CN110058232A - A kind of big strabismus sliding beam bunching mode echo-signal orientation preprocess method of satellite-borne SAR and system - Google Patents
A kind of big strabismus sliding beam bunching mode echo-signal orientation preprocess method of satellite-borne SAR and system Download PDFInfo
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Abstract
A kind of big strabismus sliding beam bunching mode echo-signal orientation preprocess method of satellite-borne SAR and system, first to echo-signal in distance to Fast Fourier Transform (FFT) is carried out, distance is transformed into frequency domain to signal, is obtained apart from frequency domain orientation time-domain signal；Then it goes range walk filter to be filtered apart from the utilization of frequency domain orientation time-domain signal to what is obtained, realizes azran uncoupling, obtain azran uncoupling signal；Then the processing of orientation sub-aperture is carried out to azran uncoupling signal, obtains no aliasing, azran uncoupling 2-d spectrum signal, which can be used for focal imaging processing.The present invention solves the problems, such as azran signal close coupling when SAR large slanting view angle machine, and solves the serious Aliasing Problem of azimuth spectrum using orientation sub-aperture path processing method, the sub-aperture path processing method can reduce orientation partition number by the filtering processing of orientation time domain sub-block simultaneously, and then improve treatment effeciency.
Description
Technical Field
The invention relates to a satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing method and system, and belongs to the technical field of echo signal processing.
Background
With the development of the satellite-borne SAR technology, the satellite-borne SAR has the capability of large squint observation in the future, multi-azimuth observation can be carried out on a target in a single-flight process by adjusting the direction of an antenna beam, and the detection flexibility of the satellite-borne SAR can be obviously improved. The sliding bunching mode can ensure a larger coverage area while improving the resolution, so the large squint sliding bunching mode is an important working mode of the future satellite-borne SAR.
Echo signal processing in a satellite-borne SAR large squint sliding bunching mode still faces a larger challenge, and is mainly embodied in aspects of echo signal azimuth spectrum mixing, azimuth distance strong coupling and the like. For the squint sliding beam-bunching mode, the total doppler bandwidth of the squint sliding beam-bunching mode includes a doppler bandwidth corresponding to 3dB of an antenna beam, a doppler bandwidth introduced by beam scanning, and a doppler bandwidth introduced by a squint angle, which are much larger than pulse repetition frequency (PFR), so that the direct implementation of azimuth fourier transform or the implementation of a conventional deskewing method still causes azimuth spectrum aliasing and fails to perform focusing processing. In addition, when the squint is large, the azimuth distance of the echo signal is strongly coupled, so that the traditional imaging processing algorithm cannot realize accurate focusing.
Disclosure of Invention
The technical problem solved by the invention is as follows: the method and the system for preprocessing the azimuth of the echo signal in the satellite-borne SAR large squint sliding bunching mode solve the problems of azimuth spectrum mixing and strong azimuth distance coupling of the echo signal in the satellite-borne SAR large squint sliding bunching mode.
The technical scheme of the invention is as follows: a satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing method comprises the following steps:
(1) carrying out fast Fourier transform on the echo signals in the distance direction to obtain distance frequency domain and azimuth time domain signals;
(2) decoupling the distance frequency domain and direction time domain signals obtained in the step (1) to obtain direction distance decoupling signals;
(3) and (3) carrying out azimuth sub-aperture processing on the signals subjected to the azimuth distance decoupling in the step (2) to obtain two-dimensional frequency spectrum signals without aliasing and with azimuth distance decoupling.
The echo signal is specifically: the echo signal is a baseband two-dimensional complex signal of a radar receiving signal after demodulation and intermediate frequency removal processing.
Echo signals can be divided into range directions and azimuth directions according to the dimensions stored in the two-dimensional matrix, the range directions refer to rows of the two-dimensional matrix, and the azimuth directions refer to columns of the two-dimensional matrix.
The distance frequency domain and azimuth time domain signal specifically comprises: the distance frequency domain azimuth time domain signal refers to a signal in which the distance direction signal is transformed into the frequency domain and the azimuth direction signal is the time domain.
The distance direction fast Fourier transform specifically comprises the following steps: and performing fast Fourier transform on the echo signals along the distance direction, namely transforming the echo signals to a distance frequency domain azimuth time domain to obtain distance frequency domain azimuth time domain signals.
Step (2) decoupling the distance frequency domain and direction time domain signals obtained in step (1) to obtain direction distance decoupling signals, which is specifically as follows:
the azimuth-distance decoupling process is: and filtering the distance frequency domain and direction time domain signals by adopting a distance-removing walking filter, relieving strong direction and distance coupling and obtaining direction and distance decoupling signals.
The distance-removing walking filter specifically comprises: the distance walk filter expression is:
wherein f isτIs the range frequency, f0Is the center frequency, fdc,refIs the doppler center frequency and t is the azimuth time.
And (3) carrying out azimuth sub-aperture processing on the azimuth distance decoupling signal in the step (2) to obtain an aliasing-free azimuth distance decoupling two-dimensional frequency spectrum signal, which is specifically as follows
(1) Carrying out azimuth time domain sub-block division on the azimuth distance outgoing coupled signal;
(2) filtering the azimuth time domain subblocks divided in the step (1);
(3) carrying out azimuth time domain subblock zero filling on the azimuth time domain subblocks subjected to the filtering processing in the step (2);
(4) carrying out azimuth fast Fourier transform on the azimuth time domain subblocks subjected to zero padding in the step (3);
(5) performing azimuth primary phase compensation on the azimuth frequency domain sub-blocks subjected to the azimuth fast Fourier transform in the step (4);
(6) and (4) carrying out azimuth distance two-dimensional frequency spectrum synthesis on each azimuth frequency domain sub-block subjected to the azimuth one-time phase compensation in the step (5) to obtain an aliasing-free and azimuth distance decoupling two-dimensional frequency spectrum signal.
And the azimuth time domain signal in the strong coupling of the mitigation distance frequency domain azimuth time domain signal is the azimuth distance signal.
A satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing system comprises: the device comprises a transformation module, a decoupling module and an aperture processing module;
the transformation module is used for carrying out fast Fourier transformation on the echo signal in the distance direction to obtain a distance frequency domain and direction time domain signal and sending the distance frequency domain and direction time domain signal to the decoupling module;
the decoupling module is used for decoupling the distance frequency domain and direction time domain signals obtained by the transformation module to obtain direction distance decoupling signals;
and the aperture processing module performs azimuth subaperture processing on the azimuth distance decoupled signal obtained by the step decoupling module to obtain an aliasing-free azimuth distance decoupled two-dimensional frequency spectrum signal.
Compared with the prior art, the invention has the advantages that:
(1) according to the echo signal azimuth preprocessing method in the spaceborne SAR large squint sliding bunching mode, provided by the invention, the range frequency domain azimuth time domain signal is filtered by utilizing the range-removing walking filter, so that the problem of strong coupling of the azimuth distance of the echo signal when the spaceborne SAR is in a large squint angle is solved;
(2) the invention solves the problem of serious aliasing of the azimuth spectrum of the echo signal by using an azimuth sub-aperture processing method;
(3) the azimuth sub-aperture processing method can reduce the division number of azimuth sub-blocks through azimuth time domain sub-block filtering processing, thereby improving the processing efficiency and having high efficiency;
(4) the two-dimensional frequency spectrum signal without aliasing and with decoupled azimuth distance can be focused by a plurality of imaging processing algorithms, and has universality;
(5) the method can be used for echo signal imaging processing of a future high-resolution large squint spaceborne SAR system, and has practicability.
Drawings
FIG. 1 is a flow chart of an echo signal azimuth preprocessing method in a satellite-borne SAR large squint sliding bunching mode according to the present invention;
FIG. 2 is a schematic diagram of a satellite-borne SAR large squint sliding bunching mode;
fig. 3 is a diagram of an azimuth-distance two-dimensional spectrum signal, in which fig. 3(a) is a diagram of an azimuth-distance two-dimensional spectrum signal without azimuth preprocessing, and fig. 3(b) is a diagram of an azimuth-distance two-dimensional spectrum signal with azimuth preprocessing.
Fig. 4 is a two-dimensional contour plot of a point target evaluation.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
The invention relates to a satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing method and a system, wherein the method comprises the following steps: (1) carrying out fast Fourier transform on the echo signals in the distance direction, and transforming the distance direction signals into a frequency domain to obtain distance frequency domain azimuth time domain signals; (2) filtering the distance frequency domain and direction time domain signals obtained in the step (1) by using a distance removal walking filter to realize direction and distance decoupling and obtain direction and distance decoupling signals; (3) and (3) carrying out azimuth sub-aperture processing on the azimuth distance decoupling signal in the step (2) to obtain an aliasing-free azimuth distance decoupling two-dimensional frequency spectrum signal, wherein the signal can be used for focusing imaging processing. The SAR sub-aperture processing method solves the problem of strong coupling of azimuth distance signals when the SAR has a large squint angle, solves the problem of serious aliasing of azimuth frequency spectrum by using the azimuth sub-aperture processing method, and can reduce the number of divided azimuth sub-blocks through azimuth time domain sub-block filtering processing, thereby improving the processing efficiency.
The invention provides an azimuth preprocessing method aiming at the problem that the azimuth spectrum aliasing and the azimuth distance strong coupling of the echo signals in the satellite-borne SAR large squint sliding bunching mode cause that the focusing processing cannot be realized, the method can realize the decoupling of the azimuth distance of the echo signals by distance walking filtering processing, and solves the problem of the azimuth spectrum aliasing of the echo signals by using an azimuth sub-aperture method, thereby obtaining two-dimensional spectrum signals without aliasing and with decoupling of the azimuth distance, and the signals can be focused by adopting a plurality of imaging processing algorithms, thereby obtaining high-quality SAR images.
A method for preprocessing the azimuth of echo signals in a satellite-borne SAR large squint sliding bunching mode is shown in a flow chart of fig. 1, and the specific steps are described as follows:
the method comprises the following steps: fast Fourier transform of the distance direction;
and performing fast Fourier transform on the echo signals in the distance direction to obtain distance frequency domain azimuth time domain signals, wherein the echo signals refer to baseband two-dimensional complex signals obtained by demodulating and removing intermediate frequency from radar receiving signals. In actual processing, echo signals are stored in a two-dimensional matrix, and the echo signals can be divided into distance directions and azimuth directions according to dimensions stored in the two-dimensional matrix, wherein the distance directions refer to rows of the two-dimensional matrix, and the azimuth directions refer to columns of the two-dimensional matrix. The distance frequency domain azimuth time domain signal refers to a signal in which the distance direction signal is transformed into the frequency domain and the azimuth direction signal is the time domain.
Step two: decoupling distance frequency domain and orientation time domain;
and decoupling the distance frequency domain and direction time domain signals obtained in the step one to obtain direction distance decoupling signals. And azimuth distance decoupling processing adopts a distance-removing walking filter to filter distance frequency domain and azimuth time domain signals, wherein the expression of the distance-removing walking filter is as follows:
wherein f isτIs the range frequency, t is the azimuth time, f0Is the center frequency, fdc,refIs the Doppler center frequency, j2＝-1。
Step three: processing the azimuth sub-aperture;
and D, performing azimuth sub-aperture processing on the azimuth distance decoupling signal obtained in the step two to obtain an aliasing-free azimuth distance decoupling two-dimensional frequency spectrum signal. According to the azimuth sub-aperture processing method, the Doppler bandwidth of the azimuth time domain sub-blocks can be reduced through azimuth time domain sub-block filtering processing, so that the division length of the azimuth time domain sub-blocks is increased, the number of the azimuth time domain sub-blocks is reduced, and the processing efficiency is improved. The azimuth subaperture processing specifically comprises the following steps:
(1) dividing azimuth time domain sub-blocks;
carrying out azimuth time domain sub-block division on the azimuth distance decoupling signal, wherein the length of the azimuth time domain sub-block is determined by the following formula:
wherein, TsubFor azimuth time domain sub-block length, fPRFFor pulse repetition frequency, B3dBDoppler bandwidth, k, corresponding to antenna 3dB beamwidthωThe frequency is modulated for the doppler caused by the beam sweep.
In the actual processing process, the corresponding sub-block division is divided in an azimuth discrete domain, and the number of pixels of the corresponding azimuth time domain sub-block can be calculated by the following formula:
wherein,denotes rounding down, Na,subThe number of the pixels of the azimuth time domain sub-block is shown.
(2) Filtering the azimuth time domain sub-blocks;
and (3) filtering the azimuth time domain sub-blocks divided in the step (1), so that the change of the azimuth frequency spectrum along the distance frequency can be eliminated. The filtering processing can be realized by filtering the azimuth time domain subblock signals by adopting an azimuth time domain subblock filter, wherein the expression of the azimuth time domain subblock filter is as follows:
wherein f isdc,kIs the Doppler center frequency, H, of the k-th azimuth sub-block2(fτAnd t) represents an azimuth time domain sub-block filter.
(3) Zero padding is carried out on the azimuth time domain sub-blocks;
and (3) carrying out azimuth time domain subblock zero filling on the azimuth time domain subblocks subjected to the filtering processing in the step (2). The purpose of zero filling is that the data length needs to be the power of 2 during fast fourier transform, and the length after zero filling of the azimuth time domain sub-block can be calculated by the following formula:
wherein, N'a,subNumber of pixels after zero padding for azimuth time domain subblocks
(4) Azimuth time domain sub-blocks carry out azimuth fast Fourier transform;
and (4) carrying out azimuth fast Fourier transform on the azimuth time domain subblocks subjected to zero filling in the step (3) to obtain an azimuth distance two-dimensional frequency domain signal of the subblocks.
(5) Azimuth primary phase compensation is carried out on the azimuth frequency domain sub-blocks;
and (3) carrying out azimuth primary phase compensation on the subblock azimuth distance two-dimensional frequency domain signal subjected to azimuth fast Fourier transform in the step (4), wherein the subblock azimuth distance two-dimensional frequency domain signal can be multiplied by an azimuth primary phase compensation filter, and the azimuth primary phase compensation filter has the expression:
wherein H3(fτ,fa) For azimuthal primary phase compensation filter, ta,kIs the azimuth-to-center time of the k-th azimuth sub-block.
(6) Synthesizing an azimuth distance two-dimensional frequency spectrum;
and (4) carrying out azimuth distance two-dimensional frequency spectrum synthesis on each azimuth frequency domain sub-block subjected to the azimuth one-time phase compensation in the step (5) to obtain an aliasing-free and azimuth distance decoupling two-dimensional frequency spectrum signal, which can be used for imaging focusing processing.
In order to verify the correctness of the method, a simulation experiment is carried out according to the parameters in table 1, and fig. 2 is a geometric schematic diagram of imaging in a spaceborne SAR squint sliding bunching mode adopted in the simulation, which shows that an antenna beam points to an underground rotating point in the running process of a satellite; fig. 3(a) and 3(b) show an azimuth distance two-dimensional spectrum signal graph without azimuth preprocessing and after azimuth preprocessing, respectively, from fig. 3(a), it can be seen that the azimuth distance two-dimensional spectrum signal is aliased in the azimuth direction and cannot be used for imaging focusing processing, from fig. 3(b), a complete azimuth distance two-dimensional spectrum signal graph without aliasing can be obtained, and the two-dimensional spectrum is not distorted, i.e. the strong coupling effect of the azimuth distance is alleviated, from fig. 4, a point target two-dimensional contour map after imaging focusing processing is performed by using the azimuth distance two-dimensional spectrum signal after azimuth preprocessing is given, from the figure, it can be seen that the target is accurately focused, from table 2, a point target evaluation result is given, which is consistent with a theoretical value, and the above experimental result verifies the effectiveness of the method provided by the present invention.
TABLE 1 preferred simulation parameters Table
TABLE 2 Point target evaluation results Table
The invention relates to a satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing system which is characterized by comprising the following components: the device comprises a transformation module, a decoupling module and an aperture processing module;
the transformation module is used for carrying out fast Fourier transformation on the echo signal in the distance direction to obtain a distance frequency domain and direction time domain signal and sending the distance frequency domain and direction time domain signal to the decoupling module;
the decoupling module is used for decoupling the distance frequency domain and direction time domain signals obtained by the transformation module to obtain direction distance decoupling signals;
and the aperture processing module performs azimuth subaperture processing on the azimuth distance decoupled signal obtained by the step decoupling module to obtain an aliasing-free azimuth distance decoupled two-dimensional frequency spectrum signal.
The echo signal is specifically: the echo signal is a baseband two-dimensional complex signal of a radar receiving signal after demodulation and intermediate frequency removal processing.
The echo signals in the transformation module can be divided into distance directions and orientation directions according to the dimensions stored in the two-dimensional matrix, wherein the distance directions refer to rows of the two-dimensional matrix, and the orientation directions refer to columns of the two-dimensional matrix.
The distance frequency domain and direction time domain signal in the decoupling module specifically comprises the following steps: the distance frequency domain azimuth time domain signal refers to a signal in which the distance direction signal is transformed into the frequency domain and the azimuth direction signal is the time domain.
In the transformation module, the distance direction fast Fourier transformation specifically comprises the following steps: and performing fast Fourier transform on the echo signals along the distance direction, namely transforming the echo signals to a distance frequency domain azimuth time domain to obtain distance frequency domain azimuth time domain signals.
Decoupling the distance frequency domain and direction time domain signals in the decoupling module to obtain direction distance decoupling signals, which is as follows:
the azimuth-distance decoupling process is: and filtering the distance frequency domain and direction time domain signals by adopting a distance-removing walking filter, relieving strong direction and distance coupling and obtaining direction and distance decoupling signals.
The distance-removing walking filter in the decoupling module specifically comprises: the distance walk filter expression is:
wherein f isτIs the range frequency, f0Is the center frequency, fdc,refIs the doppler center frequency and t is the azimuth time.
The azimuth sub-aperture processing is carried out on the azimuth distance coupled-out signal in the aperture processing module to obtain an aliasing-free and azimuth distance decoupled two-dimensional frequency spectrum signal, which is as follows
(1) Carrying out azimuth time domain sub-block division on the azimuth distance outgoing coupled signal;
(2) filtering the azimuth time domain subblocks divided in the step (1);
(3) carrying out azimuth time domain subblock zero filling on the azimuth time domain subblocks subjected to the filtering processing in the step (2);
(4) carrying out azimuth fast Fourier transform on the azimuth time domain subblocks subjected to zero padding in the step (3);
(5) performing azimuth primary phase compensation on the azimuth frequency domain sub-blocks subjected to the azimuth fast Fourier transform in the step (4);
(6) and (4) carrying out azimuth distance two-dimensional frequency spectrum synthesis on each azimuth frequency domain sub-block subjected to the azimuth one-time phase compensation in the step (5) to obtain an aliasing-free and azimuth distance decoupling two-dimensional frequency spectrum signal.
And the azimuth time domain signal in the decoupling module for relieving the strong coupling of the distance frequency domain azimuth time domain signal is the azimuth distance signal.
The distance in the transformation module is subjected to fast Fourier transformation, and the preferred scheme is;
and performing fast Fourier transform on the echo signals in the distance direction to obtain distance frequency domain azimuth time domain signals, wherein the echo signals refer to baseband two-dimensional complex signals obtained by demodulating and removing intermediate frequency from radar receiving signals. In actual processing, echo signals are stored in a two-dimensional matrix, and the echo signals can be divided into distance directions and azimuth directions according to dimensions stored in the two-dimensional matrix, wherein the distance directions refer to rows of the two-dimensional matrix, and the azimuth directions refer to columns of the two-dimensional matrix. The distance frequency domain azimuth time domain signal refers to a signal in which the distance direction signal is transformed into the frequency domain and the azimuth direction signal is the time domain.
Distance frequency domain and orientation time domain decoupling processing in decoupling module, and preferred scheme
And decoupling the distance frequency domain and direction time domain signals obtained by the transformation module to obtain direction distance decoupling signals. And azimuth distance decoupling processing adopts a distance-removing walking filter to filter distance frequency domain and azimuth time domain signals, wherein the expression of the distance-removing walking filter is as follows:
wherein f isτIs the range frequency, t is the azimuth time, f0Is the center frequency, fdc,refIs the Doppler center frequency, j2＝-1。
Processing the azimuth sub-aperture in the aperture processing module, wherein the preferred scheme is;
and carrying out azimuth sub-aperture processing on the azimuth distance decoupling signal obtained by the decoupling module to obtain an aliasing-free azimuth distance decoupling two-dimensional frequency spectrum signal. According to the azimuth sub-aperture processing method, the Doppler bandwidth of the azimuth time domain sub-blocks can be reduced through azimuth time domain sub-block filtering processing, so that the division length of the azimuth time domain sub-blocks is increased, the number of the azimuth time domain sub-blocks is reduced, and the processing efficiency is improved. The azimuth subaperture processing specifically comprises the following steps:
firstly, dividing the azimuth time domain sub-blocks, wherein the preferable scheme is as follows:
carrying out azimuth time domain sub-block division on the azimuth distance decoupling signal, wherein the length of the azimuth time domain sub-block is determined by the following formula:
wherein, TsubIs a squareBit-wise time-domain sub-block length, fPRFFor pulse repetition frequency, B3dBDoppler bandwidth, k, corresponding to antenna 3dB beamwidthωThe frequency is modulated for the doppler caused by the beam sweep.
In the actual processing process, the corresponding sub-block division is divided in an azimuth discrete domain, and the number of pixels of the corresponding azimuth time domain sub-block can be calculated by the following formula:
wherein,denotes rounding down, Na,subThe number of the pixels of the azimuth time domain sub-block is shown.
Then, the azimuth time domain sub-block filtering process, preferably:
and the divided azimuth time domain sub-blocks are filtered, so that the change of the azimuth frequency spectrum along the distance frequency can be eliminated. The filtering processing can be realized by filtering the azimuth time domain subblock signals by adopting an azimuth time domain subblock filter, wherein the expression of the azimuth time domain subblock filter is as follows:
wherein f isdc,kIs the Doppler center frequency, H, of the k-th azimuth sub-block2(fτAnd t) represents an azimuth time domain sub-block filter.
Then, the azimuth fills zero to the time domain sub-block, and the preferred scheme is:
and carrying out azimuth time domain subblock zero filling on the azimuth time domain subblocks subjected to filtering processing. The purpose of zero filling is that the data length needs to be the power of 2 during fast fourier transform, and the length after zero filling of the azimuth time domain sub-block can be calculated by the following formula:
wherein, N'a,subNumber of pixels after zero padding for azimuth time domain subblocks
Then, performing azimuth fast fourier transform on the azimuth time domain sub-blocks in azimuth direction, wherein the preferred scheme is as follows:
and carrying out azimuth fast Fourier transform on the azimuth time domain sub-blocks after zero padding to obtain azimuth distance two-dimensional frequency domain signals of the sub-blocks.
Then, the azimuth frequency domain sub-block azimuth one-time phase compensation, the preferred scheme is:
the sub-block azimuth distance two-dimensional frequency domain signals after the azimuth fast Fourier transform are subjected to azimuth one-time phase compensation, the sub-block azimuth distance two-dimensional frequency domain signals can be multiplied by an azimuth one-time phase compensation filter, and the azimuth one-time phase compensation filter has the expression:
wherein H3(fτ,fa) For azimuthal primary phase compensation filter, ta,kIs the azimuth-to-center time of the k-th azimuth sub-block.
Then, synthesizing an azimuth-distance two-dimensional frequency spectrum, wherein the preferable scheme is as follows:
and performing azimuth distance two-dimensional frequency spectrum synthesis on each azimuth frequency domain subblock subjected to the azimuth primary phase compensation to obtain an aliasing-free and azimuth distance decoupling two-dimensional frequency spectrum signal, which can be used for imaging focusing processing.
In order to verify the correctness of the method, a simulation experiment is carried out according to the parameters of table 1, fig. 2 is a geometric schematic diagram of imaging of a spaceborne SAR squint sliding bunching mode adopted in the simulation, fig. 3(a) and fig. 3(b) respectively show an azimuth distance two-dimensional spectrum signal graph without azimuth preprocessing and azimuth preprocessing, the azimuth distance two-dimensional spectrum signal graph can be seen from fig. 3(a) to be aliased along the azimuth direction and cannot be used for imaging focusing processing, while fig. 3(b) can be used for obtaining a complete non-aliased azimuth distance two-dimensional spectrum signal graph without distortion of a two-dimensional spectrum, namely, the strong coupling effect of the azimuth distance is relieved, and the azimuth spectrum is seriously aliased as can be seen from fig. 3(a), while a complete non-aliased two-dimensional signal spectrum is obtained after the azimuth preprocessing method of the present invention is adopted (fig. 3 (b)). Fig. 4 shows a two-dimensional contour diagram of a point target after imaging and focusing processing is performed on an azimuth distance two-dimensional spectrum signal after azimuth preprocessing, from which it can be seen that the target is accurately focused, further verifying the effectiveness of the azimuth preprocessing method of the present invention, and table 2 shows the evaluation result of the point target, which is consistent with the theoretical value, and the experimental result verifies the effectiveness of the method of the present invention.
TABLE 1 preferred simulation parameters Table
TABLE 2 Point target evaluation results Table
According to the echo signal azimuth preprocessing method in the spaceborne SAR large squint sliding bunching mode, provided by the invention, the range frequency domain azimuth time domain signal is filtered by utilizing the range-removing walking filter, so that the problem of strong coupling of the azimuth distance of the echo signal when the spaceborne SAR is in a large squint angle is solved; the problem of serious aliasing of the azimuth frequency spectrum of the echo signal is solved by using an azimuth sub-aperture processing method; the azimuth sub-aperture processing method can reduce the division number of azimuth sub-blocks through azimuth time domain sub-block filtering processing, thereby improving the processing efficiency and having high efficiency;
the two-dimensional frequency spectrum signal without aliasing and with decoupled azimuth distance can be focused by a plurality of imaging processing algorithms, and has universality; the method can be used for echo signal imaging processing of a future high-resolution large squint spaceborne SAR system, and has practicability.
Those skilled in the art will appreciate that those matters not described in detail in the present specification are well known in the art.
Claims (10)
1. A satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing method is characterized by comprising the following steps:
(1) carrying out fast Fourier transform on the echo signals in the distance direction to obtain distance frequency domain and azimuth time domain signals;
(2) decoupling the distance frequency domain and direction time domain signals obtained in the step (1) to obtain direction distance decoupling signals;
(3) and (3) carrying out azimuth sub-aperture processing on the signals subjected to the azimuth distance decoupling in the step (2) to obtain two-dimensional frequency spectrum signals without aliasing and with azimuth distance decoupling.
2. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 1, characterized in that: the echo signal is specifically: the echo signal is a baseband two-dimensional complex signal of a radar receiving signal after demodulation and intermediate frequency removal processing.
3. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 1, characterized in that: echo signals can be divided into range directions and azimuth directions according to the dimensions stored in the two-dimensional matrix, the range directions refer to rows of the two-dimensional matrix, and the azimuth directions refer to columns of the two-dimensional matrix.
4. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 1, characterized in that: the distance frequency domain and azimuth time domain signal specifically comprises: the distance frequency domain azimuth time domain signal refers to a signal in which the distance direction signal is transformed into the frequency domain and the azimuth direction signal is the time domain.
5. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 1, characterized in that: the distance direction fast Fourier transform specifically comprises the following steps: and performing fast Fourier transform on the echo signals along the distance direction, namely transforming the echo signals to a distance frequency domain azimuth time domain to obtain distance frequency domain azimuth time domain signals.
6. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 1, characterized in that: step (2) decoupling the distance frequency domain and direction time domain signals obtained in step (1) to obtain direction distance decoupling signals, which is specifically as follows:
the azimuth-distance decoupling process is: and filtering the distance frequency domain and direction time domain signals by adopting a distance-removing walking filter, relieving strong direction and distance coupling and obtaining direction and distance decoupling signals.
7. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 6, characterized in that: the distance-removing walking filter specifically comprises: the distance walk filter expression is:
wherein f isτIs the range frequency, f0Is the center frequency, fdc,refIs the doppler center frequency and t is the azimuth time.
8. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 6, characterized in that: and (3) carrying out azimuth sub-aperture processing on the azimuth distance decoupling signal in the step (2) to obtain an aliasing-free azimuth distance decoupling two-dimensional frequency spectrum signal, which is specifically as follows
(1) Carrying out azimuth time domain sub-block division on the azimuth distance outgoing coupled signal;
(2) filtering the azimuth time domain subblocks divided in the step (1);
(3) carrying out azimuth time domain subblock zero filling on the azimuth time domain subblocks subjected to the filtering processing in the step (2);
(4) carrying out azimuth fast Fourier transform on the azimuth time domain subblocks subjected to zero padding in the step (3);
(5) performing azimuth primary phase compensation on the azimuth frequency domain sub-blocks subjected to the azimuth fast Fourier transform in the step (4);
(6) and (4) carrying out azimuth distance two-dimensional frequency spectrum synthesis on each azimuth frequency domain sub-block subjected to the azimuth one-time phase compensation in the step (5) to obtain an aliasing-free and azimuth distance decoupling two-dimensional frequency spectrum signal.
9. The method for preprocessing the azimuth of the echo signal in the spaceborne SAR large squint sliding bunching mode according to claim 6, characterized in that: and the azimuth time domain signal in the strong coupling of the mitigation distance frequency domain azimuth time domain signal is the azimuth distance signal.
10. A satellite-borne SAR large squint sliding bunching mode echo signal azimuth preprocessing system is characterized by comprising: the device comprises a transformation module, a decoupling module and an aperture processing module;
the transformation module is used for carrying out fast Fourier transformation on the echo signal in the distance direction to obtain a distance frequency domain and direction time domain signal and sending the distance frequency domain and direction time domain signal to the decoupling module;
the decoupling module is used for decoupling the distance frequency domain and direction time domain signals obtained by the transformation module to obtain direction distance decoupling signals;
and the aperture processing module performs azimuth subaperture processing on the azimuth distance decoupled signal obtained by the step decoupling module to obtain an aliasing-free azimuth distance decoupled two-dimensional frequency spectrum signal.
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