CN112904339B - Bistatic forward-looking SAR imaging method characterized by intersection point of slope course and course - Google Patents
Bistatic forward-looking SAR imaging method characterized by intersection point of slope course and course Download PDFInfo
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Abstract
A double-base forward-looking SAR imaging method characterized by an intersection point of a slope distance process and a course comprises the following specific steps: 1) The airborne transmitter side-looking transmits a linear frequency modulation signal, and the airborne receiver side-looking receives echo data of a target; 2) The double-base forward looking SAR distance sum of each moment of azimuth slow time of the linear frequency modulation signal transmitted by the airborne transmitter is calculated; 3) Acquiring two-dimensional space-variant time domain echo data after distance direction matching filtering; 4) Calculating the intersection point position of the slope distance process and the course in the imaging coordinate system; 5) Determining the position of a scene reference point; 6) Constructing a two-dimensional space-variant matched filter function; 7) And acquiring a bistatic forward-looking SAR image. The method is used for directly calculating the Doppler parameter of echo data in the imaging of the measured data of the bistatic forward-looking SAR in the airborne radar, and has the advantages of high real-time imaging efficiency and good imaging effect.
Description
Technical Field
The invention belongs to the technical field of Radar, and further relates to a Synthetic Aperture Radar (SAR) imaging method characterized by an intersection point of an inclined distance course and a course in the technical field of Radar signal processing. The method can be used for realizing bistatic forward-looking SAR actual measurement data imaging in the airborne radar.
Background
The bistatic forward-looking SAR can work all weather and all day long, is not limited by the use environment, and the bistatic forward-looking SAR imaging technology becomes an important technology for realizing accurate guidance. How to reduce the imaging operation amount and improve the imaging speed is the key for realizing the bistatic forward-looking SAR rapid imaging. The existing bistatic forward-looking SAR imaging method has large computation amount and long processing time, and needs to be improved in the aspect of real-time property.
A foresight SAR imaging method is disclosed in a patent document 'missile-borne SAR motion compensation method based on a low-precision inertial navigation system' applied to Shanghai radio equipment research (publication No. CN111381217A, application No. 202010251164.7, application date: 2020, 04 and 01), and is suitable for both bistatic foresight SAR imaging and single-base SAR imaging. The bistatic forward-looking SAR imaging method comprises the following steps: 1. the method comprises the steps of carrying out range-to-Fourier transform on a received two-dimensional time domain signal, then carrying out range pulse pressure and range walk correction on a range frequency domain and an azimuth time domain signal in a range direction, 2, carrying out range-to-Fourier inverse transform on the signal after range-to-direction processing, and estimating Doppler instantaneous modulation frequency in SAR echo data in an azimuth direction by adopting an MD algorithm, 3, constructing an SAR azimuth matching filter function through the modulation frequency, thereby compensating phase errors caused by Doppler modulation frequency, and 4, realizing bistatic forward-looking SAR imaging. The method has the disadvantages that the calculation amount for estimating the Doppler instantaneous modulation frequency in the SAR echo data by adopting the MD algorithm is very large, the time consumption is long in the real-time imaging of the bistatic forward-looking SAR, the practicability is poor, the calculation amount for estimating the Doppler modulation frequency is reduced for realizing the real-time imaging, and the accuracy of the bistatic forward-looking SAR real-time imaging is lost.
Bengal self-force proposed a bistatic forward-looking SAR imaging method in its published paper "bistatic forward-looking high mobility platform SAR system characterization and imaging algorithm research" (Ph's paper 2016, university of Western electronic technology). The method comprises the following imaging steps: 1. carrying out distance Fourier transform on the received two-dimensional time domain signal, then carrying out distance pulse pressure and distance walk correction on the distance frequency domain and the azimuth time domain signal in the distance direction, 2, carrying out distance Fourier inverse transform on the signal after the distance direction processing, and then obtaining a reference point in the azimuth direction by adopting a reference point obtaining method. The reference point acquisition method adopts a point equal to the Doppler value of the center point of the scene as a reference point of the bistatic forward-looking SAR in the azimuth direction. 3. Calculating Doppler parameters at the time of the center of the synthetic aperture through a reference point, 4, interpolating the Doppler parameters at the time of the center of the synthetic aperture to obtain the Doppler parameters of the bistatic forward-looking SAR echo signals, and 5, compensating phase errors caused by the Doppler parameters to realize bistatic forward-looking SAR imaging. The method has the disadvantages that the Doppler parameters of the bistatic forward-looking SAR echo signals are obtained through interpolation, and the redundancy of the method is increased through the interpolation, so that the real-time imaging time consumption of the bistatic forward-looking SAR is long. For selecting a point equal to the Doppler value of the central point of the scene as a reference point of the bistatic forward-looking SAR in the azimuth direction, the imaging effect is poor at the edge with larger distance from the reference point, and the image edge is fuzzy.
Disclosure of Invention
The invention aims to provide a bistatic forward-looking SAR imaging method characterized by an intersection point of an inclined distance process and a course, aiming at overcoming the defects of the prior art, and solving the problems that in the prior art, the method for acquiring Doppler parameter operand by adopting an MD algorithm or interpolation and the contradiction between imaging precision are difficult to apply to bistatic forward-looking SAR real-time imaging, and the problem that the imaging efficiency is influenced and the edge imaging is blurred due to the fact that a reference point at the center of a synthetic aperture is selected.
The technical idea for realizing the purpose of the invention is that the characteristic of the imaging configuration of the bistatic forward-looking SAR is fully utilized, on the premise of acquiring the time domain echo data of the two-dimensional space variation, the intersection point position of the slope distance process and the course in the imaging coordinate system is calculated, the intersection point is used as the reference point of the bistatic forward-looking SAR scene, the matching filter function of the two-dimensional space variation of the time domain echo data is constructed through the position coordinates of the reference point, the bistatic forward-looking SAR image is obtained, the method of acquiring Doppler parameters by a large amount of calculation is avoided, and the problem of edge imaging blurring is effectively avoided by selecting a plurality of reference points.
The specific steps for realizing the purpose of the invention are as follows:
(1) The airborne transmitter side-looking transmits a linear frequency modulation signal, and the airborne receiver side-looking receives echo data of a target;
(2) Calculating the sum of the distance of the bistatic forward-looking SAR at each moment of the azimuth slow time of the linear frequency modulation signal transmitted by the airborne transmitter;
(3) Acquiring two-dimensional space-variant time domain echo data after distance direction matching filtering:
(3a) Carrying out range Fourier transform on the actually measured echo data to obtain echo data with a range direction as a frequency domain and an azimuth direction as a time domain, and sequentially carrying out range pulse pressure and range migration correction on the echo data by using a matched filter to obtain echo data after range direction matched filtering;
(3b) Performing Fourier inverse transformation of the distance direction on the echo data subjected to distance direction matched filtering to obtain two-dimensional space-variant time domain echo data s (t, eta):
(4) Calculating the intersection point position of the slope distance process and the course in the imaging coordinate system:
(4a) The bistatic forward-looking SAR slope sum at each time instant is calculated as R n Coordinates of the y-axis in the imaging plane coordinate system:
wherein, y (eta, R) n ) Representing the sum of the tilt distances of the bistatic forward-looking SAR and the intersection point at the eta moment as R n The coordinate value of the y axis in the imaging plane coordinate system is represented by eta, which represents the slow time of the orientation of the linear frequency modulation signal transmitted by the airborne transmitter, R n Representing the nth range bin into which the time domain echo data s (t, η) is divided in range direction, R (η) representing the bistatic forward-looking SAR range sum at time η, x T (η)，y T (η)，z T (η) represents the coordinate values of the x-axis, y-axis and z-axis of the airborne transmitter in the three-dimensional space coordinate system at time η, y R (η)，z R (η) respectively representing the coordinate values of the y-axis and the z-axis of the onboard receiver in the three-dimensional space coordinate system at time η,i denotes a first intermediate variable which is,
f denotes a second intermediate variable, F = (y) R (η)-y T (η)) 2 E denotes a third intermediate variable, E = y R (η)+y T (η), U represents a fourth intermediate variable,
g represents a fifth intermediate variable which is,
(5) Determining the position of the scene reference point:
the intersection point position of the slant range process and the course in the imaging coordinate system is used as a bistatic forward-looking SAR scene reference point to obtain the position coordinate (0, y (eta; R) of the reference point in the imaging coordinate system n ))；
(6) Constructing a two-dimensional space-variant matched filter function:
(6a) And (3) performing third-order Taylor expansion on the bistatic forward-looking SAR slant range and the bistatic forward-looking SAR slant range according to the position coordinates of the reference point in the imaging plane coordinate system:
(6b) Calculating the two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit into which the time-domain echo data s (t, eta) are divided in the distance direction:
(6c) According to the two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit, constructing a two-dimensional space-variant matched filter function H of time domain echo data:
(7) Acquiring a bistatic forward-looking SAR image:
(7a) Multiplying the time domain echo data s (t, eta) by a matched filter function H to obtain echo data u (t, eta) after two-dimensional space-variant compensation:
(7b) Performing azimuth Fourier transform on the echo data u (t, eta) to obtain echo data v (t, f) of a distance time domain and an azimuth frequency domain after two-dimensional space-variant compensation a )：
(7c) Echo data v (t, f) are processed by the PGA method a ) And carrying out self-focusing processing to obtain a bistatic forward-looking SAR image.
Compared with the prior art, the invention has the following advantages:
firstly, on the premise of acquiring time domain echo data of two-dimensional space-variant after distance direction matched filtering, the invention calculates the intersection point position of the slope distance process and the course in an imaging coordinate system, takes the intersection point as a reference point of a bistatic forward-looking SAR scene, and obtains the frequency modulation coefficient of the two-dimensional space-variant by performing three-order Taylor expansion on the basis of the slope distance of the bistatic forward-looking SAR and the slope distance of the bistatic forward-looking SAR based on the reference point, thereby effectively overcoming the problem that the computation and the imaging precision are contradicted by using an MD algorithm to estimate the Doppler frequency modulation rate in the prior art, and the problem that the method is difficult to be applied to real-time imaging of the bistatic forward-looking SAR, so that the imaging precision of the invention is higher, and the imaging speed is improved.
Secondly, because the invention constructs the two-dimensional space-variant matched filter function of the time domain echo data by utilizing the intersection point position of the slant range process and the course in the imaging coordinate system, the Doppler parameters of the bistatic forward-looking SAR echo signals obtained by interpolation in the prior art are effectively overcome, and the interpolation increases the redundancy of the method, thereby causing the time consumption of bistatic forward-looking SAR real-time imaging. The method has the advantages that the point which is equal to the Doppler value of the center point of the scene is selected as the reference point of the bistatic forward-looking SAR in the azimuth direction, the imaging effect is poor at the edge which is far away from the reference point, and the image edge is fuzzy, so that the edge imaging effect is better.
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FIG. 1 is a flow chart of the present invention;
FIG. 2 is a graph of the results of a simulation experiment of the present invention.
Detailed Description
The present invention will be described in further detail below with reference to the accompanying drawings.
The implementation steps of the present invention are described in further detail with reference to fig. 1.
Step 1, the side view of the airborne transmitter transmits a linear frequency modulation signal, and the front view of the airborne receiver receives echo data of a target.
Step 2, calculating the sum of the distance of the bistatic forward-looking SAR at each moment of the azimuth slow time of the linear frequency modulation signal transmitted by the airborne transmitter:
wherein x is b ，y b And coordinate values of the x-axis and the y-axis of the target b in the three-dimensional space coordinate system are respectively represented.
And 3, acquiring two-dimensional space-variant time domain echo data after distance direction matching filtering.
And performing range Fourier transform on the actually measured echo data to obtain echo data with a range direction as a frequency domain and a direction as a time domain, and performing range pulse pressure and range migration correction on the echo data by using a matched filter to obtain echo data after range direction matched filtering.
Performing Fourier inverse transformation of the distance direction on the echo data subjected to distance direction matched filtering to obtain two-dimensional space-variant time domain echo data s (t, eta):
s(t,η)＝sinc(B r (t-τ))w a (η)exp[jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]
wherein s (t, eta) represents a two-dimensional time domain echo signal of the bistatic forward-looking SAR at the time of the distance fast time t and eta, sinc (·) represents a sine function, and B r Representing the distance-wise bandwidth of the chirp signal emitted by the airborne transmitter, τ representing the sum of the time delays in the space of the chirp signal emitted by the airborne transmitter and the echo signal of the airborne receiver, w a (η) represents a window function at η time of the azimuth direction a of the time domain echo signal after the range direction matching filtering, exp represents an exponential operation with a natural constant e as a base, j represents an imaginary unit symbol, pi represents a circumferential ratio, and γ (η, R) n ) Representing the sum of bistatic forward-looking SAR slopes at time η as R n Quadratic frequency modulation term, ξ (η, R) of two-dimensional space-variant of time-domain echo data s (t, η) n ) Denotes the sum of bistatic foresight SAR slope at time η as R n And (3) a cubic frequency modulation term of two-dimensional space variation of the time domain echo data s (t, eta).
And 4, calculating the intersection point position of the slope distance process and the course in the imaging coordinate system.
According to the following formulaCalculating the sum of bistatic forward-looking SAR slope at each time as R n Coordinates of the y-axis in the imaging plane coordinate system:
wherein, y (eta, R) n ) Representing the sum of the tilt distances of the bistatic forward-looking SAR and the intersection point at the eta moment as R n The coordinate value of the y axis in the imaging plane coordinate system, eta represents the slow time of the orientation of the linear frequency modulation signal transmitted by the airborne transmitter, R n Representing the nth range bin into which the time-domain echo data s (t, η) is divided in the range direction, R (η) representing the bistatic forward-looking SAR range sum at time η, x T (η)，y T (η)，z T (η) represents the coordinate values of the x-axis, y-axis and z-axis of the airborne transmitter in the three-dimensional space coordinate system at time η, respectively, y R (η)，z R (η) represents the coordinate values of the y-axis and the z-axis of the onboard receiver in the three-dimensional space coordinate system at time η, respectively, I represents the first intermediate variable,
f denotes a second intermediate variable, F = (y) R -y T ) 2 Where E represents a third intermediate variable, E = y R +y T And U represents a fourth intermediate variable,
g represents a fifth intermediate variable which is,
and 5, determining the position of the scene reference point.
The intersection point position of the slant range process and the course in the imaging coordinate system is used as a bistatic forward-looking SAR scene reference point to obtain the position coordinate (0, y (eta; R) of the reference point in the imaging coordinate system n ))。
And 6, constructing a two-dimensional space-variant matched filter function.
And (3) performing third-order Taylor expansion on the bistatic forward-looking SAR slant range and the bistatic forward-looking SAR slant range according to the position coordinates of the reference point in the imaging plane coordinate system:
where K (η) represents the sum of the distance between the airborne receiver and the reference point at time η and the distance between the airborne transmitter and the reference point, R R0 Indicating the distance between the airborne receiver and the reference point when η =0,
v Ry (η)，v Rz (η) represents the flight speed of the airborne receiver at time η along the Y-axis and Z-axis, respectively, a Ry ，a Rz Representing the acceleration, R, of the airborne receiver at time η along the Y-axis and Z-axis, respectively T0 Indicating the distance between the airborne transmitter and the reference point when η =0,
v Ty (η)，v Tz (eta) represents the flight speed of the airborne transmitter along the Y-axis and Z-axis at the moment of transmitting eta, respectively, a Ty ，a Tz Denotes the acceleration of the on-board transmitter along the Y-axis and Z-axis at time η, δ denotes the sixth intermediate variable, δ =2v Ry (y R (η)-y g (η,R n ))+2v Rz (η)z R (η), α represents a seventh intermediate variable, α = a Ry (η)v Ry (η)+a Rz (η)v Rz (η), β represents an eighth intermediate variable,
ε represents the ninth intermediate variable, ε =2v Ty (y T (η)-y g (η,R n ))+2v Tz (η)z T (η), κ represents the tenth intermediate variable,
ω denotes a tenth intermediate variable, ω = a Ty (η)v Ty (η)+a Tz (η)v Tz (η)。
Calculating the two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit into which the time-domain echo data s (t, eta) are divided in the distance direction:
where λ represents the wavelength of the chirp signal transmitted by the airborne transmitter.
According to the two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit, constructing a two-dimensional space-variant matched filter function H of time domain echo data:
H＝exp[-jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]
where H represents a two-dimensional space-variant matched filter function of the time-domain echo data s (t, η).
Step 7, obtaining a bistatic forward-looking SAR image:
multiplying the time domain echo data s (t, eta) by a matched filter function H to obtain echo data u (t, eta) after two-dimensional space-variant compensation:
u(t,η)＝sinc(B r (t-τ))w a (η)exp[jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]·H
＝sinc(B r (t-τ))w a (η)
where u (t, η) represents the two-dimensional space-variant compensated echo data at the fast time t, η from the distance.
Performing azimuth Fourier transform on the echo data u (t, eta) to obtain echo data v (t, f) of a distance time domain and an azimuth frequency domain after two-dimensional space-variant compensation a )：
v(t,f a )＝sinc(B r (t-τ))sin c(T a f a )
Wherein, f a Representing two-dimensional space-variant compensated distancesAzimuthal Doppler frequency, v (t, f), of echo data from time domain, azimuthal frequency domain a ) Representing the range fast time t, azimuth Doppler frequency f a Echo data of (T) a The azimuthal synthetic aperture time of the time domain echo data s (t, η) is represented.
Echo data v (t, f) are processed by the PGA method a ) And carrying out self-focusing processing to obtain a bistatic forward-looking SAR image.
The effect of the present invention is further explained by combining the simulation experiment as follows:
1. simulation experiment conditions are as follows:
the software platform of the simulation experiment of the invention is as follows: windows 10 operating system and MATLAB R2018b.
The parameters used in the simulation experiments of the present invention are shown in table 1.
Table 1 summary of simulation experiment parameters of the invention
| Pulse width | 3e-6s | Pulse repetition frequency | 2000Hz |
| Bandwidth of | 150e6Hz | Sampling frequency | 200e6Hz |
| Carrier frequency | 16e9Hz | Speed of light | 3e8m/s |
| Scene center point coordinates | (494,5775,0) | Track angle | 87° |
| Distance direction sampling point | 4096 | Azimuth sampling point | 2048 |
| Receiving radar start coordinates | (0,0,1200) | Starting coordinate of transmitting radar | (-3667,2343,1500) |
| Receiving radar speed | (0,48,0) | Speed of transmitting radar | (0,-62,0) |
2. Simulation content and result analysis thereof:
the simulation experiment of the invention is a simulation experiment for imaging the target with large reflection coefficient in the scene formed by farmland and train tracks respectively by adopting the method of the invention and a method of the prior art, and the imaging result is shown in figure 2.
The prior art adopted in the simulation experiment of the present invention means:
a double-base SAR imaging method for estimating Doppler instantaneous modulation frequency in SAR echo data based on MD algorithm is disclosed in a patent document 'missile-borne SAR motion compensation method based on low-precision inertial navigation system' (publication No. CN111381217A, application No. 202010251164.7, application date: 2020, 04 and 01) applied by Shanghai radio equipment research.
Referring to fig. 2, for the simulation content and the result analysis thereof of the present invention:
FIG. 2 (a) is a result graph of imaging by using the prior art, and FIG. 2 (b) is a result graph of imaging by the present invention, wherein the upper right two white circles of elliptical dotted lines in the two graphs are train track images, and the lower right two white circles of elliptical dotted lines are farmland images, so that it can be seen that the train track image in the upper right white circle of elliptical dotted lines in the right graph is clearer than the train track image in the upper right white circle of elliptical dotted lines in the left graph, and the farmland image in the lower right white circle of elliptical dotted lines in the right graph is clearer than the farmland image in the lower right white circle of elliptical dotted lines in the left graph.
The simulation of the above experiment shows that: the method directly calculates the Doppler secondary frequency modulation item and the tertiary frequency modulation item of the two-dimensional space-variant of the bistatic forward-looking SAR echo data, and constructs the azimuth matched filter function through the items. In addition, the invention combines the bistatic forward-looking SAR imaging configuration to obtain the position coordinates of the bistatic forward-looking SAR scene reference point, and obtains the two-dimensional space-variant Doppler parameter of bistatic forward-looking SAR echo data according to the coordinates, so that the invention can clearly image at the edge. The problems that in the prior art, the contradiction between the Doppler parameter operation amount obtained by adopting an MD algorithm or interpolation and the imaging precision is difficult to apply to the real-time imaging of the bistatic forward-looking SAR, and the imaging efficiency is influenced and the edge imaging is blurred due to the fact that a reference point at the center of a synthetic aperture is selected are solved.
Claims (8)
1. A double-base foresight SAR imaging method characterized by intersection points of a slant range process and a course is characterized in that the intersection point position of the slant range process and the course in an imaging coordinate system is calculated, the intersection point is used as a reference point of a double-base foresight SAR scene, and then a two-dimensional space-variant matched filter function of time domain echo data is constructed through the position coordinates of the reference point, wherein the method comprises the following steps:
(1) The airborne transmitter transmits linear frequency modulation signals in a side view, and the airborne receiver receives echo data of a target in a front view;
(2) Calculating the sum of the distance of the bistatic forward-looking SAR at each time of the azimuth slow time of the linear frequency modulation signal transmitted by the airborne transmitter;
(3) Acquiring two-dimensional space-variant time domain echo data after distance direction matching filtering:
(3a) Carrying out range Fourier transform on the actually measured echo data to obtain echo data with a range direction as a frequency domain and a direction as a time domain, and sequentially carrying out range pulse pressure and range migration correction on the echo data by using a matched filter to obtain echo data after range direction matched filtering;
(3b) Performing Fourier inverse transformation of the distance direction on the echo data subjected to distance direction matched filtering to obtain two-dimensional space-variant time domain echo data s (t, eta):
(4) Calculating the intersection point position of the slope distance process and the course in the imaging coordinate system:
(4a) The bistatic forward-looking SAR slope sum at each time instant is calculated as R n Coordinates of the y-axis in the imaging plane coordinate system:
wherein, y (eta, R) n ) The sum of the slant distances of the bistatic forward-looking SAR and the intersection point at the time eta is R n The coordinate value of the y axis in the imaging plane coordinate system is represented by eta, which represents the slow time of the orientation of the linear frequency modulation signal transmitted by the airborne transmitter, R n Representing the nth range bin into which the time domain echo data s (t, η) is divided in range direction, R (η) representing the bistatic forward-looking SAR range sum at time η, x T (η)，y T (η)，z T (η) represents the coordinate values of the x-axis, y-axis and z-axis of the airborne transmitter in the three-dimensional space coordinate system at time η, respectively, y R (η)，z R (η) represents the coordinate values of the y-axis and the z-axis of the onboard receiver in the three-dimensional space coordinate system at time η, respectively, I represents the first intermediate variable,
f denotes a second intermediate variable, F = (y) R (η)-y T (η)) 2 Where E represents a third intermediate variable, E = y R (η)+y T (η), U represents a fourth intermediate variable,
g represents a fifth intermediate variable which is,
(5) Determining the position of the scene reference point:
the intersection point position of the slant range process and the course in the imaging coordinate system is used as a bistatic forward-looking SAR scene reference point to obtain the position coordinate (0, y (eta; R) of the reference point in the imaging coordinate system n ))；
(6) Constructing a two-dimensional space-variant matched filter function:
(6a) Performing third-order Taylor expansion on the slant distance of the bistatic forward-looking SAR according to the position coordinate of the reference point in the imaging plane coordinate system;
(6b) Calculating a two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit into which the time-domain echo data s (t, eta) are divided in the distance direction;
(6c) Constructing a two-dimensional space-variant matched filter function H of the time domain echo data according to the two-dimensional space-variant Doppler frequency modulation coefficient of each distance unit;
(7) Acquiring a bistatic forward-looking SAR image:
(7a) Multiplying the time domain echo data s (t, eta) by a matched filter function H to obtain echo data u (t, eta) after two-dimensional space-variant compensation;
(7b) Performing azimuth Fourier transform on the echo data u (t, eta) to obtain echo data v (t, f) of a distance time domain and an azimuth frequency domain after two-dimensional space-variant compensation a )；
(7c) Echo data v (t, f) are processed by the PGA method a ) And carrying out self-focusing processing to obtain a bistatic forward-looking SAR image.
2. The bistatic forward-looking SAR imaging method characterized by the slope-duration and heading intersection point as claimed in claim 1, wherein the bistatic forward-looking SAR distance sum in step (2) is obtained by the following formula:
wherein x is b ，y b And coordinate values of the x-axis and the y-axis of the target b in the three-dimensional space coordinate system are respectively represented.
3. The bistatic forward-looking SAR imaging method characterized by the slope-distance history and heading intersection point according to claim 2, wherein the expression of the two-dimensional space-variant time-domain echo data in step (3 b) is as follows:
s(t,η)＝sinc(B r (t-τ))w a (η)exp[jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]
wherein s (t, eta) represents a two-dimensional time domain echo signal of the bistatic forward-looking SAR at the time of the distance fast time t and eta, sinc (·) represents a sine function, B r Representing the distance-wise bandwidth of the chirp signal emitted by the airborne transmitter, τ representing the sum of the time delays of the chirp signal emitted by the airborne transmitter and the propagation in space of the echo signal of the airborne receiver, w a (η) represents a window function at η time of the azimuth direction a of the time domain echo signal after the range direction matching filtering, exp represents an exponential operation with a natural constant e as a base, j represents an imaginary unit symbol, pi represents a circumferential ratio, and γ (η, R) n ) Representing the sum of bistatic forward-looking SAR slopes at time η as R n Quadratic frequency modulation term, ξ (η, R), of two-dimensional space-variant of time-domain echo data s (t, η) n ) Denotes the sum of bistatic foresight SAR slope at time η as R n And (3) a cubic frequency modulation term of two-dimensional space variation of the time domain echo data s (t, eta).
4. The bistatic forward-looking SAR imaging method characterized by the intersection point of the slope course and the heading as claimed in claim 3, wherein the expression of the step (6 a) for performing the third order Taylor expansion on the bistatic forward-looking SAR slope sum according to the position coordinate of the reference point in the imaging plane coordinate system is as follows:
where K (η) represents the sum of the distance between the airborne receiver and the reference point at time η and the distance between the airborne transmitter and the reference point, R R0 Indicating the distance between the airborne receiver and the reference point when η =0,
v Ry (η)，v Rz (η) represents the flight speed of the airborne receiver at time η along the Y-axis and Z-axis, respectively, a Ry ，a Rz Representing the acceleration, R, of the airborne receiver at time η along the Y-axis and Z-axis, respectively T0 Indicating the distance between the airborne transmitter and the reference point when η =0,
v Ty (η)，v Tz (eta) represents the flight speed of the airborne transmitter along the Y-axis and Z-axis at the moment of transmitting eta, respectively, a Ty ，a Tz Representing the accelerations of the airborne transmitter along the Y-axis and Z-axis, respectively, at time η, δ representing a sixth intermediate variable, δ =2v Ry (y R (η)-y g (η,R n ))+2v Rz (η)z R (η), α represents a seventh intermediate variable, α = a Ry (η)v Ry (η)+a Rz (η)v Rz (η), β represents an eighth intermediate variable,
ε denotes the ninth intermediate variable, ε =2v Ty (y T (η)-y g (η,R n ))+2v Tz (η)z T (η), κ represents the tenth intermediate variable,
ω denotes a tenth intermediate variable, ω = a Ty (η)v Ty (η)+a Tz (η)v Tz (η)。
5. The bistatic forward-looking SAR imaging method characterized by the intersection point of the slant range history and the heading as claimed in claim 4, wherein the expression of the two-dimensional space-variant Doppler frequency modulation coefficient of each range unit in step (6 b) is as follows:
where λ represents the wavelength of the chirp signal transmitted by the airborne transmitter.
6. The bistatic forward-looking SAR imaging method characterized by the slope-distance history and course intersection point as claimed in claim 5, wherein the expression of the two-dimensional space-variant matched filter function H in step (6 c) is as follows:
H＝exp[-jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]
where H represents a two-dimensional space-variant matched filter function of the time-domain echo data s (t, η).
7. The bistatic forward-looking SAR imaging method characterized by the slope-duration and heading intersection point as claimed in claim 6, wherein the expression of the two-dimensional space-variant compensated echo data in step (7 a) is as follows:
u(t,η)＝sinc(B r (t-τ))w a (η)exp[jπ(γ(η,R n )η 2 +ξ(η,R n )η 3 )]·H
＝sinc(B r (t-τ))w a (η)
where u (t, η) represents the two-dimensional space-variant compensated echo data at the fast time t, η of the distance.
8. The bistatic forward-looking SAR imaging method characterized by the intersection of the slant-range history and the heading as claimed in claim 7, wherein the expression of the echo data of the distance time domain and the orientation frequency domain after the two-dimensional space-variant compensation in step (7 b) is as follows:
v(t,f a )＝sinc(B r (t-τ))sinc(T a f a )
wherein f is a An azimuth Doppler frequency v (t, f) of echo data representing a distance time domain and an azimuth frequency domain after two-dimensional space-variant compensation a ) Representing the range fast time t, azimuth Doppler frequency f a Echo data of (D), T a The azimuthal synthetic aperture time of the time domain echo data s (t, η) is represented.
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