CN102967859B - Forward-looking scanning radar imaging method - Google Patents
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Abstract
The invention discloses a forward-looking scanning radar imaging method. The method comprise the steps of initializing imaging system parameters; performing range pulse compression on echo data; subjecting a radar right forward-looking area to super-resolution imaging processing; performing back projection imaging processing on a large-azimuth area; and splicing imaging results of the two areas. According to the forward-looking scanning radar imaging method, an ill-posed problem regularization method is utilized, and accordingly, the azimuth resolution is improved, and a back projection algorithm is combined to improve the azimuth resolution of the large-azimuth area. The two imaging results are spliced into a complete scene image, so that the forward-looking scanning radar imaging scene range is expanded, and not only the defect that synthetic aperture radars cannot perform forward-looking imaging is overcome, but also the problem that azimuth imaging scene ranges of prior forward-looking scanning radars are small is solved.
Description
Technical Field
The invention belongs to the technical field of radar signal processing, and particularly relates to an imaging method in a Forward-Looking scanning radar (Forward-Looking scanning radar) technology.
Background
The microwave imaging technology has the characteristics of all-time and all-weather operation as an active aviation and aerospace remote sensing means, has wide application in the fields of geological mapping, disaster monitoring, military reconnaissance and the like, and is one of the most important means for high-resolution earth observation and global resource management at present. However, due to the limitation of the working system, the existing Synthetic Aperture Radar (SAR) cannot realize high-resolution imaging of the forward-looking area azimuth direction, so that the SAR cannot fully play a role in the aspects of forward-looking ground alignment, autonomous landing, material airdrop, missile terminal guidance and the like of an aircraft.
The foresight scanning radar is a radar foresight imaging technology based on a regularization theory of an ill-posed problem, and overcomes the problem that SAR has a foresight blind area. In the motion process of the radar platform, the antenna scans a forward-looking imaging area, distance direction high resolution is formed by transmitting a large bandwidth signal, an azimuth echo sequence after motion compensation is modeled into a convolution model of a ground scattering point and an antenna directional diagram, and finally scene information is estimated by using an ill-posed problem regularization method to complete forward-looking area azimuth direction high resolution imaging.
As the azimuth angle of an imaging scene is increased, the Doppler coherence of echoes is enhanced, so that a convolution model of echo data in a region with a larger azimuth angle is influenced, and the imaging scene range of the forward-looking scanning radar is limited. In documents "Richards m., Morris c., hayes m., Iterative enhancement of non-coherent radar data, IEEE International Conference on acoustics training and Signal Processing,1929-1932, 1986", it is pointed out that when a forward scanning radar images a region with a large azimuth angle, the regularization method for the ill-posed problem is sensitive to the doppler effect, which affects the performance of the algorithm, and the document does not propose a method for expanding the imaging scene range. The radar forward-looking imaging is realized in a large scene range, and the aircraft can play better performance in the aspects of battlefield reconnaissance and monitoring, material airdrop, earthquake disaster rescue and the like. In documents "maleke t., Oelgart, rieckw, W-band-radio system in a dual-mode receiver for automatic target detection, and european conference on Synthetic apparatus Radar, 2002", it is proposed to use a Doppler Beam Sharpening (DBS) technique to increase the forward-looking imaging scene range, where the forward-looking area doppler changes slowly, and a better forward-looking imaging result cannot be obtained by using DBS, and the imaging result precision is still poor between the forward-looking super-resolution processing area and the DBS effective processing area.
Disclosure of Invention
The invention aims to solve the defects existing in the imaging processing of the forward-looking scanning radar in the existing method, and provides a forward-looking scanning radar imaging method.
For the convenience of describing the contents of the present invention, the following terms are first explained:
the term 1: forward-looking scanning radar
The forward-looking scanning radar is a sensor system which scans a forward-looking imaging area by using an antenna and processes echo data to realize imaging in the motion process of a radar platform. The Radar system signal model is described in the literature "LiD.Y., Huang Y.L, Yang J.Y, Motion platform for forward-looking-beam Radar antenna model, IEEECIE International Conference on Radar, 1370-.
The term 2: forward looking imaging azimuth
The forward-looking imaging azimuth refers to an angle that the connecting line of an imaging target and an aircraft deviates from the flight direction by setting the flight direction of the radar platform to be 0 degrees, the left-side azimuth of the flight direction is negative, and the right-side azimuth of the flight direction is positive.
The term 3: radar angle super resolution algorithm (Radar angle super resolution algorithm)
The radar angle super-resolution algorithm is a radar imaging algorithm based on an ill-posed problem regularization theory. The forward-looking scanning radar azimuth echo sequence can be modeled into a convolution model of an original scene and a radar antenna directional pattern function, and the original scene can be estimated through an ill-posed problem regularization method. The method breaks through the limitation of the azimuth resolution of the real aperture and realizes the radar angle super-resolution.
The term 4: back Projection algorithm (Back Projection, BP)
The back Projection algorithm comes from the computer-aided tomography technology in medical imaging, which utilizes Projection slice theorem (Projection-slice theorem) to model the radar echo signals demodulated from different view angles into a slice of two-dimensional fourier transform of the original scene, and avoids the complex operation in the direct two-dimensional inverse fourier transform by the back Projection algorithm to obtain the precise estimation of the original scene, thereby realizing the high-azimuth resolution imaging processing, which is specifically referred to in the literatures "Munson d.c., O' bright.d., Jenkins w.k., a tomographics for the Projection of spotlight-mode synthetic imaging radar, Proceedings of the IEEE, vol71, no8,917-925, 1983".
The technical scheme of the invention is as follows: a forward-looking scanning radar imaging method specifically comprises the following steps:
the method comprises the following steps: the parameters of the imaging system are initialized and,
the method specifically comprises the following parameters: the platform speed, noted as V; angle of elevation of the antenna, is
The height of the platform is marked as h; carrier frequency of transmission signal, denoted f0(ii) a Pulse width, noted as Tr；
The azimuth time vector is recorded as t = [ -PRI · Na/2,-PRI·(Na/2-1),…,PRI·(Na/2-1)]Where PRI is the pulse repetition time, NaSampling points in the azimuth direction of the target echo; the distance-time vector is noted as: τ = [ -1/fs·Nf/2,-1/fs·(Nf/2-1),…,1/fs·(Nf/2-1)]Wherein f issIs the range-wise sampling rate, NfSampling points in the target echo distance direction;
the target plane point target distance history is
Wherein R is0Is the initial slant distance, theta, between the antenna and the point target at the time of azimuth time 00The azimuth angle corresponding to the target at the time point 0;
step two: the echo data is subjected to range-wise pulse compression,
the target echo expression is:
where σ (x, y) is the reflection coefficient,
is the two-way echo delay, c is the speed of light, wa[·]Representing a slow time domain window function representing the modulation of an azimuth antenna pattern function, window width TbetaRepresenting the dwell time of the spot target beam,
is an azimuth angle theta0Corresponding orientation time, rect ·]Representing a fast time domain window function with a window width Tr，KrAdjusting the frequency of a transmitting signal, wherein lambda is the wavelength of a carrier wave;
the forward-looking scanning radar echo data s (tau, t; x, y) is subjected to range-direction pulse compression, and the data after pulse compression is expressed as s1(τ,t;x,y)；
Presetting a first azimuth angle theta for determining a super-resolution imaging processing regionαAnd a second azimuth theta for determining a backprojection imaging processing regionβSatisfies thetaβGreater than thetaα；
Step three: performing super-resolution imaging processing on the forward-looking area of the radar,
data s after distance-wise pulse compression processing in step two1(τ, y; x, y), the extraction azimuth angle is- θα～θαThe data in the radar angle super-resolution processing area is defined as the following data:
wherein,respectively, is an azimuth angle-thetaα、θαB is the bandwidth of the transmitted signal at the corresponding azimuth moment;
the point target slope distance history R (t; x, y) is expanded by Taylor series and then a term is retained, so that the point target slope distance history R (t; x, y) can be obtained
Approximating the skew history as R (t; x, y) ≈ R0-Vt; using the approximated slope history to process the region data s in super resolution2(τ, t; x, y), the distance-time variable τ is replaced by a variableCarrying out scale transformation, eliminating distance walking generated by platform motion, and completing motion compensation, wherein data after the scale transformation is represented as:
antenna directional diagram function fantenAs a reference function for data s3(tau, t; x, y) regularizing the problem of unsuitability to realize the data s3(tau, t; x, y) and the imaging result is denoted as s4(τ,t;x,y)；
Step four: the backward projection algorithm is utilized to carry out imaging processing on the area with larger azimuth angle,
data s after distance-wise pulse compression processing in step two1(τ, y; x, y), the extraction azimuth angle is- θβ～-θα,θα～θβThe data in the interior is designated as a backward projection processing area, and the data is expressed as:
wherein,
respectively, is an azimuth angle-thetaβ、θβThe corresponding azimuth time;
calculating the distance from any point target on the target plane to the radar platform within the irradiation time of the antenna beam according to the position information recorded by the system when the radar platform moves
A beam dwell time vector corresponding to the point target; according toCalculating two-way time delayConstructing a phase compensation factor phi (x, y) = exp { -j2 pi f by using t (x, y)0t(x,y)}，f0Is the carrier frequency;
processing the region data s for the backprojection5(τ, t; x, y) performing distance interpolation, and interpolating the interpolated data s'5(tau, t; x, y) and phase compensation factor <math>
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</math> Coherent accumulation is carried out to obtain accurate estimation of each point target (x, y) of the imaging scene, and data s is realized5(tau, t; x, y) back projection imaging processing, and the imaging result is recorded as s6(τ,t;x,y)；
Step five: the imaging results of the two regions are spliced,
imaging result s after the three-step super-resolution processing4(tau, t; x, y) and imaging result s after the back projection processing of step four6(tau, t, x, y) is spliced, and the spliced data is the final imaging result of the forward-looking scanning radar and is marked as s7(τ,t;x,y)。
The invention has the beneficial effects that: according to the forward-looking scanning radar imaging method, the azimuth resolution is improved by adopting an ill-posed problem regularization method in a forward-looking area of the radar, the azimuth resolution of an area with a larger azimuth angle is improved by combining a back projection algorithm, the two imaging results are spliced into a complete scene image, and the forward-looking scanning radar imaging scene range is expanded. The method not only overcomes the defect that the synthetic aperture radar can not forward-view image, but also solves the problem that the existing forward-view scanning radar has small azimuth imaging scene range. The method can realize high-azimuth resolution imaging processing in an imaging scene range with a large front visual area.
Drawings
FIG. 1 is a schematic flow diagram of a forward-looking scanning radar imaging method of the present invention.
FIG. 2 is a block diagram of a forward scanning radar system employed in accordance with an exemplary embodiment of the present invention.
FIG. 3 is a table of parameters for a forward looking scanning radar system as utilized in an embodiment of the present invention.
Fig. 4 is a layout diagram of a target scene used in an embodiment of the present invention, and dotted lines are region separation lines of the super-resolution imaging process and the back-projection imaging process.
FIG. 5 is a diagram illustrating the result of imaging 9 point targets in the super-resolution imaging processing region of FIG. 4 according to an embodiment of the present invention.
FIG. 6 is a graphical representation of the results of imaging a 12 point target within the backprojection imaging processing region of FIG. 4 in an embodiment of the present invention.
FIG. 7 is a final imaging result after stitching processing of the imaging data of FIGS. 5 and 6 in an exemplary embodiment of the invention.
Detailed Description
The invention mainly adopts a simulation experiment method for verification, and all the steps and conclusions are verified to be correct on Matlab 2010. The method of the invention is further illustrated with reference to the accompanying drawings and specific examples.
The flow diagram of the forward-looking scanning radar imaging method is shown in fig. 1, and the specific process is as follows:
the method comprises the following steps: and initializing parameters of the imaging system.
The imaging geometric model diagram adopted in this embodiment is shown in fig. 2, a system coordinate system takes a ground surface point below a radar platform as a coordinate origin, the platform moves along a y-axis, an x-axis is a track cutting direction, and a z-axis is a direction perpendicular to the ground. The imaging system parameters were initialized according to the data listed in figure 3.
The target scene adopted by the embodiment is shown in fig. 4, and the black dots in the figure are 21 point targets arranged on the ground by 3 × 7. These 21 points are spaced 3 degrees apart in the theta direction (forward looking imaging azimuth) and 200 meters apart in the Y direction (along track). The zero-time position coordinate of the radar platform is (0, 5000) m, and the o point coordinate of the center of the target scene is (0,10000,0) m. The position coordinates of any point object in the scene are denoted as P (x, y).
Constructing an orientation time vector t = [ -PRI · Na/2,-PRI·(Na/2-1),…,PRI·(Na/2-1)]Where PRI is the pulse repetition time, NaAnd counting the number of sampling points in the azimuth direction of the target echo. Constructing a distance-time vector τ = [ -1/fs·Nf/2,-1/fs·(Nf/2-1),…,1/fs·(Nf/2-1)]Wherein f issIs the range-wise sampling rate, NfAnd counting the number of sampling points in the direction of the target echo distance.
Step two: and performing range-wise pulse compression on the echo data.
And simulating the echo data s (tau, t; x, y) of the point target by using Matlab according to the coordinate positions of the radar platform and the point target in the step one.
The distance direction pulse compression specifically adopts the following process:
adjusting frequency K according to the transmitted signalrDistance to reference time τrefConstructing a range-wise pulse compression reference signalUsing matched filtering to perform range-wise pulse compression on the forward-looking scanning radar echo data, and expressing the data after pulse compression as s1(τ,t;x,y)：
Wherein, sinc {. is a distance response function, and B is a transmission signal bandwidth.
Presetting a first azimuth angle theta for determining a super-resolution imaging processing regionαAnd a second azimuth theta for determining a backprojection imaging processing regionβSatisfies thetaβGreater than thetaα。
Where theta isαAnd thetaβThe antenna scanning angle information and the system parameters can be determined according to the antenna scanning angle information and the system parameters, and the antenna scanning angle information and the system parameters are generally selected according to experience, and the specific selection process is not described in detail. In the present embodiment, θαTake 5 deg. thetaβTake 10 °.
Step three: and performing super-resolution imaging processing on the forward-looking area of the radar.
According to the antenna scanning angle information obtained in the simulation, the data s after the distance direction pulse compression processing in the second step is obtained in the embodiment1In (tau, t, x, y), taking out data with azimuth angle within-5 deg, defining as radar angle super-resolution imaging processing zone, and recording data as s2(τ,t;x,y)。
The point target slope distance history R (t; x, y) is expanded by Taylor series and then a term is retained, so that the point target slope distance history R (t; x, y) can be obtained
Approximating the skew history as R (t; x, y) ≈ R0-Vt. Using t0Constructing a phase compensation factor phi by using =2 × Vt/c as a distance walk correction amountrcm=exp{2πfst0}。
Data s are paired in the distance direction2(tau, t; x, y) carrying out scale transformation, eliminating distance walking generated by platform motion, and completing motion compensation. In specific implementation, the data s of the super-resolution imaging processing area is processed2(tau, t; x, y) Fourier transforming the distance and conjugate multiplying the phase compensation factor phircmAnd then, performing inverse Fourier transform on the data along the distance direction to finish distance walk correction, and recording the motion-compensated data as s3(τ,t;x,y)。
Antenna directional diagram function fantenAs a reference function for data s3(tau, t; x, y) regularizing the problem of unsuitability to realize the data s3(tau, t; x, y) and the imaging result is denoted as s4(τ, t; x, y). FIG. 5 shows a super-resolution imaging position of a forward-looking area of a radarAnd (6) processing the result.
Step four: and imaging the area with a larger azimuth angle by utilizing a back projection algorithm.
According to the antenna scanning angle information obtained in the simulation, the data s after the distance direction pulse compression processing in the second step is obtained in the embodiment1In (tau, t, x, y), taking out the data whose azimuth angle is in-10 deg. -5 deg. and 5-10 deg., defining it as backward projection imaging processing zone, and recording the data as s5(τ,t;x,y)。
Calculating the distance from any point target on the target plane to the radar platform within the irradiation time of the antenna beam according to the position information recorded by the system when the radar platform moves
And the beam residence time vector corresponding to the point target. According to
Calculating two-way time delayConstructing a phase compensation factor phi (x, y) = exp { -j2 pi f by using t (x, y)0t(x,y)}，f0Is the carrier frequency.
Processing the region data s for the backprojection5(τ, t; x, y) distance interpolation is performed, where the interpolation may be a general interpolation method, and this embodiment adopts 8-point sinc interpolation. The interpolated data s'5(tau, t; x, y) and the phase compensation factor phi (x, y) are expressed as
Coherent accumulation is carried out to obtain accurate estimation of each point target (x, y) of the imaging scene, and data s is realized5(tau, t; x, y) back projection imaging processing, and recording the imaging result as s6(τ, t; x, y). FIG. 6 is a larger azimuthal regionAnd (5) backward projection imaging processing results.
Step five: and (5) splicing the imaging results of the two regions.
Imaging result s after the three-step super-resolution processing4(tau, t; x, y) and imaging result s after the back projection processing of step four6(tau, t, x, y) is spliced, and the spliced data is the final imaging result of the forward-looking scanning radar and is marked as s7(τ,t;x,y)。
In this embodiment, the following splicing process may be specifically adopted:
imaging result s after the three-step super-resolution processing4(tau, t; x, y) and imaging result s after the back projection processing of step four6(tau, t; x, y) are normalized respectively; using a two-dimensional sine function in a butt joint area of two imaging results, and carrying out weighted average on a single pixel to a corresponding pixel at an overlapping position to realize gray level fusion of the area and ensure seamless splicing processing of the images; the spliced data is the final imaging result of the forward-looking scanning radar and is recorded as s7(τ,t;x,y)。
FIG. 7 is a schematic representation of the final imaging results obtained in the examples using the method of the present invention. Therefore, the method realizes the high-azimuth resolution imaging processing of the forward-looking scanning radar in a larger imaging scene range.
It will be appreciated by those of ordinary skill in the art that the embodiments described herein are intended to assist the reader in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited embodiments and examples. Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.
Claims (3)
1. A forward-looking scanning radar imaging method specifically comprises the following steps:
the method comprises the following steps: the parameters of the imaging system are initialized and,
the method specifically comprises the following parameters: the platform speed, noted as V; angle of elevation of the antenna, isThe height of the platform is marked as h; carrier frequency of transmission signal, denoted f0(ii) a Pulse width, noted as Tr；
The azimuth time vector is recorded as t = [ -PRI · Na/2,-PRI·(Na/2-1),…,PRI·(Na/2-1)]Where PRI is the pulse repetition time, NaSampling points in the azimuth direction of the target echo; the distance-time vector is noted as: τ = [ -1/fs·Nf/2,-1/fs·(Nf/2-1),…,1/fs·(Nf/2-1)]Wherein f issIs the range-wise sampling rate, NfSampling points in the target echo distance direction;
the target plane point target distance history is
Wherein R is0Is the initial slant distance, theta, between the antenna and the point target at the time of azimuth time 00The azimuth angle corresponding to the target at the time point 0;
step two: the echo data is subjected to range-wise pulse compression,
the target echo expression is:
where σ (x, y) is the reflection coefficient,
is the two-way echo delay, c is the speed of light, wa[·]Representing a slow time domain window function representing the modulation of an azimuth antenna pattern function, window width TbetaRepresenting the dwell time of the spot target beam,is an azimuth angle theta0Corresponding orientation time, rect ·]Representing a fast time domain window function with a window width Tr，KrAdjusting the frequency of a transmitting signal, wherein lambda is the wavelength of a carrier wave;
the forward-looking scanning radar echo data s (tau, t; x, y) is subjected to range-direction pulse compression, and the data after pulse compression is expressed as s1(τ,t;x,y)；
Presetting a first azimuth angle theta for determining a super-resolution imaging processing regionαAnd a second azimuth theta for determining a backprojection imaging processing regionβSatisfies thetaβGreater than thetaα；
Step three: performing super-resolution imaging processing on the forward-looking area of the radar,
data s after distance-wise pulse compression processing in step two1(τ, y; x, y), the extraction azimuth angle is- θα～θαThe data in the radar angle super-resolution processing area is defined as the following data:
wherein,
respectively, is an azimuth angle-thetaα、θαB is the bandwidth of the transmitted signal at the corresponding azimuth moment;
the point target slope distance history R (t; x, y) is expanded by Taylor series and then a term is retained, so that the point target slope distance history R (t; x, y) can be obtained
Approximating the skew history as R (t; x, y) ≈ R0-Vt; using the approximated slope history to process the region data s in super resolution2(τ, t; x, y), the distance-time variable τ is replaced by a variableCarrying out scale transformation, eliminating distance walking generated by platform motion, and completing motion compensation, wherein data after the scale transformation is represented as:
antenna squareUsing the graph function as a reference function to data s3(tau, t; x, y) regularizing the problem of unsuitability to realize the data s3(tau, t; x, y) and the imaging result is denoted as s4(τ,t;x,y)；
Step four: the backward projection algorithm is utilized to carry out imaging processing on the area with larger azimuth angle,
data s after distance-wise pulse compression processing in step two1(τ, y; x, y), the extraction azimuth angle is- θβ～-θα,θα～θβThe data in the interior is designated as a backward projection processing area, and the data is expressed as:
wherein,
respectively, is an azimuth angle-thetaβ、θβThe corresponding azimuth time;
calculating the distance from any point target on the target plane to the radar platform within the irradiation time of the antenna beam according to the position information recorded by the system when the radar platform moves
A beam dwell time vector corresponding to the point target; according to
Calculating two-way time delay
Constructing a phase compensation factor phi (x, y) = exp { -j2 pi f by using t (x, y)0t(x,y)}，f0Is the carrier frequency;
in opposite directionsProjection processing region data s5(τ, t; x, y) performing distance interpolation, and interpolating the interpolated data s'5(tau, t; x, y) and phase compensation factor
Coherent accumulation is carried out to obtain accurate estimation of each point target (x, y) of the imaging scene, and data s is realized5(tau, t; x, y) back projection imaging processing, and the imaging result is recorded as s6(τ,t;x,y)；
Step five: the imaging results of the two regions are spliced,
imaging result s after the three-step super-resolution processing4(tau, t; x, y) and imaging result s after the back projection processing of step four6(tau, t, x, y) is spliced, and the spliced data is the final imaging result of the forward-looking scanning radar and is marked as s7(τ,t;x,y)。
2. The forward-looking scanning radar imaging method according to claim 1, wherein the distance-wise pulse compression of the forward-looking scanning radar echo data s (τ, t; x, y) in the second step specifically adopts the following process:
adjusting frequency K according to the transmitted signalrDistance to reference time τrefConstructing a range-wise pulse compression reference signal
The distance direction pulse compression is carried out on the forward-looking scanning radar echo data by using matched filtering, and the data after pulse compression is expressed as s1(τ,t;x,y)：
Wherein sinc {. is a distance response function.
3. The forward-looking scanning radar imaging method according to claim 1 or 2, wherein the stitching process in step 5 is as follows:
imaging result s after the three-step super-resolution processing4(tau, t; x, y) and imaging result s after the back projection processing of step four6(tau, t; x, y) are normalized respectively; two-dimensional sine functions are used in the butt joint area of the two imaging results, the single pixel is weighted and averaged to the corresponding pixel at the overlapping position, the gray level fusion of the area is realized, the spliced data is the final imaging result of the forward-looking scanning radar and is recorded as s7(τ,t;x,y)。
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