Spaceborne Bistatic SAR Non-track Mode Temporal Imaging Method

Abstract

This invention discloses a time-domain imaging method for spaceborne bistatic SAR non-track mode. The invention establishes an accurate bistatic time delay model based on a linear approximation of transmission delay, solving the problem of time-domain imaging defocusing caused by the failure of the "walk-stop" assumption in spaceborne bistatic SAR non-track mode, and achieving high-precision time-domain imaging in spaceborne bistatic SAR non-track mode.

Description

Technical Field

[0001] This invention relates to the field of synthetic aperture radar technology, and specifically to a spaceborne bistatic SAR non-track mode time-domain imaging method. Background Technology

[0002] Synthetic Aperture Radar (SAR) has advantages such as all-weather and all-time operation, high two-dimensional resolution, and strong penetration. It is also an active microwave remote sensing device, thus becoming an effective means of Earth remote sensing and playing a very important role in applications such as disaster early warning, environmental monitoring, and military reconnaissance.

[0003] Non-track imaging mode is a unique operating mode of spaceborne SAR. Traditional spaceborne SAR generates mapping strips along the satellite orbit, while spaceborne non-track SAR directly generates imaging strips distributed along the target terrain by continuously adjusting the beam pointing in the elevation and azimuth dimensions. Therefore, for certain typical "non-track" scenarios such as seismic zones and coastlines, it can fundamentally reduce echo data redundancy and significantly improve observation efficiency. Spaceborne bistatic SAR systems have flexible geometric configurations, which are conducive to multi-angle observation of targets, can obtain rich image information, and complete various remote sensing tasks; at the same time, due to its separate transmission and reception configuration, the system's survivability and anti-jamming interception capabilities are greatly improved. However, spaceborne bistatic SAR also has some problems: its observation is not flexible enough, and because its imaging strip follows the track, its observation efficiency is low for some non-track scenarios. Spaceborne bistatic SAR non-track mode combines the flexibility of bistatic observation with the efficiency of non-track mode observation, and has unique application space and development prospects in various fields of civilian and military applications. However, due to the complexity of non-track observation scenarios, the "walk-stop" assumption fails, resulting in temporal imaging defocusing problems. Summary of the Invention

[0004] In view of this, the present invention provides a spaceborne bistatic SAR non-track mode time-domain imaging method, which can realize high-precision time-domain imaging of spaceborne bistatic SAR non-track mode.

[0005] The spaceborne bistatic SAR non-track mode temporal imaging method of the present invention includes:

[0006] Step 1: Construct a spaceborne bistatic SAR transmission delay model and approximate the transmission delay using a distance-dimensional linear approximation; wherein, cubic and quartic compensation terms are added to the spaceborne bistatic SAR slant range model.

[0007] Step 2, echo preprocessing, includes line frequency modulation removal, residual video phase removal, and residual secondary phase compensation. Then, the signal is transformed into range frequency domain-azimuth time domain. Among them, after line frequency modulation removal, the signal is truncated and the aliasing part of the signal is filtered out.

[0008] Step 3: Divide the non-track imaging grid;

[0009] Specifically, a Cartesian coordinate system for non-track imaging grids is established with the scene center point of the non-track scene as the origin: the line connecting the beginning and end of the non-track beam footprint is defined as the azimuth direction, and is defined as the positive X-axis; the direction upward from the ground surface at the perpendicular origin is defined as the positive Z-axis; the Y-axis direction, i.e., the range direction, is determined according to the X-axis, Z-axis and right-hand rule; the non-track imaging grid spacing is determined according to the geometric configuration and signal transmission bandwidth of the non-track bistatic base, and a sufficient number of range imaging grid points are selected to cover the range beam coverage area, and grid points are arranged at each azimuth position along the non-track beam footprint;

[0010] Step 4, target determination for non-track imaging grid beam illumination, specifically includes:

[0011] S41, Coordinate System Rotation: Move the origin of the non-track imaging grid Cartesian coordinate system to the position of the radar platform at the corresponding azimuth time. Then rotate the coordinate system around the Z-axis, rotating the positive Y-axis to the direction in which the radar ground projection point points to the beam center. Next, rotate the coordinate system around the X-axis, pointing the positive Z-axis to the ground beam center. Then, determine the beam center rotation angle based on the radar satellite attitude information and rotate the Z-axis by the corresponding angle to obtain the imaging grid coordinates in the SAR antenna coordinate system.

[0012] S42, Illumination Judgment: Based on the two-dimensional beamwidth of the radar beam, establish the equation of the elliptical cone surface to perform beam illumination judgment on the imaging grid points established in step 3; if the Z value of the imaging grid point in the SAR antenna coordinate system is greater than the X coordinate and the Y coordinate corresponds to the Z value on the elliptical cone surface, then the target is considered to be inside the elliptical cone surface and within the beam illumination range; if the Z value of the imaging grid point in the SAR antenna coordinate system is less than the X coordinate and the Y coordinate corresponds to the Z value on the elliptical cone surface, then it is outside the beam illumination range.

[0013] Step 5: Perform back projection processing;

[0014] Step 6: Recover the Doppler phase based on the slant distance between the synthetic aperture center and the scene center to ensure that the image phase corresponds to the corresponding physical location, thus obtaining the final SAR image.

[0015] Preferably, in step 1, third and fourth order compensation terms are first added to the slant range model to obtain the slant range trajectories R from the transmitter and receiver to the target point at any time τ.T (τ) and R R (τ+τ d ), and thus the transmission delay τ is obtained. d The expression:

[0016]

[0017] Where c is the speed of light;

[0018] R T (τ) and R R (τ+τ d Substituting the expression of ) into formula (3) and simplifying it, then the transmission delay τ d Regarding distance to fast time t r Performing a Taylor series expansion and ignoring second-order and higher distance-time terms, the transmission delay is obtained as follows:

[0019] τ d (t a ,t r )≈τ dc (t a )+K τ (t a )(t r -τ c (5)

[0020] Where, τ dc (t a ) represents the transmission delay at the center; K τ For transmission delay with a fast time change rate;

[0021] Based on formula (5), a first-order fit is performed on the transmission delay of the beam center slant range at different times to obtain τ. dc (t a ) and K τ (t a This achieves a linear approximation of the distance dimension of non-track bistatic transmission delay.

[0022] Preferably, in step 2, the signal after demodulation is truncated, and the truncated range is:

[0023] T p ′=T p -2|τ d -τ c | max (10)

[0024] Among them, T p τ is the pulse duration of the signal. d For transmission delay, τ c =2R c / c, Rc This is the shortest slant distance corresponding to the reference point at the center of the scene.

[0025] Preferably, in step 2, the residual video phase removal filter constructed during residual video phase removal processing is:

[0026]

[0027] Where K is the frequency modulation frequency of the frequency-modulated continuous wave signal; f r f represents the coherent difference frequency. r =-K(τ) d -τ c ), τ d This refers to transmission delay.

[0028] Preferably, in step 2, during residual quadratic phase compensation, the constructed residual quadratic phase removal filter is:

[0029] H sec =exp{j2πKK τ (t r -τ c ) 2} (14)

[0030] Where K is the frequency modulation frequency of the frequency-modulated continuous wave signal; K τ For transmission delay, the rate of change of time is fast; t r For distance to fast time; τ c =2R c / c, where c is the speed of light.

[0031] Preferably, in step 5, range ascent sampling is first performed. Based on the coordinates of the grid points and the satellite's orbital coordinates, the transmission delay corresponding to different azimuth times is calculated to obtain τ related to the azimuth sampling time. dc and K τ This allows us to obtain the precise location of the distance migration peak for each grid point, compensate for the Doppler phase corresponding to the distance migration peak point, project the compensated echo signal onto the corresponding non-track imaging grid point, and coherently accumulate the pulses at each azimuth time to finally obtain the back projection result.

[0032] Beneficial effects:

[0033] This invention establishes an accurate bistatic delay model based on a linear approximation of transmission delay, enabling high-precision temporal imaging in spaceborne bistatic SAR non-track mode. This invention solves the problem of temporal imaging defocusing caused by the failure of the "walk-stop" assumption in spaceborne bistatic SAR non-track mode, thus overcoming the shortcomings of existing technologies. Attached Figure Description

[0034] Figure 1 This is a flowchart of the present invention;

[0035] Figure 2 A schematic diagram of the non-trajectory spatial configuration of a spaceborne bistatic SAR;

[0036] Figure 3 A schematic diagram of a non-track point target in a spaceborne bistatic SAR;

[0037] Figure 4 A time-frequency relationship diagram of non-track-based bistatic SAR.

[0038] Figure 5 This is the imaging result of a non-track point target from a spaceborne bistatic SAR. Detailed Implementation

[0039] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0040] This invention provides a spaceborne bistatic SAR non-track mode temporal imaging method, the flowchart of which is shown below. Figure 1 As shown, it specifically includes:

[0041] S1: Non-track bistatic transmission delay distance dimension linear approximation.

[0042] Non-track mode bibase model spatial configuration such as Figure 2 As shown, the imaging strait is not parallel to the satellite's flight direction and its trajectory is complex and variable. This invention employs an improved straight-track approximate slant range model, adding third and fourth order compensation terms to approximate the satellite's orbit. Assume there is a point target P within the observation strait, and the signal transmitted by the transmitter reaches point target P after a transmission delay τ. d Reaching the receiver. Let R be the slant range history from the transmitter to the target point at any time τ. T (τ), the slant range history from the receiver to the target point is R. R (τ), the expressions are shown in formula (1) and formula (2) respectively.

[0043]

[0044]

[0045] Among them, a T and b T These are the compensation coefficients for the cubic and quartic terms of the transmitter-to-target slant range, respectively, a. R and b R These are the cubic and quartic compensation coefficients for the receiver-to-target slant range, respectively. R0 represents the shortest slant range from the target center to the satellite track, and V... T V represents the speed of the transmitter. Rτ represents the velocity of the receiver. T0 This indicates the center time of the observed target P.

[0046] Assume that the transmitter transmits a signal at time τ, and the receiver receives the signal after a transmission delay of τ. d After τ+τ d The instantaneous slant range from the receiver to the target point is denoted as R when the echo signal is received at any given time. R (τ+τ d If c is the speed of light, then the transmission delay can be expressed as:

[0047]

[0048] Conventional spaceborne SAR imaging algorithms approximate the signal transmission and reception process as a "stop-go-stop" process, meaning that the satellite is assumed to transmit and receive signals from the same location. This approximation has little error under airborne platforms and low-resolution requirements. However, in spaceborne configurations, the transmission and reception platforms will undergo greater displacement during signal transmission, resulting in phase errors that cannot be ignored. Therefore, it is necessary to establish an accurate bistatic time delay model to achieve more accurate bistatic distance-migrating positioning. By approximating the transmission delay linearly in the range dimension, substituting equations (1) and (2) into equation (3) and simplifying them, the transmission delay expression is obtained as shown in equation (4).

[0049]

[0050] The transmission delay τ in formula (4) d Regarding distance to fast time t r After performing a Taylor series expansion and ignoring the second-order and higher distance-time terms, the transmission delay is expressed as:

[0051] τ d (t a ,t r )≈τ dc (t a )+K τ (t a )(t r -τ c (5)

[0052] In the formula, t a For azimuth slow time, τ c =2R c / c, R c τ is the shortest slant distance corresponding to the scene center reference point. dc (t a The transmission delay is centered on the azimuth and varies with the azimuth direction. K τ The transmission delay has a fast time change rate.

[0053] Since the shortest slant range from the target to the orbit at different azimuth positions in the non-track mode is spatially variable, a first-order fit is performed on the transmission delay of the beam center slant range at different times based on formula (5) to obtain τ. dc (t a ) and K τ (t a This allows for a linear approximation of the distance dimension of non-track bistatic transmission delay.

[0054] Assuming the backscattering coefficient of target P is σ, and the transmitter transmits a frequency-modulated continuous wave signal, the echo signal received by the receiver is expressed as shown in formula (6).

[0055]

[0056] Where K is the frequency modulation frequency of the frequency-modulated continuous wave signal, f0 is the carrier frequency, and T P This represents the pulse duration. Since the backscattering coefficient is constant and does not affect the phase, σ is neglected during subsequent normalization.

[0057] Substituting the precise time delay expression (5) into the echo expression (6), the echo signal can be expressed as:

[0058]

[0059] Among them, T p ′ represents the effective pulse width.

[0060] S2: Echo preprocessing, including line frequency modulation demodulation, residual video phase removal, and residual secondary phase compensation. The time-frequency relationship diagram of S2 preprocessing is shown below. Figure 4 As shown.

[0061] S21, Line Frequency Modulation Processing:

[0062] First, constructing the de-frequency modulation filter requires establishing a reference signal with the same form as the echo signal but a known propagation delay. The beam center slant distance at different times is selected as the propagation delay τ of the reference signal. c By setting the modulation frequency of the reference signal to be the same as that of the echo signal, the expression for the line frequency modulation filter is obtained as shown in formula (8).

[0063]

[0064] The de-modulation filter shown in formula (8) is multiplied by the echo signal shown in formula (7) using conjugate multiplication to complete the de-modulation process. The signal expression after the de-modulation process is shown in formula (9):

[0065]

[0066] To avoid signal aliasing after frequency modulation (FM) continuous wave demodulation, the signal needs to be truncated, such as... Figure 4 As shown, the cutoff range is:

[0067] T p ′=T p -2|τ d -τ c | max (10)

[0068] Wherein, the pulse duration of the signal is T. p The aliasing portion is twice the difference between the transmission delay and the slant distance transmission delay from the beam center. After filtering out the aliasing portion, the pulse width of the signal is T. p Reduced to T′ p T p With T′ p Relationship such as Figure 4 As shown.

[0069] S22, Residual video phase removal processing:

[0070] In equation (9), the phase is divided into three parts: the first exponential term contains the target azimuth information, the second exponential term contains the range migration information, and the last phase term is the residual video phase generated by the demodulation process. Since the residual video phase may affect target focusing, a residual video phase removal filter needs to be constructed, as shown in equation (11):

[0071]

[0072] Where f r The coherent difference frequency is represented by the expression shown in formula (12).

[0073] f r =-K(τ) d -τ c (12)

[0074] Multiplying the residual video phase filter by the demodulated line frequency signal removes the residual video phase, resulting in the following expression:

[0075]

[0076] S23, Residual secondary phase compensation processing:

[0077] The third phase term in formula (13) is a distance-time quadratic term generated by inter-pulse modulation, which needs to be compensated and filtered out. A residual quadratic phase filter is constructed, as shown in formula (14):

[0078] H sec =exp{j2πKK τ(t r -τ c ) 2} (14)

[0079] Multiplying the residual-removed quadratic phase filter by the signal and performing filtering yields the following expression:

[0080]

[0081] Performing a range-to-Fourier transform on the obtained results yields the signal range-frequency domain-azimuth-time domain expression:

[0082] S3(t a ,f r )=sinc{πT p ′[f r +K(τ dc -τ c )+f0K τ ]}×exp{-j2πf0(τ dc -τ c )} (16)

[0084] S3: Divide the non-track imaging grid.

[0085] A Cartesian coordinate system for the non-track imaging grid is established using the scene center point as the origin. The line connecting the beginning and end of the non-track beam footprint is defined as the azimuth direction, specifically the positive X-axis. The direction upwards from the ground surface at the origin is defined as the positive Z-axis. The Y-axis direction, i.e., the range direction, is determined according to the right-hand rule for the X and Z axes. The grid spacing for the non-track imaging grid is determined based on the geometry of the bistatic non-track system and the signal transmission bandwidth. A sufficient number of range-oriented imaging grid points are selected to cover the range beam coverage area, and these grid points are arranged along the non-track beam footprint at each azimuth position to ensure full coverage of the illuminated area.

[0086] S4: Non-track imaging grid beam illumination judgment.

[0087] Due to the spatial variation of the surface beam ellipse in non-track mode, beam determination is required during imaging to avoid Doppler aliasing. Target determination involves two steps: coordinate system rotation and beam determination.

[0088] Step 1, coordinate system rotation:

[0089] First, at the corresponding azimuth time, the origin of the non-tracking imaging grid Cartesian coordinate system is moved to the position of the radar platform at the corresponding azimuth time. Then, the coordinate system is rotated around the Z-axis, with the positive Y-axis rotated so that the radar ground projection point points towards the beam center. Next, the coordinate system is rotated around the X-axis, with the positive Z-axis pointing towards the ground beam center. Finally, based on the radar satellite's attitude information, the beam center rotation angle is determined, and the Z-axis is rotated by the corresponding angle, thus obtaining the imaging grid coordinates in the SAR antenna coordinate system.

[0090] Step 2, Irradiation determination:

[0091] Based on the two-dimensional beamwidth of the radar beam, an elliptical cone equation is established to determine the beam illumination of the imaging grid points established in S3. If the Z-value of the imaging grid point in the SAR antenna coordinate system is greater than the Z-value corresponding to the X and Y coordinates on the elliptical cone, the target is considered to be inside the elliptical cone and within the beam illumination range. If the Z-value of the imaging grid point in the SAR antenna coordinate system is less than the Z-value corresponding to the X and Y coordinates on the elliptical cone, it is outside the beam illumination range. This completes the beam illumination determination of the non-track imaging grid at different azimuth times.

[0092] S5: Back projection processing.

[0093] First, range upsampling is performed using frequency domain interpolation with zero-padding. Then, based on the coordinates of the grid points and the satellite's trajectory, the transmission delay at different azimuth times is calculated, yielding τ related to the azimuth sampling time. dc and K τ This allows us to obtain the precise distance migration peak position corresponding to each grid point. According to formula (17), the distance frequency point f corresponding to the peak position of the distance migration is... p for

[0094] f p =-K(τ) dc -τ c )-f0K τ (17)

[0095] The compensation function expression for the Doppler phase corresponding to the peak point of the distance migration is shown in formula (18):

[0096] H D =exp{-j2πf0(τ)} dc -τ c (18)

[0097] The compensated echo signal is projected onto the corresponding non-track imaging grid points, and the pulses at each azimuth time are coherently accumulated to obtain the back projection result, as shown in the following expression.

[0098]

[0099] S6: Phase-preserving processing to obtain SAR images.

[0100] After completing the back projection processing of S5, the Doppler phase needs to be recovered based on the slope distance between the synthetic aperture center and the scene center to ensure that the image phase corresponds to the corresponding physical position. The expression is shown in (20):

[0101]

[0102] This yields the final SAR image.

[0103] Example

[0104] To verify the advantages of the spaceborne bistatic SAR non-track mode time-domain imaging method, simulations were performed using the spaceborne SAR parameters in Table 1. The imaging results validated the imaging capability of the method proposed in this patent. This patent simulated 9 targets, with a theoretical ground range resolution of 2.67m and a theoretical azimuth resolution of 2.22m. The target range spacing was 2km, and the azimuth spacing was 8km. The target distribution is as follows: Figure 3 As shown in Table 2, the imaging parameters are as follows.

[0105] Table 1 List of key parameters for SAR satellites

[0106] Parameter name numerical values unit orbital height 600 km Scene tilt angle 45 deg Distance to width 10 km Azimuth width 20 km carrier frequency 10 GHz Pulse width 20 us

[0107] Table 2 Target Parameter List

[0108]

[0109] Step 1: Obtain the echo based on the parameters in Table 1 and Equation (4).

[0110] Step 2: Construct a reference signal with the same form as the echo signal but a known transmission delay. Multiply the reference signal by its conjugate with the echo signal to obtain the signal after de-frequency modulation processing, equation (9). Filter out the residual video phase term and perform secondary filtering to obtain equation (11). Perform a range-to-Fourier transform on equation (11) to obtain equation (12). The signal time-frequency relationship is shown in the figure. Figure 4 As shown.

[0111] Step 3: Generate an imaging mesh based on the scene's geographical orientation, such as... Figure 3 As shown.

[0112] Step 4: Non-trace imaging grid beam illumination judgment.

[0113] Step 5: Perform back projection imaging processing.

[0114] Step 6: Perform phase-preserving processing to obtain the final SAR image.

[0115] Figure 5 The simulation results of the nine-point target are presented. It can be seen from the figure that the point target results at different positions are well focused. The evaluation of the center point target of the scene is shown in Table 3. It can be seen that the evaluation results of the center point target of the scene meet the index requirements, and the imaging method proposed in this invention is verified.

[0116] Table 3. Point Target Assessment Results of Non-Tracking Temporal Imaging Mode for Spaceborne Bistatic SAR

[0117] Resolution (m) Peak sidelobe ratio (dB) Integral sidelobe ratio (dB) Distance 2.67 -13.29 -10.19 Orientation 2.22 -13.23 -10.14

[0118] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A spaceborne bistatic SAR non-track mode temporal imaging method, characterized in that, include: Step 1: Construct a spaceborne bistatic SAR transmission delay model and approximate the transmission delay using a range-dimensional linear approximation. Specifically, firstly, cubic and quartic compensation terms are added to the spaceborne bistatic SAR slant range model to obtain the slant range trajectories R from the transmitter and receiver to the target point at any time τ. T (τ) and R R (τ+τ d ), and thus the transmission delay τ is obtained. d The expression: Where c is the speed of light; R T (τ) and R R (τ+τ d Substituting the expression of ) into formula (3) and simplifying it, then the transmission delay τ d Regarding distance to fast time t r Performing a Taylor series expansion and ignoring second-order and higher distance-time terms, the transmission delay is obtained as follows: τ d (t a ,t r )≈τ dc (t a )+K τ (t a )(t r -τ c )(5) Where, τ dc (t a ) represents the transmission delay at the center; K τ For transmission delay with a fast time change rate; Based on formula (5), a first-order fit is performed on the transmission delay of the beam center slant range at different times to obtain τ. dc (t a ) and K τ (t a This achieves a linear approximation of the distance dimension of non-track bistatic transmission delay; Step 2, echo preprocessing, includes line modulation removal, residual video phase removal, and residual secondary phase compensation. The signal is then transformed into the range-frequency domain-azimuth time domain. After line modulation removal, the signal is truncated to filter out aliasing. The truncated range of the demodulated signal is as follows: T p ′=T p -2|t d -t c | max (10) Among them, T p τ is the pulse duration of the signal. d For transmission delay, τ c =2R c / c, R c This is the shortest slant distance corresponding to the reference point at the center of the scene; Step 3: Divide the non-track imaging grid; Specifically, a Cartesian coordinate system for non-track imaging grids is established with the scene center point of the non-track scene as the origin: the line connecting the beginning and end of the non-track beam footprint is defined as the azimuth direction, and is defined as the positive X-axis; the direction upward from the ground surface at the perpendicular origin is defined as the positive Z-axis; the Y-axis direction, i.e., the range direction, is determined according to the X-axis, Z-axis and right-hand rule; the non-track imaging grid spacing is determined according to the geometric configuration and signal transmission bandwidth of the non-track bistatic base, and a sufficient number of range imaging grid points are selected to cover the range beam coverage area, and grid points are arranged at each azimuth position along the non-track beam footprint; Step 4, target determination for non-track imaging grid beam illumination, specifically includes: S41, Coordinate System Rotation: Move the origin of the non-track imaging grid Cartesian coordinate system to the position of the radar platform at the corresponding azimuth time. Then rotate the coordinate system around the Z-axis, rotating the positive Y-axis to the direction in which the radar ground projection point points to the beam center. Next, rotate the coordinate system around the X-axis, pointing the positive Z-axis to the ground beam center. Then, determine the beam center rotation angle based on the radar satellite attitude information and rotate the Z-axis by the corresponding angle to obtain the imaging grid coordinates in the SAR antenna coordinate system. S42, Illumination Judgment: Based on the two-dimensional beamwidth of the radar beam, establish the equation of the elliptical cone surface to perform beam illumination judgment on the imaging grid points established in step 3; if the Z value of the imaging grid point in the SAR antenna coordinate system is greater than the X coordinate and the Y coordinate corresponds to the Z value on the elliptical cone surface, then the target is considered to be inside the elliptical cone surface and within the beam illumination range; if the Z value of the imaging grid point in the SAR antenna coordinate system is less than the X coordinate and the Y coordinate corresponds to the Z value on the elliptical cone surface, then it is outside the beam illumination range. Step 5: Perform back projection processing; Step 6: Recover the Doppler phase based on the slant distance between the synthetic aperture center and the scene center to ensure that the image phase corresponds to the corresponding physical location, thus obtaining the final SAR image.

2. The spaceborne bistatic SAR non-track mode time-domain imaging method as described in claim 1, characterized in that, In step 2, the residual video phase removal filter constructed during residual video phase removal processing is as follows: Where K is the frequency modulation frequency of the frequency-modulated continuous wave signal; f r f represents the coherent difference frequency. r =-K(τ) d -τ c ), τ d This refers to transmission delay.

3. The spaceborne bistatic SAR non-track mode time-domain imaging method as described in claim 1, characterized in that, In step 2, during residual quadratic phase compensation, the constructed residual quadratic phase removal filter is: H sec =exp{j2πKK τ (t r -τ c ) 2 } (14) Where K is the frequency modulation frequency of the frequency-modulated continuous wave signal; K τ For transmission delay, the rate of change of time is fast; t r For distance to fast time; τ c =2R c / c, where c is the speed of light.

4. The spaceborne bistatic SAR non-track mode time-domain imaging method as described in claim 1, characterized in that, In step 5, range ascent sampling is first performed. Based on the coordinates of the grid points and the satellite's orbital coordinates, the transmission delay corresponding to different azimuth times is calculated, and the τ related to the azimuth sampling time is obtained. dc and K τ This allows us to obtain the precise location of the distance migration peak for each grid point, compensate for the Doppler phase corresponding to the distance migration peak point, project the compensated echo signal onto the corresponding non-track imaging grid point, and coherently accumulate the pulses at each azimuth time to finally obtain the back projection result.

Images