Cooperative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning

Abstract

The present invention discloses a cooperative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning, comprising: obtaining a bistatic SAR slant range history based on a cooperative detection bistatic forward-looking SAR imaging geometric model; obtaining a baseband echo signal based on the bistatic SAR slant range history, performing range Fourier transform on the baseband echo signal and performing matched filtering processing; then sequentially performing acceleration component, linear migration reference component, and Doppler center compensation on the matched filtered signal to obtain a preprocessed signal; then sequentially performing frequency domain scale fine-tuning, high-order migration component correction, and range inverse Fourier transform on the signal to obtain a range-azimuth decoupled signal; then performing azimuth non-space-varying phase compensation and time domain scale fine-tuning processing on the decoupled signal to obtain an azimuth demodulated signal; and finally performing azimuth Fourier transform on the azimuth demodulated signal to obtain a bistatic forward-looking SAR image focused in the range-Doppler domain. The present invention improves azimuth focusing accuracy.

Description

Technical Field

[0001] The present invention belongs to the field of radar technology, and in particular relates to a collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning. Background Art

[0002] By separating the transmitter and receiver, cooperative detection bistatic synthetic aperture radar (BiSAR) can detect the non-backscatter characteristics of the target area, thereby obtaining richer scene information. Furthermore, due to its separate transmitter and receiver configuration, bistatic SAR can detect and image the position in front of the radar, thus overcoming the inherent limitation of monostatic SAR, which can only observe sideways, and expanding the application of SAR systems in reconnaissance and detection. Therefore, cooperative bistatic forward-looking SAR has become a research focus in recent years.

[0003] Compared to traditional SAR imaging, cooperative detection bistatic forward-looking SAR imaging processing faces more complex challenges. The maneuverability of the transceiver platform can cause the radar motion to deviate from the ideal straight-line trajectory. The inevitable acceleration will couple into the two-way slant range history, affecting the Doppler modulation rate and high-order Doppler parameters of the echo, leading to spectral distortion. In a cooperative detection bistatic forward-looking SAR system, one of the transceiver platforms operates in forward-looking mode. The range history of the bistatic SAR echo data will vary dramatically with azimuth and time, resulting in severe range migration. This, in turn, leads to severe coupling between range and azimuth, deforming the two-dimensional spectrum of the echo signal. Traditional methods of range frequency domain correction for movement and two-dimensional frequency domain correction for curvature will produce significant errors. In addition, in azimuth processing, the Doppler parameters of scattering points located at different azimuth positions will change with their positions, that is, the Doppler parameters are azimuthally spatially variable. At this time, it is impossible to use the same reference function to achieve focusing processing for all points on the range line. Traditional variable scaling algorithms can eliminate some spatially variable components, but they require multiple forward and inverse Fourier transforms and a large amount of zero-padding processing, which results in high algorithm complexity and low processing efficiency.

[0004] Considering the above problems, how to solve the problem of collaborative detection bistatic forward-looking SAR imaging carried on a mobile platform has become an urgent issue to be solved. Summary of the Invention

[0005] In order to solve the above problems existing in the prior art, the present invention provides a collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning. The technical problem to be solved by the present invention is achieved through the following technical solutions:

[0006] A collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning, the collaborative detection bistatic forward-looking SAR imaging processing method comprising:

[0007] S1. Based on the cooperative detection bistatic forward-looking SAR imaging geometric model, the bistatic SAR slant range history is obtained;

[0008] S2. Obtain a baseband echo signal based on the bistatic SAR slant range history, perform range Fourier transform on the baseband echo signal, and perform matched filtering to obtain a matched filtered signal;

[0009] S3, performing acceleration component, linear migration reference component and Doppler center compensation on the matched filtered signal in sequence to obtain a preprocessed signal;

[0010] S4. Performing frequency domain resizing, high-order migration component correction, and range inverse Fourier transform on the preprocessed signal in sequence to obtain a range-azimuth decoupled signal. Then, performing azimuth non-space-varying phase compensation and time domain resizing on the range-azimuth decoupled signal to obtain an azimuth demodulated signal.

[0011] S5. Perform azimuth Fourier transform on the azimuthally demodulated signal to obtain a bistatic forward-looking SAR image focused in the range Doppler domain.

[0012] In one embodiment of the present invention, step S1 includes:

[0013] S1.1. Obtaining the slant range history of cooperative bistatic SAR imaging based on the geometric model of cooperative detection bistatic forward-looking SAR imaging for a mobile platform;

[0014] S1.2, the slant range history of the cooperative bistatic SAR imaging is calculated in the azimuth slow time t m =0, and the bistatic SAR slant range history is obtained in the form of a polynomial.

[0015] In one embodiment of the present invention, step S2 includes:

[0016] S2.1. Obtain a baseband echo signal based on the bistatic SAR slant range history. The baseband echo signal is expressed as:

[0017]

[0018] Among them, ss0(t r ,t m ) represents the baseband echo signal, t r represents the distance-to-fast time variable, w r (t r ) represents the range-to-time domain window function, t m Indicates the azimuth slow time, w a (t m) represents the azimuth time domain window function, exp{·} represents the complex exponential function, which is used to characterize the phase information of the echo signal, j represents the imaginary unit, K r Indicates the range modulation frequency, f c represents the center frequency of the transmitted signal, c represents the speed of light, R bi (t m ) represents the bistatic SAR slant range history;

[0019] S2.2. Perform range-directed matched filtering on the baseband echo signal and perform range-directed Fourier transform on the baseband echo signal using the stationary phase principle to obtain an echo signal in the range-frequency domain. The echo signal in the range-frequency domain is expressed as:

[0020]

[0021] Among them, Ss1(f r ,t m ) is expressed as the echo signal in the range frequency domain, f r represents the distance frequency variable, W r (f r ) represents the distance frequency domain window function, the first exponential term Indicates the range modulation, the second exponential term Indicates azimuth modulation and range migration;

[0022] S2.3. Construct a matched filter reference function in the range-frequency domain and the azimuth-time domain, and multiply the matched filter reference function by the echo signal in the range-frequency domain to obtain a range-direction matched filtered signal. The matched filtered signal is expressed as:

[0023]

[0024] Among them, Ss2(f r ,t m ) represents the signal after matched filtering, H PC (f r ) represents the matched filter reference function.

[0025] In one embodiment of the present invention, step S3 includes:

[0026] S3.1. Construct a non-ideal motion trajectory compensation function in the range-frequency domain, azimuth-time domain, and the non-ideal motion trajectory compensation function is expressed as:

[0027]

[0028] Among them, H acc (f r ,t m ) represents the non-ideal motion trajectory compensation function, fr represents the distance frequency variable, t m represents the azimuth slow time variable, f c represents the center frequency of the transmitted signal, c represents the speed of light, k i0_acc Represents the non-ideal motion trajectory component in the i-th order Taylor expansion coefficient of the bi-base slant range history of the scene reference point;

[0029] S3.2. Multiplying the non-ideal motion trajectory compensation function by the matched filtered signal to obtain an acceleration-compensated signal;

[0030] S3.3. Construct a linear migration correction and azimuth spectrum center compensation function in the range-frequency domain and azimuth-time domain. The linear migration correction and azimuth spectrum center compensation function is expressed as:

[0031]

[0032] Among them, H com (f r ,t m ) represents the linear migration correction and azimuth spectrum center compensation function, k 10 The first-order Taylor expansion coefficient representing the bistatic slant range history of the scene reference point;

[0033] S3.4. Multiply the linear migration correction and azimuth spectrum center compensation function with the acceleration compensated signal to obtain a preprocessed signal.

[0034] In one embodiment of the present invention, step S4 includes:

[0035] S4.1. Introducing a first-stage linear rescaling process in the range-frequency domain to correct the range-migratory linear residual space-variant component of the preprocessed signal to obtain a frequency-domain rescaling signal;

[0036] S4.2. Correcting the high-order migration components of the signal after the frequency domain rescaling, and performing a range-wise inverse Fourier transform to obtain a range-azimuth decoupled signal;

[0037] S4.3. Compensating the azimuth non-space-varying phase of the range-azimuth decoupled signal to obtain a phase-compensated signal;

[0038] S4.4. Introducing a second-level nonlinear scale fine-tuning process in the two-dimensional time domain to correct the phase-compensated signal from non-uniform sampling to uniform sampling, thereby obtaining an azimuthally demodulated signal.

[0039] In one embodiment of the present invention, the signal after frequency domain scale fine-tuning is expressed as:

[0040]

[0041] Among them, Ss5(f r ,t′ m ) represents the signal after fine-tuning the frequency domain scale, f r represents the distance frequency variable, W r (f r ) represents the range frequency domain window function, t m Indicates slow time, w a (t m ) represents the azimuth time domain window function, c represents the speed of light, f c Indicates the center frequency of the transmitted signal, t′ m f c =t m f c +t m f r , k′0 represents the zero-order Taylor expansion coefficient after preprocessing, k′1 represents the first-order Taylor expansion coefficient after preprocessing, k′2 represents the second-order Taylor expansion coefficient after preprocessing, k′3 represents the third-order Taylor expansion coefficient after preprocessing, and k′4 represents the fourth-order Taylor expansion coefficient after preprocessing.

[0042] In one embodiment of the present invention, step S4.2 includes:

[0043] S4.21, the coupling phase of the signal after the frequency domain scale is fine-tuned with respect to the azimuth frequency f r =0 to perform first-order Taylor series expansion to obtain the expanded signal;

[0044] S4.22. Multiply the expanded signal by a high-order migration compensation function to obtain a signal corrected for high-order migration. The high-order migration compensation function is expressed as:

[0045]

[0046] Among them, H RCC (f r , t′ m ) represents the high-order migration compensation function, f r represents the distance frequency variable, t′ m f c =t m f c +t m f r , f c represents the center frequency of the transmitted signal, c represents the speed of light, k′ i0 Represents the Taylor expansion coefficients of each order of the slant range history of the scene reference point;

[0047] S4.23. Perform range-direction inverse Fourier transform on the signal after high-order migration correction to obtain a range-azimuth decoupled signal.

[0048] In one embodiment of the present invention, step S4.3 includes:

[0049] S4.31, the azimuth phase of the signal decoupled from the range and azimuth is about the azimuth position y n =0 and perform first-order Taylor series expansion to obtain the non-space-varying component and the space-varying component;

[0050] S4.32. Based on the relationship between the non-space-varying component and the azimuth position of the scattering point, multiply the non-space-varying phase compensation function by the two-dimensional time-domain signal to obtain the phase-compensated signal. The non-space-varying phase compensation function is expressed as:

[0051]

[0052] Among them, H Deramp (t′ m ) represents the non-space-varying phase compensation function, t′ m f c =t m f c +t m f r , f c represents the center frequency of the transmitted signal, λ represents the wavelength, k′ i0 Represents the Taylor expansion coefficients of each order of the slant range history of the scene reference point.

[0053] In one embodiment of the present invention, the azimuth demodulated signal is expressed as:

[0054]

[0055] Among them, ss9(t r ,t″ m ;y n ) represents the signal after time domain scale adjustment, A represents the constant term, sinc represents the Singer window function, B r represents the transmit signal bandwidth, t r Represents the distance-to-fast time variable, R bi0 represents the reference double base distance and t m Indicates the azimuth slow time, w a (t m ) represents the azimuth time domain window function, λ represents the wavelength, y n Indicates the position of the direction, k′ i1 The linear term coefficient of the azimuth position in the Taylor expansion coefficients of each order of the slant range history, t′ m f c =t m f c +tm f r , f c Indicates the center frequency of the transmitted signal.

[0056] Beneficial effects of the present invention:

[0057] The present invention considers the influence of acceleration parameters in the motion of a mobile platform and introduces a non-ideal motion trajectory compensation module in the preprocessing process, thereby eliminating the influence of non-ideal motion on the Doppler modulation rate and high-order Doppler parameters and restoring the distorted two-dimensional spectrum. After completing the linear migration correction process, a linear scale fine-tuning process is introduced in the range frequency domain to eliminate the space-varying component of the range linear migration and achieve range-azimuth depth decoupling. After compensating for the non-space-varying component of the azimuth phase through Deramp (phase and signal multiplication) processing, a nonlinear scale fine-tuning process is introduced again in the two-dimensional time domain to eliminate the space-varying component of the azimuth phase and improve the azimuth focusing accuracy.

[0058] The present invention eliminates the linear space-varying component of range migration through the first-stage linear scale fine-tuning to achieve range-azimuth depth decoupling; and eliminates the space-varying component of azimuth Doppler parameters through the second-stage nonlinear scale fine-tuning to achieve azimuth precise focusing. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 1 is a flow chart of a collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning provided by an embodiment of the present invention;

[0060] Figure 2 1 is a flow chart of another cooperative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning provided by an embodiment of the present invention;

[0061] Figure 3 Schematic diagram of a collaborative detection bistatic forward-looking SAR imaging geometric model provided by an embodiment of the present invention;

[0062] Figure 4 This is an azimuth profile diagram of a simulated point target provided by an embodiment of the present invention;

[0063] Figure 5 This is a contour map of a simulated point target provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0064] The present invention will be further described in detail below with reference to specific examples, but the embodiments of the present invention are not limited thereto.

[0065] Example 1

[0066] By separating the transmitter and receiver on separate moving platforms, cooperative detection bistatic SAR (SAR) can achieve high-resolution imaging at any location through signal transmission and non-backward reception of echoes, thus overcoming the inherent limitation of traditional monostatic SAR, which is limited to side-view imaging. While the cooperative detection bistatic SAR system requires additional space-time-frequency synchronization modules and a data link module compared to monostatic SAR, its advantages precisely meet the SAR requirements of cooperative detection systems. First, due to the separation of the transmitter and receiver, the receiving platform no longer needs to be equipped with a high-power transmitter, thus effectively controlling the size, weight, and cost of the receiving platform. Second, in a cooperative detection system, multiple platforms can form a "one-transmitter, multiple-receiver" multistatic SAR imaging system, acquiring scattering information from the target scene from different perspectives and providing a more comprehensive target characterization. Most importantly, cooperative detection bistatic SAR overcomes the inherent limitation of traditional monostatic SAR, which is limited to side-view imaging. The most typical bistatic forward-looking mode, through appropriate configuration, can form a well-defined range and Doppler resolution grid in front of a flying platform, generating a high-resolution two-dimensional image of the area in front of the platform. This allows for forward-looking synthetic aperture radar imaging with the assistance of other platforms. In summary, the bistatic forward-looking SAR system has great potential in collaborative detection and is one of the important development trends in future radar applications.

[0067] The existing bistatic forward-looking SAR imaging technology is as follows: first, the echo signal is processed by matched filtering and transformed into the range frequency domain for walk correction and Doppler center compensation to achieve preliminary correction of range migration; then the signal is transformed into the two-dimensional frequency domain and range curvature correction is performed with the scene center as a reference; then the signal is transformed into the range Doppler domain and a high-order nonlinear scaling factor is introduced into the range Doppler domain; then the signal is transformed into the two-dimensional time domain and azimuth frequency modulation is eliminated through Deramp processing; finally, the signal is transformed into the range Doppler domain to achieve final focusing.

[0068] Existing bistatic forward-looking SAR imaging techniques based on nonlinear scaling do not consider the impact of platform maneuverability. In reality, multidimensional acceleration affects the Doppler modulation rate and high-order Doppler parameters of the echo, leading to spectral distortion and impacting azimuth focusing. Existing techniques perform a unified range curvature correction in the two-dimensional frequency domain, using the scene center as a reference. However, due to the spatial variability of scattering points, their bistatic slant range histories vary. The curvature characteristics of different scattering points in the two-dimensional frequency domain also differ. Applying a unified range curvature correction results in significant residual migration at edge points. Existing techniques introduce high-order nonlinear scaling factors in the range-Doppler domain to eliminate the azimuth spatial variability of the Doppler parameters. However, this method can only eliminate a set of spatially varying components, while the introduced scaling factors affect the remaining order components. Furthermore, nonlinear scaling algorithms require multiple forward and inverse Fourier transforms and extensive zero-padding, resulting in high computational complexity and low processing efficiency.

[0069] In view of the above shortcomings, the present invention proposes a collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning. Figure 1 and Figure 2 , Figure 1 1 is a flow chart of a collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning provided by an embodiment of the present invention. Figure 2 This is a flow chart of another collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning provided by an embodiment of the present invention. The collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning provided by the present invention includes:

[0070] S1. Based on the cooperative detection bistatic forward-looking SAR imaging geometric model, the bistatic SAR slant range history is obtained.

[0071] S1.1. Based on the geometric model of cooperative detection bistatic forward-looking SAR imaging for mobile platforms, the slant range history of cooperative bistatic SAR imaging is obtained.

[0072] like Figure 3 As shown in the figure, a bistatic forward-looking SAR imaging geometric model for a mobile platform is established in the Cartesian coordinate system: a spatial rectangular coordinate system XOYZ is established with the central reference point of the imaging scene as the coordinate origin, where the X direction and the Y direction represent any two vertical directions in the horizontal plane, and the Z direction represents the celestial direction.

[0073] In order to more conveniently describe the dual-base imaging model of the mobile platform, the platform azimuth angle γ is defined as the angle between the ground projection component of the slant range vector and the positive direction of the X-axis, and the ground grazing angle β is the angle between the beam vector and the ground plane. The transmitting platform works in the oblique viewing mode, and the receiving platform works in the forward viewing mode. The transmitting platform and the receiving platform move along curved trajectories respectively. The radar signal reaches the imaging scene through the transmitting antenna and scatters back to the receiving antenna, forming a closed loop of transmission and reception. At the azimuth zero moment, the velocity and acceleration of the transmitter are v respectively. t =(v tx ,v ty ,v tz ) and a t =(a tx ,a ty ,a tz ), the distance from the platform to the center of the scene is R st , the ground-rubbing angle is β t , the platform azimuth is γ t ; The velocity and acceleration of the receiver are v t =(v tx ,v ty ,v tz ) and a t =(atx ,a ty ,a tz ), the distance from the platform to the center of the scene is R sr , the ground-rubbing angle is β r , the platform azimuth is 0. m represents the azimuth slow time variable, then the spatial position of the transmitting platform and the receiving platform at any time can be expressed as:

[0074]

[0075]

[0076] The target coordinate system xOy is established in the horizontal plane with the scene center reference point as the coordinate origin. The angle φ between it and the imaging coordinate system XOY is Δ It can be expressed as:

[0077]

[0078] Here, atan represents the inverse tangent function.

[0079] At this time, for any point P(x n ,y n ), its spatial position in the imaging coordinate system can be expressed as:

[0080]

[0081] Then the slant range history of cooperative bistatic SAR imaging is obtained:

[0082]

[0083] S1.2. The slant range history of cooperative bistatic SAR imaging is calculated in the azimuth slow time t m = 0, a fourth-order Taylor series expansion is performed to obtain the bistatic SAR slant range history in polynomial form. The bistatic SAR slant range history in polynomial form is expressed as:

[0084]

[0085] Among them, k i Represents the i-th order Taylor expansion coefficient, specifically expressed as:

[0086]

[0087] Among them, i! represents the factorial from 1 to i, Represents a function on the variable t m Find the i-th derivative.

[0088] S2. Based on the slant range history of the bistatic SAR, a baseband echo signal is obtained, and the baseband echo signal is subjected to range Fourier transform and matched filtering to obtain a matched filtered signal.

[0089] S2.1. Obtain a baseband echo signal based on the bistatic SAR slant range history.

[0090] Assume that the bistatic SAR transmitting antenna in the cooperative detection system transmits a linear frequency modulation (LFM) signal, and the receiving antenna receives the scattered echo from the target area. After orthogonal demodulation, the baseband echo signal ss0(t r ,t m ), which can be expressed as:

[0091]

[0092] Among them, t r represents the distance-to-fast time variable, w r (t r ) represents the range-to-time domain window function, t m Indicates the azimuth slow time, w a (t m ) represents the azimuth time domain window function, exp{·} represents the complex exponential function, which is used to characterize the phase information of the echo signal, j represents the imaginary unit, K r Indicates the range modulation frequency, f c represents the center frequency of the transmitted signal, c represents the speed of light, R bi (t m ) represents the bistatic SAR slant range history.

[0093] S2.2, perform range-direction matched filtering on the baseband echo signal, and use the stationary phase principle (POSP) to perform range-direction Fourier transform (FT) on the baseband echo signal to obtain the range-frequency domain echo signal Ss1 (f r ,t m ), the echo signal in the range frequency domain is expressed as:

[0094]

[0095] Among them, f r represents the distance frequency variable, W r (f r ) represents the distance frequency domain window function, the first exponential term Indicates the range modulation, the second exponential term Indicates azimuth modulation and range migration.

[0096] S2.3. Construct a matched filter reference function in the range-frequency domain and the azimuth-time domain, and multiply the matched filter reference function by the echo signal in the range-frequency domain to obtain a range-direction matched filtered signal. The matched filtered signal is expressed as:

[0097]

[0098] Among them, Ss2(f r ,t m ) represents the signal after matched filtering, H PC (f r ) represents the matched filter reference function,

[0099] S3. Perform acceleration component, linear migration reference component and Doppler center compensation on the matched filtered signal in sequence to obtain a pre-processed signal.

[0100] Specifically, non-ideal motion trajectory compensation function, linear migration correction and azimuth spectrum center compensation function are introduced in the range frequency domain to accurately compensate the acceleration component, linear migration reference component and Doppler center for the matched filtered signal to obtain the preprocessed signal.

[0101] S3.1. Construct a non-ideal motion trajectory compensation function in the range, frequency, azimuth and time domains.

[0102] High-order non-ideal motion trajectory compensation can restore the echo spectrum and avoid azimuth Doppler parameter mismatch caused by acceleration. The present invention extracts the non-ideal motion trajectory from the slant range history and constructs a non-ideal motion trajectory compensation function in the range frequency domain and azimuth time domain. The non-ideal motion trajectory compensation function is expressed as:

[0103]

[0104] Among them, H acc (f r ,t m ) represents the non-ideal motion trajectory compensation function, k i0_acc k represents the non-ideal motion trajectory component in the i-th order Taylor expansion coefficient of the scene reference point bi-base slant range history, i0_acc Expressed as:

[0105]

[0106] S3.2, multiply the non-ideal motion trajectory compensation function with the matched filtered signal to obtain the acceleration compensated signal Ss3 (f r ,t m ), the acceleration compensation signal Ss3 (f r ,t m ) is expressed as:

[0107]

[0108] S3.3. Construct linear migration correction and azimuth spectrum center compensation functions in the range-frequency domain and azimuth-time domain.

[0109] Linear migration correction and azimuth spectrum center compensation can eliminate most of the range migration components in bistatic SAR echoes, while shifting the two-dimensional spectrum to avoid spectrum blurring. The linear migration correction and azimuth spectrum center compensation functions constructed in the range-frequency domain and azimuth-time domain are expressed as:

[0110]

[0111] Among them, H com (f r ,t m ) represents the linear migration correction and azimuth spectrum center compensation function, k 10 The first-order Taylor expansion coefficient representing the bistatic slant range history of the scene reference point,

[0112] S3.4. Multiply the signal processed by the linear migration correction and azimuth spectrum center compensation function with the acceleration compensation function to obtain a preprocessed signal. The preprocessed signal is expressed as:

[0113]

[0114] The pre-processed signal Ss4(f r ,t m ) to get:

[0115]

[0116] Among them, the Taylor expansion coefficients of each order are updated as follows:

[0117]

[0118] S4. The preprocessed signal is subjected to frequency domain resizing fine-tuning, high-order migration component correction, and range inverse Fourier transform in sequence to obtain a range-azimuth decoupled signal. The range-azimuth decoupled signal is then subjected to azimuth non-space-varying phase compensation and time domain resizing fine-tuning to obtain an azimuth demodulated signal.

[0119] S4.1. Introducing a first-level linear scale fine-tuning process in the range-frequency domain to correct the range migration linear residual space-variant component of the pre-processed signal to obtain a frequency-domain scale-fine-tuned signal.

[0120] In bistatic forward-looking SAR imaging on a mobile platform, the range migration of scattering points exhibits two-dimensional spatial variation. This invention introduces a first-level linear scale fine-tuning process to correct for residual migration. By defining a virtual slow time, the target echo is stretched and compressed in the slow time dimension, correcting the inverted trapezoidal azimuth time domain support area to a rectangle, thereby correcting the residual movement outside the scene center. This process can be expressed as:

[0121] t′ m f c =t m f c +t m f r

[0122] Substitute the above formula into the signal Ss4(f r ,t m ) is simplified to obtain the signal after linear scale fine-tuning (i.e., the signal after frequency domain scale fine-tuning) Ss5(f r ,t′ m ), Ss5(f r ,t′ m ) is expressed as:

[0123]

[0124] At this time, there is no coupling between the range frequency and the first-order term of the azimuth slow time, and the linear coupling is completely eliminated.

[0125] S4.2. Correct the high-order migration components of the signal after fine-tuning the frequency domain scale, and perform range-direction inverse Fourier transform to obtain a range-azimuth decoupled signal.

[0126] S4.21. After completing the linear migration component compensation, the high-order migration component is processed and the coupling phase of the signal after fine-tuning the frequency domain scale is adjusted with respect to the azimuth frequency f r =0 to perform a first-order Taylor series expansion to obtain the expanded signal, which is expressed as:

[0127]

[0128] Where λ represents the wavelength, and the first exponential term Indicates azimuth modulation, the second exponential term Represents distance position and distance curvature, and constructs distance curvature compensation function with the scene center as reference

[0129] S4.22. Multiply the expanded signal by the high-order migration compensation function to obtain a signal corrected for high-order migration.

[0130] Construct a high-order migration compensation function with the scene center as the reference:

[0131]

[0132] Among them, k i ′0 represents the Taylor expansion coefficients of each order of the slant range history of the scene reference point,

[0133] The high-order migration compensation function is combined with the expanded signal Ss5(f r ,t′ m ) to obtain the signal Ss6 (f r ,t′ m ), the signal after high-order migration correction is expressed as:

[0134]

[0135] Among them, Ss6(f r ,t′ m ) represents the signal after high-order migration correction.

[0136] Step S4.23: Perform a range-direction inverse Fourier transform on the signal after high-order migration correction to obtain a range-azimuth decoupled signal (i.e., a two-dimensional time-domain signal after range-azimuth decoupling). The range-azimuth decoupled signal is expressed as:

[0137]

[0138] Among them, ss7(t r ,t′ m ) represents the range-azimuth decoupled signal, R bi0 =k′0 represents the sum of the reference dual-base distances, that is, the sum of the distances from the transmitting and receiving platform to any point in the scene at the azimuth zero moment.

[0139] S4.3. Compensate the azimuth non-space-varying phase of the range-azimuth decoupled signal to obtain a phase-compensated signal.

[0140] S4.31, the azimuth phase of the signal decoupled from the range and azimuth is about the azimuth position y n =0 and perform first-order Taylor series expansion to obtain the non-space-varying component and the space-varying component.

[0141] Take out the two-dimensional time domain signal ss7(t r ,t′ m ) is the azimuth phase of the azimuth position y n = 0 and perform first-order Taylor series expansion to obtain the non-space-varying component and the space-varying component. At this time, the azimuth phase It can be expressed as:

[0142]

[0143] Among them, the first index term Represents a constant term that is independent of focus and can be represented by a constant A. The second exponential term Represents the non-space-varying component of the azimuth phase, the third exponential term represents the space-varying component of the azimuth phase; where k′ i0 The constant term coefficient of the Taylor expansion coefficient of each order of the slant range history with respect to the azimuth position, k′ i1 The linear term coefficients of the Taylor expansion coefficients of each order of the slant range history with respect to the azimuth position,

[0144] S4.32. Based on the relationship between the non-space-varying component and the azimuth position of the scattering point, the non-space-varying phase compensation function is multiplied by the signal in the two-dimensional time domain to obtain a phase-compensated signal.

[0145] The non-space-varying component is independent of the azimuth position of the scattering point and can be directly compensated by phase multiplication to construct a non-space-varying phase compensation function. The non-space-varying phase compensation function is expressed as:

[0146]

[0147] Among them, H Deramp (t′ m ) represents the non-space-varying phase compensation function.

[0148] The non-space-varying phase compensation function is combined with the two-dimensional time domain signal ss7(t r ,t′ m ;y n ) are multiplied to obtain the phase-compensated signal, which is expressed as:

[0149]

[0150] Among them, ss8(t r ,t′ m ;y n ) represents the signal after phase compensation.

[0151] S4.4. Introduce a second-level nonlinear scaling process in the two-dimensional time domain to correct the phase-compensated signal from non-uniform sampling to uniform sampling, thereby obtaining an azimuthally demodulated signal.

[0152] For the spatial variation of the azimuth phase along the scattering point azimuth position, a second-level nonlinear scaling is introduced to correct the time domain signal from non-uniform sampling to uniform sampling, thereby eliminating the spatial variation of the Doppler modulation rate and high-order Doppler parameters. The process can be expressed as:

[0153]

[0154] Substitute the above formula into the signal ss8(t r ,t′ m ;y n ) is simplified to obtain the azimuth demodulated signal, which is expressed as:

[0155]

[0156] Among them, ss9(t r ,t″ m ;y n ) represents the signal after azimuth demodulation, sinc represents the Singer window function, B r Indicates the transmit signal bandwidth.

[0157] S5. Perform azimuth Fourier transform on the signal after time domain scale adjustment to obtain a bistatic forward-looking SAR image focused in the range Doppler domain. The bistatic forward-looking SAR image is expressed as:

[0158]

[0159] Among them, ss 10 (t r ,f a ;y n ) represents the bistatic forward-looking SAR image, T a represents the synthetic aperture time, f a represents the Doppler frequency variable, so that the image focused in the range Doppler domain is obtained.

[0160] Next, the effectiveness of the present invention is verified through point target simulation.

[0161] The simulation parameters are shown in the table.

[0162] Table 1 Simulation parameters

[0163] parameter radar system parameter launch platform Receiving platform frequency band X-band Platform speed (171,-45,-31)m / s (196,113,-39)m / s Signal bandwidth 100MHz Platform acceleration <![CDATA[(2,-1,1.5)m / s 2 ]]> <![CDATA[(1.8,1.2,-1)m / s 2 ]]> Sampling rate 125MHz Center slope distance 22km 24km PRF 2kHz Platform azimuth 20° -40° Admission Time 1.024s Mop Corner 15° 18°

[0164] In the ground scene, a set of 3×3 square dot arrays with a width of 1.5 km×1.5 km are set along the x-direction and y-direction. The scattering point located at the center coordinate (0,0) of the target coordinate system is selected as the reference point T1, the scattering point located at the coordinate (750,0) in the positive direction of the x-axis of the target coordinate system is selected as the edge point T2, and the scattering point located at the coordinate (750,750) in the first quadrant of the target coordinate system is selected as the edge point T3 to examine the imaging results.

[0165] like Figure 4 As shown in the figure, the azimuth profiles of the three selected points are given. It can be seen that the edge points have similar focusing effects to the center point, the first zero point is lower, and the first side lobe is close to the theoretical value; Figure 5As shown, the contour maps of the three selected points are given. It can be seen that the contour maps of the three points are basically consistent, and the main and side lobes are obviously separated, which illustrates the effectiveness of the method described in the present invention.

[0166] The indicators are further quantitatively evaluated, and the azimuth resolution, peak sidelobe ratio, and integrated sidelobe ratio of the three selected points are calculated as shown in Table 2. It can be seen that the performance indicator parameters of the algorithm are basically consistent with the theoretical values ​​(azimuth resolution 1.13m, peak sidelobe ratio -13.26dB, and integrated sidelobe ratio -9.80dB), further demonstrating the effectiveness of the method described in the present invention.

[0167] Table 2 Statistical results of performance index parameters

[0168] Scattering Points Azimuth resolution Peak-to-sidelobe ratio Integrated sidelobe ratio <![CDATA[T1]]> 1.13m -13.24dB -9.76dB <![CDATA[T2]]> 1.12m -13.23dB -9.78dB <![CDATA[T3]]> 1.14m -13.12dB -9.74dB

[0169] The present invention takes into account the maneuverability of the transceiver platform during system modeling and accurately compensates for non-ideal motion trajectories caused by acceleration during preprocessing. After linear migration correction and azimuth spectrum center compensation, a first-level linear scale fine-tuning process is introduced in the range frequency domain. Residual linear migration correction is achieved through azimuth time domain support area correction, and high-order migration components are uniformly processed to achieve precise range migration correction of scattering points in the entire scene. In azimuth processing, Deramp processing is used to eliminate the non-space-varying components of each order Doppler parameter. Then, a second-level nonlinear scale fine-tuning process is introduced. By defining a virtual slow time, the signal is stretched and compressed, and the signal is corrected from non-uniform sampling to uniform sampling, thereby eliminating the space-varying characteristics of the Doppler modulation frequency and high-order Doppler parameters, and finally obtaining a fine-focus image in the range Doppler domain.

[0170] In the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0171] Although the present application is described herein in conjunction with various embodiments, in the process of implementing the claimed application, those skilled in the art can understand and implement other changes to the disclosed embodiments by reviewing the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple situations. A single processor or other unit can implement several functions listed in the claims. Certain measures are recorded in different dependent claims, but this does not mean that these measures cannot be combined to produce good results.

[0172] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A collaborative detection bistatic forward-looking SAR imaging processing method based on two-level scale fine-tuning, characterized in that: The collaborative detection bistatic forward-looking SAR imaging processing method comprises: S1. Based on the cooperative detection bistatic forward-looking SAR imaging geometric model, the bistatic SAR slant range history is obtained; S2. Obtain a baseband echo signal based on the bistatic SAR slant range history, perform range Fourier transform on the baseband echo signal, and perform matched filtering to obtain a matched filtered signal; S3, performing acceleration component, linear migration reference component and Doppler center compensation on the matched filtered signal in sequence to obtain a preprocessed signal; S4. Performing frequency domain resizing, high-order migration component correction, and range inverse Fourier transform on the preprocessed signal in sequence to obtain a range-azimuth decoupled signal. Then, performing azimuth non-space-varying phase compensation and time domain resizing on the range-azimuth decoupled signal to obtain an azimuth demodulated signal. S5. Perform azimuth Fourier transform on the azimuthally demodulated signal to obtain a bistatic forward-looking SAR image focused in the range Doppler domain.

2. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 1, wherein: Step S1 includes: S1.

1. Determine the slant range history of cooperative bistatic SAR imaging based on the geometric model of cooperative detection bistatic forward-looking SAR imaging for mobile platforms. S1.2, the slant range history of the cooperative bistatic SAR imaging is calculated in azimuth slow time The fourth-order Taylor series expansion is performed at , and the bistatic SAR slant range history in polynomial form is obtained.

3. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 1, characterized in that: Step S2 includes: S2.

1. Obtain a baseband echo signal based on the bistatic SAR slant range history. The baseband echo signal is expressed as: in, represents the baseband echo signal, represents the distance-to-fast time variable, represents the range-to-time domain window function, Indicates azimuth slow time, represents the azimuth time domain window function, Represents a complex exponential function, which is used to characterize the phase information of the echo signal. j represents the imaginary unit, Indicates the range modulation frequency, Indicates the center frequency of the transmitted signal, c represents the speed of light, Indicates the bistatic SAR slant range history; S2.

2. Perform range-directed matched filtering on the baseband echo signal and perform range-directed Fourier transform on the baseband echo signal using the stationary phase principle to obtain an echo signal in the range-frequency domain. The echo signal in the range-frequency domain is expressed as: in, Represented as the echo signal in the range frequency domain, represents the distance frequency variable, Represents the distance frequency domain window function, the first exponential term Indicates the range modulation, the second exponential term Indicates azimuth modulation and range migration; S2.

3. Construct a matched filter reference function in the range-frequency domain and the azimuth-time domain, and multiply the matched filter reference function by the echo signal in the range-frequency domain to obtain a range-direction matched filtered signal. The matched filtered signal is expressed as: in, represents the signal after matched filtering, Represents the matched filter reference function.

4. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 1, wherein: Step S3 includes: S3.

1. Construct a non-ideal motion trajectory compensation function in the range-frequency domain, azimuth-time domain, and the non-ideal motion trajectory compensation function is expressed as: in, represents the non-ideal motion trajectory compensation function, represents the distance frequency variable, represents the azimuth slow time variable, Indicates the center frequency of the transmitted signal, c represents the speed of light, Indicates the first step of the bi-base slant range history of the scene reference point i The non-ideal motion trajectory components in the order Taylor expansion coefficients; S3.

2. Multiplying the non-ideal motion trajectory compensation function by the matched filtered signal to obtain an acceleration-compensated signal; S3.

3. Construct a linear migration correction and azimuth spectrum center compensation function in the range-frequency domain and azimuth-time domain. The linear migration correction and azimuth spectrum center compensation function is expressed as: in, represents the linear migration correction and azimuth spectrum center compensation function, The first-order Taylor expansion coefficient representing the bistatic slant range history of the scene reference point; S3.

4. Multiply the linear migration correction and azimuth spectrum center compensation function with the acceleration compensated signal to obtain a preprocessed signal.

5. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 1, characterized in that: Step S4 includes: S4.

1. Introducing a first-stage linear rescaling process in the range-frequency domain to correct the range-migratory linear residual space-variant component of the preprocessed signal to obtain a frequency-domain rescaling signal; S4.

2. Correcting the high-order migration components of the signal after the frequency domain rescaling, and performing a range-wise inverse Fourier transform to obtain a range-azimuth decoupled signal; S4.

3. Compensating the azimuth non-space-varying phase of the range-azimuth decoupled signal to obtain a phase-compensated signal; S4.

4. Introducing a second-level nonlinear scale fine-tuning process in the two-dimensional time domain to correct the phase-compensated signal from non-uniform sampling to uniform sampling, thereby obtaining an azimuthally demodulated signal.

6. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 1, characterized in that: The signal after frequency domain scale fine-tuning is expressed as: in, represents the signal after fine-tuning the frequency domain scale, represents the distance frequency variable, represents the range-to-frequency window function, Indicates slow time, represents the azimuth time domain window function, c represents the speed of light, Indicates the center frequency of the transmitted signal, , represents the zero-order Taylor expansion coefficient after preprocessing, represents the first-order Taylor expansion coefficient after preprocessing, represents the second-order Taylor expansion coefficient after preprocessing, represents the third-order Taylor expansion coefficient after preprocessing, Represents the fourth-order Taylor expansion coefficient after preprocessing.

7. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 5, characterized in that: Step S4.2 includes: S4.21, the coupling phase of the signal after the frequency domain scale is fine-tuned with respect to the azimuth frequency Perform a first-order Taylor series expansion to obtain the expanded signal; S4.

22. Multiply the expanded signal by a high-order migration compensation function to obtain a signal corrected for high-order migration. The high-order migration compensation function is expressed as: in, represents the high-order migration compensation function, represents the distance frequency variable, , Indicates the center frequency of the transmitted signal, c represents the speed of light, Represents the Taylor expansion coefficients of each order of the slant range history of the scene reference point, ; S4.

23. Perform range-direction inverse Fourier transform on the signal after high-order migration correction to obtain a range-azimuth decoupled signal.

8. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 5, characterized in that: Step S4.3 includes: S4.31, the azimuth phase of the signal decoupled from the range and azimuth relative to the azimuth position Perform a first-order Taylor series expansion to obtain the non-space-varying component and the space-varying component; S4.

32. Based on the relationship between the non-space-varying component and the azimuth position of the scattering point, multiply the non-space-varying phase compensation function by the two-dimensional time-domain signal to obtain the phase-compensated signal. The non-space-varying phase compensation function is expressed as: in, represents the non-space-varying phase compensation function, , Indicates the center frequency of the transmitted signal, represents the wavelength, Represents the Taylor expansion coefficients of each order of the slant range history of the scene reference point.

9. The collaborative detection bistatic forward-looking SAR imaging processing method according to claim 5, characterized in that: The signal after the azimuth demodulation is expressed as: in, represents the signal after time domain fine-tuning, represents a constant term, represents the Symge window function, represents the transmit signal bandwidth, represents the distance-to-fast time variable, represents the reference bibasic distance and, Indicates azimuth slow time, represents the azimuth time domain window function, represents the wavelength, Indicates the position of the direction, , The linear term coefficients of the Taylor expansion coefficients of each order of the slant range history with respect to the azimuth position, , , Indicates the center frequency of the transmitted signal.

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