阅读说明：本技术 一种星载聚束合成孔径雷达高分辨实时成像方法 (High-resolution real-time imaging method for satellite-borne bunching synthetic aperture radar ) 是由 刘彦斌 孙光才 邢孟道 王石语 于 2021-08-31 设计创作，主要内容包括：本发明涉及一种星载聚束合成孔径雷达高分辨实时成像方法,包括步骤：对星载SAR聚束模式回波信号在方位向上进行子孔径划分,得到子孔径回波信号；对子孔径回波信号进行方位时间尺度变换,得到方位时间尺度变换信号；对方位时间尺度信号进行高阶项相位补偿,得到高阶相位补偿信号；对高阶相位补偿信号进行距离徙动校正和距离脉压等距离向处理,得到距离向处理信号；将距离向处理信号的双曲相位转变为标准的二次相位,得到相位转换信号；对相位转变信号依次进行去调频操作和剩余相位补偿,得到目标子孔径图像；对若干目标子孔径图像进行相干叠加,得到全分辨率图像。该方法可以在子孔径数据录取的同时进行成像处理,具有很好的实时性。(The invention relates to a high-resolution real-time imaging method of satellite-borne bunching synthetic aperture radar, which comprises the following steps: sub-aperture division is carried out on the echo signals of the satellite-borne SAR bunching mode in the azimuth direction to obtain sub-aperture echo signals; carrying out azimuth time scale transformation on the sub-aperture echo signals to obtain azimuth time scale transformation signals; carrying out high-order phase compensation on the orientation time scale signal to obtain a high-order phase compensation signal; performing range migration correction and range-to-pulse equidistant distance direction processing on the high-order phase compensation signal to obtain a range direction processing signal; converting the hyperbolic phase of the distance direction processing signal into a standard secondary phase to obtain a phase conversion signal; sequentially carrying out frequency modulation removal operation and residual phase compensation on the phase transition signal to obtain a target sub-aperture image; and carrying out coherent superposition on the plurality of target sub-aperture images to obtain a full-resolution image. The method can record the sub-aperture data and perform imaging processing at the same time, and has good real-time performance.)

1. A high-resolution real-time imaging method of satellite-borne beaming synthetic aperture radar is characterized by comprising the following steps:

s1, sub-aperture division is carried out on the satellite-borne SAR bunching mode echo signals in the azimuth direction to obtain sub-aperture echo signals;

s2, performing azimuth time scale transformation on the sub-aperture echo signals to obtain azimuth time scale transformation signals;

s3, performing high-order phase compensation on the azimuth time scale signal to obtain a high-order phase compensation signal;

s4, performing range migration correction and equidistant range-direction processing to the high-order phase compensation signal to obtain a range-direction processing signal;

s5, converting the hyperbolic phase of the distance direction processing signal into a standard secondary phase to obtain a phase conversion signal;

s6, sequentially carrying out frequency modulation removing operation and residual phase compensation on the phase transition signal to obtain a target sub-aperture image;

and S7, carrying out coherent superposition on the plurality of target sub-aperture images to obtain a full-resolution image.

2. The high-resolution real-time imaging method of the spaceborne beaming synthetic aperture radar as claimed in claim 1, wherein the sub-aperture echo signals are:

wherein, w_r(. is a distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, t_subFor azimuth slow time of the sub-aperture data, γ is the modulation frequency, λ is the signal wavelength, c is the speed of light, j is the imaginary part of the signal, R t_subIs inclined toDistance history.

3. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S2 comprises:

s21, performing range-to-Fourier transform on the sub-aperture echo signal to obtain a first range-to-Fourier transform signal;

s22, performing Doppler center translation on the first distance Fourier transform signal to obtain a translation signal;

and S23, processing the translation signal in a distance frequency domain and orientation time domain by using an orientation time scale transformation method to obtain an orientation time scale transformation signal.

4. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 3, wherein the step S23 comprises:

according to slope history R t_subRule of equivalent velocity in the interior changing with azimuth time

Wherein v is₀Is the initial equivalent velocity, t_kIs the central time of the subaperture data, t_sub' Azimuth slow time for New sub-Aperture data, t_subThe azimuth slow time of the sub-aperture data is shown, and a is the equivalent acceleration;

the relation between the central time of the sub-aperture data in the new azimuth time domain and the azimuth slow time of the sub-aperture data is:

t_k+t_sub′＝t_k+t_sub+εt_k+t_sub ²

wherein ε is a/2v₀Is a scale transformation factor;

obtaining a new sub-aperture echo signal slope distance model of the azimuth time domain according to the relation between the central time of the sub-aperture data in the new azimuth time domain and the azimuth slow time of the sub-aperture data:

wherein R is_BAs the closest distance to the target, t_c′＝t_c+εt_c ²ω_rε is a scale transformation factor, w_r(. is a distance window function of the signal, beta₃Is a third order coefficient, beta₄Is a third order coefficient;

processing the translation signal by using the new sub-aperture echo signal slope distance model of the azimuth time domain to obtain an azimuth time scale transformation signal:

wherein f is_rIs the range-wise frequency, w, of the subaperture data_a(. h) is an azimuth window function, j is the imaginary part of the signal, gamma is the frequency modulation rate, c is the speed of light, f_cIs the carrier frequency, f_dcThe doppler center.

5. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S3 comprises:

s31, carrying out azimuth Fourier transform on the azimuth time scale transform signal to obtain a first azimuth Fourier transform signal;

s32, obtaining a high-order phase compensation function according to the first azimuth Fourier transform signal;

and S33, compensating the first azimuth Fourier transform signal by using the high-order phase compensation function to obtain the high-order phase compensation signal.

6. The high-resolution real-time imaging method of the spaceborne beaming synthetic aperture radar as recited in claim 5, wherein the high-order phase compensation signal is:

wherein, w_r(. is a distance window function of the signal, f_rIs the range-wise frequency, w, of the subaperture data_a(. is a function of the azimuth window, f_subFor azimuthal frequency, f, of the subaperture data_dcIs the Doppler center, j is the imaginary part of the signal, gamma is the frequency modulation rate, R_BIs the closest distance of the target, f_cIs the carrier frequency, c is the speed of light, v₀Is the initial equivalent velocity, t_kThe center time of the sub-aperture data.

7. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S4 comprises:

s41, performing distance inverse Fourier transform on the high-order phase compensation signal to obtain a first distance inverse Fourier transform signal;

s42, performing line frequency modulation scaling processing on the first distance inverse Fourier transform signal by using a line frequency modulation scaling function to obtain a line frequency modulation scaling processing signal;

s43, performing distance Fourier transform on the linear frequency modulation scale processing signal to obtain a second distance Fourier transform signal;

and S44, performing range migration correction and range equidistant processing to the second range Fourier transform signal by using an equidistant processing function to obtain the range processed signal.

8. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S5 comprises:

s51, performing distance inverse Fourier transform on the distance processed signal to obtain a second distance inverse Fourier transform signal;

and S52, converting the hyperbolic phase of the second distance inverse Fourier transform signal into a standard secondary phase by using a phase conversion function to obtain the phase conversion signal.

9. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S6 comprises:

s61, performing azimuth inverse Fourier transform on the phase conversion signal to obtain an azimuth inverse Fourier transform signal;

s62, performing frequency modulation removing processing on the azimuth direction inverse Fourier transform signal by using a frequency modulation removing processing function to obtain a frequency modulation removing processing signal;

s63, performing azimuth Fourier transform on the de-frequency modulation processing signal to obtain a second azimuth Fourier transform signal;

and S64, performing residual phase compensation on the second azimuth Fourier transform signal by using a residual phase compensation function to obtain the target sub-aperture image.

10. The method for high-resolution real-time imaging of the spaceborne beamforming synthetic aperture radar as claimed in claim 1, wherein the step S7 comprises:

carrying out coherent superposition on a plurality of target sub-aperture images in an image domain to obtain a full-resolution image:

wherein, w_r(. is a distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, f_subFor azimuthal frequency, K, of the subaperture data_scl＝-2v²/R_s，R_sAs the scene center distance, t_cAt the azimuth time of the target, j is the imaginary part of the signal.

Technical Field

The invention belongs to the technical field of radar signal processing, and particularly relates to a high-resolution real-time imaging method for a satellite-borne bunching synthetic aperture radar.

Background

A Synthetic Aperture Radar (SAR) as an active microwave imaging system can perform high-resolution imaging on a ground scene all day long. The satellite-borne SAR has wide application in the fields of topographic mapping, resource detection, disaster monitoring, target identification and the like due to the excellent global observation capability of the satellite-borne SAR. With the continuous improvement of application requirements, the satellite-borne SAR technology is also continuously developed. In order to complete some urgent observation tasks quickly and efficiently, the satellite-borne SAR needs to perform high-resolution real-time observation on a specific area, so that high-resolution real-time imaging processing in a beamforming mode is developed in the satellite-borne SAR technology. However, in the satellite-borne SAR bunching mode high-resolution real-time imaging, the motion trajectory of the satellite-borne SAR is more complex due to the long synthetic aperture time, the data volume of an echo signal is increased sharply, and great challenges are brought to the high-resolution real-time imaging. If the echo signals with such a large data volume are transmitted to the ground for processing, a large amount of data transmission time is spent, so that the direct transmission of the echo signals in the satellite-borne SAR beamforming mode high-resolution real-time imaging is not suitable. On the contrary, a more efficient method is to process the echo signal with an efficient high resolution real-time imaging algorithm, then store the imaging result as an image of several Mbytes, and finally transmit the image to the ground, which saves a lot of time compared to transmitting the echo signal directly. The core of the high-resolution real-time imaging processing of the satellite-borne SAR bunching mode is to generate a high-quality imaging result as soon as possible after the recording of echo data is completed. Where existing hardware configurations have been advanced, accelerating the generation of high quality imaging results by seeking improvements and updates in hardware may require more development costs. Therefore, it is a good choice to seek a breakthrough in high resolution real-time imaging algorithms with the goal of generating high quality imaging results as quickly as possible.

The high-resolution real-time imaging algorithm of the satellite-borne SAR spotlight mode mainly needs to solve three problems. The first problem is the azimuth spectrum aliasing caused by the azimuth bandwidth of the signal in the satellite-borne SAR beamforming mode being larger than the pulse repetition frequency. The second problem is the equivalent velocity space-variant caused by the satellite-borne SAR bending orbit. In the satellite-borne SAR bunching mode high-resolution imaging, the real track of the satellite-borne SAR in a long synthetic aperture time is not an approximate linear track any more, but a curved track; the bending track can make the equivalent speed have space variation, so that the traditional slope distance model is not accurate any more, and the focusing effect is influenced. The third problem is to improve the real-time performance of the imaging process and shorten the generation time of the imaging result after the recording of the echo data is finished.

However, most of the existing imaging algorithms can only solve the first two problems, and cannot realize the real-time performance of imaging processing, so that in actual processing, a more efficient high-resolution real-time imaging algorithm needs to be researched.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a high-resolution real-time imaging method of a satellite-borne spotlight synthetic aperture radar. The technical problem to be solved by the invention is realized by the following technical scheme:

the embodiment of the invention provides a high-resolution real-time imaging method for a satellite-borne bunching synthetic aperture radar, which comprises the following steps:

s1, sub-aperture division is carried out on the satellite-borne SAR bunching mode echo signals in the azimuth direction to obtain sub-aperture echo signals;

s2, performing azimuth time scale transformation on the sub-aperture echo signals to obtain azimuth time scale transformation signals;

s3, performing high-order phase compensation on the azimuth time scale signal to obtain a high-order phase compensation signal;

s4, performing range migration correction and equidistant range-direction processing to the high-order phase compensation signal to obtain a range-direction processing signal;

s5, converting the hyperbolic phase of the distance direction processing signal into a standard secondary phase to obtain a phase conversion signal;

s6, sequentially carrying out frequency modulation removing operation and residual phase compensation on the phase transition signal to obtain a target sub-aperture image;

and S7, carrying out coherent superposition on the plurality of target sub-aperture images to obtain a full-resolution image.

In one embodiment of the present invention, the sub-aperture echo signal is:

where wr (-) is the distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, t_subFor azimuth slow time of the sub-aperture data, γ is the modulation frequency, λ is the signal wavelength, c is the speed of light, j is the imaginary part of the signal, R t_subIs the course of the slope distance.

In one embodiment of the present invention, step S2 includes:

s21, performing range-to-Fourier transform on the sub-aperture echo signal to obtain a first range-to-Fourier transform signal;

s22, performing Doppler center translation on the first distance Fourier transform signal to obtain a translation signal;

and S23, processing the translation signal in a distance frequency domain and orientation time domain by using an orientation time scale transformation method to obtain an orientation time scale transformation signal.

In one embodiment of the present invention, step S23 includes:

according to slope history R t_subRule of equivalent velocity in the interior changing with azimuth time

Wherein v is₀Is the initial equivalent velocity, t_kIs the central time of the subaperture data, t_sub' Azimuth slow time for New sub-Aperture data, t_subThe azimuth slow time of the sub-aperture data is shown, and a is the equivalent acceleration;

the relation between the central time of the sub-aperture data in the new azimuth time domain and the azimuth slow time of the sub-aperture data is:

t_k+t_sub′＝t_k+t_sub+εt_k+t_sub ²

wherein ε is a/2v₀Is a scale transformation factor;

obtaining a new sub-aperture echo signal slope distance model of the azimuth time domain according to the relation between the central time of the sub-aperture data in the new azimuth time domain and the azimuth slow time of the sub-aperture data:

wherein R is_BIs the closest distance of the target, t_c′＝t_c+εt_c ²wr (-) with epsilon as scale transformation factor, wr (-) as a function of the distance window of the signal, beta₃Is a third order coefficient, beta₄Is a third order coefficient;

processing the translation signal by using the new sub-aperture echo signal slope distance model of the azimuth time domain to obtain an azimuth time scale transformation signal:

wherein f is_rIs the range-wise frequency, w, of the subaperture data_a(. h) is an azimuth window function, j is the imaginary part of the signal, gamma is the frequency modulation rate, c is the speed of light, f_cIs the carrier frequency, f_dcThe doppler center.

In one embodiment of the present invention, step S3 includes:

s31, carrying out azimuth Fourier transform on the azimuth time scale transform signal to obtain a first azimuth Fourier transform signal;

s32, obtaining a high-order phase compensation function according to the first azimuth Fourier transform signal;

and S33, compensating the first azimuth Fourier transform signal by using the high-order phase compensation function to obtain the high-order phase compensation signal.

In one embodiment of the present invention, the higher order phase compensation signal is:

wherein, w_r(. is a distance window function of the signal, f_rIs the range-wise frequency, w, of the subaperture data_a(. is a function of the azimuth window, f_subFor azimuthal frequency, f, of the subaperture data_dcIs the Doppler center, j is the imaginary part of the signal, gamma is the frequency modulation rate, R_BIs the closest distance of the target, f_cIs the carrier frequency, c is the speed of light, v₀Is the initial equivalent velocity, t_kThe center time of the sub-aperture data.

In one embodiment of the present invention, step S4 includes:

s41, performing distance inverse Fourier transform on the high-order phase compensation signal to obtain a first distance inverse Fourier transform signal;

s42, performing line frequency modulation scaling processing on the first distance inverse Fourier transform signal by using a line frequency modulation scaling function to obtain a line frequency modulation scaling processing signal;

s43, performing distance Fourier transform on the linear frequency modulation scale processing signal to obtain a second distance Fourier transform signal;

and S44, performing range migration correction and range equidistant processing to the second range Fourier transform signal by using an equidistant processing function to obtain the range processed signal.

In one embodiment of the present invention, step S5 includes:

s51, performing distance inverse Fourier transform on the distance processed signal to obtain a second distance inverse Fourier transform signal;

and S52, converting the hyperbolic phase of the second distance inverse Fourier transform signal into a standard secondary phase by using a phase conversion function to obtain the phase conversion signal.

In one embodiment of the present invention, step S6 includes:

s61, performing azimuth inverse Fourier transform on the phase conversion signal to obtain an azimuth inverse Fourier transform signal;

s62, performing frequency modulation removing processing on the azimuth direction inverse Fourier transform signal by using a frequency modulation removing processing function to obtain a frequency modulation removing processing signal;

s63, performing azimuth Fourier transform on the de-frequency modulation processing signal to obtain a second azimuth Fourier transform signal;

and S64, performing residual phase compensation on the second azimuth Fourier transform signal by using a residual phase compensation function to obtain the target sub-aperture image.

In one embodiment of the present invention, step S7 includes:

carrying out coherent superposition on a plurality of target sub-aperture images in an image domain to obtain a full-resolution image:

wherein, w_r(. is a distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, f_subFor azimuthal frequency, K, of the subaperture data_scl＝-2v²/R_s，R_sAs the scene center distance, t_cAt the azimuth time of the target, j is the imaginary part of the signal.

Compared with the prior art, the invention has the beneficial effects that:

the real-time imaging method divides the sub-apertures, processes the sub-aperture signals and then performs coherent superposition, each sub-aperture of the coherent superposition is independent, imaging processing can be performed while sub-aperture data is recorded, the image generation time after the recording of the echo data is finished can be shortened to the imaging processing time of one sub-aperture data, good real-time performance is achieved, and a high-efficiency high-resolution real-time imaging algorithm is achieved.

Drawings

Fig. 1 is a schematic flow chart of a high-resolution real-time imaging method of a satellite-borne beaming synthetic aperture radar according to an embodiment of the present invention;

FIG. 2 is a schematic flow chart of a specific method for high-resolution real-time imaging of a spaceborne beaming synthetic aperture radar according to an embodiment of the present invention;

FIG. 3 is a schematic view of an imaging geometry provided by an embodiment of the present invention;

fig. 4 is a beamforming mode point target imaging result obtained by the high-resolution real-time imaging method of the satellite-borne beamforming synthetic aperture radar in the embodiment;

FIG. 5 is a simulation result of the phase spread function and the profile spread function of the point target A marked in FIG. 4 in the azimuth direction and the distance direction;

6 a-6 f are imaging processes of sub-aperture coherent superposition of a point object A in an image domain;

7 a-7 f are the process of the spectral change of the azimuth signal after the coherent superposition of the sub-apertures of the point target A;

fig. 8 is a result of imaging processing performed on beamforming mode actual measurement data by the satellite-borne beamforming synthetic aperture radar high-resolution real-time imaging method according to the embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to specific examples, but the embodiments of the present invention are not limited thereto.

Example one

Referring to fig. 1 and fig. 2, fig. 1 is a schematic flow chart of a high-resolution real-time imaging method of a space-borne beamformed synthetic aperture radar according to an embodiment of the present invention, and fig. 2 is a schematic flow chart of a specific flow chart of the high-resolution real-time imaging method of the space-borne beamformed synthetic aperture radar according to an embodiment of the present invention. The real-time imaging method comprises the following steps:

and S1, sub-aperture division is carried out on the satellite-borne SAR bunching mode echo signals in the azimuth direction to obtain sub-aperture echo signals.

Specifically, taking 1024 points as an example of the echo signal, dividing the echo signal into 8 apertures by performing an equality counting, each aperture includes 128 points.

According to the method, the problem of azimuth spectrum aliasing caused by the fact that the azimuth bandwidth of the signal in the satellite-borne SAR beamforming mode is larger than the pulse repetition frequency is solved by sub-aperture division of the echo signal.

The slant range model of each sub-aperture echo signal of the satellite-borne SAR beamforming mode is represented as follows:

wherein, t_c,R_BThe position of the point target at the azimuth-distance axis,is an equivalent velocity, v₀Is the initial equivalent velocity, t_kIs the central time of the subaperture data, t_subIs the azimuth slow time of the sub-aperture data, a is the equivalent acceleration, beta₃Is a third order coefficient, beta₄Is a third order coefficient.

According to the slant range model of the sub-aperture echo signal, the expression of the sub-aperture echo signal can be obtained as follows:

wherein, w_r(. is a distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, t_subFor azimuth slow time of the sub-aperture data, γ is the modulation frequency, λ is the signal wavelength, c is the speed of light, j is the imaginary part of the signal, R t_subIs the course of the slope distance.

And S2, performing azimuth time scale transformation on the sub-aperture echo signals to obtain azimuth time scale transformation signals. The method specifically comprises the following steps:

and S21, performing range Fourier transform on the sub-aperture echo signals to obtain first range Fourier transform signals.

The first distance fourier transform signal is expressed as follows:

wherein f is_rIs the range-wise frequency, f, of the subaperture data_cIs the carrier frequency, t_subFor azimuthal slow time, w, of sub-aperture data_r(. is a distance window function of the signal, w_a(. cndot.) is an azimuth window function, j is, γ is the tuning frequency, and c is the speed of light.

S22, performing Doppler center f on the first distance Fourier transform signal_dcAnd translating to obtain a translation signal.

And S23, processing the translation signal in a distance frequency domain and orientation time domain by using an orientation time scale transformation method to obtain an orientation time scale transformation signal.

Specifically, the first distance is a slope history R t in the fourier transform signal_subThe equivalent speed is changed along with the azimuth time, therefore, the azimuth space-variant of the equivalent speed can be eliminated by adopting an azimuth time scale transformation method, and the following steps are carried out:

wherein v is₀Is the initial equivalent velocity, t_kIs the central time of the subaperture data, t_sub' Azimuth slow time for New sub-Aperture data, t_subThe azimuth slow time of the sub-aperture data is shown, and a is the equivalent acceleration;

further obtaining the relation between the center time of the sub-aperture data in the new azimuth time domain and the azimuth slow time of the sub-aperture data as follows:

t_k+t_sub＝t_k+t_sub+εt_k+t_sub ²

wherein ε is a/2v₀Is a scale transformation factor;

therefore, a new subaperture echo signal slant distance model in the azimuth time domain can be obtained according to the relation between the central time of the subaperture data in the new azimuth time domain and the azimuth slow time of the subaperture data:

wherein R is_BIs the closest distance of the target, t_c＝t_c+t_c ²wr (-) and ε are scale transformation factors, w_r(. is a distance window function of the signal, beta₃Is a third order coefficient, beta₄Is a third order coefficient.

Then, using the new sub-aperture echo signal slant range model of the azimuth time domain to the Doppler center f_dcAnd (3) carrying out azimuth time scale transformation on the translation signal after translation to obtain an azimuth time scale transformation signal:

wherein f is_rIs the range-wise frequency, w, of the subaperture data_a(. h) is an azimuth window function, j is the imaginary part of the signal, gamma is the frequency modulation rate, c is the speed of light, f_cIs the carrier frequency, f_dcThe doppler center.

According to the method, the azimuth time scale transformation is carried out on the echo signals, the azimuth space-variant property of the equivalent speed is eliminated, and the problem of the equivalent speed space-variant caused by the satellite-borne SAR curved track is solved.

For convenience of description, t is as follows_sub' and t_c' use t_subAnd t_cTo indicate.

And S3, performing high-order phase compensation on the azimuth time scale signal to obtain a high-order phase compensation signal. The method specifically comprises the following steps:

s31, carrying out azimuth Fourier transform on the azimuth time scale transform signal to obtain a first azimuth Fourier transform signal:

wherein, w_r(. is a distance window function of the signal, f_rIs the range-wise frequency, w, of the subaperture data_a(. is a function of the azimuth window, f_subFor azimuthal frequency, f, of the subaperture data_dcIs the Doppler center, j is the imaginary part of the signal, gamma is the frequency modulation rate, t_cIs the azimuth time of the target, t_kIs the center time of the subaperture data, R_BTo f is_cIs the carrier frequency, c is the speed of light, v₀To initial equivalent velocity, beta₃Is a third order coefficient, beta₄Is a third order coefficient.

S32, obtaining a high-order phase compensation function according to the first azimuth Fourier transform signal:

s33, compensating the first directional fourier transform signal in step S31 in the two-dimensional frequency domain by using the high order phase compensation function, and multiplying the first directional fourier transform signal by the high order phase compensation function to obtain the high order phase compensation signal:

wherein, w_r(. is a distance window function of the signal, f_rIs the range-wise frequency, w, of the subaperture data_a(. is a function of the azimuth window, f_sub is the azimuth frequency, fd, of the subaperture data_cIs the Doppler center, j is the imaginary part of the signal, gamma is the frequency modulation rate, R_BIs the closest distance of the target, f_cIs the carrier frequency, c is the speed of light, v₀Is the initial equivalent velocity, t_kThe center time of the sub-aperture data.

And S4, performing range migration correction and range-direction equidistant processing on the high-order phase compensation signal to obtain a range-direction processing signal. The method specifically comprises the following steps:

and S41, performing distance inverse Fourier transform on the high-order phase compensation signal to obtain a first distance inverse Fourier transform signal.

Wherein the content of the first and second substances,theta is the angle of the oblique view,f_aM＝2v₀/λ。

and S42, performing line frequency modulation scaling processing on the first distance inverse Fourier transform signal by using a line frequency modulation scaling function to obtain the line frequency modulation scaling processing signal.

Specifically, the line tone scaling function is:

wherein the content of the first and second substances,

the linear frequency scaling function is multiplied by the first distance inverse fourier transform signal in step S41 to complete the linear frequency scaling processing, and obtain the linear frequency scaling processing signal.

And S43, performing distance Fourier transform on the linear frequency modulation standard processing signal to obtain a second distance Fourier transform signal. For a specific method of fourier transform of the distance, please refer to the above steps, which are not described herein.

And S44, performing range migration correction and range equidistant processing to the second range Fourier transform signal by using an equidistant processing function to obtain the range processed signal.

Specifically, the equidistant processing function is:

and multiplying the equidistant migration processing function by the second range Fourier transform signal to complete the range migration correction and the equidistant migration processing from the pulse pressure so as to obtain a range processing signal.

And S5, converting the hyperbolic phase of the distance direction processing signal into a standard secondary phase to obtain a phase conversion signal. The method specifically comprises the following steps:

and S51, performing distance inverse Fourier transform on the distance processed signal to obtain a second distance inverse Fourier transform signal. For a specific method of inverse fourier transform of the distance, please refer to the above steps, which are not described herein.

And S52, converting the hyperbolic phase of the second distance inverse Fourier transform signal into a standard secondary phase by using a phase conversion function to obtain the phase conversion signal.

Specifically, the phase transfer function is:

wherein, K_scl＝-2v²/λR_s，R_sIs the scene center distance.

The phase conversion signal obtained by conversion is:

and S6, sequentially carrying out frequency modulation removal operation and residual phase compensation on the phase transition signal to obtain a target sub-aperture image. The method specifically comprises the following steps:

and S61, performing azimuth inverse Fourier transform on the phase conversion signal to obtain an azimuth inverse Fourier transform signal.

Specifically, the azimuth inverse fourier transform signal is:

s_k t,t_sub＝w_r t w_a t_k+t_sub-t_c×exp(jπK_scl t_k+t_sub-t_c)×exp-j2πf_dc t_k+t_sub

and S62, performing frequency modulation removal processing on the azimuth direction inverse Fourier transform signal by using a frequency modulation removal processing function to obtain a frequency modulation removal processing signal.

Specifically, the de-frequency modulation processing function is:

H₅ t,t_sub＝exp(-jπK_scl t_k+t_sub)×exp j2πf_dct_k+t_sub

using a dechirp processing function H₅ t,t_subAnd multiplying the azimuth direction inverse Fourier transform signal in the step S61 to complete the frequency modulation removing processing, and obtaining a frequency modulation removing processing signal.

And S63, performing azimuth Fourier transform on the de-frequency modulation processed signal to obtain a second azimuth Fourier transform signal. For a specific method of the direction fourier transform, please refer to the above steps, which are not described herein again.

And S64, performing residual phase compensation on the second azimuth Fourier transform signal by using a residual phase compensation function to obtain the target sub-aperture image.

In particular, the residual phase compensation function H₆ t,f_subComprises the following steps:

H₆ t,f_sub＝exp-j2πf_subt_k

and multiplying the residual phase compensation function by the second azimuth Fourier transform signal in the step S63 to complete residual phase compensation, and obtaining the target sub-aperture image.

The target sub-aperture image is a low resolution sub-aperture image, which is represented as:

s_k t,f_sub＝w_r t w_a f_sub+K_sclt_c exp jπK_sclt_c2

and S7, carrying out coherent superposition on the plurality of target sub-aperture images to obtain a full-resolution image.

Carrying out coherent superposition on a plurality of target sub-aperture images in an image domain to obtain a full-resolution image:

wherein, w_r(. is a distance window function of the signal, w_a(. h) is an azimuth window function, t is a range forward time, f_subFor azimuthal frequency, K, of the subaperture data_scl＝2v²/λR_s，R_sAs the scene center distance, t_cAt the azimuth time of the target, j is the imaginary part of the signal.

Referring to fig. 3, fig. 3 is a schematic diagram of an imaging geometry according to an embodiment of the present invention, which shows a schematic diagram of recording echo data. The real-time imaging method divides the sub-apertures, performs coherent superposition after processing the sub-aperture signals, and each sub-aperture of the coherent superposition is independent, so that imaging processing can be performed while sub-aperture data is recorded, the image generation time after the recording of the echo data is finished can be shortened to the imaging processing time of one sub-aperture data, and the real-time imaging method has good real-time performance and realizes a high-efficiency high-resolution real-time imaging algorithm.

In conclusion, the real-time imaging method of the embodiment can effectively solve three problems of the high-resolution real-time imaging in the satellite-borne SAR bunching mode: the first problem is the azimuth spectrum aliasing caused by the fact that the azimuth bandwidth of a signal in the satellite-borne SAR beamforming mode is larger than the pulse repetition frequency; the second problem is the equivalent velocity space-variant caused by the satellite-borne SAR curved track; a third problem is to improve the real-time performance of the imaging process.

Example two

On the basis of the first embodiment, the present embodiment further verifies and explains the effect of the real-time imaging method through simulation.

Simulation one: point simulation effect in bunching mode

Simulation parameter table for table-bunching mode

Mode(s)	Bunching mode
		Track height (Km)	755
Wavelength (m)	0.055
		Equivalent velocity	7063
Scene center distance (km)	928
		Distance of rotation center (km)	928
Pulse repetition frequency (Hz)	3549
		Sampling rate (MHz)	360
Bandwidth (MHz)	300
		Orientation scene (Km)	4
Distance scene (Km)	2

The point target simulation parameters of the bunching mode are shown in table one, and the lattice is distributed according to 3 (distance) × 3 (azimuth). Fig. 4 is a beamforming mode point target imaging result obtained by the high-resolution real-time imaging method of the satellite-borne beamforming synthetic aperture radar in the embodiment. Fig. 5 is a simulation result of the phase spread function and the profile spread function of the point object a marked in fig. 4 in the azimuth direction and the distance direction. The results of the peak side lobe ratio and the integrated side lobe ratio in the distance direction and the azimuth direction of the point target a are shown in table two. The peak sidelobe ratio and the integral sidelobe ratio of the point target A both meet the requirements, which shows that the method of the embodiment can enable the point target to have good focusing effect.

TABLE Peak to sidelobe ratio of two-point object A results

Fig. 6 a-6 f show the imaging process of the point object a in the image domain with coherent superposition of sub-apertures, wherein the processing of the echo data is divided into 167 sub-apertures. 6a is the imaging result of 1 sub-aperture; 6b is an imaging result obtained by coherent superposition of 34 sub-apertures; 6c is an imaging result obtained by coherent superposition of 67 sub-apertures; 6d is an imaging result obtained by coherent superposition of 100 sub-apertures; 6e is an imaging result obtained by coherent superposition of 133 sub-apertures; and 6f is the imaging result obtained by coherent superposition of all 167 sub-apertures. From 6a to 6f, it can be seen that as the number of sub-aperture stacks increases, the imaging resolution of the point target increases accordingly.

Fig. 7 a-7 f are diagrams showing the process of the spectrum change of the azimuth signal after the coherent superposition of the sub-apertures of the point target a, which correspond to fig. 6 one by one. From 7a to 7f, it can be seen that the azimuth spectrum of the point target is gradually increased as the number of sub-aperture stacks is increased.

To sum up, through point target simulation analysis, the real-time imaging method of the embodiment can obtain a full-resolution satellite-borne SAR spotlight mode image with a good focusing effect through sub-aperture splicing.

Simulation II: measured data imaging processing

To further verify the imaging method of this embodiment, the measured data in the bunching mode is processed by the real-time imaging method of this embodiment, and the measured data parameters are shown in table three. Fig. 8 is a result of imaging processing performed on beamforming mode actual measurement data by the satellite-borne beamforming synthetic aperture radar high-resolution real-time imaging method according to the embodiment of the present invention. The imaging result shows that the measured data is well focused, so that the effectiveness of the imaging method of the embodiment is verified.

Table three actual measurement data parameter table

Bandwidth of	710MHz
		Sampling rate	890MHz
Wavelength of light	0.032m
		Pulse repetition frequency	950Hz

The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions or substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.