Multi-channel spaceborne SAR (Synthetic Aperture Radar) high-squint sliding bunching imaging method

Abstract

The invention discloses a multi-channel spaceborne SAR (Synthetic Aperture Radar) high-squint sliding bunching imaging method, which solves the problems of signal time frequency inconsistency and serious imaging Doppler parameter coupling and space-variant in multi-channel spaceborne SAR high-squint sliding bunching mode imaging, and makes up the limitation of the prior art. Specifically, the method comprises the following five steps: step 1, constructing a slope distance model and an echo signal model of a multi-channel spaceborne SAR high squint sliding bunching signal; step 2, constructing a time-frequency characteristic parameter model of the multichannel spaceborne SAR high squint sliding bunching signal; step 3, carrying out azimuth preprocessing based on walking removal and spectrum reconstruction on the signal, and recovering to an equivalent single-channel signal; step 4, performing distance direction CS processing, and completing distance migration correction and distance compression; and step 5, carrying out azimuth NCS processing to complete azimuth compression.

Description

Technical Field

[0001] The present invention relates to the technical field of Synthetic Aperture Radar (SAR), and particularly relates to a multi-channel spaceborne SAR large squint sliding spotlight imaging method. Background Art

[0002] Synthetic Aperture Radar (SAR), as an all-weather and all-time observation technology, plays an important role in the fields of ocean exploration, global topographic mapping, and military monitoring due to its characteristics of being unrestricted by geography, having a wide coverage, long-range monitoring, and strong damage resistance. The SAR carried by a satellite is called spaceborne SAR.

[0003] Currently, the development focus of spaceborne SAR is concentrated on high resolution and wide-swath imaging, which can be summarized as "high resolution and wide swath". To achieve the goal of "high resolution and wide swath", researchers have conducted in-depth exploration on the imaging modes of spaceborne SAR. Based on the basic forward-looking stripmap mode, various imaging modes adapted to different observation requirements have been derived to effectively improve the two imaging indexes of resolution and swath width. The squint mode of spaceborne SAR adjusts the direction of the radar beam so that it is no longer strictly perpendicular to the satellite's motion direction, but tilts forward or backward at a certain angle. This mode can not only significantly expand the observation swath width of a single pass and effectively improve the coverage rate of earth observation, but also shorten the revisit time of a specific area by flexibly adjusting the observation angle. The multi-channel mode of spaceborne SAR receives the echoes synchronously through multiple sub-channels deployed at different azimuth positions. The system can receive echoes from multiple spatial points at each sampling moment, greatly enhancing the spatial resolution and coverage range of the signal. Thus, while maintaining or even improving the image resolution, the imaging swath width is broadened, achieving high resolution and wide swath simultaneously. The sliding spotlight mode of spaceborne SAR (abbreviated as the sliding spotlight mode) dynamically controls the radar beam to rotate at a certain angular velocity in the azimuth direction. While expanding the azimuth mapping width, it significantly extends the synthetic aperture time of the observed target area, thereby improving the azimuth resolution.

[0004] The multi-channel spaceborne SAR large squint sliding spotlight mode combines the above three imaging modes and can effectively achieve high resolution and wide swath. However, the echo signal and spectrum of this mode contain the complex characteristics of the three imaging modes, such as two-dimensional spectrum coupling, azimuth Doppler ambiguity, etc. Existing traditional algorithms are difficult to meet the imaging requirements. Therefore, it is necessary to deeply analyze the echo signal and time-frequency characteristics of the multi-channel spaceborne SAR large squint sliding spotlight mode and study the multi-channel spaceborne SAR large squint sliding spotlight imaging method. Summary of the Invention

[0005] In view of this, the present invention proposes a multi-channel spaceborne SAR large squint sliding spotlight imaging method. The specific content is as follows: constructing a slant range model and an echo signal model for the multi-channel spaceborne SAR large squint sliding spotlight signal, and constructing a time-frequency characteristic parameter model for the multi-channel spaceborne SAR large squint sliding spotlight signal; on this basis, proposing a squint multi-channel sliding spotlight imaging algorithm based on the fourth-order slant range model to realize multi-channel spaceborne SAR large squint sliding spotlight imaging.

[0006] To achieve the above object, the technical solution of the present invention is: a multi-channel spaceborne SAR large squint sliding spotlight imaging method, including:

[0007] Step 1: Construct a slant range model and an echo signal model for the multi-channel spaceborne SAR large squint sliding spotlight signal. Specifically, it includes constructing the fourth-order transmit and receive slant ranges of the multi-channel spaceborne SAR large squint sliding spotlight mode, and constructing the received echo signal model for each channel in the multi-channel spaceborne SAR large squint sliding spotlight mode.

[0008] Step 2: Construct a time-frequency characteristic parameter model for the multi-channel spaceborne SAR large squint sliding spotlight signal. Specifically, it includes constructing the Doppler center frequency, the Doppler center frequency change rate, and the total Doppler bandwidth of the multi-channel spaceborne SAR large squint sliding spotlight signal.

[0009] Step 3: Perform azimuth preprocessing based on deramping and spectrum reconstruction on the signal to restore it to an equivalent single-channel signal. Specifically, it includes deramp and linear range migration correction based on deramping and deskewing, azimuth spectrum reconstruction based on the inverse filter matrix, azimuth zero-padding based on the two-step imaging method, azimuth inverse deskewing, and azimuth compensation.

[0010] Step 4: Perform range CS processing to complete range migration correction and range compression. Specifically, it includes range chirp scaling, range matched filtering and range migration correction, and phase compensation.

[0011] Step 5: Perform azimuth NCS processing to complete azimuth compression. Specifically, it includes high-order phase compensation based on the fourth-order slant range model, azimuth non-linear chirp scaling, and azimuth compression.

[0012] Beneficial effects:

[0013] The present invention provides a multi-channel spaceborne SAR large squint sliding spotlight imaging method, and the main advantages include:

[0014] (1) Aiming at the problem of insufficient azimuth resolution of the traditional spaceborne SAR imaging algorithm, the present invention establishes the fourth-order transmit and receive slant ranges of the multi-channel spaceborne SAR large squint sliding spotlight mode, and proposes a squint multi-channel sliding spotlight imaging algorithm based on the fourth-order slant range model, realizing high-resolution imaging of the multi-channel spaceborne SAR large squint sliding spotlight.

[0015] (2) In view of the problems of inconsistent signal time-frequency, coupled and severely space-variant imaging Doppler parameters in the large squint sliding spotlight mode imaging of multi-channel spaceborne SAR, the present invention preprocesses the azimuth of the signal based on de-chirp and spectrum reconstruction to achieve the restoration of equivalent single-channel signals, thus making up for the limitations of the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is the flowchart of the large squint sliding spotlight imaging of the multi-channel spaceborne SAR according to the present invention;

[0017] Figure 2 is the schematic diagram of the geometric configuration of the large squint sliding spotlight of the multi-channel spaceborne SAR according to the present invention;

[0018] Figure 3 is the time-frequency relationship diagram of the large squint sliding spotlight mode of the multi-channel spaceborne SAR according to the present invention;

[0019] Figure 4 is the schematic diagram of the time-frequency relationship of the signal before and after the preprocessing step according to the present invention;

[0020] Figure 5 is the schematic diagram of the echo before and after the azimuth spectrum reconstruction step according to the present invention;

[0021] Figure 6 is the imaging result of the point target in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] The present invention will be described in detail below with reference to the accompanying drawings and by way of examples.

[0023] The flowchart of the large squint sliding spotlight imaging of the multi-channel spaceborne SAR according to the present invention is as Figure 1 shown, and the present invention includes the following steps:

[0024] Step 1: Construct the slant range model and echo signal model of the large squint sliding spotlight signal of the multi-channel spaceborne SAR.

[0025] The geometric configuration of the large squint sliding spotlight mode of the multi-channel spaceborne SAR is as Figure 2 shown. Taking the dual-channel mode as an example, the radar system includes three sub-channels: one transmitting channel (Tc) and two receiving channels (Rc). The left receiving channel is denoted as Rc -1 , and the right receiving channel is denoted as Rc1. d represents the distance between the receiving channel and the transmitting channel, and v r represents the equivalent velocity of the radar. The slant angle of the beam is θ, point A is the center of the wave foot, point P represents the target in the scene, and the distance from point A to point P is Y0. R represents the shortest distance from the satellite motion trajectory to the wave foot motion trajectory, and R0, R -1 , and R1 are the transmitting slant range and receiving slant ranges respectively.

[0026] The fourth-order Taylor expansion model of the transmit slant range in the multi-channel spaceborne SAR large squint sliding spotlight mode is as follows:

[0027]

[0028] Where \(t\) a is the azimuth time, \(t\) p represents the azimuth time from the zero moment to the beam center passing through the target \(P\), and \(R\) t is the slant range of the aperture center at \(t\) a =\(t\) p moment.

[0029] The receive slant range model can be expressed according to the transmit slant range model as:

[0030]

[0031] The echo signal of the \(i\)-th receive channel in the multi-channel spaceborne SAR large squint sliding spotlight mode can be expressed as

[0032]

[0033] Where \(t\) r is the range time, \(t\) a is the azimuth time, \(w\) r is the range envelope function, \(w\) a is the azimuth envelope function, \(R_0(t\) a ) and \(R\) i (\(t\) a ) are the two-way slant ranges varying with azimuth time, \(K\) r is the range modulation frequency, \(c\) is the speed of light, \(\lambda\) is the wavelength, \(A\) s is the sliding spotlight factor, and its value is the ratio of the wavefoot velocity in the actual sliding spotlight mode to the equivalent wavefoot velocity in the strip mode, \(T\) st represents the equivalent synthetic aperture time in the strip mode.

[0034] Step 2: Construct the time-frequency characteristic parameter model of the multi-channel spaceborne SAR large squint sliding spotlight signal.

[0035] According to the model established in Step 1, at the imaging moment \(t\) a , the Doppler center frequency \(f\) dc can be expressed as

[0036]

[0037] Define the change rate \(k\) rot of the Doppler center frequency as the derivative of the Doppler center frequency \(f\) dc with respect to the azimuth time \(t\) a ​

[0038]

[0039] For the point target P, by obtaining the stationary phase points in the azimuth Fourier transform, its azimuth Doppler frequency f aP can be expressed as

[0040]

[0041] where f c is the signal carrier frequency, f r is the range frequency, and Δθ ∈ [-θ azi / 2, θ azi / 2] is the angular range when the target is illuminated by the beam. The total Doppler bandwidth B total of the squinted spotlight mode can be expressed as the difference between the maximum and minimum values of the azimuth Doppler frequency, that is

[0042]

[0043] where T sl = T st / A s is the synthetic aperture time in the squinted spotlight mode. The total Doppler bandwidth of the squinted spotlight mode can be divided into three parts: the azimuth Doppler bandwidth B a , the azimuth beam scanning bandwidth B rot , and the squint bandwidth B sq . The time-frequency relationship diagram in the squinted spotlight mode is as shown in Figure 3 .

[0044] Step 3: Perform azimuth preprocessing on the signal based on deramping and spectral reconstruction to restore it to an equivalent single-channel signal.

[0045] For the multi-channel spaceborne SAR large squint sliding spotlight mode, it is necessary to preprocess the signal respectively according to the characteristics of the squint, multi-channel, and sliding spotlight modes. In the azimuth preprocessing step, it is divided into five steps in total.

[0046] (1) Deramp and linear range migration correction

[0047] This step aims at the serious range-azimuth coupling problem of the two-dimensional spectrum of the squint mode signal. By using squint minimization processing, the squint signal is multiplied by a phase factor that removes range walk to trim the two-dimensional spectrum and improve the two-dimensional orthogonality of the signal. For the problem of azimuth Doppler spectrum folding of the sliding spotlight mode signal, azimuth prefiltering in the two-step imaging processing method can be used. Figure 4 The signal time-frequency transformation diagram of the azimuth prefiltering step is given.

[0048] After the minimum squint processing and azimuth pre-filtering processing, the redundant Doppler bandwidth and the coupling of the two-dimensional spectrum can be effectively eliminated, and the squint spectrum is restored to an equivalent orthogonal spectrum. Multiply the signal of the i-th sub-channel by the deramping and de-squint function H 1i

[0049]

[0050] (2) Azimuth spectrum reconstruction

[0051] This step mainly performs spectrum reconstruction according to the characteristics of the multi-channel mode echo signal. By applying the transfer function method, the reconstruction matrix M and its inverse matrix are obtained, and the inverse matrix is multiplied by the signal to be reconstructed. The reconstruction matrix equation is given by the following formula

[0052]

[0053] where S a+i is the signal of the i-th channel after the first-step processing, and can be expressed as

[0054] S a+i =S i ·H 1i

[0055] where S0 is the non-aliased spectrum, ω = 2πf a .

[0056] The reconstruction matrix M can be expressed as

[0057]

[0058] The inverse matrix M -1 of the reconstruction matrix is called the inverse filtering matrix. Multiply this matrix by the echo spectrum of each channel after de-squinting and deramping in the first step to restore the non-aliased spectrum. Figure 5 The results before and after the azimuth spectrum reconstruction of the multi-channel mode point target echo are given.

[0059]

[0060] (3) Azimuth zero-padding

[0061] This step mainly aims at the problem of azimuth Doppler spectrum folding of the signal in the spotlight mode. This step is further divided into two sub-steps through the application of the upsampling part of the two-step imaging method: azimuth time-domain zero-padding and azimuth frequency-domain zero-padding. Assume that the original number of azimuth sampling points is N a , and the signal is converted to the range frequency domain azimuth time domain to complete the time-domain zero-padding. The number of azimuth sampling points is increased to:

[0062]

[0063] where Nc is the number of receiving channels, B total represents the total Doppler bandwidth of the squint sliding spotlight mode.

[0064] Convert the signal to the two-dimensional frequency domain through azimuth FFT. In the two-dimensional frequency domain, the number of azimuth sampling points finally increases to:

[0065]

[0066] (4) Azimuth inverse deramp

[0067] The purpose of this step is to eliminate the influence of the deramp step on the signal. Multiply the upsampled signal obtained in the third step by the inverse deramp function

[0068]

[0069] (5) Azimuth signal compensation

[0070] Perform signal compensation in the azimuth frequency domain and multiply the signal by the compensation function

[0071]

[0072] where f aup is the PRF after upsampling.

[0073] Step 4: Perform range CS processing to complete range migration correction and range compression.

[0074] After the azimuth preprocessing step, the signal is restored to an equivalent single-channel signal. Therefore, the range CS algorithm is used for range compression. The two-dimensional spectrum of the preprocessed signal is given by the following equations:

[0075]

[0076] where

[0077]

[0078] Convert the signal to the range-Doppler domain and multiply it by the range CS function

[0079]

[0080] where

[0081] R(f a ; R ref ) = R(1 + a(f a ))

[0082] R is the shortest distance between the radar motion path and the wavefoot path. R ref is the reference slant range.

[0083] Convert the signal to the two-dimensional frequency domain and multiply it by the range matching filter and range migration correction function

[0084]

[0085] Finally, convert the signal to the range-Doppler domain and multiply it by the phase compensation function

[0086]

[0087] Step 5: Perform azimuth NCS processing to complete azimuth compression.

[0088] The azimuth NCS algorithm is applied to this step. In the azimuth compression step, the slant range model is extended to the fourth order, thereby improving the accuracy of azimuth compression. The phase part of the signal after range compression is given by the following formula

[0089]

[0090] where

[0091]

[0092] where R c is the slant range at which the target point is focused after range compression, K a is the azimuth chirp rate, and are the third and fourth order phases of the signal after range compression, respectively.

[0093] First, compensate for the high-order phases and by multiplying the signal by the high-order phase compensation function

[0094]

[0095] Then convert the signal to the two-dimensional time domain and multiply it by the azimuth NCS function

[0096]

[0097] Finally, convert the signal to the range-Doppler domain and multiply it by the azimuth compression function

[0098]

[0099] After inverse Fourier transform, the image focusing is completed.

[0100] Simulation experiment: The simulation parameters of multi-channel spaceborne SAR large squint sliding spotlight imaging are shown in Table 1.

[0101] Table 1 List of Simulation Parameters for Multichannel Spaceborne SAR Large Squint Sliding Spotlight Imaging

[0102]

[0103]

[0104] Under the parameters in Table 1, using the multichannel spaceborne SAR large squint sliding spotlight imaging method described in the present invention, a group of point targets in the scene are imaged, and the imaging results are as Figure 6 shown. Select the central point target B, the range-direction edge point target C, and the azimuth-direction edge point target D in the scene for resolution, peak sidelobe ratio, and integrated sidelobe ratio evaluation. The evaluation results are shown in Table 2.

[0105] Table 2 Evaluation Results of Simulation Point Targets for Multichannel Spaceborne SAR Large Squint Sliding Spotlight Imaging

[0106]

[0107] It can be seen that the present invention provides a multichannel spaceborne SAR large squint sliding spotlight imaging method, analyzes the SAR signal model and time-frequency relationship in this mode, and proposes a squint multichannel sliding spotlight imaging algorithm based on the fourth-order slant range model, realizing high-resolution imaging of multichannel spaceborne SAR large squint sliding spotlight. The present invention solves the problems of inconsistent signal time-frequency, serious coupling and spatial variation of imaging Doppler parameters in the imaging of multichannel spaceborne SAR large squint sliding spotlight mode, and makes up for the limitations of the existing technology.

[0108] In summary, the above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A multi-channel spaceborne SAR high squint sliding spotlight imaging method, characterized in that: include: Step 1: construct the slant range model and echo signal model of the multi-channel spaceborne SAR high-squint sliding beam signal; specifically, it includes the construction of the fourth-order transmission and receiving slant range of the multi-channel spaceborne SAR high-squint sliding beam mode, and the construction of the receiving echo signal model of each channel in the multi-channel spaceborne SAR high-squint sliding beam mode; Step 2: construct the time-frequency characteristic parameter model of the multi-channel spaceborne SAR high squint sliding beam signal; specifically, it includes the construction of the Doppler center frequency, the Doppler center frequency change rate, and the total Doppler bandwidth of the multi-channel spaceborne SAR high squint sliding beam signal; Step 3: Perform azimuth preprocessing on the signal based on de-motion and spectrum reconstruction to restore it to an equivalent single-channel signal; specifically, it includes deramp and linear range motion correction based on de-motion and de-skew, azimuth spectrum reconstruction based on inverse filter matrix, azimuth zero filling based on two-step imaging method, azimuth inverse de-skew and azimuth compensation; Step 4: Perform range CS processing to complete range motion correction and range compression; specifically, it includes range linear frequency modulation, range matching filtering, range motion correction, and phase compensation; Step 5: Perform azimuth NCS processing to complete azimuth compression; specifically, it includes high-order phase compensation based on the fourth-order slant range model, azimuth nonlinear frequency modulation scaling, and azimuth compression.

2. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 1, the fourth-order Taylor expansion model of the launch slant range is as follows: where t a is the azimuth time, t p represents the azimuth time from time zero to the time when the beam center passes through the target P, R t t a =t p Slope distance from the center of the aperture at that moment.

3. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 1, the receiving slant range model is expressed according to the transmitting slant range model as follows: Where R0 is the launch slant distance, t a is the azimuth time, d is the distance between the receiving channel and the transmitting channel, v r Indicates the equivalent speed of the radar.

4. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 1, the echo signal of the i-th receiving channel is expressed as where t r is the distance to time, t a is the azimuth time, w r is the distance envelope function, w a is the azimuth envelope function, R0(t a ) and R i (t a ) is the time-varying two-way slant range in azimuth, K r is the distance modulation frequency, c is the speed of light, λ is the wavelength, A s is the sliding factor, which is the ratio of the wave foot velocity in the actual sliding mode to the equivalent wave foot velocity in the strip mode, T st Represents the equivalent synthetic aperture time in strip mode.

5. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 2, the total Doppler bandwidth of the squint sliding mode is divided into three parts: azimuth Doppler bandwidth B a 、Azimuth beam scanning bandwidth B rot , squint bandwidth B sq .

6. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step three, the multi-channel spaceborne SAR high squint sliding beam mode pre-processes the signal according to the characteristics of the squint, multi-channel and sliding beam modes respectively.

7. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step three, the orientation preprocessing step is divided into five steps: (1) deramp and linear distance migration correction; (2) azimuth spectrum reconstruction; (3) Azimuth zero filling; (4) Azimuth reverse declination; (5) Azimuth signal compensation.

8. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 4, the distance CS algorithm is used to perform distance compression.

9. The multi-channel spaceborne SAR high squint sliding spotlight imaging method according to claim 1, characterized in that: In step 5, NCS processing is performed in azimuth, and the method for completing azimuth compression is as follows: The phase part of the signal after range compression is: in Where R c K is the slant distance from the compressed target point. a is the azimuth frequency modulation, and They are the third-order and fourth-order phases of the signal after range compression. First, the high-order phase and To compensate, multiply the signal by a high-order phase compensation function The signal is then converted to the two-dimensional time domain and multiplied by the azimuth NCS function Finally, the signal is converted to the range-Doppler domain and multiplied by the azimuth compression function After inverse Fourier transform, the image is focused.

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