CN116184406A - A terahertz video SAR fast imaging method and device - Google Patents
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Abstract
本发明提供一种太赫兹视频SAR快速成像方法及装置，所述方法包括：获取太赫兹视频SAR回波数据；在以全孔径中心为原点的直角坐标系下进行子孔径划分；对子孔径的回波数据进行Dechirp处理；对Dechirp处理后的数据进行二维插值，实现极坐标到直角坐标的转换，并得到各个子孔径的波数域回波数据；将每相邻两个子孔径数据在波数域进行拼接，得到更长子孔径的波数域数据，重复进行直至所有子孔径拼接完成，得到全孔径的波数域数据；对全孔径的波数域数据进行二维逆傅里叶变换，得到全分辨率图像；对全分辨率图像进行相位误差校正，得到太赫兹视频SAR图像。本发明可在保证成像质量的同时有效地提升运算效率。
The present invention provides a terahertz video SAR fast imaging method and device. The method includes: acquiring terahertz video SAR echo data; dividing sub-apertures in a Cartesian coordinate system with the center of the full aperture as the origin; Dechirp processing is performed on the echo data; two-dimensional interpolation is performed on the data processed by Dechirp to realize the conversion from polar coordinates to rectangular coordinates, and the wave number domain echo data of each sub-aperture is obtained; each adjacent two sub-aperture data are in the wave number domain Perform splicing to obtain wavenumber domain data of longer sub-apertures, repeat until all sub-aperture splicing is completed, and obtain full-aperture wavenumber domain data; perform two-dimensional inverse Fourier transform on full-aperture wavenumber domain data to obtain full-resolution images ; Phase error correction is performed on the full-resolution image to obtain a terahertz video SAR image. The invention can effectively improve the computing efficiency while ensuring the imaging quality.
Description
Technical Field
The invention relates to the technical field of radar signal processing, in particular to a terahertz video SAR rapid imaging method and device.
Background
Terahertz (THz) waves refer to electromagnetic waves with the frequency spectrum between 100GHz and 10THz, and have the characteristics of high carrier frequency, large bandwidth, good penetrability and the like. Terahertz Video synthetic aperture radar (THz-ViSAR) imaging has significant advantages such as higher resolution, higher frame rate, higher detection probability, easier identification and the like compared with microwave synthetic aperture radar (Synthetic Aperture Radar, SAR) imaging, and therefore, is receiving more and more attention in the field of modern radar imaging.
Although there are a number of imaging algorithms currently available, the two imaging algorithms commonly used by THz-ViSAR are the polar format algorithm (Polar Format Algorithm, PFA) and the backprojection algorithm (Back Project Algorithm, BPA), respectively. PFA introduces residual phase errors with planar wavefront assumption, resulting in geometric distortion and defocus of SAR images, which makes it unsuitable for large scene and high resolution. BPA is a time domain imaging algorithm suitable for any imaging modality and any trajectory. However, the algorithm needs to traverse point by point, and has extremely high operation complexity, thereby limiting the wide application of the algorithm. For this purpose, a number of acceleration algorithms have been proposed in the academia, the most representative of which are the fast back projection algorithm (Fast Back Projection, FBP) and the fast multistage back projection algorithm (Fast Factorized Back Projection, FFBP). The FBP firstly performs sub-aperture division, reconstructs sub-images under a local polar coordinate system, and then converts the sub-images into rectangular coordinates and then performs coherent superposition to obtain a full-resolution image. It trades for an increase in computational efficiency at the expense of image quality. And the FFBP adopts the same processing mode as the FBP to obtain a sub-image, and then the final image is obtained through step-by-step image fusion. The process implements mapping between coordinates by a large number of two-dimensional interpolations, thereby inevitably introducing interpolation errors. This makes it difficult to achieve both image quality and computational efficiency in practical applications.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a terahertz video SAR rapid imaging method and device, which can effectively improve the operation efficiency while guaranteeing the imaging quality.
In order to solve the problems, the technical scheme of the invention is as follows:
a terahertz video SAR rapid imaging method comprises the following steps:
acquiring terahertz video SAR echo data;
dividing sub-apertures in a rectangular coordinate system with the full aperture center as an origin;
dechirp processing is carried out on echo data of the sub-aperture;
performing two-dimensional interpolation on the data processed by the Dechirp to realize conversion from polar coordinates to rectangular coordinates, and obtaining wave number domain echo data of each sub-aperture;
splicing every two adjacent sub-aperture data in a wave number domain to obtain wave number domain data of longer sub-apertures, and repeating until all sub-apertures are spliced to obtain wave number domain data of full apertures;
performing two-dimensional inverse Fourier transform on the wave number domain data of the full aperture to obtain a full resolution image;
and carrying out phase error correction on the full-resolution image to obtain a terahertz video SAR image.
Preferably, the step of acquiring terahertz video SAR echo data specifically includes: the terahertz video SAR transmits a linear frequency modulation pulse signal, and the expression is as follows:
wherein t is r For distance to fast time, T r Is pulse width, f c For the center frequency, the tuning frequency γ=b/T r B is bandwidth, rect (&) is a rectangular window function; for an arbitrary point target P (x, y, z) within the illuminated area, the instantaneous pitch of the radar platform to that point is:
Wherein t is a For slow azimuth time, x a Is the x-axis coordinate of the radar, v a The flying speed of the radar carrier is the flying height of the radar carrier; the echo signal of a point target can be expressed as:
wherein, the two-way delay τ=2r p And c, c is the speed of light.
Preferably, the step of dividing the sub-aperture in a rectangular coordinate system with the full aperture center as an origin specifically includes: if the full aperture length is L a The number of the sub-apertures is N, and the length of the sub-apertures is l=L a The echo data for the ith sub-aperture at this time can be expressed as:
wherein (1)>
The azimuth time corresponding to the ith sub-aperture is as follows:
Preferably, the step of performing Dechirp processing on the echo data of the sub-aperture specifically includes the following steps:
constructing a reference signal, wherein the modulation frequency and the center frequency of the reference signal are the same as those of the transmitting signal, and mixing the reference signal with the echo signal to obtain an echo difference frequency signal;
transforming the echo difference frequency signal to a distance frequency domain through distance Fourier transform;
removing residual video phase terms and diagonal phase terms in a distance frequency domain;
then the signals are transformed into a distance time domain through distance inverse Fourier transform, and the ith sub-aperture echo signal after Dechirp processing is obtained as follows:
wherein,,wave number->
Differential slant distance->
R ref Is the reference pitch of the ith sub-aperture, and +.>
Is the center time of the sub-aperture.
Preferably, the step of performing two-dimensional interpolation on the data after the Dechirp processing to realize conversion from polar coordinates to rectangular coordinates and obtain wave number domain echo data of each sub-aperture specifically includes: differential pair of skew
Performing taylor series expansion, ignoring the second and higher order terms can be expressed as:
Wherein (1)>
And the pitch angle is the azimuth angle, and at the moment, the echo signal of the ith sub-aperture is converted into:
wherein the distance wave number is K x And azimuth wavenumber K y The method comprises the following steps of:
Echo signal in polar format +.>
Echo signal s to rectangular coordinate format i (K x ,K y ) Can be achieved by two-dimensional interpolation.
Preferably, the step of splicing each two adjacent sub-aperture data in the wave number domain to obtain wave number domain data of longer sub-aperture is repeated until all sub-apertures are spliced to obtain wave number domain data of full aperture specifically includes: if the total number of stages of the sub-aperture concatenation of the algorithm is G, for stage G (g=1, 2 … G), the number of sub-apertures is K g ＝2 G-g The representation is made of a combination of a first and a second color,
the wavenumber domain echo data used to represent the q-th sub-aperture of stage G (g=1, 2 … G), then the wavenumber domain stitching process for two adjacent sub-apertures can be expressed as:
Wherein the method comprises the steps of
Wave number domain echo data of the 2q-1 th sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
Wave number domain echo data of the 2q < th > sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
The distance direction and the azimuth direction of (2) are respectively M r ×2M a Repeating until all sub-apertures are spliced to obtain wave number domain data s (K) x ,K y )。
Preferably, the pair of full pore diametersThe step of obtaining a full resolution image by performing a two-dimensional inverse fourier transform on the wavenumber domain data of (a) comprises: performing two-dimensional inverse Fourier transform on the wave number domain data of the full aperture to obtain a full resolution image: i (x, y) = ≡ζs (K) x ,K y )exp[-j(xK x +yK y )]dxdy。
Preferably, the step of performing phase error correction on the full resolution image to obtain the terahertz video SAR image specifically includes:
by the formula
Judging whether the imaging scene needs to be subjected to secondary phase error correction, if the imaging scene is larger than r π/4 Performing secondary phase error correction by spatial post-filtering, otherwise not performing correction, ρ in the formula a For azimuthal resolution, λ is wavelength, R c For the nearest diagonal of radar to the scene center, < +.>
The geometric distortion correction is realized through image domain resampling, wherein the mapping relation between the real ground coordinates (x, y) and the actual coordinates (x ', y') in the imaged image is as follows:
wherein->
Further, the invention also provides a terahertz video SAR rapid imaging device, which is characterized by comprising a processor and a memory for storing executable instructions of the processor, wherein the processor is configured to execute the terahertz video SAR rapid imaging method through executing the executable instructions.
Compared with the prior art, the invention has the advantages that: in the initial imaging stage, a global rectangular coordinate system with simpler geometric configuration and a polar coordinate algorithm with higher efficiency are adopted to process the sub-aperture data, so that the calculated amount is greatly reduced and the realization is easier; the fusion stage is realized by simple wave number domain splicing, and the introduction and accumulation of coordinate mapping and interpolation errors are avoided. Therefore, the terahertz video SAR rapid imaging method disclosed by the invention effectively improves the operation efficiency while ensuring the imaging quality.
Drawings
Other features, objects and advantages of the present invention will become more apparent upon reading of the detailed description of non-limiting embodiments, given with reference to the accompanying drawings in which:
fig. 1 is a flow chart of a terahertz video SAR rapid imaging method provided by an embodiment of the present invention;
fig. 2 is a geometric schematic diagram of a terahertz video SAR according to an embodiment of the present invention.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the present invention, but are not intended to limit the invention in any way. It should be noted that variations and modifications could be made by those skilled in the art without departing from the inventive concept. These are all within the scope of the present invention.
Specifically, fig. 1 is a flow chart of a terahertz video SAR rapid imaging method provided by an embodiment of the present invention; fig. 2 is a geometric schematic diagram of a terahertz video SAR provided by an embodiment of the present invention, and as shown in fig. 1 and fig. 2, the method for rapidly imaging the terahertz video SAR includes the following steps:
s1: acquiring terahertz video SAR echo data;
specifically, in step S1, the terahertz video SAR transmits a chirp signal, whose expression is
Wherein t is r For distance to fast time, T r Is pulse width, f c For the center frequency, the tuning frequency γ=b/T r B is bandwidth, rect (&) is a rectangular window function.
For any point target P (x, y, z) within the illuminated area, then the instantaneous pitch of the radar platform to that point is
Wherein t is a For slow azimuth time, x a Is the x-axis coordinate of the radar, v a The flying speed of the radar carrier is H, and the flying height of the radar carrier is H.
The echo signal of a point target can be expressed as:
wherein, the two-way delay τ=2r p And c, c is the speed of light.
S2: dividing sub-apertures in a rectangular coordinate system with the full aperture center as an origin;
specifically, sub-aperture division is performed in a rectangular coordinate system with the full aperture center as the origin, if the full aperture length is L a The number of the sub-apertures is N, and the length of the sub-apertures is l=L a N. The echo data for the ith sub-aperture at this time can be expressed as:
wherein,,
the azimuth time corresponding to the ith sub-aperture is in the range of
S3: dechirp processing is carried out on echo data of the sub-aperture;
specifically, the Dechirp processing of the echo data of the sub-aperture includes the following steps:
step 1: constructing a reference signal, wherein the modulation frequency and the center frequency of the reference signal are the same as those of the transmitting signal, and mixing the reference signal with the echo signal to obtain an echo difference frequency signal;
step 2: transforming the echo difference frequency signal to a distance frequency domain through distance Fourier transform;
step 3: removing residual video phase terms and diagonal phase terms in a distance frequency domain;
step 4: then the signals are transformed into a distance time domain through distance inverse Fourier transform, and an ith sub-aperture echo signal after Dechirp processing is obtained:
wherein the wave number
Differential slant distance->
R ref Is the reference pitch of the ith sub-aperture, and +.>
Is the center time of the sub-aperture.
S4: performing two-dimensional interpolation on the data processed by the Dechirp to realize conversion from polar coordinates to rectangular coordinates, and obtaining wave number domain echo data of each sub-aperture;
specifically, two-dimensional interpolation is performed on data after Dechirp processing, so that conversion from polar coordinates to rectangular coordinates is realized.
Differential pair of skew
Performing Taylor series expansion, ignoring the second and higher order terms and expressing as
Wherein,,
the pitch angle, θ is the azimuth angle.
Bringing formula (7) into formula (6) yields:
wherein the distance wave number is K x And azimuth wavenumber K y Respectively is
At this time, the echo signal of the ith sub-aperture is converted from equation (6) to equation (9), and the conversion from the polar format data to the rectangular format can be realized by two-dimensional interpolation.
And (3) respectively carrying out the processing of the step S3 and the step S4 on all the sub-aperture data to obtain wave number domain echo data of each sub-aperture.
S5: splicing every two adjacent sub-aperture data in a wave number domain to obtain wave number domain data of longer sub-apertures, and repeating until all sub-apertures are spliced to obtain wave number domain data of full apertures;
specifically, splicing every two adjacent sub-aperture data in a wave number domain to obtain wave number domain data of longer sub-apertures; and (5) repeating the step (S5) until all sub-apertures are spliced to obtain the wave number domain data of the full aperture.
Specifically, if the total number of stages of sub-aperture stitching of the algorithm is G, for stage G (g=1, 2 … G), the sub-aperture numberBy K g ＝2 G-g Representing, for example, the number of sub-apertures in stage 1 as K 1 ＝2 G-1 。
The wavenumber domain echo data used to represent the q-th sub-aperture of stage G (g=1, 2 … G), then the wavenumber domain stitching process for two adjacent sub-apertures can be expressed as:
wherein the method comprises the steps of
Wave number domain echo data of the 2q-1 th sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
Wave number domain echo data of the 2q < th > sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
The distance direction and the azimuth direction of (2) are respectively M r ×2M a . Repeating until all sub-apertures are spliced to obtain wave number domain data s (K) x ,K y )。
S6: performing two-dimensional inverse Fourier transform on the wave number domain data of the full aperture to obtain a full resolution image;
specifically, in step S6, two-dimensional inverse fourier transform is performed on the wavenumber domain data of the full aperture, to obtain a full resolution image, that is:
I(x,y)＝∫∫s(K x ,K y )exp[-j(xK x +yK y )]dxdy (11)
s7: and carrying out phase error correction on the full-resolution image to obtain a terahertz video SAR image.
Specifically, in step S7, phase error correction is performed on the full resolution image, so as to obtain a terahertz video SAR image without defocus.
In step S4, differential skew is adjusted
When the Taylor series expansion is carried out, after the second-order and above higher-order terms are ignored, linear and secondary phase errors are introduced to each sub-aperture echo signal. Wherein the linear phase error causes image distortion, the secondary phase error is a space-variant error, and the farther from the scene center, the more serious the image defocus.
The phase error correction specifically includes the steps of:
step 1: judging whether the imaging scene needs secondary phase error correction or not according to a formula (12), if the imaging scene is larger than r π/4 Performing secondary phase error correction through spatial post-filtering, otherwise, performing no correction;
ρ in the formula a For azimuthal resolution, λ is wavelength, R c For the closest slant range of the radar to the scene center,
step 2: the geometric distortion correction is realized through image domain resampling, wherein the mapping relation between the real ground coordinates (x, y) and the actual coordinates (x ', y') in the imaged image is as follows:
wherein the method comprises the steps of
Compared with the prior art, the invention has the advantages that: in the initial imaging stage, a global rectangular coordinate system with simpler geometric configuration and a polar coordinate algorithm with higher efficiency are adopted to process the sub-aperture data, so that the calculated amount is greatly reduced and the realization is easier; the fusion stage is realized by simple wave number domain splicing, and the introduction and accumulation of coordinate mapping and interpolation errors are avoided. Therefore, the terahertz video SAR multistage backward projection rapid imaging algorithm disclosed by the invention effectively improves the operation efficiency while ensuring the imaging quality.
The foregoing describes specific embodiments of the present invention. It is to be understood that the invention is not limited to the particular embodiments described above, and that various changes or modifications may be made by those skilled in the art within the scope of the appended claims without affecting the spirit of the invention. The embodiments of the present application and features in the embodiments may be combined with each other arbitrarily without conflict.
Claims (9)
1. The terahertz video SAR rapid imaging method is characterized by comprising the following steps of:
acquiring terahertz video SAR echo data;
dividing sub-apertures in a rectangular coordinate system with the full aperture center as an origin;
dechirp processing is carried out on echo data of the sub-aperture;
performing two-dimensional interpolation on the data processed by the Dechirp to realize conversion from polar coordinates to rectangular coordinates, and obtaining wave number domain echo data of each sub-aperture;
splicing every two adjacent sub-aperture data in a wave number domain to obtain wave number domain data of longer sub-apertures, and repeating until all sub-apertures are spliced to obtain wave number domain data of full apertures;
performing two-dimensional inverse Fourier transform on the wave number domain data of the full aperture to obtain a full resolution image;
and carrying out phase error correction on the full-resolution image to obtain a terahertz video SAR image.
2. The method for rapidly imaging a terahertz video SAR according to claim 1, wherein said step of acquiring terahertz video SAR echo data specifically comprises: the terahertz video SAR transmits a linear frequency modulation pulse signal, and the expression is as follows:
wherein t is r For distance to fast time, T r Is pulse width, f c For the center frequency, the tuning frequency γ=b/T r B is bandwidth, rect (&) is a rectangular window function; for an arbitrary point target P (x, y, z) within the illuminated area, the instantaneous pitch of the radar platform to that point is:
Wherein t is a For slow azimuth time, x a Is the x-axis coordinate of the radar, v a The flying speed of the radar carrier is the flying height of the radar carrier; the echo signal of a point target can be expressed as:
Wherein, the two-way delay τ=2r p And c, c is the speed of light.
3. The method for rapid imaging of terahertz video SAR according to claim 2, wherein said sub-aperture division under a rectangular coordinate system with the full aperture center as the origin specifically comprises: if the full aperture length is L a The number of the sub-apertures is N, and the length of the sub-apertures is l=L a The echo data for the ith sub-aperture at this time can be expressed as:
wherein (1)>
The azimuth time corresponding to the ith sub-aperture is as follows:
4. The terahertz video SAR rapid imaging method according to claim 1, wherein said step of Dechirp processing the echo data of the sub-aperture specifically comprises the steps of:
constructing a reference signal, wherein the modulation frequency and the center frequency of the reference signal are the same as those of the transmitting signal, and mixing the reference signal with the echo signal to obtain an echo difference frequency signal;
transforming the echo difference frequency signal to a distance frequency domain through distance Fourier transform;
removing residual video phase terms and diagonal phase terms in a distance frequency domain;
then the signals are transformed into a distance time domain through distance inverse Fourier transform, and the ith sub-aperture echo signal after Dechirp processing is obtained as follows:
wherein the wave number->
Differential slant distance->
R ref Is the reference pitch of the ith sub-aperture, and +.>
Is the center time of the sub-aperture. />
5. The method for rapid imaging of terahertz video SAR according to claim 4, wherein said two-dimensional interpolation of data after Dechirp processing to realize conversion from polar coordinates to rectangular coordinates, and obtaining wave number domain echo data of each sub-aperture specifically comprises: differential pair of skew
Performing taylor series expansion, ignoring the second and higher order terms can be expressed as:
Wherein (1)>
For pitch angle, θ is azimuth angle, and at this time, the echo signal of the ith sub-aperture is converted into:
Wherein the distance wave number is K x And azimuth wavenumber K y The method comprises the following steps of:
echo signal in polar format +.>
Echo signal s to rectangular coordinate format i (K x ,K y ) Can be achieved by two-dimensional interpolation.
6. The terahertz video SAR rapid according to claim 5The imaging method is characterized in that each two adjacent sub-aperture data are spliced in a wave number domain to obtain wave number domain data of longer sub-apertures, and the steps of repeatedly carrying out the steps until all sub-apertures are spliced to obtain wave number domain data of full apertures specifically comprise: if the total number of stages of the sub-aperture concatenation of the algorithm is G, for stage G (g=1, 2 … G), the number of sub-apertures is K g ＝2 G-g The representation is made of a combination of a first and a second color,
the wavenumber domain echo data used to represent the q-th sub-aperture of stage G (g=1, 2 … G), then the wavenumber domain stitching process for two adjacent sub-apertures can be expressed as:
wherein->
Wave number domain echo data of the 2q-1 th sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
Wave number domain echo data of the 2q < th > sub-aperture of the stage g-1, the distance direction and the azimuth direction are respectively M r ×M a ,
The distance direction and the azimuth direction of (2) are respectively M r ×2M a Repeating until all sub-apertures are spliced to obtain wave number domain data s (K) x ,K y )。
7. The method for rapid imaging of terahertz video SAR according to claim 1, wherein said step of performing two-dimensional inverse fourier transform on the wavenumber domain data of the full aperture to obtain a full resolution image specifically comprises: two-dimensional inverse Fourier transform of wave number domain data of full apertureLeaf transformation, obtaining a full resolution image: i (x, y) = ≡ζs (K) x ,K y )exp[-j(xK x +yK y )]dxdy。
8. The method for rapidly imaging the terahertz video SAR according to claim 1, wherein the step of performing phase error correction on the full resolution image to obtain the terahertz video SAR image specifically comprises:
by the formula
Judging whether the imaging scene needs to be subjected to secondary phase error correction, if the imaging scene is larger than r π/4 Performing secondary phase error correction by spatial post-filtering, otherwise not performing correction, ρ in the formula a For azimuthal resolution, λ is wavelength, R c For the nearest diagonal of radar to the scene center, < +.>
The geometric distortion correction is realized through image domain resampling, wherein the mapping relation between the real ground coordinates (x, y) and the actual coordinates (x ', y') in the imaged image is as follows:
wherein->
9. A terahertz video SAR rapid imaging apparatus, characterized in that the apparatus comprises a processor configured to execute the terahertz video SAR rapid imaging method according to any one of claims 1 to 8 via execution of executable instructions of the processor, and a memory for storing the executable instructions of the processor.
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