CN113376632A - Large squint airborne SAR imaging method based on pretreatment and improved PFA - Google Patents
Large squint airborne SAR imaging method based on pretreatment and improved PFA Download PDFInfo
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Abstract
The invention discloses a preprocessing and improved PFA-based large squint airborne SAR imaging method, which comprises the following steps: step 1: performing Doppler center frequency shift compensation on original echo data and passing through a two-stage cascaded azimuth smoothing filter; step 2: carrying out azimuth extraction on the filtered data and recovering the Doppler frequency shift characteristic; and step 3: estimating the number of shifting points for roughly correcting the range migration trajectory according to the parameters; and 4, step 4: embedding RCM rough correction operation in the distance direction processing based on PCS to obtain the maximum time domain interception point number, thereby reducing the operation amount of subsequent processing; and 5: carrying out azimuth high-precision Sinc interpolation processing to compensate the residual linear motion; step 6: and performing two-dimensional Fourier transform on the azimuth resampled signal to obtain a final imaging result. The invention improves the processing speed by reducing the data rate in two dimensions of the position and the distance through the preprocessing operation and the improvement of PFA, 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 squint airborne SAR imaging method based on pretreatment and improved PFA.
Background
Synthetic Aperture Radar (SAR) breaks through the limitation of real aperture azimuth resolution and can carry out all-weather and all-time observation on a target area. The large squint airborne SAR beam pointing has high flexibility, can perform imaging on a target to be observed in advance and for multiple times, and is widely applied to important fields of resource exploration, disaster monitoring, battlefield reconnaissance, accurate battlefield target attack and the like. The large-squint SAR imaging needs to process mass data, range migration of an echo is large, and two-dimensional coupling of a frequency domain is serious.
In addition, airborne SAR real-time imaging systems typically employ higher Pulse Repetition Frequencies (PRFs) to improve signal-to-noise ratio and reduce transmitter peak power. However, a high PRF may cause a large amount of redundancy in the azimuth direction of the echo data, and thus, the burden of subsequent processing is increased. The direct decimation method for reducing the PRF and the azimuth data rate can cause the azimuth aliasing effect of the frequency spectrum and cause the signal to noise ratio loss to be serious. Meanwhile, the effective imaging scene is far smaller than the radar range data acquisition range, so that range data redundancy is serious, and the algorithm operation amount is increased.
The PFA algorithm adopts a polar coordinate format to store data, completes two-dimensional decoupling of signals by resampling distance and direction, and can effectively solve the problem of moving of the over-resolution unit far away from the central scattering point of an imaging area. Therefore, the conventional PFA algorithm directly images the raw echo with huge data volume, which may seriously increase the computational burden, and also puts a high demand on the memory resource of the chip when implementing hardware.
Disclosure of Invention
The invention aims to solve the technical problem of providing a large squint airborne SAR imaging method based on preprocessing and improved PFA, which improves the processing speed by effectively reducing the data rate in two dimensions of azimuth and distance through preprocessing operation and improved PFA and has the characteristics of easy actual engineering realization, high algorithm efficiency and high imaging quality.
In order to solve the technical problem, the invention provides a large squint airborne SAR imaging method based on pretreatment and PFA improvement, which comprises the following steps:
step 1: performing Doppler center frequency shift compensation on original echo data and passing through a two-stage cascaded azimuth smoothing filter;
step 2: carrying out azimuth extraction on the filtered data and recovering the Doppler frequency shift characteristic;
and step 3: estimating the number of shifting points for roughly correcting the range migration trajectory according to the parameters;
and 4, step 4: embedding RCM (Range Cell Migration) coarse correction operation in the distance direction processing based on PCS (Principle of scale transformation) to obtain the maximum time domain interception point number, thereby reducing the operation amount of subsequent processing;
and 5: carrying out azimuth high-precision Sinc interpolation processing to compensate the residual linear motion;
step 6: and performing two-dimensional Fourier transform on the azimuth resampled signal to obtain a final imaging result.
Preferably, in step 1, the filter order NfoThe calculation method comprises the following steps:
wherein PRF is the pulse repetition frequency; bandwidth B of echo signald＝Bl+Bw，BlAnd BwRespectively the scene bandwidth and the bandwidth caused by the rotation of the beam angle; [ x ] of]Meaning rounding off x.
Preferably, in step 2, the orientation extraction coefficient NdsThe calculation method comprises the following steps:
wherein the content of the first and second substances,
meaning rounding down x, NdsThe extraction coefficient can ensure that the PRF after azimuth extraction is about 1.2 times of the bandwidth of an echo signal, and the Nyquist sampling theorem is satisfied, and the azimuth spectrum has no aliasing phenomenon.
Preferably, in step 3, for coarse correctionNumber of shift points n of operation0The calculation method comprises the following steps:
n0＝[2fs·(Ra-R0)/c]
wherein c is the speed of light; f. ofsDistance to sampling frequency. RaAnd R0The instantaneous distance from the antenna phase center to the scene center and the radar action distance are respectively.
Preferably, in step 4, the RCM rough correction operation is embedded in the distance-to-distance processing based on the PCS to obtain the maximum time-domain intercept point number, which specifically includes the following steps:
step 4-1: the preprocessed echo signal S (t, tau) is multiplied by a scaling function phiscl(τ)
Wherein t and tau are respectively azimuth slow time and distance fast time, and k is frequency modulation slope;
is a distance to scale transformation factor; theta and
instantaneous azimuth angle and pitch angle of the radar antenna phase center; thetarefAnd
respectively is an azimuth angle and a pitch angle of the center of the azimuth aperture at the moment;
step 4-2: distance-wise FFT operation and multiplication by a frequency-domain system function H1(fr)
In the formula (f)rIs a distance frequency variable;
step 4-3: distance-wise IFFT operation and multiplication by an inverse scaling function phiins(τ)
In the formula (f)cIs the carrier frequency;
step 4-4: performing distance direction FFT and time domain interception operation, and multiplying by frequency domain system function H2(fr)
Preferably, in step 5, the method for calculating the coordinates of the input and output interpolation points of the azimuth Sinc includes:
wherein t' is an orientation 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 invention has the beneficial effects that: the method combines the preprocessing operation with the improved PFA algorithm to realize the airborne SAR large squint imaging, effectively reduces the data rate in two dimensions of the direction and the distance through the preprocessing operation and the improved PFA to improve the processing speed, and has the characteristics of easy actual 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 diagram of the pretreatment 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 diagram of the echo of the direct azimuth extraction of the present invention.
FIG. 4(b) is a two-dimensional frequency domain diagram of the echo after being preprocessed according to the present invention.
FIG. 5 is a schematic diagram of a distance-oriented PCS processing flow integrating coarse correction and time-domain truncation according to the present invention.
FIG. 6(a) is a schematic diagram of the RCM trace before rough calibration according to the present invention.
FIG. 6(b) is a schematic diagram of the RCM trace after coarse correction according to the present invention.
Fig. 7(a) is a schematic diagram of the RCM trace of direct time domain truncation according to the present invention.
FIG. 7(b) is a schematic diagram of an RCM trace with coarse correction followed by time-domain truncation according to the present invention.
Fig. 8(a) is a schematic diagram of an actual measurement result of direct time-domain truncation according to the present invention.
FIG. 8(b) is a diagram illustrating the actual measurement result of the embedded coarse calibration operation according to the present invention.
Fig. 9(a) is a schematic diagram of a measured data processing result of the 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 large squint airborne SAR actual measurement scene according to the present invention.
Detailed Description
As shown in fig. 1, a large squint airborne SAR imaging method based on pre-processing and improved PFA comprises the following steps:
step 1: and performing Doppler central frequency shift compensation on the original echo data and passing the original echo data through a two-stage cascaded azimuth smoothing filter.
Order N of filterfoThe calculation method comprises the following steps:
wherein PRF is the pulse repetition frequency; bandwidth B of echo signald＝Bl+Bw，BlAnd BwRespectively the scene bandwidth and the bandwidth caused by the rotation of the beam angle; [ x ] of]Meaning rounding off x.
Step 2: and carrying out azimuth extraction on the filtered data and recovering the Doppler frequency shift characteristic.
Azimuth extraction coefficient NdsThe calculation method comprises the following steps:
wherein the content of the first and second substances,
indicating rounding down on x. Will NdsThe extraction coefficient can ensure that the PRF after azimuth extraction is about 1.2 times of the bandwidth of an echo signal, and can meet the Nyquist sampling theorem, and the azimuth spectrum has no aliasing phenomenon.
And step 3: and estimating the number of shifting points for roughly correcting the range migration trajectory according to the parameters.
Number of coarse correction shift points n0The calculation method comprises the following steps:
n0＝[2fs·(Ra-R0)/c]
wherein c is the speed of light; f. ofsDistance to sampling frequency. RaAnd R0The instantaneous distance from the antenna phase center to the scene center and the radar action distance are respectively.
And 4, step 4: the coarse correction and the time domain truncation operation are embedded into the distance-oriented PCS processing to eliminate the high-order distance bending of the target and reduce the subsequent processing data volume.
First, the preprocessed echo signal S (t, τ) is multiplied by a scaling function φscl(τ)
Where t and τ are the azimuth slow time and the range fast time, respectively. k is the frequency modulation slope;
is a distance to scale transformation factor; theta and
instantaneous azimuth angle and pitch angle of the radar antenna phase center; thetarefAnd
respectively is an azimuth angle and a pitch angle of the center of the azimuth aperture at the moment;
secondly, distance-wise FFT operation is performed and multiplied by a frequency domain system function H1(fr)
In the formula (f)rIs a distance frequency variable.
Then, a distance-wise IFFT operation is performed and multiplied by an inverse scaling function φins(τ)
In the formula (f)cIs the carrier frequency.
Finally, distance direction FFT and time domain interception operation are carried out and multiplied by a frequency domain system function H2(fr)
And 5: and performing azimuth high-precision Sinc interpolation processing to compensate the residual linear motion.
The calculation method of the input and output interpolation point coordinates of the azimuth Sinc comprises the following steps:
wherein t' is an orientation time variable after Keystone transformation.
Step 6: and performing two-dimensional Fourier transform on the azimuth resampled signal to obtain a final imaging result.
The preprocessing mainly comprises two times of azimuth filtering and azimuth extraction, the azimuth filtering can improve the signal-to-noise ratio and simultaneously filter redundant Doppler frequencies outside the imaging bandwidth, so that the signal-to-noise ratio loss and the azimuth spectrum aliasing phenomenon caused by the azimuth extraction are avoided, and the detailed implementation flow is shown in fig. 2. Taking the two-dimensional spectrum of the original echo in fig. 3 as an example, when the azimuth doppler bandwidth is much larger than the imaging target bandwidth, the azimuth redundant data will cause a severe computational burden on the subsequent imaging processing. The comparison results of the direct extraction mode and the output of the preprocessing are shown in fig. 4(a) and 4(b), the azimuth redundant doppler bandwidth of the preprocessed result is filtered, the noise is effectively suppressed, and the signal-to-noise ratio is improved.
The inclination of a Range Cell Migration (RCM) track under a large squint 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 present invention improves this, and performs a rough correction on the RCM trajectory before the time-domain truncation to obtain the maximum effective truncation ratio, and a specific flow is shown in fig. 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 the number of correction points; and then, moving the RCM track in the time domain to realize the rough correction of the linear walking error, thereby ensuring the correctness and the maximum efficiency of time domain interception. As can be seen from fig. 7(a) and 7(b), the effect of suppressing the range-wise redundant data by directly performing time-domain truncation without coarse correction is very limited, and the integrity of the RCM trace is easily damaged, thereby causing the range-wise entanglement phenomenon.
The large squint airborne SAR measured data is used to further verify the validity of the present invention. Comparison of the measured results of fig. 8(a) and 8(b) fully illustrates the importance of the coarse correction operation in combination with the time-domain truncation. The distance winding phenomenon caused by incomplete RCM tracks can be eliminated by embedding rough correction operation in time domain interception, and further the worsening of the SAR image focusing effect is avoided. The final output results of the invention are shown in fig. 9(a) and 9(b), which are result images of different actual measurement scenes, respectively, and the scenes such as farmlands, roads, house buildings and the like on the ground can be clearly distinguished from the images, thus the image focusing effect is good. Therefore, the method is suitable for large squint airborne SAR imaging, can effectively reduce the data rate in two dimensions of azimuth and distance to improve the processing speed, and has the characteristics of easy realization of actual engineering, high algorithm efficiency and high imaging quality.
Claims (7)
1. A large squint airborne SAR imaging method based on pretreatment and improved PFA is characterized by comprising the following steps:
step 1: performing Doppler center frequency shift compensation on original echo data and passing through a two-stage cascaded azimuth smoothing filter;
step 2: carrying out azimuth extraction on the filtered data and recovering the Doppler frequency shift characteristic;
and step 3: estimating the number of shifting points for roughly correcting the range migration trajectory according to the parameters;
and 4, step 4: embedding RCM rough correction operation in the distance direction processing based on PCS to obtain the maximum time domain interception point number, thereby reducing the operation amount of subsequent processing;
and 5: carrying out azimuth high-precision Sinc interpolation processing to compensate the residual linear motion;
step 6: and performing two-dimensional Fourier transform on the azimuth resampled signal to obtain a final imaging result.
2. The pre-processing and PFA-improved based large squint airborne SAR imaging method as claimed in claim 1, characterized in that in step 1, the filter order NfoThe calculation method comprises the following steps:
wherein PRF is the pulse repetition frequency; bandwidth B of echo signald＝Bl+Bw，BlAnd BwRespectively the scene bandwidth and the bandwidth caused by the rotation of the beam angle; [ x ] of]Meaning rounding off x.
3. The pre-treatment and PFA-modified based large squint airborne SAR imaging method as claimed in claim 1, wherein in step 2, the methodBit extraction coefficient NdsThe calculation method comprises the following steps:
wherein the content of the first and second substances,
meaning rounding down x, NdsThe extraction coefficient can ensure that the PRF after azimuth extraction is about 1.2 times of the bandwidth of an echo signal, and the Nyquist sampling theorem is satisfied, and the azimuth spectrum has no aliasing phenomenon.
4. The pre-processing and improved PFA-based large squint airborne SAR imaging method as claimed in claim 1, characterized in that in step 3, the number of shift points n for coarse correction operation0The calculation method comprises the following steps:
n0＝[2fs·(Ra-R0)/c]
wherein c is the speed of light; f. ofsDistance to sampling frequency. RaAnd R0The instantaneous distance from the antenna phase center to the scene center and the radar action distance are respectively.
5. The preprocessing and PFA-based large squint airborne SAR imaging method according to claim 1, wherein in step 4, RCM coarse correction operation is embedded in the PCS-based distance direction processing to obtain the maximum time domain intercept point number, specifically comprising the following steps:
step 4-1: the preprocessed echo signal S (t, tau) is multiplied by a scaling function phiscl(τ)
Wherein t and tau are respectively azimuth slow time and distance fast time, and k is frequency modulation slope;
is a distance to scale transformation factor; theta and
instantaneous azimuth angle and pitch angle of the radar antenna phase center; thetarefAnd
respectively is an azimuth angle and a pitch angle of the center of the azimuth aperture at the moment;
step 4-2: distance-wise FFT operation and multiplication by a frequency-domain system function H1(fr)
In the formula (f)rIs a distance frequency variable;
step 4-3: distance-wise IFFT operation and multiplication by an inverse scaling function phiins(τ)
In the formula (f)cIs the carrier frequency;
step 4-4: performing distance direction FFT and time domain interception operation, and multiplying by frequency domain system function H2(fr)
6. The preprocessing and PFA-improving-based large squint airborne SAR imaging method according to claim 1, wherein in step 5, the calculation method of the coordinates of the input and output interpolation points of the azimuth Sinc is as follows:
wherein t' is an orientation time variable after Keystone transformation.
7. The pre-processing and improved PFA-based large squint airborne SAR imaging method as claimed in claim 1, characterized in that in step 6, two-dimensional Fourier transform is performed on the two-dimensional resampled signals to obtain the final imaging result.
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