high-precision Sinc interpolation processing to compensate residual linear walking; step 6: and carrying out two-dimensional Fourier transform on the signals subjected to azimuth resampling to obtain a final imaging result. The invention reduces the data rate in the azimuth and distance dimensions by preprocessing operation and improving PFA to improve the processing speed, and has the characteristics of easy realization of actual engineering, high algorithm efficiency and high imaging quality.
Description
Technical Field
The invention relates to the technical field of radar imaging, in particular to a large strabismus airborne SAR imaging method based on pretreatment and improved PFA.
Background
Synthetic Aperture Radar (SAR) breaks the real aperture azimuth resolution limit and enables all-weather, all-day observations of the target area. The large strabismus airborne SAR beam has high pointing flexibility, can image the target to be observed in advance and for many times, and is widely applied to important fields such as resource exploration, disaster monitoring, battlefield reconnaissance, battlefield target accurate striking and the like. Large squint SAR imaging needs to process large amount of data, and the range migration of echoes is large and the frequency domain two-dimensional coupling is serious.
In addition, on-board SAR real-time imaging systems typically employ higher Pulse Repetition Frequencies (PRFs) to improve signal-to-noise ratio and reduce transmitter peak power. However, high PRF can result in significant redundancy in the echo data in the azimuth direction, adding to the burden of subsequent processing. The direct decimation to reduce PRF and azimuth data rate causes azimuth aliasing effects of the spectrum and results in serious signal-to-noise ratio loss. Meanwhile, the effective imaging scene is far smaller than the radar range of range data acquisition, so that the range data redundancy is serious, and the algorithm operand is increased.
The PFA algorithm stores data in a polar coordinate format, and two-dimensional decoupling of signals is completed by resampling the distance and the azimuth, so that the problem of moving a resolution unit far away from a scattering point at the center of an imaging area can be effectively solved. Therefore, the conventional PFA algorithm directly performs imaging processing on the original echo with huge data volume, which greatly increases the computational burden, and also puts high demands on the memory resources of the chip when the PFA algorithm is implemented in hardware.
Disclosure of Invention
The technical problem to be solved by the invention is to provide the large strabismus onboard SAR imaging method based on the pretreatment and improvement of the PFA, which can effectively reduce the data rate in the azimuth and distance dimensions by the pretreatment operation and the improvement of the PFA to improve the processing speed, and has the characteristics of easiness in practical engineering realization, high algorithm efficiency and high imaging quality.
In order to solve the technical problems, the invention provides a large strabismus airborne SAR imaging method based on pretreatment and improved PFA, which comprises the following steps:
step 1: carrying out Doppler center frequency shift compensation on the original echo data and passing through a two-stage cascade azimuth smoothing filter;
step 2: azimuth extraction is carried out on the filtered data, and Doppler frequency shift characteristics are recovered;
step 3: estimating the number of shift points for roughly correcting the range migration track according to the parameters;
step 4: embedding RCM (Range Cell Migration, distance unit migration) coarse correction operation in distance direction processing based on PCS (Principle of Chirp Scaling, scale transformation principle) to obtain maximum time domain interception point number, thereby reducing the operand of subsequent processing;
step 5: performing azimuth high-precision Sinc interpolation processing to compensate residual linear walking;
step 6: and carrying out two-dimensional Fourier transform on the signals subjected to azimuth resampling to obtain a final imaging result.
Preferably, in step 1, the filter order N fo The calculation method of (1) is as follows:
wherein PRF is pulse repetition frequency; echo signal bandwidth B d ＝B l +B w ，B l And B is connected with w The bandwidth caused by scene bandwidth and beam angle rotation are respectively; [ x ]]Meaning rounding x.
Preferably, in step 2, the azimuth extraction coefficient N ds The calculation method of (1) is as follows:
wherein,representing rounding down x, N ds The PRF after azimuth extraction can be guaranteed to be about 1.2 times of the bandwidth of the echo signal as an extraction coefficient, the Nyquist sampling theorem is satisfied, and the azimuth spectrum has no aliasing phenomenon.
Preferably, in step 3, the shift point number n for the coarse correction operation 0 The calculation method of (1) is as follows:
n 0 ＝[2f s ·(R a -R 0 )/c]
wherein c is the speed of light; f (f) s Is the distance to sampling frequency. R is R a And R is 0 The instantaneous distance from the antenna phase center to the scene center and the radar acting distance are respectively.
Preferably, in step 4, an RCM coarse correction operation is embedded in the PCS-based distance vector processing to obtain the maximum time domain intercept point number, and specifically includes the following steps:
step 4-1: the preprocessed echo signal S (t, τ) is multiplied by a scaling function Φ scl (τ)
Wherein t and τ are azimuth slow time and distance fast time, respectively, and k is frequency modulation slope;is a distance scale transformation factor; θ and->The instantaneous azimuth angle and the pitch angle of the phase center of the radar antenna; θ ref And->Respectively azimuth angle and pitch angle of azimuth aperture center moment;
step 4-2: distance-wise FFT operation and multiplication by frequency-domain system function H 1 (f r )
Wherein f r Is a distance frequency variable;
step 4-3: distance-wise IFFT operation and multiplication by an inverse scaling function phi ins (τ)
Wherein f c Is the carrier frequency;
step 4-4: performing distance FFT and time domain interception operation, and multiplying by frequency domain system function H 2 (f r )
Preferably, in step 5, the method for calculating the coordinates of the interpolation points of the input and output directions Sinc includes:
wherein t' is the azimuth time variable after Keystone transformation.
Preferably, in step 6, the two-dimensional resampled signal is subjected to two-dimensional fourier transform, so as to obtain a final imaging result.
The beneficial effects of the invention are as follows: the invention combines the pretreatment operation and the improved PFA algorithm to realize the airborne SAR large squint imaging, effectively reduces the data rate in the azimuth and distance dimensions by the pretreatment operation and the improved PFA to improve the processing speed, and has the characteristics of easy practical engineering realization, high algorithm efficiency and high imaging quality.
Drawings
FIG. 1 is a schematic flow chart of the method of the present invention.
Fig. 2 is a schematic flow chart of the preprocessing implementation of the present invention.
Fig. 3 is a two-dimensional frequency domain plot of the original echo of the present invention.
Fig. 4 (a) is a two-dimensional frequency domain plot of the echo of the direct azimuth extraction of the present invention.
Fig. 4 (b) is a two-dimensional frequency domain plot of the echo after preprocessing according to the present invention.
Fig. 5 is a schematic diagram of a distance PCS process flow in which coarse correction and time domain truncation are fused according to the present invention.
FIG. 6 (a) is a schematic view of the RCM trace before rough correction according to the present invention.
FIG. 6 (b) is a schematic diagram of the trace of the RCM after the rough correction of the present invention.
Fig. 7 (a) is a schematic diagram of an RCM trace of the present invention taken directly in the time domain.
Fig. 7 (b) is a schematic view of the RCM trace of the present invention with coarse correction followed by time domain truncation.
Fig. 8 (a) is a schematic diagram of the measured result of the direct time domain interception according to the present invention.
Fig. 8 (b) is a schematic diagram showing the actual measurement result of the embedding coarse correction operation according to the present invention.
Fig. 9 (a) is a schematic diagram of a result of processing measured data of a large squint airborne SAR measured scene according to the present invention.
Fig. 9 (b) is a schematic diagram of a processing result of two actual measurement data of the actual measurement scene of the large strabismus airborne SAR of the present invention.
Detailed Description
As shown in fig. 1, a large squint airborne SAR imaging method based on pretreatment and improved PFA includes the steps of:
step 1: the Doppler center frequency shift compensation is carried out on the original echo data, and the original echo data is passed through a two-stage cascade azimuth smoothing filter.
Filter order N fo The calculation method of (1) is as follows:
wherein PRF is pulse repetition frequency; echo signal bandwidth B d ＝B l +B w ，B l And B is connected with w The bandwidth caused by scene bandwidth and beam angle rotation are respectively; [ x ]]Meaning rounding x.
Step 2: and carrying out azimuth extraction on the filtered data and recovering Doppler frequency shift characteristics.
Azimuth extraction coefficient N ds The calculation method of (1) is as follows:
wherein,representing rounding down on x. Will N ds The PRF after azimuth extraction can be guaranteed to be about 1.2 times of the bandwidth of the echo signal as an extraction coefficient, the Nyquist sampling theorem can be satisfied, and the azimuth spectrum has no aliasing phenomenon.
Step 3: and estimating the shift points for roughly correcting the range migration track according to the parameters.
Coarse correction shift point number n 0 The calculation method of (1) is as follows:
n 0 ＝[2f s ·(R a -R 0 )/c]
wherein c is the speed of light; f (f) s Is the distance to sampling frequency. R is R a And R is 0 The instantaneous distance from the antenna phase center to the scene center and the radar acting distance are respectively.
Step 4: coarse correction and time domain truncation operations are embedded in the range-wise PCS process to eliminate high-order range warping of the target and reduce the amount of subsequent processed data.
First, the preprocessed echo signal S (t, τ) is multiplied by a scaling function φ scl (τ)
Where t and τ are azimuth slow time and distance fast time, respectively. k is the frequency modulation slope;is a distance scale transformation factor; θ and->The instantaneous azimuth angle and the pitch angle of the phase center of the radar antenna; θ ref And->Respectively azimuth angle and pitch angle of azimuth aperture center moment;
next, a distance FFT operation is performed and multiplied by a frequency domain system function H 1 (f r )
Wherein f r Is a distance frequency variable.
Then, a distance-wise IFFT operation is performed and multiplied by an inverse scaling function ins (τ)
Wherein f c Is the carrier frequency.
Finally, performing distance FFT and time domain interception operation, and multiplying by a frequency domain system function H 2 (f r )
Step 5: azimuth high-precision Sinc interpolation processing is carried out to compensate residual linear walk.
The method for calculating the coordinates of the interpolation points of the input and output of the azimuth Sinc comprises the following steps:
wherein t' is the azimuth time variable after Keystone transformation.
Step 6: and carrying out two-dimensional Fourier transform on the signals subjected to azimuth resampling to obtain a final imaging result.
The preprocessing mainly comprises twice azimuth filtering and azimuth extraction, the azimuth filtering can filter redundant Doppler frequencies outside an imaging bandwidth while improving the signal-to-noise ratio, so that the signal-to-noise ratio loss and azimuth spectrum aliasing caused by azimuth extraction are avoided, and the detailed implementation flow is shown in figure 2. Taking the two-dimensional spectrum of the original echo in fig. 3 as an example, when the azimuth doppler bandwidth is far greater than the imaging target bandwidth, the azimuth redundancy data may cause a serious computational burden on the subsequent imaging process. The comparison result of the direct extraction mode and the preprocessing output is shown in fig. 4 (a) and fig. 4 (b), the azimuth redundancy doppler bandwidth of the preprocessed result is filtered, noise is effectively suppressed, and the signal-to-noise ratio is improved.
The inclination of a Range Cell Migration (RCM) track under a large strabismus condition is seriously aggravated, and the RCM track is easily damaged by direct time domain interception, so that the image focusing quality is reduced. The invention improves the method, and the method carries out rough correction on the RCM track before time domain interception to obtain the maximum effective interception rate, and the specific flow is shown in figure 5.
To illustrate the effectiveness of the coarse correction process, fig. 6 (a) and 6 (b) show the RCM trajectories of the point targets before and after the coarse correction process, respectively. In the actual processing process, firstly estimating correction points; and then moving the RCM track in the time domain to realize rough correction of the linear walking error, thereby ensuring the accuracy and the efficiency maximization of time domain interception. As can be seen from fig. 7 (a) and 7 (b), the effect of suppressing the distance redundant data by performing time domain truncation directly without rough correction is very limited, and the integrity of the RCM track is very easily damaged, so that the distance wrapping phenomenon occurs.
The high squint airborne SAR measured data is used to further verify the validity of the present invention. The comparison of the measured results of fig. 8 (a) and 8 (b) fully demonstrates the importance of the coarse correction operation in combination with time domain truncation. The coarse correction operation is embedded in the time domain interception, so that the distance direction winding phenomenon caused by incomplete RCM track can be eliminated, and further, the degradation of SAR image focusing effect is avoided. The final output results of the invention are as shown in fig. 9 (a) and 9 (b), and are respectively result images of different actual measurement scenes, so that scenes such as farmland, roads, house buildings and the like on the ground can be clearly distinguished from the images, and the image focusing effect is good. Therefore, the method is suitable for large strabismus airborne SAR imaging, can effectively reduce the data rate in the azimuth dimension and the distance dimension to improve the processing speed, and has the characteristics of easiness in practical engineering realization, high algorithm efficiency and high imaging quality.
Claims (7)
1. A large strabismus onboard SAR imaging method based on pretreatment and improved PFA, comprising the steps of:
step 1: carrying out Doppler center frequency shift compensation on the original echo data and passing through a two-stage cascade azimuth smoothing filter;
step 2: azimuth extraction is carried out on the filtered data, and Doppler frequency shift characteristics are recovered;
step 3: estimating the number of shift points for roughly correcting the range migration track according to the parameters;
step 4: embedding RCM rough correction operation in PCS-based distance direction processing to obtain maximum time domain interception point number, thereby reducing the operand of subsequent processing;
step 5: performing azimuth high-precision Sinc interpolation processing to compensate residual linear walking;
step 6: and carrying out two-dimensional Fourier transform on the signals subjected to azimuth resampling to obtain a final imaging result.
2. The pretreatment and improvement PFA-based large strabismus onboard SAR imaging method of claim 1, wherein in step 1, the filter order N fo The calculation method of (1) is as follows:
wherein PRF is pulse repetition frequency; echo signal bandwidth B d ＝B l +B w ，B l And B is connected with w The bandwidth caused by scene bandwidth and beam angle rotation are respectively; [ x ]]Meaning rounding x.
3. The pretreatment and improvement PFA-based large strabismus onboard SAR imaging method according to claim 1, wherein in step 2, the azimuth extraction factor N ds The calculation method of (1) is as follows:
wherein,representing rounding down x, N ds As a result of extractionThe coefficient can ensure that the PRF after azimuth extraction is about 1.2 times of the bandwidth of the echo signal, and satisfies the Nyquist sampling theorem, and the azimuth spectrum has no aliasing phenomenon.
4. The large strabismus onboard SAR imaging method based on preprocessing and improving PFA according to claim 1, wherein in step 3, the shift point number n for coarse correction operation 0 The calculation method of (1) is as follows:
n 0 ＝[2f s ·(R a -R 0 )/c]
wherein c is the speed of light; f (f) s The distance sampling frequency; r is R a And R is 0 The instantaneous distance from the antenna phase center to the scene center and the radar acting distance are respectively.
5. The pretreatment and improvement PFA-based large squint airborne SAR imaging method according to claim 1, wherein in step 4, an RCM coarse correction operation is embedded in the PCS-based distance vector process to obtain the maximum time domain intercept point number, specifically comprising the steps of:
step 4-1: the preprocessed echo signal S (t, τ) is multiplied by a scaling function Φ scl (τ)
Wherein t and τ are azimuth slow time and distance fast time, respectively, and k is frequency modulation slope;is a distance scale transformation factor; θ and->The instantaneous azimuth angle and the pitch angle of the phase center of the radar antenna; θ ref And->Respectively azimuth angle and pitch angle of azimuth aperture center moment;
step 4-2: distance-wise FFT operation and multiplication by frequency-domain system function H 1 (f r )
Wherein f r Is a distance frequency variable;
step 4-3: distance-wise IFFT operation and multiplication by an inverse scaling function phi ins (τ)
Wherein f c Is the carrier frequency;
step 4-4: performing distance FFT and time domain interception operation, and multiplying by frequency domain system function H 2 (f r )
6. The pretreatment and improvement PFA-based large strabismus onboard SAR imaging method according to claim 1, wherein in step 5, the calculation method of the input and output interpolation point coordinates of the azimuth Sinc is as follows:
wherein t' is the azimuth time variable after Keystone transformation.
7. The pretreatment and improvement PFA-based large strabismus onboard SAR imaging method according to claim 1, wherein in step 6, the two-dimensional resampled signal is subjected to two-dimensional fourier transform to obtain a final imaging result.
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